Transcriber's Note

 The punctuation and spelling from the original text have been faithfully preserved. Only obvious typographical errors have been corrected. 

THE STORY OF THE HEAVENS

[Illustration: PLATE I. 

THE PLANET SATURN, IN 1872. ]

 THE

 STORY OF THE HEAVENS

 SIR ROBERT STAWELL BALL, LL. D. D. Sc. 

 _Author of_ "_Star-Land_"

 FELLOW OF THE ROYAL SOCIETY OF LONDON, HONORARY FELLOW OF THE ROYAL SOCIETY OF EDINBURGH, FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY, SCIENTIFIC ADVISER TO THE COMMISSIONERS OF IRISH LIGHTS, LOWNDEAN PROFESSOR OF ASTRONOMY AND GEOMETRY IN THE UNIVERSITY OF CAMBRIDGE, AND FORMERLY ROYAL ASTRONOMER OF IRELAND

 _WITH TWENTY-FOUR COLOURED PLATES AND NUMEROUS ILLUSTRATIONS_

 NEW AND REVISED EDITION

 CASSELL AND COMPANY, LIMITED _LONDON, PARIS, NEW YORK & MELBOURNE_ 1900

 ALL RIGHTS RESERVED

[Illustration: LA·BELLE SAUVAGE]

PREFACE TO ORIGINAL EDITION. 

I have to acknowledge the kind aid which I have received in thepreparation of this book. 

Mr. Nasmyth has permitted me to use some of the beautiful drawings ofthe Moon, which have appeared in the well-known work published by him inconjunction with Mr. Carpenter. To this source I am indebted for PlatesVII. , VIII. , IX. , X. , and Figs. 28, 29, 30. 

Professor Pickering has allowed me to copy some of the drawings made atHarvard College Observatory by Mr. Trouvelot, and I have availed myselfof his kindness for Plates I. , IV. , XII. , XV. 

I am indebted to Professor Langley for Plate II. , to Mr. De la Rue forPlates III. And XIV. , to Mr. T. E. Key for Plate XVII. , to ProfessorSchiaparelli for Plate XVIII. , to the late Professor C. Piazzi Smyth forFig. 100, to Mr. Chambers for Fig. 7, which has been borrowed from his"Handbook of Descriptive Astronomy, " to Dr. Stoney for Fig. 78, and toDr. Copeland and Dr. Dreyer for Fig. 72. I have to acknowledge thevaluable assistance derived from Professor Newcomb's "PopularAstronomy, " and Professor Young's "Sun. " In revising the volume I havehad the kind aid of the Rev. Maxwell Close. 

I have also to thank Dr. Copeland and Mr. Steele for their kindness inreading through the entire proofs; while I have also occasionallyavailed myself of the help of Mr. Cathcart. 

 ROBERT S. BALL. 

 OBSERVATORY, DUNSINK, CO. DUBLIN. _12th May, 1886. _

NOTE TO THIS EDITION. 

I have taken the opportunity in the present edition to revise the workin accordance with the recent progress of astronomy. I am indebted tothe Royal Astronomical Society for the permission to reproduce somephotographs from their published series, and to Mr. Henry F. Griffiths, for beautiful drawings of Jupiter, from which Plate XI. Was prepared. 

 ROBERT S. BALL. 

 CAMBRIDGE, _1st May, 1900_. 

CONTENTS. 

 PAGE

 INTRODUCTION 1

 CHAPTER

 I. THE ASTRONOMICAL OBSERVATORY 9

 II. THE SUN 29

 III. THE MOON 70

 IV. THE SOLAR SYSTEM 107

 V. THE LAW OF GRAVITATION 122

 VI. THE PLANET OF ROMANCE 150

 VII. MERCURY 155

 VIII. VENUS 167

 IX. THE EARTH 192

 X. MARS 208

 XI. THE MINOR PLANETS 229

 XII. JUPITER 245

 XIII. SATURN 268

 XIV. URANUS 298

 XV. NEPTUNE 315

 XVI. COMETS 336

 XVII. SHOOTING STARS 372

 XVIII. THE STARRY HEAVENS 409

 XIX. THE DISTANT SUNS 425

 XX. DOUBLE STARS 434

 XXI. THE DISTANCES OF THE STARS 441

 XXII. STAR CLUSTERS AND NEBULÆ 461

 XXIII. THE PHYSICAL NATURE OF THE STARS 477

 XXIV. THE PRECESSION AND NUTATION OF THE EARTH'S AXIS 492

 XXV. THE ABERRATION OF LIGHT 503

 XXVI. THE ASTRONOMICAL SIGNIFICANCE OF HEAT 513

 XXVII. THE TIDES 531

 APPENDIX 558

LIST OF PLATES. 

PLATE

 I. The Planet Saturn _Frontispiece_

 II. A Typical Sun-spot _To face page_ 9

 A. The Sun " " 44

 III. Spots and Faculæ on the Sun " " 37

 IV. Solar Prominences or Flames " " 57

 V. The Solar Corona " " 62

 VI. Chart of the Moon's Surface " " 81

 B. Portion of the Moon " " 88

 VII. The Lunar Crater Triesnecker " " 93

 VIII. A Normal Lunar Crater " " 97

 IX. The Lunar Crater Plato " " 102

 X. The Lunar Crater Tycho " " 106

 XI. The Planet Jupiter " " 254

 XII. Coggia's Comet " " 340

 C. Comet A. , 1892, 1 Swift " " 358

 XIII. Spectra of the Sun and of three Stars " " 47

 D. The Milky Way, near Messier II. " " 462

 XIV. The Great Nebula in Orion " " 466

 XV. The Great Nebula in Andromeda " " 468

 E. Nebulæ in the Pleiades " " 472

 F. ô Centauri " " 474

 XVI. Nebulæ observed with Lord Rosse's Telescope " " 476

 XVII. The Comet of 1882 " " 357

 XVIII. Schiaparelli's Map of Mars " " 221

LIST OF ILLUSTRATIONS. 

 FIG. PAGE

 1. Principle of the Refracting Telescope 11

 2. Dome of the South Equatorial at Dunsink Observatory, Co. Dublin 12

 3. Section of the Dome of Dunsink Observatory 13

 4. The Telescope at Yerkes Observatory, Chicago 15

 5. Principle of Herschel's Reflecting Telescope 16

 6. South Front of the Yerkes Observatory, Chicago 17

 7. Lord Rosse's Telescope 18

 8. Meridian Circle 20

 9. The Great Bear 27

 10. Comparative Sizes of the Earth and the Sun 30

 11. The Sun, photographed September 22, 1870 33

 12. Photograph of the Solar Surface 35

 13. An ordinary Sun-spot 36

 14. Scheiner's Observations on Sun-spots 38

 15. Zones on the Sun's Surface in which Spots appear 39

 16. Texture of the Sun and a small Spot 43

 17. The Prism 45

 18. Dispersion of Light by the Prism 46

 19. Prominences seen in Total Eclipses 53

 20. View of the Corona in a Total Eclipse 62

 21. View of Corona during Eclipse of January 22, 1898 63

 22. The Zodiacal Light in 1874 69

 23. Comparative Sizes of the Earth and the Moon 73

 24. The Moon's Path around the Sun 76

 25. The Phases of the Moon 76

 26. The Earth's Shadow and Penumbra 78

 27. Key to Chart of the Moon (Plate VI. ) 81

 28. Lunar Volcano in Activity: Nasmyth's Theory 97

 29. Lunar Volcano: Subsequent Feeble Activity 97

 30. " " Formation of the Level Floor by Lava 98

 31. Orbits of the Four Interior Planets 115

 32. The Earth's Movement 116

 33. Orbits of the Four Giant Planets 117

 34. Apparent Size of the Sun from various Planets 118

 35. Comparative Sizes of the Planets 119

 36. Illustration of the Moon's Motion 130

 37. Drawing an Ellipse 137

 38. Varying Velocity of Elliptic Motion 140

 39. Equal Areas in Equal Times 141

 40. Transit of the Planet of Romance 153

 41. Variations in Phase and apparent Size of Mercury 160

 42. Mercury as a Crescent 161

 43. Venus, May 29, 1889 170

 44. Different Aspects of Venus in the Telescope 171

 45. Venus on the Sun at the Transit of 1874 177

 46. Paths of Venus across the Sun in the Transits of 1874 and 1882 179

 47. A Transit of Venus, as seen from Two Localities 183

 48. Orbits of the Earth and of Mars 210

 49. Apparent Movements of Mars in 1877 212

 50. Relative Sizes of Mars and the Earth 216

 51, 52. Drawings of Mars 217

 53. Elevations and Depressions on the Terminator of Mars 217

 54. The Southern Polar Cap on Mars 217

 55. The Zone of Minor Planets between Mars and Jupiter 234

 56. Relative Dimensions of Jupiter and the Earth 246

 57-60. The Occultation of Jupiter 255

 61. Jupiter and his Four Satellites 258

 62. Disappearances of Jupiter's Satellites 259

 63. Mode of Measuring the Velocity of Light 264

 64. Saturn 270

 65. Relative Sizes of Saturn and the Earth 273

 66. Method of Measuring the Rotation of Saturn's Rings 288

 67. Method of Measuring the Rotation of Saturn's Rings 289

 68. Transit of Titan and its Shadow 295

 69. Parabolic Path of a Comet 339

 70. Orbit of Encke's Comet 346

 71. Tail of a Comet directed from the Sun 363

 72. Bredichin's Theory of Comets' Tails 366

 73. Tails of the Comet of 1858 367

 74. The Comet of 1744 368

 75. The Path of the Fireball of November 6, 1869 375

 76. The Orbit of a Shoal of Meteors 378

 77. Radiant Point of Shooting Stars 381

 78. The History of the Leonids 385

 79. Section of the Chaco Meteorite 398

 80. The Great Bear and Pole Star 410

 81. The Great Bear and Cassiopeia 411

 82. The Great Square of Pegasus 413

 83. Perseus and its Neighbouring Stars 415

 84. The Pleiades 416

 85. Orion, Sirius, and Neighbouring Stars 417

 86. Castor and Pollux 418

 87. The Great Bear and the Lion 419

 88. Boötes and the Crown 420

 89. Virgo and Neighbouring Constellations 421

 90. The Constellation of Lyra 422

 91. Vega, the Swan, and the Eagle 423

 92. The Orbit of Sirius 426

 93. The Parallactic Ellipse 444

 94. 61 Cygni and the Comparison Stars 447

 95. Parallax in Declination of 61 Cygni 450

 96. Globular Cluster in Hercules 463

 97. Position of the Great Nebula in Orion 466

 98. The Multiple Star th Orionis 467

 99. The Nebula N. G. C. 1499 471

 100. Star-Map, showing Precessional Movement 493

 101. Illustration of the Motion of Precession 495

THE

STORY OF THE HEAVENS. 

"The Story of the Heavens" is the title of our book. We have indeed awondrous story to narrate; and could we tell it adequately it wouldprove of boundless interest and of exquisite beauty. It leads to thecontemplation of grand phenomena in nature and great achievements ofhuman genius. 

Let us enumerate a few of the questions which will be naturally asked byone who seeks to learn something of those glorious bodies which adornour skies: What is the Sun--how hot, how big, and how distant? Whencecomes its heat? What is the Moon? What are its landscapes like? How doesour satellite move? How is it related to the earth? Are the planetsglobes like that on which we live? How large are they, and how far off?What do we know of the satellites of Jupiter and of the rings of Saturn?How was Uranus discovered? What was the intellectual triumph whichbrought the planet Neptune to light? Then, as to the other bodies of oursystem, what are we to say of those mysterious objects, the comets? Canwe discover the laws of their seemingly capricious movements? Do we knowanything of their nature and of the marvellous tails with which they areoften decorated? What can be told about the shooting-stars which sooften dash into our atmosphere and perish in a streak of splendour? Whatis the nature of those constellations of bright stars which have beenrecognised from all antiquity, and of the host of smaller stars whichour telescopes disclose? Can it be true that these countless orbs arereally majestic suns, sunk to an appalling depth in the abyss ofunfathomable space? What have we to tell of the different varieties ofstars--of coloured stars, of variable stars, of double stars, ofmultiple stars, of stars that seem to move, and of stars that seem atrest? What of those glorious objects, the great star clusters? What ofthe Milky Way? And, lastly, what can we learn of the marvellous nebulæwhich our telescopes disclose, poised at an immeasurable distance? Suchare a few of the questions which occur when we ponder on the mysteriesof the heavens. 

The history of Astronomy is, in one respect, only too like many otherhistories. The earliest part of it is completely and hopelessly lost. The stars had been studied, and some great astronomical discoveries hadbeen made, untold ages before those to which our earliest historicalrecords extend. For example, the observation of the apparent movement ofthe sun, and the discrimination between the planets and the fixed stars, are both to be classed among the discoveries of prehistoric ages. Nor isit to be said that these achievements related to matters of an obviouscharacter. Ancient astronomy may seem very elementary to those of thepresent day who have been familiar from childhood with the great truthsof nature, but, in the infancy of science, the men who made suchdiscoveries as we have mentioned must have been sagacious philosophers. 

Of all the phenomena of astronomy the first and the most obvious is thatof the rising and the setting of the sun. We may assume that in the dawnof human intelligence these daily occurrences would form one of thefirst problems to engage the attention of those whose thoughts roseabove the animal anxieties of everyday existence. A sun sets anddisappears in the west. The following morning a sun rises in the east, moves across the heavens, and it too disappears in the west; the sameappearances recur every day. To us it is obvious that the sun, whichappears each day, is the same sun; but this would not seem reasonable toone who thought his senses showed him that the earth was a flat plain ofindefinite extent, and that around the inhabited regions on all sidesextended, to vast distances, either desert wastes or trackless oceans. How could that same sun, which plunged into the ocean at a fabulousdistance in the west, reappear the next morning at an equally greatdistance in the east? The old mythology asserted that after the sun haddipped in the western ocean at sunset (the Iberians, and other ancientnations, actually imagined that they could hear the hissing of thewaters when the glowing globe was plunged therein), it was seized byVulcan and placed in a golden goblet. This strange craft with itsastonishing cargo navigated the ocean by a northerly course, so as toreach the east again in time for sunrise the following morning. Amongthe earlier physicists of old it was believed that in some manner thesun was conveyed by night across the northern regions, and that darknesswas due to lofty mountains, which screened off the sunbeams during thevoyage. 

In the course of time it was thought more rational to suppose that thesun actually pursued his course below the solid earth during the courseof the night. The early astronomers had, moreover, learned to recognisethe fixed stars. It was noticed that, like the sun, many of these starsrose and set in consequence of the diurnal movement, while the moonobviously followed a similar law. Philosophers thus taught that thevarious heavenly bodies were in the habit of actually passing beneaththe solid earth. 

By the acknowledgment that the whole contents of the heavens performedthese movements, an important step in comprehending the constitution ofthe universe had been decidedly taken. It was clear that the earth couldnot be a plane extending to an indefinitely great distance. It was alsoobvious that there must be a finite depth to the earth below our feet. Nay, more, it became certain that whatever the shape of the earth mightbe, it was at all events something detached from all other bodies, andpoised without visible support in space. When this discovery was firstannounced it must have appeared a very startling truth. It was sodifficult to realise that the solid earth on which we stand reposed onnothing! What was to keep it from falling? How could it be sustainedwithout tangible support, like the legendary coffin of Mahomet? Butdifficult as it may have been to receive this doctrine, yet itsnecessary truth in due time commanded assent, and the science ofAstronomy began to exist. The changes of the seasons and the recurrenceof seed-time and harvest must, from the earliest times, have beenassociated with certain changes in the position of the sun. In thesummer at mid-day the sun rises high in the heavens, in the winter it isalways low. Our luminary, therefore, performs an annual movement up anddown in the heavens, as well as a diurnal movement of rising andsetting. But there is a third species of change in the sun's position, which is not quite so obvious, though it is still capable of beingdetected by a few careful observations, if combined with a philosophicalhabit of reflection. The very earliest observers of the stars can hardlyhave failed to notice that the constellations visible at night variedwith the season of the year. For instance, the brilliant figure ofOrion, though so well seen on winter nights, is absent from the summerskies, and the place it occupied is then taken by quite different groupsof stars. The same may be said of other constellations. Each season ofthe year can thus be characterised by the sidereal objects that areconspicuous by night. Indeed, in ancient days, the time for commencingthe cycle of agricultural occupations was sometimes indicated by theposition of the constellations in the evening. 

By reflecting on these facts the early astronomers were enabled todemonstrate the apparent annual movement of the sun. There could be norational explanation of the changes in the constellations with theseasons, except by supposing that the place of the sun was altering, soas to make a complete circuit of the heavens in the course of the year. This movement of the sun is otherwise confirmed by looking at the westafter sunset, and watching the stars. As the season progresses, it maybe noticed each evening that the constellations seem to sink lower andlower towards the west, until at length they become invisible from thebrightness of the sky. The disappearance is explained by the suppositionthat the sun appears to be continually ascending from the west to meetthe stars. This motion is, of course, not to be confounded with theordinary diurnal rising and setting, in which all the heavenly bodiesparticipate. It is to be understood that besides being affected by thecommon motion our luminary has a slow independent movement in theopposite direction; so that though the sun and a star may set at thesame time to-day, yet since by to-morrow the sun will have moved alittle towards the east, it follows that the star must then set a fewminutes before the sun. [1]

The patient observations of the early astronomers enabled the sun'strack through the heavens to be ascertained, and it was found that inits circuit amid the stars and constellations our luminary invariablyfollowed the same path. This is called the _ecliptic_, and theconstellations through which it passes form a belt around the heavensknown as the _zodiac_. It was anciently divided into twelve equalportions or "signs, " so that the stages on the sun's great journey couldbe conveniently indicated. The duration of the year, or the periodrequired by the sun to run its course around the heavens, seems to havebeen first ascertained by astronomers whose names are unknown. The skillof the early Oriental geometers was further evidenced by theirdetermination of the position of the ecliptic with regard to thecelestial equator, and by their success in the measurement of the anglebetween these two important circles on the heavens. 

The principal features of the motion of the moon have also been noticedwith intelligence at an antiquity more remote than history. Theattentive observer perceives the important truth that the moon does notoccupy a fixed position in the heavens. During the course of a singlenight the fact that the moon has moved from west to east across theheavens can be perceived by noting its position relatively to adjacentstars. It is indeed probable that the motion of the moon was a discoveryprior to that of the annual motion of the sun, inasmuch as it is theimmediate consequence of a simple observation, and involves but littleexercise of any intellectual power. In prehistoric times also, the timeof revolution of the moon had been ascertained, and the phases of oursatellite had been correctly attributed to the varying aspect underwhich the sun-illuminated side is turned towards the earth. 

But we are far from having exhausted the list of great discoveries whichhave come down from unknown antiquity. Correct explanations had beengiven of the striking phenomenon of a lunar eclipse, in which thebrilliant surface is plunged temporarily into darkness, and also of thestill more imposing spectacle of a solar eclipse, in which the sunitself undergoes a partial or even a total obscuration. Then, too, theacuteness of the early astronomers had detected the five wandering starsor planets: they had traced the movements of Mercury and Venus, Mars, Jupiter, and Saturn. They had observed with awe the variousconfigurations of these planets: and just as the sun, and in a lesserdegree the moon, were intimately associated with the affairs of dailylife, so in the imagination of these early investigators the movementsof the planets were thought to be pregnant with human weal or human woe. At length a certain order was perceived to govern the apparentlycapricious movements of the planets. It was found that they obeyedcertain laws. The cultivation of the science of geometry went hand inhand with the study of astronomy: and as we emerge from the dimprehistoric ages into the historical period, we find that the theory ofthe phenomena of the heavens possessed already some degree of coherence. 

Ptolemy, following Pythagoras, Plato, and Aristotle, acknowledged thatthe earth's figure was globular, and he demonstrated it by the samearguments that we employ at the present day. He also discerned how thismighty globe was isolated in space. He admitted that the diurnalmovement of the heavens could be accounted for by the revolution of theearth upon its axis, but unfortunately he assigned reasons for thedeliberate rejection of this view. The earth, according to him, was afixed body; it possessed neither rotation round an axis nor translationthrough space, but remained constantly at rest in what he supposed to bethe centre of the universe. According to Ptolemy's theory the sun andthe moon moved in circular orbits around the earth in the centre. Theexplanation of the movements of the planets he found to be morecomplicated, because it was necessary to account for the fact that aplanet sometimes advanced and that it sometimes retrograded. The ancientgeometers refused to believe that any movement, except revolution in acircle, was possible for a celestial body: accordingly a contrivance wasdevised by which each planet was supposed to revolve in a circle, ofwhich the centre described another circle around the earth. 

Although the Ptolemaic doctrine is now known to be framed on quite anextravagant estimate of the importance of the earth in the scheme of theheavens, yet it must be admitted that the apparent movements of thecelestial bodies can be thus accounted for with considerable accuracy. This theory is described in the great work known as the "Almagest, "which was written in the second century of our era, and was regarded forfourteen centuries as the final authority on all questions of astronomy. 

Such was the system of Astronomy which prevailed during the Middle Ages, and was only discredited at an epoch nearly simultaneous with that ofthe discovery of the New World by Columbus. The true arrangement of thesolar system was then expounded by Copernicus in the great work to whichhe devoted his life. The first principle established by these laboursshowed the diurnal movement of the heavens to be due to the rotation ofthe earth on its axis. Copernicus pointed out the fundamental differencebetween real motions and apparent motions; he proved that theappearances presented in the daily rising and setting of the sun and thestars could be accounted for by the supposition that the earth rotated, just as satisfactorily as by the more cumbrous supposition of Ptolemy. He showed, moreover, that the latter supposition must attribute analmost infinite velocity to the stars, so that the rotation of theentire universe around the earth was clearly a preposterous supposition. The second great principle, which has conferred immortal glory onCopernicus, assigned to the earth its true position in the universe. Copernicus transferred the centre, about which all the planets revolve, from the earth to the sun; and he established the somewhat humiliatingtruth, that our earth is merely a planet pursuing a track between thepaths of Venus and of Mars, and subordinated like all the other planetsto the supreme sway of the Sun. 

This great revolution swept from astronomy those distorted views of theearth's importance which arose, perhaps not unnaturally, from the factthat we happen to be domiciled on that particular planet. Theachievements of Copernicus were soon to be followed by the invention ofthe telescope, that wonderful instrument by which the modern science ofastronomy has been created. To the consideration of this importantsubject we shall devote the first chapter of our book. 

[Illustration: PLATE II. 

A TYPICAL SUN-SPOT. 

(AFTER LANGLEY. )]

CHAPTER I. 

THE ASTRONOMICAL OBSERVATORY. 

 Early Astronomical Observations--The Observatory of Tycho Brahe--The Pupil of the Eye--Vision of Faint Objects--The Telescope--The Object-Glass--Advantages of Large Telescopes--The Equatorial--The Observatory--The Power of a Telescope--Reflecting Telescopes--Lord Rosse's Great Reflector at Parsonstown--How the mighty Telescope is used--Instruments of Precision--The Meridian Circle--The Spider Lines--Delicacy of pointing a Telescope--Precautions necessary in making Observations--The Ideal Instrument and the Practical One--The Elimination of Error--Greenwich Observatory--The ordinary Opera-Glass as an Astronomical Instrument--The Great Bear--Counting the Stars in the Constellation--How to become an Observer. 

The earliest rudiments of the Astronomical Observatory are as littleknown as the earliest discoveries in astronomy itself. Probably thefirst application of instrumental observation to the heavenly bodiesconsisted in the simple operation of measuring the shadow of a post castby the sun at noonday. The variations in the length of this shadowenabled the primitive astronomers to investigate the apparent movementsof the sun. But even in very early times special astronomicalinstruments were employed which possessed sufficient accuracy to add tothe amount of astronomical knowledge, and displayed considerableingenuity on the part of the designers. 

Professor Newcomb[2] thus writes: "The leader was Tycho Brahe, who wasborn in 1546, three years after the death of Copernicus. His attentionwas first directed to the study of astronomy by an eclipse of the sun onAugust 21st, 1560, which was total in some parts of Europe. Astonishedthat such a phenomenon could be predicted, he devoted himself to a studyof the methods of observation and calculation by which the predictionwas made. In 1576 the King of Denmark founded the celebrated observatoryof Uraniborg, at which Tycho spent twenty years assiduously engaged inobservations of the positions of the heavenly bodies with the bestinstruments that could then be made. This was just before the inventionof the telescope, so that the astronomer could not avail himself of thatpowerful instrument. Consequently, his observations were superseded bythe improved ones of the centuries following, and their celebrity andimportance are principally due to their having afforded Kepler the meansof discovering his celebrated laws of planetary motion. "

The direction of the telescope to the skies by Galileo gave a wonderfulimpulse to the study of the heavenly bodies. This extraordinary man isprominent in the history of astronomy, not alone for his connection withthis supreme invention, but also for his achievements in the moreabstract parts of astronomy. He was born at Pisa in 1564, and in 1609the first telescope used for astronomical observation was constructed. Galileo died in 1642, the year in which Newton was born. It was Galileowho laid with solidity the foundations of that science of Dynamics, ofwhich astronomy is the most splendid illustration; and it was he who, bypromulgating the doctrines taught by Copernicus, incurred the wrath ofthe Inquisition. 

The structure of the human eye in so far as the exquisite adaptation ofthe pupil is concerned presents us with an apt illustration of theprinciple of the telescope. To see an object, it is necessary that thelight from it should enter the eye. The portal through which the lightis admitted is the pupil. In daytime, when the light is brilliant, theiris decreases the size of the pupil, and thus prevents too much lightfrom entering. At night, or whenever the light is scarce, the eye oftenrequires to grasp all it can. The pupil then expands; more and morelight is admitted according as the pupil grows larger. The illuminationof the image on the retina is thus effectively controlled in accordancewith the requirements of vision. 

A star transmits to us its feeble rays of light, and from those raysthe image is formed. Even with the most widely-opened pupil, it may, however, happen that the image is not bright enough to excite thesensation of vision. Here the telescope comes to our aid: it catches allthe rays in a beam whose original dimensions were far too great to allowof its admission through the pupil. The action of the lensesconcentrates those rays into a stream slender enough to pass through thesmall opening. We thus have the brightness of the image on the retinaintensified. It is illuminated with nearly as much light as would becollected from the same object through a pupil as large as the greatlenses of the telescope. 

[Illustration: Fig. 1. --Principle of the Refracting Telescope. ]

In astronomical observatories we employ telescopes of two entirelydifferent classes. The more familiar forms are those known as_refractors_, in which the operation of condensing the rays of light isconducted by refraction. The character of the refractor is shown in Fig. 1. The rays from the star fall upon the object-glass at the end of thetelescope, and on passing through they become refracted into aconverging beam, so that all intersect at the focus. Diverging fromthence, the rays encounter the eye-piece, which has the effect ofrestoring them to parallelism. The large cylindrical beam which poureddown on the object-glass has been thus condensed into a small one, whichcan enter the pupil. It should, however, be added that the compositenature of light requires a more complex form of object-glass than thesimple lens here shown. In a refracting telescope we have to employ whatis known as the achromatic combination, consisting of one lens of flintglass and one of crown glass, adjusted to suit each other with extremecare. 

[Illustration: Fig. 2. --The Dome of the South Equatorial at DunsinkObservatory Co Dublin. ]

[Illustration: Fig. 3. --Section of the Dome of Dunsink Observatory. ]

The appearance of an astronomical observatory, designed to accommodatean instrument of moderate dimensions, is shown in the adjoining figures. The first (Fig. 2) represents the dome erected at Dunsink Observatoryfor the equatorial telescope, the object-glass of which was presented tothe Board of Trinity College, Dublin, by the late Sir James South. Themain part of the building is a cylindrical wall, on the top of whichreposes a hemispherical roof. In this roof is a shutter, which can beopened so as to allow the telescope in the interior to obtain a view ofthe heavens. The dome is capable of revolving so that the opening may beturned towards that part of the sky where the object happens to besituated. The next view (Fig. 3) exhibits a section through the dome, showing the machinery by which the attendant causes it to revolve, aswell as the telescope itself. The eye of the observer is placed at theeye-piece, and he is represented in the act of turning a handle, whichhas the power of slowly moving the telescope, in order to adjust theinstrument accurately on the celestial body which it is desired toobserve. The two lenses which together form the object-glass of thisinstrument are twelve inches in diameter, and the quality of thetelescope mainly depends on the accuracy with which these lenses havebeen wrought. The eye-piece is a comparatively simple matter. Itconsists merely of one or two small lenses; and various eye-pieces canbe employed, according to the magnifying power which may be desired. Itis to be observed that for many purposes of astronomy high magnifyingpowers are not desirable. There is a limit, too, beyond which themagnification cannot be carried with advantage. The object-glass canonly collect a certain quantity of light from the star; and if themagnifying power be too great, this limited amount of light will bethinly dispersed over too large a surface, and the result will be foundunsatisfactory. The unsteadiness of the atmosphere still further limitsthe extent to which the image may be advantageously magnified, for everyincrease of power increases in the same degree the atmosphericdisturbance. 

A telescope mounted in the manner here shown is called an _equatorial_. The convenience of this peculiar style of supporting the instrumentconsists in the ease with which the telescope can be moved so as tofollow a star in its apparent journey across the sky. The necessarymovements of the tube are given by clockwork driven by a weight, sothat, once the instrument has been correctly pointed, the star willremain in the observer's field of view, and the effect of the apparentdiurnal movement will be neutralised. The last refinement in thisdirection is the application of an electrical arrangement by which thedriving of the instrument is controlled from the standard clock of theobservatory. 

[Illustration: Fig. 4. --The Telescope at Yerkes Observatory, Chicago. 

(_From the Astrophysical Journal, Vol. Vi. , No. 1. _)]

The power of a refracting telescope--so far as the expression has anydefinite meaning--is to be measured by the diameter of its object-glass. There has, indeed, been some honourable rivalry between the variouscivilised nations as to which should possess the greatest refractingtelescope. Among the notable instruments that have been successfullycompleted is that erected in 1881 by Sir Howard Grubb, of Dublin, at thesplendid observatory at Vienna. Its dimensions may be estimated from thefact that the object-glass is two feet and three inches in diameter. Many ingenious contrivances help to lessen the inconvenience incident tothe use of an instrument possessing such vast proportions. Among them wemay here notice the method by which the graduated circles attached tothe telescope are brought within view of the observer. These circles arenecessarily situated at parts of the instrument which lie remote fromthe eye-piece where the observer is stationed. The delicate marks andfigures are, however, easily read from a distance by a small auxiliarytelescope, which, by suitable reflectors, conducts the rays of lightfrom the circles to the eye of the observer. 

[Illustration: Fig. 5. --Principle of Herschel's Refracting Telescope. ]

Numerous refracting telescopes of exquisite perfection have beenproduced by Messrs. Alvan Clark, of Cambridgeport, Boston, Mass. One oftheir most famous telescopes is the great Lick Refractor now in use onMount Hamilton in California. The diameter of this object-glass isthirty-six inches, and its focal length is fifty-six feet two inches. Astill greater effort has recently been made by the same firm in therefractor of forty inches aperture for the Yerkes Observatory of theUniversity of Chicago. The telescope, which is seventy-five feet inlength, is mounted under a revolving dome ninety feet in diameter, andin order to enable the observer to reach the eye-piece without usingvery large step-ladders, the floor of the room can be raised and loweredthrough a range of twenty-two feet by electric motors. This is shown inFig. 4, while the south front of the Yerkes Observatory is representedin Fig. 6. 

[Illustration: Fig. 6. --South Front of the Yerkes Observatory, Chicago. 

(_From the Astrophysical Journal, Vol. Vi. , No. 1. _)]

[Illustration: Fig. 7. --Lord Rosse's Telescope. ]

Within the last few years two fine telescopes have been added to theinstrumental equipment of the Royal Observatory, Greenwich, both by SirH. Grubb. One of these, containing a 28-inch object-glass, has beenerected on a mounting originally constructed for a smaller instrument bySir G. Airy. The other, presented by Sir Henry Thompson, is of 26 inchesaperture, and is adapted for photographic work. 

There is a limit to the size of the refractor depending upon thematerial of the object-glass. Glass manufacturers seem to experienceunusual difficulties in their attempts to form large discs of opticalglass pure enough and uniform enough to be suitable for telescopes. These difficulties are enhanced with every increase in the size of thediscs, so that the cost has a tendency to increase at a very muchgreater rate. It may be mentioned in illustration that the price paidfor the object-glass of the Lick telescope exceeded ten thousand pounds. 

There is, however, an alternative method of constructing a telescope, inwhich the difficulty we have just mentioned does not arise. Theprinciple of the simplest form of _reflector_ is shown in Fig. 5, whichrepresents what is called the Herschelian instrument. The rays of lightfrom the star under observation fall on a mirror which is both carefullyshaped and highly polished. After reflection, the rays proceed to afocus, and diverging from thence, fall on the eye-piece, by which theyare restored to parallelism, and thus become adapted for reception inthe eye. It was essentially on this principle (though with a secondaryflat mirror at the upper end of the tube reflecting the rays at a rightangle to the side of the tube, where the eye-piece is placed) that SirIsaac Newton constructed the little reflecting telescope which is nowtreasured by the Royal Society. A famous instrument of the Newtoniantype was built, half a century ago, by the late Earl of Rosse, atParsonstown. It is represented in Fig. 7. The colossal aperture of thisinstrument has never been surpassed; it has, indeed, never beenrivalled. The mirror or speculum, as it is often called, is a thickmetallic disc, composed of a mixture of two parts of copper with one oftin. This alloy is so hard and brittle as to make the necessarymechanical operations difficult to manage. The material admits, however, of a brilliant polish, and of receiving and retaining an accuratefigure. The Rosse speculum--six feet in diameter and three tons inweight--reposes at the lower end of a telescope fifty-five feet long. The tube is suspended between two massive castellated walls, which forman imposing feature on the lawn at Birr Castle. This instrument cannotbe turned about towards every part of the sky, like the equatorials wehave recently been considering. The great tube is only capable ofelevation in altitude along the meridian, and of a small lateralmovement east and west of the meridian. Every star or nebula visible inthe latitude of Parsonstown (except those very near the pole) can, however, be observed in the great telescope, if looked for at the righttime. 

[Illustration: Fig. 8. --Meridian Circle. ]

Before the object reaches the meridian, the telescope must be adjustedat the right elevation. The necessary power is transmitted by a chainfrom a winch at the northern end of the walls to a point near the upperend of the tube. By this contrivance the telescope can be raised orlowered, and an ingenious system of counterpoises renders the movementequally easy at all altitudes. The observer then takes his station inone of the galleries which give access to the eye-piece; and when theright moment has arrived, the star enters the field of view. Powerfulmechanism drives the great instrument, so as to counteract the diurnalmovement, and thus the observer can retain the object in view until hehas made his measurements or finished his drawing. 

Of late years reflecting telescopes have been generally made withmirrors of glass covered with a thin film of silver, which is capable ofreflecting much more light than the surface of a metallic mirror. Amonggreat reflectors of this kind we may mention two, of three and five feetaperture respectively, with which Dr. Common has done valuable work. 

We must not, however, assume that for the general work in an observatorya colossal instrument is the most suitable. The mighty reflector, orrefractor, is chiefly of use where unusually faint objects are beingexamined. For work in which accurate measurements are made of objectsnot particularly difficult to see, telescopes of smaller dimensions aremore suitable. The fundamental facts about the heavenly bodies have beenchiefly learned from observations obtained with instruments of moderateoptical power, specially furnished so as to enable precise measures ofposition to be secured. Indeed, in the early stages of astronomy, important determinations of position were effected by contrivanceswhich showed the direction of the object without any telescopic aid. 

Perhaps the most valuable measurements obtained in our modernobservatories are yielded by that instrument of precision known as the_meridian circle_. It is impossible, in any adequate account of theStory of the Heavens, to avoid some reference to this indispensable aidto astronomical research, and therefore we shall give a brief account ofone of its simpler forms, choosing for this purpose a great instrumentin the Paris Observatory, which is represented in Fig. 8. 

The telescope is attached at its centre to an axis at right angles toits length. Pivots at each extremity of this axis rotate upon fixedbearings, so that the movements of the telescope are completelyrestricted to the plane of the meridian. Inside the eye-piece of thetelescope extremely fine vertical fibres are stretched. The observerwatches the moon, or star, or planet enter the field of view; and henotes by the clock the exact time, to the fraction of a second, at whichthe object passes over each of the lines. A silver band on the circleattached to the axis is divided into degrees and subdivisions of adegree, and as this circle moves with the telescope, the elevation atwhich the instrument is pointed will be indicated. For reading thedelicately engraved marks and figures on the silver, microscopes arenecessary. These are shown in the sketch, each one being fixed into anaperture in the wall which supports one end of the instrument. At theopposite side is a lamp, the light from which passes through theperforated axis of the pivot, and is thence ingeniously deflected bymirrors so as to provide the requisite illumination for the lines at thefocus. 

The fibres which the observer sees stretched over the field of view ofthe telescope demand a few words of explanation. We require for thispurpose a material which shall be very fine and fairly durable, as wellas somewhat elastic, and of no appreciable weight. These conditionscannot be completely fulfilled by any metallic wire, but they areexquisitely realised in the beautiful thread which is spun by thespider. The delicate fibres are stretched with nice skill across thefield of view of the telescope, and cemented in their proper places. With instruments so beautifully appointed we can understand theprecision attained in modern observations. The telescope is directedtowards a star, and the image of the star is a minute point of light. When that point coincides with the intersection of the two centralspider lines the telescope is properly sighted. We use the word sighteddesignedly, because we wish to suggest a comparison between the sightingof a rifle at the target and the sighting of a telescope at a star. Instead of the ordinary large bull's-eye, suppose that the target onlyconsisted of a watch-dial, which, of course, the rifleman could not seeat the distance of any ordinary range. But with the telescope of themeridian circle the watch-dial would be visible even at the distance ofa mile. The meridian circle is indeed capable of such precision as asighting instrument that it could be pointed separately to each of twostars which subtend at the eye an angle no greater than that subtendedby an adjoining pair of the sixty minute dots around the circumferenceof a watch-dial a mile distant from the observer. 

This power of directing the instrument so accurately would be of butlittle avail unless it were combined with arrangements by which, whenonce the telescope has been pointed correctly, the position of the starcan be ascertained and recorded. One element in the determination of theposition is secured by the astronomical clock, which gives the momentwhen the object crosses the central vertical wire; the other element isgiven by the graduated circle which reads the angular distance of thestar from the zenith or point directly overhead. 

Superb meridian instruments adorn our great observatories, and arenightly devoted to those measurements upon which the great truths ofastronomy are mainly based. These instruments have been constructed withrefined skill; but it is the duty of the painstaking astronomer todistrust the accuracy of his instrument in every conceivable way. Thegreat tube may be as rigid a structure as mechanical engineers canproduce; the graduations on the circle may have been engraved by themost perfect of dividing machines; but the conscientious astronomer willnot be content with mere mechanical precision. That meridian circlewhich, to the uninitiated, seems a marvellous piece of workmanship, possessing almost illimitable accuracy, is viewed in a very differentlight by the astronomer who makes use of it. No one can appreciate morefully than he the skill of the artist who has made that meridian circle, and the beautiful contrivances for illumination and reading off whichgive to the instrument its perfection; but while the astronomerrecognises the beauty of the actual machine he is using, he has alwaysbefore his mind's eye an ideal instrument of absolute perfection, towhich the actual meridian circle only makes an approximation. 

Contrasted with the ideal instrument, the finest meridian circle islittle more than a mass of imperfections. The ideal tube is perfectlyrigid, the actual tube is flexible; the ideal divisions of the circleare perfectly uniform, the actual divisions are not uniform. The idealinstrument is a geometrical embodiment of perfect circles, perfectstraight lines, and perfect right angles; the actual instrument can onlyshow approximate circles, approximate straight lines, and approximateright angles. Perhaps the spider's part of the work is on the whole thebest; the stretched web gives us the nearest mechanical approach to aperfectly straight line; but we mar the spider's work by not being ableto insert those beautiful threads with perfect uniformity, while ourattempts to adjust two of them across the field of view at right anglesdo not succeed in producing an angle of exactly ninety degrees. 

Nor are the difficulties encountered by the meridian observer due solelyto his instrument. He has to contend against his own imperfections; hehas often to allow for personal peculiarities of an unexpected nature;the troubles that the atmosphere can give are notorious; while thelevelling of his instrument warns him that he cannot even rely on thesolid earth itself. We learn that the earthquakes, by which the solidground is sometimes disturbed, are merely the more conspicuousinstances of incessant small movements in the earth which every night inthe year derange the delicate adjustment of the instrument. 

When the existence of these errors has been recognised, the first greatstep has been taken. By an alliance between the astronomer and themathematician it is possible to measure the discrepancies between theactual meridian circle and the instrument that is ideally perfect. Oncethis has been done, we can estimate the effect which the irregularitiesproduce on the observations, and finally, we succeed in purging theobservations from the grosser errors by which they are contaminated. Wethus obtain results which are not indeed mathematically accurate, butare nevertheless close approximations to those which would be obtainedby a perfect observer using an ideal instrument of geometrical accuracy, standing on an earth of absolute rigidity, and viewing the heavenswithout the intervention of the atmosphere. 

In addition to instruments like those already indicated, astronomershave other means of following the motions of the heavenly bodies. Withinthe last fifteen years photography has commenced to play an importantpart in practical astronomy. This beautiful art can be utilised forrepresenting many objects in the heavens by more faithful pictures thanthe pencil of even the most skilful draughtsman can produce. Photographyis also applicable for making charts of any region in the sky which itis desired to examine. When repeated pictures of the same region aremade from time to time, their comparison gives the means of ascertainingwhether any star has moved during the interval. The amount and directionof this motion may be ascertained by a delicate measuring apparatusunder which the photographic plate is placed. 

If a refracting telescope is to be used for taking celestialphotographs, the lenses of the object-glass must be specially designedfor this purpose. The rays of light which imprint an image on theprepared plate are not exactly the same as those which are chieflyconcerned in the production of the image on the retina of the human eye. A reflecting mirror, however, brings all the rays, both those which arechemically active and those which are solely visual, to one and thesame focus. The same reflecting instrument may therefore be used eitherfor looking at the heavens or for taking pictures on a photographicplate which has been substituted for the observer's eye. 

A simple portrait camera has been advantageously employed for obtainingstriking photographs of larger areas of the sky than can be grasped in along telescope; but for purposes of accurate measurement those takenwith the latter are incomparably better. 

It is needless to say that the photographic apparatus, whatever it maybe, must be driven by delicately-adjusted clockwork to counteract theapparent daily motion of the stars caused by the rotation of the earth. The picture would otherwise be spoiled, just as a portrait is ruined ifthe sitter does not remain quiet during the exposure. 

Among the observatories in the United Kingdom the Royal Observatory atGreenwich is of course the most famous. It is specially remarkable amongall the similar institutions in the world for the continuity of itslabours for several generations. Greenwich Observatory was founded in1675 for the promotion of astronomy and navigation, and the observationshave from the first been specially arranged with the object ofdetermining with the greatest accuracy the positions of the principalfixed stars, the sun, the moon, and the planets. In recent years, however, great developments of the work of the Observatory have beenwitnessed, and the most modern branches of the science are nowassiduously pursued there. 

The largest equatorial at Greenwich is a refractor of twenty-eightinches aperture and twenty-eight feet long, constructed by Sir HowardGrubb. A remarkable composite instrument from the same celebratedworkshop has also been recently added to our national institution. Itconsists of a great refractor specially constructed for photography, oftwenty-six inches aperture (presented by Sir Henry Thompson) and areflector of thirty inches diameter, which is the product of Dr. Common's skill. The huge volume published annually bears witness to theassiduity with which the Astronomer Royal and his numerous staff ofassistant astronomers make use of the splendid means at their disposal. 

The southern part of the heavens, most of which cannot be seen in thiscountry, is watched from various observatories in the southernhemisphere. Foremost among them is the Royal Observatory at the Cape ofGood Hope, which is furnished with first-class instruments. We maymention a great photographic telescope, the gift of Mr. M'Clean. Astronomy has been greatly enriched by the many researches made by Dr. Gill, the director of the Cape Observatory. 

[Illustration: Fig. 9. --The Great Bear. ]

It is not, however, necessary to use such great instruments to obtainsome idea of the aid the telescope will afford. The most suitableinstrument for commencing astronomical studies is within ordinary reach. It is the well-known binocular that a captain uses on board ship; or ifthat cannot be had, then the common opera-glass will answer nearly aswell. This is, no doubt, not so powerful as a telescope, but it has somecompensating advantages. The opera-glass will enable us to survey alarge region of the sky at one glance, while a telescope, generallyspeaking, presents a much smaller field of view. 

Let us suppose that the observer is provided with an opera-glass and isabout to commence his astronomical studies. The first step is to becomeacquainted with the conspicuous group of seven stars represented in Fig. 9. This group is often called the Plough, or Charles's Wain, butastronomers prefer to regard it as a portion of the constellation of theGreat Bear (Ursa Major). There are many features of interest in thisconstellation, and the beginner should learn as soon as possible toidentify the seven stars which compose it. Of these the two markeda and b, at the head of the Bear, are generally called the"pointers. " They are of special use, because they serve to guide the eyeto that most important star in the whole sky, known as the "pole star. "

Fix the attention on that region in the Great Bear, which forms a sortof rectangle, of which the stars a b g d are the corners. The next finenight try to count how many stars are visible within that rectangle. Ona very fine night, without a moon, perhaps a dozen might be perceived, or even more, according to the keenness of the eyesight. But when theopera-glass is directed to the same part of the constellation anastonishing sight is witnessed. A hundred stars can now be seen with thegreatest ease. 

But the opera-glass will not show nearly all the stars in this region. Any good telescope will reveal many hundreds too faint for the feeblerinstrument. The greater the telescope the more numerous the stars: sothat seen through one of the colossal instruments the number would haveto be reckoned in thousands. 

We have chosen the Great Bear because it is more generally known thanany other constellation. But the Great Bear is not exceptionally rich instars. To tell the number of the stars is a task which no man hasaccomplished; but various estimates have been made. Our great telescopescan probably show at least 50, 000, 000 stars. 

The student who uses a good refracting telescope, having an object-glassnot less than three inches in diameter, will find occupation for many afine evening. It will greatly increase the interest of his work if hehave the charming handbook of the heavens known as Webb's "CelestialObjects for Common Telescopes. "

CHAPTER II. 

THE SUN. 

 The vast Size of the Sun--Hotter than Melting Platinum--Is the Sun the Source of Heat for the Earth?--The Sun is 92, 900, 000 miles distant--How to realise the magnitude of this distance--Day and Night--Luminous and Non-Luminous Bodies--Contrast between the Sun and the Stars--The Sun a Star--Granulated Appearance of the Sun--The Spots on the Sun--Changes in the Form of a Spot--The Faculæ--The Rotation of the Sun on its Axis--View of a Typical Sun-Spot--Periodicity of the Sun-Spots--Connection between the Sun-Spots and Terrestrial Magnetism--Principles of Spectrum Analysis--Substances present in the Sun--Spectrum of a Spot--The Prominences surrounding the Sun--Total Eclipse of the Sun--Size and Movement of the Prominences--Their connection with the Spots--Spectroscopic Measurement of Motion on the Sun--The Corona surrounding the Sun--Constitution of the Sun. 

In commencing our examination of the orbs which surround us, wenaturally begin with our peerless sun. His splendid brilliance gives himthe pre-eminence over all other celestial bodies. 

The dimensions of our luminary are commensurate with his importance. Astronomers have succeeded in the difficult task of ascertaining theexact figures, but they are so gigantic that the results are hard torealise. The diameter of the orb of day, or the length of the axis, passing through the centre from one side to the other, is 866, 000 miles. Yet this bare statement of the dimensions of the great globe fails toconvey an adequate idea of its vastness. If a railway were laid roundthe sun, and if we were to start in an express train moving sixty milesan hour, we should have to travel for five years without intermissionnight or day before we had accomplished the journey. 

When the sun is compared with the earth the bulk of our luminary becomesstill more striking. Suppose his globe were cut up into one millionparts, each of these parts would appreciably exceed the bulk of ourearth. Fig. 10 exhibits a large circle and a very small one, marked Sand E respectively. These circles show the comparative sizes of the twobodies. The mass of the sun does not, however, exceed that of the earthin the same proportion. Were the sun placed in one pan of a mightyweighing balance, and were 300, 000 bodies as heavy as our earth placedin the other, the luminary would turn the scale. 

[Illustration: Fig. 10. --Comparative Size of the Earth and the Sun. ]

The sun has a temperature far surpassing any that we artificiallyproduce, either in our chemical laboratories or our metallurgicalestablishments. We can send a galvanic current through a piece ofplatinum wire. The wire first becomes red hot, then white hot; then itglows with a brilliance almost dazzling until it fuses and breaks. Thetemperature of the melting platinum wire could hardly be surpassed inthe most elaborate furnaces, but it does not attain the temperature ofthe sun. 

It must, however, be admitted that there is an apparent discrepancybetween a fact of common experience and the statement that the sunpossesses the extremely high temperature that we have just tried toillustrate. "If the sun were hot, " it has been said, "then the nearer weapproach to him the hotter we should feel; yet this does not seem to bethe case. On the top of a high mountain we are nearer to the sun, andyet everybody knows that it is much colder up there than in the valleybeneath. If the mountain be as high as Mont Blanc, then we are certainlytwo or three miles nearer the glowing globe than we were at thesea-level; yet, instead of additional warmth, we find eternal snow. " Asimple illustration may help to lessen this difficulty. In a greenhouseon a sunshiny day the temperature is much hotter than it is outside. Theglass will permit the hot sunbeams to enter, but it refuses to allowthem out again with equal freedom, and consequently the temperaturerises. The earth may, from this point of view, be likened to agreenhouse, only, instead of the panes of glass, our globe is envelopedby an enormous coating of air. On the earth's surface, we stand, as itwere, inside the greenhouse, and we benefit by the interposition of theatmosphere; but when we climb very high mountains, we gradually passthrough some of the protecting medium, and then we suffer from the cold. If the earth were deprived of its coat of air, it seems certain thateternal frost would reign over whole continents as well as on the topsof the mountains. 

The actual distance of the sun from the earth is about 92, 900, 000 miles;but by merely reciting the figures we do not receive a vivid impressionof the real magnitude. It would be necessary to count as quickly aspossible for three days and three nights before one million wascompleted; yet this would have to be repeated nearly ninety-three timesbefore we had counted all the miles between the earth and the sun. 

Every clear night we see a vast host of stars scattered over the sky. Some are bright, some are faint, some are grouped into remarkable forms. With regard to this multitude of brilliant points we have now to ask animportant question. Are they bodies which shine by their own light likethe sun, or do they only shine with borrowed light like the moon? Theanswer is easily stated. Most of those bodies shine by their own light, and they are properly called _stars_. 

Suppose that the sun and the multitude of stars, properly so called, areeach and all self-luminous brilliant bodies, what is the greatdistinction between the sun and the stars? There is, of course, a vastand obvious difference between the unrivalled splendour of the sun andthe feeble twinkle of the stars. Yet this distinction does notnecessarily indicate that our luminary has an intrinsic splendoursuperior to that of the stars. The fact is that we are nestled upcomparatively close to the sun for the benefit of his warmth and light, while we are separated from even the nearest of the stars by a mightyabyss. If the sun were gradually to retreat from the earth, his lightwould decrease, so that when he had penetrated the depths of space to adistance comparable with that by which we are separated from the stars, his glory would have utterly departed. No longer would the sun seem tobe the majestic orb with which we are familiar. No longer would he be asource of genial heat, or a luminary to dispel the darkness of night. Our great sun would have shrunk to the insignificance of a star, not sobright as many of those which we see every night. 

Momentous indeed is the conclusion to which we are now led. That myriadhost of stars which studs our sky every night has been elevated intovast importance. Each one of those stars is itself a mighty sun, actually rivalling, and in many cases surpassing, the splendour of ourown luminary. We thus open up a majestic conception of the vastdimensions of space, and of the dignity and splendour of the myriadglobes by which that space is tenanted. 

There is another aspect of the picture not without its utility. We mustfrom henceforth remember that our sun is only a star, and not aparticularly important star. If the sun and the earth, and all which itcontains, were to vanish, the effect in the universe would merely bethat a tiny star had ceased its twinkling. Viewed simply as a star, thesun must retire to a position of insignificance in the mighty fabric ofthe universe. But it is not as a star that we have to deal with the sun. To us his comparative proximity gives him an importance incalculablytranscending that of all the other stars. We imagined ourselves to bewithdrawn from the sun to obtain his true perspective in the universe;let us now draw near, and give him that attention which his supremeimportance to us merits. 

[Illustration: Fig. 11. --The Sun, photographed on September 22, 1870. ]

To the unaided eye the sun appears to be a flat circle. If, however, itbe examined with the telescope, taking care of course to interpose apiece of dark-coloured glass, or to employ some similar precaution toscreen the eye from injury, it will then be perceived that the sun isnot a flat surface, but a veritable glowing globe. 

The first question which we must attempt to answer enquires whether theglowing matter which forms the globe is a solid mass, or, if not solid, which is it, liquid or gaseous? At the first glance we might think thatthe sun cannot be fluid, and we might naturally imagine that it was asolid ball of some white-hot substance. But this view is not correct;for we can show that the sun is certainly not a solid body in so far atleast as its superficial parts are concerned. 

A general view of the sun as shown by a telescope of moderate dimensionsmay be seen in Fig. 11, which is taken from a photograph obtained by Mr. Rutherford at New York on the 22nd of September, 1870. It is at onceseen that the surface of the luminary is by no means of uniform textureor brightness. It may rather be described as granulated or mottled. Thisappearance is due to the luminous clouds which float suspended in asomewhat less luminous layer of gas. It is needless to say that thesesolar clouds are very different from the clouds which we know so well inour own atmosphere. Terrestrial clouds are, of course, formed fromminute drops of water, while the clouds at the surface of the sun arecomposed of drops of one or more chemical elements at an exceedinglyhigh temperature. 

The granulated appearance of the solar surface is beautifully shown inthe remarkable photographs on a large scale which M. Janssen, of Meudon, has succeeded in obtaining during the last twenty years. We are enabledto reproduce one of them in Fig. 12. It will be observed that theinterstices between the luminous dots are of a greyish tint, the generaleffect (as remarked by Professor Young) being much like that of roughdrawing paper seen from a little distance. We often notice places overthe surface of such a plate where the definition seems to beunsatisfactory. These are not, however, the blemishes that might atfirst be supposed. They arise neither from casual imperfections of thephotographic plate nor from accidents during the development; theyplainly owe their origin to some veritable cause in the sun itself, norshall we find it hard to explain what that cause must be. As we shallhave occasion to mention further on, the velocities with which theglowing gases on the sun are animated must be exceedingly great. Even inthe hundredth part of a second (which is about the duration of theexposure of this plate) the movements of the solar clouds aresufficiently great to produce the observed indistinctness. 

[Illustration: Fig. 12. --Photograph of the Solar Surface. (_ByJanssen. _)]

Irregularly dispersed over the solar surface small dark objects calledsun-spots are generally visible. These spots vary greatly both as tosize and as to number. Sun-spots were first noticed in the beginning ofthe seventeenth century, shortly after the invention of the telescope. Their general appearance is shown in Fig. 13, in which the dark centralnucleus appears in sharp contrast with the lighter margin or penumbra. Fig. 16 shows a small spot developing out of one of the pores orinterstices between the granules. 

[Illustration: Fig. 13. --An Ordinary Sun-spot. ]

The earliest observers of these spots had remarked that they seem tohave a common motion across the sun. In Fig. 14 we give a copy of aremarkable drawing by Father Scheiner, showing the motion of two spotsobserved by him in March, 1627. The figure indicates the successivepositions assumed by the spots on the several days from the 2nd to the16th March. Those marks which are merely given in outline represent theassumed positions on the 11th and the 13th, on which days it happenedthat the weather was cloudy, so that no observations could be made. Itis invariably found that these objects move in the samedirection--namely, from the eastern to the western limb[3] of the sun. They complete the journey across the face of the sun in twelve orthirteen days, after which they remain invisible for about the samelength of time until they reappear at the eastern limb. These earlyobservers were quick to discern the true import of their discovery. Theydeduced from these simple observations the remarkable fact that the sun, like the earth, performs a rotation on its axis, and in the samedirection. But there is the important difference between these rotationsthat whereas the earth takes only twenty-four hours to turn once round, the solar globe takes about twenty-six days to complete one of its muchmore deliberate rotations. 

[Illustration: PLATE III. 

SPOTS AND FACULÆ ON THE SUN. 

(FROM A PHOTOGRAPH BY MR. WARREN DE LA RUE, 20TH SEPT. , 1861. )]

If we examine sun-spots under favourable atmospheric conditions andwith a telescope of fairly large aperture, we perceive a great amount ofinteresting detail which is full of information with regard to thestructure of the sun. The penumbra of a spot is often found to be madeup of filaments directed towards the middle of the spot, and generallybrighter at their inner ends, where they adjoin the nucleus. In aregularly formed spot the outline of the penumbra is of the same generalform as that of the nucleus, but astronomers are frequently deeplyinterested by witnessing vast spots of very irregular figure. In suchcases the bright surface-covering of the sun (the photosphere, as it iscalled) often encroaches on the nucleus and forms a peninsula stretchingout into, or even bridging across, the gloomy interior. This is wellshown in Professor Langley's fine drawing (Plate II. ) of a veryirregular spot which he observed on December 23-24, 1873. 

The details of a spot vary continually; changes may often be noticedeven from day to day, sometimes from hour to hour. A similar remark maybe made with respect to the bright streaks or patches which arefrequently to be observed especially in the neighbourhood of spots. These bright marks are known by the name of _faculæ_ (little torches). They are most distinctly seen near the margin of the sun, where thelight from its surface is not so bright as it is nearer to the centre ofthe disc. The reduction of light at the margin is due to the greaterthickness of absorbing atmosphere round the sun, through which the lightemitted from the regions near the margin has to pass in starting on itsway towards us. 

None of the markings on the solar disc constitute permanent features onthe sun. Some of these objects may no doubt last for weeks. It has, indeed, occasionally happened that the same spot has marked the solarglobe for many months; but after an existence of greater or lessduration those on one part of the sun may disappear, while as frequentlyfresh marks of the same kind become visible in other places. Theinference from these various facts is irresistible. They tell us thatthe visible surface of the sun is not a solid mass, is not even a liquidmass, but that the globe, so far as we can see it, consists of matter inthe gaseous, or vaporous, condition. 

[Illustration: Fig. 14. --Scheiner's Observations on Sun-spots. ]

It often happens that a large spot divides into two or more separateportions, and these have been sometimes seen to fly apart with avelocity in some cases not less than a thousand miles an hour. "Attimes, though very rarely" (I quote here Professor Young, [4] to whom Iam frequently indebted), "a different phenomenon of the most surprisingand startling character appears in connection with these objects:patches of intense brightness suddenly break out, remaining visible fora few minutes, moving, while they last, with velocities as great as onehundred miles _a second_. "

[Illustration: Fig. 15. --Zones on the Sun's Surface in which Spotsappear. ]

"One of these events has become classical. It occurred on the forenoon(Greenwich time) of September 1st, 1859, and was independently witnessedby two well-known and reliable observers--Mr. Carrington and Mr. Hodgson--whose accounts of the matter may be found in the MonthlyNotices of the Royal Astronomical Society for November, 1859. Mr. Carrington at the time was making his usual daily observations upon theposition, configuration, and size of the spots by means of an image ofthe solar disc upon a screen--being then engaged upon that eight years'series of observations which lie at the foundation of so much of ourpresent solar science. Mr. Hodgson, at a distance of many miles, was atthe same time sketching details of sun-spot structure by means of asolar eye-piece and shade-glass. They simultaneously saw two luminousobjects, shaped something like two new moons, each about eight thousandmiles in length and two thousand wide, at a distance of some twelvethousand miles from each other. These burst suddenly into sight at theedge of a great sun-spot with a dazzling brightness at least five or sixtimes that of the neighbouring portions of the photosphere, and movedeastward over the spot in parallel lines, growing smaller and fainter, until in about five minutes they disappeared, after traversing a courseof nearly thirty-six thousand miles. "

The sun-spots do not occur at all parts of the sun's surfaceindifferently. They are mainly found in two zones (Fig. 15) on each sideof the solar equator between the latitudes of 10° and 30°. On theequator the spots are rare except, curiously enough, near the time whenthere are few spots elsewhere. In high latitudes they are never seen. Closely connected with these peculiar principles of their distributionis the remarkable fact that spots in different latitudes do not indicatethe same values for the period of rotation of the sun. By watching aspot near the sun's equator Carrington found that it completed arevolution in twenty-five days and two hours. At a latitude of 20° theperiod is about twenty-five days and eighteen hours, at 30° it is noless than twenty-six days and twelve hours, while the comparatively fewspots observed in the latitude of 45° require twenty-seven and a halfdays to complete their circuit. 

As the sun, so far at least as its outer regions are concerned, is amass of gas and not a solid body, there would be nothing incredible inthe supposition that spots are occasionally endowed with movements oftheir own like ships on the ocean. It seems, however, from the factsbefore us that the different zones on the sun, corresponding to what wecall the torrid and temperate zones on the earth, persist in rotatingwith velocities which gradually decrease from the equator towards thepoles. It seems probable that the interior parts of the sun do notrotate as if the whole were a rigidly connected mass. The mass of thesun, or at all events its greater part, is quite unlike a rigid body, and the several portions are thus to some extent free for independentmotion. Though we cannot actually see how the interior parts of the sunrotate, yet here the laws of dynamics enable us to infer that theinterior layers of the sun rotate more rapidly than the outer layers, and thus some of the features of the spot movements can be accountedfor. But at present it must be confessed that there are greatdifficulties in the way of accounting for the distribution of spots andthe law of rotation of the sun. 

In the year 1826 Schwabe, a German astronomer, commenced to keep aregular register of the number of spots visible on the sun. Afterwatching them for seventeen years he was able to announce that thenumber of spots seemed to fluctuate from year to year, and that therewas a period of about ten years in their changes. Subsequentobservations have confirmed this discovery, and old books andmanuscripts have been thoroughly searched for information of earlydate. Thus a more or less complete record of the state of the sun asregards spots since the beginning of the seventeenth century has beenput together. This has enabled astronomers to fix the period of therecurring maximum with greater accuracy. 

The course of one of the sun-spot cycles may be described as follows:For two or three years the spots are both larger and more numerous thanon the average; then they begin to diminish, until in about six or sevenyears from the maximum they decline to a minimum; the number of thespots then begins to increase, and in about four and a half years themaximum is once more attained. The length of the cycle is, on anaverage, about eleven years and five weeks, but both its length and theintensity of the maxima vary somewhat. For instance, a great maximumoccurred in the summer of 1870, after which a very low minimum occurredin 1879, followed by a feeble maximum at the end of 1883; next came anaverage minimum about August, 1889, followed by the last observedmaximum in January, 1894. It is not unlikely that a second period ofabout sixty or eighty years affects the regularity of the eleven-yearperiod. Systematic observations carried on through a great many years tocome will be required to settle this question, as the observations ofsun-spots previous to 1826 are far too incomplete to decide the issueswhich arise. 

A curious connection seems to exist between the periodicity of the spotsand their distribution over the surface of the sun. When a minimum isabout to pass away the spots generally begin to show themselves inlatitudes about 30° north and south of the sun's equator; they thengradually break out somewhat nearer to the equator, so that at the timeof maximum frequency most of them appear at latitudes not greater than16°. This distance from the sun's equator goes on decreasing till thetime of minimum. Indeed, the spots linger on very close to the equatorfor a couple of years more, until the outbreak signalising thecommencement of another period has commenced in higher latitudes. 

We have still to note an extraordinary feature which points to anintimate connection between the phenomena of sun-spots and the purelyterrestrial phenomena of magnetism. It is of course well known that theneedle of a compass does not point exactly to the north, but divergesfrom the meridian by an angle which is different in different places andis not even constant at the same place. For instance, at Greenwich theneedle at present points in a direction 17° West of North, but thisamount is subject to very slow and gradual changes, as well as to verysmall daily oscillations. It was found about fifty years ago by Lamont(a Bavarian astronomer, but a native of Scotland) that the extent ofthis daily oscillation increases and decreases regularly in a periodwhich he gave as 10-1/3 years, but which was subsequently found to be11-1/10 years, exactly the same as the period of the spots on the sun. From a diligent study of the records of magnetic observations it hasbeen found that the time of sun-spot maximum always coincides almostexactly with that of maximum daily oscillation of the compass needle, while the minima agree similarly. This close relationship between theperiodicity of sun-spots and the daily movements of the magnetic needleis not the sole proof we possess that there is a connection of some sortbetween solar phenomena and terrestrial magnetism. A time of maximumsun-spots is a time of great magnetic activity, and there have even beenspecial cases in which a peculiar outbreak on the sun has beenassociated with remarkable magnetic phenomena on the earth. A veryinteresting instance of this kind is recorded by Professor Young, who, when observing at Sherman on the 3rd August, 1872, perceived a veryviolent disturbance of the sun's surface. He was told the same day by amember of his party, who was engaged in magnetic observations and whowas quite in ignorance of what Professor Young had seen, that he hadbeen obliged to desist from his magnetic work in consequence of theviolent motion of his magnet. It was afterwards found from thephotographic records at Greenwich and Stonyhurst that the magnetic"storm" observed in America had simultaneously been felt in England. Asimilar connection between sun-spots and the aurora borealis has alsobeen noticed, this fact being a natural consequence of the well-knownconnection between the aurora and magnetic disturbances. On the otherhand, it must be confessed that many striking magnetic storms haveoccurred without any corresponding solar disturbance, [5] but even thosewho are inclined to be sceptical as to the connection between these twoclasses of phenomena in particular cases can hardly doubt the remarkableparallelism between the general rise and fall in the number of sun-spotsand the extent of the daily movements of the compass needle. 

[Illustration: Fig. 16. --The Texture of the Sun and a small Spot. ]

We have now described the principal solar phenomena with which thetelescope has made us acquainted. But there are many questions connectedwith the nature of the sun which not even the most powerful telescopewould enable us to solve, but which the spectroscope has given us themeans of investigating. 

What we receive from the sun is warmth and light. The intensely heatedmass of the sun radiates forth its beams in all directions withboundless prodigality. Each beam we feel to be warm, and we see to bebrilliantly white, but a more subtle analysis than mere feeling or merevision is required. Each sunbeam bears marks of its origin. These marksare not visible until a special process has been applied, but then thesunbeam can be made to tell its story, and it will disclose to us muchof the nature of the constitution of the great luminary. 

We regard the sun's light as colourless, just as we speak of water astasteless, but both of those expressions relate rather to our ownfeelings than to anything really characteristic of water or of sunlight. We regard the sunlight as colourless because it forms, as it were, thebackground on which all other colours are depicted. The fact is, thatwhite is so far from being colourless that it contains every known hueblended together in certain proportions. The sun's light is reallyextremely composite; Nature herself tells us this if we will but giveher the slightest attention. Whence come the beautiful hues with whichwe are all familiar? Look at the lovely tints of a garden; the red ofthe rose is not in the rose itself. All the rose does is to grasp thesunbeams which fall upon it, extract from these beams the red which theycontain, and radiate that red light to our eyes. Were there not red raysconveyed with the other rays in the sunbeam, there could be no red roseto be seen by sunlight. 

The principle here involved has many other applications; a lady willoften say that a dress which looks very well in the daylight does notanswer in the evening. The reason is that the dress is intended to showcertain colours which exist in the sunlight; but these colours are notcontained to the same degree in gaslight, and consequently the dress hasa different hue. The fault is not in the dress, the fault lies in thegas; and when the electric light is used it sends forth beams morenearly resembling those from the sun, and the colours of the dressappear with all their intended beauty. 

The most glorious natural indication of the nature of the sunlight isseen in the rainbow. Here the sunbeams are refracted and reflected fromtiny globes of water in the clouds; these convey to us the sunlight, andin doing so decompose the white beams into the seven primary hues--red, orange, yellow, green, blue, indigo, and violet. 

[Illustration: PLATE A. 

THE SUN. 

_Royal Observatory, Greenwich, July 8, 1892. _]

[Illustration: Fig. 17. --The Prism. ]

The bow set in the cloud is typical of that great department of modernscience of which we shall now set forth the principles. The globes ofwater decompose the solar beams; and we follow the course suggested bythe rainbow, and analyse the sunlight into its constituents. We areenabled to do this with scientific accuracy when we employ thatremarkable key to Nature's secrets known as the spectroscope. The beamsof white sunlight consist of innumerable beams of every hue in intimateassociation. Every shade of red, of yellow, of blue, and of green, canbe found in a sunbeam. The magician's wand, with which we strike thesunbeam and sort the tangled skein into perfect order, is the simpleinstrument known as the glass prism. We have represented this instrumentin its simplest form in the adjoining figure (Fig. 17). It is a piece ofpure and homogeneous glass in the shape of a wedge. When a ray of lightfrom the sun or from any source falls upon the prism, it passes throughthe transparent glass and emerges on the other side; a remarkable changeis, however, impressed on the ray by the influence of the glass. It isbent by refraction from the path it originally pursued, and is compelledto follow a different path. If, however, the prism bent all rays oflight equally, then it would be of no service in the analysis of light;but it fortunately happens that the prism acts with varying efficiencyon the rays of different hues. A red ray is not refracted so much as ayellow ray; a yellow ray is not refracted so much as a blue one. Itconsequently happens that when the composite beam of sunlight, in whichall the different rays are blended, passes through the prism, theyemerge in the manner shown in the annexed figure (Fig. 18). Here then wehave the source of the analysing power of the prism; it bends thedifferent hues unequally and consequently the beam of compositesunlight, after passing through the prism, no longer shows mere whitelight, but is expanded into a coloured band of light, with hues like therainbow, passing from deep red at one end through every intermediategrade to the violet. 

[Illustration: Fig. 18. --Dispersion of Light by the Prism. ]

We have in the prism the means of decomposing the light from the sun, orthe light from any other source, into its component parts. Theexamination of the quality of the light when analysed enables us tolearn something of the constitution of the body from which this lighthas emanated. Indeed, in some simple cases the mere colour of a lightwill be sufficient to indicate the source from which it has come. Thereis, for instance, a splendid red light sometimes seen in displays offireworks, due to the metal strontium. The eye can identify the elementby the mere colour of the flame. There is also a characteristic yellowlight produced by the flame of common salt burned with spirits of wine. Sodium is the important constituent of salt, so here we recogniseanother substance merely by the colour it emits when burning. We mayalso mention a third substance, magnesium, which burns with a brilliantwhite light, eminently characteristic of the metal. 

[Illustration: PLATE XIII. 

SPECTRA OF THE SUN AND STARS. 

I. SUN. II. SIRIUS. III. ALDEBARAN. IV. BETELGEUZE. ]

The three metals, strontium, sodium, and magnesium, may thus beidentified by the colours they produce when incandescent. In this simpleobservation lies the germ of the modern method of research known asspectrum analysis. We may now examine with the prism the colours of thesun and the colours of the stars, and from this examination we can learnsomething of the materials which enter into their composition. We arenot restricted to the use of merely a single prism, but we may arrangethat the light which it is desired to analyse shall pass through severalprisms in succession in order to increase the _dispersion_ or thespreading out of the different colours. To enter the spectroscope thelight first passes through a narrow slit, and the rays are then renderedparallel by passing through a lens; these parallel rays next passthrough one or more prisms, and are finally viewed through a smalltelescope, or they may be intercepted by a photographic plate on which apicture will then be made. If the beam of light passing through the slithas radiated from an incandescent solid or liquid body, or from a gasunder high pressure, the coloured band or _spectrum_ is found to containall the colours indicated on Plate XIII. , without any interruptionbetween the colours. This is known as a continuous spectrum. But if weexamine light from a gas under low pressure, as can be done by placing asmall quantity of the gas in a glass tube and making it glow by anelectric current, we find that it does not emit rays of all colours, butonly rays of certain distinct colours which are different for differentgases. The spectrum of a gas, therefore, consists of a number ofdetached luminous lines. 

When we study the sunlight through the prism, it is found that thespectrum does not extend quite continuously from one end to the other, but is shaded over by a multitude of dark lines, only a few of which areshown in the adjoining plate. (Plate XIII. ) These lines are a permanentfeature in the solar spectrum. They are as characteristic of thesunlight as the prismatic colours themselves, and are full of interestand information with regard to the sun. These lines are the charactersin which the history and the nature of the sun are written. Viewedthrough an instrument of adequate power, dark lines are to be foundcrossing the solar spectrum in hundreds and in thousands. They are ofevery variety of strength and faintness; their distribution seems guidedby no simple law. At some parts of the spectrum there are but few lines;in other regions they are crowded so closely together that it isdifficult to separate them. They are in some places exquisitely fine anddelicate, and they never fail to excite the admiration of every one wholooks at this interesting spectacle in a good instrument. 

There can be no better method of expounding the rather difficult subjectof spectrum analysis than by actually following the steps of theoriginal discovery which first gave a clear demonstration of thesignificance of the dark "Fraunhofer" lines. Let us concentrate ourattention specially upon that line of the solar spectrum marked D. This, when seen in the spectroscope, is found to consist of two lines, verydelicately separated by a minute interval, one of these lines beingslightly thicker than the other. Suppose that while the attention isconcentrated on these lines the flame of an ordinary spirit-lampcoloured by common salt be held in front of the instrument, so that theray of direct solar light passes through the flame before entering thespectroscope. The observer sees at once the two lines known as D flashout with a greatly increased blackness and vividness, while there is noother perceptible effect on the spectrum. A few trials show that thisintensification of the D lines is due to the vapour of sodium arisingfrom the salt burning in the lamp through which the sunlight has passed. 

It is quite impossible that this marvellous connection between sodiumand the D lines of the spectrum can be merely casual. Even if there wereonly a single line concerned, it would be in the highest degree unlikelythat the coincidence should arise by accident; but when we find thesodium affecting both of the two close lines which form D, ourconviction that there must be some profound connection between theselines and sodium rises to absolute certainty. Suppose that the sunlightbe cut off, and that all other light is excluded save that emanatingfrom the glowing vapour of sodium in the spirit flame. We shall thenfind, on looking through the spectroscope, that we no longer obtain allthe colours of the rainbow; the light from the sodium is concentratedinto two bright yellow lines, filling precisely the position which thedark D lines occupied in the solar spectrum, and the darkness of whichthe sodium flame seemed to intensify. 

We must here endeavour to remove what may at first sight appear to be aparadox. How is it, that though the sodium flame produces two _bright_lines when viewed in the absence of other light, yet it actually appearsto intensify the two _dark_ lines in the sun's spectrum? The explanationof this leads us at once to the cardinal doctrine of spectrum analysis. The so-called dark lines in the solar spectrum are only dark _bycontrast_ with the brilliant illumination of the rest of the spectrum. Agood deal of solar light really lies in the dark lines, though notenough to be seen when the eye is dazzled by the brilliancy around. Whenthe flame of the spirit-lamp charged with sodium intervenes, it sendsout a certain amount of light, which is entirely localised in these twolines. So far it would seem that the influence of the sodium flame oughtto be manifested in diminishing the darkness of the lines and renderingthem less conspicuous. As a matter of fact, they are far moreconspicuous with the sodium flame than without it. This arises from thefact that the sodium flame possesses the remarkable property of cuttingoff the sunlight which was on its way to those particular lines; sothat, though the sodium contributes some light to the lines, yet itintercepts a far greater quantity of the light that would otherwise haveilluminated those lines, and hence they became darker with the sodiumflame than without it. 

We are thus conducted to a remarkable principle, which has led to theinterpretation of the dark lines in the spectrum of the sun. We findthat when the sodium vapour is heated, it gives out light of a veryparticular type, which, viewed through the prism, is concentrated in twolines. But the sodium vapour possesses also this property, that lightfrom the sun can pass through it without any perceptible absorption, except of those particular rays which are of the same characters as thetwo lines in question. In other words, we say that if the heated vapourof a substance gives a spectrum of bright lines, corresponding to lightsof various kinds, this same vapour will act as an opaque screen tolights of those special kinds, while remaining transparent to light ofevery other description. 

This principle is of such importance in the theory of spectrum analysisthat we add a further example. Let us take the element iron, which in avery striking degree illustrates the law in question. In the solarspectrum some hundreds of the dark lines are known to correspond withthe spectrum of iron. This correspondence is exhibited in a vivid mannerwhen, by a suitable contrivance, the light of an electric spark frompoles of iron is examined in the spectroscope side by side with thesolar spectrum. The iron lines in the sun are identical in position withthe lines in the spectrum of glowing iron vapour. But the spectrum ofiron, as here described, consists of bright lines; while those withwhich it is compared in the sun are dark on a bright background. Theycan be completely understood if we suppose the vapour arising fromintensely heated iron to be present in the atmosphere which surroundsthe luminous strata on the sun. This vapour would absorb or stopprecisely the same rays as it emits when incandescent, and hence welearn the important fact that iron, no less than sodium, must, in oneform or another, be a constituent of the sun. 

Such is, in brief outline, the celebrated discovery of modern timeswhich has given an interpretation to the dark lines of the solarspectrum. The spectra of a large number of terrestrial substances havebeen examined in comparison with the solar spectrum, and thus it hasbeen established that many of the elements known on the earth arepresent in the sun. We may mention calcium, iron, hydrogen, sodium, carbon, nickel, magnesium, cobalt, aluminium, chromium, strontium, manganese, copper, zinc, cadmium, silver, tin, lead, potassium. Some ofthe elements which are of the greatest importance on the earth wouldappear to be missing from the sun. Sulphur, phosphorus, mercury, gold, nitrogen may be mentioned among the elements which have hitherto givenno indication of their being solar constituents. 

It is also possible that the lines of a substance in the sun'satmosphere may be so very bright that the light of the continuousspectrum, on which they are superposed, is not able to "reverse"them--_i. E. _ turn them into dark lines. We know, for instance, that thebright lines of sodium vapour may be made so intensely bright that thespectrum of an incandescent lime-cylinder placed behind the sodiumvapour does not reverse these lines. If, then, we make the sodium linesfainter, they may be reduced to exactly the intensity prevailing in thatpart of the spectrum of the lime-light, in which case the lines, ofcourse, could not be distinguished. The question as to what elements arereally missing from the sun must therefore, like many other questionsconcerning our great luminary, at present be considered an open one. Weshall shortly see that an element previously unknown has actually beendiscovered by means of a line representing it in the solar spectrum. 

Let us now return to the sun-spots and see what the spectroscope canteach us as to their nature. We attach a powerful spectroscope to theeye-end of a telescope in order to get as much light as possibleconcentrated on the slit; the latter has therefore to be placed exactlyat the focus of the object-glass. The instrument is then pointed to aspot, so that its image falls on the slit, and the presence of the darkcentral part called the _umbra_ reveals itself by a darkish stripe whichtraverses the ordinary sun-spectrum from end to end. It is bordered onboth sides by the spectrum of the _penumbra_, which is much brighterthan that of the umbra, but fainter than that of the adjoining regionsof the sun. 

From the fact that the spectrum is darkened we learn that there isconsiderable general absorption of light in the umbra. This absorptionis not, however, such as would be caused by the presence of volumes ofminute solid or liquid particles like those which constitute smoke orcloud. This is indicated by the fact, first discovered by Young in1883, that the spectrum is not uniformly darkened as it would be if theabsorption were caused by floating particles. In the course ofexamination of many large and quiescent spots, he perceived that themiddle green part of the spectrum was crossed by countless fine, darklines, generally touching each other, but here and there separated bybright intervals. Each line is thicker in the middle (corresponding tothe centre of the spot) and tapers to a fine thread at each end; indeed, most of these lines can be traced across the spectrum of the penumbraand out on to that of the solar surface. The absorption would thereforeseem to be caused by gases at a much lower temperature than that of thegases present outside the spot. 

In the red and yellow parts of the spot-spectrum, which have beenspecially studied for many years by Sir Norman Lockyer at the SouthKensington Observatory, interesting details are found which confirm thisconclusion. Many of the dark lines are not thicker and darker in thespot than they are in the ordinary sun-spectrum, while others are verymuch thickened in the spot-spectrum, such as the lines of iron, calcium, and sodium. The sodium lines are sometimes both widened and doublyreversed--that is, on the thick dark line a bright line is superposed. The same peculiarity is not seldom seen in the notable calcium lines Hand K at the violet end of the spectrum. These facts indicate thepresence of great masses of the vapours of sodium and calcium over thenucleus. The observations at South Kensington have also brought to lightanother interesting peculiarity of the spot-spectra. At the time ofminimum frequency of spots the lines of iron and other terrestrialelements are prominent among the most widened lines; at the maxima thesealmost vanish, and the widening is found only amongst lines of unknownorigin. 

The spectroscope has given us the means of studying other interestingfeatures on the sun, which are so faint that in the full blaze ofsunlight they cannot be readily observed with a mere telescope. We can, however, see them easily enough when the brilliant body of the sun isobscured during the rare occurrence of a total eclipse. The conditionsnecessary for the occurrence of an eclipse will be more fullyconsidered in the next chapter. For the present it will be sufficient toobserve that by the movement of the moon it may so happen that the mooncompletely hides the sun, and thus for certain parts of the earthproduces what we call a total eclipse. The few minutes during which atotal eclipse lasts are of much interest to the astronomer. Darknessreigns over the landscape, and in that darkness rare and beautifulsights are witnessed. 

[Illustration: Fig. 19. --Prominences seen in Total Eclipse. ]

We have in Fig. 19 a diagram of a total eclipse, showing some of theremarkable objects known as prominences (_a_, _b_, _c_, _d_, _e_) whichproject from behind the dark body of the moon. That they do not belongto the moon, but are solar appendages of some sort, is easilydemonstrated. They first appear on the eastern limb at the commencementof totality. Those first seen are gradually more or less covered by theadvancing moon, while others peep out behind the western limb of themoon, until totality is over and the sunlight bursts out again, whenthey all instantly vanish. 

The first total eclipse which occurred after the spectroscope had beenplaced in the hands of astronomers was in 1868. On the 18th August inthat year a total eclipse was visible in India. Several observers, armedwith spectroscopes, were on the look-out for the prominences, and wereable to announce that their spectrum consisted of detached bright lines, thus demonstrating that these objects were masses of glowing gas. On thefollowing day the illustrious astronomer, Janssen, one of the observersof the eclipse, succeeded in seeing the lines in full sunlight, as henow knew exactly where to look for them. Many months before the eclipseSir Norman Lockyer had been preparing to search for the prominences, ashe expected them to yield a line spectrum which would be readilyvisible, if only the sun's ordinary light could be sufficiently winnowedaway. He proposed to effect this by using a spectroscope of greatdispersion, which would spread out the continuous spectrum considerablyand make it fainter. The effect of the great dispersion on the isolatedbright lines he expected to see would be only to widen the intervalsbetween them without interfering with their brightness. The newspectroscope, which he ordered to be constructed for this purpose, wasnot completed until some weeks after the eclipse was over, though beforethe news of Janssen's achievement reached Europe from India. When thatnews did arrive Sir N. Lockyer had already found the spectrum of unseenprominences at the sun's limb. The honour of the practical applicationof a method of observing solar prominences without the help of aneclipse must therefore be shared between the two astronomers. 

When a spectroscope is pointed to the margin of the sun so that the slitis radial, certain short luminous lines become visible which lie exactlyin the prolongation of the corresponding dark lines in the solarspectrum. From due consideration of the circumstances it can be shownthat the gases which form the prominences are also present as acomparatively shallow atmospheric layer all round the great luminary. This layer is about five or six thousand miles deep, and is situatedimmediately above the dense layer of luminous clouds which forms thevisible surface of the sun and which we call the photosphere. Thegaseous envelope from which the prominences spring has been called thechromosphere on account of the coloured lines displayed in its spectrum. Such lines are very numerous, but those pertaining to the singlesubstance, hydrogen, predominate so greatly that we may say thechromosphere consists chiefly of this element. It is, however, to benoted that calcium and one other element are also invariably present, while iron, manganese and magnesium are often apparent. The remarkableelement, of which we have not yet mentioned the name, has had anastonishing history. 

During the eclipse of 1868 a fine yellow line was noticed among thelines of the prominence spectrum, and it was not unnaturally at firstassumed that it must be the yellow sodium line. But when carefulobservations were afterwards made without hurry in full sunshine, andaccurate measures were obtained, it was at once remarked that this linewas not identical with either of the components of the double sodiumline. The new line was, no doubt, quite close to the sodium lines, butslightly towards the green part of the spectrum. It was also noticedthere was not generally any corresponding line to be seen among the darklines in the ordinary solar spectrum, though a fine dark one has now andthen been detected, especially near a sun-spot. Sir Norman Lockyer andSir Edward Frankland showed that this was not produced by any knownterrestrial element. It was, therefore, supposed to be caused by somehitherto unknown body to which the name of _helium_, or the sun element, was given. About a dozen less conspicuous lines were graduallyidentified in the spectrum of the prominences and the chromosphere, which appeared also to be caused by this same mysterious helium. Thesesame remarkable lines have in more recent years also been detected inthe spectra of various stars. 

This gas so long known in the heavens was at last detected on earth. InApril, 1895, Professor Ramsay, who with Lord Rayleigh had discovered thenew element argon, detected the presence of the famous helium line inthe spectrum of the gas liberated by heating the rare mineral known ascleveite, found in Norway. Thus this element, the existence of which hadfirst been detected on the sun, ninety-three million miles away, has atlast been proved to be a terrestrial element also. 

When it was announced by Runge that the principal line in the spectrumof the terrestrial helium had a faint and very close companion line onthe red-ward side, some doubt seemed at first to be cast on the identityof the new terrestrial gas discovered by Ramsay with the helium of thechromosphere. The helium line of the latter had never been noticed to bedouble. Subsequently, however, several observers provided with verypowerful instruments found that the famous line in the chromospherereally had a very faint companion line. Thus the identity between thecelestial helium and the gas found on our globe was established in themost remarkable manner. Certain circumstances have seemed to indicatethat the new gas might possibly be a mixture of two gases of differentdensities, but up to the present this has not been proved to be thecase. 

After it had been found possible to see the spectra of prominenceswithout waiting for an eclipse, Sir W. Huggins, in an observation on the13th of February, 1869, successfully applied a method for viewing theremarkable solar objects themselves instead of their mere spectra infull sunshine. It is only necessary to adjust the spectroscope so thatone of the brightest lines--_e. G. _ the red hydrogen line--is in themiddle of the field of the viewing telescope, and then to open wide theslit of the spectroscope. A red image of the prominence will then bedisplayed instead of the mere line. In fact, when the slit is openedwide, the prisms produce a series of detached images of the prominenceunder observation, one for each kind of light which the object emits. 

We have spoken of the spectroscope as depending upon the action of glassprisms. It remains to be added that in the highest class ofspectroscopes the prisms are replaced by ruled gratings from which thelight is reflected. The effect of the ruling is to produce by what isknown as diffraction the required breaking up of the beam of light intoits constituent parts. 

[Illustration: PLATE IV. 

SOLAR PROMINENCES. 

(DRAWN BY TROUVELOT AT HARVARD COLLEGE, CAMBRIDGE, U. S. , IN 1872. )]

Majestic indeed are the proportions of some of those mighty prominenceswhich leap from the luminous surface; yet they flicker, as do ourterrestrial flames, when we allow them time comparable to their giganticdimensions. Drawings of the same prominence made at intervals of a fewhours, or even less, often show great changes. The magnitude of thedisplacements that have been noticed sometimes attains many thousands ofmiles, and the actual velocity with which such masses move frequentlyexceeds 100 miles a second. Still more violent are the convulsions when, from the surface of the chromosphere, as from a mighty furnace, vastincandescent masses of gas are projected upwards. Plate IV. Gives a viewof a number of prominences as seen by Trouvelot at Harvard CollegeObservatory, Cambridge, U. S. A. Trouvelot has succeeded in exhibiting inthe different pictures the wondrous variety of aspect which theseobjects assume. The dimensions of the prominences may be inferred fromthe scale appended to the plate. The largest of those here shown isfully 80, 000 miles high; and trustworthy observers have recordedprominences of an altitude even much greater. The rapid changes whichthese objects sometimes undergo are well illustrated in the two sketcheson the left of the lowest line, which were drawn on April 27th, 1872. These are both drawings of the same prominence taken at an interval nogreater than twenty minutes. This mighty flame is so vast that itslength is ten times as great as the diameter of the earth, yet in thisbrief period it has completely changed its aspect; the upper part of theflame has, indeed, broken away, and is now shown in that part of thedrawing between the two figures on the line above. The same plate alsoshows various instances of the remarkable spike-like objects, taken, however, at different times and at various parts of the sun. Thesespikes attain altitudes not generally greater than 20, 000 miles, thoughsometimes they soar aloft to stupendous distances. 

We may refer to one special object of this kind, the remarkable historyof which has been chronicled by Professor Young. On October 7th, 1880, aprominence was seen, at about 10. 30 a. M. , on the south-east limb of thesun. It was then about 40, 000 miles high, and attracted no specialattention. Half an hour later a marvellous transformation had takenplace. During that brief interval the prominence became very brilliantand doubled its length. For another hour the mighty flame still soaredupwards, until it attained the unprecedented elevation of 350, 000miles--a distance more than one-third the diameter of the great luminaryitself. At this climax the energy of the mighty outbreak seems to haveat last become exhausted: the flame broke up into fragments, and by12. 30--an interval of only two hours from the time when it was firstnoticed--the phenomenon had completely faded away. 

No doubt this particular eruption was exceptional in its vehemence, andin the vastness of the changes of which it was an indication. Thevelocity of upheaval must have been at least 200, 000 miles an hour, or, to put it in another form, more than fifty miles a second. This mightyflame leaped from the sun with a velocity more than 100 times as greatas that of the swiftest bullet ever fired from a rifle. 

The prominences may be generally divided into two classes. We have firstthose which are comparatively quiescent, and in form somewhat resemblethe clouds which float in our earth's atmosphere. The second class ofprominences are best described as eruptive. They are, in fact, thrown upfrom the chromosphere like gigantic jets of incandescent material. Thesetwo classes of objects differ not only in appearance but also in thegases of which they are composed. The cloud-like prominences consistmainly of hydrogen, with helium and calcium, while many metals arepresent in the eruptive discharges. The latter are never seen in theneighbourhood of the sun's poles, but generally appear close to asun-spot, thus confirming the conclusion that the spots are associatedwith violent disturbances on the surface of the sun. When a spot hasreached the limb of the sun it is frequently found to be surrounded byprominences. It has even been possible in a few instances to detectpowerful gaseous eruptions in the neighbourhood of a spot, thespectroscope rendering them visible against the background of the solarsurface just as the prominences are observed at the limb against thebackground of the sky. 

In order to photograph a prominence we have, of course, to substitute aphotographic plate for the observer's eye. Owing, however, to thedifficulty of preventing the feeble light from the prominence from beingoverpowered by extraneous light, the photography of these bodies was notvery successful until Professor Hale, of Chicago, designed hisspectro-heliograph. In this instrument there is (in addition to theusual slit through which the light falls on the prisms, or grating, ) asecond slit immediately in front of the photographic plate through whichthe light of a given wave-length can be permitted to pass to theexclusion of all the rest. The light chosen for producing an image ofthe prominences is that radiated in the remarkable "K line, " due tocalcium. This lies at the extreme end of the violet. The light from thatpart of the spectrum, though it is invisible to the eye, is much moreactive photographically than the light from the red, yellow, or greenparts of the spectrum. The front slit is adjusted so that the K linefalls upon the second slit, and as the front slit is slowly swept byclockwork over the whole of a prominence, the second slit keeps pacewith it by a mechanical contrivance. 

If the image of the solar disc is hidden by a screen of exactly theproper size, the slits may be made to sweep over the whole sun, thusgiving us at one exposure a picture of the chromospheric ring round thesun's limb with its prominences. The screen may now be withdrawn, andthe slits may be made to sweep rapidly over the disc itself. They revealthe existence of glowing calcium vapours in many parts of the surface ofthe sun. Thus we get a striking picture of the sun as drawn by thisparticular light. In this manner Professor Hale confirmed theobservation made long before by Professor Young, that the spectra offaculæ always show the two great calcium bands. 

The velocity with which a prominence shoots upward from the sun's limbcan, of course, be measured directly by observations of the ordinarykind with a micrometer. The spectroscope, however, enables us toestimate the speed with which disturbances at the surface of the suntravel in the direction towards the earth or from the earth. We canmeasure this speed by watching the peculiar behaviour of the spectrallines representing the rapidly moving masses. This opens up a remarkableline of investigation with important applications in many branches ofastronomy. 

It is, of course, now generally understood that the sensation of lightis caused by waves or undulations which impinge on the retina of theeye after having been transmitted through that medium which we call theether. To the different colours correspond different wave-lengths--thatis to say, different distances between two successive waves. A beam ofwhite light is formed by the union of innumerable different waves whoselengths have almost every possible value lying between certain limits. The wave-length of red light is such that there are 33, 000 waves in aninch, while that of violet light is but little more than half that ofred light. The position of a line in the spectrum depends solely on thewave-length of the light to which it is due. Suppose that the source oflight is approaching directly towards the observer; obviously the wavesfollow each other more closely than if the source were at rest, and thenumber of undulations which his eye receives in a second must beproportionately increased. Thus the distance between two successiveether waves will be very slightly diminished. A well-known phenomenon ofa similar character is the change of pitch of the whistle of alocomotive engine as it rushes past. This is particularly noticeable ifthe observer happens to be in a train which is moving rapidly in theopposite direction. In the case of sound, of course, the vibrations orwaves take place in the air and not in the ether. But the effect ofmotion to or from the observer is strictly analogous in the two cases. As, however, light travels 186, 000 miles a second, the source of lightwill also have to travel with a very high velocity in order to produceeven the smallest perceptible change in the position of a spectral line. 

We have already seen that enormously high velocities are by no meansuncommon in some of these mighty disturbances on the sun; accordingly, when we examine the spectrum of a sun-spot, we often see that some ofthe lines are shifted a little towards one end of the spectrum andsometimes towards the other, while in other cases the lines are seen tobe distorted or twisted in the most fantastic manner, indicating veryviolent local commotions. If the spot happens to be near the centre ofthe sun's disc, the gases must be shooting upwards or downwards toproduce these changes in the lines. The velocities indicated inobservations of this class sometimes amount to as much as two or eventhree hundred miles per second. We find it difficult to conceive theenormous internal pressures which are required to impel such mightymasses of gases aloft from the photosphere with speeds so terrific, orthe conditions which bring about the downrush of such gigantic masses ofvapour from above. In the spectra of the prominences on the sun's limbalso we often see the bright lines bent or shifted to one side. In suchcases what we witness is evidently caused by movements along the surfaceof the chromosphere, conveying materials towards us or away from us. 

An interesting application of this beautiful method of measuring thespeed of moving bodies has been made in various attempts to determinethe period of rotation of the sun spectroscopically. As the sun turnsround on its axis, a point on the eastern limb is moving towards theobserver and a point on the western limb is moving away from him. Ineach case the velocity is a little over a mile per second. At theeastern limb the lines in the solar spectrum are very slightly shiftedtowards the violet end of the spectrum, while the lines in the spectrumof the western limb are equally shifted towards the red end. By aningenious optical contrivance it is possible to place the spectra fromthe two limbs side by side, which doubles the apparent displacement, andthus makes it much more easy to measure. Even with this contrivance thevisual quantities to be measured remain exceedingly minute. All theparts of the instrument have to be most accurately adjusted, and theobservations are correspondingly delicate. They have been attempted byvarious observers. Among the most successful investigations of this kindwe may mention that of the Swedish astronomer, Dunér, who, by pointinghis instrument to a number of places on the limb, found values in goodagreement with the peculiar law of rotation which has been deduced fromthe motion of sun-spots. This result is specially interesting, as itshows that the atmospheric layers, in which that absorption takes placewhich produces the dark lines in the spectrum, shares in the motion ofthe photosphere at the same latitude. 

[Illustration: Fig. 20. --View of the Corona (and a Comet) in a TotalEclipse. ]

[Illustration: PLATE V. 

TOTAL SOLAR ECLIPSE, JULY 29TH, 1878. 

THE CORONA FROM THE PHOTOGRAPHS. 

(HARKNESS. )]

We have yet to mention one other striking phenomenon which is among thechief attractions to observers of total eclipses, and which it hashitherto not been found possible to see in full daylight. This is thecorona or aureole of light which is suddenly seen to surround the sun inan eclipse when the moon has completely covered the last remainingcrescent of the sun. A general idea of the appearance of the corona isgiven in Fig. 20, and we further present in Plate V. The drawing of thecorona made by Professor Harkness from a comparison of a large number ofphotographs obtained at different places in the United States during thetotal eclipse of July 29th, 1878. In Fig. 21 we are permitted by thekindness of Mr. And Mrs. Maunder to reproduce the remarkable photographof the corona which they obtained in India during the eclipse of January22nd, 1898. 

[Illustration: Fig. 21. --View of Corona during the Eclipse of Jan. 22nd, 1898 (_Reproduced by kind permission of Mr. And Mrs. Maunder and of theproprietors of "Knowledge. _")]

The part of the corona nearest the sun is very bright, though not sobrilliant as the prominences, which (as Professor Young says) blazethrough it like carbuncles. This inner portion is generally of fairlyregular outline, forming a white ring about a tenth part of the solardiameter in width. The outer parts of the corona are usually veryirregular and very extensive. They are often interrupted by narrow"rifts, " or narrow dark bands, which reach from the limb of the sunthrough the entire corona. On the other hand, there are also sometimesnarrow bright streamers, inclined at various angles to the limb of thesun and not seldom curved. In the eclipses of 1867, 1878, and 1889, allof which occurred at periods of sun-spot minimum, the corona showed longand faint streamers nearly in the direction of the sun's equator, andshort but distinct brushes of light near the poles. In the eclipses of1870, 1882, and 1893, near sun-spot maxima, the corona was moreregularly circular, and chiefly developed over the spot zones. We havehere another proof (if one were necessary) of the intimate connectionbetween the periodicity of the spots and the development of all othersolar phenomena. 

In the spectrum of the corona there is a mysterious line in the green, as to the origin of which nothing is at present certainly known. It isbest seen during eclipses occurring near the time of sun-spot maximum. It is presented in the ordinary solar spectrum as a very thin, darkline, which generally remains undisturbed even when lines of hydrogenand other substances are twisted and distorted by the violent rush ofdisturbed elements. The line is always present among the bright lines ofthe chromosphere spectrum. In addition to it the corona shows a fewother bright lines, belonging, no doubt, to the same unknown element("coronium"), and also a faint continuous spectrum, in which even a fewof the more prominent dark lines of the solar spectrum have beensometimes detected. This shows that in addition to glowing gas(represented by the bright lines) the corona also contains a great dealof matter like dust, or fog, the minute particles of which are capableof reflecting the sunlight and thereby producing a feeble continuousspectrum. This matter seems to form the principal constituent of thelong coronal rays and streamers, as the latter are not visible in thedetached images of the corona which appear instead of the bright lineswhen the corona is viewed, or photographed, during an eclipse, in aspectroscope without a slit. If the long rays were composed of the gasor gases which constitute the inner corona, it is evident that theyought to appear in these detached images. As to the nature of the forceswhich are continually engaged in shooting out these enormously longstreamers, we have at present but little information. It is, however, certain that the extensive atmospheric envelope round the sun, whichshows itself as the inner corona, must be extremely attenuated. Cometshave on several occasions been known to rush through this coronalatmosphere without evincing the slightest appreciable diminution intheir speed from the resistance to which they were exposed. 

We have accumulated by observation a great number of facts concerningthe sun, but when we try to draw from these facts conclusions as to thephysical constitution of that great body, it cannot be denied that thedifficulties seem to be very great indeed. We find that the bestauthorities differ considerably in the opinions they entertain as to itsnature. We shall here set forth the principal conclusions as to whichthere is little or no controversy. 

We shall see in a following chapter that astronomers have been able todetermine the relative densities of the bodies in the solar system; inother words, they have found the relation between the quantities ofmatter contained in an equally large volume of each. It has thus beenascertained that the average density of the sun is about a quarter thatof the earth. If we compare the weight of the sun with that of anequally great globe of water, we find that the luminary would be barelyone and a half times as heavy as the water. Of course, the actual massof the sun is very enormous; it is no less than 330, 000 times as greatas that of the earth. The solar material itself is, however, relativelylight, so that the sun is four times as big as it would have to be if, while its weight remained the same, its density equalled that of theearth. Bearing in mind this lightness of the sun, and also theexceedingly high temperature which we know to prevail there, no otherconclusion seems possible than that the body of the sun must be in agaseous state. The conditions under which such gases exist in the sunare, no doubt, altogether different from those with which we areacquainted on the earth. At the surface of the sun the force of gravityis more than twenty-seven times as great as it is on the earth. A personwho on the earth could just lift twenty-seven equal pieces of metalwould, if he were transferred to the sun, only be able to lift one ofthe pieces at a time. The pressure of the gases below the surface musttherefore be very great, and it might be supposed that they would becomeliquefied in consequence. It was, however, discovered by Andrews that solong as a gas is kept at a temperature higher than a certain point, known as the "critical temperature" (which is different for differentgases), the gas will not be turned into a liquid however great be thepressure to which it is submitted. The temperature on the sun cannot belower than the critical temperatures of the gases there existing; so itwould seem that even the enormous pressure can hardly reduce the gasesin the great luminary to the liquid form. 

Of the interior of the sun we can, of course, expect to learn little ornothing. What we observe is the surface-layer, the so-calledphotosphere, in which the cold of space produces the condensation of thegases into those luminous clouds which we see in our drawings andphotographs as "rice grains" or "willow leaves. " It has been suggestedby Dr. Johnstone Stoney (and afterwards by Professor Hastings, ofBaltimore) that these luminous clouds are mainly composed of carbon withthose of the related elements silicon and boron, the boiling points ofwhich are much higher than those of other elements which might beconsidered likely to form the photospheric clouds. The low atomic weightof carbon must also have the effect of giving the molecules of thiselement a very high velocity, and thereby enabling them to work theirway into the upper regions, where the temperature has so fallen that thevapour becomes chilled into cloud. A necessary consequence of the rapidcooling of these clouds, and the consequent radiation of heat on alarge scale, would be the formation of what we may perhaps describe assmoke, which settles by degrees through the intervals between the clouds(making these intervals appear darker) until it is again volatilised onreaching a level of greater heat below the clouds. This same smoke isprobably the cause of the well-known fact that the solar limb isconsiderably fainter than the middle of the disc. This seems to arisefrom the greater absorption caused by the longer distance which a ray oflight from a point near the limb has to travel through this layer ofsmoke before reaching the earth. It is shown that this absorption cannotbe attributed to a gaseous atmosphere, since this would have the effectof producing more dark absorption lines in the spectrum. There wouldthus be a marked difference between the solar spectrum from a part nearthe middle of the disc and the spectrum from a part near the limb. This, however, we do not find to be the case. 

With regard to the nature of sun-spots, the idea first suggested bySecchi and Lockyer, that they represent down rushes of cooler vapoursinto the photosphere (or to its surface), seems on the whole to accordbest with the observed phenomena. We have already mentioned that thespots are generally accompanied by faculæ and eruptive prominences intheir immediate neighbourhood, but whether these eruptions are caused bythe downfall of the vapour which makes the photospheric matter "splashup" in the vicinity, or whether the eruptions come first, and bydiminishing the upward pressure from below form a "sink, " into whichoverlying cooler vapour descends, are problems as to which opinions arestill much divided. 

A remarkable appendage to the sun, which extends to a distance very muchgreater than that of the corona, produces the phenomenon of the zodiacallight. A pearly glow is sometimes seen in the spring to spread over apart of the sky in the vicinity of the point where the sun hasdisappeared after sunset. The same spectacle may also be witnessedbefore sunrise in the autumn, and it would seem as if the materialproducing the zodiacal light, whatever it may be, had a lens-shapedform with the sun in the centre. The nature of this object is still amatter of uncertainty, but it is probably composed of a kind of dust, asthe faint spectrum it affords is of a continuous type. A view of thezodiacal light is shown in Fig. 22. 

In all directions the sun pours forth, with the most prodigalliberality, its torrents of light and of heat. The earth can only graspthe merest fraction, less than the 2, 000, 000, 000th part of the whole. Our fellow planets and the moon also intercept a trifle; but how smallis the portion of the mighty flood which they can utilise! The sip thata flying swallow takes from a river is as far from exhausting the waterin the river as are the planets from using all the heat which streamsfrom the sun. 

The sun's gracious beams supply the magic power that enables the corn togrow and ripen. It is the heat of the sun which raises water from theocean in the form of vapour, and then sends down that vapour as rain torefresh the earth and to fill the rivers which bear our ships down tothe ocean. It is the heat of the sun beating on the large continentswhich gives rise to the breezes and winds that waft our vessels acrossthe deep; and when on a winter's evening we draw around the fire andfeel its invigorating rays, we are only enjoying sunbeams which shone onthe earth countless ages ago. The heat in those ancient sunbeamsdeveloped the mighty vegetation of the coal period, and in the form ofcoal that heat has slumbered for millions of years, till we now call itagain into activity. It is the power of the sun stored up in coal thaturges on our steam-engines. It is the light of the sun stored up in coalthat beams from every gaslight in our cities. 

For the power to live and move, for the plenty with which we aresurrounded, for the beauty with which nature is adorned, we areimmediately indebted to one body in the countless hosts of space, andthat body is the sun. 

[Illustration: Fig. 22. --The Zodiacal Light in 1874. ]

CHAPTER III. 

THE MOON. 

 The Moon and the Tides--The Use of the Moon in Navigation--The Changes of the Moon--The Moon and the Poets--Whence the Light of the Moon?--Sizes of the Earth and the Moon--Weight of the Moon--Changes in Apparent Size--Variations in its Distance--Influence of the Earth on the Moon--The Path of the Moon--Explanation of the Moon's Phases--Lunar Eclipses--Eclipses of the Sun, how produced--Visibility of the Moon in a Total Eclipse--How Eclipses are Predicted--Uses of the Moon in finding Longitude--The Moon not connected with the Weather--Topography of the Moon--Nasmyth's Drawing of Triesnecker--Volcanoes on the Moon--Normal Lunar Crater--Plato--The Shadows of Lunar Mountains--The Micrometer--Lunar Heights--Former Activity on the Moon--Nasmyth's View of the Formation of Craters--Gravitation on the Moon--Varied Sizes of the Lunar Craters--Other Features of the Moon--Is there Life on the Moon?--Absence of Water and of Air--Dr. Stoney's Theory--Explanation of the Rugged Character of Lunar Scenery--Possibility of Life on Distant Bodies in Space. 

If the moon were suddenly struck out of existence, we should beimmediately apprised of the fact by a wail from every seaport in thekingdom. From London and from Liverpool we should hear the samestory--the rise and fall of the tide had almost ceased. The ships indock could not get out; the ships outside could not get in; and themaritime commerce of the world would be thrown into dire confusion. 

The moon is the principal agent in causing the daily ebb and flow of thetide, and this is the most important work which our satellite has to do. The fleets of fishing boats around the coasts time their daily movementsby the tide, and are largely indebted to the moon for bringing them inand out of harbour. Experienced sailors assure us that the tides are ofthe utmost service to navigation. The question as to how the moon causesthe tides is postponed to a future chapter, in which we shall alsosketch the marvellous part which the tides seem to have played in theearly history of our earth. 

Who is there that has not watched, with admiration, the beautiful seriesof changes through which the moon passes every month? We first see heras an exquisite crescent of pale light in the western sky after sunset. If the night is fine, the rest of the moon is visible inside thecrescent, being faintly illumined by light reflected from our own earth. Night after night she moves further and further to the east, until shebecomes full, and rises about the same time that the sun sets. From thetime of the full the disc of light begins to diminish until the lastquarter is reached. Then it is that the moon is seen high in the heavensin the morning. As the days pass by, the crescent shape is againassumed. The crescent wanes thinner and thinner as the satellite drawscloser to the sun. Finally she becomes lost in the overpowering light ofthe sun, again to emerge as the new moon, and again to go through thesame cycle of changes. 

The brilliance of the moon arises solely from the light of the sun, which falls on the not self-luminous substance of the moon. Out of thevast flood of light which the sun pours forth with such prodigality intospace the dark body of the moon intercepts a little, and of that littleit reflects a small fraction to illuminate the earth. The moon sheds somuch light, and seems so bright, that it is often difficult at night toremember that the moon has no light except what falls on it from thesun. Nevertheless, the actual surface of the brightest full moon isperhaps not much brighter than the streets of London on a clear sunshinyday. A very simple observation will suffice to show that the moon'slight is only sunlight. Look some morning at the moon in daylight, andcompare the moon with the clouds. The brightness of the moon and of theclouds are directly comparable, and then it can be readily comprehendedhow the sun which illuminates the clouds has also illumined the moon. Anattempt has been made to form a comparative estimate of the brightnessof the sun and the full moon. If 600, 000 full moons were shining atonce, their collective brilliancy would equal that of the sun. 

The beautiful crescent moon has furnished a theme for many a poet. Indeed, if we may venture to say so, it would seem that some poets haveforgotten that the moon is not to be seen every night. A poeticaldescription of evening is almost certain to be associated with theappearance of the moon in some phase or other. We may cite one notableinstance in which a poet, describing an historical event, has enshrinedin exquisite verse a statement which cannot be correct. Every child whospeaks our language has been taught that the burial of Sir John Mooretook place

 "By the struggling moonbeams' misty light. "

There is an appearance of detail in this statement which wears the garbof truth. We are not inclined to doubt that the night was misty, nor asto whether the moonbeams had to struggle into visibility; the questionat issue is a much more fundamental one. We do not know who was thefirst to raise the point as to whether any moon shone on that memorableevent at all or not; but the question having been raised, the NauticalAlmanac immediately supplies an answer. From it we learn in language, whose truthfulness constitutes its only claim to be poetry, that themoon was new at one o'clock in the morning of the day of the battle ofCorunna (16th January, 1809). The ballad evidently implies that thefuneral took place on the night following the battle. We are thereforeassured that the moon can hardly have been a day old when the hero wasconsigned to his grave. But the moon in such a case is practicallyinvisible, and yields no appreciable moonbeams at all, misty orotherwise. Indeed, if the funeral took place at the "dead of night, " asthe poet asserts, then the moon must have been far below the horizon atthe time. [6]

In alluding to this and similar instances, Mr. Nasmyth gives a word ofadvice to authors or to artists who desire to bring the moon on a scenewithout knowing as a matter of fact that our satellite was actuallypresent. He recommends them to follow the example of Bottom in _AMidsummer's Night's Dream_, and consult "a calendar, a calendar! Look inthe almanac; find out moonshine, find out moonshine!"

[Illustration: Fig. 23. --Comparative Sizes of the Earth and the Moon. ]

Among the countless host of celestial bodies--the sun, the moon, theplanets, and the stars--our satellite enjoys one special claim on ourattention. The moon is our nearest permanent neighbour. It is justpossible that a comet may occasionally approach the earth more closelythan the moon but with this exception the other celestial bodies are allmany hundreds or thousands, or even many millions, of times further fromus than the moon. 

It is also to be observed that the moon is one of the smallest visibleobjects which the heavens contain. Every one of the thousands of starsthat can be seen with the unaided eye is enormously larger than oursatellite. The brilliance and apparent vast proportions of the moonarise from the fact that it is only 240, 000 miles away, which is adistance almost immeasurably small when compared with the distancesbetween the earth and the stars. 

Fig. 23 exhibits the relative sizes of the earth and its attendant. Thesmall globe shows the moon, while the larger globe represents the earth. When we measure the actual diameters of the two globes, we find that ofthe earth to be 7, 918 miles and of the moon 2, 160 miles, so that thediameter of the earth is nearly four times greater than the diameter ofthe moon. If the earth were cut into fifty pieces, all equally large, then one of these pieces rolled into a globe would equal the size of themoon. The superficial extent of the moon is equal to about onethirteenth part of the surface of the earth. The hemisphere ourneighbour turns towards us exhibits an area equal to about onetwenty-seventh part of the area of the earth. This, to speakapproximately, is about double the actual extent of the continent ofEurope. The average materials of the earth are, however, much heavierthan those contained in the moon. It would take more than eighty globes, each as ponderous as the moon, to weigh down the earth. 

Amid the changes which the moon presents to us, one obvious fact standsprominently forth. Whether our satellite be new or full, at firstquarter or at last, whether it be high in the heavens or low near thehorizon, whether it be in process of eclipse by the sun, or whether thesun himself is being eclipsed by the moon, the apparent size of thelatter is nearly constant. We can express the matter numerically. Aglobe one foot in diameter, at a distance of 111 feet from the observer, would under ordinary circumstances be just sufficient to hide the discof the moon; occasionally, however, the globe would have to be broughtin to a distance of only 103 feet, or occasionally it might have to bemoved out to so much as 118 feet, if the moon is to be exactly hidden. It is unusual for the moon to approach either of its extreme limits ofposition, so that the distance from the eye at which the globe must besituated so as to exactly cover the moon is usually more than 105 feet, and less than 117 feet. These fluctuations in the apparent size of oursatellite are contained within such narrow limits that in the firstglance at the subject they may be overlooked. It will be easily seenthat the apparent size of the moon must be connected with its realdistance from the earth. Suppose, for the sake of illustration, that themoon were to recede into space, its size would seem to dwindle, and longere it had reached the distance of even the very nearest of the othercelestial bodies it would have shrunk into insignificance. On the otherhand, if the moon were to come nearer to the earth, its apparent sizewould gradually increase until, when close to our globe, it would seemlike a mighty continent stretching over the sky. We find that theapparent size of the moon is nearly constant, and hence we infer thatthe average distance of the same body is also nearly constant. Theaverage value of that distance is 239, 000 miles. In rare circumstancesit may approach to a distance but little more than 221, 000 miles, orrecede to a distance hardly less than 253, 000 miles, but the ordinaryfluctuations do not exceed more than about 13, 000 miles on either sideof its mean value. 

From the moon's incessant changes we perceive that she is in constantmotion, and we now further see that whatever these movements may be, theearth and the moon must at present remain at _nearly_ the same distanceapart. If we further add that the path pursued by the moon around theheavens lies nearly in a plane, then we are forced to the conclusionthat our satellite must be revolving in a nearly circular path aroundthe earth at the centre. It can, indeed, be shown that the constantdistance of the two bodies involves as a necessary condition therevolution of the moon around the earth. The attraction between the moonand the earth tends to bring the two bodies together. The only way bywhich such a catastrophe can be permanently avoided is by making thesatellite move as we actually find it to do. The attraction between theearth and the moon still exists, but its effect is not then shown inbringing the moon in towards the earth. The attraction has now to exertits whole power in restraining the moon in its circular path; were theattraction to cease, the moon would start off in a straight line, andrecede never to return. 

[Illustration: Fig. 24. --The Moon's Path around the Sun. ]

The fact of the moon's revolution around the earth is easilydemonstrated by observations of the stars. The rising and setting of oursatellite is, of course, due to the rotation of the earth, and thisapparent diurnal movement the moon possesses in common with the sun andwith the stars. It will, however, be noticed that the moon iscontinually changing its place among the stars. Even in the course of asingle night the displacement will be conspicuous to a careful observerwithout the aid of a telescope. The moon completes each revolutionaround the earth in a period of 27·3 days. 

[Illustration: Fig. 25. --The Phases of the Moon. ]

In Fig. 24 we have a view of the relative positions of the earth, thesun, and the moon, but it is to be observed that, for the convenience ofillustration, we have been obliged to represent the orbit of the moon ona much larger scale than it ought to be in comparison with the distanceof the sun. That half of the moon which is turned towards the sun isbrilliantly illuminated, and, according as we see more or less of thatbrilliant half, we say that the moon is more or less full, the several"phases" being visible in the succession shown by the numbers in Fig. 25. A beginner sometimes finds considerable difficulty in understandinghow the light on the full moon at night can have been derived from thesun. "Is not, " he will say, "the earth in the way? and must it notintercept the sunlight from every object on the other side of the earthto the sun?" A study of Fig. 24 will explain the difficulty. The planein which the moon revolves does not coincide with the plane in which theearth revolves around the sun. The line in which the plane of theearth's motion is intersected by that of the moon divides the moon'spath into two semicircles. We must imagine the moon's path to be tilteda little, so that the upper semicircle is somewhat above the plane ofthe paper, and the other semicircle below. It thus follows that when themoon is in the position marked full, under the circumstances shown inthe figure, the moon will be just above the line joining the earth andthe sun; the sunlight will thus pass over the earth to the moon, and themoon will be illuminated. At new moon, the moon will be under the linejoining the earth and the sun. 

As the relative positions of the earth and the sun are changing, ithappens twice in each revolution that the sun comes into the position ofthe line of intersection of the two planes. If this occurs at the timeof full moon, the earth lies directly between the moon and the sun; themoon is thus plunged into the shadow of the earth, the light from thesun is intercepted, and we say that the moon is eclipsed. The moonsometimes only partially enters the earth's shadow, in which case theeclipse is a partial one. When, on the other hand, the sun is situatedon the line of intersection at the time of new moon, the moon liesdirectly between the earth and the sun, and the dark body of the moonwill then cut off the sunlight from the earth, producing a solareclipse. Usually only a part of the sun is thus obscured, forming thewell-known partial eclipse; if, however, the moon pass centrally overthe sun, then we must have one or other of two very remarkable kinds ofeclipse. Sometimes the moon entirely blots out the sun, and thus isproduced the sublime spectacle of a total eclipse, which tells us somuch as to the nature of the sun, and to which we have already referredin the last chapter. Even when the moon is placed centrally over thesun, a thin rim of sunlight is occasionally seen round the margin of themoon. We then have what is known as an annular eclipse. 

It is remarkable that the moon is sometimes able to hide the suncompletely, while on other occasions it fails to do so. It happens thatthe average apparent size of the moon is nearly equal to the averageapparent size of the sun, but, owing to the fluctuations in theirdistances, the actual apparent sizes of both bodies undergo certainchanges. On certain occasions the apparent size of the moon is greaterthan that of the sun. In this case a central passage produces a totaleclipse; but it may also happen that the apparent size of the sunexceeds that of the moon, in which case a central passage can onlyproduce an annular eclipse. 

[Illustration: Fig. 26. --Form of the Earth's Shadow, showing thePenumbra, or partially shaded region. Within the Penumbra, the Moon isvisible; in the Shadow it is nearly invisible. ]

There are hardly any more interesting celestial phenomena than thedifferent descriptions of eclipses. The almanac will always give timelynotice of the occurrence, and the more striking features can be observedwithout a telescope. In an eclipse of the moon (Fig. 26) it isinteresting to note the moment when the black shadow is first detected, to watch its gradual encroachment over the bright surface of the moon, to follow it, in case the eclipse is total, until there is only a thincrescent of moonlight left, and to watch the final extinction of thatcrescent when the whole moon is plunged into the shadow. But now aspectacle of great interest and beauty is often manifested; for thoughthe moon is so hidden behind the earth that not a single direct ray ofthe sunlight could reach its surface, yet we often find that the moonremains visible, and, indeed, actually glows with a copper-coloured huebright enough to permit several of the markings on the surface to bediscerned. 

This illumination of the moon even in the depth of a total eclipse isdue to the sunbeams which have just grazed the edge of the earth. Indoing so they have become bent by the refraction of the atmosphere, andhave thus been turned inwards into the shadow. Such beams have passedthrough a prodigious thickness of the earth's atmosphere, and in thislong journey through hundreds of miles of air they have become tingedwith a ruddy or copper-like hue. Nor is this property of our atmospherean unfamiliar one. The sun both at sunrise and at sunset glows with alight which is much more ruddy than the beams it dispenses at noonday. But at sunset or at sunrise the rays which reach our eyes have much moreof our atmosphere to penetrate than they have at noon, and accordinglythe atmosphere imparts to them that ruddy colour so characteristic andoften so lovely. If the spectrum of the sun when close to the horizon isexamined it is seen to be filled with numerous dark lines and bandssituated chiefly towards the blue and violet end. These are caused bythe increased absorption which the light suffers in the atmosphere, andgive rise to the preponderating red light on the sun under suchconditions. In the case of the eclipsed moon, the sunbeams have to takean atmospheric journey more than double as long as that at sunrise orsunset, and hence the ruddy glow of the eclipsed moon may be accountedfor. 

The almanacs give the full particulars of each eclipse that happens inthe corresponding year. These predictions are reliable, becauseastronomers have been carefully observing the moon for ages, and havelearned from these observations not only how the moon moves at present, but also how it will move for ages to come. The actual calculations areso complicated that we cannot here discuss them. There is, however, oneleading principle about eclipses which is so simple that we must referto it. The eclipses occurring this year have no very obvious relationto the eclipses that occurred last year, or to those that will occurnext year. Yet, when we take a more extended view of the sequence ofthese phenomena, a very definite principle becomes manifest. If weobserve all the eclipses in a period of eighteen years, or nineteenyears, then we can predict, with at least an approximation to the truth, all the future eclipses for many years. It is only necessary torecollect that in 6, 585-1/3 days after one eclipse a nearly similareclipse follows. For instance, a beautiful eclipse of the moon occurredon the 5th of December, 1881. If we count back 6, 585 days from thatdate, or, that is, eighteen years and eleven days, we come to November24th, 1863, and a similar eclipse of the moon took place then. Again, there were four eclipses in the year 1881. If we add 6, 585-1/3 days tothe date of each eclipse, it will give the dates of all the foureclipses in the year 1899. It was this rule which enabled the ancientastronomers to predict the recurrence of eclipses, at a time when themotions of the moon were not understood nearly so well as they now are. 

During a long voyage, and perhaps in critical circumstances, the moonwill often render invaluable information to the sailor. To navigate aship, suppose from Liverpool to China, the captain must frequentlydetermine the precise position which his ship then occupies. If he couldnot do this, he would never find his way across the trackless ocean. Observations of the sun give him his latitude and tell him his localtime, but the captain further requires to know the Greenwich time beforehe can place his finger at a point of the chart and say, "My ship ishere. " To ascertain the Greenwich time the ship carries a chronometerwhich has been carefully rated before starting, and, as a precaution, two or three chronometers are usually provided to guard against the riskof error. An unknown error of a minute in the chronometer might perhapslead the vessel fifteen miles from its proper course. 

[Illustration: PLATE VI. 

CHART OF THE MOON'S SURFACE. ]

[Illustration: Fig. 27. --Key to Chart of the Moon (Plate VI. ). ]

It is important to have the means of testing the chronometers during theprogress of the voyage; and it would be a great convenience if everycaptain, when he wished, could actually consult some infallible standardof Greenwich time. We want, in fact, a Greenwich clock which may bevisible over the whole globe. There is such a clock; and, like any otherclock, it has a face on which certain marks are made, and a hand whichtravels round that face. The great clock at Westminster shrinks intoinsignificance when compared with the mighty clock which the captainuses for setting his chronometer. The face of this stupendous dial isthe face of the heavens. The numbers engraved on the face of a clock arereplaced by the twinkling stars; while the hand which moves over thedial is the beautiful moon herself. When the captain desires to testhis chronometer, he measures the distance of the moon from aneighbouring star. For example, he may see that the moon is threedegrees from the star Regulus. In the Nautical Almanac he finds theGreenwich time at which the moon was three degrees from Regulus. Comparing this with the indications of the chronometer, he finds therequired correction. 

There is one widely-credited myth about the moon which must be regardedas devoid of foundation. The idea that our satellite and the weatherbear some relation has no doubt been entertained by high authority, andappears to be an article in the belief of many an excellent mariner. Careful comparison between the state of the weather and the phases ofthe moon has, however, quite discredited the notion that any connectionof the kind does really exist. 

We often notice large blank spaces on maps of Africa and of Australiawhich indicate our ignorance of parts of the interior of those greatcontinents. We can find no such blank spaces in the map of the moon. Astronomers know the surface of the moon better than geographers knowthe interior of Africa. Every spot on the face of the moon which is aslarge as an English parish has been mapped, and all the more importantobjects have been named. 

A general map of the moon is shown in Plate VI. It has been based upondrawings made with small telescopes, and it gives an entire view of thatside of our satellite which is presented towards us. The moon is shownas it appears in an astronomical telescope, which inverts everything, sothat the south is at the top and the north at the bottom (to showobjects upright a telescope requires an additional pair of lenses in theeye-piece, and as this diminishes the amount of light reaching the eyethey are dispensed with in astronomical telescopes). We can see on themap some of the characteristic features of lunar scenery. Those darkregions so conspicuous in the ordinary full moon are easily recognisedon the map. They were thought to be seas by astronomers before the daysof telescopes, and indeed the name "Mare" is still retained, though itis obvious that they contain no water at present. The map also showscertain ridges or elevated portions, and when we apply measurement tothese objects we learn that they must be mighty mountain ranges. But themost striking features on the moon are those ring-like objects which arescattered over the surface in profusion. These are known as the lunarcraters. 

To facilitate reference to the chief points of interest we have arrangedan index map (Fig. 27) which will give a clue to the names of theseveral objects depicted upon the plate. The so-called seas arerepresented by capital letters; so that A is the Mare Crisium, and H theOceanus Procellarum. The ranges of mountains are indicated by smallletters; thus a on the index is the site of the so-called Caucasusmountains, and similarly the Apennines are denoted by _c_. The numerouscraters are distinguished by numbers; for example, the feature on themap corresponding to 20 on the index is the crater designated Ptolemy. 

 A. Mare Crisium. B. Mare Foecunditatis. C. Mare Tranquillitatis. D. Mare Serenitatis. E. Mare Imbrium. F. Sinus Iridum. G. Mare Vaporum. H. Oceanus Procellarum. I. Mare Humorum. J. Mare Nubium. K. Mare Nectaris. 

 _a. _ Caucasus. _b. _ Alps. _c. _ Apennines. _d. _ Carpathians. _f. _ Cordilleras & D'Alembert mountains. _g. _ Rook mountains. _h. _ Doerfel mountains. _i. _ Leibnitz mountains. 

 1. Posidonius. 2. Linné. 3. Aristotle. 4. Great Valley of the Alps. 5. Aristillus. 6. Autolycus. 7. Archimedes. 8. Plato. 9. Eratosthenes. 10. Copernicus. 11. Kepler. 12. Aristarchus. 13. Grimaldi. 14. Gassendi. 15. Schickard. 16. Wargentin. 17. Clavius. 18. Tycho. 19. Alphonsus. 20. Ptolemy. 21. Catharina. 22. Cyrillus. 23. Theophilus. 24. Petavius. 25. Hyginus. 26. Triesnecker. 

In every geographical atlas there is a map showing the two hemispheresof the earth, the eastern and the western. In the case of the moon wecan only give a map of one hemisphere, for the simple reason that themoon always turns the same side towards us, and accordingly we never geta view of the other side. This is caused by the interesting circumstancethat the moon takes exactly the same time to turn once round its ownaxis as it takes to go once round the earth. The rotation is, however, performed with uniform speed, while the moon does not move in its orbitwith a perfectly uniform velocity (_see_ Chapter IV. ). The consequenceis that we now get a slight glimpse round the east limb, and now asimilar glimpse round the west limb, as if the moon were shaking itshead very gently at us. But it is only an insignificant margin of thefar side of the moon which this _libration_ permits us to examine. 

Lunar objects are well suited for observation when the sunlight fallsupon them in such a manner as to exhibit strongly contrasted lights andshadows. It is impossible to observe the moon satisfactorily when it isfull, for then no conspicuous shadows are cast. The most opportunemoment for seeing any particular lunar object is when it lies just atthe illuminated side of the boundary between light and shade, for thenthe features are brought out with exquisite distinctness. 

Plate VII. [7] gives an illustration of lunar scenery, the objectrepresented being known to astronomers by the name of Triesnecker. Thedistrict included is only a very small fraction of the entire surface ofthe moon, yet the actual area is very considerable, embracing as it doesmany hundreds of square miles. We see in it various ranges of lunarmountains, while the central object in the picture is one of thoseremarkable lunar craters which we meet with so frequently in every lunarlandscape. This crater is about twenty miles in diameter, and it has alofty mountain in the centre, the peak of which is just illuminated bythe rising sun in that phase of our satellite which is represented inthe picture. 

A typical view of a lunar crater is shown in Plate VIII. This is, nodoubt, a somewhat imaginary sketch. The point of view from which theartist is supposed to have taken the picture is one quite unattainableby terrestrial astronomers, yet there can be little doubt that it is afair representation of objects on the moon. We should, however, recollect the scale on which it is drawn. The vast crater must be manymiles across, and the mountain at its centre must be thousands of feethigh. The telescope will, even at its best, only show the moon as wellas we could see it with the unaided eye if it were 250 miles awayinstead of being 240, 000. We must not, therefore, expect to see anydetails on the moon even with the finest telescopes, unless they werecoarse enough to be visible at a distance of 250 miles. England fromsuch a point of view would only show London as a coloured spot, incontrast with the general surface of the country. 

We return, however, from a somewhat fancy sketch to a more prosaicexamination of what the telescope does actually reveal. Plate IX. Represents the large crater Plato, so well known to everyone who uses atelescope. The floor of this remarkable object is nearly flat, and thecentral mountain, so often seen in other craters, is entirely wanting. We describe it more fully in the general list of lunar objects. 

The mountain peaks on the moon throw long, well-defined shadows, characterised by a sharpness which we do not find in the shadows ofterrestrial objects. The difference between the two cases arises fromthe absence of air from the moon. Our atmosphere diffuses a certainamount of light, which mitigates the blackness of terrestrial shadowsand tends to soften their outline. No such influences are at work on themoon, and the sharpness of the shadows is taken advantage of in ourattempts to measure the heights of the lunar mountains. 

It is often easy to compute the altitude of a church steeple, a loftychimney, or any similar object, from the length of its shadow. Thesimplest and the most accurate process is to measure at noon the numberof feet from the base of the object to the end of the shadow. Theelevation of the sun at noon on the day in question can be obtained fromthe almanac, and then the height of the object follows by a simplecalculation. Indeed, if the observations can be made either on the 6thof April or the 6th of September, at or near the latitude of London, then calculations would be unnecessary. The noonday length of the shadowon either of the dates named is equal to the altitude of the object. Insummer the length of the noontide shadow is less than the altitude; inwinter the length of the shadow exceeds the altitude. At sunrise orsunset the shadows are, of course, much longer than at noon, and it isshadows of this kind that we observe on the moon. The necessarymeasurements are made by that indispensable adjunct to the equatorialtelescope known as the _micrometer_. 

This word denotes an instrument for measuring _small_ distances. In onesense the term is not a happy one. The objects to which the astronomerapplies the micrometer are usually anything but small. They aregenerally of the most transcendent dimensions, far exceeding the moon orthe sun, or even our whole system. Still, the name is not altogetherinappropriate, for, vast though the objects may be, they generally seemminute, even in the telescope, on account of their great distance. 

We require for such measurements an instrument capable of the greatestnicety. Here, again, we invoke the aid of the spider, to whoseassistance in another department we have already referred. In the filarmicrometer two spider lines are parallel, and one intersects them atright angles. One or both of the parallel lines can be moved by means ofscrews, the threads of which have been shaped by consummate workmanship. The distance through which the line has been moved is accuratelyindicated by noting the number of revolutions and parts of a revolutionof the screw. Suppose the two lines be first brought into coincidence, and then separated until the apparent length of the shadow of themountain on the moon is equal to the distance between the lines: we thenknow the number of revolutions of the micrometer screw which isequivalent to the length of the shadow. The number of miles on the moonwhich correspond to one revolution of the screw has been previouslyascertained by other observations, and hence the length of the shadowcan be determined. The elevation of the sun, as it would have appearedto an observer at this point of the moon, at the moment when themeasures were being made, is also obtainable, and hence the actualelevation of the mountain can be calculated. By measurements of thiskind the altitudes of other lunar objects, such, for example, as theheight of the rampart surrounding a circular-walled plane, can bedetermined. 

The beauty and interest of the moon as a telescopic object induces us togive to the student a somewhat detailed account of the more remarkablefeatures which it presents. Most of the objects we are to describe canbe effectively exhibited with very moderate telescopic power. It is, however, to be remembered that all of them cannot be well seen at onetime. The region most distinctly shown is the boundary between light anddarkness. The student will, therefore, select for observation suchobjects as may happen to lie near that boundary at the time when he isobserving. 

1. _Posidonius. _--The diameter of this large crater is nearly 60 miles. Although its surrounding wall is comparatively slender, it is sodistinctly marked as to make the object very conspicuous. As sofrequently happens in lunar volcanoes, the bottom of the crater is belowthe level of the surrounding plain, in the present instance to theextent of nearly 2, 500 feet. 

2. _Linné. _--This small crater lies in the Mare Serenitatis. About sixtyyears ago it was described as being about 6-1/2 miles in diameter, andseems to have been sufficiently conspicuous. In 1866 Schmidt, of Athens, announced that the crater had disappeared. Since then an exceedinglysmall shallow depression has been visible, but the whole object is nowvery inconsiderable. This seems to be the most clearly attested case ofchange in a lunar object. Apparently the walls of the crater havetumbled into the interior and partly filled it up, but many astronomersdoubt that a change has really taken place, as Schröter, a Hanoverianobserver at the end of the eighteenth century, appears not to have seenany conspicuous crater in the place, though it must be admitted thathis observations are rather incomplete. To give some idea of Schmidt'samazing industry in lunar researches, it may be mentioned that in sixyears he made nearly 57, 000 individual settings of his micrometer in themeasurement of lunar altitudes. His great chart of the mountains in themoon is based on no less than 2, 731 drawings and sketches, if those arecounted twice that may have been used for two divisions of the map. 

3. _Aristotle. _--This great philosopher's name has been attached to agrand crater 50 miles in diameter, the interior of which, although veryhilly, shows no decidedly marked central cone. But the lofty wall of thecrater, exceeding 10, 500 feet in height, overshadows the floor socontinuously that its features are never seen to advantage. 

4. _The Great Valley of the Alps. _--A wonderfully straight valley, witha width ranging from 3-1/2 to 6 miles, runs right through the lunarAlps. It is, according to Mädler, at least 11, 500 feet deep, and over 80miles in length. A few low ridges which are parallel to the sides of thevalley may possibly be the result of landslips. 

5. _Aristillus. _--Under favourable conditions Lord Rosse's greattelescope has shown the exterior of this magnificent crater to be scoredwith deep gullies radiating from its centre. Aristillus is about 34miles wide and 10, 000 feet in depth. 

6. _Autolycus_ is somewhat smaller than the foregoing, to which it formsa companion in accordance with what Mädler thought a well-definedrelation amongst lunar craters, by which they frequently occurred inpairs, with the smaller one more usually to the south. Towards the edgethis arrangement is generally rather apparent than real, and is merely aresult of foreshortening. 

7. _Archimedes. _--This large plain, about 50 miles in diameter, has itsvast smooth interior divided by unequally bright streaks into sevendistinct zones, running east and west. There is no central mountain orother obvious internal sign of former activity, but its irregular wallrises into abrupt towers, and is marked outside by decided terraces. 

[Illustration: PLATE B. 

PORTION OF THE MOON. 

(ALPS, ARCHIMEDES, APENNINES. )

_Messrs. Loewy & Puiseux_. ]

8. _Plato. _--We have already referred to this extensive circular plain, which is noticeable with the smallest telescope. The average height ofthe rampart is about 3, 800 feet on the eastern side; the western side issomewhat lower, but there is one peak rising to the height of nearly7, 300 feet. The plain girdled by this vast rampart is of ampleproportions. It is a somewhat irregular circle, about 60 miles indiameter, and containing an area of 2, 700 square miles. On its floor theshadows of the western wall are shown in Plate IX. , as are also three ofthe small craters, of which a large number have been detected bypersevering observers. The narrow sharp line leading from the crater tothe left is one of those remarkable "clefts" which traverse the moon inso many directions. Another may be seen further to the left. Above Platoare several detached mountains, the loftiest of which is Pico, about8, 000 feet in height. Its long and pointed shadow would at first sightlead one to suppose that it must be very steep; but Schmidt, whospecially studied the inclinations of the lunar slopes, is of opinionthat it cannot be nearly so steep as many of the Swiss mountains thatare frequently ascended. As many as thirty minute craters have beencarefully observed on the floor of Plato, and variations have beenthought by Mr. W. H. Pickering to be perceptible. 

9. _Eratosthenes. _--This profound crater, upwards of 37 miles indiameter, lies at the end of the gigantic range of the Apennines. Notimprobably, Eratosthenes once formed the volcanic vent for thestupendous forces that elevated the comparatively craterless peaks ofthese great mountains. 

10. _Copernicus. _--Of all the lunar craters this is one of the grandestand best known. The region to the west is dotted over with innumerableminute craterlets. It has a central many-peaked mountain about 2, 400feet in height. There is good reason to believe that the terracing shownin its interior is mainly due to the repeated alternate rise, partialcongelation, and subsequent retreat of a vast sea of lava. At full moonthe crater of Copernicus is seen to be surrounded by radiating streaks. 

11. _Kepler. _--Although the internal depth of this crater is scarcelyless than 10, 000 feet, it has but a very low surrounding wall, which isremarkable for being covered with the same glistening substance thatalso forms a system of bright rays not unlike those surrounding the lastobject. 

12. _Aristarchus_ is the most brilliant of the lunar craters, beingspecially vivid with a low power in a large telescope. So bright is it, indeed, that it has often been seen on the dark side just after newmoon, and has thus given rise to marvellous stories of active lunarvolcanoes. To the south-east lies another smaller crater, Herodotus, north of which is a narrow, deep valley, nowhere more than 2-1/2 milesbroad, which makes a remarkable zigzag. It is one of the largest of thelunar "clefts. "

13. _Grimaldi_ calls for notice as the darkest object of its size in themoon. Under very exceptional circumstances it has been seen with thenaked eye, and as its area has been estimated at nearly 14, 000 squaremiles, it gives an idea of how little unaided vision can discern in themoon; it must, however, be added that we always see Grimaldiconsiderably foreshortened. 

14. The great crater _Gassendi_ has been very frequently mapped onaccount of its elaborate system of "clefts. " At its northern end itcommunicates with a smaller but much deeper crater, that is often filledwith black shadow after the whole floor of Gassendi has beenilluminated. 

15. _Schickard_ is one of the largest walled plains on the moon, about134 miles in breadth. Within its vast expanse Mädler detected 23 minorcraters. With regard to this object Chacornac pointed out that, owing tothe curvature of the surface of the moon, a spectator at the centre ofthe floor "would think himself in a boundless desert, " because thesurrounding wall, although in one place nearly 10, 000 feet high, wouldlie entirely beneath his horizon. 

16. Close to the foregoing is _Wargentin_. There can be little doubtthat this is really a huge crater almost filled with congealed lava, asthere is scarcely any fall towards the interior. 

17. _Clavius. _--Near the 60th parallel of lunar south latitude liesthis enormous enclosure, the area of which is not less than 16, 500square miles. Both in its interior and on its walls are many peaks andsecondary craters. The telescopic view of a sunrise upon the surface ofClavius is truly said by Mädler to be indescribably magnificent. One ofthe peaks rises to a height of 24, 000 feet above the bottom of one ofthe included craters. Mädler even expressed the opinion that in thiswild neighbourhood there are craters so profound that no ray of sunlightever penetrated their lowest depths, while, as if in compensation, thereare peaks whose summits enjoy a mean day almost twice as long as theirnight. 

18. If the full moon be viewed through an opera-glass or any smallhand-telescope, one crater is immediately seen to be conspicuous beyondall others, by reason of the brilliant rays or streaks that radiate fromit. This is the majestic _Tycho_, 17, 000 feet in depth and 50 miles indiameter (Plate X. ). A peak 6, 000 feet in height rises in the centre ofits floor, while a series of terraces diversity its interior slopes; butit is the mysterious bright rays that chiefly surprise us. When the sunrises on Tycho, these streaks are utterly invisible; indeed, the wholeobject is then so obscure that it requires a practised eye to recogniseTycho amidst its mountainous surroundings. But as soon as the sun hasattained a height of about 30° above its horizon, the rays emerge fromtheir obscurity and gradually increase in brightness until the moonbecomes full, when they are the most conspicuous objects on her surface. They vary in length, from a few hundred miles to two or, in oneinstance, nearly three thousand miles. They extend indifferently acrossvast plains, into the deepest craters, or over the loftiest elevations. We know of nothing on our earth to which they can be compared. As theserays are only seen about the time of full moon, their visibilityobviously depends on the light falling more or less closely in the lineof sight, quite regardless of the inclination of the surfaces, mountainsor valleys, on which they appear. Each small portion of the surface ofthe streak must therefore be of a form which is symmetrical to thespectator from whatever point it is seen. The sphere alone appears tofulfil this condition, and Professor Copeland therefore suggests thatthe material constituting the surface of the streak must be made up of alarge number of more or less completely spherical globules. The streaksmust represent parts of the lunar surface either pitted with minutecavities of spherical figure, or strewn over with minute transparentspheres. [8]

Near the centre of the moon's disc is a fine range of ring plains fullyopen to our view under all illuminations. Of these, two may bementioned--_Alphonsus_ (19), the floor of which is strangelycharacterised by two bright and several dark markings which cannot beexplained by irregularities in the surface. --_Ptolemy_ (20). Besidesseveral small enclosed craters, its floor is crossed by numerous lowridges, visible when the sun is rising or setting. 

21, 22, 23. --When the moon is five or six days old this beautiful groupof three craters will be favourably placed for observation. They arenamed _Catharina_, _Cyrillus_, and _Theophilus_. Catharina, the mostsoutherly of the group, is more than 16, 000 feet deep, and connectedwith Cyrillus by a wide valley; but between Cyrillus and Theophilusthere is no such connection. Indeed, Cyrillus looks as if its hugesurrounding ramparts, as high as Mont Blanc, had been completelyfinished before the volcanic forces commenced the formation ofTheophilus, the rampart of which encroaches considerably on its olderneighbour. Theophilus stands as a well-defined circular crater about 64miles in diameter, with an internal depth of 14, 000 to 18, 000 feet, anda beautiful central group of mountains, one-third of that height, on itsfloor. Although Theophilus is the deepest crater we can see in the moon, it has suffered little or no deformation from secondary eruptions, whilethe floor and wall of Catharina show complete sequences of lessercraters of various sizes that have broken in upon and partly destroyedeach other. In the spring of the year, when the moon is somewhat beforethe first quarter, this instructive group of extinct volcanoes can beseen to great advantage at a convenient hour in the evening. 

[Illustration: PLATE VII. 

TRIESNECKER. 

(AFTER NASMYTH. )]

24. _Petavius_ is remarkable not only for its great size, but also forthe rare feature of having a double rampart. It is a beautiful objectsoon after new moon, or just after full moon, but disappears absolutelywhen the sun is more than 45° above its horizon. The crater floor isremarkably convex, culminating in a central group of hills intersectedby a deep cleft. 

25. _Hyginus_ is a small crater near the centre of the moon's disc. Oneof the largest of the lunar chasms passes right through it, making anabrupt turn as it does so. 

26. _Triesnecker. _--This fine crater has been already described, but isagain alluded to in order to draw attention to the elaborate system ofchasms so conspicuously shown in Plate VII. That these chasms aredepressions is abundantly evident by the shadows inside. Very oftentheir margins are appreciably raised. They seem to be fractures in themoon's surface. 

Of the various mountains that are occasionally seen as projections onthe actual edge of the moon, those called after Leibnitz (_i_) seem tobe the highest. Schmidt found the highest peak to be upwards of 41, 900feet above a neighbouring valley. In comparing these altitudes withthose of mountains on our earth, we must for the latter add the depth ofthe sea to the height of the land. Reckoned in this way, our highestmountains are still higher than any we know of in the moon. 

We must now discuss the important question as to the origin of theseremarkable features on the surface of the moon. We shall admit at theoutset that our evidence on this subject is only indirect. To establishby unimpeachable evidence the volcanic origin of the remarkable lunarcraters, it would seem almost necessary that volcanic outbursts shouldhave been witnessed on the moon, and that such outbursts should havebeen seen to result in the formation of the well-known ring, with orwithout the mountain rising from the centre. To say that nothing of thekind has ever been witnessed would be rather too emphatic a statement. On certain occasions careful observers have reported the occurrence ofminute local changes on the moon. As we have already remarked, a craternamed Linné, of dimensions respectable, no doubt, to a lunar inhabitant, but forming a very inconsiderable telescopic object, was thought to haveundergone some change. On another occasion a minute crater was thoughtto have arisen near the well-known object named Hyginus. The mereenumeration of such instances gives real emphasis to the statement thatthere is at the present time no appreciable source of disturbance of themoon's surface. Even were these trifling cases of suspected changereally established--and this is perhaps rather farther than manyastronomers would be willing to go--they are still insignificant whencompared with the mighty phenomena that gave rise to the host of greatcraters which cover so large a portion of the moon's surface. 

We are led inevitably to the conclusion that our satellite must haveonce possessed much greater activity than it now displays. We can alsogive a reasonable, or, at all events, a plausible, explanation of thecessation of that activity in recent times. Let us glance at two otherbodies of our system, the earth and the sun, and compare them with themoon. Of the three bodies, the sun is enormously the largest, while themoon is much less than the earth. We have also seen that though the sunmust have a very high temperature, there can be no doubt that it isgradually parting with its heat. The surface of the earth, formed as itis of solid rocks and clay, or covered in great part by the vast expanseof ocean, bears but few obvious traces of a high temperature. Nevertheless, it is highly probable from ordinary volcanic phenomenathat the interior of the earth still possesses a temperature ofincandescence. 

A large body when heated takes a longer time to cool than does a smallbody raised to the same temperature. A large iron casting will take daysto cool; a small casting will become cold in a few hours. Whatever mayhave been the original source of heat in our system--a question whichwe are not now discussing--it seems demonstrable that the differentbodies were all originally heated, and have now for ages been graduallycooling. The sun is so vast that he has not yet had time to cool; theearth, of intermediate bulk, has become cold on the outside, while stillretaining vast stores of internal heat; while the moon, the smallestbody of all, has lost its heat to such an extent that changes ofimportance on its surface can no longer be originated by internal fires. 

We are thus led to refer the origin of the lunar craters to some ancientepoch in the moon's history. We have no moans of knowing the remotenessof that epoch, but it is reasonable to surmise that the antiquity of thelunar volcanoes must be extremely great. At the time when the moon wassufficiently heated to originate those convulsions, of which the mightycraters are the survivals, the earth must also have been much hotterthan it is at present. When the moon possessed sufficient heat for itsvolcanoes to be active, the earth was probably so hot that life wasimpossible on its surface. This supposition would point to an antiquityfor the lunar craters far too great to be estimated by the centuries andthe thousands of years which are adequate for such periods as those withwhich the history of human events is concerned. It seems not unlikelythat millions of years may have elapsed since the mighty craters ofPlato or of Copernicus consolidated into their present form. 

We shall now attempt to account for the formation of the lunar craters. The most probable views on the subject seem to be those which have beenset forth by Mr. Nasmyth, though it must be admitted that his doctrinesare by no means free from difficulty. According to his theory we canexplain how the rampart around the lunar crater has been formed, and howthe great mountain arose which so often adorns the centre of the plain. The view in Fig. 28 contains an imaginary sketch of a volcanic vent onthe moon in the days when the craters were active. The eruption is hereshown in the fulness of its energy, when the internal forces are hurlingforth ashes or stones which fall at a considerable distance from thevent. The materials thus accumulated constitute the rampart surroundingthe crater. 

The second picture (Fig. 29) depicts the crater in a later stage of itshistory. The prodigious explosive power has now been exhausted, and hasperhaps been intermitted for some time. Again, the volcano bursts intoactivity, but this time with only a small part of its original energy. Acomparatively feeble eruption now issues from the same vent, depositsmaterials close around the orifice, and raises a mountain in the centre. Finally, when the activity has subsided, and the volcano is silent andstill, we find the evidence of the early energy testified to by therampart which surrounds the ancient crater, and by the mountain whichadorns the interior. The flat floor which is found in some of thecraters may not improbably have arisen from an outflow of lava which hasafterwards consolidated. Subsequent outbreaks have also occurred in manycases. 

One of the principal difficulties attending this method of accountingfor the structure of a crater arises from the great size which some ofthese objects attain. There are ancient volcanoes on the moon forty orfifty miles in diameter; indeed, there is one well-formed ring, with amountain rising in the centre, the diameter of which is no less thanseventy-eight miles (Petavius). It seems difficult to conceive how ablowing cone at the centre could convey the materials to such a distanceas the thirty-nine miles between the centre of Petavius and the rampart. The explanation is, however, facilitated when it is borne in mind thatthe force of gravitation is much less on the moon than on the earth. 

[Illustration: PLATE VIII. 

A NORMAL LUNAR CRATER. ]

[Illustration: Fig. 28. --Volcano in Activity. ]

[Illustration: Fig. 29. --Subsequent Feeble Activity. ]

Have we not already seen that our satellite is so much smaller than theearth that eighty moons rolled into one would not weigh as much as theearth? On the earth an ounce weighs an ounce and a pound weighs a pound;but a weight of six ounces here would only weigh one ounce on the moon, and a weight of six pounds here would only weigh one pound on the moon. A labourer who can carry one sack of corn on the earth could, with thesame exertion, carry six sacks of corn on the moon. A cricketer who canthrow a ball 100 yards on the earth could with precisely the sameexertion throw the same ball 600 yards on the moon. Hiawatha could shootten arrows into the air one after the other before the first reachedthe ground; on the moon he might have emptied his whole quiver. Thevolcano, which on the moon drove projectiles to the distance ofthirty-nine miles, need only possess the same explosive power as wouldhave been sufficient to drive the missiles six or seven miles on theearth. A modern cannon properly elevated would easily achieve this feat. 

[Illustration: Fig. 30. --Formation of the Level Floor by Lava. ]

It must also be borne in mind that there are innumerable craters on themoon of the same general type but of the most varied dimensions; from atiny telescopic object two or three miles in diameter, we can point outgradually ascending stages until we reach the mighty Petavius justconsidered. With regard to the smaller craters, there is obviouslylittle or no difficulty in attributing to them a volcanic origin, and asthe continuity from the smallest to the largest craters is unbroken, itseems quite reasonable to suppose that even the greatest has arisen inthe same way. 

It should, however, be remarked that some lunar features might beexplained by actions from without rather than from within. Mr. G. K. Gilbert has marshalled the evidence in support of the belief that lunarsculptures arise from the impact of bodies falling on the moon. The MareImbrium, according to this view, has been the seat of a collision towhich the surrounding lunar scenery is due. Mr. Gilbert explains thefurrows as hewn out by mighty projectiles moving with such velocities asmeteors possess. 

The lunar landscapes are excessively weird and rugged. They alwaysremind us of sterile deserts, and we cannot fail to notice the absenceof grassy plains or green forests such as we are familiar with on ourglobe. In some respects the moon is not very differently circumstancedfrom the earth. Like it, the moon has the pleasing alternations of dayand night, though the day in the moon is as long as twenty-nine of ourdays, and the night of the moon is as long as twenty-nine of our nights. We are warmed by the rays of the sun; so, too, is the moon; but, whatever may be the temperature during the long day on the moon, itseems certain that the cold of the lunar night would transcend thatknown in the bleakest regions of our earth. The amount of heat radiatedto us by the moon has been investigated by Lord Rosse, and more recentlyby Professor Langley. Though every point on the moon's surface isexposed to the sunlight for a fortnight without any interruption, theactual temperature to which the soil is raised cannot be a high one. Themoon does not, like the earth, possess a warm blanket, in the shape ofan atmosphere, which can keep in and accumulate the heat received. 

Even our largest telescopes can tell nothing directly as to whether lifecan exist on the moon. The mammoth trees of California might be growingon the lunar mountains, and elephants might be walking about on theplains, but our telescopes could not show them. The smallest object thatwe can see on the moon must be about as large as a good-sized cathedral, so that organised beings resembling in size any that we are familiarwith, if they existed, could not make themselves visible as telescopicobjects. 

We are therefore compelled to resort to indirect evidence as to whetherlife would be possible on the moon. We may say at once that astronomersbelieve that life, as we know it, could not exist. Among the necessaryconditions of life, water is one of the first. Take every form ofvegetable life, from the lichen which grows on the rock to the gianttree of the forest, and we find the substance of every plant containswater, and could not exist without it. Nor is water less necessary tothe existence of animal life. Deprived of this element, all organiclife, the life of man himself, would be inconceivable. 

Unless, therefore, water be present in the moon, we shall be bound toconclude that life, as we know it, is impossible. If anyone stationed onthe moon were to look at the earth through a telescope, would he be ableto see any water here? Most undoubtedly he would. He would see theclouds and he would notice their incessant changes, and the clouds alonewould be almost conclusive evidence of the existence of water. Anastronomer on the moon would also see our oceans as coloured surfaces, remarkably contrasted with the land, and he would perhaps frequently seean image of the sun, like a brilliant star, reflected from some smoothportion of the sea. In fact, considering that much more than half of ourglobe is covered with oceans, and that most of the remainder is liableto be obscured by clouds, the lunar astronomer in looking at our earthwould often see hardly anything but water in one form or other. Verylikely he would come to the conclusion that our globe was only fitted tobe a residence for amphibious animals. 

But when we look at the moon with our telescopes we see no directevidence of water. Close inspection shows that the so-called lunar seasare deserts, often marked with small craters and rocks. The telescopereveals no seas and no oceans, no lakes and no rivers. Nor is thegrandeur of the moon's scenery ever impaired by clouds over her surface. Whenever the moon is above our horizon, and terrestrial clouds are outof the way, we can see the features of our satellite's surface withdistinctness. There are no clouds in the moon; there are not even themists or the vapours which invariably arise wherever water is present, and therefore astronomers have been led to the conclusion that thesurface of the globe which attends the earth is a sterile and awaterless desert. 

Another essential element of organic life is also absent from the moon. Our globe is surrounded with a deep clothing of air resting on thesurface, and extending above our heads to the height of about 200 or 300miles. We need hardly say how necessary air is to life, and therefore weturn with interest to the question as to whether the moon can besurrounded with an atmosphere. Let us clearly understand the problem weare about to consider. Imagine that a traveller started from the earthon a journey to the moon; as he proceeded, the air would graduallybecome more and more rarefied, until at length, when he was a fewhundred miles above the earth's surface, he would have left the lastperceptible traces of the earth's envelope behind him. By the time hehad passed completely through the atmosphere he would have advanced onlya very small fraction of the whole journey of 240, 000 miles, and therewould still remain a vast void to be traversed before the moon would bereached. If the moon were enveloped in the same way as the earth, then, as the traveller approached the end of his journey, and came within afew hundred miles of the moon's surface, he would meet again with tracesof an atmosphere, which would gradually increase in density until hearrived at the moon's surface. The traveller would thus have passedthrough one stratum of air at the beginning of his journey, and throughanother at the end, while the main portion of the voyage would have beenthrough space more void than that to be found in the exhausted receiverof an air-pump. 

Such would be the case if the moon were coated with an atmosphere likethat surrounding our earth. But what are the facts? The traveller as hedrew near the moon would seek in vain for air to breathe at allresembling ours. It is possible that close to the surface there arefaint traces of some gaseous material surrounding the moon, but it canonly be equal to a very small fractional part of the ample clothingwhich the earth now enjoys. For all purposes of respiration, as weunderstand the term, we may say that there is no air on the moon, and aninhabitant of our earth transferred thereto would be as certainlysuffocated as he would be in the middle of space. 

It may, however, be asked how we learn this. Is not air transparent, andhow, therefore, could our telescopes be expected to show whether themoon really possessed such an envelope? It is by indirect, butthoroughly reliable, methods of observation that we learn the destitutecondition of our satellite. There are various arguments to be adduced;but the most conclusive is that obtained on the occurrence of what iscalled an "occultation. " It sometimes happens that the moon comesdirectly between the earth and a star, and the temporary extinction ofthe latter is an "occultation. " We can observe the moment when thephenomenon takes place, and the suddenness of the disappearance of thestar is generally remarked. If the moon were enveloped in a copiousatmosphere, the interposition of this gaseous mass by the movement ofthe moon would produce a gradual evanescence of the star wholly wantingthe abruptness which marks the obscuration. [9]

Let us consider how we can account for the absence of an atmosphere fromthe moon. What we call a gas has been found by modern research to be acollection of an immense number of molecules, each of which is inexceedingly rapid motion. This motion is only pursued for a shortdistance in one direction before a molecule comes into collision withsome other molecule, whereby the directions and velocities of theindividual molecules are continually changed. There is a certain averagespeed for each gas which is peculiar to the molecules of that gas at acertain temperature. When several gases are mixed, as oxygen andnitrogen are in our atmosphere, the molecules of each gas continue tomove with their own characteristic velocities. So far as we can estimatethe temperature at the boundary of the earth's atmosphere, we may assumethat the average of the velocities of the oxygen molecules there foundis about a quarter of a mile per second. The velocities for nitrogen aremuch the same, while the average speed of a molecule of hydrogen isabout one mile per second, being, in fact, by far the greatest molecularvelocity possessed by any gas. 

[Illustration: PLATE IX. 

PLATO. 

(AFTER NASMYTH. )]

A stone thrown into the air soon regains the earth. A rifle bulletfired vertically upwards will ascend higher and higher, until at lengthits motion ceases, it begins to return, and falls to the ground. Let usfor the moment suppose that we had a rifle of infinite strength andgunpowder of unlimited power. As we increase the charge we find that thebullet will ascend higher and higher, and each time it will take alonger period before it returns to the ground. The descent of the bulletis due to the attraction of the earth. Gravitation must necessarily acton the projectile throughout its career, and it gradually lessens thevelocity, overcomes the upward motion, and brings the bullet back. Itmust be remembered that the efficiency of the attraction decreases whenthe height is increased. Consequently when the body has a prodigiouslygreat initial velocity, in consequence of which it ascends to anenormous height, its return is retarded by a twofold cause. In the firstplace, the distance through which it has to be recalled is greatlyincreased, and in the second place the efficiency of gravitation ineffecting its recall has decreased. The greater the velocity, thefeebler must be the capacity of gravitation for bringing back the body. We can conceive the speed to be increased to that point at which thegravitation, constantly declining as the body ascends, is never quiteable to neutralise the velocity, and hence we have the remarkable caseof a body projected away never to return. 

It is possible to exhibit this reasoning in a numerical form, and toshow that a velocity of six or seven miles a second directed upwardswould suffice to convey a body entirely away from the gravitation of theearth. This speed is far beyond the utmost limits of our artillery. Itis, indeed, at least a dozen times as swift as a cannon shot; and evenif we could produce it, the resistance of the air would present aninsuperable difficulty. Such reflections, however, do not affect theconclusion that there is for each planet a certain specific velocityappropriate to that body, and depending solely upon its size and mass, with which we should have to discharge a projectile, in order to preventthe attraction of that body from pulling the projectile back again. 

It is a simple matter of calculation to determine this "criticalvelocity" for any celestial body. The greater the body the greater ingeneral must be the initial speed which will enable the projectile toforsake for ever the globe from which it has been discharged. As wehave already indicated, this speed is about seven miles per second onthe earth. It would be three on the planet Mercury, three and a half onMars, twenty-two on Saturn, and thirty-seven on Jupiter; while for amissile to depart from the sun without prospect of return, it must leavethe brilliant surface at a speed not less than 391 miles per second. 

Supposing that a quantity of free hydrogen was present in ouratmosphere, its molecules would move with an average velocity of aboutone mile per second. It would occasionally happen by a combination ofcircumstances that a molecule would attain a speed which exceeded sevenmiles a second. If this happened on the confines of the atmosphere whereit escaped collision with other molecules, the latter object would flyoff into space, and would not be recaptured by the earth. By incessantrepetitions of this process, in the course of countless ages, all themolecules of hydrogen gas would escape from the earth, and in thismanner we may explain the fact that there is no free hydrogen present inthe earth's atmosphere. [10]

The velocities which can be attained by the molecules of gases otherthan hydrogen are far too small to permit of their escape from theattraction of the earth. We therefore find oxygen, nitrogen, watervapour, and carbon dioxide remaining as permanent components of our air. On the other hand, the enormous mass of the sun makes the "criticalvelocity" at the surface of that body to be so great (391 miles persecond) that not even the molecules of hydrogen can possibly emulate it. Consequently, as we have seen, hydrogen is a most important component ofthe sun's atmospheric envelope. 

If we now apply this reasoning to the moon, the critical velocity isfound by calculation to be only a mile and a half per second. This seemsto be well within the maximum velocities attainable by the molecules ofoxygen, nitrogen, and other gases. It therefore follows that none ofthese gases could remain permanently to form an atmosphere at thesurface of so small a body as the moon. This seems to be the reason whythere are no present traces of any distinct gaseous surroundings to oursatellite. 

The absence of air and of water from the moon explains the sublimeruggedness of the lunar scenery. We know that on the earth the action ofwind and of rain, of frost and of snow, is constantly tending to weardown our mountains and reduce their asperities. No such agents are atwork on the moon. Volcanoes sculptured the surface into its presentcondition, and, though they have ceased to operate for ages, the tracesof their handiwork seem nearly as fresh to-day as they were when themighty fires were extinguished. 

"The cloud-capped towers, the gorgeous palaces, the solemn temples" havebut a brief career on earth. It is chiefly the incessant action of waterand of air that makes them vanish like the "baseless fabric of avision. " On the moon these causes of disintegration and of decay are allabsent, though perhaps the changes of temperature in the transition fromlunar day to lunar night would be attended with expansions andcontractions that might compensate in some slight degree for the absenceof more potent agents of dissolution. 

It seems probable that a building on the moon would remain for centuryafter century just as it was left by the builders. There need be noglass in the windows, for there is no wind and no rain to keep out. There need not be fireplaces in the rooms, for fuel cannot burn withoutair. Dwellers in a lunar city would find that no dust could rise, noodours be perceived, no sounds be heard. 

Man is a creature adapted for life under circumstances which are verynarrowly limited. A few degrees of temperature more or less, a slightvariation in the composition of air, the precise suitability of food, make all the difference between health and sickness, between life anddeath. Looking beyond the moon, into the length and breadth of theuniverse, we find countless celestial globes with every conceivablevariety of temperature and of constitution. Amid this vast number ofworlds with which space is tenanted, are there any inhabited by livingbeings? To this great question science can make no response: we cannottell. Yet it is impossible to resist a conjecture. We find our earthteeming with life in every part. We find life under the most variedconditions that can be conceived. It is met with under the burning heatof the tropics and in the everlasting frost at the poles. We find lifein caves where not a ray of light ever penetrates. Nor is it wanting inthe depths of the ocean, at the pressure of tons on the square inch. Whatever may be the external circumstances, Nature generally providessome form of life to which those circumstances are congenial. 

It is not at all probable that among the million spheres of the universethere is a single one exactly like our earth--like it in the possessionof air and of water, like it in size and in composition. It does notseem probable that a man could live for one hour on any body in theuniverse except the earth, or that an oak-tree could live in any othersphere for a single season. Men can dwell on the earth, and oak-treescan thrive therein, because the constitutions of the man and of the oakare specially adapted to the particular circumstances of the earth. 

Could we obtain a closer view of some of the celestial bodies, we shouldprobably find that they, too, teem with life, but with life speciallyadapted to the environment--life in forms strange and weird; life farstranger to us than Columbus found it to be in the New World when hefirst landed there. Life, it may be, stranger than ever Dante describedor Doré sketched. Intelligence may also have a home among those spheresno less than on the earth. There are globes greater and globesless--atmospheres greater and atmospheres less. The truest philosophy onthis subject is crystallised in the language of Tennyson:--

 "This truth within thy mind rehearse, That in a boundless universe Is boundless better, boundless worse. 

 "Think you this mould of hopes and fears Could find no statelier than his peers In yonder hundred million spheres?"

[Illustration: PLATE X. 

TYCHO AND ITS SURROUNDINGS. 

(AFTER NASMYTH. )]

CHAPTER IV. 

THE SOLAR SYSTEM. 

 Exceptional Importance of the Sun and Moon--The Course to be pursued--The Order of Distance--The Neighbouring Orbs--How are they to be discriminated?--The Planets Venus and Jupiter attract Notice by their Brilliancy--Sirius not a Neighbour--The Planets Saturn and Mercury--Telescopic Planets--The Criterion as to whether a Body is to be ranked as a Neighbour--Meaning of the word _Planet_--Uranus and Neptune--Comets--The Planets are illuminated by the Sun--The Stars are not--The Earth is really a Planet--The Four Inner Planets, Mercury, Venus, the Earth, and Mars--Velocity of the Earth--The Outer Planets, Jupiter, Saturn, Uranus, Neptune--Light and Heat received by the Planets from the Sun--Comparative Sizes of the Planets--The Minor Planets--The Planets all revolve in the same Direction--The Solar System--An Island Group in Space. 

In the two preceding chapters of this work we have endeavoured todescribe the heavenly bodies in the order of their relative importanceto mankind. Could we doubt for a moment as to which of the many orbs inthe universe should be the first to receive our attention? We do not nowallude to the intrinsic significance of the sun when compared with otherbodies or groups of bodies scattered through space. It may be thatnumerous globes rival the sun in real splendour, in bulk, and in mass. We shall, in fact, show later on in this volume that this is the case;and we shall then be in a position to indicate the true rank of the sunamid the countless hosts of heaven. But whatever may be the importanceof the sun, viewed merely as one of the bodies which teem through space, there can be no hesitation in asserting how immeasurably his influenceon the earth surpasses that of all other bodies in the universetogether. It was therefore natural--indeed inevitable--that our firstexamination of the orbs of heaven should be directed to that mighty bodywhich is the source of our life itself. 

Nor could there be much hesitation as to the second step which ought tobe taken. The intrinsic importance of the moon, when compared with othercelestial bodies, may be small; it is, indeed, as we shall afterwardssee, almost infinitesimal. But in the economy of our earth the moon hasplayed, and still plays, a part second only in importance to that of thesun himself. The moon is so close to us that her brilliant rays pale toinvisibility countless orbs of a size and an intrinsic splendourincomparably greater than her own. The moon also occupies an exceptionalposition in the history of astronomy; for the law of gravitation, thegreatest discovery that science has yet witnessed, was chieflyaccomplished by observations of the moon. It was therefore natural thatan early chapter in our Story of the Heavens should be devoted to a bodythe interest of which approximated so closely to that of the sunhimself. 

But the sun and the moon having been partly described (we shallafterwards have to refer to them again), some hesitation is natural inthe choice of the next step. The two great luminaries being abstractedfrom our view, there remains no other celestial body of such exceptionalinterest and significance as to make it quite clear what course topursue; we desire to unfold the story of the heavens in the most naturalmanner. If we made the attempt to describe the celestial bodies in theorder of their actual magnitude, our ignorance must at once pronouncethe task to be impossible. We cannot even make a conjecture as to whichbody in the heavens is to stand first on the list. Even if thatmightiest body be within reach of our telescopes (in itself a highlyimprobable supposition), we have not the least idea in what part of theheavens it is to be sought. And even if this were possible--if we wereable to arrange all the visible bodies rank by rank in the order oftheir magnitude and their splendour--still the scheme would beimpracticable, for of most of them we know little or nothing. 

We are therefore compelled to adopt a different method of procedure, andthe simplest, as well as the most natural, will be to follow as far aspossible the order of distance of the different bodies. We have alreadyspoken of the moon as the nearest neighbour to the earth; we shall nextconsider some of the other celestial bodies which are comparatively nearto us; then, as the subject unfolds, we shall discuss the objectsfurther and further away, until towards the close of the volume we shallbe engaged in considering the most distant bodies in the universe whichthe telescope has yet revealed to us. 

Even when we have decided on this principle, our course is still notfree from ambiguity. Many of the bodies in the heavens are in motion, sothat their relative distances from the earth are in continual change;this is, however, a difficulty which need not detain us. We shall makeno attempt to adhere closely to the principle in all details. It will besufficient if we first describe those great bodies--not a very numerousclass--which are, comparatively speaking, in our vicinity, though stillat varied distances; and then we shall pass on to the uncounted bodieswhich are separated from us by distances so vast that the imagination isbaffled in the attempt to realise them. 

Let us, then, scan the heavens to discover those orbs which lie in ourneighbourhood. The sun has set, the moon has not risen; a cloudless skydiscloses a heaven glittering with countless gems of light. Some aregrouped together into well-marked constellations; others seem scatteredpromiscuously, with every degree of lustre, from the very brightest downto the faintest point that the eye can just glimpse. Amid all this hostof objects, how are we to identify those which lie nearest to the earth?Look to the west: and there, over the spot where the departing sunbeamsstill linger, we often see the lovely evening star shining forth. Thisis the planet Venus--a beauteous orb, twin-sister to the earth. Thebrilliancy of this planet, its rapid changes both in position and inlustre, would suggest at once that it was much nearer to the earth thanother star-like objects. This presumption has been amply confirmed bycareful measurements, and therefore Venus is to be included in the listof the orbs which constitute our neighbours. 

Another conspicuous planet--almost rivalling Venus in lustre, andvastly surpassing Venus in the magnificence of its proportions and itsretinue--has borne from antiquity the majestic name of Jupiter. No doubtJupiter is much more distant from us than Venus. Indeed, he is always atleast twice as far, and sometimes as much as ten times. But still wemust include Jupiter among our neighbours. Compared with the host ofstars which glitter on the heavens, Jupiter must be regarded as quitecontiguous. The distance of the great planet requires, it is true, hundreds of millions of miles for its expression; yet, vast as is thatdistance, it would have to be multiplied by tens of thousands, orhundreds of thousands, before it would be long enough to span the abysswhich intervenes between the earth and the nearest of the stars. 

Venus and Jupiter have invited our attention by their exceptionalbrilliancy. We should, however, fall into error if we assumed generallythat the brightest objects were those nearest to the earth. An observerunacquainted with astronomy might not improbably point to the DogStar--or Sirius, as astronomers more generally know it--as an objectwhose exceptional lustre showed it to be one of our neighbours. This, however, would be a mistake. We shall afterwards have occasion to refermore particularly to this gem of our southern skies, and then it willappear that Sirius is a mighty globe far transcending our own sun insize as well as in splendour, but plunged into the depths of space tosuch an appalling distance that his enfeebled rays, when they reach theearth, give us the impression, not of a mighty sun, but only of abrilliant star. 

The principle of selection, by which the earth's neighbours can bediscriminated, will be explained presently; in the meantime, it will besufficient to observe that our list is to be augmented first by theaddition of the unique object known as Saturn, though its brightness isfar surpassed by that of Sirius, as well as by a few other stars. Thenwe add Mars, an object which occasionally approaches so close to theearth that it shines with a fiery radiance which would hardly prepareus for the truth that this planet is intrinsically one of the smallestof the celestial bodies. Besides the objects we have mentioned, theancient astronomers had detected a fifth, known as Mercury--a planetwhich is usually invisible amid the light surrounding the sun. Mercury, however, occasionally wanders far enough from our luminary to be seenbefore sunrise or after sunset. These five--Mercury, Venus, Mars, Jupiter, and Saturn--comprised the planets known from remote antiquity. 

We can, however, now extend the list somewhat further by adding to itthe telescopic objects which have in modern times been found to be amongour neighbours. Here we must no longer postpone the introduction of thecriterion by which we can detect whether a body is near the earth ornot. The brighter planets can be recognised by the steady radiance oftheir light as contrasted with the incessant twinkling of the stars. Alittle attention devoted to any of the bodies we have named will, however, point out a more definite contrast between the planets and thestars. 

Observe, for instance, Jupiter, on any clear night when the heavens canbe well seen, and note his position with regard to the constellations inhis neighbourhood--how he is to the right of this star, or to the leftof that; directly between this pair, or directly pointed to by that. Wethen mark down the place of Jupiter on a celestial map, or we make asketch of the stars in the neighbourhood showing the position of theplanet. After a month or two, when the observations are repeated, theplace of Jupiter is to be compared again with those stars by which itwas defined. It will be found that, while the stars have preserved theirrelative positions, the place of Jupiter has changed. Hence this body iswith propriety called a _planet_, or a wanderer, because it isincessantly moving from one part of the starry heavens to another. Bysimilar comparisons it can be shown that the other bodies we havementioned--Venus and Mercury, Saturn and Mars--are also wanderers, andbelong to that group of heavenly bodies known as planets. Here, then, wehave the simple criterion by which the earth's neighbours are readily tobe discriminated from the stars. Each of the bodies near the earth is aplanet, or a wanderer, and the mere fact that a body is a wanderer isalone sufficient to prove it to be one of the class which we are nowstudying. 

Provided with this test, we can at once make an addition to our list ofneighbours. Amid the myriad orbs which the telescope reveals, weoccasionally find one which is a wanderer. Two other mighty planets, known as Uranus and Neptune, must thus be added to the five alreadymentioned, making in all a group of seven great planets. A vastlygreater number may also be reckoned when we admit to our view bodieswhich not only seem to be minute telescopic objects, but really aresmall globes when compared with the mighty bulk of our earth. Theselesser planets, to the number of more than four hundred, are also amongthe earth's neighbours. 

We should remark that another class of heavenly bodies widely differingfrom the planets must also be included in our system. These are thecomets, and, indeed, it may happen that one of these erratic bodies willsometimes draw nearer to the earth than even the closest approach evermade by a planet. These mysterious visitors will necessarily engage agood deal of our attention later on. For the present we confine ourattention to those more substantial globes, whether large or small, which are always termed planets. 

Imagine for a moment that some opaque covering could be clasped aroundour sun so that all his beams were extinguished. That our earth would beplunged into the darkness of midnight is of course an obviousconsequence. A moment's consideration will show that the moon, shiningas it does by the reflected rays of the sun, would become totallyinvisible. But would this extinction of the sunlight have any othereffect? Would it influence the countless brilliant points that stud theheavens at midnight? Such an obscuration of the sun would indeed producea remarkable effect on the sky at night, which a little attention woulddisclose. The stars, no doubt, would not exhibit the slightest change inbrilliancy. Each star shines by its own light and is not indebted to thesun. The constellations would thus twinkle on as before, but awonderful change would come over the planets. Were the sun to beobscured, the planets would also disappear from view. The midnight skywould thus experience the effacement of the planets one by one, whilethe stars would remain unaltered. It may seem difficult to realise howthe brilliancy of Venus or the lustre of Jupiter have their originsolely in the beams which fall upon these bodies from the distant sun. The evidence is, however, conclusive on the question; and it will beplaced before the reader more fully when we come to discuss the severalplanets in detail. 

Suppose that we are looking at Jupiter high in mid-heavens on a winter'snight, it might be contended that, as the earth lies between Jupiter andthe sun, it must be impossible for the rays of the sun to fall upon theplanet. This is, perhaps, not an unnatural view for an inhabitant ofthis earth to adopt until he has become acquainted with the relativesizes of the various bodies concerned, and with the distances by whichthose bodies are separated. But the question would appear in a widelydifferent form to an inhabitant of the planet Jupiter. If such a beingwere asked whether he suffered much inconvenience by the intrusion ofthe earth between himself and the sun, his answer would be something ofthis kind:--"No doubt such an event as the passage of the earth betweenme and the sun is possible, and has occurred on rare occasions separatedby long intervals; but so far from the transit being the cause of anyinconvenience, the whole earth, of which you think so much, is really sominute, that when it did come in front of the sun it was merely seen asa small telescopic point, and the amount of sunlight which itintercepted was quite inappreciable. "

The fact that the planets shine by the sun's light points at once to thesimilarity between them and our earth. We are thus led to regard our sunas a central fervid globe associated with a number of much smallerbodies, each of which, being dark itself, is indebted to the sun bothfor light and for heat. 

That was, indeed, a grand step in astronomy which demonstrated thenature of the solar system. The discovery that our earth must be aglobe isolated in space was in itself a mighty exertion of humanintellect; but when it came to be recognised that this globe was but oneof a whole group of similar objects, some smaller, no doubt, but othersvery much larger, and when it was further ascertained that these bodieswere subordinated to the supreme control of the sun, we have a chain ofdiscoveries that wrought a fundamental transformation in humanknowledge. 

We thus see that the sun presides over a numerous family. The members ofthat family are dependent upon the sun, and their dimensions aresuitably proportioned to their subordinate position. Even Jupiter, thelargest member of that family, does not contain one-thousandth part ofthe material which forms the vast bulk of the sun. Yet the bulk ofJupiter alone would exceed that of the rest of the planets were they allrolled together. 

Around the central luminary in Fig. 31 we have drawn four circles indotted lines which sufficiently illustrate the orbits in which thedifferent bodies move. The innermost of these four paths represents theorbit of the planet Mercury. The planet moves around the sun in thispath, and regains the place from which it started in eighty-eight days. 

The next orbit, proceeding outwards from the sun, is that of the planetVenus, which we have already referred to as the well-known Evening Star. Venus completes the circuit of its path in 225 days. One step furtherfrom the sun and we come to the orbit of another planet. This body isalmost the same size as Venus, and is therefore much larger thanMercury. The planet now under consideration accomplishes each revolutionin 365 days. This period sounds familiar to our ears. It is the lengthof the year; and the planet is the earth on which we stand. There is animpressive way in which to realise the length of the road along whichthe earth has to travel in each annual journey. The circumference of acircle is about three and one-seventh times the diameter of the samefigure; so that taking the distance from the earth to the centre of thesun as 92, 900, 000 miles, the diameter of the circle which the earthdescribes around the sun will be 185, 800, 000 miles, and consequentlythe circumference of the mighty circle in which the earth moves roundthe sun is fully 583, 000, 000 miles. The earth has to travel thisdistance every year. It is merely a sum in division to find how far wehave to move each second in order to accomplish this long journey in atwelvemonth. It will appear that the earth must actually completeeighteen miles every second, as otherwise it would not finish itsjourney within the allotted time. 

[Illustration: Fig. 31. --The Orbits of the Four Interior Planets. ]

Pause for a moment to think what a velocity of eighteen miles a secondreally implies. Can we realise a speed so tremendous? Let us compare itwith our ordinary types of rapid movement. Look at that express trainhow it crashes under the bridge, how, in another moment, it is lost toview! Can any velocity be greater than that? Let us try it by figures. The train moves a mile a minute; multiply that velocity by eighteen andit becomes eighteen miles a _minute_, but we must further multiply itby sixty to make it eighteen miles a _second_. The velocity of theexpress train is not even the thousandth part of the velocity of theearth. Let us take another illustration. We stand at the rifle ranges tosee a rifle fired at a target 1, 000 feet away, and we find that a secondor two is sufficient to carry the bullet over that distance. The earthmoves nearly one hundred times as fast as the rifle bullet. 

[Illustration: Fig. 32. --The Earth's Movement. ]

Viewed in another way, the stupendous speed of the earth does not seemimmoderate. The earth is a mighty globe, so great indeed that even whenmoving at this speed it takes almost eight minutes to pass over its owndiameter. If a steamer required eight minutes to traverse a distanceequal to its own length, its pace would be less than a mile an hour. Toillustrate this method of considering the subject, we show here a viewof the progress made by the earth (Fig. 32). The distance between thecentres of these circles is about six times the diameter; and, accordingly, if they be taken to represent the earth, the time requiredto pass from one position to the other is about forty-eight minutes. 

Outside the path of the earth, we come to the orbit of the fourthplanet, Mars, which requires 687 days, or nearly two years, to completeits circuit round the sun. With our arrival at Mars we have gained thelimit to the inner portion of the solar system. 

The four planets we have mentioned form a group in themselves, distinguished by their comparative nearness to the sun. They are allbodies of moderate dimensions. Venus and the Earth are globes of aboutthe same size. Mercury and Mars are both smaller objects which lie, sofar as bulk is concerned, between the earth and the moon. The fourplanets which come nearest to the sun are vastly surpassed in bulk andweight by the giant bodies of our system--the stately group of Jupiterand Saturn, Uranus and Neptune. 

[Illustration: Fig. 33. --The Orbits of the Four Giant Planets. ]

These giant planets enjoy the sun's guidance equally with their weakerbrethren. In the diagram on this page (Fig. 33) parts of the orbits ofthe great outer planets are represented. The sun, as before, presides atthe centre, but the inner planets would on this scale be so close to thesun that it is only possible to represent the orbit of Mars. After theorbit of Mars comes a considerable interval, not, however, devoid ofplanetary activity, and then follow the orbits of Jupiter and Saturn;further still, we have Uranus, a great globe on the verge of unassistedvision; and, lastly, the whole system is bounded by the grand orbit ofNeptune--a planet of which we shall have a marvellous story to narrate. 

The various circles in Fig. 34 show the apparent sizes of the sun asseen from the different planets. Taking the circle corresponding to theearth to represent the amount of heat and light which the earth derivesfrom the sun then the other circles indicate the heat and the lightenjoyed by the corresponding planets. The next outer planet to the earthis Mars, whose share of solar blessings is not so very inferior to thatof the earth; but we fail to see how bodies so remote as Jupiter orSaturn can enjoy climates at all comparable with those of the planetswhich are more favourably situated. 

[Illustration: Fig. 34. --Comparative Apparent Size of the Sun as seenfrom the Various Planets. ]

Fig. 35 shows a picture of the whole family of planets surrounding thesun--represented on the same scale, so as to exhibit their comparativesizes. Measured by bulk, Jupiter is more than 1, 200 times as great asthe earth, so that it would take at least 1, 200 earths rolled into oneto form a globe equal to the globe of Jupiter. Measured by weight, thedisparity between the earth and Jupiter, though still enormous, is notquite so great; but this is a matter to be discussed more fully in alater chapter. 

[Illustration: Fig. 35. --Comparative Sizes of the Planets. ]

Even in this preliminary survey of the solar system we must not omit torefer to the planets which attract our attention, not by their bulk, butby their multitude. In the ample zone bounded on the inside by the orbitof Mars and on the outside by the orbit of Jupiter it was thought at onetime that no planet revolved. Modern research has shown that this regionis tenanted, not by one planet, but by hundreds. The discovery of theseplanets is a charge which has been undertaken by various diligentastronomers of the present day, while the discussion of their movementsaffords labour to other men of science. We shall find something to learnfrom the study of these tiny bodies, and especially from another smallplanet called Eros, which lies nearer to the earth than the limit aboveindicated. A chapter will be devoted to these objects. 

But we do not propose to enter deeply into the mere statistics of theplanetary system at present. Were such our intention, the tables at theend of the volume would show that ample materials are available. Astronomers have taken an inventory of each of the planets. They havemeasured their distances, the shapes of their orbits and the positionsof those orbits, their times of revolution, and, in the case of all thelarger planets, their sizes and their weights. Such results are ofinterest for many purposes. It is, however, the more general features ofthe science which at present claim our attention. 

Let us, in conclusion, note one or two important truths with referenceto our planetary system. We have seen that all the planets revolve innearly circular paths around the sun. We have now to add another factpossessing much significance. Each of the planets pursues its path inthe same direction. It thus happens that one such body may overtakeanother, but it can never happen that two planets pass by each other asdo the trains on adjacent lines of railway. We shall subsequently findthat the whole welfare of our system, nay, its continuous existence, isdependent upon this remarkable uniformity taken in conjunction withother features of the system. 

Such is our solar system; a mighty organised group of planetscirculating under the control of the sun, and completely isolated fromall external interference. No star, no constellation, has anyappreciable influence on our solar system. We constitute a little islandgroup, separated from the nearest stars by the most amazing distances. It may be that as the other stars are suns, so they too may have systemsof planets circulating around them; but of this we know nothing. Of thestars we can only say that they appear to us as points of light, and anyplanets they may possess must for ever remain invisible to us, even ifthey were many times larger than Jupiter. 

We need not repine at this limitation to our possible knowledge, forjust as we find in the solar system all that is necessary for our dailybodily wants, so shall we find ample occupation for whatever facultieswe may possess in endeavouring to understand those mysteries of theheavens which lie within our reach. 

CHAPTER V. 

THE LAW OF GRAVITATION. 

 Gravitation--The Falling of a Stone to the Ground--All Bodies fall equally, Sixteen Feet in a Second--Is this true at Great Heights?--Fall of a Body at a Height of a Quarter of a Million Miles--How Newton obtained an Answer from the Moon--His Great Discovery--Statement of the Law of Gravitation--Illustrations of the Law--How is it that all the Bodies in the Universe do not rush Together?--The Effect of Motion--How a Circular Path can be produced by Attraction--General Account of the Moon's Motion--Is Gravitation a Force of Great Intensity?--Two Weights of 50 lbs. --Two Iron Globes, 53 Yards in Diameter, and a Mile apart, attract with a Force of 1 lb. --Characteristics of Gravitation--Orbits of the Planets not strictly Circles--The Discoveries of Kepler--Construction of an Ellipse--Kepler's First Law--Does a Planet move Uniformly?--Law of the Changes of Velocity--Kepler's Second Law--The Relation between the Distances and the Periodic Times--Kepler's Third Law--Kepler's Laws and the Law of Gravitation--Movement in a Straight Line--A Body unacted on by Disturbing Forces would move in a Straight Line with Constant Velocity--Application to the Earth and the Planets--The Law of Gravitation deduced from Kepler's Laws--Universal Gravitation. 

Our description of the heavenly bodies must undergo a slightinterruption, while we illustrate with appropriate detail an importantprinciple, known as the law of gravitation, which underlies the whole ofastronomy. By this law we can explain the movements of the moon aroundthe earth, and of the planets around the sun. It is accordinglyincumbent upon us to discuss this subject before we proceed to the moreparticular account of the separate planets. We shall find, too, that thelaw of gravitation sheds some much-needed light on the nature of thestars situated at the remotest distances in space. It also enables us tocast a glance through the vistas of time past, and to trace withplausibility, if not with certainty, certain early phases in the historyof our system. The sun and the moon, the planets and the comets, thestars and the nebulæ, all alike are subject to this universal law, whichis now to engage our attention. 

What is more familiar than the fact that when a stone is dropped itwill fall to the ground? No one at first thinks the matter even worthyof remark. People are often surprised at seeing a piece of iron drawn toa magnet. Yet the fall of a stone to the ground is the manifestation ofa force quite as interesting as the force of magnetism. It is the earthwhich draws the stone, just as the magnet draws the iron. In each casethe force is one of attraction; but while the magnetic attraction isconfined to a few substances, and is of comparatively limitedimportance, the attraction of gravitation is significant throughout theuniverse. 

Let us commence with a few very simple experiments upon the force ofgravitation. Hold in the hand a small piece of lead, and then allow itto drop upon a cushion. The lead requires a certain time to move fromthe fingers to the cushion, but that time is always the same when theheight is the same. Take now a larger piece of lead, and hold one piecein each hand at the same height. If both are released at the samemoment, they will both reach the cushion simultaneously. It might havebeen thought that the heavy body would fall more quickly than the lightbody; but when the experiment is tried, it is seen that this is not thecase. Repeat the experiment with various other substances. An ordinarymarble will be found to fall in the same time as the piece of lead. Witha piece of cork we again try the experiment, and again obtain the sameresult. At first it seems to fail when we compare a feather with thepiece of lead; but that is solely on account of the air, which resiststhe feather more than it resists the lead. If, however, the feather beplaced upon the top of a penny, and the penny be horizontal whendropped, it will clear the air out of the way of the feather in itsdescent, and then the feather will fall as quickly as the penny, asquickly as the marble, or as quickly as the lead. 

If the observer were in a gallery when trying these experiments, and ifthe cushion were sixteen feet below his hands, then the time the marblewould take to fall through the sixteen feet would be one second. Thetime occupied by the cork or by the lead would be the same; and even thefeather itself would fall through sixteen feet in one second, if itcould be screened from the interference of the air. Try this experimentwhere we like, in London, or in any other city, in any island orcontinent, on board a ship at sea, at the North Pole, or the South Pole, or the equator, it will always be found that any body, of any size orany material, will fall about sixteen feet in one second of time. 

Lest any erroneous impression should arise, we may just mention that thedistance traversed in one second does vary slightly at different partsof the earth, but from causes which need not at this moment detain us. We shall for the present regard sixteen feet as the distance throughwhich any body, free from interference, would fall in one second at anypart of the earth's surface. But now let us extend our view above theearth's surface, and enquire how far this law of sixteen feet in asecond may find obedience elsewhere. Let us, for instance, ascend to thetop of a mountain and try the experiment there. It would be found thatat the top of the mountain a marble would take a little longer to fallthrough sixteen feet than the same marble would if let fall at its base. The difference would be very small; but yet it would be measurable, andwould suffice to show that the power of the earth to pull the marble tothe ground becomes somewhat weakened at a point high above the earth'ssurface. Whatever be the elevation to which we ascend, be it either thetop of a high mountain, or the still greater altitudes that have beenreached in balloon ascents, we shall never find that the tendency ofbodies to fall to the ground ceases, though no doubt the higher we gothe more is that tendency weakened. It would be of great interest tofind how far this power of the earth to draw bodies towards it canreally extend. We cannot attain more than about five or six miles abovethe earth's surface in a balloon; yet we want to know what would happenif we could ascend 500 miles, or 5, 000 miles, or still further, into theregions of space. 

Conceive that a traveller were endowed with some means of soaring aloftfor miles and thousands of miles, still up and up, until at length hehad attained the awful height of nearly a quarter of a million of milesabove the ground. Glancing down at the surface of that earth, which isat such a stupendous depth beneath, he would be able to see a wonderfulbird's-eye view. He would lose, no doubt, the details of towns andvillages; the features in such a landscape would be whole continents andwhole oceans, in so far as the openings between the clouds would permitthe earth's surface to be exposed. 

At this stupendous elevation he could try one of the most interestingexperiments that was ever in the power of a philosopher. He could testwhether the earth's attraction was felt at such a height, and he couldmeasure the amount of that attraction. Take for the experiment a cork, amarble, or any other object, large or small; hold it between thefingers, and let it go. Everyone knows what would happen in such a casedown here; but it required Sir Isaac Newton to tell what would happen insuch a case up there. Newton asserts that the power of the earth toattract bodies extends even to this great height, and that the marblewould fall. This is the doctrine that we can now test. We are ready forthe experiment. The marble is released, and, lo! our first exclamationis one of wonder. Instead of dropping instantly, the little objectappears to remain suspended. We are on the point of exclaiming that wemust have gone beyond the earth's attraction, and that Newton is wrong, when our attention is arrested; the marble is beginning to move, soslowly that at first we have to watch it carefully. But the pacegradually improves, so that the attraction is beyond all doubt, until, gradually acquiring more and more velocity, the marble speeds on itslong journey of a quarter of a million of miles to the earth. 

But surely, it will be said, such an experiment must be entirelyimpossible; and no doubt it cannot be performed in the way described. The bold idea occurred to Newton of making use of the moon itself, whichis almost a quarter of a million of miles above the earth, for thepurpose of answering the question. Never was our satellite put to suchnoble use before. It is actually at each moment falling in towards theearth. We can calculate how much it is deflected towards the earth ineach second, and thus obtain a measure of the earth's attractive power. From such enquiries Newton was able to learn that a body released at thedistance of 240, 000 miles above the surface of the earth would still beattracted by the earth, that in virtue of the attraction the body wouldcommence to move off towards the earth--not, indeed, with the velocitywith which a body falls in experiments on the surface, but with a verymuch lesser speed. A body dropped down from the distance of the moonwould commence its long journey so slowly that a _minute_, instead of a_second_, would have elapsed before the distance of sixteen feet hadbeen accomplished. [11]

It was by pondering on information thus won from the moon that Newtonmade his immortal discovery. The gravitation of the earth is a forcewhich extends far and wide through space. The more distant the body, theweaker the gravitation becomes; here Newton found the means ofdetermining the great problem as to the law according to which theintensity of the gravitation decreased. The information derived from themoon, that a body 240, 000 miles away requires a minute to fall through aspace equal to that through which it would fall in a second down here, was of paramount importance. In the first place, it shows that theattractive power of the earth, by which it draws all bodies earthwards, becomes weaker at a distance. This might, indeed, have been anticipated. It is as reasonable to suppose that as we retreated further and furtherinto the depths of space the power of attraction should diminish, asthat the lustre of light should diminish as we recede from it; and it isremarkable that the law according to which the attraction of gravitationdecreases with the increase of distance is precisely the same as thelaw according to which the brilliancy of a light decreases as itsdistance increases. 

The law of nature, stated in its simplest form, asserts that theintensity of gravitation varies inversely as the square of the distance. Let me endeavour to elucidate this somewhat abstract statement by one ortwo simple illustrations. Suppose a body were raised above the surfaceof the earth to a height of nearly 4, 000 miles, so as to be at analtitude equal to the radius of the earth. In other words, a body sosituated would be twice as far from the centre of the earth as a bodywhich lay on the surface. The law of gravitation says that the intensityof the attraction is then to be decreased to one-fourth part, so thatthe pull of the earth on a body 4, 000 miles high is only one quarter ofthe pull of the earth on that body so long as it lies on the ground. Wemay imagine the effect of this pull to be shown in different ways. Allowthe body to fall, and in the interval of one second it will only dropthrough four feet, a mere quarter of the distance that gravity wouldcause near the earth's surface. 

We may consider the matter in another way by supposing that theattraction of the earth is measured by one of those little weighingmachines known as a spring balance. If a weight of four pounds be hungon such a contrivance, at the earth's surface, the index of course showsa weight of four pounds; but conceive this balance, still bearing theweight appended thereto, were to be carried up and up, the _indicated_strain would become less and less, until by the time the balance reached4, 000 miles high, where it was _twice_ as far away from the earth'scentre as at first, the indicated strain would be reduced to the_fourth_ part, and the balance would only show one pound. If we couldimagine the instrument to be carried still further into the depths ofspace, the indication of the scale would steadily continue to decline. By the time the apparatus had reached a distance of 8, 000 miles high, being then _three_ times as far from the earth's centre as at first, thelaw of gravitation tells us that the attraction must have decreased toone-ninth part. The strain thus shown on the balance would be only theninth part of four pounds, or less than half a pound. But let the voyagebe once again resumed, and let not a halt be made this time until thebalance and its four-pound weight have retreated to that orbit which themoon traverses in its monthly course around the earth. The distance thusattained is about sixty times the radius of the earth, and consequentlythe attraction of gravitation is diminished in the proportion of one tothe square of sixty; the spring will then only be strained by theinappreciable fraction of 1-3, 600 part of four pounds. It thereforeappears that a weight which on the earth weighed a ton and a half would, if raised 240, 000 miles, weigh less than a pound. But even at this vastdistance we are not to halt; imagine that we retreat still further andfurther; the strain shown by the balance will ever decrease, but it willstill exist, no matter how far we go. Astronomy appears to teach us thatthe attraction of gravitation can extend, with suitably enfeebledintensity, across the most profound gulfs of space. 

The principle of gravitation is of far wider scope than we have yetindicated. We have spoken merely of the attraction of the earth, and wehave stated that this force extends throughout space. But the law ofgravitation is not so limited. Not only does the earth attract everyother body, and every other body attract the earth, but each of thesebodies attracts the other; so that in its more complete shape the law ofgravitation announces that "every body in the universe attracts everyother body with a force which varies inversely as the square of thedistance. "

It is impossible for us to over-estimate the importance of this law. Itsupplies the clue by which we can unravel the complicated movements ofthe planets. It has led to marvellous discoveries, in which the law ofgravitation has enabled us to anticipate the telescope, and to feel theexistence of bodies before those bodies have even been seen. 

An objection which may be raised at this point must first be dealt with. It seems to be, indeed, a plausible one. If the earth attracts the moon, why does not the moon tumble down on the earth? If the earth isattracted by the sun, why does it not tumble into the sun? If the sun isattracted by other stars, why do they not rush together with a frightfulcollision? It may not unreasonably be urged that if all these bodies inthe heavens are attracting each other, it would seem that they must allrush together in consequence of that attraction, and thus weld the wholematerial universe into a single mighty mass. We know, as a matter offact, that these collisions do not often happen, and that there isextremely little likelihood of their taking place. We see that althoughour earth is said to have been attracted by the sun for countless ages, yet the earth is just as far from the sun as ever it was. Is not this inconflict with the doctrine of universal gravitation? In the early daysof astronomy such objections would be regarded, and doubtless wereregarded, as well-nigh insuperable; even still we occasionally hear themraised, and it is therefore the more incumbent on us to explain how ithappens that the solar system has been able to escape from thecatastrophe by which it seems to be threatened. 

There can be no doubt that if the moon and the earth had been initiallyplaced _at rest_, they would have been drawn together by their mutualattraction. So, too, if the system of planets surrounding the sun hadbeen left initially _at rest_ they would have dashed into the sun, andthe system would have been annihilated. It is the fact that the planetsare _moving_, and that the moon is _moving_, which has enabled thesebodies successfully to resist the attraction in so far, at least, asthat they are not drawn thereby to total destruction. 

It is so desirable that the student should understand clearly how acentral attraction is compatible with revolution in a nearly circularpath, that we give an illustration to show how the moon pursues itsmonthly orbit under the guidance and the control of the attractingearth. 

[Illustration: Fig. 36. --Illustration of the Moon's Motion. ]

The imaginary sketch in Fig. 36 denotes a section of the earth with ahigh mountain thereon. [12] If a cannon were stationed on the top of themountain at C, and if the cannonball were fired off in the direction C Ewith a moderate charge of powder, the ball would move down along thefirst curved path. If it be fired a second time with a heavier charge, the path will be along the second curved line, and the ball would againfall to the ground. But let us try next time with a charge still furtherincreased, and, indeed, with a far stronger cannon than any piece ofordnance ever yet made. The velocity of the projectile must now beassumed to be some miles per second, but we can conceive that the speedshall be so adjusted that the ball shall move along the path C D, alwaysat the same height above the earth, though still curving, as everyprojectile must curve, from the horizontal line in which it moved at thefirst moment. Arrived at D, the ball will still be at the same heightabove the surface, and its velocity must be unabated. It will thereforecontinue in its path and move round another quadrant of the circlewithout getting nearer to the surface. In this manner the projectilewill travel completely round the whole globe, coming back again to C andthen taking another start in the same path. If we could abolish themountain and the cannon at the top, we should have a body revolving forever around the earth in consequence of the attraction of gravitation. 

Make now a bold stretch of the imagination. Conceive a terrific cannoncapable of receiving a round bullet not less than 2, 000 miles indiameter. Discharge this enormous bullet with a velocity of about 3, 000feet per second, which is two or three times as great as the velocityactually attainable in modern artillery. Let this notable bullet befired horizontally from some station nearly a quarter of a million milesabove the surface of the earth. That fearful missile would sweep rightround the earth in a nearly circular orbit, and return to where itstarted in about four weeks. It would then commence another revolution, four weeks more would find it again at the starting point, and thismotion would go on for ages. 

Do not suppose that we are entirely romancing. We cannot indeed show thecannon, but we can point to a great projectile. We see it every month;it is the beautiful moon herself. No one asserts that the moon was evershot from such a cannon; but it must be admitted that she moves as ifshe had been. In a later chapter we shall enquire into the history ofthe moon, and show how she came to revolve in this wonderful manner. 

As with the moon around the earth, so with the earth around the sun. Theillustration shows that a circular or nearly circular motion harmoniseswith the conception of the law of universal gravitation. 

We are accustomed to regard gravitation as a force of stupendousmagnitude. Does not gravitation control the moon in its revolutionaround the earth? Is not even the mighty earth itself retained in itspath around the sun by the surpassing power of the sun's attraction? Nodoubt the actual force which keeps the earth in its path, as well asthat which retains the moon in our neighbourhood, is of vast intensity, but that is because gravitation is in such cases associated with bodiesof enormous mass. No one can deny that all bodies accessible to ourobservation appear to attract each other in accordance with the law ofgravitation; but it must be confessed that, unless one or both of theattracting bodies is of gigantic dimensions, the intensity is almostimmeasurably small. 

Let us attempt to illustrate how feeble is the gravitation betweenmasses of easily manageable dimensions. Take, for instance, two ironweights, each weighing about 50lb. , and separated by a distance of onefoot from centre to centre. There is a certain attraction of gravitationbetween these weights. The two weights are drawn together, yet they donot move. The attraction between them, though it certainly exists, is anextremely minute force, not at all comparable as to intensity withmagnetic attraction. Everyone knows that a magnet will draw a piece ofiron with considerable vigour, but the intensity of gravitation is verymuch less on masses of equal amount. The attraction between these two50lb. Weights is less than the ten-millionth part of a single pound. Such a force is utterly infinitesimal in comparison with the frictionbetween the weights and the table on which they stand, and hence thereis no response to the attraction by even the slightest movement. Yet, ifwe can conceive each of these weights mounted on wheels absolutelydevoid of friction, and running on absolutely perfect horizontal rails, then there is no doubt that the bodies would slowly commence to drawtogether, and in the course of time would arrive in actual contact. 

If we desire to conceive gravitation as a force of measurable intensity, we must employ masses immensely more ponderous than those 50lb. Weights. Imagine a pair of globes, each composed of 417, 000 tons of cast iron, and each, if solid, being about 53 yards in diameter. Imagine theseglobes placed at a distance of one mile apart. Each globe attracts theother by the force of gravitation. It does not matter that buildings andobstacles of every description intervene; gravitation will pass throughsuch impediments as easily as light passes through glass. No screen canbe devised dense enough to intercept the passage of this force. Each ofthese iron globes will therefore under all circumstances attract theother; but, notwithstanding their ample proportions, the intensity ofthat attraction is still very small, though appreciable. The attractionbetween these two globes is a force no greater than the pressure exertedby a single pound weight. A child could hold back one of these massiveglobes from its attraction by the other. Suppose that all was clear, andthat friction could be so neutralised as to permit the globes to followthe impulse of their mutual attractions. The two globes will thencommence to approach, but the masses are so large, while the attractionis so small, that the speed will be accelerated very slowly. Amicroscope would be necessary to show when the motion has actuallycommenced. An hour and a half must elapse before the distance isdiminished by a single foot; and although the pace improvessubsequently, yet three or four days must elapse before the two globeswill come together. 

The most remarkable characteristic of the force of gravitation must behere specially alluded to. The intensity appears to depend only on thequantity of matter in the bodies, and not at all on the nature of thesubstances of which these bodies are composed. We have described the twoglobes as made of cast iron, but if either or both were composed of leador copper, of wood or stone, of air or water, the attractive power wouldstill be the same, provided only that the masses remain unaltered. Inthis we observe a profound difference between the attraction ofgravitation and magnetic attraction. In the latter case the attractionis not perceptible at all in the great majority of substances, and isonly considerable in the case of iron. 

In our account of the solar system we have represented the moon asrevolving around the earth in a _nearly_ circular path, and the planetsas revolving around the sun in orbits which are also approximatelycircular. It is now our duty to give a more minute description of theseremarkable paths; and, instead of dismissing them as being _nearly_circles, we must ascertain precisely in what respects they differtherefrom. 

If a planet revolved around the sun in a truly circular path, of whichthe sun was always at the centre, it is then obvious that the distancefrom the sun to the planet, being always equal to the radius of thecircle, must be of constant magnitude. Now, there can be no doubt thatthe distance from the sun to each planet is approximately constant; butwhen accurate observations are made, it becomes clear that the distanceis not absolutely so. The variations in distance may amount to manymillions of miles, but, even in extreme cases, the variation in thedistance of the planet is only a small fraction--usually a very smallfraction--of the total amount of that distance. The circumstances varyin the case of each of the planets. The orbit of the earth itself issuch that the distance from the earth to the sun departs but little fromits mean value. Venus makes even a closer approach to perfectly circularmovement; while, on the other hand, the path of Mars, and much more thepath of Mercury, show considerable relative fluctuations in the distancefrom the planet to the sun. 

It has often been noticed that many of the great discoveries in sciencehave their origin in the nice observation and explanation of minutedepartures from some law approximately true. We have in this departmentof astronomy an excellent illustration of this principle. The orbits ofthe planets are nearly circles, but they are not exactly circles. Now, why is this? There must be some natural reason. That reason has beenascertained, and it has led to several of the grandest discoveries thatthe mind of man has ever achieved in the realms of Nature. 

In the first place, let us see the inferences to be drawn from the factthat the distance of a planet from the sun is not constant. The motionin a circle is one of such beauty and simplicity that we are reluctantto abandon it, unless the necessity for doing so be made clearlyapparent. Can we not devise any way by which the circular motion mightbe preserved, and yet be compatible with the fluctuations in thedistance from the planet to the sun? This is clearly impossible with thesun at the centre of the circle. But suppose the sun did not occupy thecentre, while the planet, as before, revolved around the sun. Thedistance between the two bodies would then necessarily fluctuate. Themore eccentric the position of the sun, the larger would be theproportionate variation in the distance of the planet when at thedifferent parts of its orbit. It might further be supposed that byplacing a series of circles around the sun the various planetary orbitscould be accounted for. The centre of the circle belonging to Venus isto coincide very nearly with the centre of the sun, and the centres ofthe orbits of all the other planets are to be placed at such suitabledistances from the sun as will render a satisfactory explanation of thegradual increase and decrease of the distance between the two bodies. 

There can be no doubt that the movements of the moon and of the planetswould be, to a large extent, explained by such a system of circularorbits; but the spirit of astronomical enquiry is not satisfied withapproximate results. Again and again the planets are observed, and againand again the observations are compared with the places which theplanets would occupy if they moved in accordance with the system hereindicated. The centres of the circles are moved hither and thither, their radii are adjusted with greater care; but it is all of no avail. The observations of the planets are minutely examined to see if they canbe in error; but of errors there are none at all sufficient to accountfor the discrepancies. The conclusion is thus inevitable--astronomersare forced to abandon the circular motion, which was thought to possesssuch unrivalled symmetry and beauty, and are compelled to admit that theorbits of the planets are not circular. 

Then if these orbits be not circles, what are they? Such was the greatproblem which Kepler proposed to solve, and which, to his immortalglory, he succeeded in solving and in proving to demonstration. Thegreat discovery of the true shape of the planetary orbits stands out asone of the most conspicuous events in the history of astronomy. It may, in fact, be doubted whether any other discovery in the whole range ofscience has led to results of such far-reaching interest. 

We must here adventure for a while into the field of science known asgeometry, and study therein the nature of that curve which thediscovery of Kepler has raised to such unparalleled importance. Thesubject, no doubt, is a difficult one, and to pursue it with any detailwould involve us in many abstruse calculations which would be out ofplace in this volume; but a general sketch of the subject isindispensable, and we must attempt to render it such justice as may becompatible with our limits. 

The curve which represents with perfect fidelity the movements of aplanet in its revolution around the sun belongs to that well-known groupof curves which mathematicians describe as the conic sections. Theparticular form of conic section which denotes the orbit of a planet isknown by the name of the _ellipse_: it is spoken of somewhat lessaccurately as an oval. The ellipse is a curve which can be readilyconstructed. There is no simpler method of doing so than that which isfamiliar to draughtsmen, and which we shall here briefly describe. 

We represent on the next page (Fig. 37) two pins passing through a sheetof paper. A loop of twine passes over the two pins in the manner hereindicated, and is stretched by the point of a pencil. With a little carethe pencil can be guided so as to keep the string stretched, and itspoint will then describe a curve completely round the pins, returning tothe point from which it started. We thus produce that celebratedgeometrical figure which is called an ellipse. 

It will be instructive to draw a number of ellipses, varying in eachcase the circumstances under which they are formed. If, for instance, the pins remain placed as before, while the length of the loop isincreased, so that the pencil is farther away from the pins, then itwill be observed that the ellipse has lost some of its elongation, andapproaches more closely to a circle. On the other hand, if the length ofthe cord in the loop be lessened, while the pins remain as before, theellipse will be found more oval, or, as a mathematician would say, its_eccentricity_ is increased. It is also useful to study the changeswhich the form of the ellipse undergoes when one of the pins is altered, while the length of the loop remains unchanged. If the two pins bebrought nearer together the eccentricity will decrease, and the ellipsewill approximate more closely to the shape of a circle. If the pins beseparated more widely the eccentricity of the ellipse will be increased. That the circle is an extreme form of ellipse will be evident, if wesuppose the two pins to draw in so close together that they becomecoincident; the point will then simply trace out a circle as the pencilmoves round the figure. 

[Illustration: Fig. 37. --Drawing an Ellipse. ]

The points marked by the pins obviously possess very remarkablerelations with respect to the curve. Each one is called a _focus_, andan ellipse can only have one pair of foci. In other words, there is buta single pair of positions possible for the two pins, when an ellipse ofspecified size, shape, and position is to be constructed. 

The ellipse differs principally from a circle in the circumstance thatit possesses variety of form. We can have large and small ellipses justas we can have large and small circles, but we can also have ellipses ofgreater or less eccentricity. If the ellipse has not the perfectsimplicity of the circle it has, at least, the charm of variety whichthe circle has not. The oval curve has also the beauty derived from anoutline of perfect grace and an association with ennobling conceptions. 

The ancient geometricians had studied the ellipse: they had noticed itsfoci; they were acquainted with its geometrical relations; and thusKepler was familiar with the ellipse at the time when he undertook hiscelebrated researches on the movements of the planets. He had found, aswe have already indicated, that the movements of the planets could notbe reconciled with circular orbits. What shape of orbit should next betried? The ellipse was ready to hand, its properties were known, and thecomparison could be made; memorable, indeed, was the consequence of thiscomparison. Kepler found that the movement of the planets could beexplained, by supposing that the path in which each one revolved was anellipse. This in itself was a discovery of the most commandingimportance. On the one hand it reduced to order the movements of thegreat globes which circulate round the sun; while on the other, it tookthat beautiful class of curves which had exercised the geometricaltalents of the ancients, and assigned to them the dignity of definingthe highways of the universe. 

But we have as yet only partly enunciated the first discovery of Kepler. We have seen that a planet revolves in an ellipse around the sun, andthat the sun is, therefore, at some point in the interior of theellipse--but at what point? Interesting, indeed, is the answer to thisquestion. We have pointed out how the foci possess a geometricalsignificance which no other points enjoy. Kepler showed that the sunmust be situated in one of the foci of the ellipse in which each planetrevolves. We thus enunciate the first law of planetary motion in thefollowing words:--

 _Each planet revolves around the sun in an elliptic path, having the sun at one of the foci. _

We are now enabled to form a clear picture of the orbits of the planets, be they ever so numerous, as they revolve around the sun. In the firstplace, we observe that the ellipse is a plane curve; that is to say, each planet must, in the course of its long journey, confine itsmovements to one plane. Each planet has thus a certain planeappropriated to it. It is true that all these planes are very nearlycoincident, at least in so far as the great planets are concerned; butstill they are distinct, and the only feature in which they all agree isthat each one of them passes through the sun. All the elliptic orbits ofthe planets have one focus in common, and that focus lies at the centreof the sun. 

It is well to illustrate this remarkable law by considering thecircumstances of two or three different planets. Take first the case ofthe earth, the path of which, though really an ellipse, is very nearlycircular. In fact, if it were drawn accurately to scale on a sheet ofpaper, the difference between the elliptic orbit and the circle wouldhardly be detected without careful measurement. In the case of Venus theellipse is still more nearly a circle, and the two foci of the ellipseare very nearly coincident with the centre of the circle. On the otherhand, in the case of Mercury, we have an ellipse which departs from thecircle to a very marked extent, while in the orbits of some of the minorplanets the eccentricity is still greater. It is extremely remarkablethat every planet, no matter how far from the sun, should be found tomove in an ellipse of some shape or other. We shall presently show thatnecessity compels each planet to pursue an elliptic path, and that noother form of path is possible. 

Started on its elliptic path, the planet pursues its stately course, andafter a certain duration, known as the _periodic time_, regains theposition from which its departure was taken. Again the planet traces outanew the same elliptic path, and thus, revolution after revolution, anidentical track is traversed around the sun. Let us now attempt tofollow the body in its course, and observe the history of its motionduring the time requisite for the completion of one of its circuits. Thedimensions of a planetary orbit are so stupendous that the planet mustrun its course very rapidly in order to finish the journey within theallotted time. The earth, as we have already seen, has to move eighteenmiles a second to accomplish one of its voyages round the sun in thelapse of 365-1/4 days. The question then arises as to whether the rateat which a planet moves is uniform or not. Does the earth, for instance, actually move at all times with the velocity of eighteen miles asecond, or does our planet sometimes move more rapidly and sometimesmore slowly, so that the average of eighteen miles a second is stillmaintained? This is a question of very great importance, and we are ableto answer it in the clearest and most emphatic manner. The velocity of aplanet is _not_ uniform, and the variations of that velocity can beexplained by the adjoining figure (Fig. 38). 

[Illustration: Fig. 38. --Varying Velocity of Elliptic Motion. ]

Let us first of all imagine the planet to be situated at that part ofits path most distant from the sun towards the right of the figure. Inthis position the body's velocity is at its lowest; as the planet beginsto approach the sun the speed gradually improves until it attains itsmean value. After this point has been passed, and the planet is nowrapidly hurrying on towards the sun, the velocity with which it movesbecomes gradually greater and greater, until at length, as it dashesround the sun, its speed attains a maximum. After passing the sun, thedistance of the planet from the luminary increases, and the velocity ofthe motion begins to abate; gradually it declines until the mean valueis again reached, and then it falls still lower, until the body recedesto its greatest distance from the sun, by which time the velocity hasabated to the value from which we supposed it to commence. We thusobserve that the nearer the planet is to the sun the quicker it moves. We can, however, give numerical definiteness to the principle accordingto which the velocity of the planet varies. The adjoining figure (Fig. 39) shows a planetary orbit, with, of course, the sun at the focus S. Wehave taken two portions, A B and C D, round the ellipse, and joinedtheir extremities to the focus. Kepler's second law may be stated inthese words:--

 "_Every planet moves round the sun with such a velocity at every point, that a straight line drawn from it to the sun passes over equal areas in equal times. _"

[Illustration: Fig. 39. --Equal Areas in Equal Times. ]

For example, if the two shaded portions, A B S and D C S, are equal inarea, then the times occupied by the planet in travelling over theportions of the ellipse, A B and C D, are equal. If the one area begreater than the other, then the times required are in the proportion ofthe areas. 

This law being admitted, the reason of the increase in the planet'svelocity when it approaches the sun is at once apparent. To accomplish adefinite area when near the sun, a larger arc is obviously necessarythan at other parts of the path. At the opposite extremity, a small arcsuffices for a large area, and the velocity is accordingly less. 

These two laws completely prescribe the motion of a planet round thesun. The first defines the path which the planet pursues; the seconddescribes how the velocity of the body varies at different points alongits path. But Kepler added to these a third law, which enables us tocompare the movements of two different planets revolving round the samesun. Before stating this law, it is necessary to explain exactly what ismeant by the _mean_ distance of a planet. In its elliptic path thedistance from the sun to the planet is constantly changing; but it isnevertheless easy to attach a distinct meaning to that distance which isan average of all the distances. This average is called the meandistance. The simplest way of finding the mean distance is to add thegreatest of these quantities to the least, and take half the sum. Wehave already defined the periodic time of the planet; it is the numberof days which the planet requires for the completion of a journey roundits path. Kepler's third law establishes a relation between the meandistances and the periodic times of the various planets. That relationis stated in the following words:--

 "_The squares of the periodic times are proportional to the cubes of the mean distances. _"

Kepler knew that the different planets had different periodic times; healso saw that the greater the mean distance of the planet the greaterwas its periodic time, and he was determined to find out the connectionbetween the two. It was easily found that it would not be true to saythat the periodic time is merely proportional to the mean distance. Werethis the case, then if one planet had a distance twice as great asanother, the periodic time of the former would have been double that ofthe latter; observation showed, however, that the periodic time of themore distant planet exceeded twice, and was indeed nearly three times, that of the other. By repeated trials, which would have exhausted thepatience of one less confident in his own sagacity, and less assured ofthe accuracy of the observations which he sought to interpret, Kepler atlength discovered the true law, and expressed it in the form we havestated. 

To illustrate the nature of this law, we shall take for comparison theearth and the planet Venus. If we denote the mean distance of the earthfrom the sun by unity then the mean distance of Venus from the sun is0·7233. Omitting decimals beyond the first place, we can represent theperiodic time of the earth as 365·3 days, and the periodic time ofVenus as 224·7 days. Now the law which Kepler asserts is that the squareof 365·3 is to the square of 224·7 in the same proportion as unity is tothe cube of 0·7233. The reader can easily verify the truth of thisidentity by actual multiplication. It is, however, to be rememberedthat, as only four figures have been retained in the expressions of theperiodic times, so only four figures are to be considered significant inmaking the calculations. 

The most striking manner of making the verification will be to regardthe time of the revolution of Venus as an unknown quantity, and deduceit from the known revolution of the earth and the mean distance ofVenus. In this way, by assuming Kepler's law, we deduce the cube of theperiodic time by a simple proportion, and the resulting value of 224·7days can then be obtained. As a matter of fact, in the calculations ofastronomy, the distances of the planets are usually ascertained fromKepler's law. The periodic time of the planet is an element which can bemeasured with great accuracy; and once it is known, then the square ofthe mean distance, and consequently the mean distance itself, isdetermined. 

Such are the three celebrated laws of Planetary Motion, which havealways been associated with the name of their discoverer. The profoundskill by which these laws were elicited from the mass of observations, the intrinsic beauty of the laws themselves, their widespreadgenerality, and the bond of union which they have established betweenthe various members of the solar system, have given them quite anexceptional position in astronomy. 

As established by Kepler, these planetary laws were merely the resultsof observation. It was found, as a matter of fact, that the planets didmove in ellipses, but Kepler assigned no reason why they should adoptthis curve rather than any other. Still less was he able to offer areason why these bodies should sweep over equal areas in equal times, orwhy that third law was invariably obeyed. The laws as they came fromKepler's hands stood out as three independent truths; thoroughlyestablished, no doubt, but unsupported by any arguments as to why thesemovements rather than any others should be appropriate for therevolutions of the planets. 

It was the crowning triumph of the great law of universal gravitation toremove this empirical character from Kepler's laws. Newton's granddiscovery bound together the three isolated laws of Kepler into onebeautiful doctrine. He showed not only that those laws are true, but heshowed why they must be true, and why no other laws could have beentrue. He proved to demonstration in his immortal work, the "Principia, "that the explanation of the famous planetary laws was to be sought inthe attraction of gravitation. Newton set forth that a power ofattraction resided in the sun, and as a necessary consequence of thatattraction every planet must revolve in an elliptic orbit round the sun, having the sun as one focus; the radius of the planet's orbit must sweepover equal areas in equal times; and in comparing the movements of twoplanets, it was necessary to have the squares of the periodic timesproportional to the cubes of the mean distances. 

As this is not a mathematical treatise, it will be impossible for us todiscuss the proofs which Newton has given, and which have commanded theimmediate and universal acquiescence of all who have taken the troubleto understand them. We must here confine ourselves only to a very briefand general survey of the subject, which will indicate the character ofthe reasoning employed, without introducing details of a technicalcharacter. 

Let us, in the first place, endeavour to think of a globe freely poisedin space, and completely isolated from the influence of every other bodyin the universe. Let us imagine that this globe is set in motion by someimpulse which starts it forward on a rapid voyage through the realms ofspace. When the impulse ceases the globe is in motion, and continues tomove onwards. But what will be the path which it pursues? We are soaccustomed to see a stone thrown into the air moving in a curved path, that we might naturally think a body projected into free space willalso move in a curve. A little consideration will, however, show thatthe cases are very different. In the realms of free space we find noconception of upwards or downwards; all paths are alike; there is noreason why the body should swerve to the right or to the left; and hencewe are led to surmise that in these circumstances a body, once startedand freed from all interference, would move in a straight line. It istrue that this statement is one which can never be submitted to the testof direct experiment. Circumstanced as we are on the surface of theearth, we have no means of isolating a body from external forces. Theresistance of the air, as well as friction in various other forms, noless than the gravitation towards the earth itself, interfere with ourexperiments. A stone thrown along a sheet of ice will be exposed to butlittle resistance, and in this case we see that the stone will take astraight course along the frozen surface. A stone similarly cast intoempty space would pursue a course absolutely rectilinear. This wedemonstrate, not by any attempts at an experiment which wouldnecessarily be futile, but by indirect reasoning. The truth of thisprinciple can never for a moment be doubted by one who has duly weighedthe arguments which have been produced in its behalf. 

Admitting, then, the rectilinear path of the body, the next questionwhich arises relates to the velocity with which that movement isperformed. The stone gliding over the smooth ice on a frozen lake will, as everyone has observed, travel a long distance before it comes torest. There is but little friction between the ice and the stone, butstill even on ice friction is not altogether absent; and as thatfriction always tends to stop the motion, the stone will at length bebrought to rest. In a voyage through the solitudes of space, a bodyexperiences no friction; there is no tendency for the velocity to bereduced, and consequently we believe that the body could journey on forever with unabated speed. No doubt such a statement seems at variancewith our ordinary experience. A sailing ship makes no progress on thesea when the wind dies away. A train will gradually lose its velocitywhen the steam has been turned off. A humming-top will slowly expend itsrotation and come to rest. From such instances it might be plausiblyargued that when the force has ceased to act, the motion that the forcegenerated gradually wanes, and ultimately vanishes. But in all thesecases it will be found, on reflection, that the decline of the motion isto be attributed to the action of resisting forces. The sailing ship isretarded by the rubbing of the water on its sides; the train is checkedby the friction of the wheels, and by the fact that it has to force itsway through the air; and the atmospheric resistance is mainly the causeof the stopping of the humming-top, for if the air be withdrawn, bymaking the experiment in a vacuum, the top will continue to spin for agreatly lengthened period. We are thus led to admit that a body, onceprojected freely in space and acted upon by no external resistance, willcontinue to move on for ever in a straight line, and will preserveunabated to the end of time the velocity with which it originallystarted. This principle is known as the _first law of motion_. 

Let us apply this principle to the important question of the movement ofthe planets. Take, for instance, the case of our earth, and let usdiscuss the consequences of the first law of motion. We know that theearth is moving each moment with a velocity of about eighteen miles asecond, and the first law of motion assures us that if this globe weresubmitted to no external force, it would for ever pursue a straighttrack through the universe, nor would it depart from the precisevelocity which it possesses at the present moment. But is the earthmoving in this manner? Obviously not. We have already found that ourglobe is moving round the sun, and the comprehensive laws of Kepler havegiven to that motion the most perfect distinctness and precision. Theconsequence is irresistible. The earth cannot be free from externalforce. Some potent influence on our globe must be in ceaseless action. That influence, whatever it may be, constantly deflects the earth fromthe rectilinear path which it tends to pursue, and constrains it totrace out an ellipse instead of a straight line. 

The great problem to be solved is now easily stated. There must be someexternal agent constantly influencing the earth. What is that agent, whence does it proceed, and to what laws is it submitted? Nor is thequestion confined to the earth. Mercury and Venus, Mars, Jupiter, andSaturn, unmistakably show that, as they are not moving in rectilinearpaths, they must be exposed to some force. What is this force whichguides the planets in their paths? Before the time of Newton thisquestion might have been asked in vain. It was the splendid genius ofNewton which supplied the answer, and thus revolutionised the whole ofmodern science. 

The data from which the question is to be answered must be obtained fromobservation. We have here no problem which can be solved by meremathematical meditation. Mathematics is no doubt a useful, indeed, anindispensable, instrument in the enquiry; but we must not attribute tomathematics a potency which it does not possess. In a case of this kind, all that mathematics can do is to interpret the results obtained byobservation. The data from which Newton proceeded were the observedphenomena in the movement of the earth and the other planets. Thosefacts had found a succinct expression by the aid of Kepler's laws. Itwas, accordingly, the laws of Kepler which Newton took as the basis ofhis labours, and it was for the interpretation of Kepler's laws thatNewton invoked the aid of that celebrated mathematical reasoning whichhe created. 

The question is then to be approached in this way: A planet beingsubject to _some_ external influence, we have to determine what thatinfluence is, from our knowledge that the path of each planet is anellipse, and that each planet sweeps round the sun over equal areas inequal times. The influence on each planet is what a mathematician wouldcall a force, and a force must have a line of direction. The most simpleconception of a force is that of a pull communicated along a rope, andthe direction of the rope is in this case the direction of the force. Let us imagine that the force exerted on each planet is imparted by aninvisible rope. Kepler's laws will inform us with regard to thedirection of this rope and the intensity of the strain transmittedthrough it. 

The mathematical analysis of Kepler's laws would be beyond the scope ofthis volume. We must, therefore, confine ourselves to the results towhich they lead, and omit the details of the reasoning. Newton firsttook the law which asserted that the planet moved over equal areas inequal times, and he showed by unimpeachable logic that this at once gavethe direction in which the force acted on the planet. He showed that theimaginary rope by which the planet is controlled must be invariablydirected towards the sun. In other words, the force exerted on eachplanet was at all times pointed from the planet towards the sun. 

It still remained to explain the intensity of the force, and to show howthe intensity of that force varied when the planet was at differentpoints of its path. Kepler's first law enables this question to beanswered. If the planet's path be elliptic, and if the force be alwaysdirected towards the sun at one focus of that ellipse, then mathematicalanalysis obliges us to say that the intensity of the force must varyinversely as the square of the distance from the planet to the sun. 

The movements of the planets, in conformity with Kepler's laws, wouldthus be accounted for even in their minutest details, if we admit thatan attractive power draws the planet towards the sun, and that theintensity of this attraction varies inversely as the square of thedistance. Can we hesitate to say that such an attraction does exist? Wehave seen how the earth attracts a falling body; we have seen how theearth's attraction extends to the moon, and explains the revolution ofthe moon around the earth. We have now learned that the movement of theplanets round the sun can also be explained as a consequence of this lawof attraction. But the evidence in support of the law of universalgravitation is, in truth, much stronger than any we have yet presented. We shall have occasion to dwell on this matter further on. We shall shownot only how the sun attracts the planets, but how the planets attracteach other; and we shall find how this mutual attraction of the planetshas led to remarkable discoveries, which have elevated the law ofgravitation beyond the possibility of doubt. 

Admitting the existence of this law, we can show that the planets mustrevolve around the sun in elliptic paths with the sun in the commonfocus. We can show that they must sweep over equal areas in equal times. We can prove that the squares of the periodic times must be proportionalto the cubes of their mean distances. Still further, we can show how themysterious movements of comets can be accounted for. By the same greatlaw we can explain the revolutions of the satellites. We can account forthe tides, and for other phenomena throughout the Solar System. Finally, we shall show that when we extend our view beyond the limits of ourSolar System to the beautiful starry systems scattered through space, wefind even there evidence of the great law of universal gravitation. 

CHAPTER VI. 

THE PLANET OF ROMANCE. 

 Outline of the Subject--Is Mercury the Planet nearest the Sun?--Transit of an Interior Planet across the Sun--Has a Transit of Vulcan ever been seen?--Visibility of Planets during a Total Eclipse of the Sun--Professor Watson's Researches in 1878. 

Provided with a general survey of the Solar System, and with such anoutline of the law of universal gravitation as the last chapter hasafforded us, we commence the more detailed examination of the planetsand their satellites. We shall begin with the planets nearest to thesun, and then we shall gradually proceed outwards to one planet afteranother, until we reach the confines of the system. We shall find muchto occupy our attention. Each planet is itself a globe, and it will befor us to describe as much as is known of it. The satellites by which somany of the planets are accompanied possess many points of interest. Thecircumstances of their discovery, their sizes, their movements, andtheir distances must be duly considered. It will also be found that themovements of the planets present much matter for reflection andexamination. We shall have occasion to show how the planets mutuallydisturb each other, and what remarkable consequences have arisen fromthese influences. We must also occasionally refer to the importantproblems of celestial measuring and celestial weighing. We must show howthe sizes, the weights, and the distances of the various members of oursystem are to be discovered. The greater part of our task willfortunately lead us over ground which is thoroughly certain, and wherethe results have been confirmed by frequent observation. It happens, however, that at the very outset of our course we are obliged to dealwith observations which are far from certain. The existence of a planetmuch closer to the sun than those hitherto known has been asserted bycompetent authority. The question is still unsettled, but the planetcannot at present be found. Hence it is that we have called the subjectof this chapter, The Planet of Romance. 

It had often been thought that Mercury, long supposed to be the nearestplanet to the sun, was perhaps not really the body entitled to thatdistinction. Mercury revolves round the sun at an average distance ofabout 36, 000, 000 miles. In the interval between it and the sun theremight have been one or many other planets. There might have been onerevolving at ten million miles, another at fifteen, and so on. But didsuch planets exist? Did even one planet revolve inside the orbit ofMercury? There were certain reasons for believing in such a planet. Inthe movements of Mercury indications were perceptible of an influencethat it was at one time thought might have been accounted for by thesupposition of an interior planet. [13] But there was necessarily a greatdifficulty about seeing this object. It must always be close to the sun, and even in the best telescope it is generally impossible to see astar-like point in that position. Nor could such a planet be seen aftersunset, for under the most favourable conditions it would set almostimmediately after the sun, and a like difficulty would make it invisibleat sunrise. 

Our ordinary means of observing a planet have therefore completelyfailed. We are compelled to resort to extraordinary methods if we wouldseek to settle the great question as to the existence of theintra-Mercurial planets. There are at least two lines of observationwhich might be expected to answer our purpose. 

An opportunity for the first would arise when it happened that theunknown planet came directly between the earth and the sun. In thediagram (Fig. 40) we show the sun at the centre; the internal dottedcircle denotes the orbit of the unknown planet, which has received thename of Vulcan before even its very existence has been at allsatisfactorily established. The outer circle denotes the orbit of theearth. As Vulcan moves more rapidly than the earth, it will frequentlyhappen that the planet will overtake the earth, so that the three bodieswill have the positions represented in the diagram. It would not, however, necessarily follow that Vulcan was exactly between the earthand the luminary. The path of the planet may be tilted, so that, as seenfrom the earth, Vulcan would be over or under the sun, according tocircumstances. 

If, however, Vulcan really does exist, we might expect that sometimesthe three bodies will be directly in line, and this would then give thedesired opportunity of making the telescopic discovery of the planet. Weshould expect on such an occasion to observe the planet as a dark spot, moving slowly across the face of the sun. The two other planets interiorto the earth, namely, Mercury and Venus, are occasionally seen in theact of transit; and there cannot be a doubt that if Vulcan exists, itstransits across the sun must be more numerous than those of Mercury, andfar more numerous than those of Venus. On the other hand, it mayreasonably be anticipated that Vulcan is a small globe, and as it willbe much more distant from us than Mercury at the time of its transit, wecould not expect that the transit of the planet of romance would be atall comparable as a spectacle with those of either of the two otherbodies we have named. 

The question arises as to whether telescopic research has ever disclosedanything which can be regarded as a transit of Vulcan. On this point itis not possible to speak with any certainty. It has, on more than oneoccasion, been asserted by observers that a spot has been seentraversing the sun, and from its shape and general appearance they havepresumed it to have been an intra-Mercurial planet. But a closeexamination of the circumstances in which such observations have beenmade has not tended to increase confidence in this presumption. Suchdiscoveries have usually been made by persons little familiar withtelescopic observations. It is certainly a significant fact that, notwithstanding the diligent scrutiny to which the sun has beensubjected during the past century by astronomers who have speciallydevoted themselves to this branch of research, no telescopic discoveryof Vulcan on the sun has been announced by any really experiencedastronomer. The last announcement of a planet having crossed the sundates from 1876, and was made by a German amateur, but what he thoughtto have been a planet was promptly shown to have been a small sun-spot, which had been photographed at Greenwich in the course of the dailyroutine work, and had also been observed at Madrid. From an examinationof the whole subject, we are inclined to believe that there is not atthis moment any reliable telescopic evidence of the transit of anintra-Mercurial planet over the face of the central luminary. 

[Illustration: Fig. 40. --The Transit of the Planet of Romance. ]

But there is still another method by which we might reasonably hope todetect new planets in the vicinity of the sun. This method is, however, but seldom available. It is only possible when the sun is totallyeclipsed. 

When the moon is interposed directly between the earth and the sun, thebrightness of day is temporarily exchanged for the gloom of night. Ifthe sky be free from clouds the stars spring forth, and can be seenaround the obscured sun. Even if a planet were quite close to theluminary it would be visible on such an occasion if its magnitude werecomparable with that of Mercury. Careful preparation is necessary whenit is proposed to make a trial of this kind. The danger to be speciallyavoided is that of confounding the planet with the ordinary stars, whichit will probably resemble. The late distinguished American astronomer, Professor Watson, specially prepared to devote himself to this researchduring the notable total eclipse in 1878. When the eclipse occurred thelight of the sun vanished and the stars burst forth. Among themProfessor Watson saw an object which to him seemed to be the long-soughtintra-Mercurial planet. We should add that this zealous observer sawanother object which he at first took to be the star known as Zeta inthe constellation Cancer. When he afterwards found that the recordedplace of this object did not agree so well as he expected with the knownposition of this star, he came to the conclusion that it could not beZeta but must be some other unknown planet. The relative positions ofthe two objects which he took to be planets agree, however, sufficientlywell, considering the difficulties of the observation, with the relativepositions of the stars Theta and Zeta Cancri, and it can now hardly bedoubted that Watson merely saw these two stars. He maintained, however, that he had noticed Theta Cancri as well as the two planets, but withoutrecording its position. There is, however, a third star, known as 20Cancri, near the same place, and this Watson probably mistook for Theta. It is necessary to record that Vulcan has not been observed, thoughspecially looked for, during the eclipses which have occurred since1878, and it is accordingly the general belief among astronomers that noplanet has yet been detected within the orbit of Mercury. 

CHAPTER VII. 

MERCURY. 

 The Ancient Astronomical Discoveries--How Mercury was first found--Not easily seen--Mercury was known from the earliest ages--Skill necessary in the Discovery--The Distinction of Mercury from a Star--Mercury in the East and in the West--The Prediction--How to Observe Mercury--Its Telescopic Appearance--Difficulty of Observing its Appearance--Orbit of Mercury--Velocity of the Planet--Can there be Life on the Planet?--Changes in its Temperature--Transit of Mercury over the Sun--Gassendi's Observations--Rotation of Mercury--The Weight of Mercury. 

Long and glorious is the record of astronomical discovery. Thediscoveries of modern days have succeeded each other with such rapidity, they have so often dazzled our imaginations with their brilliancy, thatwe are sometimes apt to think that astronomical discovery is a purelymodern product. But no idea could be more fundamentally wrong. While weappreciate to the utmost the achievements of modern times, let usendeavour to do justice to the labours of the astronomers of antiquity. 

And when we speak of the astronomers of antiquity, let us understandclearly what is meant. The science is now growing so rapidly that eachcentury witnesses a surprising advance; each generation, each decade, each year, has its own rewards for those diligent astronomers by whomthe heavens are so carefully scanned. We must, however, project ourglance to a remote epoch in time past, if we would view the memorablediscovery of Mercury. Compared with it, the discoveries of Newton are tobe regarded as very modern achievements; even the announcement of theCopernican system of the heavens is itself a recent event in comparisonwith the detection of this planet now to be discussed. 

By whom was this great discovery made? Let us see if the question canbe answered by the examination of astronomical records. At the close ofhis memorable life Copernicus was heard to express his sincere regretthat he never enjoyed an opportunity of beholding the planet Mercury. Hehad specially longed to see this body, the movements of which were tosuch a marked extent illustrative of the theory of the celestial motionswhich it was his immortal glory to have established, but he had neverbeen successful. Mercury is not generally to be seen so easily as aresome of the other planets, and it may well have been that the vapoursfrom the immense lagoon at the mouth of the Vistula obscured the horizonat Frauenburg, where Copernicus dwelt, and thus his opportunities ofviewing Mercury were probably even rarer than they are at other places. 

The existence of Mercury was certainly quite a familiar fact in the timeof Copernicus, and therefore we must look to some earlier epoch for itsdiscovery. In the scanty astronomical literature of the Middle Ages wefind occasional references to the existence of this object. We can traceobservations of Mercury through remote centuries to the commencement ofour era. Records from dates still earlier are not wanting, until atlength we come on an observation which has descended to us for more than2, 000 years, having been made in the year 265 before the Christian era. It is not pretended, however, that this observation records the_discovery_ of the planet. Earlier still we find the chief of theastronomers at Nineveh alluding to Mercury in a report which he made toAssurbanipal, the King of Assyria. It does not appear in the leastdegree likely that the discovery was even then a recent one. It may havebeen that the planet was independently discovered in two or morelocalities, but all records of such discoveries are totally wanting; andwe are ignorant alike of the names of the discoverers, of the nations towhich they belonged, and of the epochs at which they lived. 

Although this discovery is of such vast antiquity, although it was madebefore correct notions were entertained as to the true system of theuniverse, and, it is needless to add, long before the invention of thetelescope, yet it must not be assumed that the detection of Mercury wasby any means a simple or obvious matter. This will be manifest when wetry to conceive the manner in which the discovery must probably havebeen made. 

Some primæval astronomer, long familiar with the heavens, had learned torecognise the various stars and constellations. Experience had impressedupon him the permanence of these objects; he had seen that Siriusinvariably appeared at the same seasons of the year, and he had noticedhow it was placed with regard to Orion and the other neighbouringconstellations. In the same manner each of the other bright stars was tohim a familiar object always to be found in a particular region of theheavens. He saw how the stars rose and set in such a way, that thougheach star appeared to move, yet the relative positions of the stars wereincapable of alteration. No doubt this ancient astronomer was acquaintedwith Venus; he knew the evening star; he knew the morning star; and hemay have concluded that Venus was a body which oscillated from one sideof the sun to the other. 

We can easily imagine how the discovery of Mercury was made in the clearskies over an Eastern desert. The sun has set, the brief twilight hasalmost ceased, when lo, near that part of the horizon where the glow ofthe setting sun still illuminates the sky, a bright star is seen. Theprimæval astronomer knows that there is no bright star at this place inthe heavens. If the object of his attention be not a star, what, then, can it be? Eager to examine this question, the heavens are watched nextnight, and there again, higher above the horizon, and more brilliantstill, is the object seen the night before. Each successive night thebody gains more and more lustre, until at length it becomes aconspicuous gem. Perhaps it will rise still higher and higher; perhapsit will increase till it attains the brilliancy of Venus itself. Suchwere the surmises not improbably made by those who first watched thisobject; but they were not realised. After a few nights of exceptionalsplendour the lustre of this mysterious orb declines. The planet againdraws near the horizon at sunset, until at length it sets so soon afterthe sun that it has become invisible. Is it lost for ever? Years mayelapse before another opportunity of observing the object after sunsetmay be available; but then again it will be seen to run through the sameseries of changes, though, perhaps, under very different circumstances. The greatest height above the horizon and the greatest brightness bothvary considerably. 

Long and careful observations must have been made before the primævalastronomer could assure himself that the various appearances might allbe attributed to a single body. In the Eastern deserts the phenomena ofsunrise must have been nearly as familiar as those of sunset, and in theclear skies, at the point where the sunbeams were commencing to dawnabove the horizon, a bright star-like point might sometimes beperceived. Each successive day this object rose higher and higher abovethe horizon before the moment of sunrise, and its lustre increased withthe distance; then again it would draw in towards the sun, and returnfor many months to invisibility. Such were the data which were presentedto the mind of the primitive astronomer. One body was seen after sunset, another body was seen before sunrise. To us it may seem an obviousinference from the observed facts that the two bodies were identical. The inference is a correct one, but it is in no sense an obvious one. Long and patient observation established the remarkable law that one ofthese bodies was never seen until the other had disappeared. Hence itwas inferred that the phenomena, both at sunrise and at sunset, were dueto the same body, which oscillated to and fro about the sun. 

We can easily imagine that the announcement of the identity of these twoobjects was one which would have to be carefully tested before it couldbe accepted. How are the tests to be applied in a case of this kind?There can hardly be a doubt that the most complete and convincingdemonstration of scientific truth is found in the fulfilment ofprediction. When Mercury had been observed for years, a certainregularity in the recurrence of its visibility was noticed. Once aperiodicity had been fully established, prediction became possible. Thetime when Mercury would be seen after sunset, the time when it would beseen before sunrise, could be foretold with accuracy! When it was foundthat these predictions were obeyed to the letter--that the planet wasalways seen when looked for in accordance with the predictions--it wasimpossible to refuse assent to the hypothesis on which these predictionswere based. Underlying that hypothesis was the assumption that all thevarious appearances arose from the oscillations of a single body, andhence the discovery of Mercury was established on a basis as firm as thediscovery of Jupiter or of Venus. 

In the latitudes of the British Islands it is generally possible to seeMercury some time during the course of the year. It is not practicableto lay down, within reasonable limits, any general rule for finding thedates at which the search should be made; but the student who isdetermined to see the planet will generally succeed with a littlepatience. He must first consult an almanac which gives the positions ofthe body, and select an occasion when Mercury is stated to be an eveningor a morning star. Such an occasion during the spring months isespecially suitable, as the elevation of Mercury above the horizon isusually greater then than at other seasons; and in the evening twilight, about three-quarters of an hour after sunset, a view of this shy butbeautiful object will reward the observer's attention. 

To those astronomers who are provided with equatorial telescopes suchinstructions are unnecessary. To enjoy a telescopic view of Mercury, wefirst turn to the Nautical Almanac, and find the position in which theplanet lies. If it happen to be above the horizon, we can at once directthe telescope to the place, and even in broad daylight the planet willvery often be seen. The telescopic appearance of Mercury is, however, disappointing. Though it is much larger than the moon, yet it issufficiently far off to seem insignificant. There is, however, onefeature in a view of this planet which would immediately attractattention. Mercury is not usually observed to be a circular object, butmore or less crescent-shaped, like a miniature moon. The phases of theplanet are also to be accounted for on exactly the same principles asthe phases of the moon. Mercury is a globe composed, like our earth, ofmaterials possessing in themselves no source of illumination. Onehemisphere of the planet must necessarily be turned towards the sun, andthis side is accordingly lighted up brilliantly by the solar rays. Whenwe look at Mercury we see nothing of the non-illuminated side, and thecrescent is due to the foreshortened view which we obtain of theilluminated part. The planet is such a small object that, in the glitterof the naked-eye view, the _shape_ of the luminous body cannot bedefined. Indeed, even in the much larger crescent of Venus, the aid ofthe telescope has to be invoked before the characteristic form can beobserved. Beyond, however, the fact that Mercury is a crescent, and thatit undergoes varying phases in correspondence with the changes in itsrelative position to the earth and the sun, we cannot see much of theplanet. It is too small and too bright to admit of easy delineation ofdetails on its surface. No doubt attempts have been made, andobservations have been recorded, as to certain very faint and indistinctmarkings on the planet, but such statements must be received with greathesitation. 

[Illustration: Fig. 41. --The Movement of Mercury, showing the Variationsin Phase and in apparent size. ]

[Illustration: Fig. 42. --Mercury as a Crescent. ]

The facts which have been thoroughly established with regard to Mercuryare mainly numerical statements as to the path it describes around thesun. The time taken by the planet to complete one of its revolutions iseighty-eight days nearly. The average distance from the sun is about36, 000, 000 miles, and the mean velocity with which the body moves isover twenty-nine miles a second. We have already alluded to the mostcharacteristic and remarkable feature of the orbit of Mercury. Thatorbit differs from the paths of all the other large planets by its muchgreater departure from the circular form. In the majority of cases theplanetary orbits are so little elliptic that a diagram of the orbitdrawn accurately to scale would not be perceived to differ from a circleunless careful measurements were made. In the case of Mercury thecircumstances are different. The elliptic form of the path would bequite unmistakable by the most casual observer. The distance from thesun to the planet fluctuates between very considerable limits. Thelowest value it can attain is about 30, 000, 000 miles; the highest valueis about 43, 000, 000 miles. In accordance with Kepler's second law, thevelocity of the planet must exhibit corresponding changes. It must sweeprapidly around that part of his path near the sun, and more slowly roundthe remote parts of his path. The greatest velocity is aboutthirty-five miles a second, and the least is twenty-three miles asecond. 

For an adequate conception of the movements of Mercury we ought not todissociate the velocity from the true dimensions of the body by which itis performed. No doubt a speed of twenty-nine miles a second is enormouswhen compared with the velocities with which daily life makes usfamiliar. The speed of the planet is not less than a hundred times asgreat as the velocity of the rifle bullet. But when we compare the sizesof the bodies with their velocities, the velocity of Mercury seemsrelatively much less than that of the bullet. A rifle bullet traverses adistance equal to its own diameter many thousands of times in a second. But even though Mercury is moving so much faster, yet the dimensions ofthe planet are so considerable that a period of two minutes will berequired for it to move through a distance equal to its diameter. Viewing the globe of the planet as a whole, the velocity of its movementis but a stately and dignified progress appropriate to its dimensions. 

As we can learn little or nothing of the true surface of Mercury, it isutterly impossible for us to say whether life can exist on the surfaceof that planet. We may, however, reasonably conclude that there cannotbe life on Mercury in any respect analogous to the life which we know onthe earth. The heat of the sun and the light of the sun beat down onMercury with an intensity many times greater than that which weexperience. When this planet is at its utmost distance from the sun theintensity of solar radiation is even then more than four times greaterthan the greatest heat which ever reaches the earth. But when Mercury, in the course of its remarkable changes of distance, draws in to thewarmest part of its orbit, it is exposed to a terrific scorching. Theintensity of the sun's heat must then be not less than nine times asgreat as the greatest radiation to which we are exposed. 

These tremendous climatic changes succeed each other much more rapidlythan do the variations of our seasons. On Mercury the interval betweenmidsummer and midwinter is only forty-four days, while the whole yearis only eighty-eight days. Such rapid variations in solar heat must inthemselves exercise a profound effect on the habitability of Mercury. Mr. Ledger well remarks, in his interesting work, [14] that if there beinhabitants on Mercury the notions of "perihelion" and "aphelion, " whichare here often regarded as expressing ideas of an intricate or reconditecharacter, must on the surface of that planet be familiar to everybody. The words imply "near the sun, " and "away from the sun;" but we do notassociate these expressions with any obvious phenomena, because thechanges in the distance of the earth from the sun are inconsiderable. But on Mercury, where in six weeks the sun rises to more than double hisapparent size, and gives more than double the quantity of light and ofheat, such changes as are signified by perihelion and aphelion embodyideas obviously and intimately connected with the whole economy of theplanet. 

It is nevertheless rash to found any inferences as to climate merelyupon the proximity or the remoteness of the sun. Climate depends uponother matters besides the sun's distance. The atmosphere surrounding theearth has a profound influence on heat and cold, and if Mercury have anatmosphere--as has often been supposed--its climate may be therebymodified to any necessary extent. It seems, however, hardly possible tosuppose that any atmosphere could form an adequate protection for theinhabitants from the violent and rapid fluctuations of solar radiation. All we can say is, that the problem of life in Mercury belongs to theclass of unsolved, and perhaps unsolvable, mysteries. 

It was in the year 1629 that Kepler made an important announcement as toimpending astronomical events. He had been studying profoundly themovements of the planets; and from his study of the past he had venturedto predict the future. Kepler announced that in the year 1631 theplanets Venus and Mercury would both make a transit across the sun, andhe assigned the dates to be November 7th for Mercury, and December 6thfor Venus. This was at the time a very remarkable prediction. We are soaccustomed to turn to our almanacs and learn from them all theastronomical phenomena which are anticipated during the year, that weare apt to forget how in early times this was impossible. It has onlybeen by slow degrees that astronomy has been rendered so perfect as toenable us to foretell, with accuracy, the occurrence of the moredelicate phenomena. The prediction of those transits by Kepler, someyears before they occurred, was justly regarded at the time as a mostremarkable achievement. 

The illustrious Gassendi prepared to apply the test of actualobservation to the announcements of Kepler. We can now assign the timeof the transit accurately to within a few minutes, but in those earlyattempts equal precision was not practicable. Gassendi considered itnecessary to commence watching for the transit of Mercury two whole daysbefore the time indicated by Kepler, and he had arranged an ingeniousplan for making his observations. The light of the sun was admitted intoa darkened room through a hole in the shutter, and an image of the sunwas formed on a white screen by a lens. This is, indeed, an admirableand a very pleasing way of studying the surface of the sun, and even atthe present day, with our best telescopes, one of the methods of viewingour luminary is founded on the same principle. 

Gassendi commenced his watch on the 5th of November, and carefullystudied the sun's image at every available opportunity. It was not, however, until five hours after the time assigned by Kepler that thetransit of Mercury actually commenced. Gassendi's preparations had beenmade with all the resources which he could command, but these resourcesseem very imperfect when compared with the appliances of our modernobservatories. He was anxious to note the time when the planet appeared, and for this purpose he had stationed an assistant in the room beneath, who was to observe the altitude of the sun at the moment indicated byGassendi. The signal to the assistant was to be conveyed by a veryprimitive apparatus. Gassendi was to stamp on the floor when thecritical moment had arrived. In spite of the long delay, whichexhausted the patience of the assistant, some valuable observations wereobtained, and thus the first passage of a planet across the sun wasobserved. 

The transits of Mercury are not rare phenomena (there have been thirteenof them during the nineteenth century), and they are chiefly ofimportance on account of the accuracy which their observation infusesinto our calculations of the movements of the planet. It has often beenhoped that the opportunities afforded by a transit would be availablefor procuring information as to the physical character of the globe ofMercury, but these hopes have not been realised. 

Spectroscopic observations of Mercury are but scanty. They seem toindicate that water vapour is a probable constituent in the atmosphereof Mercury, as it is in our own. 

A distinguished Italian astronomer, Professor Schiaparelli, some yearsago announced a remarkable discovery with respect to the rotation of theplanet Mercury. He found that the planet rotates on its axis in the sameperiod as it revolves around the sun. The practical consequence of theidentity between these two periods is that Mercury always turns the sameface to the sun. If our earth were to rotate in a similar fashion, thenthe hemisphere directed to the sun would enjoy eternal day, while theopposite hemisphere would be relegated to perpetual night. According tothis discovery, Mercury revolves around the sun in the same way as themoon revolves around the earth. As the velocity with which Mercurytravels round the sun is very variable, owing to the highly ellipticshape of its orbit, while the rotation about its axis is performed withuniform speed, it follows that rather more than a hemisphere (aboutfive-eighths of the surface) enjoys more or less the light of the sun inthe course of a Mercurial year. 

This important discovery of Schiaparelli has lately been confirmed by anAmerican astronomer, Mr. Lowell, of Arizona, U. S. A. , who observed theplanet under very favourable conditions with a refractor of twenty-fourinches aperture. He has detected on the globe of Mercury certain narrow, dark lines, the very slow shifting of which points to a period ofrotation about its axis exactly coincident with the period ofrevolution round the sun. The same observer shows that the axis ofrotation of Mercury is perpendicular to the plane of the orbit. Mr. Lowell has perceived no sign of clouds or obscurations, and indeed noindication of any atmospheric envelope; the surface of Mercury iscolourless, "a geography in black and white. "

We may assert that, there is a strong _à priori_ probability in favourof the reality of Schiaparelli's discovery. Mercury, being one of theplanets devoid of a moon, will be solely influenced by the sun in so faras tidal phenomena are concerned. Owing, moreover, to the proximity ofMercury to the sun, the solar tides on that planet possess an especialvehemence. As the tendency of tides is to make Mercury present aconstant face to the sun, there need be little hesitation in acceptingtestimony that tides have wrought exactly the result that we know theywere competent to perform. 

Here we take leave of the planet Mercury--an interesting and beautifulobject, which stimulates our intellectual curiosity, while at the sametime it eludes our attempts to make a closer acquaintance. There is, however, one point of attainable knowledge which we must mention inconclusion. It is a difficult, but not by any means an impossible, taskto weigh Mercury in the celestial balance, and determine his mass incomparison with the other globes of our system. This is a delicateoperation, but it leads us through some of the most interesting paths ofastronomical discovery. The weight of the planet, as recently determinedby Von Asten, is about one twenty-fourth part of the weight of theearth, but the result is more uncertain than the determinations of themass of any of the other larger planets. 

CHAPTER VIII. 

VENUS. 

 Interest attaching to this Planet--The Unexpectedness of its Appearance--The Evening Star--Visibility in Daylight--Lighted only by the Sun--The Phases of Venus--Why the Crescent is not Visible to the Unaided Eye--Variations in the Apparent Size of the Planet--The Rotation of Venus--Resemblance of Venus to the Earth--The Transit of Venus--Why of such Especial Interest--The Scale of the Solar System--Orbits of the Earth and Venus not in the same Plane--Recurrence of the Transits in Pairs--Appearance of Venus in Transit--Transits of 1874 and 1882--The Early Transits of 1631 and 1639--The Observations of Horrocks and Crabtree--The Announcement of Halley--How the Track of the Planet differs from Different Places--Illustrations of Parallax--Voyage to Otaheite--The Result of Encke--Probable Value of the Sun's Distance--Observations at Dunsink of the Last Transit of Venus--The Question of an Atmosphere to Venus--Other Determinations of the Sun's Distance--Statistics about Venus. 

It might, for one reason, have been not inappropriate to have commencedour review of the planetary system by the description of the planetVenus. This body is not especially remarkable for its size, for thereare other planets hundreds of times larger. The orbit of Venus is nodoubt larger than that of Mercury, but it is much smaller than that ofthe outer planets. Venus has not even the splendid retinue of minorattendants which gives such dignity and such interest to the mightyplanets of our system. Yet the fact still remains that Venus is peerlessamong the planetary host. We speak not now of celestial bodies only seenin the telescope; we refer to the ordinary observation which detectedVenus ages before telescopes were invented. 

Who has not been delighted with the view of this glorious object? It isnot to be seen at all times. For months together the star of evening ishidden from mortal gaze. Its beauties are even enhanced by the capriceand the uncertainty which attend its appearance. We do not say thatthere is any caprice in the movements of Venus, as known to those whodiligently consult their almanacs. The movements of the lovely planetare there prescribed with a prosaic detail hardly in harmony with thecharacter usually ascribed to the Goddess of Love. But to those who donot devote particular attention to the stars, the very unexpectedness ofits appearance is one of its greatest charms. Venus has not beennoticed, not been thought of, for many months. It is a beautifully clearevening; the sun has just set. The lover of nature turns to admire thesunset, as every lover of nature will. In the golden glory of the west abeauteous gem is seen to glitter; it is the evening star--the planetVenus. A few weeks later another beautiful sunset is seen, and now theplanet is no longer a point low down in the western glow; it has risenhigh above the horizon, and continues a brilliant object long after theshades of night have descended. Again, a little later, and Venus hasgained its full brilliancy and splendour. All the heavenly host--evenSirius and even Jupiter--must pale before the splendid lustre of Venus, the unrivalled queen of the firmament. 

After weeks of splendour, the height of Venus at sunset diminishes, andits lustre begins gradually to decline. It sinks to invisibility, and isforgotten by the great majority of mankind; but the capricious goddesshas only moved from one side of the sky to the other. Ere the sun rises, the morning star will be seen in the east. Its splendour graduallyaugments until it rivals the beauty of the evening star. Then again theplanet draws near to the sun, and remains lost to view for many months, until the same cycle of changes recommences, after an interval of a yearand seven months. 

When Venus is at its brightest it can be easily seen in broad daylightwith the unaided eye. This striking spectacle proclaims in anunmistakable manner the unrivalled supremacy of this planet as comparedwith its fellow-planets and with the fixed stars. Indeed, at this timeVenus is from forty to sixty times more brilliant than any stellarobject in the northern heavens. 

The beautiful evening star is often such a very conspicuous object thatit may seem difficult at first to realise that the body is notself-luminous. Yet it is impossible to doubt that the planet is reallyonly a dark globe, and to that extent resembles our own earth. Thebrilliance of the planet is not so very much greater than that of theearth on a sunshiny day. The splendour of Venus entirely arises from thereflected light of the sun, in the manner already explained with respectto the moon. 

We cannot distinguish the characteristic crescent shape of the planetwith the unaided eye, which merely shows a brilliant point too small topossess sensible form. This is to be explained on physiological grounds. The optical contrivances in the eye form an image of the planet on theretina which is necessarily very small. Even when Venus is nearest tothe earth the diameter of the planet subtends an angle not much morethan one minute of arc. On the delicate membrane a picture of Venus isthus drawn about one six-thousandth part of an inch in diameter. Greatas may be the delicacy of the retina, it is not adequate to theperception of form in a picture so minute. The nervous structure, whichhas been described as the source of vision, forms too coarse a canvasfor the reception of the details of this tiny picture. Hence it is thatto the unaided eye the brilliant Venus appears merely as a bright spot. Ordinary vision cannot tell what shape it has; still less can it revealthe true beauty of the crescent. 

If the diameter of Venus were several times as great as it actually is;were this body, for instance, as large as Jupiter or some of the othergreat planets, then its crescent could be readily discerned by theunaided eye. It is curious to speculate on what might have been thehistory of astronomy had Venus only been as large as Jupiter. Wereeveryone able to see the crescent form without a telescope, it wouldthen have been an elementary and almost obvious truth that Venus must bea dark body revolving round the sun. The analogy between Venus and ourearth would have been at once perceived; and the doctrine which was leftto be discovered by Copernicus in comparatively modern times might notimprobably have been handed down to us with the other discoveries whichhave come from the ancient nations of the East. 

[Illustration: Fig. 43. Venus, May 29th, 1889. ]

Perhaps the most perfect drawing of Venus that has been hithertoobtained is that made (Fig. 43) by Professor E. E. Barnard, on 29th May, 1889, with a 12-inch equatorial, at the Lick Observatory, which for thispurpose and on this occasion Professor Barnard found to be superior tothe 36-inch. The markings shown seem undoubtedly to exist on the planet, and in 1897 Professor Barnard writes: "The circumstances under whichthis drawing was made are memorable with me, for I never afterwards hadsuch perfect conditions to observe Venus. "

In Fig. 44 we show three views of Venus under different aspects. Theplanet is so much closer to the earth when the crescent is seen, that itappears to be part of a much larger circle than that made by Venus whenmore nearly full. This drawing shows the different aspects of the globein their true relative proportions. It is very difficult to perceivedistinctly any markings on the brilliantly lighted surface. Sometimesobservers have seen spots or other features, and occasionally thepointed extremities of the horns have been irregular, as if to show thatthe surface of Venus is not smooth. Some observers report having seenwhite spots at the poles of Venus, in some degree resembling the moreconspicuous features of the same character to be seen on Mars. 

[Illustration: Fig. 44. --Different Aspects of Venus in the Telescope. ]

As it is so very difficult to see any markings on Venus, we are hardlyyet able to give a definite answer to the important question as to theperiod of rotation of this planet round its axis. Various observersduring the last two hundred years have from very insufficient dataconcluded that Venus rotated in about twenty-three hours. Schiaparelli, of Milan, turned his attention to this planet in 1877 and noticed a darkshade and two bright spots, all situated not far from the southern endof the crescent. This most painstaking astronomer watched thesemarkings for three months, and found that there was no changeperceptible in the position which they occupied. This was particularlythe case when he continued his watch for some consecutive hours. Thisfact seemed to show conclusively that Venus could not rotate intwenty-three hours nor in any other short period. Week after week thespots remained unaltered, until Schiaparelli felt convinced that hisobservations could only be reconciled with a period of rotation betweensix and nine months. He naturally concluded that the period was 225days--that is to say, the period which Venus takes to complete onerevolution round the sun; in other words, Venus always turns the sameface to the sun. 

This remarkable result was confirmed by observations made at Nice; butit has been vigorously assailed by several observers, who maintain thattheir own drawings can only agree with a period about equal to that ofthe rotation of our own earth. Schiaparelli's result is, however, wellsupported by the letters of Mr. Lowell. He has published a number ofdrawings of Venus made with his 24-inch refractor, and he finds that therotation is performed in the same time as the planet's orbitalrevolution, the axis of rotation being perpendicular to the plane of theorbit. The markings seen by Mr. Lowell were long and streaky, and theywere always visible whenever his own atmospheric conditions were fairlygood. 

We have seen that the moon revolves so as to keep the same face alwaysturned towards the earth. We have now seen that the planets Venus andMercury each appear to revolve in such a way that they keep the sameface towards the sun. All these phenomena are of profound interest inthe higher departments of astronomical research. They are not merecoincidences. They arise from the operation of the tides, in a mannerthat will be explained in a later chapter. 

It happens that our earth and Venus are very nearly equal in bulk. Thedifference is hardly perceptible, but the earth has a diameter a fewmiles greater than that of Venus. There are indications of the existenceof an atmosphere around Venus, and the evidence of the spectroscopeshows that water vapour is there present. 

If there be oxygen in the atmosphere of Venus, then it would seempossible that there might be life on that globe not essentiallydifferent in character from some forms of life on the earth. No doubtthe sun's heat on Venus is greatly in excess of the sun's heat withwhich we are acquainted, but this is not an insuperable difficulty. Wesee at present on the earth, life in very hot regions and life in verycold regions. Indeed, with each approach to the Equator we find lifemore and more exuberant; so that, if water be present on the surface ofVenus and if oxygen be a constituent of its atmosphere, we might expectto find in that planet a luxuriant tropical life, of a kind perhapsanalogous in some respects to life on the earth. 

In our account of the planet Mercury, as well as in the briefdescription of the hypothetical planet Vulcan, it has been necessary toallude to the phenomena presented by the transit of a planet over theface of the sun. Such an event is always of interest to astronomers, andespecially so in the case of Venus. We have in recent years had theopportunity of witnessing two of these rare occurrences. It is perhapsnot too much to assert that the transits of 1874 and 1882 have receiveda degree of attention never before accorded to any astronomicalphenomenon. 

The transit of Venus cannot be described as a very striking or beautifulspectacle. It is not nearly so fine a sight as a great comet or a showerof shooting stars. Why is it, then, that it is regarded as of so muchscientific importance? It is because the phenomenon helps us to solveone of the greatest problems which has ever engaged the mind of man. Bythe transit of Venus we may determine the scale on which our solarsystem is constructed. Truly this is a noble problem. Let us dwell uponit for a moment. In the centre of our system we have the sun--a majesticglobe more than a million times as large as the earth. Circling roundthe sun we have the planets, of which our earth is but one. There arehundreds of small planets. There are a few comparable with our earth;there are others vastly surpassing the earth. Besides the planets thereare other bodies in our system. Many of the planets are accompanied bysystems of revolving moons. There are hundreds, perhaps thousands, ofcomets. Each member of this stupendous host moves in a prescribed orbitaround the sun, and collectively they form the solar system. 

It is comparatively easy to learn the proportions of this system, tomeasure the relative distances of the planets from the sun, and even therelative sizes of the planets themselves. Peculiar difficulties are, however, experienced when we seek to ascertain the actual _size_ of thesystem as well as its shape. It is this latter question which thetransit of Venus offers us a method of solving. 

Look, for instance, at an ordinary map of Europe. We see the variouscountries laid down with precision; we can tell the courses of therivers; we can say that France is larger than England, and Russia largerthan France; but no matter how perfectly the map be constructed, something else is necessary before we can have a complete conception ofthe dimensions of the country. We must know _the scale on which the mapis drawn_. The map contains a reference line with certain marks upon it. This line is to give the scale of the map. Its duty is to tell us thatan inch on the map corresponds with so many miles on the actual surface. Unless it be supplemented by the scale, the map would be quite uselessfor many purposes. Suppose that we consulted it in order to choose aroute from London to Vienna, we can see at once the direction to betaken and the various towns and countries to be traversed; but unless werefer to the little scale in the corner, the map will not tell how manymiles long the journey is to be. 

A map of the solar system can be readily constructed. We can draw on itthe orbits of some of the planets and of their satellites, and we caninclude many of the comets. We can assign to the planets and to theorbits their proper proportions. But to render the map quite efficientsomething more is necessary. We must have the scale which is to tell ushow many millions of miles on the heavens correspond to one inch of themap. It is at this point we encounter a difficulty. There are, however, several ways of solving the problem, though they are all difficult andlaborious. The most celebrated method (though far from the best) is thatpresented on an occasion of the transit of Venus. Herein, then, lies theimportance of this rare event. It is one of the best-known means offinding the actual scale on which our system is constructed. Observe thefull importance of the problem. Once the scale has been determined, thenall is known. We know the size of the sun; we know his distance; we knowthe bulk of Jupiter, and the distances at which his satellites revolve;we know the dimensions of the comets, and the number of miles to whichthey recede in their wanderings; we know the velocity of the shootingstars; and we learn the important lesson that our earth is but one ofthe minor members of the sun's family. 

As the path of Venus lies inside that of the earth, and as Venus movesmore quickly than the earth, it follows that the earth is frequentlypassed by the planet, and just at the critical moment it will sometimeshappen that the earth, the planet, and the sun lie in the same straightline. We can then see Venus on the face of the sun, and this is thephenomenon which we call the _transit of Venus_. It is, indeed, quiteplain that if the three bodies were exactly in a line, an observer onthe earth, looking at the planet, would see it brought out vividlyagainst the brilliant background of the sun. 

Considering that the earth is overtaken by Venus once every nineteenmonths, it might seem that the transits of the planet should occur withcorresponding frequency. This is not the case; the transit of Venus isan exceedingly rare occurrence, and a hundred years or more will oftenelapse without a single one taking place. The rarity of these phenomenaarises from the fact that the path of the planet is inclined to theplane of the earth's orbit; so that for half of its path Venus is abovethe plane of the earth's orbit, and in the other half it is below. WhenVenus overtakes the earth, the line from the earth to Venus willtherefore usually pass over or under the sun. If, however, it shouldhappen that Venus overtakes the earth at or near either of the points inwhich the plane of the orbit of Venus passes through that of the earth, then the three bodies will be in line, and a transit of Venus will bethe consequence. The rarity of the occurrence of a transit need nolonger be a mystery. The earth passes through one of the critical partsevery December, and through the other every June. If it happens that theconjunction of Venus occurs on, or close to, June 6th or December 7th, then a transit of Venus will occur at that conjunction, but in no othercircumstances. 

The most remarkable law with reference to the repetition of thephenomenon is the well-known eight-year interval. The transits may beall grouped together into pairs, the two transits of any single pairbeing separated by an interval of eight years. For instance, a transitof Venus took place in 1761, and again in 1769. No further transitsoccurred until those witnessed in 1874 and in 1882. Then, again, comes along interval, for another transit will not occur until 2004, but itwill be followed by another in 2012. 

This arrangement of the transits in pairs admits of a very simpleexplanation. It happens that the periodic time of Venus bears aremarkable relation to the periodic time of the earth. The planetaccomplishes thirteen revolutions around the sun in very nearly the sametime that the earth requires for eight revolutions. If, therefore, Venusand the earth were in line with the sun in 1874, then in eight yearsmore the earth will again be found in the same place; and so will Venus, for it has just been able to accomplish thirteen revolutions. A transitof Venus having occurred on the first occasion, a transit must alsooccur on the second. 

It is not, however, to be supposed that every eight years the planetswill again resume the same position with sufficient precision for aregular eight-year transit interval. It is only approximately true thatthirteen revolutions of Venus are coincident with eight revolutions ofthe earth. Each recurrence of conjunction takes place at a slightlydifferent position of the planets, so that when the two planets cametogether again in the year 1890 the point of conjunction was so farremoved from the critical point that the line from the earth to Venusdid not intersect the sun, and thus, although Venus passed very near thesun, yet no transit took place. 

[Illustration: Fig. 45. --Venus on the Sun at the Transit of 1874. ]

Fig. 45 represents the transit of Venus in 1874. It is taken from aphotograph obtained, during the occurrence, by M. Janssen. His telescopewas directed towards the sun during the eventful minutes while itlasted, and thus an image of the sun was depicted on the photographicplate placed in the telescope. The lighter circle represents the disc ofthe sun. On that disc we see the round, sharp image of Venus, showingthe characteristic appearance of the planet during the progress of thetransit. The only other features to be noticed are a few of the solarspots, rather dimly shown, and a network of lines which were marked on aglass plate across the field of view of the telescope to facilitatemeasurements. 

The adjoining sketch (Fig. 46) exhibits the course which the planetpursued in its passage across the sun on the two occasions in 1874 and1882. Our generation has had the good fortune to witness the twooccurrences indicated on this picture. The white circle denotes the discof the sun; the planet encroaches on the white surface, and at first islike a bite out of the sun's margin. Gradually the black spot steals infront of the sun, until, after nearly half an hour, the black disc isentirely visible. Slowly the planet wends its way across, followed byhundreds of telescopes from every accessible part of the globe whencethe phenomenon is visible, until at length, in the course of a fewhours, it emerges at the other side. 

It will be useful to take a brief retrospect of the different transitsof Venus of which there is any historical record. They are not numerous. Hundreds of such phenomena have occurred since man first came on theearth. It was not until the approach of the year 1631 that attentionbegan to be directed to the matter, though the transit which undoubtedlyoccurred in that year was not noticed by anyone. The success of Gassendiin observing the transit of Mercury, to which we have referred in thelast chapter, led him to hope that he would be equally fortunate inobserving the transit of Venus, which Kepler had also foretold. Gassendilooked at the sun on the 4th, 5th, and 6th December. He looked at itagain on the 7th, but he saw no sign of the planet. We now know thereason. The transit of Venus took place during the night, between the6th and the 7th, and must therefore have been invisible to Europeanobservers. 

Kepler had not noticed that another transit would occur in 1639. Thisdiscovery was made by another astronomer, and it is the one with whichthe history of the subject may be said to commence. It was the firstoccasion on which the phenomenon was ever actually witnessed; nor was itthen seen by many. So far as is known, it was witnessed by only twopersons. 

[Illustration: Fig. 46. --The Path of Venus across the Sun in theTransits of 1874 and 1882. ]

A young and ardent English astronomer, named Horrocks, had undertakensome computations about the motions of Venus. He made the discovery thatthe transit of Venus would be repeated in 1639, and he prepared toverify the fact. The sun rose bright on the morning of the day--whichhappened to be a Sunday. The clerical profession, which Horrocksfollowed, here came into collision with his desires as an astronomer. Hetells us that at nine he was called away by business of the highestimportance--referring, no doubt, to his official duties; but the servicewas quickly performed, and a little before ten he was again on thewatch, only to find the brilliant face of the sun without any unusualfeature. It was marked with a spot, but nothing that could be mistakenfor a planet. Again, at noon, came an interruption; he went to church, but he was back by one. Nor were these the only impediments to hisobservations. The sun was also more or less clouded over during part ofthe day. However, at a quarter past three in the afternoon his clericalwork was over; the clouds had dispersed, and he once more resumed hisobservations. To his intense delight he then saw on the sun the round, dark spot, which was at once identified as the planet Venus. Theobservations could not last long; it was the depth of winter, and thesun was rapidly setting. Only half an hour was available, but he hadmade such careful preparations beforehand that it sufficed to enable himto secure some valuable measurements. 

Horrocks had previously acquainted his friend, William Crabtree, withthe impending occurrence. Crabtree was therefore on the watch, andsucceeded in seeing the transit; a striking picture of Crabtree's famousobservation is shown in one of the beautiful frescoes in the Town Hallat Manchester. But to no one else had Horrocks communicated theintelligence; as he says, "I hope to be excused for not informing otherof my friends of the expected phenomenon, but most of them care littlefor trifles of this kind, rather preferring their hawks and hounds, tosay no worse; and although England is not without votaries of astronomy, with some of whom I am acquainted, I was unable to convey to them theagreeable tidings, having myself had so little notice. "

It was not till long afterwards that the full importance of the transitof Venus was appreciated. Nearly a century had rolled away when thegreat astronomer, Halley (1656-1742), drew attention to the subject. Thenext transit was to occur in 1761, and forty-five years before thatevent Halley explained his celebrated method of finding the distance ofthe sun by means of the transit of Venus. [15] He was then a man sixtyyears of age; he could have no expectation that he would live to witnessthe event; but in noble language he commends the problem to the noticeof the learned, and thus addresses the Royal Society of London:--"Andthis is what I am now desirous to lay before this illustrious Society, which I foretell will continue for ages, that I may explain beforehandto young astronomers, who may, perhaps, live to observe these things, amethod by which the immense distance of the sun may be trulyobtained. .. . I recommend it, therefore, again and again to those curiousastronomers who, when I am dead, will have an opportunity of observingthese things, that they would remember this my admonition, anddiligently apply themselves with all their might in making theobservations, and I earnestly wish them all imaginable success--in thefirst place, that they may not by the unseasonable obscurity of a cloudysky be deprived of this most desirable sight, and then that, havingascertained with more exactness the magnitudes of the planetary orbits, it may redound to their immortal fame and glory. " Halley lived to a goodold age, but he died nineteen years before the transit occurred. 

The student of astronomy who desires to learn how the transit of Venuswill tell the distance from the sun must prepare to encounter ageometrical problem of no little complexity. We cannot give to thesubject the detail that would be requisite for a full explanation. Allwe can attempt is to render a general account of the method, sufficientto enable the reader to see that the transit of Venus really doescontain all the elements necessary for the solution of the problem. 

We must first explain clearly the conception which is known toastronomers by the name of _parallax_; for it is by parallax that thedistance of the sun, or, indeed, the distance of any other celestialbody, must be determined. Let us take a simple illustration. Stand neara window whence you can look at buildings, or the trees, the clouds, orany distant objects. Place on the glass a thin strip of paper verticallyin the middle of one of the panes. Close the right eye, and note withthe left eye the position of the strip of paper relatively to theobjects in the background. Then, while still remaining in the sameposition, close the left eye and again observe the position of thestrip of paper with the right eye. You will find that the position ofthe paper on the background has changed. As I sit in my study and lookout of the window I see a strip of paper, with my right eye, in front ofa certain bough on a tree a couple of hundred yards away; with my lefteye the paper is no longer in front of that bough, it has moved to aposition near the outline of the tree. This apparent displacement of thestrip of paper, relatively to the distant background, is what is calledparallax. 

Move closer to the window, and repeat the observation, and you find that_the apparent displacement of the strip increases_. Move away from thewindow, and the displacement decreases. Move to the other side of theroom, the displacement is much less, though probably still visible. Wethus see that the change in the apparent place of the strip of paper, asviewed with the right eye or the left eye, varies in amount as thedistance changes; but it varies in the opposite way to the distance, foras either becomes greater the other becomes less. We can thus associatewith each particular distance a corresponding particular displacement. From this it will be easy to infer that if we have the means ofmeasuring the amount of displacement, then we have the means ofcalculating the distance from the observer to the window. 

It is this principle, applied on a gigantic scale, which enables us tomeasure the distances of the heavenly bodies. Look, for instance, at theplanet Venus; let this correspond to the strip of paper, and let thesun, on which Venus is seen in the act of transit, be the background. Instead of the two eyes of the observer, we now place two observatoriesin distant regions of the earth; we look at Venus from one observatory, we look at it from the other; we measure the amount of the displacement, and from that we calculate the distance of the planet. All depends, then, on the means which we have of measuring the displacement of Venusas viewed from the two different stations. There are various ways ofaccomplishing this, but the most simple is that originally proposed byHalley. 

From the observatory at A Venus seems to pursue the upper of the twotracks shown in the adjoining figure (Fig. 47). From the observatory atB it follows the lower track, and it is for us to measure the distancebetween the two tracks. This can be accomplished in several ways. Suppose the observer at A notes the time that Venus has occupied incrossing the disc, and that similar observations be made at B. As thetrack seen from B is the larger, it must follow that the time observedat B will be greater than that at A. When the observations from thedifferent hemispheres are compared, the _times_ observed will enable thelengths of the tracks to be calculated. The lengths being known, theirplaces on the circular disc of the sun are determined, and hence theamount of displacement of Venus in transit is ascertained. Thus it isthat the distance of Venus is measured, and the scale of the solarsystem is known. 

[Illustration: Fig. 47. --To Illustrate the Observation of the Transit ofVenus from Two Localities, A and B, on the Earth. ]

The two transits to which Halley's memorable researches referredoccurred in the years 1761 and 1769. The results of the first were notvery successful, in spite of the arduous labours of those who undertookthe observations. The transit of 1769 is of particular interest, notonly for the determination of the sun's distance, but also because itgave rise to the first of the celebrated voyages of Captain Cook. It wasto see the transit of Venus that Captain Cook was commissioned to sailto Otaheite, and there, on the 3rd of June, on a splendid day in thatexquisite climate, the phenomenon was carefully observed and measured bydifferent observers. Simultaneously with these observations others wereobtained in Europe and elsewhere, and from the combination of all theobservations an approximate knowledge of the sun's distance was gained. The most complete discussion of these observations did not, however, take place for some time. It was not until the year 1824 that theillustrious Encke computed the distance of the sun, and gave as thedefinite result 95, 000, 000 miles. 

For many years this number was invariably adopted, and many of thepresent generation will remember how they were taught in theirschool-days that the sun was 95, 000, 000 miles away. At length doubtsbegan to be whispered as to the accuracy of this result. The doubtsarose in different quarters, and were presented with different degreesof importance; but they all pointed in one direction, they all indicatedthat the distance of the sun was not really so great as the result whichEncke had obtained. It must be remembered that there are several ways offinding the distance of the sun, and it will be our duty to allude tosome other methods later on. It has been ascertained that the resultobtained by Encke from the observations made in 1761 and 1769, withinstruments inferior to our modern ones, was too great, and that thedistance of the sun may probably be now stated at 92, 000, 000 miles. 

I venture to record our personal experience of the last transit ofVenus, which we had the good fortune to view from Dunsink Observatory onthe afternoon of the 6th of December, 1882. 

The morning of the eventful day appeared to be about as unfavourablefor a grand astronomical spectacle as could well be imagined. Snow, acouple of inches thick, covered the ground, and more was falling, withbut little intermission, all the forenoon. It seemed almost hopelessthat a view of the phenomenon could be obtained from that observatory;but it is well in such cases to bear in mind the injunction given to theobservers on a celebrated eclipse expedition. They were instructed, nomatter what the day should be like, that they were to make all theirpreparations precisely as they would have done were the sun shining withundimmed splendour. By this advice no doubt many observers haveprofited; and we acted upon it with very considerable success. 

There were at that time at the observatory two equatorials, one of theman old, but tolerably good, instrument, of about six inches aperture;the other the great South equatorial, of twelve inches aperture, alreadyreferred to. At eleven o'clock the day looked worse than ever; but we atonce proceeded to make all ready. I stationed Mr. Rambaut at the smallequatorial, while I myself took charge of the South instrument. The snowwas still falling when the domes were opened; but, according to ourprearranged scheme, the telescopes were directed, not indeed upon thesun, but to the place where we knew the sun was, and the clockwork wasset in motion which carried round the telescopes, still constantlypointing towards the invisible sun. The predicted time of the transithad not yet arrived. 

The eye-piece employed on the South equatorial must also receive a briefnotice. It will, of course, be obvious that the full glare of the sunhas to be greatly mitigated before the eye can view it with impunity. The light from the sun falls upon a piece of transparent glass inclinedat a certain angle, and the chief portion of the sun's heat, as well asa certain amount of its light, pass through the glass and are lost. Acertain fraction of the light is, however, reflected from the glass, andenters the eye-piece. This light is already much reduced in intensity, but it undergoes as much further reduction as we please by an ingeniouscontrivance. The glass which reflects the light does so at what iscalled the polarising angle, and between the eye-piece and the eye is aplate of tourmaline. This plate of tourmaline can be turned round by theobserver. In one position it hardly interferes with the polarised lightat all, while in the position at right angles thereto it cuts off nearlythe whole of it. By simply adjusting the position of the tourmaline, theobserver has it in his power to render the image of any brightness thatmay be convenient, and thus the observations of the sun can be conductedwith the appropriate degree of illumination. 

But such appliances seemed on this occasion to be a mere mockery. Thetourmaline was all ready, but up to one o'clock not a trace of the suncould be seen. Shortly after one o'clock, however, we noticed that theday was getting lighter; and, on looking to the north, whence the windand the snow were coming, we saw, to our inexpressible delight, that theclouds were clearing. At length, the sky towards the south began toimprove, and at last, as the critical moment approached, we could detectthe spot where the sun was becoming visible. But the predicted momentarrived and passed, and still the sun had not broken through the clouds, though every moment the certainty that it would do so became moreapparent. The external contact was therefore missed. We tried to consoleourselves by the reflection that this was not, after all, a veryimportant phase, and hoped that the internal contact would be moresuccessful. 

At length the struggling beams pierced the obstruction, and I saw theround, sharp disc of the sun in the finder, and eagerly glanced at thepoint on which attention was concentrated. Some minutes had now elapsedsince the predicted moment of first contact, and, to my delight, I sawthe small notch in the margin of the sun showing that the transit hadcommenced, and that the planet was then one-third on the sun. But thecritical moment had not yet arrived. By the expression "first internalcontact" we are to understand the moment when the planet has completelyentered _on_ the sun. This first contact was timed to occur twenty-oneminutes later than the external contact already referred to. But theclouds again disappointed our hope of seeing the internal contact. Whilesteadily looking at the exquisitely beautiful sight of the gradualadvance of the planet, I became aware that there were other objectsbesides Venus between me and the sun. They were the snowflakes, whichagain began to fall rapidly. I must admit the phenomenon was singularlybeautiful. The telescopic effect of a snowstorm with the sun as abackground I had never before seen. It reminded me of the golden rainwhich is sometimes seen falling from a flight of sky-rockets duringpyrotechnic displays; I would gladly have dispensed with the spectacle, for it necessarily followed that the sun and Venus again disappearedfrom view. The clouds gathered, the snowstorm descended as heavily asever, and we hardly dared to hope that we should see anything more; 1hr. 57 min. Came and passed, the first internal contact was over, andVenus had fully entered on the sun. We had only obtained a brief view, and we had not yet been able to make any measurements or otherobservations that could be of service. Still, to have seen even a partof a transit of Venus is an event to remember for a lifetime, and wefelt more delight than can be easily expressed at even this slight gleamof success. 

But better things were in store. My assistant came over with the reportthat he had also been successful in seeing Venus in the same phase as Ihad. We both resumed our posts, and at half-past two the clouds began todisperse, and the prospect of seeing the sun began to improve. It wasnow no question of the observations of contact. Venus by this time waswell on the sun, and we therefore prepared to make observations with themicrometer attached to the eye-piece. The clouds at length dispersed, and at this time Venus had so completely entered on the sun that thedistance from the edge of the planet to the edge of the sun was abouttwice the diameter of the planet. We measured the distance of the inneredge of Venus from the nearest limb of the sun. These observations wererepeated as frequently as possible, but it should be added that theywere only made with very considerable difficulty. The sun was now verylow, and the edges of the sun and of Venus were by no means of thatsteady character which is suitable for micrometrical measurement. Themargin of the luminary was quivering, and Venus, though no doubt it wassometimes circular, was very often distorted to such a degree as to makethe measures very uncertain. 

We succeeded in obtaining sixteen measures altogether; but the sun wasnow getting low, the clouds began again to interfere, and we saw thatthe pursuit of the transit must be left to the thousands of astronomersin happier climes who had been eagerly awaiting it. But before thephenomena had ceased I spared a few minutes from the somewhat mechanicalwork at the micrometer to take a view of the transit in the morepicturesque form which the large field of the finder presented. The sunwas already beginning to put on the ruddy hues of sunset, and there, farin on its face, was the sharp, round, black disc of Venus. It was theneasy to sympathise with the supreme joy of Horrocks, when, in 1639, hefor the first time witnessed this spectacle. The intrinsic interest ofthe phenomenon, its rarity, the fulfilment of the prediction, the nobleproblem which the transit of Venus helps us to solve, are all present toour thoughts when we look at this pleasing picture, a repetition ofwhich will not occur again until the flowers are blooming in the June ofA. D. 2004. 

The occasion of a transit of Venus also affords an opportunity ofstudying the physical nature of the planet, and we may here brieflyindicate the results that have been obtained. In the first place, atransit will throw some light on the question as to whether Venus isaccompanied by a satellite. If Venus were attended by a small body inclose proximity, it would be conceivable that in ordinary circumstancesthe brilliancy of the planet would obliterate the feeble beam of raysfrom the minute companion, and thus the satellite would remainundiscovered. It was therefore a matter of great interest to scrutinisethe vicinity of the planet while in the act of transit. If a satelliteexisted--and the existence of one or more of such bodies has often beensuspected--then it would be capable of detection against the brilliantbackground of the sun. Special attention was directed to this pointduring the recent transits, but no satellite of Venus was to be found. It seems, therefore, to be very unlikely that Venus can be attended byany companion globe of appreciable dimensions. 

The observations directed to the investigation of the atmospheresurrounding Venus have been more successful. If the planet were devoidof an atmosphere, then it would be totally invisible just beforecommencing to enter on the sun, and would relapse into totalinvisibility as soon as it had left the sun. The observations madeduring the transits are not in conformity with such suppositions. Special attention has been directed to this point during the recenttransits. The result has been very remarkable, and has proved in themost conclusive manner the existence of an atmosphere around Venus. Asthe planet gradually moved off the sun, the circular edge of the planetextending out into the darkness was seen to be bounded by a circular arcof light, and Dr. Copeland, who observed this transit in very favourablecircumstances, was actually able to follow the planet until it hadpassed entirely away from the sun, at which time the globe, thoughitself invisible, was distinctly marked by the girdle of light by whichit was surrounded. This luminous circle is inexplicable save by thesupposition that the globe of Venus is surrounded by an atmosphericshell in the same way as the earth. 

It may be asked, what is the advantage of devoting so much time andlabour to a celestial phenomenon like the transit of Venus which has solittle bearing on practical affairs? What does it matter whether the sunbe 95, 000, 000 miles off, or whether it be only 93, 000, 000, or any otherdistance? We must admit at once that the enquiry has but a slenderbearing on matters of practical utility. No doubt a fanciful personmight contend that to compute our nautical almanacs with perfectaccuracy we require a precise knowledge of the distance of the sun. Ourvast commerce depends on skilful navigation, and one factor necessaryfor success is the reliability of the "Nautical Almanac. " The increasedperfection of the almanac must therefore bear some relation to increasedperfection in navigation. Now, as good authorities tell us that inrunning for a harbour on a tempestuous night, or in other criticalemergencies, even a yard of sea-room is often of great consequence, soit may conceivably happen that to the infinitesimal influence of thetransit of Venus on the "Nautical Almanac" is due the safety of agallant vessel. 

But the time, the labour, and the money expended in observing thetransit of Venus are really to be defended on quite different grounds. We see in it a fruitful source of information. It tells us the distanceof the sun, which is the foundation of all the great measurements of theuniverse. It gratifies the intellectual curiosity of man by a view ofthe true dimensions of the majestic solar system, in which the earth isseen to play a dignified, though still subordinate, part; and it leadsus to a conception of the stupendous scale on which the universe isconstructed. 

It is not possible for us, with a due regard to the limits of thisvolume, to protract any longer our discussion of the transit of Venus. When we begin to study the details of the observations, we areimmediately confronted with a multitude of technical and intricatematters. Unfortunately, there are very great difficulties in making theobservations with the necessary precision. The moments when Venus enterson and leaves the solar disc cannot be very accurately observed, partlyowing to a peculiar optical illusion known as "the black drop, " wherebyVenus seems to cling to the sun's limb for many seconds, partly owing tothe influence of the planet's atmosphere, which helps to make theobserved time of contact uncertain. These circumstances make itdifficult to determine the distance of the sun from observations oftransits of Venus with the accuracy which modern science requires. Itseems therefore likely that the final determination of the sun'sdistance will be obtained in quite a different manner. This will beexplained in Chapter XI. , and hence we feel the less reluctance inpassing any from the consideration of the transit of Venus as a methodof celestial surveying. 

We must now close our description of this lovely planet; but beforedoing so, let us add--or in some cases repeat--a few statistical factsas to the size and the dimensions of the planet and its orbit. 

The diameter of Venus is about 7, 660 miles, and the planet shows nomeasurable departure from the globular form, though we can hardly doubtthat its polar diameter must really be somewhat shorter than theequatorial diameter. This diameter is only about 258 miles less thanthat of the earth. The mass of Venus is about three-quarters of the massof the earth; or if, as is more usual, we compare the mass of Venus withthe sun, it is to be represented by the fraction 1 divided by 425, 000. It is to be observed that the mass of Venus is not quite so great incomparison with its bulk as might have been expected. The density ofthis planet is about 0·850 of that of the earth. Venus would weigh 4·81times as much as a globe of water of equal size. The gravitation at itssurface will, to a slight extent, be less than the gravitation at thesurface of the earth. A body here falls sixteen feet in a second; a bodylet fall at the surface of Venus would fall about three feet less. Itseems not unlikely that the time of rotation of Venus may be equal tothe period of its revolution around the sun. 

The orbit of Venus is remarkable for the close approach which it makesto a circle. The greatest distance of this planet from the sun does notexceed the least distance by one per cent. Its mean distance from thesun is about 67, 000, 000 miles, and the movement in the orbit amounts toa mean velocity of nearly 22 miles per second, the entire journey beingaccomplished in 224·70 days. 

CHAPTER IX. 

THE EARTH. 

 The Earth is a great Globe--How the Size of the Earth is Measured--The Base Line--The Latitude found by the Elevation of the Pole--A Degree of the Meridian--The Earth not a Sphere--The Pendulum Experiment--Is the Motion of the Earth slow or fast?--Coincidence of the Axis of Rotation and the Axis of Figure--The Existence of Heat in the Earth--The Earth once in a Soft Condition--Effects of Centrifugal Force--Comparison with the Sun and Jupiter--The Protuberance of the Equator--The Weighing of the Earth--Comparison between the Weight of the Earth and an equal Globe of Water--Comparison of the Earth with a Leaden Globe--The Pendulum--Use of the Pendulum in Measuring the Intensity of Gravitation--The Principle of Isochronism--Shape of the Earth measured by the Pendulum. 

That the earth must be a round body is a truth immediately suggested bysimple astronomical considerations. The sun is round, the moon is round, and telescopes show that the planets are round. No doubt comets are notround, but then a comet seems to be in no sense a solid body. We can seeright through one of these frail objects, and its weight is too smallfor our methods of measurement to appreciate. If, then, all the solidbodies we can see are round globes, is it not likely that the earth is aglobe also? But we have far more direct information than mere surmise. 

There is no better way of actually seeing that the surface of the oceanis curved than by watching a distant ship on the open sea. When the shipis a long way off and is still receding, its hull will graduallydisappear, while the masts will remain visible. On a fine summer's daywe can often see the top of the funnel of a steamer appearing above thesea, while the body of the steamer is below. To see this best the eyeshould be brought as close as possible to the surface of the sea. If thesea were perfectly flat, there would be nothing to obscure the body ofthe vessel, and it would therefore be visible so long as the funnelremains visible. If the sea be really curved, the protuberant partintercepts the view of the hull, while the funnel is still to be seen. 

We thus learn how the sea is curved at every part, and therefore it isnatural to suppose that the earth is a sphere. When we make more carefulmeasurements we find that the globe is not perfectly round. It isflattened to some extent at each of the poles. This may be easilyillustrated by an indiarubber ball, which can be compressed on twoopposite sides so as to bulge out at the centre. The earth is similarlyflattened at the poles, and bulged out at the equator. The divergence ofthe earth from the truly globular form is, however, not very great, andwould not be noticed without very careful measurements. 

The determination of the size of the earth involves operations of nolittle delicacy. Very much skill and very much labour have been devotedto the work, and the dimensions of the earth are known with a highdegree of accuracy, though perhaps not with all the precision that wemay ultimately hope to attain. The scientific importance of an accuratemeasurement of the earth can hardly be over-estimated. The radius of theearth is itself the unit in which many other astronomical magnitudes areexpressed. For example, when observations are made with the view offinding the distance of the moon, the observations, when discussed andreduced, tell us that the distance of the moon is equal to fifty-ninetimes the equatorial radius of the earth. If we want to find thedistance of the moon in miles, we require to know the number of miles inthe earth's radius. 

A level part of the earth's surface having been chosen, a line a fewmiles long is measured. This is called the base, and as all thesubsequent measures depend ultimately on the base, it is necessary thatthis measurement shall be made with scrupulous accuracy. To measure aline four or five miles long with such precision as to exclude anyerrors greater than a few inches demands the most minute precautions. Wedo not now enter upon a description of the operations that arenecessary. It is a most laborious piece of work, and many ponderousvolumes have been devoted to the discussion of the results. But when afew base lines have been obtained in different places on the earth'ssurface, the measuring rods are to be laid aside, and the subsequenttask of the survey of the earth is to be conducted by the measurement ofangles from one station to another and trigonometrical calculationsbased thereon. Starting from a base line a few miles long, distances ofgreater length are calculated, until at length stretches 100 miles long, or even more, can be accomplished. It is thus possible to find thelength of a long line running due north and south. 

So far the work has been merely that of the terrestrial surveyor. Thedistance thus ascertained is handed over to the astronomer to deducefrom it the dimensions of the earth. The astronomer fixes hisobservatory at the northern end of the long line, and proceeds todetermine his latitude by observation. There are various ways by whichthis can be accomplished. They will be found fully described in works onpractical astronomy. We shall here only indicate in a very brief mannerthe principle on which such observations are to be made. 

Everyone ought to be familiar with the Pole Star, which, though by nomeans the most brilliant, is probably the most important star in thewhole heavens. In these latitudes we are accustomed to find the PoleStar at a considerable elevation, and there we can invariably find it, always in the same place in the northern sky. But suppose we start on avoyage to the southern hemisphere: as we approach the equator we find, night after night, the Pole Star coming closer to the horizon. At theequator it is on the horizon; while if we cross the line, we find onentering the southern hemisphere that this useful celestial body hasbecome invisible. This is in itself sufficient to show us that the earthcannot be the flat surface that untutored experience seems to indicate. 

On the other hand, a traveller leaving England for Norway observes thatthe Pole Star is every night higher in the heavens than he has beenaccustomed to see it. If he extend his journey farther north, the sameobject will gradually rise higher and higher, until at length, whenapproaching the pole of the earth, the Pole Star is high up over hishead. We are thus led to perceive that the higher our latitude, thehigher, in general, is the elevation of the Pole Star. But we cannot useprecise language until we replace the twinkling point by the pole of theheavens itself. The pole of the heavens is near the Pole Star, whichitself revolves around the pole of the heavens, as all the other starsdo, once every day. The circle described by the Pole Star is, however, so small that, unless we give it special attention, the motion will notbe perceived. The true pole is not a visible point, but it is capable ofbeing accurately defined, and it enables us to state with the utmostprecision the relation between the pole and the latitude. The statementis, that the elevation of the pole above the horizon is equal to thelatitude of the place. 

The astronomer stationed at one end of the long line measures theelevation of the pole above the horizon. This is an operation of somedelicacy. In the first place, as the pole is invisible, he has to obtainits position indirectly. He measures the altitude of the Pole Star whenthat altitude is greatest, and repeats the operation twelve hours later, when the altitude of the Pole Star is least; the mean between the two, when corrected in various ways which it is not necessary for us now todiscuss, gives the true altitude of the pole. Suffice it to say that bysuch operations the latitude of one end of the line is determined. Theastronomer then, with all his equipment of instruments, moves to theother end of the line. He there repeats the process, and he finds thatthe pole has now a different elevation, corresponding to the differentlatitude. The difference of the two elevations thus gives him anaccurate measure of the number of degrees and fractional parts of adegree between the latitudes of the two stations. This can be comparedwith the actual distance in miles between the two stations, which hasbeen ascertained by the trigonometrical survey. A simple calculationwill then show the number of miles and fractional parts of a milecorresponding to one degree of latitude--or, as it is more usuallyexpressed, the length of a degree of the meridian. 

This operation has to be repeated in different parts of the earth--inthe northern hemisphere and in the southern, in high latitudes and inlow. If the sea-level over the entire earth were a perfect sphere, animportant consequence would follow--the length of a degree of themeridian would be everywhere the same. It would be the same in Peru asin Sweden, the same in India as in England. But the lengths of thedegrees are not all the same, and hence we learn that our earth is notreally a sphere. The measured lengths of the degrees enable us to see towhat extent the shape of the earth departs from a perfect sphere. Nearthe pole the length of a degree is longer than near the equator. Thisshows that the earth is flattened at the poles and protuberant at theequator, and it provides the means by which we may calculate the actuallengths of the polar and the equatorial axes. In this way the equatorialdiameter has been found equal to 7, 927 miles, while the polar diameteris 27 miles shorter. 

The polar axis of the earth may be defined as the diameter about whichthe earth rotates. This axis intersects the surface at the north andsouth poles. The time which the earth occupies in making a completerotation around this axis is called a sidereal day. The sidereal day isa little shorter than the ordinary day, being only 23 hours, 56 minutes, and 4 seconds. The rotation is performed just as if a rigid axis passedthrough the centre of the earth; or, to use the old and homelyillustration, the earth rotates just as a ball of worsted may be made torotate around a knitting-needle thrust through its centre. 

It is a noteworthy circumstance that the axis about which the earthrotates occupies a position identical with that of the shortest diameterof the earth as found by actual surveying. This is a coincidence whichwould be utterly inconceivable if the shape of the earth was not in someway physically connected with the fact that the earth is rotating. Whatconnection can then be traced? Let us enquire into the subject, and weshall find that the shape of the earth is a consequence of its rotation. 

The earth at the present time is subject, at various localities, tooccasional volcanic outbreaks. The phenomena of such eruptions, theallied occurrence of earthquakes, the well-known fact that the heatincreases the deeper we descend into the earth, the existence of hotsprings, the geysers found in Iceland and elsewhere, all testify to thefact that heat exists in the interior of the earth. Whether that heatbe, as some suppose, universal in the interior of the earth, or whetherit be merely local at the several places where its manifestations arefelt, is a matter which need not now concern us. All that is necessaryfor our present purpose is the admission that heat is present to someextent. 

This internal heat, be it much or little, has obviously a differentorigin from the heat which we know on the surface. The heat we enjoy isderived from the sun. The internal heat cannot have been derived fromthe sun; its intensity is far too great, and there are other insuperabledifficulties attending the supposition that it has come from the sun. Where, then, has this heat come from? This is a question which atpresent we can hardly answer--nor, indeed, does it much concern ourargument that we should answer it. The fact being admitted that the heatis there, all that we require is to apply one or two of the well-knownthermal laws to the interpretation of the facts. We have first toconsider the general principle by which heat tends to diffuse itself andspread away from its original source. The heat, deep-seated in theinterior of the earth, is transmitted through the superincumbent rocks, and slowly reaches the surface. It is true that the rocks and materialswith which our earth is covered are not good conductors of heat; most ofthem are, indeed, extremely inefficient in this way; but, good or bad, they are in some shape conductors, and through them the heat must creepto the surface. 

It cannot be urged against this conclusion that we do not feel thisheat. A few feet of brickwork will so confine the heat of a mighty blastfurnace that but little will escape through the bricks; but _some_ heatdoes escape, and the bricks have never been made, and never could bemade, which would absolutely intercept all the heat. If a few feet ofbrickwork can thus nearly mask the heat of a furnace, cannot some scoresof miles of rock nearly mask the heat in the depths of the earth, eventhough that heat were seven times hotter than the mightiest furnace thatever existed? The heat would escape slowly, and perhaps imperceptibly, but, unless all our knowledge of nature is a delusion, no rocks, howeverthick, can prevent, in the course of time, the leakage of the heat tothe surface. When this heat arrives at the surface of the earth it must, in virtue of another thermal law, gradually radiate away and be lost tothe earth. 

It would lead us too far to discuss fully the objections which mayperhaps be raised against what we have here stated. It is often saidthat the heat in the interior of the earth is being produced by chemicalcombination or by mechanical process, and thus that the heat may beconstantly renewed as fast or even faster than it escapes. This, however, is more a difference in form than in substance. If heat beproduced in the way just supposed (and there can be no doubt that theremay be such an origin for some of the heat in the interior of the globe)there must be a certain expenditure of chemical or mechanical energiesthat produces a certain exhaustion. For every unit of heat which escapesthere will either be a loss of an unit of heat from the globe, or, whatcomes nearly to the same thing, a loss of an unit of heat-making powerfrom the chemical or the mechanical energies. The substantial result isthe same; the heat, actual or potential, of the earth must bedecreasing. It should, of course, be observed that a great part of thethermal losses experienced by the earth is of an obvious character, andnot dependent upon the slow processes of conduction. Each outburst of avolcano discharges a stupendous quantity of heat, which disappears veryspeedily from the earth; while in the hot springs found in so manyplaces there is a perennial discharge of the same kind, which in thecourse of years attains enormous proportions. 

The earth is thus losing heat, while it never acquires any freshsupplies of the same kind to replace the losses. The consequence isobvious; the interior of the earth must be growing colder. No doubt thisis an extremely slow process; the life of an individual, the life of anation, perhaps the life of the human race itself, has not been longenough to witness any pronounced change in the store of terrestrialheat. But the law is inevitable, and though the decline in heat may beslow, yet it is continuous, and in the lapse of ages must necessarilyproduce great and important results. 

It is not our present purpose to offer any forecast as to the changeswhich must necessarily arise from this process. We wish at presentrather to look back into past time and see what consequences we maylegitimately infer. Such intervals of time as we are familiar with inordinary life, or even in ordinary history, are for our present purposequite inappreciable. As our earth is daily losing internal heat, or theequivalent of heat, it must have contained more heat yesterday than itdoes to-day, more last year than this year, more twenty years ago thanten years ago. The effect has not been appreciable in historic time; butwhen we rise from hundreds of years to thousands of years, fromthousands of years to hundreds of thousands of years, and from hundredsof thousands of years to millions of years, the effect is not onlyappreciable, but even of startling magnitude. 

There must have been a time when the earth contained much more heat thanat present. There must have been a time when the surface of the earthwas sensibly hot from this source. We cannot pretend to say how manythousands or millions of years ago this epoch must have been; but we maybe sure that earlier still the earth was even hotter, until at length weseem to see the temperature increase to a red heat, from a red heat welook back to a still earlier age when the earth was white hot, backfurther till we find the surface of our now solid globe was actuallymolten. We need not push the retrospect any further at present, stillless is it necessary for us to attempt to assign the probable origin ofthat heat. This, it will be observed, is not required in our argument. We find heat now, and we know that heat is being lost every day. Fromthis the conclusion that we have already drawn seems inevitable, andthus we are conducted back to some remote epoch in the abyss of timepast when our solid earth was a globe molten and soft throughout. 

A dewdrop on the petal of a flower is nearly globular; but it is notquite a globe, because the gravitation presses it against the flower andsomewhat distorts the shape. A falling drop of rain is a globe; a dropof oil suspended in a liquid with which it does not mix forms a globe. Passing from small things to great things, let us endeavour to conceivea stupendous globe of molten matter. Let that globe be as large as theearth, and let its materials be so soft as to obey the forces ofattraction exerted by each part of the globe on all the other parts. There can be no doubt as to the effect of these attractions; they wouldtend to smooth down any irregularities on the surface just in the sameway as the surface of the ocean is smooth when freed from the disturbinginfluences of the wind. We might, therefore, expect that our moltenglobe, isolated from all external interference, would assume the form ofa sphere. 

But now suppose that this great sphere, which we have hitherto assumedto be at rest, is made to rotate round an axis passing through itscentre. We need not suppose that this axis is a material object, nor arewe concerned with any supposition as to how the velocity of rotation wascaused. We can, however, easily see what the consequence of the rotationwould be. The sphere would become deformed, the centrifugal force wouldmake the molten body bulge out at the equator and flatten down at thepoles. The greater the velocity of rotation the greater would be thebulging. To each velocity of rotation a certain degree of bulging wouldbe appropriate. The molten earth thus bulged out to an extent which wasdependent upon the fact that it turned round once a day. Now supposethat the earth, while still rotating, commences to pass from the liquidto the solid state. The form which the earth would assume onconsolidation would, no doubt, be very irregular on the surface; itwould be irregular in consequence of the upheavals and the outburstsincident to the transformation of so mighty a mass of matter; butirregular though it be, we can be sure that, on the whole, the form ofthe earth's surface would coincide with the shape which it had assumedby the movement of rotation. Hence we can explain the protuberant formof the equator of the earth, and we can appeal to that form incorroboration of the view that this globe was once in a soft or moltencondition. 

The argument may be supported and illustrated by comparing the shape ofour earth with the shapes of some of the other celestial bodies. Thesun, for instance, seems to be almost a perfect globe. No measures thatwe can make show that the polar diameter of the sun is shorter than theequatorial diameter. But this is what we might have expected. No doubtthe sun is rotating on its axis, and, as it is the rotation that causesthe protuberance, why should not the rotation have deformed the sun likethe earth? The probability is that a difference really does existbetween the two diameters of the sun, but that the difference is toosmall for us to measure. It is impossible not to connect this with the_slowness_ of the sun's rotation. The sun takes twenty-five days tocomplete a rotation, and the protuberance appropriate to so low avelocity is not appreciable. 

On the other hand, when we look at one of the quickly-rotating planets, we obtain a very different result. Let us take the very strikinginstance which is presented in the great planet Jupiter. Viewed in thetelescope, Jupiter is at once seen not to be a globe. The difference isso conspicuous that accurate measures are not necessary to show that thepolar diameter of Jupiter is shorter than the equatorial diameter. Thedeparture of Jupiter from the truly spherical shape is indeed muchgreater than the departure of the earth. It is impossible not to connectthis with the much more rapid rotation of Jupiter. We shall presentlyhave to devote a chapter to the consideration of this splendid orb. Wemay, however, so far anticipate what we shall then say as to state thatthe time of Jupiter's rotation is under ten hours, and thisnotwithstanding the fact that Jupiter is more than one thousand timesgreater than the earth. His enormously rapid rotation has caused him tobulge out at the equator to a remarkable extent. 

The survey of our earth and the measurement of its dimensions havingbeen accomplished, the next operation for the astronomer is thedetermination of its weight. Here, indeed, is a problem which taxes theresources of science to the very uttermost. Of the interior of the earthwe know little--I might almost say we know nothing. No doubt we sinkdeep mines into the earth. These mines enable us to penetrate half amile, or even a whole mile, into the depths of the interior. But thisis, after all, only a most insignificant attempt to explore the interiorof the earth. What is an advance of one mile in comparison with thedistance to the centre of the earth? It is only about onefour-thousandth part of the whole. Our knowledge of the earth merelyreaches to an utterly insignificant depth below the surface, and we havenot a conception of what may be the nature of our globe only a few milesbelow where we are standing. Seeing, then, our almost complete ignoranceof the solid contents of the earth, does it not seem a hopeless task toattempt to weigh the entire globe? Yet that problem has been solved, andthe result is known--not, indeed, with the accuracy attained in otherastronomical researches, but still with tolerable approximation. 

It is needless to enunciate the weight of the earth in our ordinaryunits. The enumeration of billions of tons does not convey any distinctimpression. It is a far more natural course to compare the mass of theearth with that of an equal globe of water. We should be prepared tofind that our earth was heavier than a like volume of water. The rockswhich form its surface are heavier, bulk for bulk, than the oceans whichrepose on those rocks. The abundance of metals in the earth, the gradualincrease in the density of the earth, which must arise from the enormouspressure at great depths--all these considerations will prepare us tolearn that the earth is very much heavier than a globe of water of equalsize. 

Newton supposed that the earth was between five and six times as heavyas an equal bulk of water. Nor is it hard to see that such a suggestionis plausible. The rocks and materials on the surface are usually abouttwo or three times as heavy as water, but the density of the interiormust be much greater. There is good reason to believe that down in theremote depths of the earth there is a very large proportion of iron. Aniron earth would weigh about seven times as much as an equal globe ofwater. We are thus led to see that the earth's weight must be probablymore than three, and probably less than seven, times an equal globe ofwater; and hence, in fixing the density between five and six, Newtonadopted a result plausible at the moment, and since shown to be probablycorrect. Several methods have been proposed by which this importantquestion can be solved with accuracy. Of all these methods we shall hereonly describe one, because it illustrates, in a very remarkable manner, the law of universal gravitation. 

In the chapter on Gravitation it was pointed out that the intensity ofthis force between two masses of moderate dimensions was extremelyminute, and the difficulty in weighing the earth arises from this cause. The practical application of the process is encumbered by multitudinousdetails, which it will be unnecessary for us to consider at present. Theprinciple of the process is simple enough. To give definiteness to ourdescription, let us conceive a large globe about two feet in diameter;and as it is desirable for this globe to be as heavy as possible, let ussuppose it to be made of lead. A small globe brought near the large oneis attracted by the force of gravitation. The amount of this attractionis extremely small, but, nevertheless, it can be measured by a refinedprocess which renders extremely small forces sensible. The intensity ofthe attraction depends both on the masses of the globes and on theirdistance apart, as well as on the force of gravitation. We can alsoreadily measure the attraction of the earth upon the small globe. Thisis, in fact, nothing more nor less than the weight of the small globe inthe ordinary acceptation of the word. We can thus compare theattraction exerted by the leaden globe with the attraction exerted bythe earth. 

If the centre of the earth and the centre of the leaden globe were atthe same distance from the attracted body, then the intensity of theirattractions would give at once the ratio of their masses by simpleproportion. In this case, however, matters are not so simple: the leadenball is only distant by a few inches from the attracted ball, while thecentre of the earth's attraction is nearly 4, 000 miles away at thecentre of the earth. Allowance has to be made for this difference, andthe attraction of the leaden sphere has to be reduced to what it wouldbe were it removed to a distance of 4, 000 miles. This can fortunately beeffected by a simple calculation depending upon the general law that theintensity of gravitation varies inversely as the square of the distance. We can thus, partly by calculation and partly by experiment, compare theintensity of the attraction of the leaden sphere with the attraction ofthe earth. It is known that the attractions are proportional to themasses, so that the comparative masses of the earth and of the leadensphere have been measured; and it has been ascertained that the earth isabout half as heavy as a globe of lead of equal size would be. We maythus state finally that the mass of the earth is about five and a halftimes as great as the mass of a globe of water equal to it in bulk. 

In the chapter on Gravitation we have mentioned the fact that a body letfall near the surface of the earth drops through sixteen feet in thefirst second. This distance varies slightly at different parts of theearth. If the earth were a perfect sphere, then the attraction would bethe same at every part, and the body would fall through the samedistance everywhere. The earth is not round, so the distance which thebody falls in one second differs slightly at different places. At thepole the radius of the earth is shorter than at the equator, andaccordingly the attraction of the earth at the pole is greater than atthe equator. Had we accurate measurements showing the distance a bodywould fall in one second both at the pole and at the equator, we shouldhave the means of ascertaining the shape of the earth. 

It is, however, difficult to measure correctly the distance a body willfall in one second. We have, therefore, been obliged to resort to othermeans for determining the force of attraction of the earth at theequator and other accessible parts of its surface. The methods adoptedare founded on the pendulum, which is, perhaps, the simplest andcertainly one of the most useful of philosophical instruments. The idealpendulum is a small and heavy weight suspended from a fixed point by afine and flexible wire. If we draw the pendulum aside from its verticalposition and then release it, the weight will swing to and fro. 

For its journey to and fro the pendulum requires a small period of time. It is very remarkable that this period does not depend appreciably onthe length of the circular arc through which the pendulum swings. Toverify this law we suspend another pendulum beside the first, both beingof the same length. If we draw both pendulums aside and then releasethem, they swing together and return together. This might have beenexpected. But if we draw one pendulum a great deal to one side, and theother only a little, the two pendulums still swing sympathetically. This, perhaps, would not have been expected. Try it again, with even astill greater difference in the arc of vibration, and still we see thetwo weights occupy the same time for the swing. 

We can vary the experiment in another way. Let us change the weights onthe pendulums, so that they are of unequal size, though both of iron. Shall we find any difference in the periods of vibration? We try again:the period is the same as before; swing them through different arcs, large or small, the period is still the same. But it may be said thatthis is due to the fact that both weights are of the same material. Tryit again, using a leaden weight instead of one of the iron weights; theresult is identical. Even with a ball of wood the period of oscillationis the same as that of the ball of iron, and this is true no matter whatbe the arc through which the vibration takes place. 

If, however, we change the _length_ of the wire by which the weight issupported, then the period will not remain unchanged. This can be veryeasily illustrated. Take a short pendulum with a wire only one-fourth ofthe length of that of the long one; suspend the two close together, andcompare the periods of vibration of the short pendulum with that of thelong one, and we find that the former has a period only half that of thelatter. We may state the result generally, and say that the time ofvibration of a pendulum is proportional to the square root of itslength. If we quadruple the length of the suspending cord we double thetime of its vibration; if we increase the length of the pendulumninefold, we increase its period of vibration threefold. 

It is the gravitation of the earth which makes the pendulum swing. Thegreater the attraction, the more rapidly will the pendulum oscillate. This may be easily accounted for. If the earth pulls the weight downvery vigorously, the time will be short; if the power of the earth'sattraction be lessened, then it cannot pull the weight down so quickly, and the period will be lengthened. 

The time of vibration of the pendulum can be determined with greataccuracy. Let it swing for 10, 000 oscillations, and measure the timethat these oscillations have consumed. The arc through which thependulum swings may not have remained quite constant, but this does notappreciably affect the _time_ of its oscillation. Suppose that an errorof a second is made in the determination of the time of 10, 000oscillations; this will only entail an error of the ten-thousandth partof the second in the time of a single oscillation, and will afford acorrespondingly accurate determination of the force of gravity at theplace where the experiment was made. 

Take a pendulum to the equator. Let it perform 10, 000 oscillations, anddetermine carefully the _time_ that these oscillations have required. Bring the same pendulum to another part of the earth, and repeat theexperiment. We have thus a means of comparing the gravitation at the twoplaces. There are, no doubt, a multitude of precautions to be observedwhich need not here concern us. It is not necessary to enter intodetails as to the manner in which the motion of the pendulum is to besustained, nor as to the effect of changes of temperature in thealteration of its length. It will suffice for us to see how the time ofthe pendulum's swing can be measured accurately, and how from thatmeasurement the intensity of gravitation can be calculated. 

The pendulum thus enables us to make a gravitational survey of thesurface of the earth with the highest degree of accuracy. We cannot, however, infer that gravity alone affects the oscillations of thependulum. We have seen how the earth rotates on its axis, and we haveattributed the bulging of the earth at the equator to this influence. But the centrifugal force arising from the rotation has the effect ofdecreasing the apparent weight of bodies, and the change is greatest atthe equator, and lessens gradually as we approach the poles. From thiscause alone the attraction of the pendulum at the equator is less thanelsewhere, and therefore the oscillations of the pendulum will take alonger time there than at other localities. A part of the apparentchange in gravitation is accordingly due to the centrifugal force; butthere is, in addition, a real alteration. 

In a work on astronomy it does not come within our scope to enter intofurther detail on the subject of our planet. The surface of the earth, its contour and its oceans, its mountain chains and its rivers, are forthe physical geographer; while its rocks and their contents, itsvolcanoes and its earthquakes, are to be studied by the geologists andthe physicists. 

CHAPTER X. 

MARS. 

 Our nearer Neighbours in the Heavens--Surface of Mars can be Examined in the Telescope--Remarkable Orbit of Mars--Resemblance of Mars to a Star--Meaning of Opposition--The Eccentricity of the Orbit of Mars--Different Oppositions of Mars--Apparent Movements of the Planet--Effect of the Earth's Movement--Measurement of the Distance of Mars--Theoretical Investigation of the Sun's Distance--Drawings of the Planet--Is there Snow on Mars?--The Rotation of the Planet--Gravitation on Mars--Has Mars any Satellites?--Prof. Asaph Hall's great Discovery--The Revolutions of the Satellites--Deimos and Phobos--"Gulliver's Travels. "

The special relation in which we stand to one planet of our system hasnecessitated a somewhat different treatment of that globe from thetreatment appropriate to the others. We discussed Mercury and Venus asdistant objects known chiefly by telescopic research, and bycalculations of which astronomical observations were the foundation. Ourknowledge of the earth is of a different character, and attained in adifferent way. Yet it was necessary for symmetry that we should discussthe earth after the planet Venus, in order to give to the earth its trueposition in the solar system. But now that the earth has been passed inour outward progress from the sun, we come to the planet Mars; and hereagain we resume, though in a somewhat modified form, the methods thatwere appropriate to Venus and to Mercury. 

Venus and Mars have, from one point of view, quite peculiar claims onour attention. They are our nearest planetary neighbours, on eitherside. We may naturally expect to learn more of them than of the otherplanets farther off. In the case of Venus, however, this anticipationcan hardly be realised, for, as we have already pointed out, its denseatmosphere prevents us from making a satisfactory telescopicexamination. When we turn to our other planetary neighbour, Mars, we areenabled to learn a good deal with regard to his appearance. Indeed, withthe exception of the moon, we are better acquainted with the details ofthe surface of Mars than with those of any other celestial body. 

This beautiful planet offers many features for consideration besidesthose presented by its physical structure. The orbit of Mars is one ofremarkable proportions, and it was by the observations of this orbitthat the celebrated laws of Kepler were discovered. During theoccasional approaches of Mars to the earth it has been possible tomeasure its distance with accuracy, and thus another method of findingthe sun's distance has arisen which, to say the least, may compete inprecision with that afforded by the transit of Venus. It must also beobserved that the greatest achievement in pure telescopic research whichthis century has witnessed was that of the discovery of the satellitesof Mars. 

To the unaided eye this planet generally appears like a star of thefirst magnitude. It is usually to be distinguished by its ruddy colour, but the beginner in astronomy cannot rely on its colour only for theidentification of Mars. There are several stars nearly, if not quite, asruddy as this globe. The bright star Aldebaran, the brightest star inthe constellation of the Bull, has often been mistaken for the planet. It often resembles Betelgeuze, a brilliant point in the constellation ofOrion. Mistakes of this kind will be impossible if the learner has firststudied the principal constellations and the more brilliant stars. Hewill then find great interest in tracing out the positions of theplanets, and in watching their ceaseless movements. 

[Illustration: Fig. 48. --The Orbits of the Earth and of Mars, showingthe Favourable Opposition of 1877. ]

The position of each orb can always be ascertained from the almanac. Sometimes the planet will be too near the sun to be visible. It willrise with the sun and set with the sun, and consequently will not beabove the horizon during the night. The best time for seeing one of theplanets situated like Mars will be during what is called its opposition. This state of things occurs when the earth intervenes directly betweenthe planet and the sun. In this case, the distance from Mars to theearth is less than at any other time. There is also another advantage inviewing Mars during opposition. The planet is then at one side of theearth and the sun at the opposite side, so that when Mars is high in theheavens the sun is directly beneath the earth; in other words, theplanet is then at its greatest elevation above the horizon at midnight. Some oppositions of Mars are, however, much more favourable than others. This is distinctly shown in Fig. 48, which represents the orbit of Marsand the orbit of the Earth accurately drawn to scale. It will be seenthat while the orbit of the earth is very nearly circular, the orbit ofMars has a very decided degree of eccentricity; indeed, with theexception of the orbit of Mercury, that of Mars has the greatesteccentricity of any orbit of the larger planets in our system. 

The value of an opposition of Mars for telescopic purposes will varygreatly according to circumstances. The favourable oppositions will bethose which occur as near as possible to the 26th of August. The otherextreme will be found in an opposition which occurs near the 22nd ofFebruary. In the latter case the distance between the planet and theearth is nearly twice as great as the former. The last opposition whichwas suitable for the highest class of work took place in the year 1877. Mars was then a magnificent object, and received much, and deserved, attention. The favourable oppositions follow each other at somewhatirregular intervals; the last occurred in the year 1892, and anotherwill take place in the year 1909. 

The apparent movements of Mars are by no means simple. We can imaginethe embarrassment of the early astronomer who first undertook the taskof attempting to decipher these movements. The planet is seen to be abrilliant and conspicuous object. It attracts the astronomer'sattention; he looks carefully, and he sees how it lies among theconstellations with which he is familiar. A few nights later he observesthe same body again; but is it exactly in the same place? He thinks not. He notes more carefully than before the place of the planet. He sees howit is situated with regard to the stars. Again, in a few days, hisobservations are repeated. There is no longer a trace of doubt about thematter--Mars has decidedly changed his position. It is veritably awanderer. 

Night after night the primitive astronomer is at his post. He notes thechanges of Mars. He sees that it is now moving even more rapidly than itwas at first. Is it going to complete the circuit of the heavens? Theastronomer determines to watch the orb and see whether this surmise isjustified. He pursues his task night after night, and at length hebegins to think that the body is not moving quite so rapidly as atfirst. A few nights more, and he is sure of the fact: the planet ismoving more slowly. Again a few nights more, and he begins to surmisethat the motion may cease; after a short time the motion does cease, andthe object seems to rest; but is it going to remain at rest for ever?Has its long journey been finished? For many nights this seems to be thecase, but at length the astronomer suspects that the planet must becommencing to move backwards. A few nights more, and the fact isconfirmed beyond possibility of doubt, and the extraordinary discoveryof the direct and the retrograde movement of Mars has been accomplished. 

[Illustration: Fig. 49. --The Apparent Movements of Mars In 1877. ]

In the greater part of its journey around the heavens Mars seems to movesteadily from the west to the east. It moves backwards, in fact, as themoon moves and as the sun moves. It is only during a comparatively smallpart of its path that those elaborate movements are accomplished whichpresented such an enigma to the primitive observer. We show in theadjoining picture (Fig. 49) the track of the actual journey which Marsaccomplished in the opposition of 1877. The figure only shows that partof its path which presents the anomalous features; the rest of the orbitis pursued, not indeed with uniform velocity, but with unaltereddirection. 

This complexity of the apparent movements of Mars seems at first sightfatal to the acceptance of any simple and elementary explanation of theplanetary motion. If the motion of Mars were purely elliptic, how, itmay well be said, could it perform this extraordinary evolution? Theelucidation is to be found in the fact that the earth on which we standis itself in motion. Even if Mars were at rest, the fact that the earthmoves would make the planet appear to move. The apparent movements ofMars are thus combined with the real movements. This circumstance willnot embarrass the geometer. He is able to disentangle the true movementof the planet from its association with the apparent movement, and toaccount completely for the complicated evolutions exhibited by Mars. Could we transfer our point of view from the ever-shifting earth to animmovable standpoint, we should then see that the shape of the orbit ofMars was an ellipse, described around the sun in conformity with thelaws which Kepler discovered by observations of this planet. 

Mars takes 687 days to travel round the sun, its average distance fromthat body being 141, 500, 000 miles. Under the most favourablecircumstances the planet, at the time of opposition, may approach theearth to a distance not greater than about 35, 500, 000 miles. No doubtthis seems an enormous distance, when estimated by any standard adaptedfor terrestrial measurements; it is, however, hardly greater than thedistance of Venus when nearest, and it is much less than the distancefrom the earth to the sun. 

We have explained how the _form_ of the solar system is known fromKepler's laws, and how the absolute size of the system and of itsvarious parts can be known when the direct measurement of any one parthas been accomplished. A close approach of Mars affords a favourableopportunity for measuring his distance, and thus, in a different way, solving the same problem as that investigated by the transit of Venus. We are thus led a second time to a knowledge of the distance of the sunand the distances of the planets generally, and to many other numericalfacts about the solar system. 

On the occasion of the opposition of Mars in 1877 a successful attemptwas made to apply this refined process to the solution of the problem ofcelestial measurement. It cannot be said to have been the first occasionon which this method was suggested, or even practically attempted. Theobservations of 1877 were, however, conducted with such skill and withsuch minute attention to the necessary precautions as to render them animportant contribution to astronomy. Dr. David Gill, now her Majesty'sAstronomer at the Cape of Good Hope, undertook a journey to the Islandof Ascension for the purpose of observing the parallax of Mars in 1877. On this occasion Mars approached to the earth so closely as to afford anadmirable opportunity for the application of the method. Dr. Gillsucceeded in obtaining a valuable series of measurements, and from themhe concluded the distance of the sun with an accuracy somewhat superiorto that attainable by the transit of Venus. 

There is yet another method by which Mars can be made to give usinformation as to the distance of the sun. This method is one of somedelicacy, and is interesting from its connection with the loftiestenquiries in mathematical astronomy. It was foreshadowed in theDynamical theory of Newton, and was wrought to perfection by Le Verrier. It is based upon the great law of gravitation, and is intimatelyassociated with the splendid discoveries in planetary perturbation whichform so striking a chapter in modern astronomical discovery. 

There is a certain relation between two quantities which at first sightseems quite independent. These quantities are the mass of the earth andthe distance of the sun. The distance of the sun bears to a certaindistance (which can be calculated when we know the intensity ofgravitation at the earth's surface, the size of the earth and the lengthof the year) the same proportion that the cube root of the sun's massbears to the cube root of that of the earth. There is no uncertaintyabout this result, and the consequence is obvious. If we have the meansof weighing the earth in comparison with the sun, then the distance ofthe sun can be immediately deduced. How are we to place our great earthin the weighing scales? This is the problem which Le Verrier has shownus how to solve, and he does so by invoking the aid of the planet Mars. 

If Mars in his revolution around the sun were solely swayed by theattraction of the sun, he would, in accordance with the well-known lawsof planetary motion, follow for ever the same elliptic path. At the endof one century, or even of many centuries, the shape, the size, and theposition of that ellipse would remain unaltered. Fortunately for ourpresent purpose, a disturbance in the orbit of Mars is produced by theearth. Although the mass of our globe is so much less than that of thesun, yet the earth is still large enough to exercise an appreciableattraction on Mars. The ellipse described by the planet is consequentlynot invariable. The shape of that ellipse and its position graduallychange, so that the position of the planet depends to some extent uponthe mass of the earth. The place in which the planet is found can bedetermined by observation; the place which the planet would have had ifthe earth were absent can be found by calculation. The differencebetween the two is due to the attraction of the earth, and, when it hasbeen measured, the mass of the earth can be ascertained. The amount ofdisplacement increases from one century to another, but as the rate ofgrowth is small, ancient observations are necessary to enable themeasures to be made with accuracy. 

A remarkable occurrence which took place more than two centuries agofortunately enables the place of Mars to be determined with greatprecision at that date. On the 1st of October, 1672, three independentobservers witnessed the occultation of a star in Aquarius by the ruddyplanet. The place of the star is known with accuracy, and hence we areprovided with the means of indicating the exact point in the heavensoccupied by Mars on the day in question. From this result, combined withthe modern meridian observations, we learn that the displacement of Marsby the attraction of the earth has, in the lapse of two centuries, grownto about five minutes of arc (294 seconds). It has been maintained thatthis cannot be erroneous to the extent of more than a second, and henceit would follow that the earth's mass is determined to about onethree-hundredth part of its amount. If no other error were present, thiswould give the sun's distance to about one nine-hundredth part. 

[Illustration: Fig. 50. --Relative Sizes of Mars and the Earth. ]

Notwithstanding the intrinsic beauty of this method, and the very highauspices under which it has been introduced, it is, we think, at presenthardly worthy of reliance in comparison with some of the other methods. As the displacement of Mars, due to the perturbing influence of theearth, goes on increasing continually, it will ultimately attainsufficient magnitude to give a very exact value of the earth's mass, andthen this method will give us the distance of the sun with greatprecision. But interesting and beautiful though this method may be, wemust as yet rather regard it as a striking confirmation of the law ofgravitation than as affording an accurate means of measuring the sun'sdistance. 

[Illustration: Fig. 51. --Drawing of Mars (July 30th, 1894). ]

[Illustration: Fig. 52. --Drawing of Mars (August 16th, 1894). ]

[Illustration: Fig. 53. --Elevations and Depressions on the "Terminator"of Mars (August 24th, 1894). ]

[Illustration: Fig. 54. --The Southern Polar Cap on Mars (July 1, 1894). ]

The close approaches of Mars to the earth afford us opportunities formaking a careful telescopic scrutiny of his surface. It must not beexpected that the details on Mars could be inspected with the sameminuteness as those on the moon. Even under the most favourablecircumstances, Mars is still more than a hundred times as far as themoon, and, therefore, the features of the planet have to be at least onehundred times as large if they are to be seen as distinctly as thefeatures on the moon. Mars is much smaller than the earth. The diameterof the planet is 4, 200 miles, but little more than half that of theearth. Fig. 50 shows the comparative sizes of the two bodies. We herereproduce two of the remarkable drawings[16] of Mars made by ProfessorWilliam H. Pickering at the Lowell Observatory, Flagstaff A. T. Fig. 51was taken on the 30th of July, 1894, and Fig. 52 on the 16th of August, 1894. 

The southern polar cap on Mars, as seen by Professor William H. Pickering at Lowell Observatory on the 1st of July, 1894, is representedin Fig. 54. [17] The remarkable black mark intruding into the polar areawill be noticed. In Fig. 53 are shown a series of unusually markedelevations and depressions upon the "terminator" of the planet, drawn asaccurately as possible to scale by the same skilful hand on the 24th ofAugust, 1894. 

In making an examination of the planet it is to be observed that it doesnot, like the moon, always present the same face towards the observer. Mars rotates upon an axis in exactly the same manner as the earth. It isnot a little remarkable that the period required by Mars for thecompletion of one rotation should be only about half an hour greaterthan the period of rotation of the earth. The exact period is 24 hours, 37 minutes, 22-3/4 seconds. It therefore follows that the aspect of theplanet changes from hour to hour. The western side gradually sinks fromview, the eastern side gradually assumes prominence. In twelve hours theaspect of the planet is completely changed. These changes, together withthe inevitable effects of foreshortening, render it often difficult tocorrelate the objects on the planet with those on the maps. The latter, it must be confessed, fall short of the maps of the moon in definitenessand in certainty; yet there is no doubt that the main features of theplanet are to be regarded as thoroughly established, and someastronomers have given names to all the prominent objects. 

The markings on the surface of Mars are of two classes. Some of them areof an iron-grey hue verging on green, while the others are generallydark yellow or orange, occasionally verging on white. The former haveusually been supposed to represent the tracts of ocean, the latter thecontinental masses on the ruddy planet. We possess a great number ofdrawings of Mars, the earliest being taken in the middle of theseventeenth century. Though these early sketches are very rough, and arenot of much value for the solution of questions of topography, they havebeen found very useful in aiding us to fix the period of rotation of theplanet on its axis by comparison with our modern drawings. 

Early observers had already noticed that each of the poles of Mars isdistinguished by a white spot. It is, however, to William Herschel thatwe owe the first systematic study of these remarkable polar caps. Thisillustrious astronomer was rewarded by a very interesting discovery. Hefound that these arctic tracts on Mars vary both in extent anddistinctness with the seasons of the hemisphere on which they aresituated. They attain a maximum development from three to six monthsafter the winter solstice on that planet, and then diminish until theyare smallest about three to six months after the summer solstice. Theanalogy with the behaviour of the masses of snow and ice which surroundour own poles is complete, and there has until lately been hardly anydoubt that the white polar spots of Mars are somewhat similarlyconstituted. 

As the period of revolution of Mars around the sun is so much longerthan our year, 687 days instead of 365, the seasons of the planet are, of course, also much longer than the terrestrial seasons. In thenorthern hemisphere of Mars the summer lasts for no fewer than 381 days, and the winter must be 306 days. In both hemispheres the white polar capin the course of the long winter season increases until it reaches adiameter of 45° to 50°, while the long summer reduces it to a small areaonly 4° or 5° in diameter. It is remarkable that one of these whiteregions--that at the south pole--seems not to be concentric with thepole, but is placed so much to one side that the south pole of Marsappears to be quite free from ice or snow once a year. 

Although many valuable observations of Mars were made in the course ofthe nineteenth century, it is only since the very favourable oppositionof 1877 that the study of the surface of Mars has made that immenseprogress which is one of the most remarkable features of modernastronomy. Among the observers who produced valuable drawings of theplanet in 1877 we may mention Mr. Green, whose exquisite pictures werepublished by the Royal Astronomical Society, and Professor Schiaparelli, of Milan, who almost revolutionised our knowledge of this planet. Schiaparelli had a refractor of only eight inches aperture at hisdisposal, but he was doubtless much favoured by the purity of theItalian sky, which enabled him to detect in the bright portions of thesurface of Mars a considerable number of long, narrow lines. To these hegave the name of canals, inasmuch as they issued from the so-calledoceans, and could be traced across the reputed continents forconsiderable distances, which sometimes reached thousands of miles. 

The canals seemed to form a kind of network, which connected the variousseas with each other. A few of the more conspicuous of these so-calledcanals appeared indeed on some of the drawings made by Dawes and othersbefore Schiaparelli's time. It was, however, the illustrious Italianastronomer who detected that these narrow lines are present in suchgreat numbers as to form a notable feature of the planet. Some of theseremarkable features are shown in Figs. 51 and 52, which are copied fromdrawings made by Professor William H. Pickering at the LowellObservatory in 1894. 

Great as had been the surprise of astronomers when Schiaparelli firstproclaimed the discovery of these numerous canals, it was, perhaps, surpassed by the astonishment with which his announcement was receivedin 1882 that most of the canals had become double. Between December, 1881, and February, 1882, thirty of these duplications appear to havetaken place. Nineteen of these were cases of a well-traced parallel linebeing formed near a previously existing canal. The remaining canals wereless certainly established, or were cases where the two lines did notseem to be quite parallel. A copy of the map of Mars which Schiaparelliformed from his observations of 1881-82 is given in Plate XVIII. Itbrings out clearly these strange double canals, so unlike any featuresthat we know on any other globe. 

[Illustration: PLATE XVIII. 

SCHIAPARELLI'S MAP OF MARS IN 1881-82. ]

Subsequent observations by Schiaparelli and several other observersseem to indicate that this phenomenon of the duplication of the canalsis of a periodic character. It is produced about the times when Marspasses through its equinoxes. One of the two parallel lines is oftensuperposed as exactly as possible upon the track of the old canal. Itdoes, however, sometimes happen that both the lines occupy oppositesides of the former canal and are situated on entirely new ground. Thedistance between the two lines varies from about 360 miles as a maximumdown to the smallest limit distinguishable in our large telescopes, which is something less than thirty miles. The breadth of each of theseremarkable channels may range from the limits of visibility, say, up tomore than sixty miles. 

The duplication of the canals is perhaps the most difficult problemwhich Mars offers to us for solution. Even if we admit that the canalsthemselves represent inlets or channels through which the melted polarsnow makes its way across the equatorial continents, it is not easy tosee how the duplicate canals can arise. This is especially true in thosecases where the original channel seems to vanish and to be replaced bytwo quite new canals, each about the breadth of the English Channel, andlying one on each side of the course of the old one. The very obviousexplanation that the whole duplication is an optical illusion has beenbrought forward more than once, but never in a conclusive manner. Wemust, perhaps, be content to let the solution of this matter rest forthe present, in the hope that the extraordinary attention which thisplanet is now receiving will in due time explain the present enigma. 

The markings on the surface of this planet are, generally speaking, of apermanent character, so that when we compare drawings made one or twohundred years ago with drawings made more recently we can recognise ineach the same features. This permanence is, however, not nearly soabsolute as it is in the case of the moon. In addition to the canalswhich we have already considered, many other parts of the surface ofMars alter their outlines from time to time. This is particularly thecase with those dark spots which we call oceans, the contours of whichsometimes undergo modifications in matters of detail which are quiteunmistakable. Changes of colour are often observed on parts of theplanet, and though some of these observations may perhaps be attributedto the influence of our own atmosphere on the planet's appearance, theycannot be all thus accounted for. Some of the phenomena must certainlybe due to actual changes which have taken place on the surface of Mars. 

As an example of such changes, we may refer to the north-western part ofthe notable feature, to which Schiaparelli has given the name of _Syrtismajor_. [18] This has at various times been recorded as grey, green, blue, brown, and even violet. When this region (about the time of theautumnal equinox of the northern hemisphere) is situated in the middleof the visible disc, the eastern part is distinctly greener than thewestern. As the season progresses this characteristic colour getsfeebler, until the green tint is to be perceived only on the shores ofthe Syrtis. The atmosphere of Mars is usually very transparent, andfortunately allows us to scrutinise the surface of the planet withoutputting obstacles in the way m the shape of Martian clouds. Such clouds, however, are not invariably absent. Our view of the surface isoccasionally obstructed in such a manner as to make it certain thatclouds or mist in the atmosphere of Mars must be the cause of thetrouble. 

Would we form an idea of the physical constitution of the surface ofMars, then the question as to the character of the atmosphere of theplanet is among the first to be considered. Spectroscopic observationsdo not in this case render us much assistance. Of course, we know thatthe planet has no intrinsic light. It merely shines by reflectedsunlight. The hemisphere which is turned towards the sun is bright, andthe hemisphere which is turned away from the sun is dark. The spectrumought, therefore, like that of the moon, to be an exact though faintcopy of the solar spectrum, unless the sun's rays, by passing twicethrough the atmosphere of Mars, suffered some absorption which couldgive rise to additional dark lines. Some of the earlier observersthought that they could distinctly make out some such lines due, as wassupposed, to water vapour. The presence of such lines is, however, denied by Mr. Campbell, of the Lick Observatory, and Professor Keeler, at the Allegheny Observatory, [19] who, with their unrivalledopportunities, both instrumental and climatic, could find no differencebetween the spectra of Mars and the moon. If Mars had an atmosphere ofappreciable extent, its absorptive effect should be noticeable, especially at the limb of the planet; but Mr. Campbell's observations donot show any increased absorption at the limb. It would therefore seemthat Mars cannot have an extensive atmosphere, and this conclusion isconfirmed in several other ways. 

The distinctness with which we see the surface of this planet tends toshow that the atmosphere must be very thin as compared with our own. There can hardly be any doubt that an observer on Mars with a goodtelescope would be unable to distinguish much of the features of theearth's surface. This would be the case not only by reason of the strongabsorption of the light during the double passage through ouratmosphere, but also on account of the great diffusion of the lightcaused by this same atmosphere. Also, it is needless to say, the greatamount of cloud generally floating over the earth would totally obscuremany parts of our planet from a Martian observer. But though, as alreadymentioned, we occasionally find parts of Mars rendered indistinct, itmust be acknowledged that the clouds on Mars are very slight. We shouldexpect that the polar caps, if composed of snow, would, when melting, produce clouds which would more or less hide the polar regions from ourinspection; yet nothing of the kind has ever been seen. 

We have seen that there are very grave doubts as to the existence ofwater on Mars. No doubt we have frequently spoken of the dark markingsas "oceans" and of the bright parts as "continents. " That this languagewas just has been the opinion of astronomers for a very long time. A fewyears ago Mr. Schaeberle, of the Lick Observatory, came to the veryopposite conclusion. He contended that the dark parts were thecontinents and the bright ones were the oceans of water, or some otherfluid. He pointed to the irregular shading of the dark parts, which doesnot suggest the idea of light reflected from a spherical surface ofwater, especially as the contrasts between light and shade are strongestabout the middle of the disc. 

It is also to be noticed that the dark regions are not infrequentlytraversed by still darker streaks, which can be traced for hundreds ofmiles almost in straight lines, while the so-called canals in the brightparts often seem to be continuations of these same lines. Mr. Schaeberletherefore suggests that the canals may be chains of mountains stretchingover sea and land! The late Professor Phillips and Mr. H. D. Taylor havepointed out that if there were lakes or seas in the tropical regions ofMars we should frequently see the sun directly reflected from them, thusproducing a bright, star-like point which could not escape observation. Even moderately disturbed water would make its presence known in thismanner, and yet nothing of the kind has ever been recorded. 

On the question as to the possibility of life on Mars a few words may beadded. If we could be certain of the existence of water on Mars, thenone of the fundamental conditions would be fulfilled; and even thoughthe atmosphere on Mars had but few points of resemblance either incomposition or in density to the atmosphere of the earth, life mightstill be possible. Even if we could suppose that a man would findsuitable nutriment for his body and suitable air for his respiration, itseems very doubtful whether he would be able to live. Owing to the smallsize of Mars and the smallness of its mass in comparison with theearth, the intensity of the gravitation on the neighbouring planet wouldbe different from the attraction on the surface of the earth. We havealready alluded to the small gravitation on the moon, and in a lesserdegree the same remarks will apply to Mars. A body which weighs on theearth two pounds would on the surface of Mars weigh rather less than onepound. Nearly the same exertion which will raise a 56-lb. Weight on theearth would lift two similar weights on Mars. 

The earth is attended by one moon. Jupiter is attended by fourconspicuous moons. Mars is a planet revolving between the orbits of theearth and of Jupiter. It is a body of the same general type as the earthand Jupiter. It is ruled by the same sun, and all three planets formpart of the same system; but as the earth has one moon and Jupiter fourmoons, why should not Mars also have a moon? No doubt Mars is a smallbody, less even than the earth, and much less than Jupiter. We could notexpect Mars to have large moons, but why should it be unlike its twoneighbours, and not have any moon at all? So reasoned astronomers, butuntil modern times no satellite of Mars could be found. For centuriesthe planet has been diligently examined with this special object, and asfailure after failure came to be recorded, the conclusion seemed almostto be justified that the chain of analogical reasoning had broken down. The moonless Mars was thought to be an exception to the rule that allthe great planets outside Venus were dignified by an attendant retinueof satellites. It seemed almost hopeless to begin again a research whichhad often been tried, and had invariably led to disappointment; yet, fortunately, the present generation has witnessed still one more attack, conducted with perfect equipment and with consummate skill This attempthas obtained the success it so well merited, and the result has been thememorable detection of two satellites of Mars. 

This discovery was made by Professor Asaph Hall, the distinguishedastronomer at the observatory of Washington. Mr. Hall was provided withan instrument of colossal proportions and of exquisite workmanship, known as the great Washington refractor. It is the product of thecelebrated workshop of Messrs. Alvan Clark and Sons, from which so manylarge telescopes have proceeded, and in its noble proportions farsurpassed any other telescope ever devoted to the same research. Theobject-glass measures twenty-six inches in diameter, and is hardly lessremarkable for the perfection of its definition than for its size. Buteven the skill of Mr. Hall, and the space-penetrating power of histelescope, would not have been able on ordinary occasions to discoverthe satellites of Mars. Advantage was accordingly taken of thatmemorable opposition of Mars in 1877, when, as we have alreadydescribed, the planet came unusually near the earth. 

Had Mars been attended by a moon one-hundredth part of the bulk of ourmoon it must long ago have been discovered. Mr. Hall, therefore, knewthat if there were any satellites they must be extremely small bodies, and he braced himself for a severe and diligent search. Thecircumstances were all favourable. Not only was Mars as near as it wellcould be to the earth; not only was the great telescope at Washingtonthe most powerful refractor then in existence; but the situation ofWashington is such that Mars was seen from the observatory at a highelevation. It was while the British Association were meeting atPlymouth, in 1877, that a telegram flashed across the Atlantic. Brilliant success had rewarded Mr. Hall's efforts. He had hoped todiscover one satellite. The discovery of even one would have made thewhole scientific world ring; but fortune smiled on Mr. Hall. Hediscovered first one satellite, and then he discovered a second; and, inconnection with these satellites, he further discovered a unique fact inthe solar system. 

Deimos, the outer of the satellites, revolves around the planet in theperiod of 30 hours, 17 mins. , 54 secs. ; it is the inner satellite, Phobos, which has commanded the more special attention of everyastronomer in the world. Mars turns round on his axis in a Martial day, which is very nearly the same length as our day of twenty-four hours. The inner satellite of Mars moves round in 7 hours, 39 mins. , 14 secs. Phobos, in fact, revolves three times round Mars in the same time thatMars can turn round once. This circumstance is unparalleled in the solarsystem; indeed, as far as we know, it is unparalleled in the universe. In the case of our own planet, the earth rotates twenty-seven times forone revolution of the moon. To some extent the same may be said ofJupiter and of Saturn; while in the great system of the sun himself andthe planets, the sun rotates on his axis several times for eachrevolution of even the most rapidly moving of the planets. There is noother known case where the satellite revolves around the primary morequickly than the primary rotates on its axis. The anomalous movement ofthe satellite of Mars has, however, been accounted for. In a subsequentchapter we shall again allude to this, as it is connected with animportant department of modern astronomy. 

The satellites are so small that we are unable to measure theirdiameters directly, but from observations of their brightness it isevident that their diameters cannot exceed twenty or thirty miles, andmay be even smaller. Owing to their rapid motion the two satellites mustpresent some remarkable peculiarities to an observer on Mars. Phobosrises in the west, passes across the heavens, and sets in the east afterabout five and a half hours, while Deimos rises in the east and remainsmore than two days above the horizon. 

As the satellites revolve in paths vertically above the equator of theirprimary, the one less than 4, 000 miles and the other only some 14, 500miles above the surface, it follows that they can never be visible fromthe poles of Mars; indeed, to see Phobos, the observer's planetarylatitude must not be above 68-3/4°. If it were so, the satellite wouldbe hidden by the body of Mars, just as we, in the British Islands, wouldbe unable to see an object revolving round the earth a few hundred milesabove the equator. 

Before passing from the attractive subject of the satellites, we mayjust mention two points of a literary character. Mr. Hall consulted hisclassical friends as to the designation to be conferred on the twosatellites. Homer was referred to, and a passage in the "Iliad"suggested the names of Deimos and Phobos. These personages were theattendants of Mars, and the lines in which they occur have been thusconstrued by my friend Professor Tyrrell:--

 "Mars spake, and called Dismay and Rout To yoke his steeds, and he did on his harness sheen. "

A curious circumstance with respect to the satellites of Mars will befamiliar to those who are acquainted with "Gulliver's Travels. " Theastronomers on board the flying Island of Laputa had, according toGulliver, keen vision and good telescopes. The traveller says that theyhad found two satellites to Mars, one of which revolved around him inten hours, and the other in twenty-one and a half. The author has thusnot only made a correct guess about the number of the satellites, but heactually stated the periodic time with considerable accuracy! We do notknow what can have suggested the latter guess. A few years ago anyastronomer reading the voyage to Laputa would have said this was absurd. There might be two satellites to Mars, no doubt; but to say that one ofthem revolves in ten hours would be to assert what no one could believe. Yet the truth has been even stranger than the fiction. 

And now we must bring to a close our account of this beautiful andinteresting planet. There are many additional features over which we aretempted to linger, but so many other bodies claim our attention in thesolar system, so many other bodies which exceed Mars in size andintrinsic importance, that we are obliged to desist. Our next step willnot, however, at once conduct us to the giant planets. We find outsideMars a host of objects, small indeed, but of much interest; and withthese we shall find abundant occupation for the following chapter. 

CHAPTER XI. 

THE MINOR PLANETS. 

 The Lesser Members of our System--Bode's Law--The Vacant Region in the Planetary System--The Research--The Discovery of Piazzi--Was the small Body a Planet?--The Planet becomes Invisible--Gauss undertakes the Search by Mathematics--The Planet Recovered--Further Discoveries--Number of Minor Planets now known--The Region to be Searched--The Construction of the Chart for the Search for Small Planets--How a Minor Planet is Discovered--Physical Nature of the Minor Planets--Small Gravitation on the Minor Planets--The Berlin Computations--How the Minor Planets tell us the Distance of the Sun--Accuracy of the Observations--How they may be Multiplied--Victoria and Sappho--The most Perfect Method. 

In our chapters on the Sun and Moon, on the Earth and Venus, and onMercury and Mars, we have been discussing the features and the movementsof globes of vast dimensions. The least of all these bodies is the moon, but even that globe is 2, 000 miles from one side to the other. Inapproaching the subject of the minor planets we must be prepared to findobjects of dimensions quite inconsiderable in comparison with the greatspheres of our system. No doubt these minor planets are all of them somefew miles, and some of them a great many miles, in diameter. Were theyclose to the earth they would be conspicuous, and even splendid, objects; but as they are so distant they do not, even in our greatesttelescopes, become very remarkable, while to the unaided eye they arealmost all invisible. 

In the diagram (p. 234) of the orbits of the various planets, it isshown that a wide space exists between the orbit of Mars and that ofJupiter. It was often surmised that this ample region must be tenantedby some other planet. The presumption became much stronger when aremarkable law was discovered which exhibited, with considerableaccuracy, the relative distances of the great planets of our system. Take the series of numbers, 0, 3, 6, 12, 24, 48, 96, whereof each number(except the second) is double of the number which precedes it. If we nowadd four to each, we have the series 4, 7, 10, 16, 28, 52, 100. With theexception of the fifth of these numbers (28), they are all sensiblyproportional to the distances of the various planets from the sun. Infact, the distances are as follows:--Mercury, 3·9; Venus, 7·2; Earth, 10; Mars, 15·2; Jupiter, 52·9; Saturn, 95·4. Although we have nophysical reason to offer why this law--generally known as Bode's--shouldbe true, yet the fact that it is so nearly true in the case of all theknown planets tempts us to ask whether there may not also be a planetrevolving around the sun at the distance represented by 28. 

So strongly was this felt at the end of the eighteenth century that someenergetic astronomers decided to make a united effort to search for theunknown planet. It seemed certain that the planet could not be a largeone, as otherwise it must have been found long ago. If it should exist, then means were required for discriminating between the planet and thehosts of stars strewn along its path. 

The search for the small planet was soon rewarded by a success which hasrendered the evening of the first day in the nineteenth centurymemorable in astronomy. It was in the pure skies of Palermo that theobservatory was situated where the memorable discovery of the firstknown minor planet was made by Piazzi. This laborious and accomplishedastronomer had organised an ingenious system of exploring the heavenswhich was eminently calculated to discriminate a planet among the starryhost. On a certain night he would select a series of stars to the numberof fifty, more or less, according to circumstances. With his meridiancircle he determined the places of the chosen objects. The followingnight, or, at all events, as soon as convenient, he re-observed thewhole fifty stars with the same instrument and in the same manner, andthe whole operation was afterwards repeated on two, or perhaps more, nights. When the observations were compared together he was inpossession of some four or more places of each one of the stars ondifferent nights, and the whole series was complete. He was perseveringenough to carry on these observations for very many groups, and atlength he was rewarded by a success which amply compensated him for allhis toil. 

It was on the 1st of January, 1801, that Piazzi commenced for the onehundred and fifty-ninth time to observe a new series. Fifty stars thisnight were viewed in his telescope, and their places were carefullyrecorded. Of these objects the first twelve were undoubtedly stellar, and so to all appearance was the thirteenth, a star of the eighthmagnitude in the constellation of Taurus. There was nothing todistinguish the telescopic appearance of this object from all the otherswhich preceded or followed it. The following night Piazzi, according tohis custom, re-observed the whole fifty stars, and he did the same againon the 3rd of January, and once again on the 4th. He then, as usual, brought together the four places he had found for each of the severalbodies. When this was done it was at once seen that the thirteenthobject on the list was quite a different body from the remainder andfrom all the other stars which he had ever observed before. The fourplaces of this mysterious object were all different; in other words, itwas in movement, and was therefore a planet. 

A few days' observation sufficed to show how this little body, afterwards called Ceres, revolved around the sun, and how it circulatedin that vacant path intermediate between the path of Mars and the pathof Jupiter. Great, indeed, was the interest aroused by this discoveryand the influence which it has exercised on the progress of astronomy. The majestic planets of our system had now to admit a much more humbleobject to a share of the benefits dispensed by the sun. 

After Piazzi had obtained a few further observations, the season forobserving this part of the heavens passed away, and the new planet ofcourse ceased to be visible. In a few months, no doubt, the same part ofthe sky would again be above the horizon after dark, and the stars wouldof course be seen as before. The planet, however, was moving, and wouldcontinue to move, and by the time the next season had arrived it wouldhave passed off into some distant region, and would be again confoundedwith the stars which it so closely resembled. How, then, was the planetto be pursued through its period of invisibility and identified when itagain came within reach of observation?

This difficulty attracted the attention of astronomers, and they soughtfor some method by which the place of the planet could be recovered soas to prevent Piazzi's discovery from falling into oblivion. A youngGerman mathematician, whose name was Gauss, opened his distinguishedcareer by a successful attempt to solve this problem. A planet, as wehave shown, describes an ellipse around the sun, and the sun lies at afocus of that curve. It can be demonstrated that when three positions ofa planet are known, then the ellipse in which the planet moves iscompletely determined. Piazzi had on each occasion measured the placewhich it then occupied. This information was available to Gauss, and theproblem which he had to solve may be thus stated. Knowing the place ofthe planet on three nights, it is required, without any furtherobservations, to tell what the place of the planet will be on a specialoccasion some months in the future. Mathematical calculations, based onthe laws of Kepler, will enable this problem to be solved, and Gausssucceeded in solving it. Gauss demonstrated that though the telescope ofthe astronomer was unable to detect the wanderer during its season ofinvisibility, yet the pen of the mathematician could follow it withunfailing certainty. When, therefore, the progress of the seasonspermitted the observations to be renewed, the search was recommenced. The telescope was directed to the point which Gauss's calculationsindicated, and there was the little Ceres. Ever since its re-discovery, the planet has been so completely bound in the toils of mathematicalreasoning that its place every night of the year can be indicated with afidelity approaching to that attainable in observing the moon or thegreat planets of our system. 

The discovery of one minor planet was quickly followed by similarsuccesses, so that within seven years Pallas, Juno, and Vesta were addedto the solar system. The orbits of all these bodies lie in the regionbetween the orbit of Mars and of Jupiter, and for many years it seems tohave been thought that our planetary system was now complete. Fortyyears later systematic research was again commenced. Planet after planetwas added to the list; gradually the discoveries became a stream ofincreasing volume, until in 1897 the total number reached about 430. Their distribution in the solar system is somewhat as represented inFig. 55. By the improvement of astronomical telescopes, and by thedevotion with which certain astronomers have applied themselves to thisinteresting research, a special method of observing has been created forthe distinct purpose of searching out these little objects. 

It is known that the paths in which all the great planets move throughthe heavens coincide very nearly with the path which the sun appears tofollow among the stars, and which is known as the ecliptic. It isnatural to assume that the small planets also move in the same greathighway, which leads them through all the signs of the zodiac insuccession. Some of the small planets, no doubt, deviate rather widelyfrom the track of the sun, but the great majority are approximately nearit. This consideration at once simplifies the search for new planets. Acertain zone extending around the heavens is to be examined, but thereis in general little advantage in pushing the research into other partsof the sky. 

The next step is to construct a map containing all the stars in thisregion. This is a task of very great labour; the stars visible in thelarge telescopes are so numerous that many tens of thousands, perhaps weshould say hundreds of thousands, are included in the region so narrowlylimited. The fact is that many of the minor planets now known areobjects of extreme minuteness; they can only be seen with very powerfultelescopes, and for their detection it is necessary to use charts onwhich even the faintest stars have been depicted. Many astronomers haveconcurred in the labour of producing these charts; among them may bementioned Palisa, of Vienna, who by means of his charts has foundeighty-three minor planets, and the late Professor Peters, of Clinton, New York, who in a similar way found forty-nine of these bodies. 

[Illustration: Fig. 55. --The Zone of Minor Planets between Mars andJupiter. ]

The astronomer about to seek for a new planet directs his telescopetowards that part of the sun's path which is on the meridian atmidnight; there, if anywhere, lies the chance of success, because thatis the region in which such a body is nearer to the earth than at anyother part of its course. He steadfastly compares his chart with theheavens, and usually finds the stars in the heavens and the stars in thechart to correspond; but sometimes it will happen that a point in theheavens is missing from the chart. His attention is at once arrested; hefollows the object with care, and if it moves it is a planet. Still hecannot be sure that he has really made a discovery; he has found aplanet, no doubt, but it may be one of the large number already known. To clear up this point he must undertake a further, and sometimes a verylaborious, enquiry; he must search the Berlin Year-Book and otherephemerides of such planets and see whether it is possible for one ofthem to have been in the position on the night in question. If he canascertain that no previously discovered body could have been there, heis then entitled to announce to his brother astronomers the discovery ofa new member of the solar system. It seems certain that all the moreimportant of the minor planets have been long since discovered. Therecent additions to the list are generally extremely minute objects, beyond the powers of small telescopes. 

Since 1891 the method of searching for minor planets which we have justdescribed has been almost abandoned in favour of a process greatlysuperior. It has been found feasible to employ photography for makingcharts of the heavens. A photographic plate is exposed in the telescopeto a certain region of the sky sufficiently long to enable very fainttelescopic stars to imprint their images. Care has to be taken that theclock which moves the camera shall keep pace most accurately with therotation of the earth, so that fixed stars appear on the plate as sharppoints. If, on developing the plate, a star is found to have left atrail, it is evident that this star must during the time of exposure(generally some hours) have had an independent motion of its own; inother words, it must be a planet. For greater security a second pictureis generally taken of the same region after a short interval. If theplace occupied by the trail on the first plate is now vacant, while onthe second plate a new trail appears in a line with the first one, thereremains no possible doubt that we have genuine indications of a planet, and that we have not been led astray by some impurity on the plate or bya few minute stars which happened to lie very closely together. Wolf, of Heidelberg, and following in his footsteps Charlois, of Nice, have inthis manner discovered a great number of new minor planets, while theyhave also recovered a good many of those which had been lost sight ofowing to an insufficiency of observations. 

On the 13th of August, 1898, Herr G. Witt, of the observatory of Uraniain Berlin, discovered a new asteroid by the photographic method. Thisobject was at first regarded merely as forming an addition of no specialimportance to the 432 asteroids whose discovery had preceded it. Itreceived, as usual, a provisional designation in accordance with asimple alphabetical device. This temporary label affixed to Witt'sasteroid was "D Q. " But the formal naming of the asteroid has nowsuperseded this label. Herr Witt has given to his asteroid the name of"Eros. " This has been duly accepted by astronomers, and thus for alltime the planet is to be known. 

The feature which makes the discovery of Eros one of the most remarkableincidents in recent astronomy is that on those rare occasions when thisasteroid comes nearest to the earth it is closer to the earth than theplanet Mars can ever be. Closer than the planet Venus can ever be. Closer than any other known asteroid can ever be. Thus we assign to Erosthe exceptional position of being our nearest planetary neighbour in thewhole host of heaven. Under certain circumstances it will have adistance from the earth not exceeding one-seventh of the mean distanceof the sun. 

Of the physical composition of the asteroids and of the character oftheir surfaces we are entirely ignorant. It may be, for anything we cantell, that these planets are globes like our earth in miniature, diversified by continents and by oceans. If there be life on suchbodies, which are often only a few miles in diameter, that life must besomething totally different from anything with which we are familiar. Setting aside every other difficulty arising from the possible absenceof water and from the great improbability of finding there an atmosphereof a density and a composition suitable for respiration, gravitationitself would prohibit organic beings adapted for this earth fromresiding on a minor planet. 

Let us attempt to illustrate this point, and suppose that we take thecase of a minor planet eight miles in diameter, or, in round numbers, one-thousandth part of the diameter of the earth. If we further supposethat the materials of the planet are of the same nature as thesubstances in the earth, it is easy to prove that the gravity on thesurface of the planet will be only one-thousandth part of the gravity ofthe earth. It follows that the weight of an object on the earth would bereduced to the thousandth part if that object were transferred to theplanet. This would not be disclosed by an ordinary weighing scales, where the weights are to be placed in one pan and the body to be weighedin the other. Tested in this way, a body would, of course, weighprecisely the same anywhere; for if the gravitation of the body isaltered, so is also in equal proportion the gravitation of thecounterpoising weights. But, weighed with a spring balance, the changewould be at once evident, and the effort with which a weight could beraised would be reduced to one-thousandth part. A load of one thousandpounds could be lifted from the surface of the planet by the same effortwhich would lift one pound on the earth; the effects which this wouldproduce are very remarkable. 

In our description of the moon it was mentioned (p. 103) that we cancalculate the velocity with which it would be necessary to discharge aprojectile so that it would never again fall back on the globe fromwhich it was expelled. We applied this reasoning to explain why the moonhas apparently altogether lost any atmosphere it might have oncepossessed. 

If we assume for the sake of illustration that the densities of allplanets are identical, then the law which expresses the criticalvelocity for each planet can be readily stated. It is, in fact, simplyproportional to the diameter of the globe in question. Thus, for a minorplanet whose diameter was one-thousandth part of that of the earth, orabout eight miles, the critical velocity would be the thousandth part ofsix miles a second--that is, about thirty feet per second. This is a lowvelocity compared with ordinary standards. A child easily tosses a ballup fifteen or sixteen feet high, yet to carry it up this height it mustbe projected with a velocity of thirty feet per second. A child, standing upon a planet eight miles in diameter, throws his ballvertically upwards; up and up the ball will soar to an amazingelevation. If the original velocity were less than thirty feet persecond, the ball would at length cease to move, would begin to turn, andfall with a gradually accelerating pace, until at length it regained thesurface with a speed equal to that with which it had been projected. Ifthe original velocity had been as much as, or more than, thirty feet persecond, then the ball would soar up and up never to return. In a futurechapter it will be necessary to refer again to this subject. 

A few of the minor planets appear in powerful telescopes as discs withappreciable dimensions, and they have even been measured with themicrometer. In this way Professor Barnard, late of the Lick Observatory, determined the following values for the diameters of the four firstdiscovered minor planets:--

 Ceres 485 miles. Pallas 304 miles. Juno 118 miles. Vesta 243 miles. 

The value for Juno is, however, very uncertain, and by far the greaternumber of the minor planets are very much smaller than the figures heregiven would indicate. It is possible by a certain calculation to form anestimate of the aggregate mass of all the minor planets, inasmuch asobservations disclose to us the extent of their united disturbinginfluences on the motion of Mars. In this manner Le Verrier concludedthat the collected mass of the small planets must be about equal toone-fourth of the mass of the earth. Harzer, repeating the enquiry in animproved manner, deduced a collected mass one-sixth of that of theearth. There can be no doubt that the total mass of all the minorplanets at present known is not more than a very small fraction of theamount to which these calculations point. We therefore conclude thatthere must be a vast number of minor planets which have not yet beenrecognised in the observatory. These unknown planets must be extremelyminute. 

The orbits of this group of bodies differ in remarkable characteristicsfrom those of the larger planets. Some of them are inclined at angles of30° to the plane of the earth's orbit, the inclinations of the greatplanets being not more than a few degrees. Some of the orbits of theminor planets are also greatly elongated ellipses, while, of course, theorbits of the large planets do not much depart from the circular form. The periods of revolution of these small objects round the sun rangefrom three years to nearly nine years. 

A great increase in the number of minor planets has rewarded the zeal ofthose astronomers who have devoted their labours to this subject. Theirsuccess has entailed a vast amount of labour on the computers of the"Berlin Year-Book. " That useful work occupies in this respect a positionwhich has not been taken by our own "Nautical Almanac, " nor by thesimilar publications of other countries. A skilful band of computersmake it their duty to provide for the "Berlin Year-Book" detailedinformation as to the movements of the minor planets. As soon as a fewcomplete observations have been obtained, the little object passes intothe secure grasp of the mathematician; he is able to predict its careerfor years to come, and the announcements with respect to all the knownminor planets are to be found in the annual volumes of the work referredto. 

The growth of discovery has been so rapid that the necessary labour forthe preparation of such predictions is now enormous. It must beconfessed that many of the minor planets are very faint and otherwisedevoid of interest, so that astronomers are sometimes tempted to concurwith the suggestion that a portion of the astronomical labour nowdevoted to the computation of the paths of these bodies might be moreprofitably applied. For this it would be only necessary to cast adriftall the less interesting members of the host, and allow them to pursuetheir paths unwatched by the telescope, or by the still more ceaselesstables of the mathematical computer. 

The sun, which controls the mighty orbs of our system, does not disdainto guide, with equal care, the tiny globes which form the minor planets. At certain times some of them approach near enough to the earth to meritthe attention of those astronomers who are specially interested indetermining the dimensions of the solar system. The observations are ofsuch a nature that they can be made with considerable precision; theycan also be multiplied to any extent that may be desired. Some of theselittle bodies have consequently a great astronomical future, inasmuch asthey seem destined to indicate the true distance from the earth to thesun more accurately than Venus or than Mars. The smallest of theseplanets will not answer for this purpose; they can only be seen inpowerful telescopes, and they do not admit of being measured with thenecessary accuracy. It is also obvious that the planets to be chosen forobservation must come as near the earth as possible. In favourablecircumstances, some of the minor planets will approach the earth to adistance which is about three-quarters of the distance of the sun. Thesevarious conditions limit the number of bodies available for this purposeto about a dozen, of which one or two will usually be suitably placedeach year. 

For the determination of the sun's distance this method by the minorplanets offers unquestionable advantages. The orb itself is a minutestar-like point in the telescope, and the measures are made from it tothe stars which are seen near it. A few words will, perhaps, benecessary at this place as to the nature of the observations referredto. When we speak of the measures from the planet to the star, we do notrefer to what would be perhaps the most ordinary acceptation of theexpression. We do _not_ mean the actual measurement of the number ofmiles in a straight line between the planet and the star. This element, even if attainable, could only be the result of a protracted series ofobservations of a nature which will be explained later on when we cometo speak of the distances of the stars. The measures now referred to areof a more simple character; they are merely to ascertain the apparentdistance of the objects expressed in angular measure. This angularmeasurement is of a wholly different character from the linearmeasurement, and the two methods may, indeed, lead to results that wouldat first seem paradoxical. 

We may take, as an illustration, the case of the group of stars formingthe Pleiades, and those which form the Great Bear. The latter is a largegroup, the former is a small one. But why do we think the words largeand small rightly applied here? Each pair of stars of the Great Bearmakes a large angle with the eye. Each pair of stars in the Pleiadesmakes a small angle, and it is these angles which are the direct objectof astronomical measurement. We speak of the distance of two stars, meaning thereby the angle which is bounded by the two lines from the eyeto the two stars. This is what our instruments are able to measure, andit is to be observed that no reference to linear magnitude is implied. Indeed, if we are to mention actual dimensions, it is quite possible, for anything we can tell, that the Pleiades may form a much larger groupthan the Great Bear, and that the apparent superiority of the latter ismerely due to its being closer to us. The most accurate of these angularmeasures are obtained when two stars, or two star-like points, are soclose together as to enable them to be included in one field of view ofthe telescope. There are special forms of apparatus which enable theastronomer in this case to give to his observations a precisionunattainable in the measurement of objects less definitely marked, or ata greater apparent distance. The determination of the distance of thesmall star-like planet from a star is therefore characterised by greataccuracy. 

But there is another and, perhaps, a weightier argument in favour of thedetermination of the scale of the solar system by this process. The realstrength of the minor planet method rests hardly so much on theindividual accuracy of the observations, as on the fact that from thenature of the method a considerable number of repetitions can beconcentrated on the result. It will, of course, be understood that whenwe speak of the accuracy of an observation, it is not to be presumedthat it can ever be entirely free from error. Errors always exist, andthough they may be small, yet if the quantity to be measured is minute, an error of intrinsic insignificance may amount to an appreciablefraction of the whole. The one way by which their effect can be subduedis by taking the mean of a large number of observations. This is thereal source of the value of the minor planet method. We have not to waitfor the occurrence of rare events like the transit of Venus. Each yearwill witness the approach of some one or more minor planets sufficientlyclose to the earth to render the method applicable. The variedcircumstances attending each planet, and the great variety of theobservations which may be made upon it, will further conduce toeliminate error. 

As the planet pursues its course through the sky, which is everywherestudded over with countless myriads of minute stars, it is evident thatthis body, itself so like a star, will always have some stars in itsimmediate neighbourhood. As the movements of the planet are well known, we can foretell where it will be on each night that it is to beobserved. It is thus possible to prearrange with observers inwidely-different parts of the earth as to the observations to be made oneach particular night. 

An attempt has been made, on the suggestion of Dr. Gill, to carry outthis method on a scale commensurate with its importance. The planetsIris, Victoria, and Sappho happened, in the years 1888 and 1889, toapproach so close to the earth that arrangements were made forsimultaneous measurements in both the northern and the southernhemispheres. A scheme was completely drawn up many months before theobservations were to commence. Each observer who participated in thework was thus advised beforehand of the stars which were to be employedeach night. Viewed from any part of the earth, from the Cape of GoodHope or from Great Britain, the positions of the stars remain absolutelyunchanged. Their distance is so stupendous that a change of place on theearth displaces them to no appreciable extent. But the case is differentwith a minor planet. It is hardly one-millionth part of the distance ofthe stars, and the displacement of the planet when viewed from the Capeand when viewed from Europe is a measurable quantity. 

The magnitude we are seeking is to be elicited by comparison between themeasurements made in the northern hemisphere with those made in thesouthern. The observations in the two localities must be as nearlysimultaneous as possible, due allowance being made for the motion of theplanet in whatever interval may have elapsed. Although every precautionis taken to eliminate the errors of each observation, yet the factremains that we compare the measures made by observers in the northernhemisphere with those made by different observers, using of coursedifferent instruments, thousands of miles away. But in this respect weare at no greater disadvantage than in observing the transit of Venus. 

It is, however, possible to obviate even this objection, and thus togive the minor planet method a supremacy over its rival which cannot bedisputed. The difficulty would be overcome if we could arrange that anastronomer, after making a set of observations on a fine night in thenorthern hemisphere, should be instantly transferred, instruments andall, to the southern station, and there repeat the observations. Anequivalent transformation can be effected without any miraculous agency, and in it we have undoubtedly the most perfect mode of measuring thesun's distance with which we are acquainted. This method has alreadybeen applied with success by Dr. Gill in the case of Juno, and there areother members of the host of minor planets still more favourablycircumstanced. 

Consider, for instance, a minor planet, which sometimes approaches towithin 70, 000, 000 miles of the earth. When the opposition is drawingnear, a skilled observer is to be placed at some suitable station nearthe equator. The instrument he is to use should be that marvellous pieceof mechanical and optical skill known as the heliometer. [20] It can beused to measure the angular distance between objects too far apart forthe filar micrometer. The measurements are to be made in the evening assoon as the planet has risen high enough to enable it to be seendistinctly. The observer and the observatory are then to be transferredto the other side of the earth. How is this to be done? Say, rather, howwe could prevent it from being done. Is not the earth rotating on itsaxis, so that in the course of a few hours the observatory on theequator is carried bodily round for thousands of miles? As the morningapproaches the observations are to be repeated. The planet is found tohave changed its place very considerably with regard to the stars. Thisis partly due to its own motion, but it is also largely due to theparallactic displacement arising from the rotation of the earth, whichmay amount to so much as twenty seconds. The measures on a single nightwith the heliometer should not have a mean error greater than one-fifthof a second, and we might reasonably expect that observations could besecured on about twenty-five nights during the opposition. Four suchgroups might be expected to give the sun's distance without anyuncertainty greater than the thousandth part of the total amount. Thechief difficulty of the process arises from the movement of the planetduring the interval which divides the evening from the morningobservations. This drawback can be avoided by diligent and repeatedmeasurements of the place of the planet with respect to the stars amongwhich it passes. 

In the monumental piece of work which issued in 1897 from the CapeObservatory, under the direction of Dr. Gill, the final results from theobservations of Iris, Victoria, and Sappho have been obtained. From thisit appears that the angle which the earth's equatorial radius subtendsat the centre of the sun when at its mean distance has the value8"·802. If we employ the best value of the earth's equatorial radius weobtain 92, 870, 000 miles as the mean distance of the centre of the sunfrom the centre of the earth. This is probably the most accuratedetermination of the scale of the solar system which has yet been made. 

CHAPTER XII. 

JUPITER. 

 The Great Size of Jupiter--Comparison of his Diameter with that of the Earth--Dimensions of the Planet and his Orbit--His Rotation--Comparison of his Weight and Bulk with that of the Earth--Relative Lightness of Jupiter--How Explained--Jupiter still probably in a Heated Condition--The Belts on Jupiter--Spots on his Surface--Time of Rotation of different Spots various--Storms on Jupiter--Jupiter not Incandescent--The Satellites--Their Discovery--Telescopic Appearance--Their Orbits--The Eclipses and Occultations--A Satellite in Transit--The Velocity of Light Discovered--How is this Velocity to be Measured Experimentally?--Determination of the Sun's Distance by the Eclipses of Jupiter's Satellites--Jupiter's Satellites demonstrating the Copernican System. 

In our exploration of the beautiful series of bodies which form thesolar system, we have proceeded step by step outwards from the sun. Inthe pursuit of this method we have now come to the splendid planetJupiter, which wends its majestic way in a path immediately outsidethose orbits of the minor planets which we have just been considering. Great, indeed, is the contrast between these tiny globes and thestupendous globe of Jupiter. Had we adopted a somewhat different methodof treatment--had we, for instance, discussed the various bodies of ourplanetary system in the order of their magnitude--then the minor planetswould have been the last to be considered, while the leader of the hostwould be Jupiter. To this position Jupiter is entitled without anapproach to rivalry. The next greatest on the list, the beautiful andinteresting Saturn, comes a long distance behind. Another great descentin the scale of magnitude has to be made before we reach Uranus andNeptune, while still another step downwards must be made before wereach that lesser group of planets which includes our earth. Soconspicuously does Jupiter tower over the rest, that even if Saturn wereto be augmented by all the other globes of our system rolled into one, the united mass would still not equal the great globe of Jupiter. 

[Illustration: Fig. 56. --The Relative Dimensions of Jupiter and theEarth. ]

The adjoining picture (Fig. 56) shows the relative dimensions of Jupiterand the earth, and it conveys to the eye a more vivid impression of theenormous bulk of Jupiter than we can readily obtain by merelyconsidering the numerical statements by which his bulk is to beaccurately estimated. As, however, it will be necessary to place thenumerical facts before our readers, we do so at the outset of thischapter. 

Jupiter revolves in an elliptic orbit around the sun in the focus, at amean distance of 483, 000, 000 miles. The path of Jupiter is thus about5·2 times as great in diameter as the path pursued by the earth. Theshape of Jupiter's orbit departs very appreciably from a circle, thegreatest distance from the sun being 5·45, while the least distance isabout 4·95, the earth's distance from the sun being taken as unity. Inthe most favourable circumstances for seeing Jupiter at opposition, itmust still be about four times as far from the earth as the earth isfrom the sun. This great globe will also illustrate the law that themore distant a planet is, the slower is the velocity with which itsorbital motion is accomplished. While the earth passes over eighteenmiles each second, Jupiter only accomplishes eight miles. Thus for atwofold reason the time occupied by an exterior planet in completing arevolution is greater than the period of the earth. Not only has theouter planet to complete a longer course than the earth, but the speedis less; it thus happens that Jupiter requires 4, 332·6 days, or aboutfifty days less than twelve years, to make a circuit of the heavens. 

The mean diameter of the great planet is about 87, 000 miles. We say the_mean_ diameter, because there is a conspicuous difference in the caseof Jupiter between his equatorial and his polar diameters. We havealready seen that there is a similar difference in the case of theearth, where we find the polar diameter to be shorter than theequatorial; but the inequality of these two dimensions is very muchlarger in Jupiter than in the earth. The equatorial diameter of Jupiteris 89, 600 miles, while the polar is not more than 84, 400 miles. Theellipticity of Jupiter indicated by these figures is sufficiently markedto be obvious without any refined measures. Around the shortest diameterthe planet spins with what must be considered an enormous velocity whenwe reflect on the size of the globe. Each rotation is completed in about9 hrs. 55 mins. 

We may naturally contrast the period of rotation of Jupiter with themuch slower rotation of our earth in twenty-four hours. The differencebecomes much more striking if we consider the relative speeds at whichan object on the equator of the earth and on that of Jupiter actuallymoves. As the diameter of Jupiter is nearly eleven times that of theearth, it will follow that the speed of the equator on Jupiter must beabout twenty-seven times as great as that on the earth. It is no doubtto this high velocity of rotation that we must ascribe the extraordinaryellipticity of Jupiter; the rapid rotation causes a great centrifugalforce, and this bulges out the pliant materials of which he seems to beformed. 

Jupiter is not, so far as we can see, a solid body. This is an importantcircumstance; and therefore it will be necessary to discuss the matterat some little length, as we here perceive a wide contrast between thisgreat planet and the other planets which have previously occupied ourattention. From the measurements already given it is easy to calculatethe bulk or the volume of Jupiter. It will be found that this planet isabout 1, 300 times as large as the earth; in other words, it would take1, 300 globes, each as large as our earth, all rolled into one, to form asingle globe as large as Jupiter. 

If the materials of which Jupiter is composed were of a nature analogousto the materials of the earth, we might expect that the weight of theplanet would exceed the weight of the earth in something like theproportion of their volumes. This is the matter now proposed to bebrought to trial. Here we may at once be met with the query, as to howwe are to find the weight of Jupiter. It is not even an easy matter toweigh the earth on which we stand. How, then, can we weigh a mightyplanet vastly larger than the earth, and distant from us by somehundreds of millions of miles? Truly, this is a bold problem. Yet theintellectual resources of man have proved sufficient to achieve thisfeat of celestial engineering. They are not, it is true, actually ableto make the ponderous weighing scales in which the great planet is to becast, but they are able to divert to this purpose certain naturalphenomena which yield the information that is required. 

Such investigations are based on the principle of universal gravitation. The mass of Jupiter attracts other masses in the solar system. Theefficiency of that attraction is more particularly shown on the bodieswhich are near the planet. In virtue of this attraction certainmovements are performed by those bodies. We can observe their characterwith our telescopes, we can ascertain their amount, and from ourmeasurements we can calculate the mass of the body by which themovements have been produced. This is the sole method which we possessfor the investigation of the masses of the planets; and though it maybe difficult in its application--not only from the observations whichare required, but also from the intricacy and the profundity of thecalculations to which those observations must be submitted--yet, in thecase of Jupiter at least, there is no uncertainty about the result. 

The task is peculiarly simplified in the case of the greatest planet ofour system by the beautiful system of moons with which he is attended. These little moons revolve under the guidance of Jupiter, and theirmovements are not otherwise interfered with so as to prevent their usefor our present purpose. It is from the observations of the satellitesof Jupiter that we are enabled to measure his attractive power, andthence to calculate the mass of the mighty planet. 

To those not specially conversant with the principles of mechanics, itmay seem difficult to realise the degree of accuracy of which such amethod is capable. Yet there can be no doubt that his moons inform us ofthe mass of Jupiter, and do not leave a margin of inaccuracy so great asone hundredth part of the total amount. If other confirmation be needed, then it is forthcoming in abundance. A minor planet occasionally drawsnear the orbit of Jupiter and experiences his attraction; the planet isforced to swerve from its path, and the amount of the deviation can bemeasured. From that measurement the mass of Jupiter can be computed by acalculation, of which it would be impossible to give an account in thisplace. The mass of Jupiter, as determined by this method, agrees withthe mass obtained in a totally different manner from the satellites. 

Nor have we yet exhausted the resources of astronomy in its bearing onthis question. We can discard the planetary system, and invite theassistance of a comet which, flashing through the orbits of the planets, occasionally experiences large and sometimes enormous disturbances. Forthe present it suffices to remark, that on one or two occasions it hashappened that venturous comets have been near enough to Jupiter to bemuch disturbed by his attraction, and then to proclaim in their alteredmovements the magnitude of the mass which has affected them. Thesatellites of Jupiter, the minor planets, and the comets, all tell theweight of the giant orb; and, as they all concur in the result (at leastwithin extremely narrow limits), we cannot hesitate to conclude that themass of the greatest planet of our system has been determined withaccuracy. 

The results of these measures must now be stated. They show, of course, that Jupiter is vastly inferior to the sun--that, in fact, it would takeabout 1, 047 Jupiters, all rolled into one, to form a globe equal in_weight_ to the sun. They also show us that it would take 316 globes asheavy as our Earth to counterbalance the weight of Jupiter. 

No doubt this proves Jupiter to be a body of magnificent proportions;but the remarkable circumstance is not that Jupiter should be 316 timesas heavy as the earth, but that he is not a great deal more. Have we notstated that Jupiter is 1, 300 times as _large_ as the earth? How thencomes it that he is only 316 times as _heavy_? This points at once tosome fundamental contrast between the constitution of Jupiter and of theearth. How are we to account for this difference? We can conceive of twoexplanations. In the first place, it might be supposed that Jupiter isconstituted of materials partly or wholly unknown on the earth. Thereis, however, an alternative supposition at once more philosophical andmore consistent with the evidence. It is true that we know little ornothing of what the elementary substances on Jupiter may be, but one ofthe great discoveries of modern astronomy has taught us something of theelementary bodies present in other bodies of the universe, and hasdemonstrated that to a large extent they are identical with theelementary bodies on the earth. If Jupiter be composed of bodiesresembling those on the earth, there is one way, and only one, in whichwe can account for the disparity between his size and his mass. Perhapsthe best way of stating the argument will be found in a glance at theremote history of the earth itself, for it seems not impossible that thepresent condition of Jupiter was itself foreshadowed by the condition ofour earth countless ages ago. 

In a previous chapter we had occasion to point out how the earth seemedto be cooling from an earlier and highly heated condition. The furtherwe look back, the hotter our globe seems to have been; and if we projectour glance back to an epoch sufficiently remote, we see that it mustonce have been so hot that life on its surface would have beenimpossible. Back still earlier, we find the heat to have been such thatwater could not rest on the earth; and hence it seems likely that atsome incredibly remote epoch all the oceans now reposing in the deeps onthe surface, and perhaps a considerable portion of its now solid crust, must have been in a state of vapour. Such a transformation of the globewould not alter its _mass_, for the materials weigh the same whatever betheir condition as to temperature, but it would alter the _size_ of ourglobe to a very considerable extent. If these oceans were transformedinto vapour, then the atmosphere, charged with mighty clouds, would havea bulk some hundreds of times greater than that which it has at present. Viewed from a distant planet, the cloud-laden atmosphere would indicatethe visible size of our globe, and its average density would accordinglyappear to be very much less than it is at present. 

From these considerations it will be manifest that the discrepancybetween the size and the weight of Jupiter, as contrasted with ourearth, would be completely removed if we supposed that Jupiter was atthe present day a highly heated body in the condition of our earthcountless ages ago. Every circumstance of the case tends to justify thisargument. We have assigned the smallness of the moon as a reason why themoon has cooled sufficiently to make its volcanoes silent and still. Inthe same way the smallness of the earth, as compared with Jupiter, accounts for the fact that Jupiter still retains a large part of itsoriginal heat, while the smaller earth has dissipated most of its store. This argument is illustrated and strengthened when we introduce otherplanets into the comparison. As a general rule we find that the smallerbodies, like the earth and Mars, have a high density, indicative of alow temperature, while the giant planets, like Jupiter and Saturn, havea low density, suggesting that they still retain a large part of theiroriginal heat. We say "original heat" for the want, perhaps, of a morecorrect expression; it will, however, indicate that we do not in theleast refer to the solar heat, of which, indeed, the great outer planetsreceive much less than those nearer the sun. Where the original heat mayhave come from is a matter still confined to the province ofspeculation. 

A complete justification of these views with regard to Jupiter is to befound when we make a minute telescopic scrutiny of its surface; and itfortunately happens that the size of the planet is so great that, evenat a distance of more millions of miles than there are days in the year, we can still trace on its surface some significant features. 

Plate XI. Gives a series of four different views of Jupiter. They havebeen taken from a series of admirable drawings of the great planet madeby Mr. Griffiths in 1897. The first picture shows the appearance of theglobe at 10h. 20m. Greenwich time on February 17th, 1897, through apowerful refracting telescope. We at once notice in this drawing thatthe outline of Jupiter is distinctly elliptical. The surface of theplanet usually shows the remarkable series of belts here represented. They are nearly parallel to each other and to the planet's equator. 

When Jupiter is observed for some hours, the appearance of the beltsundergoes certain changes. These are partly due to the regular rotationof the planet on its axis, which, in a period of less than five hours, will completely carry away the hemisphere we first saw, and replace itby the hemisphere originally at the other side. But besides the changesthus arising, the belts and other features on the planet are also veryvariable. Sometimes new stripes or marks appear, and old ones disappear;in fact, a thorough examination of Jupiter will demonstrate theremarkable fact that there are no permanent features whatever to bediscerned. We are here immediately struck by the contrast betweenJupiter and Mars; on the smaller planet the main topographical outlinesare almost invariable, and it has been feasible to construct maps of thesurface with tolerably accurate detail; a map of Jupiter is, however, animpossibility--the drawing of the planet which we make to-night will bedifferent from the drawing of the same hemisphere made a few weekshence. 

It should, however, be noticed that objects occasionally appear on theplanet which seem of a rather more persistent character than the belts. We may especially mention the object known as the great oblong Red Spot, which has been a very remarkable feature upon the southern hemisphere ofJupiter since 1878. This object, which has attracted a great deal ofattention from observers, is about 30, 000 miles long by about 7, 000 inbreadth. Professor Barnard remarks that the older the spots on Jupiterare, the more ruddy do they tend to become. 

The conclusion is irresistibly forced upon us that when we view thesurface of Jupiter we are not looking at any solid body. The want ofpermanence in the features of the planet would be intelligible if whatwe see be merely an atmosphere laden with clouds of impenetrabledensity. The belts especially support this view; we are at once remindedof the equatorial zones on our own earth, and it is not at all unlikelythat an observer sufficiently remote from the earth to obtain a justview of its appearance would detect upon its surface more or lessperfect cloud-belts suggestive of those on Jupiter. A view of our earthwould be, as it were, intermediate between a view of Jupiter and ofMars. In the latter case the appearance of the permanent features of theplanet is only to a trifling extent obscured by clouds floating over thesurface. Our earth would always be partly, and often perhaps verylargely, covered with cloud, while Jupiter seems at all times completelyenveloped. 

From another class of observations we are also taught the importanttruth that Jupiter is not, superficially at least, a solid body. Theperiod of rotation of the planet around its axis is derived from theobservation of certain marks, which present sufficient definiteness andsufficient permanence to be suitable for the purpose. Suppose one ofthese objects to lie at the centre of the planet's disc; its position iscarefully measured, and the time is noted. As the hours pass on, themark moves to the edge of the disc, then round the other side of theplanet, and back again to the visible disc. When it has returned to theposition originally occupied the time is again taken, and the intervalwhich has elapsed is called the period of rotation of the spot. 

If Jupiter were a solid, and if these features were engraved upon itssurface, then it is perfectly clear that the time of rotation as foundby any one spot must coincide precisely with the time yielded by anyother spot; but this is not observed to be the case. In fact, it wouldbe nearer the truth to say that each spot gives a special period of itsown. Nor are the differences very minute. It has been found that thetime in which the red spot (the latitude of which is about 25° south) iscarried round is five minutes longer than that required by some peculiarwhite marks near the equator. The red spot has now been watched forabout twenty years, and during most of that time has had a tendency torotate more and more slowly, as may be seen from the following values ofits rotation period:--

 In 1879, 9h. 55m. 33·9s. In 1886, 9h. 55m. 40·6s. In 1891, 9h. 55m. 41·7s. 

Since 1891 this tendency seems to have ceased, while the spot has beengradually fading away. Generally speaking, we may say that theequatorial regions rotate in about 9h. 50m. 20s. , and the temperatezones in about 9h. 55m. 40s. Remarkable exceptions are occasionally metwith. Some small black spots in north latitude 22°, which broke out in1880 and again in 1891, rotated in 9h. 48m. To 9h. 49-1/2m. It may, therefore, be regarded as certain that the globe of Jupiter, so far aswe can see it, is not a solid body. It consists, on the exterior at allevents, of clouds and vaporous masses, which seem to be agitated bystorms of the utmost intensity, if we are to judge from the ceaselesschanges of the planet's surface. 

[Illustration: PLATE XI. 

Feb. 2nd. Feb. 4th. 

Feb. 12th. Feb. 28th. 

THE PLANET JUPITER. 

1897. ]

[Illustration: Fig. 57. --The Occultation of Jupiter (1). ]

[Illustration: Fig. 58. --The Occultation of Jupiter (2). ]

[Illustration: Fig. 59. --The Occultation of Jupiter (3). ]

[Illustration: Fig. 60. --The Occultation of Jupiter (4). ]

Various photographs of Jupiter have been obtained; those which have beentaken at the Lick Observatory being specially interesting andinstructive. Pictures of the planet obtained with the camera inremarkable circumstances are represented in Figs. 57-60, which weretaken by Professor Wm. H. Pickering at Arequipa, Peru, on the 12th ofAugust, 1892. [21] The small object with the belts is the planet Jupiter. The large advancing disc (of which only a small part can be shown) isthe moon. The phenomenon illustrated is called the "occultation" of theplanet. The planet is half-way behind the moon in Fig. 59, while in Fig. 60 half of the planet is still hidden by the dark limb of the moon. 

It is well known that the tempests by which the atmosphere surroundingthe earth is convulsed are to be ultimately attributed to the heat ofthe sun. It is the rays from the great luminary which, striking on thevast continents, warm the air in contact therewith. This heated airbecomes lighter, and rises, while air to supply its place must flow inalong the surface. The currents so produced form a breeze or a wind;while, under exceptional circumstances, we have the phenomena ofcyclones and hurricanes, all originated by the sun's heat. Need we addthat the rains, which so often accompany the storms, have also arisenfrom the solar beams, which have distilled from the wide expanse ofocean the moisture by which the earth is refreshed?

The storms on Jupiter seem to be vastly greater than those on the earth. Yet the intensity of the sun's heat on Jupiter is only a merefraction--less, indeed, than the twenty-fifth part--of that received bythe earth. It is incredible that the motive power of the appallingtempests on the great planet can be entirely, or even largely, due tothe feeble influence of solar heat. We are, therefore, led to seek forsome other source of such disturbances. What that source is to be willappear obvious when we admit that Jupiter still retains a largeproportion of primitive internal heat. Just as the sun itself isdistracted by violent tempests as an accompaniment of its intenseinternal fervour, so, in a lesser degree, do we observe the samephenomena in Jupiter. It may also be noticed that the spots on the sunusually lie in more or less regular zones, parallel to its equator, thearrangement being in this respect not dissimilar to that of the belts onJupiter. 

It being admitted that the mighty planet still retains some of itsinternal heat, the question remains as to how much. It is, of course, obvious that the heat of the planet is inconsiderable when comparedwith the heat of the sun. The brilliance of Jupiter, which makes it anobject of such splendour in our midnight sky, is derived from the samegreat source which illuminates the earth, the moon, or the otherplanets. Jupiter, in fact, shines by reflected sunlight, and not invirtue of any intrinsic light in his globe. A beautiful proof of thistruth is familiar to every user of a telescope. The little satellites ofthe planet sometimes intrude between him and the sun, and cast a shadowon Jupiter. The shadow is black, or, at all events, it seems black, relatively to the brilliant surrounding surface of the planet. It must, therefore, be obvious that Jupiter is indebted to the sun for itsbrilliancy. The satellites supply another interesting proof of thistruth. One of these bodies sometimes enters the shadow of Jupiter andlo! the little body vanishes. It does so because Jupiter has cut off thesupply of sunlight which previously rendered the satellite visible. Butthe planet is not himself able to offer the satellite any light incompensation for the sunlight which he has intercepted. [22]

Enough, however, has been demonstrated to enable us to pronounce on thequestion as to whether the globe of Jupiter can be inhabited by livingcreatures resembling those on this earth. Obviously this cannot be so. The internal heat and the fearful tempests seem to preclude thepossibility of organic life on the great planet, even were there notother arguments tending to the same conclusion. It may, however, becontended, with perhaps some plausibility, that Jupiter has in thedistant future the prospect of a glorious career as the residence oforganic life. The time will assuredly come when the internal heat mustdecline, when the clouds will gradually condense into oceans. On thesurface dry land may then appear, and Jupiter be rendered habitable. 

From this sketch of the planet itself we now turn to the interesting andbeautiful system of five satellites by which Jupiter is attended. Wehave, indeed, already found it necessary to allude more than once tothese little bodies, but not to such an extent as to interfere with themore formal treatment which they are now to receive. 

The discovery of the four chief satellites may be regarded as animportant epoch in the history of astronomy. They are objects situatedin a remarkable manner on the border-line which divides the objectsvisible to the unaided eye from those which require telescopic aid. Ithas been frequently asserted that these objects have been seen with theunaided eye; but without entering into any controversy on the matter, itis sufficient to recite the well-known fact that, although Jupiter hadbeen a familiar object for countless centuries, yet the sharpest eyesunder the clearest skies never discovered the satellites until Galileoturned the newly invented telescope upon them. This tube was no doubt avery feeble instrument, but very little power suffices to show objectsso dose to the limit of visibility. 

[Illustration: Fig. 61. --Jupiter and his Four Satellites as seen in aTelescope of Low Power. ]

The view of the planet and its elaborate system of satellites as shownin a telescope of moderate power, is represented in Fig. 61. We here seethe great globe, and nearly in a line parsing through its centre liefour small objects, three on one side and one on the other. These littlebodies resemble stars, but they can be distinguished therefrom by theirceaseless movements around the planet, which they never fail toaccompany during his entire circuit of the heavens. There is no morepleasing spectacle for the student than to follow with his telescope themovements of this beautiful system. 

[Illustration: Fig. 62. --Disappearances of Jupiter's Satellites. ]

In Fig. 62 we have represented some of the various phenomena which thesatellites present. The long black shadow is that produced by theinterposition of Jupiter in the path of the sun's rays. In consequenceof the great distance of the sun this shadow will extend, in the form ofa very elongated cone, to a distance far beyond the orbit of the outersatellite. The second satellite is immersed in this shadow, andconsequently eclipsed. The eclipse of a satellite must not be attributedto the intervention of the body of Jupiter between the satellite and theearth. Such an occurrence is called an occultation, and the thirdsatellite is shown in this condition. The second and the thirdsatellites are thus alike invisible, but the cause of the invisibilityis quite different in the two cases. The eclipse is much the morestriking phenomenon of the two, because the satellite, at the moment itplunges into the darkness, may be still at some apparent distance fromthe edge of the planet, and is thus seen up to the moment of theeclipse. In an occultation the satellite in passing behind the planetis, at the time of disappearance, close to the planet's bright edge, and the extinction of the light from the small body cannot be observedwith the same impressiveness as the occurrence of an eclipse. 

A satellite also assumes another remarkable situation when in the courseof transit over the face of the planet. The satellite itself is notalways very easy to see in such circumstances, but the beautiful shadowwhich it casts forms a sharp black spot on the brilliant orb: thesatellite will, indeed, frequently cast a visible shadow when it passesbetween the planet and the sun, even though it be not actually at themoment in front of the planet, as it is seen from the earth. 

The periods in which the four principal moons of Jupiter revolve aroundtheir primary are respectively, 1 day 18 hrs. 27 min. 34 secs. For thefirst; 3 days 13 hrs. 13 min. 42 secs. , for the second; 7 days 3 hrs. 42min. 33 secs, for the third; and 16 days 16 hrs. 32 min. 11 secs. Forthe fourth. We thus observe that the periods of Jupiter's satellites aredecidedly briefer than that of our moon. Even the satellite most distantfrom the great planet requires for each revolution less than two-thirdsof an ordinary lunar month. The innermost of these bodies, revolving asit does in less than two days, presents a striking series of ceaselessand rapid changes, and it becomes eclipsed during every revolution. Thedistance from the centre of Jupiter to the orbit of the innermost ofthese four attendants is a quarter of a million miles, while the radiusof the outermost is a little more than a million miles. The second ofthe satellites proceeding outwards from the planet is almost the samesize as our moon; the other three bodies are larger; the third being thegreatest of all (about 3, 560 miles in diameter). Owing to the minutenessof the satellites as seen from the earth, it is extremely difficult toperceive any markings on their surfaces, but the few observations madeseem to indicate that the satellites (like our moon) always turn thesame face towards their primary. Professor Barnard has, with the greatLick refractor, seen a white equatorial belt on the first satellite, while its poles were very dark. Mr. Douglass, observing with Mr. Lowell's great refractor, has also reported certain streaky markings onthe third satellite. 

A very interesting astronomical discovery was that made by ProfessorE. E. Barnard in 1892. He detected with the 36-inch Lick refractor anextremely minute fifth satellite to Jupiter at a distance of 112, 400miles, and revolving in a period of 11 hrs. 57 min. 22·6 secs. It canonly be seen with the most powerful telescopes. 

The eclipses of Jupiter's satellites had been observed for many years, and the times of their occurrence had been recorded. At length it wasperceived that a certain order reigned among the eclipses of thesebodies, as among all other astronomical phenomena. When once the lawsaccording to which the eclipses recurred had been perceived, the usualconsequence followed. It became possible to foretell the time at whichthe eclipses would occur in future. Predictions were accordingly made, and it was found that they were approximately verified. Furtherimprovements in the calculations were then perfected, and it was soughtto predict the times with still greater accuracy. But when it came tonaming the actual minute at which the eclipse should occur, expectationswere not always realised. Sometimes the eclipse was five or ten minutestoo soon. Sometimes it was five or ten minutes too late. Discrepanciesof this kind always demand attention. It is, indeed, by the right use ofthem that discoveries are often made, and one of the most interestingexamples is that now before us. 

The irregularity in the occurrence of the eclipses was at lengthperceived to observe certain rules. It was noticed that when the earthwas near to Jupiter the eclipse generally occurred before the predictedtime; while when the earth happened to be at the side of its orbit awayfrom Jupiter, the eclipse occurred after the predicted time. Once thiswas proved, the great discovery was quickly made by Roemer, a Danishastronomer, in 1675. When the satellite enters the shadow, its lightgradually decreases until it disappears. It is the last ray of lightfrom the eclipsed satellite that gives the time of the eclipse; but thatray of light has to travel from the satellite to the earth, and enterour telescope, before we can note the occurrence. It used to be thoughtthat light travelled instantaneously, so that the moment the eclipseoccurred was assumed to be the moment when the eclipse was seen in thetelescope. This was now perceived to be incorrect. It was found thatlight took time to travel. When the earth was comparatively near Jupiterthe light had only a short journey, the intelligence of the eclipsearrived quickly, and the eclipse was seen sooner than the calculationsindicated. When the earth occupied a position far from Jupiter, thelight had a longer journey, and took more than the average time, so thatthe eclipse was later than the prediction. This simple explanationremoved the difficulty attending the predictions of the eclipses of thesatellites. But the discovery had a significance far more momentous. Welearned from it that light had a measurable velocity, which, accordingto recent researches, amounts to 186, 300 miles per second. 

One of the most celebrated attempts to ascertain the distance of the sunis derived from a combination of experiments on the velocity of lightwith astronomical measurements. This is a method of considerablerefinement and interest, and although it does not so fulfil all thenecessary conditions as to make it perfectly satisfactory, yet it isimpossible to avoid some reference to it here. Notwithstanding that thevelocity of light is so stupendous, it has been found possible tomeasure that velocity by actual trial. This is one of the most delicateexperimental researches that have ever been undertaken. If it bedifficult to measure the speed of a rifle bullet, what shall we say ofthe speed of a ray of light, which is nearly a million times as great?How shall we devise an apparatus subtle enough to determine the velocitywhich would girdle the earth at the equator no less than seven times ina single second of time? Ordinary contrivances for measurement are herefutile; we have to devise an instrument of a wholly different character. 

In the attempt to discover the speed of a moving body we first mark outa certain distance, and then measure the time which the body requires totraverse that distance. We determine the velocity of a railway train bythe time it takes to pass from one mile-post to the next. We learn thespeed of a rifle bullet by an ingenious contrivance really founded onthe same principle. The greater the velocity, the more desirable is itthat the distance traversed during the experiment shall be as large aspossible. In dealing with the measurement of the velocity of light, wetherefore choose for our measured distance the greatest length that maybe convenient. It is, however, necessary that the two ends of the lineshall be visible from each other. A hill a mile or two away will form asuitable site for the distant station, and the distance of the selectedpoint on the hill from the observer must be carefully measured. 

The problem is now easily stated. A ray of light is to be sent from theobserver to the distant station, and the time occupied by that ray inthe journey is to be measured. We may suppose that the observer, by asuitable contrivance, has arranged a lantern from which a thin ray oflight issues. Let us assume that this travels all the way to the distantstation, and there falls upon the surface of a reflecting mirror. Instantly it will be diverted by reflection into a new directiondepending upon the inclination of the mirror. By suitable adjustment thelatter can be so placed that the light shall fall perpendicularly uponit, in which case the ray will of course return along the direction inwhich it came. Let the mirror be fixed in this position throughout thecourse of the experiments. It follows that a ray of light starting fromthe lantern will be returned to the lantern after it has made thejourney to the distant station and back again. Imagine, then, a littleshutter placed in front of the lantern. We open the shutter, the raystreams forth to the remote reflector, and back again through theopening. But now, after having allowed the ray to pass through theshutter, suppose we try and close it before the ray has had time to getback again. What fingers could be nimble enough to do this? Even if thedistant station were ten miles away, so that the light had a journey often miles in going to the mirror and ten miles in coming back, yet thewhole course would be accomplished in about the nine-thousandth part ofa second--a period so short that even were it a thousand times as longit would hardly enable manual dexterity to close the aperture. Yet ashutter can be constructed which shall be sufficiently delicate for thepurpose. 

[Illustration: Fig. 63. --Mode of Measuring the Velocity of Light. ]

The principle of this beautiful method will be sufficiently obvious fromthe diagram on this page (Fig. 63), which has been taken from Newcomb's"Popular Astronomy. " The figure exhibits the lantern and the observer, and a large wheel with projecting teeth. Each tooth as it passes roundeclipses the beam of light emerging from the lantern, and also the eye, which is of course directed to the mirror at the distant station. In theposition of the wheel here shown the ray from the lantern will pass tothe mirror and back so as to be visible to the eye; but if the wheel berotating, it may so happen that the beam after leaving the lantern willnot have time to return before the next tooth of the wheel comes infront of the eye and screens it. If the wheel be urged still faster, thenext tooth may have passed the eye, so that the ray again becomesvisible. The speed at which the wheel is rotating can be measured. Wecan thus determine the time taken by one of the teeth to pass in frontof the eye; we have accordingly a measure of the time occupied by theray of light in the double journey, and hence we have a measurement ofthe velocity of light. 

It thus appears that we can tell the velocity of light either by theobservations of Jupiter's satellites or by experimental enquiry. If wetake the latter method, then we are entitled to deduce remarkableastronomical consequences. We can, in fact, employ this method forsolving that great problem so often referred to--the distance from theearth to the sun--though it cannot compete in accuracy with some of theother methods. 

The dimensions of the solar system are so considerable that a sunbeamrequires an appreciable interval of time to span the abyss whichseparates the earth from the sun. Eight minutes is approximately theduration of the journey, so that at any moment we see the sun as itappeared eight minutes earlier to an observer in its immediateneighbourhood. In fact, if the sun were to be suddenly blotted out itwould still be seen shining brilliantly for eight minutes after it hadreally disappeared. We can determine this period from the eclipses ofJupiter's satellites. 

So long as the satellite is shining it radiates a stream of light acrossthe vast space between Jupiter and the earth. When the eclipse hascommenced, the little orb is no longer luminous, but there is, nevertheless, a long stream of light on its way, and until all this haspoured into our telescopes we still see the satellite shining as before. If we could calculate the moment when the eclipse really took place, andif we could observe the moment at which the eclipse is seen, thedifference between the two gives the time which the light occupies onthe journey. This can be found with some accuracy; and, as we alreadyknow the velocity of light, we can ascertain the distance of Jupiterfrom the earth; and hence deduce the scale of the solar system. It must, however, be remarked that at both extremities of the process there arecharacteristic sources of uncertainty. The occurrence of the eclipse isnot an instantaneous phenomenon. The satellite is large enough torequire an appreciable time in crossing the boundary which defines theshadow, so that the observation of an eclipse cannot be sufficientlyprecise to form the basis of an important and accurate measurement. [23]Still greater difficulties accompany the attempt to define the truemoment of the occurrence of the eclipse as it would be seen by anobserver in the vicinity of the satellite. For this we should require afar more perfect theory of the movements of Jupiter's satellites than isat present attainable. This method of finding the sun's distance holdsout no prospect of a result accurate to the one-thousandth part of itsamount, and we may discard it, inasmuch as the other methods availableseem to admit of much higher accuracy. 

The four chief satellites of Jupiter have special interest for themathematician, who finds in them a most striking instance of theuniversality of the law of gravitation. These bodies are, of course, mainly controlled in their movements by the attraction of the greatplanet; but they also attract each other, and certain curiousconsequences are the result. 

The mean motion of the first satellite in each day about the centre ofJupiter is 203°·4890. That of the second is 101°·3748, and that of thethird is 50°·3177. These quantities are so related that the followinglaw will be found to be observed:

The mean motion of the first satellite added to twice the mean motion ofthe third is exactly equal to three times the mean motion of the second. 

There is another law of an analogous character, which is thus expressed(the mean longitude being the angle between a fixed line and the radiusto the mean place of the satellite): If to the mean longitude of thefirst satellite we add twice the mean longitude of the third, andsubtract three times the mean longitude of the second, the difference isalways 180°. 

It was from observation that these principles were first discovered. Laplace, however, showed that if the satellites revolved nearly in thisway, then their mutual perturbations, in accordance with the law ofgravitation, would preserve them in this relative position for ever. 

We shall conclude with the remark, that the discovery of Jupiter'ssatellites afforded the great confirmation of the Copernican theory. Copernicus had asked the world to believe that our sun was a greatglobe, and that the earth and all the other planets were small bodiesrevolving round the great one. This doctrine, so repugnant to thetheories previously held, and to the immediate evidence of our senses, could only be established by a refined course of reasoning. Thediscovery of Jupiter's satellites was very opportune. Here we had anexquisite ocular demonstration of a system, though, of course, on a muchsmaller scale, precisely identical with that which Copernicus hadproposed. The astronomer who had watched Jupiter's moons circling aroundtheir primary, who had noticed their eclipses and all the interestingphenomena attendant on them, saw before his eyes, in a manner whollyunmistakable, that the great planet controlled these small bodies, andforced them to revolve around him, and thus exhibited a miniature of thegreat solar system itself. "As in the case of the spots on the sun, Galileo's announcement of this discovery was received with incredulityby those philosophers of the day who believed that everything in naturewas described in the writings of Aristotle. One eminent astronomer, Clavius, said that to see the satellites one must have a telescope whichwould produce them; but he changed his mind as soon as he saw themhimself. Another philosopher, more prudent, refused to put his eye tothe telescope lest he should see them and be convinced. He died shortlyafterwards. 'I hope, ' said the caustic Galileo, 'that he saw them whileon his way to heaven'"[24]

CHAPTER XIII. 

SATURN. 

 The Position of Saturn in the System--Saturn one of the Three most Interesting Objects in the Heavens--Compared with Jupiter--Saturn to the Unaided Eye--Statistics relating to the Planet--Density of Saturn--Lighter than Water--The Researches of Galileo--What he found in Saturn--A Mysterious Object--The Discoveries made by Huyghens half a Century later--How the Existence of the Ring was Demonstrated--Invisibility of the Rings every Fifteen Years--The Rotation of the Planet--The Celebrated Cypher--The Explanation--Drawing of Saturn--The Dark Line--W. Herschel's Researches--Is the Division in the Ring really a Separation?--Possibility of Deciding the Question--The Ring in a Critical Position--Are there other Divisions in the Ring?--The Dusky Ring--Physical Nature of Saturn's Rings--Can they be Solid?--Can they even be Slender Rings?--A Fluid?--True Nature of the Rings--A Multitude of Small Satellites--Analogy of the Rings of Saturn to the Group of Minor Planets--Problems Suggested by Saturn--The Group of Satellites to Saturn--The Discoveries of Additional Satellites--The Orbit of Saturn not the Frontier of our System. 

At a profound distance in space, which, on an average, is 886, 000, 000miles, the planet Saturn performs its mighty revolution around the sunin a period of twenty-nine and a half years. This gigantic orbit formedthe boundary to the planetary system, so far as it was known to theancients. 

Although Saturn is not so great a body as Jupiter, yet it vastly exceedsthe earth in bulk and in mass, and is, indeed, much greater than any oneof the planets, Jupiter alone excepted. But while Saturn must yield thepalm to Jupiter so far as mere dimensions are concerned, yet it will begenerally admitted that even Jupiter, with all the retinue by which heis attended, cannot compete in beauty with the marvellous system ofSaturn. To the present writer it has always seemed that Saturn is one ofthe three most interesting celestial objects visible to observers innorthern latitudes. The other two will occupy our attention in futurechapters. They are the great nebula in Orion, and the star cluster inHercules. 

So far as the globe of Saturn is concerned, we do not meet with anyfeatures which give to the planet any exceptional interest. The globe isless than that of Jupiter, and as the latter is also much nearer to us, the apparent size of Saturn is in a twofold way much smaller than thatof Jupiter. It should also be noticed that, owing to the greaterdistance of Saturn from the sun, its intrinsic brilliancy is less thanthat of Jupiter. There are, no doubt, certain marks and bands often tobe seen on Saturn, but they are not nearly so striking nor socharacteristic as the ever-variable belts upon Jupiter. The telescopicappearance of the globe of Saturn must also be ranked as greatlyinferior in interest to that of Mars. The delicacy of detail which wecan see on Mars when favourably placed has no parallel whatever in thedim and distant Saturn. Nor has Saturn, regarded again merely as aglobe, anything like the interest of Venus. The great splendour of Venusis altogether out of comparison with that of Saturn, while the brilliantcrescent of the evening star is infinitely more pleasing than anytelescopic view of the globe of Saturn. Yet even while we admit all thisto the fullest extent, it does not invalidate the claim of Saturn to beone of the most supremely beautiful and interesting objects in theheavens. This interest is not due to his globe; it is due to thatmarvellous system of rings by which Saturn is surrounded--a systemwonderful from every point of view, and, so far as our knowledge goes, without a parallel in the wide extent of the universe. 

[Illustration: Fig. 64. Saturn. (July 2nd, 1894. 36-in. Equatorial. )(Prof. E. E. Barnard. )]

To the unaided eye Saturn usually appears like a star of the firstmagnitude. Its light alone would hardly be sufficient to discriminate itfrom many of the brighter fixed stars. Yet the ancients were acquaintedwith Saturn, and they knew it as a planet. It was included with theother four great planets--Mercury, Venus, Mars, and Jupiter--in thegroup of wanderers, which were bound to no fixed points of the sky likethe stars. On account of the great distance of Saturn, its movements aremuch slower than those of the other planets known to the ancients. Twenty-nine years and a half are required for this distant object tocomplete its circuit of the heavens; and, though this movement is slowcompared with the incessant changes of Venus, yet it is rapid enough toattract the attention of any careful observer. In a single year Saturnmoves through a distance of about twelve degrees, a quantitysufficiently large to be conspicuous to casual observation. Even in amonth, or sometimes in a week, the planet traverses an arc of the skywhich can be detected by anyone who will take the trouble to mark theplace of the planet with regard to the stars in its vicinity. Those whoare privileged to use accurate astronomical instruments can readilydetect the motion of Saturn in a few hours. 

The average distance from the sun to Saturn is about 886 millions ofmiles. The path of Saturn, as of every other planet, is really anellipse with the sun in one focus. In the case of Saturn the shape ofthis ellipse is very appreciably different from a purely circular path. Around this path Saturn moves with an average velocity of 5·96 miles persecond. 

The mean diameter of the globe of Saturn is about 71, 000 miles. Itsequatorial diameter is about 75, 000 miles, and its polar diameter 67, 000miles--the ratio of these numbers being approximately that of 10 to 9. It is thus obvious that Saturn departs from the truly spherical shape toa very marked extent. The protuberance at its equator must, no doubt, beattributed to the high velocity with which the planet is rotating. Thevelocity of rotation of Saturn is more than double as fast as that ofthe earth, though it is not quite so fast as that of Jupiter. Saturnmakes one complete rotation in about 10 hrs. 14 min. Mr. StanleyWilliams has, however, observed with great care a number of spots whichhe has discovered, and he finds that some of these spots in about 27°north latitude indicate rotation in a period of 10 hrs. 14 mins. To 15min. , while equatorial spots require no more than 10 hrs. 12 min. To 13min. There is, however, the peculiarity that spots in the same latitude, but at different parts of the planet, rotate at rates which differ by aminute or more, while the period found by various groups of spots seemsto change from year to year. 

These facts prove that Saturn and the spots do not form a rigid system. The lightness of this planet is such as to be wholly incompatible withthe supposition that its globe is constituted of solid materials at allcomparable with those of which the crust of our earth is composed. Thesatellites, which surround Saturn and form a system only lessinteresting than the renowned rings themselves, enable us to weigh theplanet in comparison with the sun, and hence to deduce its actual massrelatively to the earth. The result is not a little remarkable. Itappears that the density of the earth is eight times as great as that ofSaturn. In fact, the density of the latter is less than that of wateritself, so that a mighty globe of water, equal in bulk to Saturn, wouldactually weigh more. If we could conceive a vast ocean into which aglobe equal to Saturn in size and weight were cast, the great globewould not sink like our earth or like any of the other planets; it wouldfloat buoyantly at the surface with one-fourth of its bulk out of thewater. 

We thus learn with high probability that what our telescopes show uponSaturn is not a solid surface, but merely a vast envelope of cloudssurrounding a heated interior. It is impossible to resist the suggestionthat this planet, like Jupiter, has still retained its heat because itsmass is so large. We must, however, allude to a circumstance whichperhaps may seem somewhat inconsistent with the view here taken. We havefound that Jupiter and Saturn are, both of them, much less dense thanthe earth. When we compare the two planets together, it appears thatSaturn is much less dense than Jupiter. In fact, every cubic mile ofJupiter weighs nearly twice as much as each cubic mile of Saturn. Thiswould seem to point to the conclusion that Saturn is the more heated ofthe two bodies. Yet, as Jupiter is the larger, it might more reasonablyhave been expected to be hotter than the other planet. We do not attemptto reconcile this discrepancy; in fact, in our ignorance as to thematerial constitution of these bodies, it would be idle to discuss thequestion. 

Even if we allow for the lightness of Saturn, as compared bulk for bulkwith the earth, yet the volume of Saturn is so enormous that the planetweighs more than ninety-five times as much as the earth. The adjoiningview represents the relative sizes of Saturn and the earth (Fig. 65). 

[Illustration: Fig. 65. --Relative Sizes of Saturn and the Earth. ]

As the unaided eye discloses none of those marvels by which Saturn issurrounded, the interest which attaches to this planet may be said tocommence from the time when it began to be observed with the telescope. The history must be briefly alluded to, for it was only by degrees thatthe real nature of this complicated object was understood. When Galileocompleted his little refracting telescope, which, though it onlymagnified thirty times, was yet an enormous addition to the powers ofunaided vision, he made with it his memorable review of the heavens. Hesaw the spots on the sun and the mountains on the moon; he noticed thecrescent of Venus and the satellites of Jupiter. Stimulated andencouraged by such brilliant discoveries, he naturally sought to examinethe other planets, and accordingly directed his telescope to Saturn. Here, again, Galileo at once made a discovery. He saw that Saturnpresented a visible form like the other planets, but that it differedfrom any other telescopic object, inasmuch as it appeared to him to becomposed of three bodies which always touched each other and alwaysmaintained the same relative positions. These three bodies were in aline--the central one was the largest, and the two others were east andwest of it. There was nothing he had hitherto seen in the heavens whichfilled his mind with such astonishment, and which seemed so whollyinexplicable. 

In his endeavours to understand this mysterious object, Galileocontinued his observations during the year 1610, and, to his amazement, he saw the two lesser bodies gradually become smaller and smaller, until, in the course of the two following years, they had entirelyvanished, and the planet simply appeared with a round disc like Jupiter. Here, again, was a new source of anxiety to Galileo. He had at that dayto contend against the advocates of the ancient system of astronomy, whoderided his discoveries and refused to accept his theories. He hadannounced his observation of the composite nature of Saturn; he had nowto tell of the gradual decline and the ultimate extinction of these twoauxiliary globes, and he naturally feared that his opponents would seizethe opportunity of pronouncing that the whole of his observations wereillusory. [25] "What, " he remarks, "is to be said concerning so strange ametamorphosis? Are the two lesser stars consumed after the manner of thesolar spots? Have they vanished and suddenly fled? Has Saturn perhaps, devoured his own children? Or were the appearances indeed illusion orfraud, with which the glasses have so long deceived me, as well as manyothers to whom I have shown them? Now, perhaps, is the time come torevive the well-nigh withered hopes of those who, guided by moreprofound contemplations, have discovered the fallacy of the newobservations, and demonstrated the utter impossibility of theirexistence. I do not know what to say in a case so surprising, sounlooked for, and so novel. The shortness of the time, the unexpectednature of the event, the weakness of my understanding, and the fear ofbeing mistaken, have greatly confounded me. "

But Galileo was not mistaken. The objects were really there when hefirst began to observe, they really did decline, and they reallydisappeared; but this disappearance was only for a time--they again cameinto view. They were then subjected to ceaseless examination, untilgradually their nature became unfolded. With increased telescopic powerit was found that the two bodies which Galileo had described as globeson either side of Saturn were not really spherical--they were rather twoluminous crescents with the concavity of each turned towards the centralglobe. It was also perceived that these objects underwent a remarkableseries of periodic changes. At the beginning of such a series the planetwas found with a truly circular disc. The appendages first appeared astwo arms extending directly outwards on each side of the planet; thenthese arms gradually opened into two crescents, resembling handles tothe globe, and attained their maximum width after about seven or eightyears; then they began to contract, until after the lapse of about thesame time they vanished again. 

The true nature of these objects was at length discovered by Huyghens in1655, nearly half a century after Galileo had first detected theirappearance. He perceived the shadow thrown by the ring upon the globe, and his explanation of the phenomena was obtained in a veryphilosophical manner. He noticed that the earth, the sun, and the moonrotated upon their axes, and he therefore regarded it as a general lawthat each one of the bodies in the system rotates about an axis. It istrue, observations had not yet been made which actually showed thatSaturn was also rotating; but it would be highly, nay, indeed, infinitely, improbable that any planet should be devoid of suchmovement. All the analogies of the system pointed to the conclusion thatthe velocity of rotation would be considerable. One satellite of Saturnwas already known to revolve in a period of sixteen days, being littlemore than half our month. Huyghens assumed--and it was a most reasonableassumption--that Saturn in all probability rotated rapidly on its axis. It was also to be observed that if these remarkable appendages wereattached by an actual bodily connection to the planet they must rotatewith Saturn. If, however, the appendages were not actually attached itwould still be necessary that they should rotate if the analogy ofSaturn to other objects in the system were to be in any degreepreserved. We see satellites near Jupiter which revolve around him. Wesee, nearer home, how the moon revolves around the earth. We see how allthe planetary system revolves around the sun. All these considerationswere present to Huyghens when he came to the conclusion that, whetherthe curious appendages were actually attached to the planet or werephysically free from it, they must still be in rotation. 

Provided with such reasonings, it soon became easy to conjecture thetrue nature of the Saturnian system. We have seen how the appendagesdeclined to invisibility once every fifteen years, and then graduallyreappeared in the form, at first, of rectilinear arms projectingoutwards from the planet. The progressive development is a slow one, andfor weeks and months, night after night, the same appearance ispresented with but little change. But all this time both Saturn and themysterious objects around him are rotating. Whatever these may be, theypresent the same appearance to the eye, notwithstanding their ceaselessmotion of rotation. 

What must be the shape of an object which satisfies the conditions hereimplied? It will obviously not suffice to regard the projections as twospokes diverging from the planet. They would change from visibility toinvisibility in every rotation, and thus there would be ceaselessalterations of the appearance instead of that slow and gradual changewhich requires fifteen years for a complete period. There are, indeed, other considerations which preclude the possibility of the objects beinganything of this character, for they are always of the same length ascompared with the diameter of the planet. A little reflection will showthat one supposition--and indeed only one--will meet all the facts ofthe case. If there were a thin symmetrical ring rotating in its ownplane around the equator of Saturn, then the persistence of the objectfrom night to night would be accounted for. This at once removes thegreater part of the difficulty. For the rest, it was only necessary tosuppose that the ring was so thin that when turned actually edgewise tothe earth it became invisible, and then as the illuminated side of theplane became turned more and more towards the earth the appendages tothe planet gradually increased. The handle-shaped appearance which theobject periodically assumed demonstrated that the ring could not beattached to the globe. 

At length Huyghens found that he had the clue to the great enigma whichhad perplexed astronomers for the last fifty years. He saw that the ringwas an object of astonishing interest, unique at that time, as it is, indeed, unique still. He felt, however, that he had hardly demonstratedthe matter with all the certainty which it merited, and which he thoughtthat by further attention he could secure. Yet he was loath to hazardthe loss of his discovery by an undue postponement of its announcement, lest some other astronomer might intervene. How, then, was he to securehis priority if the discovery should turn out correct, and at the sametime be enabled to perfect it at his leisure? He adopted the course, usual at the time, of making his first announcement in cipher, andaccordingly, on March 5th, 1656, he published a tract, which containedthe following proposition:--

 aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s ttttt uuuuu

Perhaps some of those curious persons whose successors now devote somuch labour to double acrostics may have pondered on this renownedcryptograph, and even attempted to decipher it. But even if suchattempts were made, we do not learn that they were successful. A fewyears of further study were thus secured to Huyghens. He tested histheory in every way that he could devise, and he found it verified inevery detail. He therefore thought that it was needless for him anylonger to conceal from the world his great discovery, and accordingly inthe year 1659--about three years after the appearance of hiscryptograph--he announced the interpretation of it. By restoring theletters to their original arrangement the discovery was enunciated inthe following words:--"_Annulo cingitur_, _tenui_, _plano_, _nusquamcohærente_, _ad eclipticam inclinato_, " which may be translated into thestatement:--"The planet is surrounded by a slender flat ring everywheredistinct from its surface, and inclined to the elliptic. "

Huyghens was not content with merely demonstrating how fully thisassumption explained all the observed phenomena. He submitted it to thefurther and most delicate test which can be applied to any astronomicaltheory. He attempted by its aid to make a prediction the fulfilment ofwhich would necessarily give his theory the seal of certainty. From hiscalculations he saw that the planet would appear circular about July orAugust in 1671. This anticipation was practically verified, for the ringwas seen to vanish in May of that year. No doubt, with our moderncalculations founded on long-continued and accurate observation, we arenow enabled to make forecasts as to the appearance or the disappearanceof Saturn's ring with far greater accuracy; but, remembering the earlystage in the history of the planet at which the prediction of Huyghenswas made, we must regard its fulfilment as quite sufficient, and asconfirming in a satisfactory manner the theory of Saturn and his ring. 

The ring of Saturn having thus been thoroughly established as a fact incelestial architecture, each generation of astronomers has laboured tofind out more and more of its marvellous features. In the frontispiece(Plate I. ) we have a view of the planet as seen at the Harvard CollegeObservatory, U. S. A. , between July 28th and October 20th, 1872. It hasbeen drawn by the skilful astronomer and artist--Mr. L. Trouvelot--andgives a faithful and beautiful representation of this unique object. 

Fig. 64 is a drawing of the same object taken on July 2nd, 1894, byProf. E. E. Barnard, at the Lick Observatory. 

The next great discovery in the Saturnian system after those of Huyghensshowed that the ring surrounding the planet was marked by a darkconcentric line, which divided it into two parts--the outer beingnarrower than the inner. This line was first seen by J. D. Cassini, whenSaturn emerged from the rays of the sun in 1675. That this black lineis not merely a black mark on the ring, but that it is actually aseparation, was rendered very probable by the researches of Maraldi in1715, followed many years later by those of Sir William Herschel, who, with that thoroughness which was a marked characteristic of the man, made a minute and scrupulous examination of Saturn. Night after night hefollowed it for hours with his exquisite instruments, and considerablyadded to our knowledge of the planet and his system. 

Herschel devoted very particular attention to the examination of theline dividing the ring. He saw that the colour of this line was not tobe distinguished from the colour of the space intermediate between theglobe and the ring. He observed it for ten years on the northern face ofthe ring, and during that time it continued to present the same breadthand colour and sharpness of outline. He was then fortunate enough toobserve the southern side of the ring. There again could the black linebe seen, corresponding both in appearance and in position with the darkline as seen on the northern side. No doubt could remain as to the factthat Saturn was girdled by two concentric rings equally thin, the outeredge of one closely approaching to the inner edge of the other. 

At the same time it is right to add that the only absolutelyindisputable proof of the division between the rings has not yet beenyielded by the telescope. The appearances noted by Herschel would beconsistent with the view that the black line was merely a part of thering extending through its thickness, and composed of materials verymuch less capable of reflecting light than the rest of the ring. It isstill a matter of doubt how far it is ever possible actually to seethrough the dark line. There is apparently only one satisfactory methodof accomplishing this. It would only occur in rare circumstances, and itdoes not seem that the opportunity has as yet arisen. Suppose that inthe course of its motion through the heavens the path of Saturn happenedto cross directly between the earth and a fixed star. The telescopicappearance of a star is merely a point of light much smaller than theglobes and rings of Saturn. If the ring passed in front of the star andthe black line on the ring came over the star, we should, if the blackline were really an opening, see the star shining through the narrowaperture. 

Up to the present, we believe, there has been no opportunity ofsubmitting the question of the duplex character of the ring to thiscrucial test. Let us hope that as there are now so many telescopes inuse adequate to deal with the subject, there may, ere long, beobservations made which will decide the question. It can hardly beexpected that a very small star would be suitable. No doubt thesmallness of the star would render the observations more delicate andprecise if the star were visible; but we must remember that it will bethrown into contrast with the bright rings of Saturn on each margin sothat unless the star were of considerable magnitude it would hardlyanswer. It has, however, been recently observed that the globe of theplanet can be, in some degree, discerned through the dark line; this ispractically a demonstration of the fact that the line is at all eventspartly transparent. 

The outer ring is also divided into two by a line much fainter than thatjust described. It requires a good telescope and a fine night, combinedwith a favourable position of the planet, to render this line awell-marked object. It is most easily seen at the extremities of thering most remote from the planet. To the present writer, who hasexamined the planet with the twelve-inch refractor of the Southequatorial at Dunsink Observatory, this outer line appears as broad asthe well-known line; but it is unquestionably fainter, and has a moreshaded appearance. It certainly does not suggest the appearance of beingactually an opening in the ring, and it is often invisible for a longtime. It seems rather as if the ring were at this place thinner and lesssubstantial without being actually void of substance. 

On these points it may be expected that much additional information willbe acquired when next the ring places itself in such a position that itsplane, if produced, would pass between the earth and the sun. Suchoccasions are but rare, and even when they do occur it may happen thatthe planet will not be well placed for observation. The next reallygood opportunity will not be till 1907. In this case the sunlightilluminates one side of the ring, while it is the other side of the ringthat is presented towards the earth. Powerful telescopes are necessaryto deal with the planet under such circumstances; but it may bereasonably hoped that the questions relating to the division of thering, as well as to many other matters, will then receive some furtherelucidation. 

Occasionally, other divisions of the ring, both inner and outer, havebeen recorded. It may, at all events, be stated that no such divisionscan be regarded as permanent features. Yet their existence has been sofrequently enunciated by skilful observers that it is impossible todoubt that they have been sometimes seen. 

It was about 200 years after Huyghens had first explained the truetheory of Saturn that another very important discovery was effected. Ithad, up to the year 1850, been always supposed that the two rings, divided by the well-known black line, comprised the entire ring systemsurrounding the planet. In the year just mentioned, Professor Bond, thedistinguished astronomer of Cambridge, Mass. , startled the astronomicalworld by the announcement of his discovery of a third ring surroundingSaturn. As so often happens in such cases, the same object wasdiscovered independently by another--an English astronomer named Dawes. This third ring lies just inside the inner of the two well-known rings, and extends to about half the distance towards the body of the planet. It seems to be of a totally different character from the two other ringsin so far as they present a comparatively substantial appearance. Weshall, indeed, presently show that they are not solid--not even liquidbodies--but still, when compared with the third ring, the others were ofa substantial character. They can receive and exhibit the deeply-markedshadow of Saturn, and they can throw a deep and black shadow upon Saturnthemselves; but the third ring is of a much less compact texture. It hasnot the brilliancy of the others, it is rather of a dusky, semi-transparent appearance, and the expression "crape ring, " by whichit is often designated, is by no means inappropriate. It is thefaintness of this crape ring which led to its having been so frequentlyoverlooked by the earlier observers of Saturn. 

It has often been noticed that when an astronomical discovery has beenmade with a good telescope, it afterwards becomes possible for the sameobject to be observed with instruments of much inferior power. No doubt, when the observer knows what to look for, he will often be able to seewhat would not otherwise have attracted his attention. It may beregarded as an illustration of this principle, that the crape ring ofSaturn has become an object familiar to those who are accustomed to workwith good telescopes; but it may, nevertheless, be doubted whether theease and distinctness with which the crape ring is now seen can beentirely accounted for by this supposition. Indeed, it seems possiblethat the crape ring has, from some cause or other, gradually become moreand more visible. The supposed increased brightness of the crape ring isone of those arguments now made use of to prove that in all probabilitythe rings of Saturn are at this moment undergoing gradualtransformation; but observations of Hadley show that the crape ring wasseen by him in 1720, and it was previously seen by Campani and Picard, as a faint belt crossing the planet. The partial transparency of thecrape ring was beautifully illustrated in an observation by ProfessorBarnard of the eclipse of Iapetus on November 1st, 1889. The satellitewas faintly visible in the shadow of the crape ring, while whollyinvisible in the shadow of the better known rings. 

The various features of the rings are well shown in the drawing ofTrouvelot already referred to. We here see the inner and the outer ring, and the line of division between them. We see in the outer ring thefaint traces of the line by which it is divided, and inside the innerring we have a view of the curious and semi-transparent crape ring. Theblack shadow of the planet is cast upon the ring, thus proving that thering, no less than the body of the planet, shines only in virtue of thesunlight which falls upon it. This shadow presents some anomalousfeatures, but its curious irregularity may be, to some extent, anoptical illusion. 

There can be no doubt that any attempt to depict the rings of Saturnonly represents the salient features of that marvellous system. We aresituated at such a great distance that all objects not of colossaldimensions are invisible. We have, indeed, only an outline, which makesus wish to be able to fill in the details. We long, for instance, to seethe actual texture of the rings, and to learn of what materials they aremade; we wish to comprehend the strange and filmy crape ring, so unlikeany other object known to us in the heavens. There is no doubt that muchmay even yet be learned under all the disadvantageous conditions of ourposition; there is still room for the labour of whole generations ofastronomers provided with splendid instruments. We want accuratedrawings of Saturn under every conceivable aspect in which it may bepresented. We want incessantly repeated measurements, of the mostfastidious accuracy. These measures are to tell us the sizes and theshapes of the rings; they are to measure with fidelity the position ofthe dark lines and the boundaries of the rings. These measures are to beprotracted for generations and for centuries; then and then only canterrestrial astronomers learn whether this elaborate system has reallythe attributes of permanence, or whether it may be undergoing changes. 

We have been accustomed to find that the law of universal gravitationpervades every part of our system, and to look to gravitation for theexplanation of many phenomena otherwise inexplicable. We have goodreasons for knowing that in this marvellous Saturnian system the law ofgravitation is paramount. There are satellites revolving around Saturnas well as a ring; these satellites move, as other satellites do, inconformity with the laws of Kepler; and, therefore, any theory as to thenature of Saturn's ring must be formed subject to the condition that itshall be attracted by the gigantic planet situated in its interior. 

To a hasty glance nothing might seem easier than to reconcile thephenomena of the ring with the attraction of the planet. We mightsuppose that the ring stands at rest symmetrically around the planet. Atits centre the planet pulls in the ring equally on all sides, so thatthere is no tendency in it to move in one way rather than another; and, therefore, it will stay at rest. This will not do. A ring composed ofmaterials almost infinitely rigid might possibly, under suchcircumstances, be for a moment at rest; but it could not remainpermanently at rest any more than can a needle balanced vertically onits point. In each case the equilibrium is unstable. If the slightestcause of disturbance arise, the equilibrium is destroyed, and the ringwould inevitably fall in upon the planet. Such causes of derangement areincessantly present, so that unstable equilibrium cannot be anappropriate explanation of the phenomena. 

Even if this difficulty could be removed, there is still another, whichwould be quite insuperable if the ring be composed of any materials withwhich we are acquainted. Let us ponder for a moment on the matter, as itwill lead up naturally to that explanation of the rings of Saturn whichis now most generally accepted. 

Imagine that you stood on the planet Saturn, near his equator; over yourhead stretches the ring, which sinks down to the horizon in the east andin the west. The half-ring above your horizon would then resemble amighty arch, with a span of about a hundred thousand miles. Everyparticle of this arch is drawn towards Saturn by gravitation, and if thearch continue to exist, it must do so in obedience to the ordinarymechanical laws which regulate the railway arches with which we arefamiliar. 

The continuance of these arches depends upon the resistance of thestones forming them to a crushing force. Each stone of an arch issubjected to a vast pressure, but stone is a material capable ofresisting such pressure, and the arch remains. The wider the span of thearch the greater is the pressure to which each stone is exposed. Atlength a span is reached which corresponds to a pressure as great as thestones can safely bear, and accordingly we thus find the limiting spanover which a single arch of masonry can be constructed. Apply theseprinciples to the stupendous arch formed by the ring of Saturn. It canbe shown that the pressure on the materials of the arch capable ofspanning an abyss of such awful magnitude would be something so enormousthat no materials we know of would be capable of bearing it. Were thering formed of the toughest steel that was ever made, the pressure wouldbe so great that the metal would be squeezed like a liquid, and themighty structure would collapse and fall down on the surface of theplanet. It is not credible that any materials could exist capable ofsustaining a stress so stupendous. The law of gravitation accordinglybids us search for a method by which the intensity of this stress can bemitigated. 

One method is at hand, and is obviously suggested by analogous phenomenaeverywhere in our system. We have spoken of the ring as if it were atrest; let us now suppose it to be animated by a motion of rotation inits plane around Saturn as a centre. Instantly we have a force developedantagonistic to the gravitation of Saturn. This force is the so-calledcentrifugal force. If we imagine the ring to rotate, the centrifugalforce at all points acts in an opposite direction to the attractiveforce, and hence the enormous stress on the ring can be abated and onedifficulty can be overcome. 

We can thus attribute to each ring a rotation which will partly relieveit from the stress the arch would otherwise have to sustain. But wecannot admit that the difficulty has been fully removed. Suppose thatthe outer ring revolve at such a rate as shall be appropriate toneutralise the gravitation on its outer edge, the centrifugal force willbe less at the interior of the ring, while the gravitation will begreater; and hence vast stresses will be set up in the interior parts ofthe outer ring. Suppose the ring to rotate at such a rate as would beadequate to neutralise the gravitation at its inner margin; then thecentrifugal force at the outer parts will largely exceed thegravitation, and there will be a tendency to disruption of the ringoutwards. 

To obviate this tendency we may assume the outer parts of each ring torotate more slowly than the inner parts. This naturally requires thatthe parts of the ring shall be mobile relatively to one another, andthus we are conducted to the suggestion that perhaps the rings arereally composed of matter in a fluid state. The suggestion is, at firstsight, a plausible one; each part of each ring would then move with anappropriate velocity, and the rings would thus exhibit a number ofconcentric circular currents with different velocities. Themathematician can push this inquiry a little farther, and he can studyhow this fluid would behave under such circumstances. His symbols canpursue the subject into the intricacies which cannot be described ingeneral language. The mathematician finds that waves would originate inthe supposed fluid, and that as these waves would lead to disruption ofthe rings, the fluid theory must be abandoned. 

But we can still make one or two more suppositions. What if it be reallytrue that the ring consist of an incredibly large number of concentricrings, each animated precisely with the velocity which would be suitableto the production of a centrifugal force just adequate to neutralise theattraction? No doubt this meets many of the difficulties: it is alsosuggested by those observations which have shown the presence of severaldark lines on the ring. Here again dynamical considerations must beinvoked for the reply. Such a system of solid rings is not compatiblewith the laws of dynamics. 

We are, therefore, compelled to make one last attempt, and still furtherto subdivide the ring. It may seem rather startling to abandon entirelythe supposition that the ring is in any sense a continuous body, butthere remains no alternative. Look at it how we will, we seem to beconducted to the conclusion that the ring is really an enormous shoal ofextremely minute bodies; each of these little bodies pursues an orbit ofits own around the planet, and is, in fact, merely a satellite. Thesebodies are so numerous and so close together that they seem to us to becontinuous, and they may be very minute--perhaps not larger than theglobules of water found in an ordinary cloud over the surface of theearth, which, even at a short distance, seems like a continuous body. 

Until a few years ago this theory of the constitution of Saturn's rings, though unassailable from a mathematical point of view, had never beenconfirmed by observation. The only astronomer who maintained that he hadactually seen the rings rotate was W. Herschel, who watched the motionof some luminous points on the ring in 1789, at which time the plane ofthe ring happened to pass through the earth. From these observationsHerschel concluded that the ring rotated in ten hours and thirty-twominutes. But none of the subsequent observers, even though they may havewatched Saturn with instruments very superior to that used by Herschel, were ever able to succeed in verifying his rotation of these appendagesof Saturn. If the ring were composed of a vast number of small bodies, then the third law of Kepler will enable us to calculate the time whichthese tiny satellites would require to travel completely round theplanet. It appears that any satellite situated at the outer edge of thering would require as long a period as 13 hrs. 46 min. , those about themiddle would not need more than 10 hrs. 28 min. , while those at theinner edge of the ring would accomplish their rotation in 7 hrs. 28 min. Even our mightiest telescopes, erected in the purest skies and employedby the most skilful astronomers, refuse to display this extremelydelicate phenomenon. It would, indeed, have been a repetition on a grandscale of the curious behaviour of the inner satellite of Mars, whichrevolves round its primary in a shorter time than the planet itselftakes to turn round on its own axis. 

[Illustration: Fig. 66. --Prof. Keeler's Method of Measuring theRotation of Saturn's Ring. ]

But what the telescope could not show, the spectroscope has latelydemonstrated in a most effective and interesting manner. We haveexplained in the chapter on the sun how the motion of a source of lightalong the line of vision, towards or away from the observer, produces aslight shift in the position of the lines of the spectrum. By themeasurement of the displacement of the lines the direction and amount ofthe motion of the source of light may be determined. We illustrated themethod by showing how it had actually been used to measure the speed ofrotation of the solar surface. In 1895 Professor Keeler, [26] Director ofthe Allegheny Observatory, succeeded in measuring the rotation ofSaturn's ring in this manner. He placed the slit of his spectroscopeacross the ball, in the direction of the major axis of the ellipticfigure which the effect of perspective gives the ring as shown by theparallel lines in Fig. 66 stretching from E to W. His photographicplate should then show three spectra close together, that of the ball ofSaturn in the middle, separated by dark intervals from the narrowerspectra above and below it of the two handles (or ansæ, as they aregenerally called) of the ring. In Fig. 67 we have represented thebehaviour of any one line of the spectrum under various suppositions asto rotation or non-rotation of Saturn and the ring. At the top (1) wesee how each line would look if there was no rotatory motion; the threelines produced by ring, planet, and ring are in a straight line. Ofcourse the spectrum, which is practically a very faint copy of the solarspectrum, shows the principal dark Fraunhofer lines, so that the readermust imagine these for himself, parallel to the one we show in thefigure. But Saturn and the ring are not standing still, they arerotating, the eastern part (at E) moving towards us, and the westernpart (W) moving away from us. [27] At E the line will therefore beshifted towards the violet end of the spectrum and at W towards the red, and as the actual linear velocity is greater the further we get awayfrom the centre of Saturn (assuming ring and planet to rotate together), the lines would be turned as in Fig. 67 (2), but the three would remainin a straight line. If the ring consisted of two independent ringsseparated by Cassini's division and rotating with different velocities, the lines would be situated as in Fig. 67 (3), the lines due to theinner ring being more deflected than those due to the outer ring, owingto the greater velocity of the inner ring. 

[Illustration: Fig. 67. --Prof. Keeler's Method of Measuring the Rotationof Saturn's Ring. ]

Finally, let us consider the case of the rings, consisting ofinnumerable particles moving round the planet in accordance withKepler's third law. The actual velocities of these particles would beper second:--

 At outer edge of ring 10·69 miles. At middle of ring 11·68 miles. At inner edge of ring 13·01 miles. Rotation speed at surface of planet 6·38 miles. 

The shifting of the lines of the spectrum should be in accordance withthese velocities, and it is easy to see that the lines ought to lie asin the fourth figure. When Professor Keeler came to examine thephotographed spectra, he found the lines of the three spectra tiltedprecisely in this manner, showing that the outer edge of the ring wastravelling round the planet with a smaller linear velocity than theinner one, as it ought to do if the sources of light (or, rather, thereflectors of sunlight) were independent particles free to moveaccording to Kepler's third law, and as it ought not to do if the ring, or rings, were rigid, in which case the outer edge would have thegreatest linear speed, as it had to travel through the greatestdistance. Here, at last, was the proof of the meteoritic composition ofSaturn's ring. Professor Keeler's beautiful discovery has since beenverified by repeated observations at the Allegheny, Lick, Paris, andPulkova Observatories; the actual velocities resulting from the observeddisplacements of the lines have been measured and found to agree well(within the limits of the errors of observation) with the calculatedvelocities, so that this brilliant confirmation of the mathematicaldeductions of Clerk Maxwell is raised beyond the possibility of doubt. 

The spectrum of Saturn is so faint that only the strongest lines of thesolar spectrum can be seen in it, but the atmosphere of the planet seemsto exert a considerable amount of general absorption in the blue andviolet parts of the spectrum, which is especially strong near theequatorial belt, while a strong band in the red testifies to the densityof the atmosphere. This band is not seen in the spectrum of the rings, around which there can therefore be no atmosphere. 

As Saturn's ring is itself unique, we cannot find elsewhere any verypertinent illustration of a structure so remarkable as that now claimedfor the ring. Yet the solar system does show some analogous phenomena. There is, for instance, one on a very grand scale surrounding the sunhimself. We allude to the multitude of minor planets, all confinedwithin a certain region of the system. Imagine these planets to bevastly increased in number, and those orbits which are much inclined tothe rest flattened down and otherwise adjusted, and we should have aring surrounding the sun, thus producing an arrangement not dissimilarfrom that now attributed to Saturn. 

It is tempting to linger still longer over this beautiful system, tospeculate on the appearance which the ring would present to aninhabitant of Saturn, to conjecture whether it is to be regarded as apermanent feature of our system in the same way as we attributepermanence to our moon or to the satellites of Jupiter. Looked at fromevery point of view, the question is full of interest, and it providesoccupation abundant for the labours of every type of astronomer. If hebe furnished with a good telescope, then has he ample duties to fulfilin the task of surveying, of sketching, and of measuring. If he be oneof those useful astronomers who devote their energies not to actualtelescopic work, but to forming calculations based on the observationsof others, then the beautiful system of Saturn provides copiousmaterial. He has to foretell the different phases of the ring, toannounce to astronomers when each feature can be best seen, and at whathour each element can be best determined. He has also to predict thetimes of the movements of Saturn's satellites, and the other phenomenaof a system more elaborate than that of Jupiter. 

Lastly, if the astronomer be one of that class--perhaps, from somepoints of view, the highest class of all--who employ the most profoundresearches of the human intellect to unravel the dynamical problems ofastronomy, he, too, finds in Saturn problems which test to the utmost, even if they do not utterly transcend, the loftiest flights of analysis. He discovers in Saturn's ring an object so utterly unlike anything else, that new mathematical weapons have to be forged for the encounter. Hefinds in the system so many extraordinary features, and such delicacy ofadjustment, that he is constrained to admit that if he did not actuallysee Saturn's rings before him, he would not have thought that such asystem was possible. The mathematician's labours on this wondrous systemare at present only in their infancy. Not alone are the researches of soabstruse a character as to demand the highest genius for this branch ofscience, but even yet the materials for the inquiry have not beenaccumulated. In a discussion of this character, observation must precedecalculation. The scanty observations hitherto obtained, however they mayillustrate the beauty of the system, are still utterly insufficient toform the basis of that great mathematical theory of Saturn which musteventually be written. 

But Saturn possesses an interest for a far more numerous class ofpersons than those who are specially devoted to astronomy. It is ofinterest, it must be of interest, to every cultivated person who has theslightest love for nature. A lover of the picturesque cannot beholdSaturn in a telescope without feelings of the liveliest emotion; while, if his reading and reflection have previously rendered him aware of thecolossal magnitude of the object at which he is looking, he will beconstrained to admit that no more remarkable spectacle is presented inthe whole of nature. 

We have pondered so long over the fascinations of Saturn's ring that wecan only give a very brief account of that system of satellites by whichthe planet is attended. We have already had occasion to allude more thanonce to these bodies; it only remains now to enumerate a few furtherparticulars. 

It was on the 25th of March, 1655, that the first satellite of Saturnwas detected by Huyghens, to whose penetration we owe the discovery ofthe true form of the ring. On the evening of the day referred to, Huyghens was examining Saturn with a telescope constructed with his ownhands, when he observed a small star-like object near the planet. Thenext night he repeated his observations, and it was found that the starwas accompanying the planet in its progress through the heavens. Thisshowed that the little object was really a satellite to Saturn, andfurther observations revealed the fact that it was revolving around himin a period of 15 days, 22 hours, 41 minutes. Such was the commencementof that numerous series of discoveries of satellites which accompanySaturn. One by one they were detected, so that at the present time nofewer than nine are known to attend the great planet through hiswanderings. The subsequent discoveries were, however, in no case made byHuyghens, for he abandoned the search for any further satellites ongrounds which sound strange to modern ears, but which were quite inkeeping with the ideas of his time. It appears that from some principleof symmetry, Huyghens thought that it would accord with the fitness ofthings that the number of satellites, or secondary planets, should beequal in number to the primary planets themselves. The primary planets, including the earth, numbered six; and Huyghens' discovery now broughtthe total number of satellites to be also six. The earth had one, Jupiter had four, Saturn had one, and the system was complete. 

Nature, however, knows no such arithmetical doctrines as those whichHuyghens attributed to her. Had he been less influenced by suchprejudices, he might, perhaps, have anticipated the labours of Cassini, who, by discovering other satellites of Saturn, demonstrated theabsurdity of the doctrine of numerical equality between planets andsatellites. As further discoveries were made, the number of satelliteswas at first raised above the number of planets; but in recent times, when the swarm of minor planets came to be discovered, the number ofplanets speedily reached and speedily passed the number of theirattendant satellites. 

It was in 1671, about sixteen years after the discovery of the firstsatellite of Saturn, that a second was discovered by Cassini. This isthe outermost of the older satellites; it takes 79 days to travel roundSaturn. In the following year he discovered another; and twelve yearslater, in 1684, still two more; thus making a total of five satellitesto this planet. 

[Illustration: Fig. 68. --Transit of Titan and its Shadow, by F. TerbyLouvain, 12th April, 1892. ]

The complexity of the Saturnian system had now no rival in the heavens. Saturn had five satellites, and Jupiter had but four, while at least oneof the satellites of Saturn, named Titan, was larger than any satelliteof Jupiter. [28] Some of the discoveries of Cassini had been made withtelescopes of quite monstrous dimensions. The length of the instrument, or rather the distance at which the object-glass was placed, was onehundred feet or more from the eye of the observer. It seemed hardlypossible to push telescopic research farther with instruments of thiscumbrous type. At length, however, the great reformation in theconstruction of astronomical instruments began to dawn. In the hands ofHerschel, it was found possible to construct reflecting telescopes ofmanageable dimensions, which were both more powerful and more accuratethan the long-focussed lenses of Cassini. A great instrument of thiskind, forty feet long, just completed by Herschel, was directed toSaturn on the 28th of August, 1789. Never before had the wondrous planetbeen submitted to a scrutiny so minute. Herschel was familiar with thelabours of his predecessors. He had often looked at Saturn and his fivemoons in inferior telescopes; now again he saw the five moons and astar-like object so near the plane of the ring that he conjectured thisto be a sixth satellite. A speedy method of testing this conjecture wasat hand. Saturn was then moving rapidly over the heavens. If this newobject were in truth a satellite, then it must be carried on by Saturn. Herschel watched with anxiety to see whether this would be the case. Ashort time sufficed to answer the question; in two hours and a half theplanet had moved to a distance quite appreciable, and had carried withhim not only the five satellites already known, but also this sixthobject. Had this been a star it would have been left behind; it was notleft behind, and hence it, too, was a satellite. Thus, after the longlapse of a century, the telescopic discovery of satellites to Saturnrecommenced. Herschel, as was his wont, observed this object withunremitting ardour, and discovered that it was much nearer to Saturnthan any of the previously known satellites. In accordance with thegeneral law, that the nearer the satellite the shorter the period ofrevolution, Herschel found that this little moon completed a revolutionin about 1 day, 8 hours, 53 minutes. The same great telescope, used withthe same unrivalled skill, soon led Herschel to a still more interestingdiscovery. An object so small as only to appear like a very minutepoint in the great forty-foot reflector was also detected by Herschel, and was by him proved to be a satellite, so close to the planet that itcompleted a revolution in the very brief period of 22 hours and 37minutes. This is an extremely delicate object, only to be seen by thebest telescopes in the brief intervals when it is not entirely screenedfrom view by the ring. 

Again another long interval elapsed, and for almost fifty years theSaturnian system was regarded as consisting of the series of rings andof the seven satellites. The next discovery has a singular historicalinterest. It was made simultaneously by two observers--Professor Bond, of Cambridge, Mass. , and Mr. Lassell, of Liverpool--for on the 19thSeptember, 1848, both of these astronomers verified that a small pointwhich they had each seen on previous nights was really a satellite. Thisobject is, however, at a considerable distance from the planet, andrequires 21 days, 7 hours, 28 minutes for each revolution; it is theseventh in order from the planet. 

Yet one more extremely faint outer satellite was discerned byphotography on the 16th, 17th, and 18th August, 1898, by Professor W. H. Pickering. This object is much more distant from the planet than thelarger and older satellites. Its motion has not yet been fullydetermined, but probably it requires not less than 490 days to perform asingle revolution. 

From observations of the satellites it has been found that 3, 500 globesas heavy as Saturn would weigh as much as the sun. 

A law has been observed by Professor Kirkwood, which connects togetherthe movements of the four interior satellites of Saturn. This law isfulfilled in such a manner as leads to the supposition that it arisesfrom the mutual attraction of the satellites. We have already describeda similar law relative to three of the satellites of Jupiter. Theproblem relating to Saturn, involving as it does no fewer than foursatellites, is one of no ordinary complexity. It involves the theory ofPerturbations to a greater degree than that to which mathematicians areaccustomed in their investigation of the more ordinary features of oursystem. To express this law it is necessary to have recourse to thedaily movements of the satellites; these are respectively--

 SATELLITE. DAILY MOVEMENT. I. 382°·2. II. 262°·74. III. 190°·7. IV. 131°·4. 

The law states that if to five times the movement of the first satellitewe add that of the third and four times that of the fourth, the wholewill equal ten times the movement of the second satellite. Thecalculation stands thus:--

 5 times I. Equals 1911°·0 III. Equals 190°·7 II. 262°·74 4 times IV. Equals 525°·6 10 -------- -------- 2627°·3 equal 2627°·4 nearly. 

Nothing can be simpler than the verification of this law; but the taskof showing the physical reason why it should be fulfilled has not yetbeen accomplished. 

Saturn was the most distant planet known to the ancients. It revolves inan orbit far outside the other ancient planets, and, until the discoveryof Uranus in the year 1781, the orbit of Saturn might well be regardedas the frontier of the solar system. The ringed planet was indeed aworthy object to occupy a position so distinguished. But we now knowthat the mighty orbit of Saturn does not extend to the frontiers of thesolar system; a splendid discovery, leading to one still more splendid, has vastly extended the boundary, by revealing two mighty planets, revolving in dim telescopic distance, far outside the path of Saturn. These objects have not the beauty of Saturn; they are, indeed, in nosense effective telescopic pictures. Yet these outer planets awaken aninterest of a most special kind. The discovery of each is a classicalevent in the history of astronomy, and the opinion has been maintained, and perhaps with reason, that the discovery of Neptune, the more remoteof the two, is the greatest achievement in astronomy made since the timeof Newton. 

CHAPTER XIV

URANUS. 

 Contrast between Uranus and the other great Planets--William Herschel--His Birth and Parentage--Herschel's Arrival in England--His Love of Learning--Commencement of his Astronomical Studies--The Construction of Telescopes--Construction of Mirrors--The Professor of Music becomes an Astronomer--The Methodical Research--The 13th March, 1781--The Discovery of Uranus--Delicacy of Observation--Was the Object a Comet?--The Significance of this Discovery--The Fame of Herschel--George III. And the Bath Musician--The King's Astronomer at Windsor--The Planet Uranus--Numerical Data with reference thereto--The Four Satellites of Uranus--Their Circular Orbits--Early Observations of Uranus--Flamsteed's Observations--Lemonnier saw Uranus--Utility of their Measurements--The Elliptic Path--The Great Problem thus Suggested. 

To the present writer it has always seemed that the history of Uranus, and of the circumstances attending its discovery, forms one of the mostpleasing and interesting episodes in the whole history of science. Wehere occupy an entirely new position in the study of the solar system. All the other great planets were familiarly known from antiquity, however erroneous might be the ideas entertained in connection withthem. They were conspicuous objects, and by their movements could hardlyfail to attract the attention of those whose pursuits led them toobserve the stars. But now we come to a great planet, the very existenceof which was utterly unknown to the ancients; and hence, in approachingthe subject, we have first to describe the actual discovery of thisobject, and then to consider what we can learn as to its physicalnature. 

We have, in preceding pages, had occasion to mention the revered name ofWilliam Herschel in connection with various branches of astronomy; butwe have hitherto designedly postponed any more explicit reference tothis extraordinary man until we had arrived at the present stage of ourwork. The story of Uranus, in its earlier stages at all events, is thestory of the early career of William Herschel. It would be alikeimpossible and undesirable to attempt to separate them. 

William Herschel, the illustrious astronomer, was born at Hanover in1738. His father was an accomplished man, pursuing, in a somewhat humblemanner, the calling of a professor of music. He had a family of tenchildren, of whom William was the fourth; and it may be noted that allthe members of the family of whom any record has been preservedinherited their father's musical talents, and became accomplishedperformers. Pleasing sketches have been given of this interestingfamily, of the unusual aptitude of William, of the long discussions onmusic and on philosophy, and of the little sister Caroline, destined inlater years for an illustrious career. William soon learned all that hismaster could teach him in the ordinary branches of knowledge, and by theage of fourteen he was already a competent performer on the oboe and theviol. He was engaged in the Court orchestra at Hanover, and was also amember of the band of the Hanoverian Guards. Troublous times were soonto break up Herschel's family. The French invaded Hanover, theHanoverian Guards were overthrown in the battle of Hastenbeck, and youngWilliam Herschel had some unpleasant experience of actual warfare. Hishealth was not very strong, and he decided that he would make a changein his profession. His method of doing so is one which his biographerscan scarcely be expected to defend; for, to speak plainly, he deserted, and succeeded in making his escape to England. It is stated onunquestionable authority that on Herschel's first visit to King GeorgeIII. , more than twenty years afterwards, his pardon was handed to him bythe King himself, written out in due form. 

At the age of nineteen the young musician began to seek his fortunes inEngland. He met at first with very considerable hardship, but industryand skill conquered all difficulties, and by the time he was twenty-sixyears of age he was thoroughly settled in England, and doing well in hisprofession. In the year 1766 we find Herschel occupying a position ofsome distinction in the musical world; he had become the organist of theOctagon Chapel at Bath, and his time was fully employed in givinglessons to his numerous pupils, and with his preparation for concertsand oratorios. 

Notwithstanding his busy professional life, Herschel still retained thatinsatiable thirst for knowledge which he had when a boy. Every moment hecould snatch from his musical engagements was eagerly devoted to study. In his desire to perfect his knowledge of the more abstruse parts of thetheory of music he had occasion to learn mathematics; from mathematicsthe transition to optics was a natural one; and once he had commenced tostudy optics, he was of course brought to a knowledge of the telescope, and thence to astronomy itself. 

His beginnings were made on a very modest scale. It was through a smalland imperfect telescope that the great astronomer obtained his firstview of the celestial glories. No doubt he had often before looked atthe heavens on a clear night, and admired the thousands of stars withwhich they were adorned; but now, when he was able to increase hispowers of vision even to a slight extent, he obtained a view whichfascinated him. The stars he had seen before he now saw far moredistinctly; but, more than this, he found that myriads of otherspreviously invisible were now revealed to him. Glorious, indeed, is thisspectacle to anyone who possesses a spark of enthusiasm for naturalbeauty. To Herschel this view immediately changed the whole current ofhis life. His success as a professor of music, his oratorios, and hispupils were speedily to be forgotten, and the rest of his life was to bedevoted to the absorbing pursuit of one of the noblest of the sciences. 

Herschel could not remain contented with the small and imperfectinstrument which first interested him. Throughout his career hedetermined to see everything for himself in the best manner which hisutmost powers could command. He at once decided to have a betterinstrument, and he wrote to a celebrated optician in London with theview of making a purchase. But the price which the optician demandedseemed more than Herschel thought he could or ought to give. Instantlyhis resolution was taken. A good telescope he must have, and as he couldnot buy one he resolved to make one. It was alike fortunate, both forHerschel and for science, that circumstances impelled him to thisdetermination. Yet, at first sight, how unpromising was the enterprise!That a music teacher, busily employed day and night, should, withoutprevious training, expect to succeed in a task where the highestmechanical and optical skill was required, seemed indeed unlikely. Butenthusiasm and genius know no insuperable difficulties. From conductinga brilliant concert in Bath, when that city was at the height of itsfame, Herschel would rush home, and without even delaying to take offhis lace ruffles, he would plunge into his manual labours of grindingspecula and polishing lenses. No alchemist of old was ever more deeplyabsorbed in a project for turning lead into gold than was Herschel inhis determination to have a telescope. He transformed his home into alaboratory; of his drawing-room he made a carpenter's shop. Turninglathes were the furniture of his best bedroom. A telescope he must have, and as he progressed he determined, not only that he should have a goodtelescope, but a very good one; and as success cheered his efforts heultimately succeeded in constructing the greatest telescope that theworld had up to that time ever seen. Though it is as an astronomer thatwe are concerned with Herschel, yet we must observe even as a telescopemaker also great fame and no small degree of commercial success flowedin upon him. When the world began to ring with his glorious discoveries, and when it was known that he used no other telescopes than those whichwere the work of his own hands, a demand sprang up for instruments ofhis construction. It is stated that he made upwards of eighty largetelescopes, as well as many others of smaller size. Several of theseinstruments were purchased by foreign princes and potentates. [29] Wehave never heard that any of these illustrious personages becamecelebrated astronomers, but, at all events, they seem to have paidHerschel handsomely for his skill, so that by the sale of largetelescopes he was enabled to realise what may be regarded as a fortunein the moderate horizon of the man of science. 

Up to the middle of his life Herschel was unknown to the public exceptas a laborious musician, with considerable renown in his profession, notonly in Bath, but throughout the West of England. His telescope-makingwas merely the occupation of his spare moments, and was unheard of bymost of those who knew and respected his musical attainments. It was in1774 that Herschel first enjoyed a view of the heavens through aninstrument built with his own hands. It was but a small one incomparison with those which he afterwards fashioned, but at once heexperienced the advantage of being his own instrument maker. Night afternight he was able to add the improvements which experience suggested; atone time he was enlarging the mirrors; at another he was reconstructingthe mounting, or trying to remedy defects in the eye-pieces. Withunwearying perseverance he aimed at the highest excellence, and witheach successive advance he found that he was able to pierce further intothe sky. His enthusiasm attracted a few friends who were, like himself, ardently attached to science. The mode in which he first made theacquaintance of Sir William Watson, who afterwards became his warmestfriend, was characteristic of both. Herschel was observing the mountainsin the moon, and as the hours passed on, he had occasion to bring histelescope into the street in front of his house to enable him tocontinue his work. Sir William Watson happened to pass by, and wasarrested by the unusual spectacle of an astronomer in the public street, at the dead of night, using a large and quaint-looking instrument. Having a taste for astronomy, Sir William stopped, and when Herscheltook his eye from the telescope, asked if he might be allowed to have alook at the moon. The request was readily granted. Probably Herschelfound but few in the gay city who cared for such matters; he was quicklydrawn to Sir W. Watson, who at once reciprocated the feeling, and thusbegan a friendship which bore important fruit in Herschel's subsequentcareer. 

At length the year 1781 approached, which was to witness his greatachievement. Herschel had made good use of seven years' practicalexperience in astronomy, and he had completed a telescope of exquisiteoptical perfection, though greatly inferior in size to some of thosewhich he afterwards erected. With this reflector Herschel commenced amethodical piece of observation. He formed the scheme of systematicallyexamining all the stars which were above a certain degree of brightness. It does not quite appear what object Herschel proposed to himself whenhe undertook this labour, but, in any case, he could hardly haveanticipated the extraordinary success with which the work was to becrowned. In the course of this review the telescope was directed to astar; that star was examined; then another was brought into the field ofview, and it too was examined. Every star under such circumstancesmerely shows itself as a point of light; the point may be brilliant ornot, according as the star is bright or not; the point will also, ofcourse, show the colour of the star, but it cannot exhibit recognisablesize or shape. The greater, in fact, the perfection of the telescope, the smaller is the telescopic image of a star. 

How many stars Herschel inspected in this review we are not told; but atall events, on the ever-memorable night of the 13th of March, 1781, hewas pursuing his self-allotted task among the hosts in the constellationGemini. Doubtless, one star after another was admitted to view, and wasallowed to pass away. At length, however, an object was placed in thefield which differed from every other star. It was not a mere point oflight; it had a minute, but still a perfectly recognisable, disc. We saythe disc was perfectly recognisable, but we should be careful to addthat it was so in the excellent telescope of Herschel alone. Otherastronomers had seen this object before. Its position had actually beenmeasured no fewer than nineteen times before the Bath musician, with hishome-made telescope, looked at it, but the previous observers had onlyseen it in small meridian instruments with low magnifying powers. Evenafter the discovery was made, and when well-trained observers with goodinstruments looked again under the direction of Herschel, one afteranother bore testimony to the extraordinary delicacy of the greatastronomer's perception, which enabled him almost at the first glance todiscriminate between it and a star. 

If not a star, what, then, could it be? The first step to enable thisquestion to be answered was to observe the body for some time. ThisHerschel did. He looked at it one night after another, and soon hediscovered another fundamental difference between this object and anordinary star. The stars are, of course, characterised by their fixity, but this object was not fixed; night after night the place it occupiedchanged with respect to the stars. No longer could there be any doubtthat this body was a member of the solar system, and that an interestingdiscovery had been made; many months, however, elapsed before Herschelknew the real merit of his achievement. He did not realise that he hadmade the superb discovery of another mighty planet revolving outsideSaturn; he thought that it could only be a comet. No doubt this objectlooked very different from a great comet, decorated with a tail. It wasnot, however, so entirely different from some forms of telescopic cometsas to make the suggestion of its being a body of this kind unlikely; andthe discovery was at first announced in accordance with this view. Timewas necessary before the true character of the object could beascertained. It must be followed for a considerable distance along itspath, and measures of its position at different epochs must be effected, before it is practicable for the mathematician to calculate the pathwhich the body pursues; once, however, attention was devoted to thesubject, many astronomers aided in making the necessary observations. These were placed in the hands of mathematicians, and the result wasproclaimed that this body was not a comet, but that, like all theplanets, it revolved in nearly a circular path around the sun, and thatthe path lay millions of miles outside the path of Saturn, which had solong been regarded as the boundary of the solar system. 

It is hardly possible to over-estimate the significance of this splendiddiscovery. The five planets had been known from all antiquity; they wereall, at suitable seasons, brilliantly conspicuous to the unaided eye. But it was now found that, far outside the outermost of these planetsrevolved another splendid planet, larger than Mercury or Mars, larger--far larger--than Venus and the earth, and only surpassed in bulkby Jupiter and by Saturn. This superb new planet was plunged into spaceto such a depth that, notwithstanding its noble proportions, it seemedmerely a tiny star, being only on rare occasions within reach of theunaided eye. This great globe required a period of eighty-four years tocomplete its majestic path, and the diameter of that path was3, 600, 000, 000 miles. 

Although the history of astronomy is the record of brilliantdiscoveries--of the labours of Copernicus, and of Kepler--of thetelescopic achievements of Galileo, and the splendid theory ofNewton--of the refined discovery of the aberration of light--of manyother imperishable triumphs of intellect--yet this achievement of theorganist at the Octagon Chapel occupies a totally different positionfrom any other. There never before had been any historic record of thediscovery of one of the bodies of the particular system to which theearth belongs. The older planets were no doubt discovered by someone, but we can say little more about these discoveries than we can about thediscovery of the sun or of the moon; all are alike prehistoric. Here wasthe first recorded instance of the discovery of a planet which, like theearth, revolves around the sun, and, like our earth, may conceivably bean inhabited globe. So unique an achievement instantly arrested theattention of the whole scientific world. The music-master at Bath, hitherto unheard of as an astronomer, was speedily placed in the veryforemost rank of those entitled to the name. On all sides the greatestinterest was manifested about the unknown philosopher. The name ofHerschel, then unfamiliar to English ears, appeared in every journal, and a curious list has been preserved of the number of blunders whichwere made in spelling the name. The different scientific societieshastened to convey their congratulations on an occasion so memorable. 

Tidings of the discovery made by the Hanoverian musician reached theears of George III. , and he sent for Herschel to come to the Court, thatthe King might learn what his achievement actually was from thediscoverer's own lips. Herschel brought with him one of his telescopes, and he provided himself with a chart of the solar system, with which toexplain precisely wherein the significance of the discovery lay. TheKing was greatly interested in Herschel's narrative, and not less inHerschel himself. The telescope was erected at Windsor, and, under theastronomer's guidance, the King was shown Saturn and other celebratedobjects. It is also told how the ladies of the Court the next day askedHerschel to show them the wonders which had so pleased the King. Thetelescope was duly erected in a window of one of the Queen's apartments, but when evening arrived the sky was found to be overcast with clouds, and no stars could be seen. This was an experience with which Herschel, like every other astronomer, was unhappily only too familiar. But it isnot every astronomer who would have shown the readiness of Herschel inescaping gracefully from the position. He showed to his lady pupils theconstruction of the telescope; he explained the mirror, and how he hadfashioned it and given the polish; and then, seeing the clouds wereinexorable, he proposed that, as he could not show them the real Saturn, he should exhibit an artificial one as the best substitute. Thepermission granted, Herschel turned the telescope away from the sky, andpointed it towards the wall of a distant garden. On looking into thetelescope there was Saturn, his globe and his system of rings, sofaithfully shown that, says Herschel, even a skilful astronomer mighthave been deceived. The fact was that during the course of the dayHerschel saw that the sky would probably be overcast in the evening, andhe had provided for the emergency by cutting a hole in a piece ofcardboard, the shape of Saturn, which was then placed against thedistant garden wall, and illuminated by a lamp at the back. 

This visit to Windsor was productive of consequences momentous toHerschel, momentous to science. He had made so favourable an impression, that the King proposed to create for him the special appointment ofKing's Astronomer at Windsor. The King was to provide the means forerecting the great telescopes, and he allocated to Herschel a salary of£200 a year, the figures being based, it must be admitted, on a somewhatmoderate estimate of the requirements of an astronomer's household. Herschel mentioned these particulars to no one save to his constant andgenerous friend, Sir W. Watson, who exclaimed, "Never bought monarchhonour so cheap. " To other enquirers, Herschel merely said that the Kinghad provided for him. In accepting this post, the great astronomer tookno doubt a serious step. He at once sacrificed entirely his musicalcareer, now, from many sources, a lucrative one; but his determinationwas speedily taken. The splendid earnest that he had already given ofhis devotion to astronomy was, he knew, only the commencement of aseries of memorable labours. He had indeed long been feeling that it washis bounden duty to follow that path in life which his genius indicated. He was no longer a young man. He had attained middle age, and the yearshad become especially precious to one who knew that he had still alife-work to accomplish. He at one stroke freed himself from alldistractions; his pupils and concerts, his whole connection at Bath, were immediately renounced; he accepted the King's offer with alacrity, and after one or two changes settled permanently at Slough, nearWindsor. 

It has, indeed, been well remarked that the most important event inconnection with the discovery of Uranus was the discovery of Herschel'sunrivalled powers of observation. Uranus must, sooner or later, havebeen found. Had Herschel not lived, we would still, no doubt, have knownUranus long ere this. The really important point for science was thatHerschel's genius should be given full scope, by setting him free fromthe engrossing details of an ordinary professional calling. Thediscovery of Uranus secured all this, and accordingly obtained forastronomy all Herschel's future labours. [30]

Uranus is so remote that even the best of our modern telescopes cannotmake of it a striking picture. We can see, as Herschel did, that it hasa measurable disc, and from measurements of that disc we conclude thatthe diameter of the planet is about 31, 700 miles. This is about fourtimes as great as the diameter of the earth, and we accordingly see thatthe volume of Uranus must be about sixty-four times as great as that ofthe earth. We also find that, like the other giant planets, Uranus seemsto be composed of materials much lighter, on the whole, than those wefind here; so that, though sixty-four times as large as the earth, Uranus is only fifteen times as heavy. If we may trust to the analogiesof what we see everywhere else in our system, we can feel but littledoubt that Uranus must rotate about an axis. The ordinary means ofdemonstrating this rotation can be hardly available in a body whosesurface appears so small and so faint. The period of rotation isaccordingly unknown. The spectroscope tells us that a remarkableatmosphere, containing apparently some gases foreign to our own, deeplyenvelops Uranus. 

There is, however, one feature about Uranus which presents many pointsof interest to those astronomers who are possessed of telescopes ofunusual size and perfection. Uranus is accompanied by a system ofsatellites, some of which are so faint as to require the closestscrutiny for their detection. The discovery of these satellites was oneof the subsequent achievements of Herschel. It is, however, remarkablethat even his penetration and care did not preserve him from errorswith regard to these very delicate objects. Some of the points which hethought to be satellites must, it would now seem, have been merely starsenormously more distant, which happened to lie in the field of view. Ithas been since ascertained that the known satellites of Uranus are fourin number, and their movements have been made the subject of prolongedand interesting telescopic research. The four satellites bear the namesof Ariel, Umbriel, Titania, and Oberon. Arranged in order of theirdistance from the central body, Ariel, the nearest, accomplishes itsjourney in 2 days and 12 hours. Oberon, the most distant, completes itsjourney in 13 days and 11 hours. 

The law of Kepler declares that the path of a satellite around itsprimary, no less than of the primary around the sun, must be an ellipse. It leaves, however, boundless latitude in the actual eccentricity of thecurve. The ellipse may be nearly a circle, it may be absolutely acircle, or it may be something quite different from a circle. The pathspursued by the planets are, generally speaking, nearly circles; but wemeet with no exact circle among planetary orbits. So far as we atpresent know, the closest approach made to a perfectly circular movementis that by which the satellites of Uranus revolve around their primary. We are not prepared to say that these paths are absolutely circular. Allthat can be said is that our telescopes fail to show any measurabledeparture therefrom. It is also to be noted as an interestingcircumstance that the orbits of the satellites of Uranus all lie in thesame plane. This is not true of the orbits of the planets around thesun, nor is it true of the orbits of any other system of satellitesaround their primary. The most singular circumstance attending theUranian system is, however, found in the position which this planeoccupies. This is indeed almost as great an anomaly in our system as arethe rings of Saturn themselves. We have already had occasion to noticethat the plane in which the earth revolves around the sun is very nearlycoincident with the planes in which all the other great planets revolve. The same is true, to a large extent, of the orbits of the minor planets;though here, no doubt, we meet with a few cases in which the plane ofthe orbit is inclined at no inconsiderable angle to the plane in whichthe earth moves. The plane in which the moon revolves also approximatesto this system of planetary planes. So, too, do the orbits of thesatellites of Saturn and of Jupiter, while even the more recentlydiscovered satellites of Mars form no exception to the rule. The wholesolar system--at least so far as the great planets are concerned--wouldrequire comparatively little alteration if the orbits were to beentirely flattened down into one plane. There are, however, some notableexceptions to this rule. The satellites of Uranus revolve in a planewhich is far from coinciding with the plane to which all other orbitsapproximate. In fact, the paths of the satellites of Uranus lie in aplane nearly at right angles to the orbit of Uranus. We are not in aposition to give any satisfactory explanation of this circumstance. Itis, however, evident that in the genesis of the Uranian system theremust have been some influence of a quite exceptional and localcharacter. 

Soon after the discovery of the planet Uranus, in 1781, sufficientobservations were accumulated to enable the orbit it follows to bedetermined. When the path was known, it was then a mere matter ofmathematical calculation to ascertain where the planet was situated atany past time, and where it would be situated at any future time. Aninteresting enquiry was thus originated as to how far it might bepossible to find any observations of the planet made previously to itsdiscovery by Herschel. Uranus looks like a star of the sixth magnitude. Not many astronomers were provided with telescopes of the perfectionattained by Herschel, and the personal delicacy of perceptioncharacteristic of Herschel was a still more rare possession. It was, therefore, to be expected that, if such previous observations existed, they would merely record Uranus as a star visible, and indeed bright, ina moderate telescope, but still not claiming any exceptional attentionover thousands of apparently similar stars. Many of the earlyastronomers had devoted themselves to the useful and laborious work offorming catalogues of stars. In the preparation of a star catalogue, thetelescope was directed to the heavens, the stars were observed, theirplaces were carefully measured, the brightness of the star was alsoestimated, and thus the catalogue was gradually compiled in which eachstar had its place faithfully recorded, so that at any future time itcould be identified. The stars were thus registered, by hundreds and bythousands, at various dates from the birth of accurate astronomy tillthe present time. The suggestion was then made that, as Uranus looked solike a star, and as it was quite bright enough to have engaged theattention of astronomers possessed of even very moderate instrumentalpowers, there was a possibility that it had already been observed, andthus actually lay recorded as a star in some of the older catalogues. This was indeed an idea worthy of every attention, and pregnant with themost important consequences in connection with the immortal discovery tobe discussed in our next chapter. But how was such an examination of thecatalogues to be conducted? Uranus is constantly moving about; does itnot seem that there is every element of uncertainty in such aninvestigation? Let us consider a notable example. 

The great national observatory at Greenwich was founded in 1675, and thefirst Astronomer-Royal was the illustrious Flamsteed, who in 1676commenced that series of observations of the heavenly bodies which hasbeen continued to the present day with such incalculable benefits toscience. At first the instruments were of a rather primitivedescription, but in the course of some years Flamsteed succeeded inprocuring instruments adequate to the production of a catalogue ofstars, and he devoted himself with extraordinary zeal to theundertaking. It is in this memorable work, the "Historia Coelestis" ofFlamsteed, that the earliest observation of Uranus is recorded. In thefirst place it was known that the orbit of this body, like the orbit ofevery other great planet, was inclined at a very small angle to theecliptic. It hence follows that Uranus is at all times only to be metwith along the ecliptic, and it is possible to calculate where theplanet has been in each year. It was thus seen that in 1690 the planetwas situated in that part of the ecliptic where Flamsteed was at thesame date making his observations. It was natural to search theobservations of Flamsteed, and see whether any of the so-called starscould have been Uranus. An object was found in the "HistoriaCoelestis" which occupied a position identical with that which Uranusmust have filled on the same date. Could this be Uranus? A decisive testwas at once available. The telescope was directed to the spot in theheavens where Flamsteed saw a sixth-magnitude star. If that were reallya star, then would it still be visible. The trial was made: no such starcould be found, and hence the presumption that this was really Uranuscould hardly be for a moment doubted. Speedily other confirmation flowedin. It was shown that Uranus had been observed by Bradley and by TobiasMayer, and it also became apparent that Flamsteed had observed Uranusnot only once, but that he had actually measured its place four times inthe years 1712 and 1715. Yet Flamsteed was never conscious of thediscovery that lay so nearly in his grasp. He was, of course, under theimpression that all these observations related to different stars. Astill more remarkable case is that of Lemonnier, who had actuallyobserved Uranus twelve times, and even recorded it on four consecutivedays in January, 1769. If Lemonnier had only carefully looked over hisown work; if he had perceived, as he might have done, how the star heobserved yesterday was gone to-day, while the star visible to-day hadmoved away by to-morrow, there is no doubt that Uranus would have beendiscovered, and William Herschel would have been anticipated. WouldLemonnier have made as good use of his fame as Herschel did? This seemsa question which can never be decided, but those who estimate Herschelas the present writer thinks he ought to be estimated, will probablyagree in thinking that it was most fortunate for science that Lemonnierdid _not_ compare his observations. [31]

These early accidental observations of Uranus are not merely to beregarded as matters of historical interest or curiosity. That they areof the deepest importance with regard to the science itself a few wordswill enable us to show. It is to be remembered that Uranus requires noless than eighty-four years to accomplish his mighty revolution aroundthe sun. The planet has completed one entire revolution since itsdiscovery, and up to the present time (1900) has accomplished more thanone-third of another. For the careful study of the nature of the orbit, it was desirable to have as many measurements as possible, and extendingover the widest possible interval. This was in a great measure securedby the identification of the early observations of Uranus. Anapproximate knowledge of the orbit was quite capable of giving theplaces of the planet with sufficient accuracy to identify it when metwith in the catalogues. But when by their aid the actual observationshave been discovered, they tell us precisely the place of Uranus; andhence, instead of our knowledge of the planet being limited to butlittle more than one revolution, we have at the present time informationwith regard to it extending over considerably more than two revolutions. 

From the observations of the planet the ellipse in which it moves can beascertained. We can compute this ellipse from the observations madeduring the time since the discovery. We can also compute the ellipsefrom the early observations made before the discovery. If Kepler's lawswere rigorously verified, then, of course, the ellipse performed in thepresent revolution must differ in no respect from the ellipse performedin the preceding, or indeed in any other revolution. We can test thispoint in an interesting manner by comparing the ellipse derived from theancient observations with that deduced from the modern ones. Theseellipses closely resemble each other; they are nearly the same; but itis most important to observe that they are not _exactly_ the same, evenwhen allowance has been made for every known source of disturbance inaccordance with the principles explained in the next chapter. The law ofKepler seems thus not absolutely true in the case of Uranus. Here is, indeed, a matter demanding our most earnest and careful attention. Havewe not repeatedly laid down the universality of the laws of Kepler incontrolling the planetary motions? How then can we reconcile this lawwith the irregularities proved beyond a doubt to exist in the motions ofUranus?

Let us look a little more closely into the matter. We know that the lawsof Kepler are a consequence of the laws of gravitation. We know that theplanet moves in an elliptic path around the sun, in virtue of the sun'sattraction, and we know that the ellipse will be preserved without theminutest alteration if the sun and the planet be left to their mutualattractions, and if no other force intervene. We can also calculate theinfluence of each of the known planets on the form and position of theorbit. But when allowance is made for all such perturbing influences itis found that the observed and computed orbits do not agree. Theconclusion is irresistible. Uranus does not move solely in consequenceof the sun's attraction and that of the planets of our system interiorto Uranus; there must therefore be some further influence acting uponUranus besides those already known. To the development of this subjectthe next chapter will be devoted. 

CHAPTER XV. 

NEPTUNE. 

 Discovery of Neptune--A Mathematical Achievement--The Sun's Attraction--All Bodies attract--Jupiter and Saturn--The Planetary Perturbations--Three Bodies--Nature has simplified the Problem--Approximate Solution--The Sources of Success--The Problem Stated for the Earth--The Discoveries of Lagrange--The Eccentricity--Necessity that all the Planets revolve in the same Direction--Lagrange's Discoveries have not the Dramatic Interest of the more Recent Achievements--The Irregularities of Uranus--The Unknown Planet must revolve outside the Path of Uranus--The Data for the Problem--Le Verrier and Adams both investigate the Question--Adams indicates the Place of the Planet--How the Search was to be conducted--Le Verrier also solves the Problem--The Telescopic Discovery of the Planet--The Rival Claims--Early Observation of Neptune--Difficulty of the Telescopic Study of Neptune--Numerical Details of the Orbit--Is there any Outer Planet?--Contrast between Mercury and Neptune. 

We describe in this chapter a discovery so extraordinary that the wholeannals of science may be searched in vain for a parallel. We are nothere concerned with technicalities of practical astronomy. Neptune wasfirst revealed by profound mathematical research rather than by minutetelescopic investigation. We must develop the account of this strikingepoch in the history of science with the fulness of detail which iscommensurate with its importance; and it will accordingly be necessary, at the outset of our narrative, to make an excursion into a difficultbut attractive department of astronomy, to which we have as yet madelittle reference. 

The supreme controlling power in the solar system is the attraction ofthe sun. Each planet of the system experiences that attraction, and, invirtue thereof, is constrained to revolve around the sun in an ellipticpath. The efficiency of a body as an attractive agent is directlyproportional to its mass, and as the mass of the sun is more than athousand times as great as that of Jupiter, which, itself, exceeds thatof all the other planets collectively, the attraction of the sun isnecessarily the chief determining force of the movements in our system. The law of gravitation, however, does not merely say that the sunattracts each planet. Gravitation is a doctrine much more general, forit asserts that every body in the universe attracts every other body. Inobedience to this law, each planet must be attracted, not only by thesun, but by innumerable bodies, and the movement of the planet must bethe joint effect of all such attractions. As for the influence of thestars on our solar system, it may be at once set aside as inappreciable. The stars are no doubt enormous bodies, in many cases possiblytranscending the sun in magnitude, but the law of gravitation tells usthat the intensity of the attraction decreases as the square of thedistance increases. Most of the stars are a million times as remote asthe sun, and consequently their attraction is so slight as to beabsolutely inappreciable in the discussion of this question. The onlyattractions we need consider are those which arise from the action ofone body of the system upon another. Let us take, for instance, the twolargest planets of our system, Jupiter and Saturn. Each of these globesrevolves mainly in consequence of the sun's attraction, but every planetalso attracts every other, and the consequence is that each one isslightly drawn away from the position it would have otherwise occupied. In the language of astronomy, we would say that the path of Jupiter isperturbed by the attraction of Saturn; and, conversely, that the path ofSaturn is perturbed by the attraction of Jupiter. 

For many years these irregularities of the planetary motions presentedproblems with which astronomers were not able to cope. Gradually, however, one difficulty after another has been vanquished, and thoughthere are no doubt some small irregularities still outstanding whichhave not been completely explained, yet all the larger and moreimportant phenomena of the kind are well understood. The subject is oneof the most difficult which the astronomer has to encounter in thewhole range of his science. He has here to calculate what effect oneplanet is capable of producing on another planet. Such calculationsbristle with formidable difficulties, and can only be overcome byconsummate skill in the loftiest branches of mathematics. Let us statewhat the problem really is. 

When two bodies move in virtue of their mutual attraction, both of themwill revolve in a curve which admits of being exactly ascertained. Eachpath is, in fact, an ellipse, and they must have a common focus at thecentre of gravity of the two bodies, considered as a single system. Inthe case of a sun and a planet, in which the mass of the sunpreponderates enormously over the mass of the planet, the centre ofgravity of the two lies very near the centre of the sun; the path of thegreat body is in such a case very small in comparison with the path ofthe planet. All these matters admit of perfectly accurate calculation ofa somewhat elementary character. But now let us add a third body to thesystem which attracts each of the others and is attracted by them. Inconsequence of this attraction, the third body is displaced, andaccordingly its influence on the others is modified; they in turn actupon it, and these actions and reactions introduce endless complexityinto the system. Such is the famous "problem of three bodies, " which hasengaged the attention of almost every great mathematician since the timeof Newton. Stated in its mathematical aspect, and without having itsintricacy abated by any modifying circumstances, the problem is one thatdefies solution. Mathematicians have not yet been able to deal with themutual attractions of three bodies moving freely in space. If the numberof bodies be greater than three, as is actually the case in the solarsystem, the problem becomes still more hopeless. 

Nature, however, has in this matter dealt kindly with us. She has, it istrue, proposed a problem which cannot be accurately solved; but she hasintroduced into the problem, as proposed in the solar system, certainspecial features which materially reduce the difficulty. We are stillunable to make what a mathematician would describe as a rigoroussolution of the question; we cannot solve it with the completeness of asum in arithmetic; but we can do what is nearly if not quite as useful. We can solve the problem approximately; we can find out what the effectof one planet on the other is _very nearly_, and by additional labour wecan reduce the limits of uncertainty to as low a point as may bedesired. We thus obtain a practical solution of the problem adequate forall the purposes of science. It avails us little to know the place of aplanet with absolute mathematical accuracy. If we can determine what wewant with so close an approximation to the true position that notelescope could possibly disclose the difference, then every practicalend will have been attained. The reason why in this case we are enabledto get round the difficulties which we cannot surmount lies in theexceptional character of the problem of three bodies as exhibited in thesolar system. In the first place, the sun is of such pre-eminent massthat many matters may be overlooked which would be of moment were herivalled in mass by any of the planets. Another source of our successarises from the small inclinations of the planetary orbits to eachother; while the fact that the orbits are nearly circular also greatlyfacilitates the work. The mathematicians who may reside in some of theother parts of the universe are not equally favoured. Among the siderealsystems we find not a few cases where the problem of three bodies, oreven of more than three, would have to be faced without any of thealleviating circumstances which our system presents. In such groups asthe marvellous star Th Orionis, we have three or four bodiescomparable in size, which must produce movements of the utmostcomplexity. Even if terrestrial mathematicians shall ever have thehardihood to face such problems, there is no likelihood of their beingable to do so for ages to come; such researches must repose on accurateobservations as their foundation; and the observations of these distantsystems are at present utterly inadequate for the purpose. 

The undisturbed revolution of a planet around the sun, in conformitywith Kepler's law, would assure for that planet permanent conditions ofclimate. The earth, for instance, if guided solely by Kepler's laws, would return each day of the year exactly to the same position which ithad on the same day of last year. From age to age the quantity of heatreceived by the earth would remain constant if the sun continuedunaltered, and the present climate might thus be preserved indefinitely. But since the existence of planetary perturbation has become recognised, questions arise of the gravest importance with reference to the possibleeffects which such perturbations may have. We now see that the path ofthe earth is not absolutely fixed. That path is deranged by Venus and byMars; it is deranged, it must be deranged, by every planet in oursystem. It is true that in a year, or even in a century, the amount ofalteration produced is not very great; the ellipse which represents thepath of our earth this year does not differ considerably from theellipse which represented the movement of the earth one hundred yearsago. But the important question arises as to whether the slightdifference which does exist may not be constantly increasing, and maynot ultimately assume such proportions as to modify our climates, oreven to render life utterly impossible. Indeed, if we look at thesubject without attentive calculation, nothing would seem more probablethan that such should be the fate of our system. This globe revolves ina path inside that of the mighty Jupiter. It is, therefore, constantlyattracted by Jupiter, and when it overtakes the vast planet, and comesbetween him and the sun, then the two bodies are comparatively closetogether, and the earth is pulled outwards by Jupiter. It might besupposed that the tendency of such disturbances would be to draw theearth gradually away from the sun, and thus to cause our globe todescribe a path ever growing wider and wider. It is not, however, possible to decide a dynamical question by merely superficial reasoningof this character. The question has to be brought before the tribunal ofmathematical analysis, where every element in the case is duly takeninto account. Such an enquiry is by no means a simple one. It worthilyoccupied the splendid talents of Lagrange and Laplace, whosediscoveries in the theory of planetary perturbation are some of the mostremarkable achievements in astronomy. 

We cannot here attempt to describe the reasoning which these greatmathematicians employed. It can only be expressed by the formulæ of themathematician, and would then be hardly intelligible without previousyears of mathematical study. It fortunately happens, however, that theresults to which Lagrange and Laplace were conducted, and which havebeen abundantly confirmed by the labours of other mathematicians, admitof being described in simple language. 

Let us suppose the case of the sun, and of two planets circulatingaround him. These two planets are mutually disturbing each other, butthe amount of the disturbance is small in comparison with the effect ofthe sun on each of them. Lagrange demonstrated that, though the ellipsein which each planet moved was gradually altered in some respects by theattraction of the other planet, yet there is one feature of the curvewhich the perturbation is powerless to alter permanently: the longestaxis of the ellipse, and, therefore, the mean distance of the planetfrom the sun, which is equal to one-half of it, must remain unchanged. This is really a discovery as important as it was unexpected. It at onceremoves all fear as to the effect which perturbations can produce on thestability of the system. It shows that, notwithstanding the attractionsof Mars and of Venus, of Jupiter and of Saturn, our earth will for evercontinue to revolve at the same mean distance from the sun, and thus thesuccession of the seasons and the length of the year, so far as thiselement at least is concerned, will remain for ever unchanged. 

But Lagrange went further into the enquiry. He saw that the meandistance did not alter, but it remained to be seen whether theeccentricity of the ellipse described by the earth might not be affectedby the perturbations. This is a matter of hardly less consequence thanthat just referred to. Even though the earth preserved the same averagedistance from the sun, yet the greatest and least distance might bewidely unequal: the earth might pass very close to the sun at one partof its orbit, and then recede to a very great distance at the oppositepart. So far as the welfare of our globe and its inhabitants isconcerned, this is quite as important as the question of the meandistance; too much heat in one half of the year would afford butindifferent compensation for too little during the other half. Lagrangesubmitted this question also to his analysis. Again he vanquished themathematical difficulties, and again he was able to give assurance ofthe permanence of our system. It is true that he was not this time ableto say that the eccentricity of each path will remain constant; this isnot the case. What he does assert, and what he has abundantly proved, isthat the eccentricity of each orbit will always remain small. We learnthat the shape of the earth's orbit gradually swells and graduallycontracts; the greatest length of the ellipse is invariable, butsometimes it approaches more to a circle, and sometimes becomes moreelliptical. These changes are comprised within narrow limits; so that, though they may probably correspond with measurable climatic changes, yet the safety of the system is not imperilled, as it would be if theeccentricity could increase indefinitely. Once again Lagrange appliedthe resources of his calculus to study the effect which perturbationscan have on the inclination of the path in which the planet moves. Theresult in this case was similar to that obtained with respect to theeccentricities. If we commence with the assumption that the mutualinclinations of the planets are small, then mathematics assure us thatthey must always remain small. We are thus led to the conclusion thatthe planetary perturbations are unable to affect the stability of thesolar system. 

We shall perhaps more fully appreciate the importance of these memorableresearches if we consider how easily matters might have been otherwise. Let us suppose a system resembling ours in every respect save one. Letthat system have a sun, as ours has; a system of planets and ofsatellites like ours. Let the masses of all the bodies in thishypothetical system be identical with the masses in our system, and letthe distances and the periodic times be the same in the two cases. Letall the planes of the orbits be similarly placed; and yet thishypothetical system might contain seeds of decay from which ours isfree. There is one point in the imaginary scheme which we have not yetspecified. In our system all the planets revolve in the _same direction_around the sun. Let us suppose this law violated in the hypotheticalsystem by reversing one planet on its path. That slight change alonewould expose the system to the risk of destruction by the planetaryperturbations. Here, then, we find the necessity of that remarkableuniformity of the directions in which the planets revolve around thesun. Had these directions not been uniform, our system must, in allprobability, have perished ages ago, and we should not be here todiscuss perturbations or any other subject. 

Great as was the success of the eminent French mathematician who madethese beautiful discoveries, it was left for this century to witness thecrowning triumph of mathematical analysis applied to the law ofgravitation. The work of Lagrange lacks the dramatic interest of thediscovery made by Le Verrier and Adams, which gave still wider extent tothe solar system by the discovery of the planet Neptune revolving faroutside Uranus. 

We have already alluded to the difficulties which were experienced whenit was sought to reconcile the early observations of Uranus with thosemade since its discovery. We have shown that the path in which thisplanet revolved experienced change, and that consequently Uranus must beexposed to the action of some other force besides the sun's attraction. 

The question arises as to the nature of these disturbing forces. Fromwhat we have already learned of the mutual deranging influence betweenany two planets, it seems natural to inquire whether the irregularitiesof Uranus could not be accounted for by the attraction of the otherplanets. Uranus revolves just outside Saturn. The mass of Saturn is muchlarger than the mass of Uranus. Could it not be that Saturn draws Uranusaside, and thus causes the changes? This is a question to be decided bythe mathematician. He can compute what Saturn is able to do, and hefinds, no doubt, that Saturn is capable of producing some displacementof Uranus. In a similar manner Jupiter, with his mighty mass, acts onUranus, and produces a disturbance which the mathematician calculates. When the figures had been worked out for all the known planets they wereapplied to Uranus, and we might expect to find that they would fullyaccount for the observed irregularities of his path. This was, however, not the case. After every known source of disturbance had been carefullyallowed for, Uranus was still shown to be influenced by some furtheragent; and hence the conclusion was established that Uranus must beaffected by some unknown body. What could this unknown body be, andwhere must it be situated? Analogy was here the guide of those whospeculated on this matter. We know no cause of disturbance of a planet'smotion except it be the attraction of another planet. Could it be thatUranus was really attracted by some other planet at that time utterlyunknown? This suggestion was made by many astronomers, and it waspossible to determine some conditions which the unknown body shouldfulfil. In the first place its orbit must lie outside the orbit ofUranus. This was necessary, because the unknown planet must be a largeand massive one to produce the observed irregularities. If, therefore, it were nearer than Uranus, it would be a conspicuous object, and musthave been discovered long ago. Other reasonings were also available toshow that if the disturbances of Uranus were caused by the attraction ofa planet, that body must revolve outside the globe discovered byHerschel. The general analogies of the planetary system might also beinvoked in support of the hypothesis that the path of the unknownplanet, though necessarily elliptic, did not differ widely from acircle, and that the plane in which it moved must also be nearlycoincident with the plane of the earth's orbit. 

The measured deviations of Uranus at the different points of its orbitwere the sole data available for the discovery of the new planet. Wehave to fit the orbit of the unknown globe, as well as the mass of theplanet itself, in such a way as to account for the variousperturbations. Let us, for instance, assume a certain distance for thehypothetical body, and try if we can assign both an orbit and a mass forthe planet, at that distance, which shall account for the perturbations. Our first assumption is perhaps too great. We try again with a lesserdistance. We can now represent the observations with greater accuracy. Athird attempt will give the result still more closely, until at lengththe distance of the unknown planet is determined. In a similar way themass of the body can be also determined. We assume a certain value, andcalculate the perturbations. If the results seem greater than thoseobtained by observations, then the assumed mass is too great. We amendthe assumption, and recompute with a lesser amount, and so on until atlength we determine a mass for the planet which harmonises with theresults of actual measurement. The other elements of the unknownorbit--its eccentricity and the position of its axis--are all to beascertained in a similar manner. At length it appeared that theperturbations of Uranus could be completely explained if the unknownplanet had a certain mass, and moved in an orbit which had a certainposition, while it was also manifest that no very different orbit orgreatly altered mass would explain the observed facts. 

These remarkable computations were undertaken quite independently by twoastronomers--one in England and one in France. Each of them attacked, and each of them succeeded in solving, the great problem. The scientificmen of England and the scientific men of France joined issue on thequestion as to the claims of their respective champions to the greatdiscovery; but in the forty years which have elapsed since thesememorable researches the question has gradually become settled. It isthe impartial verdict of the scientific world outside England andFrance, that the merits of this splendid triumph of science must bedivided equally between the late distinguished Professor J. C. Adams, ofCambridge, and the late U. J. J. Le Verrier, the director of the ParisObservatory. 

Shortly after Mr. Adams had taken his degree at Cambridge, in 1843, whenhe obtained the distinction of Senior Wrangler, he turned his attentionto the perturbations of Uranus, and, guided by these perturbationsalone, commenced his search for the unknown planet. Long and arduous wasthe enquiry--demanding an enormous amount of numerical calculation, aswell as consummate mathematical resource; but gradually Mr. Adamsovercame the difficulties. As the subject unfolded itself, he saw howthe perturbations of Uranus could be fully explained by the existence ofan exterior planet, and at length he had ascertained, not alone theorbit of this outer body, but he was even able to indicate the part ofthe heavens in which the unknown globe must be sought. With hisresearches in this advanced condition, Mr. Adams called on theAstronomer-Royal, Sir George Airy, at Greenwich, in October, 1845, andplaced in his hands the computations which indicated with marvellousaccuracy the place of the yet unobserved planet. It thus appears thatseven months before anyone else had solved this problem Mr. Adams hadconquered its difficulties, and had actually located the planet in aposition but little more than a degree distant from the spot which it isnow known to have occupied. All that was wanted to complete thediscovery, and to gain for Professor Adams and for English science theundivided glory of this achievement, was a strict telescopic searchthrough the heavens in the neighbourhood indicated. 

Why, it may be said, was not such an enquiry instituted at once? Nodoubt this would have been done, if the observatories had been generallyfurnished forty years ago with those elaborate star-charts which theynow possess. In the absence of a chart (and none had yet been publishedof the part of the sky where the unknown planet was) the search for theplanet was a most tedious undertaking. It had been suggested that thenew globe could be detected by its visible disc; but it must beremembered that even Uranus, so much closer to us, had a disc so smallthat it was observed nearly a score of times without particular notice, though it did not escape the eagle glance of Herschel. There remainedthen only one available method of finding Neptune. It was to construct achart of the heavens in the neighbourhood indicated, and then to comparethis chart night after night with the stars in the heavens. Beforerecommending the commencement of a labour so onerous, theAstronomer-Royal thought it right to submit Mr. Adams's researches to acrucial preliminary test. Mr. Adams had shown how his theory rendered anexact account of the perturbations of Uranus in longitude. TheAstronomer-Royal asked Mr. Adams whether he was able to give an equallyclear explanation of the notable variations in the distance of Uranus. There can be no doubt that his theory would have rendered a satisfactoryaccount of these variations also; but, unfortunately, Mr. Adams seemsnot to have thought the matter of sufficient importance to give theAstronomer-Royal any speedy reply, and hence it happened that no lessthan nine months elapsed between the time when Mr. Adams firstcommunicated his results to the Astronomer-Royal and the time when thetelescopic search for the planet was systematically commenced. Up tothis time no account of Mr. Adams's researches had been published. Hislabours were known to but few besides the Astronomer-Royal and ProfessorChallis of Cambridge, to whom the duty of making the search wasafterwards entrusted. 

In the meantime the attention of Le Verrier, the great Frenchmathematician and astronomer, had been specially directed by Arago tothe problem of the perturbations of Uranus. With exhaustive analysis LeVerrier investigated every possible known source of disturbance. Theinfluences of the older planets were estimated once more with everyprecision, but only to confirm the conclusion already arrived at as totheir inadequacy to account for the perturbations. Le Verrier thencommenced the search for the unknown planet by the aid of mathematicalinvestigation, in complete ignorance of the labours of Adams. InNovember, 1845, and again on the 1st of June, 1846, portions of theFrench astronomer's results were announced. The Astronomer-Royal thenperceived that his calculations coincided practically with those ofAdams, insomuch that the places assigned to the unknown planet by thetwo astronomers were not more than a degree apart! This was, indeed, aremarkable result. Here was a planet unknown to human sight, yet felt, as it were, by mathematical analysis with a certainty so great that twoastronomers, each in total ignorance of the other's labours, concurredin locating the planet in almost the same spot of the heavens. Theexistence of the new globe was thus raised nearly to a certainty, and itbecame incumbent on practical astronomers to commence the searchforthwith. In June, 1846, the Astronomer-Royal announced to the visitorsof the Greenwich Observatory the close coincidence between thecalculations of Le Verrier and of Adams, and urged that a strictscrutiny of the region indicated should be at once instituted. ProfessorChallis, having the command of the great Northumberland equatorialtelescope at Cambridge, was induced to undertake the work, and on the29th July, 1846, he began his labours. 

The plan of search adopted by Professor Challis was an onerous one. Hefirst took the theoretical place of the planet, as given by Mr. Adams, and after allowing a very large margin for the uncertainties of acalculation so recondite, he marked out a certain region of the heavens, near the ecliptic, in which it might be anticipated that the unknownplanet must be found. He then determined to observe all the stars inthis region and measure their relative positions. When this work wasonce done it was to be repeated a second time. His scheme evencontemplated a third complete set of observations of the stars containedwithin this selected region. There could be no doubt that this processwould determine the planet if it were bright enough to come within thelimits of stellar magnitude which Professor Challis adopted. The globewould be detected by its motion relatively to the stars, when the threeseries of measures came to be compared. The scheme was organised sothoroughly that it must have led to the expected discovery--in fact, itafterwards appeared that Professor Challis did actually observe theplanet more than once, and a subsequent comparison of its positions mustinfallibly have led to the detection of the new globe. 

Le Verrier was steadily maturing his no less elaborate investigations inthe same direction. He felt confident of the existence of the planet, and he went so far as to predict not only the situation of the globe buteven its actual appearance. He thought the planet would be large enough(though still of course only a telescopic object) to be distinguishedfrom the stars by the possession of a disc. These definite predictionsstrengthened the belief that we were on the verge of another greatdiscovery in the solar system, so much so that when Sir John Herscheladdressed the British Association on the 10th of September, 1846, heuttered the following words:--"The past year has given to us the newplanet Astræa--it has done more, it has given us the probable prospectof another. We see it as Columbus saw America from the shores of Spain. Its movements have been felt trembling along the far-reaching line ofour analysis, with a certainty hardly inferior to ocular demonstration. "

The time of the discovery was now rapidly approaching. On the 18th ofSeptember, 1846, Le Verrier wrote to Dr. Galle of the BerlinObservatory, describing the place of the planet indicated by hiscalculations, and asking him to make its telescopic discovery. Therequest thus preferred was similar to that made on behalf of Adams toProfessor Challis. Both at Berlin and at Cambridge the telescopicresearch was to be made in the same region of the heavens. The Berlinastronomers were, however, fortunate in possessing an invaluable aid tothe research which was not at the time in the hands of ProfessorChallis. We have mentioned how the search for a telescopic planet can befacilitated by the use of a carefully-executed chart of the stars. Infact, a mere comparison of the chart with the sky is all that isnecessary. It happened that the preparation of a series of star chartshad been undertaken by the Berlin Academy of Sciences some yearspreviously. On these charts the place of every star, down even to thetenth magnitude, had been faithfully engraved. This work was one of muchutility, but its originators could hardly have anticipated the brilliantdiscovery which would arise from their years of tedious labour. It wasfound convenient to publish such an extensive piece of surveying work byinstalments, and accordingly, as the chart was completed, it issued fromthe press sheet by sheet. It happened that just before the news of LeVerrier's labours reached Berlin the chart of that part of the heavenshad been engraved and printed. 

It was on the 23rd of September that Le Verrier's letter reached Dr. Galle at Berlin. The sky that night was clear, and we can imagine withwhat anxiety Dr. Galle directed his telescope to the heavens. Theinstrument was pointed in accordance with Le Verrier's instructions. Thefield of view showed a multitude of stars, as does every part of theheavens. One of these was really the planet. The new chart was unrolled, and, star by star, the heavens were compared with it. As theidentification of the stars went on, one object after another was foundto lie in the heavens as it was engraved on the chart, and was of courserejected. At length a star of the eighth magnitude--a brilliantobject--was brought into review. The chart was examined, but there wasno star there. This object could not have been in its present place whenthe chart was formed. The object was therefore a wanderer--a planet. Yetit was necessary to be cautious in such a matter. Many possibilities hadto be guarded against. It was, for instance, at least conceivable thatthe object was really a star which, by some mischance, eluded thecareful eye of the astronomer who had constructed the map. It was evenpossible that the star might be one of the large class of variableswhich alternate in brightness, and it might have been too faint to havebeen visible when the chart was made. Or it might be one of the minorplanets moving between Mars and Jupiter. Even if none of theseexplanations would answer, it was still necessary to show that theobject was moving with that particular velocity and in that particulardirection which the theory of Le Verrier indicated. The lapse of asingle day was sufficient to dissipate all doubts. The next night theobject was again observed. It had moved, and when its motion wasmeasured it was found to accord precisely with what Le Verrier hadforetold. Indeed, as if no circumstance in the confirmation should bewanting, the diameter of the planet, as measured by the micrometers atBerlin, proved to be practically coincident with that anticipated by LeVerrier. 

The world speedily rang with the news of this splendid achievement. Instantly the name of Le Verrier rose to a pinnacle hardly surpassed bythat of any astronomer of any age or country. The circumstances of thediscovery were highly dramatic. We picture the great astronomer buriedin profound meditation for many months; his eyes are bent, not on thestars, but on his calculations. No telescope is in his hand; the humanintellect is the instrument he alone uses. With patient labour, guidedby consummate mathematical artifice, he manipulates his columns offigures. He attempts one solution after another. In each he learnssomething to avoid; by each he obtains some light to guide him in hisfuture labours. At length he begins to see harmony in those resultswhere before there was but discord. Gradually the clouds disperse, andhe discerns with a certainty little short of actual vision the planetglittering in the far depths of space. He rises from his desk andinvokes the aid of a practical astronomer; and lo! there is the planetin the indicated spot. The annals of science present no such spectacleas this. It was the most triumphant proof of the law of universalgravitation. The Newtonian theory had indeed long ere this attained animpregnable position; but, as if to place its truth in the mostconspicuous light, this discovery of Neptune was accomplished. 

For a moment it seemed as if the French were to enjoy the undividedhonour of this splendid triumph; nor would it, indeed, have beenunfitting that the nation which gave birth to Lagrange and to Laplace, and which developed the great Newtonian theory by their immortallabours, should have obtained this distinction. Up to the time of thetelescopic discovery of the planet by Dr. Galle at Berlin, no publicannouncement had been made of the labours of Challis in searching forthe planet, nor even of the theoretical researches of Adams on whichthose observations were based. But in the midst of the pæans of triumphwith which the enthusiastic French nation hailed the discovery of LeVerrier, there appeared a letter from Sir John Herschel in the_Athenæum_ for 3rd October, 1846, in which he announced the researchesmade by Adams, and claimed for him a participation in the glory of thediscovery. Subsequent enquiry has shown that this claim was a just one, and it is now universally admitted by all independent authorities. Yetit will easily be imagined that the French _savants_, jealous of thefame of their countryman, could not at first be brought to recognise aclaim so put forward. They were asked to divide the unparalleled honourbetween their own illustrious countryman and a young foreigner of whombut few had ever heard, and who had not even published a line of hiswork, nor had any claim been made on his part until after the work hadbeen completely finished by Le Verrier. The demand made on behalf ofAdams was accordingly refused any acknowledgment in France; and anembittered controversy was the consequence. Point by point the Englishastronomers succeeded in establishing the claim of their countryman. Itwas true that Adams had not published his researches to the world, buthe had communicated them to the Astronomer-Royal, the official head ofthe science in this country. They were also well known to ProfessorChallis, the Professor of Astronomy at Cambridge. Then, too, the work ofAdams was published, and it was found to be quite as thorough and quiteas successful as that of Le Verrier. It was also found that the methodof search adopted by Professor Challis not only must have beeneventually successful, but that it actually was in a sense alreadysuccessful. When the telescopic discovery of the planet had beenachieved, Challis turned naturally to see whether he had observed thenew globe also. It was on the 1st October that he heard of the successof Dr. Galle, and by that time Challis had accumulated observations inconnection with this research of no fewer than 3, 150 stars. Among themhe speedily found that an object observed on the 12th of August was notin the same place on the 30th of July. This was really the planet; andits discovery would thus have been assured had Challis had time tocompare his measurements. In fact, if he had only discussed hisobservations at once, there cannot be much doubt that the entire gloryof the discovery would have been awarded to Adams. He would then havebeen first, no less in the theoretical calculations than in the opticalverification of the planet's existence. It may also be remarked thatChallis narrowly missed making the discovery of Neptune in another way. Le Verrier had pointed out in his paper the possibility of detecting thesought-for globe by its disc. Challis made the attempt, and before theintelligence of the actual discovery at Berlin had reached him he hadmade an examination of the region indicated by Le Verrier. About 300stars passed through the field of view, and among them he selected oneon account of its disc; it afterwards appeared that this was indeed theplanet. 

If the researches of Le Verrier and of Adams had never been undertakenit is certain that the distant Neptune must have been some timediscovered; yet that might have been made in a manner which every truelover of science would now deplore. We hear constantly that new minorplanets are observed, yet no one attaches to such achievements afraction of the consequence belonging to the discovery of Neptune. Thedanger was, that Neptune should have been merely dropped upon by simplesurvey work, just as Uranus was discovered, or just as the hosts ofminor planets are now found. In this case Theoretical Astronomy, thegreat science founded by Newton, would have been deprived of its mostbrilliant illustration. 

Neptune had, in fact, a very narrow escape on at least one previousoccasion of being discovered in a very simple way. This was shown whensufficient observations had been collected to enable the path of theplanet to be calculated. It was then possible to trace back themovements of the planet among the stars and thus to institute a searchin the catalogues of earlier astronomers to see whether they containedany record of Neptune, erroneously noted as a star. Several suchinstances have been discovered. I shall, however, only refer to one, which possesses a singular interest. It was found that the place of theplanet on May 10th, 1795, must have coincided with that of a so-calledstar recorded on that day in the "Histoire Céleste" of Lalande. Byactual examination of the heavens it further appeared that there was nostar in the place indicated by Lalande, so the fact that here wasreally an observation of Neptune was placed quite beyond doubt. Whenreference was made to the original manuscripts of Lalande, a matter ofgreat interest was brought to light. It was there found that he hadobserved the same star (for so he regarded it) both on May 8th and onMay 10th; on each day he had determined its position, and bothobservations are duly recorded. But when he came to prepare hiscatalogue and found that the places on the two occasions were different, he discarded the earlier result, and merely printed the latter. 

Had Lalande possessed a proper confidence in his own observations, animmortal discovery lay in his grasp; had he manfully said, "I was righton the 10th of May and I was right on the 8th of May; I made no mistakeon either occasion, and the object I saw on the 8th must have movedbetween that and the 10th, " then he must without fail have foundNeptune. But had he done so, how lamentable would have been the loss toscience! The discovery of Neptune would then merely have been anaccidental reward to a laborious worker, instead of being one of themost glorious achievements in the loftiest department of human reason. 

Besides this brief sketch of the discovery of Neptune, we have butlittle to tell with regard to this distant planet. If we fail to see inUranus any of those features which make Mars or Venus, Jupiter orSaturn, such attractive telescopic objects, what can we expect to findin Neptune, which is half as far again as Uranus? With a good telescopeand a suitable magnifying power we can indeed see that Neptune has adisc, but no features on that disc can be identified. We areconsequently not in a position to ascertain the period in which Neptunerotates around its axis, though from the general analogy of the systemwe must feel assured that it really does rotate. More successful havebeen the attempts to measure the diameter of Neptune, which is found tobe about 35, 000 miles, or more than four times the diameter of theearth. It would also seem that, like Jupiter and like Saturn, the planetmust be enveloped with a vast cloud-laden atmosphere, for the meandensity of the globe is only about one-fifth that of the earth. Thisgreat globe revolves around the sun at a mean distance of no less than2, 800 millions of miles, which is about thirty times as great as themean distance from the earth to the sun. The journey, thoughaccomplished at the rate of more than three miles a second, is yet solong that Neptune requires almost 165 years to complete one revolution. Since its discovery, some fifty years ago, Neptune has moved throughabout one-third of its path, and even since the date when it was firstcasually seen by Lalande, in 1795, it has only had time to traversethree-fifths of its mighty circuit. 

Neptune, like our earth, is attended by a single satellite; thisdelicate object was discovered by Mr. Lassell with his two-footreflecting telescope shortly after the planet itself became known. Themotion of the satellite of Neptune is nearly circular. Its orbit isinclined at an angle of about 35° to the Ecliptic, and it is speciallynoteworthy that, like the satellites of Uranus, the direction of themotion runs counter to the planetary movements generally. The satelliteperforms its journey around Neptune in a period of a little less thansix days. By observing the motions of this moon we are enabled todetermine the mass of the planet, and thus it appears that the weight ofNeptune is about one nineteen-thousandth part of that of the sun. 

No planets beyond Neptune have been seen, nor is there at present anygood ground for believing in their existence as visual objects. In thechapter on the minor planets I have entered into a discussion of the wayin which these objects are discovered. It is by minute and diligentcomparison of the heavens with elaborate star charts that these bodiesare brought to light. Such enquiries would be equally efficacious insearching for an ultra-Neptunian planet; in fact, we could design nobetter method to seek for such a body, if it existed, than that which isat this moment in constant practice at many observatories. The laboursof those who search for small planets have been abundantly rewarded withdiscoveries now counted by hundreds. Yet it is a noteworthy fact thatall these planets are limited to one region of the solar system. It hassometimes been conjectured that time may disclose perturbations in theorbit of Neptune, and that these perturbations may lead to the discoveryof a planet still more remote, even though that planet be so distant andso faint that it eludes all telescopic research. At present, however, such an enquiry can hardly come within the range of practical astronomy. Its movements have no doubt been studied minutely, but it must describea larger part of its orbit before it would be feasible to conclude, fromthe perturbations of its path, the existence of an unknown and stillmore remote planet. 

We have thus seen that the planetary system is bounded on one side byMercury and on the other by Neptune. The discovery of Mercury was anachievement of prehistoric times. The early astronomer who accomplishedthat feat, when devoid of instrumental assistance and unsupported byaccurate theoretical knowledge, merits our hearty admiration for hisuntutored acuteness and penetration. On the other hand, the discovery ofthe exterior boundary of the planetary system is worthy of specialattention from the fact that it was founded solely on profoundtheoretical learning. 

Though we here close our account of the planets and their satellites, wehave still two chapters to add before we shall have completed what is tobe said with regard to the solar system. A further and notable class ofbodies, neither planets nor satellites, own allegiance to the sun, andrevolve round him in conformity with the laws of universal gravitation. These bodies are the comets, and their somewhat more humble associates, the shooting stars. We find in the study of these objects many mattersof interest, which we shall discuss in the ensuing chapters. 

CHAPTER XVI. 

COMETS. 

 Comets contrasted with Planets in Nature as well as in their Movements--Coggia's Comet--Periodic Returns--The Law of Gravitation--Parabolic and Elliptic Orbits--Theory in Advance of Observations--Most Cometary Orbits are sensibly Parabolic--The Labours of Halley--The Comet of 1682--Halley's Memorable Prediction--The Retardation produced by Disturbance--Successive Returns of Halley's Comet--Encke's Comet--Effect of Perturbations--Orbit of Encke's Comet--Attraction of Mercury and of Jupiter--How the Identity of the Comet is secured--How to weigh Mercury--Distance from the Earth to the Sun found by Encke's Comet--The Disturbing Medium--Remarkable Comets--Spectrum of a Comet--Passage of a Comet between the Earth and the Stars--Can the Comet be weighed?--Evidence of the Small Mass of the Comet derived from the Theory of Perturbation--The Tail of the Comet--Its Changes--Views as to its Nature--Carbon present in Comets--Origin of Periodic Comets. 

In our previous chapters, which treated of the sun and the moon, theplanets and their satellites, we found in all cases that the celestialbodies with which we were concerned were nearly globular in form, andmany are undoubtedly of solid substance. All these objects possess adensity which, even if in some cases it be much less than that of theearth, is still hundreds of times greater than the density of merelygaseous materials. We now, however, approach the consideration of aclass of objects of a widely different character. We have no longer todeal with globular objects possessing considerable mass. Comets are ofaltogether irregular shape; they are in large part, at all events, formed of materials in the utmost state of tenuity, and their masses areso small that no means we possess have enabled them to be measured. Notonly are comets different in constitution from planets or from the othermore solid bodies of our system, but the movements of such bodies arequite distinct from the orderly return of the planets at their appointedseasons. The comets appear sometimes with almost startlingunexpectedness; they rapidly swell in size to an extent that insuperstitious ages called forth the utmost terror; presently theydisappear, in many cases never again to return. Modern science has, nodoubt, removed a great deal of the mystery which once invested the wholesubject of comets. Their movements are now to a large extent explained, and some additions have been made to our knowledge of their nature, though we must still confess that what we do know bears but a very smallproportion to what remains unknown. 

Let me first describe in general terms the nature of a comet, in so faras its structure is disclosed by the aid of a powerful refractingtelescope. We represent in Plate XII. Two interesting sketches made atHarvard College Observatory of the great comet of 1874, distinguished bythe name of its discoverer Coggia. 

We see here the head of the comet, containing as its brightest spot whatis called the nucleus, and in which the material of the comet seems tobe much denser than elsewhere. Surrounding the nucleus we find certaindefinite layers of luminous material, the coma, or head, from 20, 000 to1, 000, 000 miles in diameter, from which the tail seems to stream away. This view may be regarded as that of a typical object of this class, butthe varieties of structure presented by different comets are almostinnumerable. In some cases we find the nucleus absent; in other cases wefind the tail to be wanting. The tail is, no doubt, a conspicuousfeature in those great comets which receive universal attention; but inthe small telescopic objects, of which a few are generally found everyyear, this feature is usually absent. Not only do comets present greatvarieties in appearance, but even the aspect of a single objectundergoes great change. The comet will sometimes increase enormously inbulk; sometimes it will diminish; sometimes it will have a large tail, or sometimes no tail at all. Measurements of a comet's size are almostfutile; they may cease to be true even during the few hours in which acomet is observed in the course of a night. It is, in fact, impossibleto identify a comet by any description of its personal appearance. Yetthe question as to identity of a comet is often of very greatconsequence. We must provide means by which it can be established, entirely apart from what the comet may look like. 

It is now well known that several of these bodies make periodic returns. After having been invisible for a certain number of years, a comet comesinto view, and again retreats into space to perform another revolution. The question then arises as to how we are to recognise the body when itdoes come back? The personal features of its size or brightness, thepresence or absence of a tail, large or small, are fleeting charactersof no value for such a purpose. Fortunately, however, the law ofelliptic motion established by Kepler has suggested the means ofdefining the identity of a comet with absolute precision. 

After Newton had made his discovery of the law of gravitation, andsucceeded in demonstrating that the elliptic paths of the planets aroundthe sun were necessary consequences of that law, he was naturallytempted to apply the same reasoning to explain the movements of comets. Here, again, he met with marvellous success, and illustrated his theoryby completely explaining the movements of the remarkable body which wasvisible from December, 1680, to March, 1681. 

[Illustration: Fig. 69. --The Parabolic Path of a Comet. ]

There is a certain beautiful curve known to geometricians by the name ofthe parabola. Its form is shown in the adjoining figure; it is a curvedline which bends in towards and around a certain point known as thefocus. This would not be the occasion for any allusion to thegeometrical properties of this curve; they should be sought in works onmathematics. It will here be only necessary to point to the connectionwhich exists between the parabola and the ellipse. In a former chapterwe have explained the construction of the latter curve, and we haveshown how it possesses two foci. Let us suppose that a series ofellipses are drawn, each of which has a greater distance between itsfoci than the preceding one. Imagine the process carried on until atlength the distance between the foci became enormously great incomparison with the distance from each focus to the curve, then each endof this long ellipse will practically have the same form as a parabola. We may thus look on the latter curve represented in Fig. 69 as being oneend of an ellipse of which the other end is at an indefinitely greatdistance. In 1681 Doerfel, a clergyman of Saxony, proved that the greatcomet then recently observed moved in a parabola, in the focus of whichthe sun was situated. Newton showed that the law of gravitation wouldpermit a body to move in an ellipse of this very extreme type no lessthan in one of the more ordinary proportions. An object revolving in aparabolic orbit about the sun at the focus moves in gradually towardsthe sun, sweeps around the great luminary, and then begins to retreat. There is a necessary distinction between parabolic and elliptic motion. In the latter case the body, after its retreat to a certain distance, will turn round and again draw in towards the sun; in fact, it must makeperiodic circuits of its orbit, as the planets are found to do. But inthe case of the true parabola the body can never return; to do so itwould have to double the distant focus, and as that is infinitelyremote, it could not be reached except in the lapse of infinite time. 

The characteristic feature of the movement in a parabola may be thusdescribed. The body draws in gradually towards the focus from anindefinitely remote distance on one side, and after passing round thefocus gradually recedes to an indefinitely remote distance on the otherside, never again to return. When Newton had perceived that parabolicmotion of this type could arise from the law of gravitation, it at onceoccurred to him (independently of Doerfel's discovery, of which he wasnot aware) that by its means the movements of a comet might beexplained. He knew that comets must be attracted by the sun; he saw thatthe usual course of a comet was to appear suddenly, to sweep around thesun and then retreat, never again to return. Was this really a case ofparabolic motion? Fortunately, the materials for the trial of thisimportant suggestion were ready to his hand. He was able to availhimself of the known movements of the comet of 1680, and of observationsof several other bodies of the same nature which had been collected bythe diligence of astronomers. With his usual sagacity, Newton devised amethod by which, from the known facts, the path which the comet pursuescould be determined. He found that it was a parabola, and that thevelocity of the comet was governed by the law that the straight linefrom the sun to the comet swept over equal areas in equal times. Herewas another confirmation of the law of universal gravitation. In thiscase, indeed, the theory may be said to have been actually in advance ofcalculation. Kepler had determined from observation that the paths ofthe planets were ellipses, and Newton had shown how this fact was aconsequence of the law of gravitation. But in the case of the cometstheir highly erratic orbits had never been reduced to geometrical formuntil the theory of Newton showed him that they were parabolic, and thenhe invoked observation to verify the anticipations of his theory. 

[Illustration: PLATE XII. 

COGGIA'S COMET. 

(AS SEEN ON JUNE 10TH AND JULY 9TH, 1874. )]

The great majority of comets move in orbits which cannot be sensiblydiscriminated from parabolæ, and any body whose orbit is of thischaracter can only be seen at a single apparition. The theory ofgravitation, though it admits the parabola as a possible orbit for acomet, does not assert that the path must necessarily be of this type. We have pointed out that this curve is only a very extreme type ofellipse, and it would still be in perfect accordance with the law ofgravitation for a comet to pursue a path of any elliptical form, provided that the sun was placed at the focus, and that the comet obeyedthe rule of describing equal areas in equal times. If a body move in anelliptic path, then it will return to the sun again, and consequently weshall have periodical visits from the same object. 

An interesting field of enquiry was here presented to the astronomer. Nor was it long before the discovery of a periodic comet was made whichillustrated, in a striking manner, the soundness of the anticipationjust expressed. The name of the celebrated astronomer Halley is, perhaps, best known from its association with the great comet whoseperiodicity was discovered by his calculations. When Halley learned fromthe Newtonian theory the possibility that a comet might move in anelliptic orbit, he undertook a most laborious investigation; hecollected from various records of observed comets all the reliableparticulars that could be obtained, and thus he was enabled toascertain, with tolerable accuracy, the nature of the paths pursued byabout twenty-four large comets. One of these was the great body of 1682, which Halley himself observed, and whose path he computed in accordancewith the principles of Newton. Halley then proceeded to investigatewhether this comet of 1682 could have visited our system at any previousepoch. To answer this question he turned to the list of recorded cometswhich he had so carefully compiled, and he found that his comet veryclosely resembled, both in appearance and in orbit, a comet observed in1607, and also another observed in 1531. Could these three bodies beidentical? It was only necessary to suppose that a comet, instead ofrevolving in a parabolic orbit, really revolved in an extremelyelongated ellipse, and that it completed each revolution in a period ofabout seventy-five or seventy-six years. He submitted this hypothesis toevery test that he could devise; he found that the orbits, determined oneach of the three occasions, were so nearly identical that it would becontrary to all probability that the coincidence should be accidental. Accordingly, he decided to submit his theory to the most supreme testknown to astronomy. He ventured to make a prediction which posteritywould have the opportunity of verifying. If the period of the comet wereseventy-five or seventy-six years, as the former observations seemed toshow, then Halley estimated that, if unmolested, it ought to return in1757 or 1758. There were, however, certain sources of disturbance whichhe pointed out, and which would be quite powerful enough to affectmaterially the time of return. The comet in its journey passes near thepath of Jupiter, and experiences great perturbations from that mightyplanet. Halley concluded that the expected return might be accordinglydelayed till the end of 1758 or the beginning of 1759. 

This prediction was a memorable event in the history of astronomy, inasmuch as it was the first attempt to foretell the apparition of oneof those mysterious bodies whose visits seemed guided by no fixed law, and which were usually regarded as omens of awful import. Halley feltthe importance of his announcement. He knew that his earthly coursewould have run long before the comet had completed its revolution; and, in language almost touching, the great astronomer writes: "Wherefore ifit should return according to our prediction about the year 1758, impartial posterity will not refuse to acknowledge that this was firstdiscovered by an Englishman. "

As the time drew near when this great event was expected, it awakenedthe liveliest interest among astronomers. The distinguishedmathematician Clairaut undertook to compute anew, by the aid of improvedmethods, the effect which would be wrought on the comet by theattraction of the planets. His analysis of the perturbations wassufficient to show that the object would be kept back for 100 days bySaturn, and for 518 days by Jupiter. He therefore gave some additionalexactness to the prediction of Halley, and finally concluded that thiscomet would reach the perihelion, or the point of its path nearest tothe sun, about the middle of April, 1759. The sagacious astronomer (who, we must remember, lived long before the discovery of Uranus and ofNeptune) further adds that as this body retreats so far, it may possiblybe subject to influences of which we do not know, or to the disturbanceeven of some planet too remote to be ever perceived. He, accordingly, qualified his prediction with the statement that, owing to these unknownpossibilities, his calculations might be a month wrong one way or theother. Clairaut made this memorable communication to the Academy ofSciences on the 14th of November, 1758. The attention of astronomers wasimmediately quickened to see whether the visitor, who last appearedseventy-six years previously, was about to return. Night after night theheavens were scanned. On Christmas Day in 1758 the comet was firstdetected, and it passed closest to the sun about midnight on the 12th ofMarch, just a month earlier than the time announced by Clairaut, butstill within the limits of error which he had assigned as beingpossible. 

The verification of this prediction was a further confirmation of thetheory of gravitation. Since then, Halley's comet has returned onceagain, in 1835, in circumstances somewhat similar to those justnarrated. Further historical research has also succeeded in identifyingHalley's comet with numerous memorable apparitions of comets in formertimes. It has even been shown that a splendid object, which appearedeleven years before the commencement of the Christian era, was merelyHalley's comet in one of its former returns. Among the most celebratedvisits of this body was that of 1066, when the apparition attracteduniversal attention. A picture of the comet on this occasion forms aquaint feature in the Bayeux Tapestry. The next return of Halley's cometis expected about the year 1910. 

There are now several comets known which revolve in elliptic paths, andare, accordingly, entitled to be termed periodic. These objects arechiefly telescopic, and are thus in strong contrast to the splendidcomet of Halley. Most of the other periodic comets have periods muchshorter than that of Halley. Of these objects, by far the mostcelebrated is that known as Encke's comet, which merits our carefulattention. 

The object to which we refer has had a striking career during which ithas provided many illustrations of the law of gravitation. We are nothere concerned with the prosaic routine of a mere planetary orbit. Aplanet is mainly subordinated to the compelling sway of the sun'sgravitation. It is also to some slight extent affected by theattractions which it experiences from the other planets. Mathematicianshave long been accustomed to anticipate the movements of these globes byactual calculation. They know how the place of the planet isapproximately decided by the sun's attraction, and they can discriminatethe different adjustments which that place is to receive in consequenceof the disturbances produced by the other planets. The capabilities ofthe planets for producing disturbance are greatly increased when thedisturbed body follows the eccentric path of a comet. It is frequentlyfound that the path of such a body comes very near the track of aplanet, so that the comet may actually sweep by the planet itself, evenif the two bodies do not actually run into collision. On such anoccasion the disturbing effect is enormously augmented, and we thereforeturn to the comets when we desire to illustrate the theory of planetaryperturbations by some striking example. 

Having decided to choose a comet, the next question is, _What_ comet?There cannot here be much room for hesitation. Those splendid cometswhich appear so capriciously may be at once excluded. They are visitorsapparently coming for the first time, and retreating without anydistinct promise that mankind shall ever see them again. A comet of thiskind moves in a parabolic path, sweeps once around the sun, and thenceretreats into the space whence it came. We cannot study the effect ofperturbations on a comet completely until it has been watched duringsuccessive returns to the sun. Our choice is thus limited to thecomparatively small class of objects known as periodic comets; and, froma survey of the entire group, we select the most suitable to ourpurpose. It is the object generally known as Encke's comet, for, thoughEncke was not the discoverer, yet it is to his calculations that thecomet owes its fame. This body is rendered more suitable for our purposeby the researches to which it has recently given rise. 

In the year 1818 a comet was discovered by the painstaking astronomerPons at Marseilles. We are not to imagine that this body produced asplendid spectacle. It was a small telescopic object, not unlike one ofthose dim nebulæ which are scattered in thousands over the heavens. Thecomet is, however, readily distinguished from a nebula by its movementrelatively to the stars, while the nebula remains at rest for centuries. The position of this comet was ascertained by its discoverer, as well asby other astronomers. Encke found from the observations that the cometreturned to the sun once in every three years and a few months. This wasa startling announcement. At that time no other comet of short periodhad been detected, so that this new addition to the solar systemawakened the liveliest interest. The question was immediately raised asto whether this comet, which revolved so frequently, might not have beenobserved during previous returns. The historical records of theapparitions of comets are counted by hundreds, and how among this hostare we to select those objects which were identical with the cometdiscovered by Pons?

[Illustration: Fig. 70. --The Orbit of Encke's Comet. ]

We may at once relinquish any hope of identification from drawings ofthe object, but, fortunately, there is one feature of a comet on whichwe can seize, and which no fluctuations of the actual structure canmodify or disguise. The path in which the body travels through space isindependent of the bodily changes in its structure. The shape of thatpath and its position depend entirely upon those other bodies of thesolar system which are specially involved in the theory of Encke'scomet. In Fig. 70 we show the orbits of three of the planets. They havebeen chosen with such proportions as shall make the innermost representthe orbit of Mercury; the next is the orbit of the earth, while theoutermost is the orbit of Jupiter. Besides these three we perceive inthe figure a much more elliptical path, representing the orbit ofEncke's comet, projected down on the plane of the earth's motion. Thesun is situated at the focus of the ellipse. The comet is constrained torevolve in this curve by the attraction of the sun, and it requires alittle more than three years to accomplish a complete revolution. Itpasses close to the sun at perihelion, at a point inside the path ofMercury, while at its greatest distance it approaches the path ofJupiter. This elliptic orbit is mainly determined by the attraction ofthe sun. Whether the comet weighed an ounce, a ton, a thousand tons, ora million tons, whether it was a few miles, or many thousands of milesin diameter, the orbit would still be the same. It is by the shape ofthis ellipse, by its actual size, and by the position in which it lies, that we identify the comet. It had been observed in 1786, 1795, and1805, but on these occasions it had not been noticed that the comet'spath deviated from the parabola. 

Encke's comet is usually so faint that even the most powerful telescopein the world would not show a trace of it. After one of its periodicalvisits, the body withdraws until it recedes to the outermost part of itspath, then it will turn, and again approach the sun. It would seem thatit becomes invigorated by the sun's rays, and commences to dilate undertheir genial influence. While moving in this part of its path the cometlessens its distance from the earth. It daily increases in splendour, until at length, partly by the intrinsic increase in brightness andpartly by the decrease in distance from the earth, it comes within therange of our telescopes. We can generally anticipate when this willoccur, and we can tell to what point of the heavens the telescope is tobe pointed so as to discern the comet at its next return to perihelion. The comet cannot elude the grasp of the mathematician. He can tell whenand where the comet is to be found, but no one can say what it will belike. 

Were all the other bodies of the system removed, then the path ofEncke's comet must be for ever performed in the same ellipse and withabsolute regularity. The chief interest for our present purpose lies notin the regularity of its path, but in the _irregularities_ introducedinto that path by the presence of the other bodies of the solar system. Let us, for instance, follow the progress of the comet through itsperihelion passage, in which the track lies near that of the planetMercury. It will usually happen that Mercury is situated in a distantpart of its path at the moment the comet is passing, and the influenceof the planet will then be comparatively small. It may, however, sometimes happen that the planet and the comet come close together. Oneof the most interesting instances of a close approach to Mercury tookplace on the 22nd November, 1848. On that day the comet and the planetwere only separated by an interval of about one-thirtieth of the earth'sdistance from the sun, _i. E. _ about 3, 000, 000 miles. On several otheroccasions the distance between Encke's comet and Mercury has been lessthan 10, 000, 000 miles--an amount of trifling import in comparison withthe dimensions of our system. Approaches so close as this are fraughtwith serious consequences to the movements of the comet. Mercury, thougha small body, is still sufficiently massive. It always attracts thecomet, but the efficacy of that attraction is enormously enhanced whenthe comet in its wanderings comes near the planet. The effect of thisattraction is to force the comet to swerve from its path, and to impresscertain changes upon its velocity. As the comet recedes, the disturbinginfluence of Mercury rapidly abates, and ere long becomes insensible. But time cannot efface from the orbit of the comet the effect which thedisturbance of Mercury has actually accomplished. The disturbed orbit isdifferent from the undisturbed ellipse which the comet would haveoccupied had the influence of the sun alone determined its shape. We areable to calculate the movements of the comet as determined by the sun. We can also calculate the effects arising from the disturbance producedby Mercury, provided we know the mass of the latter. 

Though Mercury is one of the smallest of the planets, it is perhaps themost troublesome to the astronomer. It lies so close to the sun that itis seen but seldom in comparison with the other great planets. Its orbitis very eccentric, and it experiences disturbances by the attraction ofother bodies in a way not yet fully understood. A special difficulty hasalso been found in the attempt to place Mercury in the weighing scales. We can weigh the whole earth, we can weigh the sun, the moon, and evenJupiter and other planets, but Mercury presents difficulties of apeculiar character. Le Verrier, however, succeeded in devising a methodof weighing it. He demonstrated that our earth is attracted by thisplanet, and he showed how the amount of attraction may be disclosed byobservations of the sun, so that, from an examination of theobservations, he made an approximate determination of the mass ofMercury. Le Verrier's result indicated that the weight of the planet wasabout the fourteenth part of the weight of the earth. In other words, ifour earth was placed in a balance, and fourteen globes, each equal toMercury, were laid in the other, the scales would hang evenly. It wasnecessary that this result should be received with great caution. Itdepended upon a delicate interpretation of somewhat precariousmeasurements. It could only be regarded as of provisional value, to bediscarded when a better one should be obtained. 

The approach of Encke's comet to Mercury, and the elaborateinvestigations of Von Asten and Backlund, in which the observations ofthe body were discussed, have thrown much light on the subject; but, owing to a peculiarity in the motion of this comet, which we shallpresently mention, the difficulties of this investigation are enormous. Backlund's latest result is, that the sun is 9, 700, 000 times as heavy asMercury, and he considers that this is worthy of great confidence. Thereis a considerable difference between this result (which makes the earthabout thirty times as heavy as Mercury) and that of Le Verrier; and, onthe other hand, Haerdtl has, from the motion of Winnecke's periodiccomet, found a value of the mass of Mercury which is not very differentfrom Le Verrier's. Mercury is, however, the only planet about the massof which there is any serious uncertainty, and this must not make usdoubt the accuracy of this delicate weighing-machine. Look at the orbitof Jupiter, to which Encke's comet approaches so nearly when it retreatsfrom the sun. It will sometimes happen that Jupiter and the comet are inclose proximity, and then the mighty planet seriously disturbs thepliable orbit of the comet. The path of the latter bears unmistakabletraces of the Jupiter perturbations, as well as of the Mercuryperturbations. It might seem a hopeless task to discriminate between theinfluences of the two planets, overshadowed as they both are by thesupreme control of the sun, but contrivances of mathematical analysisare adequate to deal with the problem. They point out how much is due toMercury, how much is due to Jupiter; and the wanderings of Encke's cometcan thus be made to disclose the mass of Jupiter as well as that ofMercury. Here we have a means of testing the precision of our weighingappliances. The mass of Jupiter can be measured by his moons, in the waymentioned in a previous chapter. As the satellites revolve round andround the planet, they furnish a method of measuring his weight by therapidity of their motion. They tell us that if the sun were placed inone scale of the celestial balance, it would take 1, 047 bodies equal toJupiter in the other to weigh him down. Hardly a trace of uncertaintyclings to this determination, and it is therefore of great interest totest the theory of Encke's comet by seeing whether it gives an accordantresult. The comparison has been made by Von Asten. Encke's comet tellsus that the sun is 1, 050 times as heavy as Jupiter; so the results arepractically identical, and the accuracy of the indications of the cometare confirmed. But the calculation of the perturbations of Encke's cometis so extremely intricate that Asten's result is not of great value. From the motion of Winnecke's periodic comet, Haerdtl has found that thesun is 1, 047·17 times as heavy as Jupiter, in perfect accordance withthe best results derived from the attraction of Jupiter on hissatellites and the other planets. 

We have hitherto discussed the adventures of Encke's comet in caseswhere they throw light on questions otherwise more or less known to us. We now approach a celebrated problem, on which Encke's comet is our onlyauthority. Every 1, 210 days that comet revolves completely around itsorbit, and returns again to the neighbourhood of the sun. The movementsof the comet are, however, somewhat irregular. We have already explainedhow perturbations arise from Mercury and from Jupiter. Furtherdisturbances arise from the attraction of the earth and of the otherremaining planets; but all these can be allowed for, and then we areentitled to expect, if the law of gravitation be universally true, thatthe comet shall obey the calculations of mathematics. Encke's comet hasnot justified this anticipation; at each revolution the period isgetting steadily shorter! Each time the comet comes back to perihelionin two and a half hours less than on the former occasion. Two and a halfhours is, no doubt, a small period in comparison with that of an entirerevolution; but in the region of its path visible to us the comet ismoving so quickly that its motion in two and a half hours isconsiderable. This irregularity cannot be overlooked, inasmuch as it hasbeen confirmed by the returns during about twenty revolutions. It hassometimes been thought that the discrepancies might be attributed tosome planetary perturbations omitted or not fully accounted for. Themasterly analysis of Von Asten and Backlund has, however, disposed ofthis explanation. They have minutely studied the observations down to1891, but only to confirm the reality of this diminution in the periodictime of Encke's comet. 

An explanation of these irregularities was suggested by Encke long ago. Let us briefly attempt to describe this memorable hypothesis. When wesay that a body will move in an elliptic path around the sun in virtueof gravitation, it is always assumed that the body has a free coursethrough space. It is assumed that there is no friction, no air, or othersource of disturbance. But suppose that this assumption should beincorrect; suppose that there really is some medium pervading spacewhich offers resistance to the comet in the same way as the air impedesthe flight of a rifle bullet, what effect ought such a medium toproduce? This is the idea which Encke put forward. Even if the greaterpart of space be utterly void, so that the path of the filmy and almostspiritual comet is incapable of feeling resistance, yet in theneighbourhood of the sun it was supposed that there might be some mediumof excessive tenuity capable of affecting so light a body. It can bedemonstrated that a resisting medium such as we have supposed wouldlessen the size of the comet's path, and diminish the periodic time. This hypothesis has, however, now been abandoned. It has always appearedstrange that no other comet showed the least sign of being retarded bythe assumed resisting medium. But the labours of Backlund have nowproved beyond a doubt that the acceleration of the motion of Encke'scomet is not a constant one, and cannot be accounted for by assuming aresisting medium distributed round the sun, no matter how we imaginethis medium to be constituted with regard to density at differentdistances from the sun. Backlund found that the acceleration was fairlyconstant from 1819 to 1858; it commenced to decrease between 1858 and1862, and continued to diminish till some time between 1868 and 1871, since which time it has remained fairly constant. He considers that theacceleration can only be produced by the comet encountering periodicallya swarm of meteors, and if we could only observe the comet during itsmotion through the greater part of its orbit we should be able to pointout the locality where this encounter takes place. 

We have selected the comets of Halley and of Encke as illustrations ofthe class of periodic comets, of which, indeed, they are the mostremarkable members. Another very remarkable periodic comet is that ofBiela, of which we shall have more to say in the next chapter. Of themuch more numerous class of non-periodic comets, examples in abundancemay be cited. We shall mention a few which have appeared during thepresent century. There is first the splendid comet of 1843, whichappeared suddenly in February of that year, and was so brilliant that itcould be seen during full daylight. This comet followed a path whichcould not be certainly distinguished from a parabola, though there is nodoubt that it might have been a very elongated ellipse. It is frequentlyimpossible to decide a question of this kind, during the briefopportunities available for finding the place of the comet. We can onlysee the object during a very small arc of its orbit, and even then it isnot a very well-defined point which admits of being measured with theprecision attainable in observations of a star or a planet. This cometof 1843 is, however, especially remarkable for the rapidity with whichit moved, and for the close approach which it made to the sun. The heatto which it was exposed during its passage around the sun must havebeen enormously greater than the heat which can be raised in ourmightiest furnaces. If the materials had been agate or cornelian, or themost infusible substances known on the earth, they would have been fusedand driven into vapour by the intensity of the sun's rays. 

The great comet of 1858 was one of the celestial spectacles of moderntimes. It was first observed on June 2nd of that year by Donati, whosename the comet has subsequently borne; it was then merely a faintnebulous spot, and for about three months it pursued its way across theheavens without giving any indications of the splendour which it was sosoon to attain. The comet had hardly become visible to the unaided eyeat the end of August, and was then furnished with only a very smalltail, but as it gradually drew nearer and nearer to the sun inSeptember, it soon became invested with splendour. A tail of majesticproportions was quickly developed, and by the middle of October, whenthe maximum brightness was attained, its length extended over an arc offorty degrees. The beauty and interest of this comet were greatlyenhanced by its favourable position in the sky at a season when thenights were sufficiently dark. 

On the 22nd May, 1881, Mr. Tebbutt, of Windsor, in New South Wales, discovered a comet which speedily developed into one of the mostinteresting celestial objects seen by this generation. About the 22nd ofJune it became visible from these latitudes in the northern sky atmidnight. Gradually it ascended higher and higher until it passed aroundthe pole. The nucleus of the comet was as bright as a star of the firstmagnitude, and its tail was about 20° long. On the 2nd of September itceased to be visible to the unaided eye, but remained visible intelescopes until the following February. This was the first comet whichwas successfully photographed, and it may be remarked that cometspossess very little actinic power. It has been estimated that moonlightpossesses an intensity 300, 000 times greater than that of a comet wherethe purposes of photography are concerned. 

Another of the bodies of this class which have received great anddeserved attention was that discovered in the southern hemisphere earlyin September, 1882. It increased so much in brilliancy that it was seenin daylight by Mr. Common on the 17th of that month, while on the sameday the astronomers at the Cape of Good Hope were fortunate enough tohave observed the body actually approach the sun's limb, where it ceasedto be visible. We know that the comet must have passed between the earthand the sun, and it is very interesting to learn from the Cape observersthat it was totally invisible when it was actually projected on thesun's disc. The following day it was again visible to the naked eye infull daylight, not far from the sun, and valuable spectroscopicobservations were secured at Dunecht and Palermo. At that time the cometwas rushing through the part of its orbit closest to the sun, and abouta week later it began to be visible in the morning before sunrise, nearthe eastern horizon, exhibiting a fine long tail. (_See_ Plate XVII. )The nucleus gradually lengthened until it broke into four separatepieces, lying in a straight line, while the comet's head becameenveloped in a sort of faint, nebulous tube, pointing towards the sun. Several small detached nebulous masses became also visible, whichtravelled along with the comet, though not with the same velocity. Thecomet became invisible to the naked eye in February, and was lastobserved telescopically in South America on the 1st June, 1883. 

There is a remarkable resemblance between the orbit of this comet andthe orbits in which the comet of 1668, the great comet of 1843, and agreat comet seen in 1880 in the southern hemisphere, travelled round thesun. In fact, these four comets moved along very nearly the same trackand rushed round the sun within a couple of hundred thousand miles ofthe surface of the photosphere. It is also possible that the cometwhich, according to Aristotle, appeared in the year 372 B. C. Followedthe same orbit. And yet we cannot suppose that all these wereapparitions of one and the same comet, as the observations of the cometof 1882 give the period of revolution of that body equal to about 772years. It is not impossible that the comets of 1843 and 1880 are one andthe same, but in both years the observations extend over too short atime to enable us to decide whether the orbit was a parabola or anellipse. But as the comet of 1882 was in any case a distinct body, itseems more likely that we have here a family of comets approaching thesun from the same region of space and pursuing almost the same course. We know a few other instances of such resemblances between the orbits ofdistinct comets. 

Of other interesting comets seen within the last few years we maymention one discovered by Mr. Holmes in London on the 6th November, 1892. It was then situated not far from the bright nebula in theconstellation Andromeda, and like it was just visible to the naked eye. The comet became gradually fainter and more diffused, but on the 16thJanuary following it appeared suddenly with a central condensation, likea star of the eighth magnitude, surrounded by a small coma. Gradually itexpanded again, and grew fainter, until it was last observed on the 6thApril. [32] The orbit was found to be an ellipse more nearly circularthan the orbit of any other known comet, the period being nearly sevenyears. Another comet of 1892 is remarkable as having been discovered byProfessor Barnard, of the Lick Observatory, on a photograph of a regionin Aquila; he was at once able to distinguish the comet from a nebula byits motion. 

Since 1864 the light of every comet which has made its appearance hasbeen analysed by the spectroscope. The slight surface-brightness ofthese bodies renders it necessary to open the slit of the spectroscoperather wide, and the dispersion employed cannot be very great, whichagain makes accurate measurements difficult. The spectrum of a comet ischiefly characterised by three bright bands shading gradually offtowards the violet, and sharply defined on the side towards the red. This appearance is caused by a large number of fine and close lines, whose intensity and distance apart decrease towards the violet. Thesethree bands reveal the existence of hydrocarbon in comets. 

The important _rôle_ which we thus find carbon playing in theconstitution of comets is especially striking when we reflect on thesignificance of the same element on the earth. We see it as the chiefconstituent of all vegetable life, we find it to be invariably presentin animal life. It is an interesting fact that this element, of suchtranscendent importance on the earth, should now have been proved to bepresent in these wandering bodies. The hydrocarbon bands are, however, not always the only features visible in cometary spectra. In a cometseen in the spring months of 1882, Professor Copeland discovered that anew bright yellow line, coinciding in position with the D-line ofsodium, had suddenly appeared, and it was subsequently, both by him andby other observers, seen beautifully double. In fact, sodium was sostrongly represented in this comet, that both the head and the tailcould be perfectly well seen in sodium light by merely opening the slitof the spectroscope very wide, just as a solar prominence may be seen inhydrogen light. The sodium line attained its greatest brilliance at thetime when the comet was nearest to the sun, while the hydrocarbon bandswere either invisible or very faint. The same connection between theintensity of the sodium line and the distance from the sun was noticedin the great September comet of 1882. 

The spectrum of the great comet of 1882 was observed by Copeland andLohse on the 18th September in daylight, and, in addition to the sodiumline, they saw a number of other bright lines, which seemed to be due toiron vapour, while the only line of manganese visible at the temperatureof a Bunsen burner was also seen. This very remarkable observation wasmade less than a day after the perihelion passage, and illustrates thewonderful activity in the interior of a comet when very close to thesun. 

[Illustration: PLATE XVII. 

THE COMET OF 1882, AS SEEN FROM STREATHAM, NOV. 4TH, 4 A. M. 

FROM A DRAWING BY T. E. KEY. ]

In addition to the bright lines comets generally show a faintcontinuous spectrum, in which dark Fraunhofer lines can occasionally bedistinguished. Of course, this shows that the continuous spectrum is toa great extent due to reflected sunlight, but there is no doubt thatpart of it is often due to light actually developed in the comets. Thiswas certainly the case in the first comet of 1884, as a sudden outburstof light in this body was accompanied by a considerable increase ofbrightness of the continuous spectrum. A change in the relativebrightness of the three hydrocarbon bands indicated a considerable riseof temperature, during the continuance of which the comet emitted whitelight. 

As comets are much nearer to the earth than the stars, it willoccasionally happen that the comet must arrive at a position directlybetween the earth and a star. There is quite a similar phenomenon in themovement of the moon. A star is frequently occulted in this way, and theobservations of such phenomena are familiar to astronomers; but when acomet passes in front of a star the circumstances are widely different. The star is indeed seen nearly as well through the comet as it would beif the comet were entirely out of the way. This has often been noticed. One of the most celebrated observations of this kind was made by thelate Sir John Herschel on Biela's comet, which is one of the periodicclass, and will be alluded to in the next chapter. The illustriousastronomer saw on one occasion this object pass over a star cluster. Itconsisted of excessively minute stars, which could only be seen by apowerful telescope, such as the one Sir John was using. The faintesthaze or the merest trace of a cloud would have sufficed to hide all thestars. It was therefore with no little interest that the astronomerwatched the progress of Biela's comet. Gradually the wanderer encroachedon the group of stars, so that if it had any appreciable solidity thenumerous twinkling points would have been completely screened. But whatwere the facts? Down to the most minute star in that cluster, down tothe smallest point of light which the great telescope could show, everyobject in the group was distinctly seen to twinkle right through themass of Biela's comet. 

This was an important observation. We must recollect that the veil drawnbetween the cluster and the telescope was not a thin curtain; it was avolume of cometary substance many thousands of miles in thickness. Contrast, then, the almost inconceivable tenuity of a comet with theclouds to which we are accustomed. A cloud a few hundred feet thickwill hide not only the stars, but even the great sun himself. Thelightest haze that ever floated in a summer sky would do more to screenthe stars from our view than would one hundred thousand miles of suchcometary material as was here interposed. 

The great comet of Donati passed over many stars which were visibledistinctly through its tail. Among these stars was a very brightone--the well-known Arcturus. The comet, fortunately, happened to passover Arcturus, and though nearly the densest part of the comet wasinterposed between the earth and the star, yet Arcturus twinkled on withundiminished lustre through the thickness of this stupendous curtain. Recent observations have, however, shown that stars in some casesexperience change in lustre when the denser part of the comet passesover them. It is, indeed, difficult to imagine that a star would remainvisible if the nucleus of a really large comet passed over it; but itdoes not seem that an opportunity of testing this supposition has yetarisen. 

As a comet contains transparent gaseous material we might expect thatthe place of a star would be deranged when the comet approached it. Therefractive power of air is very considerable. When we look at thesunset, we see the sun appearing to pass below the horizon; yet the sunhas actually sunk beneath the horizon before any part of its diskappears to have commenced its descent. The refractive power of the airbends the luminous rays round and shows the sun, though it is directlyscreened by the intervening obstacles. The refractive power of thematerial of comets has been carefully tested. A comet has been observedto approach two stars; one of which was seen through the comet, whilethe other could be observed directly. If the body had any appreciablequantity of gas in its composition the relative places of the two starswould be altered. This question has been more than once submitted to thetest of actual measurement. It has sometimes been found that noappreciable change of position could be detected, and that accordinglyin such cases the comet has no perceptible density. Careful measurementsof the great comet in 1881 showed, however, that in the neighbourhood ofthe nucleus there was some refractive power, though quite insignificantin comparison with the refraction of our atmosphere. 

[Illustration: PLATE C. 

COMET A 1892, 1. SWIFT. 

_Photographed by E. E. Barnard, 7th April, 1892. _]

From these considerations it will probably be at once admitted that the_mass_ of a comet must be indeed a very small quantity in comparisonwith its bulk. When we attempt actually to weigh the comet, our effortshave proved abortive. We have been able to weigh the mighty planetsJupiter and Saturn; we have been even able to weigh the vast sunhimself; the law of gravitation has provided us with a stupendousweighing apparatus, which has been applied in all these cases withsuccess, but the same methods applied to comets are speedily seen to beillusory. No weighing machinery known to the astronomer is delicateenough to determine the weight of a comet. All that we can accomplish inany circumstances is to weigh one heavenly body in comparison withanother. Comets seem to be almost imponderable when estimated by suchrobust masses as those of the earth, or any of the other great planets. Of course, it will be understood that when we say the weight of a cometis inappreciable, we mean with regard to the other bodies of our system. Perhaps no one now doubts that a great comet must really weigh tons;though whether those tons are to be reckoned in tens, in hundreds, inthousands, or in millions, the total seems quite insignificant whencompared with the weight of a body like the earth. 

The small mass of comets is also brought before us in a very strikingway when we recall what has been said in the last chapter on theimportant subject of the planetary perturbations. We have there treatedof the permanence of our system, and we have shown that this permanencedepends upon certain laws which the planetary motions must invariablyfulfil. The planets move nearly in circles, their orbits are all nearlyin the same plane, and they all move in the same direction. Thepermanence of the system would be imperilled if any one of theseconditions was not fulfilled. In that discussion we made no allusion tothe comets. Yet they are members of our system, and they far outnumberthe planets. The comets repudiate these rules of the road which theplanets so rigorously obey. Their orbits are never like circles; theyare, indeed, more usually parabolic, and thus differ as widely aspossible from the circular path. Nor do the planes of the orbits ofcomets affect any particular aspect; they are inclined at all sorts ofangles, and the directions in which they move seem to be mere matters ofcaprice. All these articles of the planetary convention are violated bycomets, but yet our system lasts; it has lasted for countless ages, andseems destined to last for ages to come. The comets are attracted by theplanets, and conversely, the comets must attract the planets, and mustperturb their orbits to some extent; but to what extent? If comets movedin orbits subject to the same general laws which characterise planetarymotion, then our argument would break down. The planets might experienceconsiderable derangements from cometary attraction, and yet in the lapseof time those disturbances would neutralise each other, and thepermanence of the system would be unaffected. But the case is verydifferent when we deal with the actual cometary orbits. If comets couldappreciably disturb planets, those disturbances would not neutraliseeach other, and in the lapse of time the system would be wrecked by acontinuous accumulation of irregularities. The facts, however, show thatthe system has lived, and is living, notwithstanding comets; and hencewe are forced to the conclusion that their masses must be insignificantin comparison with those of the great planetary bodies. 

These considerations exhibit the laws of universal gravitation and theirrelations to the permanence of our system in a very striking light. Ifwe include the comets, we may say that the solar system includes manythousands of bodies, in orbits of all sizes, shapes, and positions, onlyagreeing in the fact that the sun occupies a focus common to all. Themajority of these bodies are imponderable in comparison with planets, and their orbits are placed anyhow, so that, although they may suffermuch from the perturbations of the other bodies, they can in no caseinflict any appreciable disturbance. There are, however, a few greatplanets capable of producing vast disturbances; and if their orbits werenot properly adjusted, chaos would sooner or later be the result. By themutual adaptations of their orbits to a nearly circular form, to anearly coincident plane, and to a uniformity of direction, a permanenttruce has been effected among the great planets. They cannot nowpermanently disorganise each other, while the slight mass of the cometsrenders them incompetent to do so. The stability of the great planets isthus assured; but it is to be observed that there is no guarantee ofstability for comets. Their eccentric and irregular paths may undergothe most enormous derangements; indeed, the history of astronomycontains many instances of the vicissitudes to which a cometary careeris exposed. 

Great comets appear in the heavens in the most diverse circumstances. There is no part of the sky, no constellation or region, which is notliable to occasional visits from these mysterious bodies. There is noseason of the year, no hour of the day or of the night when comets maynot be seen above the horizon. In like manner, the size and aspect ofthe comets are of every character, from the dim spot just visible to aneye fortified by a mighty telescope, up to a gigantic and brilliantobject, with a tail stretching across the heavens for a distance whichis as far as from the horizon to the zenith. So also the direction ofthe tail of the comet seems at first to admit of every possibleposition: it may stand straight up in the heavens, as if the comet wereabout to plunge below the horizon; it may stream down from the head ofthe comet, as if the body had been shot up from below; it may slope tothe right or to the left. Amid all this variety and seeming caprice, canwe discover any feature common to the different phenomena? We shall findthat there is a very remarkable law which the tails of comets obey--alaw so true and satisfactory, that if we are given the place of a cometin the heavens, it is possible at once to point out in what directionthe tail will lie. 

A beautiful comet appears in summer in the northern sky. It is nearmidnight; we are gazing on the faintly luminous tail, which stands upstraight and points towards the zenith; perhaps it may be curved alittle or possibly curved a good deal, but still, on the whole, it isdirected from the horizon to the zenith. We are not here referring toany particular comet. Every comet, large or small, that appears in thenorth must at midnight have its tail pointed up in a nearly verticaldirection. This fact, which has been verified on numerous occasions, isa striking illustration of the law of direction of comets' tails. Thinkfor one moment of the facts of the case. It is summer; the twilight atthe north shows the position of the sun, and the tail of the cometpoints directly away from the twilight and away from the sun. Takeanother case. It is evening; the sun has set, the stars have begun toshine, and a long-tailed comet is seen. Let that comet be high or low, north or south, east or west, its tail invariably points _away_ fromthat point in the west where the departing sunlight still lingers. Again, a comet is watched in the early morning, and if the eye be movedfrom the place where the first streak of dawn is appearing to the headof the comet, then along that direction, streaming away from the sun, isfound the tail of the comet. This law is of still more generalapplication. At any season, at any hour of the night, the tail of acomet is directed away from the sun. 

More than three hundred years ago this fact in the movement of cometsarrested the attention of those who pondered on the movements of theheavenly bodies. It is a fact patent to ordinary observation, it givessome degree of consistency to the multitudinous phenomena of comets, andit must be made the basis of our enquiries into the structure of thetails. 

In the adjoining figure, Fig. 71, we show a portion of the parabolicorbit of a comet, and we also represent the position of the tail of thecomet at various points of its path. It would be, perhaps, going too farto assert that throughout the whole vast journey of the comet, its tailmust always be directed from the sun. In the first place, it must berecollected that we can only see the comet during that small part of itsjourney when it is approaching to or receding from the sun. It is alsoto be remembered that, while actually passing round the sun, thebrilliancy of the comet is so overpowered by the sun that the cometoften becomes invisible, just as the stars are invisible in daylight. Indeed, in certain cases, jets of cometary material are actuallyprojected towards the sun. 

[Illustration: Fig. 71. --The Tail of a Comet directed from the Sun. ]

In a hasty consideration of the subject, it might be thought that as thecomet was dashing along with enormous velocity the tail was merelystreaming out behind, just as the shower of sparks from a rocket arestrewn along the path which it follows. This would be an entirelyerroneous analogy; the comet is moving not through an atmosphere, butthrough open space, where there is no medium sufficient to sweep thetail into the line of motion. Another very remarkable feature is thegradual growth of the tail as the comet approaches the sun. While thebody is still at a great distance it has usually no perceptible tail, but as it draws in the tail gradually develops, and in some casesreaches stupendous dimensions. It is not to be supposed that thisincrease is a mere optical consequence of the diminution of distance. Itcan be shown that the growth of the tail takes place much more rapidlythan it would be possible to explain in this way. We are thus led toconnect the formation of the tail with the approach to the sun, and weare accordingly in the presence of an enigma without any analogy amongthe other bodies of our system. 

That the comet as a whole is attracted by the sun there can be no doubtwhatever. The fact that the comet moves in an ellipse or in a parabolaproves that the two bodies act and react on each other in obedience tothe law of universal gravitation. But while this is true of the comet asa whole, it is no less certain that the tail of the comet is _repelled_by the sun. It is impossible to speak with certainty as to how thiscomes about, but the facts of the case seem to point to an explanationof the following kind. 

We have seen that the spectroscope has proved with certainty thepresence of hydrocarbon and other gases in comets. But we are not toconclude from this that comets are merely masses of gas moving throughspace. Though the total quantity of matter in a comet, as we have seen, is exceedingly small, it is quite possible that the comet may consist ofa number of widely scattered particles of appreciable density; indeed, we shall see in the next chapter, when describing the remarkablerelationship between comets and meteors, that we have reason to believethis to be the case. We may therefore look on a comet as a swarm of tinysolid particles, each surrounded by gas. 

When we watch a great comet approaching the sun the nucleus is firstseen to become brighter and more clearly defined; at a later stageluminous matter appears to be projected from it towards the sun, oftenin the shape of a fan or a jet, which sometimes oscillates to and frolike a pendulum. In the head of Halley's comet, for instance, Besselobserved in October, 1835, that the jet in the course of eight hoursswung through an angle of 36°. On other occasions concentric arcs oflight are formed round the nucleus, one after another, getting fainteras they travel further from the nucleus. Evidently the material of thefan or the arcs is repelled by the nucleus of the comet; but it is alsorepelled by the sun, and this latter repulsive force compels theluminous matter to overcome the attraction of gravitation, and to turnback all round the nucleus in the direction away from the sun. In thismanner the tail is formed. (_See_ Plate XII. ) The mathematical theory ofthe formation of comets' tails has been developed on the assumption thatthe matter which forms the tail is repelled both by the nucleus and bythe sun. This investigation was first undertaken by the great astronomerBessel, in his memoir on the appearance of Halley's comet in 1835, andit has since been considerably developed by Roche and the Russianastronomer Bredichin. Though we are, perhaps, hardly in a position toaccept this theory as absolutely true, we can assert that it accountswell for the principal phenomena observed in the formation of comets'tails. 

Professor Bredichin has conducted his labours in the philosophicalmanner which has led to many other great discoveries in science. He hascarefully collated the measurements and drawings of the tails of variouscomets. One result has been obtained from this preliminary part of hisenquiry, which possesses a value that cannot be affected even if theulterior portion of his labours should be found to requirequalification. In the examination of the various tails, he observed thatthe curvilinear shapes of the outlines fall into one or other of threespecial types. In the first we have the straightest tails, which pointalmost directly away from the sun. In the second are classed tailswhich, after starting away from the sun, are curved backwards from thedirection towards which the comet is moving. In the third we find theappendage still more curved in towards the comet's path. It can be shownthat the tails of comets can almost invariably be identified with one orother of these three types; and in cases where the comet exhibits twotails, as has sometimes happened, then they will be found to belong totwo of the types. 

The adjoining diagram (Fig. 72) gives a sketch of an imaginary cometfurnished with tails of the three different types. The direction inwhich the comet is moving is shown by the arrow-head on the line passingthrough the nucleus. Bredichin concludes that the straightest of thethree tails, marked as Type I. , is most probably due to the elementhydrogen; the tails of the second form are due to the presence of someof the hydrocarbons in the body of the comet; while the small tails ofthe third type may be due to iron or to some other element with a highatomic weight. It will, of course, be understood that this diagram doesnot represent any actual comet. 

[Illustration: Fig. 72. --Bredichin's Theory of Comets' Tails. ]

[Illustration: Fig. 73. --Tails of the Comet of 1858. ]

An interesting illustration of this theory is afforded in the case ofthe celebrated comet of 1858 already referred to, of which a drawing isshown in Fig. 73. We find here, besides the great tail, which is thecharacteristic feature of the body, two other faint streaks of light. These are the edges of the hollow cone which forms a tail of Type I. When we look through the central regions it will be easily understoodthat the light is not sufficiently intense to be visible; at the edges, however, a sufficient thickness of the cometary matter is presented, andthus we have the appearance shown in this figure. It would seem thatDonati's comet possessed one tail due to hydrogen, and another due tosome of the compounds of carbon. The carbon compounds involved appear tobe of considerable variety, and there is, in consequence, a dispositionin the tails of the second type to a more indefinite outline than in thehydrogen tails. Cases have been recorded in which several tails havebeen seen simultaneously on the same comet. The most celebrated of theseis that which appeared in the year 1744. Professor Bredichin has devotedspecial attention to the theory of this marvellous object, and he hasshown with a high degree of probability how the multiform tail could beaccounted for. The adjoining figure (Fig. 74) is from a sketch of thisobject made on the morning of the 7th March by Mademoiselle Kirch at theBerlin Observatory. The figure shows eleven streaks, of which the firstten (counting from the left) represent the bright edges of five of thetails, while the sixth and shortest tail is at the extreme right. Sketches of this rare phenomenon were also made by Chéseaux at Lausanneand De L'Isle at St. Petersburg. Before the perihelion passage thecomet had only had one tail, but a very splendid one. 

[Illustration: Fig. 74. --The Comet of 1744. ]

It is possible to submit some of the questions involved to the test ofcalculation, and it can be shown that the repulsive force adequate toproduce the straight tail of Type I. Need only be about twelve times aslarge as the attraction of gravitation. Tails of the second type couldbe produced by a repulsive force which was about equal to gravitation, while tails of the third type would only require a repulsive force aboutone-quarter the power of gravitation. [33] The chief repulsive forceknown in nature is derived from electricity, and it has naturally beensurmised that the phenomena of comets' tails are due to the electriccondition of the sun and of the comet. It would be premature to assertthat the electric character of the comet's tail has been absolutelydemonstrated; all that can be said is that, as it seems to account forthe observed facts, it would be undesirable to introduce some merehypothetical repulsive force. It must be remembered that on quite othergrounds it is known that the sun is the seat of electric phenomena. 

As the comet gradually recedes from the sun the repulsive force becomesweaker, and accordingly we find that the tail of the comet declines. Ifthe comet be a periodic one, the same series of changes may take placeat its next return to perihelion. A new tail is formed, which alsogradually disappears as the comet regains the depths of space. If we mayemploy the analogy of terrestrial vapours to guide us in our reasoning, then it would seem that, as the comet retreats, its tail would condenseinto myriads of small particles. Over these small particles the law ofgravitation would resume its undivided sway, no longer obscured by thesuperior efficiency of the repulsion. The mass of the comet is, however, so extremely small that it would not be able to recall these particlesby the mere force of attraction. It follows that, as the comet at eachperihelion passage makes a tail, it must on each occasion expend acorresponding quantity of tail-making material. Let us suppose that thecomet was endowed in the beginning with a certain capital of thoseparticular materials which are adapted for the production of tails. Eachperihelion passage witnesses the formation of a tail, and theexpenditure of a corresponding amount of the capital. It is obvious thatthis operation cannot go on indefinitely. In the case of the greatmajority of comets the visits to perihelion are so extremely rare thatthe consequences of the extravagance are not very apparent; but to thoseperiodic comets which have short periods and make frequent returns, theconsequences are precisely what might have been anticipated: thetail-making capital has been gradually squandered, and thus at length wehave the spectacle of a comet without any tail at all. We can evenconceive that a comet may in this manner be completely dissipated, andwe shall see in the next chapter how this fate seems to have overtakenBiela's periodic comet. 

But as it sweeps through the solar system the comet may chance to passvery near one of the larger planets, and, in passing, its motion may beseriously disturbed by the attraction of the planet. If the velocity ofthe comet is accelerated by this disturbing influence, the orbit will bechanged from a parabola into another curve known as a hyperbola, and thecomet will swing round the sun and pass away never to return. But if theplanet is so situated as to retard the velocity of the comet, theparabolic orbit will be changed into an ellipse, and the comet willbecome a periodic one. We can hardly doubt that some periodic cometshave been "captured" in this manner and thereby made permanent membersof our solar system, if we remark that the comets of short periods (fromthree to eight years) come very near the orbit of Jupiter at some pointor other of their paths. Each of them must, therefore, have been nearthe giant planet at some moment during their past history. Similarly theother periodic comets of longer period approach near to the orbits ofeither Saturn, Uranus, or Neptune, the last-mentioned planet beingprobably responsible for the periodicity of Halley's comet. We have, indeed, on more than one occasion, actually witnessed the violentdisturbance of a cometary orbit. The most interesting case is that ofLexell's comet. In 1770 the French astronomer Messier (who devotedhimself with great success to the discovery of comets) detected a cometfor which Lexell computed the orbit, and found an ellipse with a periodof five years and some months. Yet the comet had never been seen before, nor did it ever come back again. Long afterwards it was found, from mostlaborious investigations by Burckhardt and Le Verrier, that the comethad moved in a totally different orbit previous to 1767. But at thebeginning of the year 1767 it happened to come so close to Jupiter thatthe powerful attraction of this planet forced it into a new orbit, witha period of five and a half years. It passed the perihelion on the 13thAugust, 1770, and again in 1776, but in the latter year it was notconveniently situated for being seen from the earth. In the summer of1779 the comet was again in the neighbourhood of Jupiter, and was thrownout of its elliptic orbit, so that we have never seen it since, or, perhaps, it would be safer to say that we have not with certaintyidentified Lexell's comet with any comet observed since then. We arealso, in the case of several other periodic comets, able to fix in asimilar way the date when they started on their journeys in theirpresent elliptic orbits. 

Such is a brief outline of the principal facts known with regard tothese interesting but perplexing bodies. We must be content with therecital of what we know, rather than hazard guesses about matters beyondour reach. We see that they are obedient to the great laws ofgravitation, and afford a striking illustration of their truth. We haveseen how modern science has dissipated the superstition with which, inearlier ages, the advent of a comet was regarded. We no longer regardsuch a body as a sign of impending calamity; we may rather look upon itas an interesting and a beautiful visitor, which comes to please us andto instruct us, but never to threaten or to destroy. 

CHAPTER XVII. 

SHOOTING STARS. 

 Small Bodies of our System--Their Numbers--How they are Observed--The Shooting Star--The Theory of Heat--A Great Shooting Star--The November Meteors--Their Ancient History--The Route followed by the Shoal--Diagram of the Shoal of Meteors--How the Shoal becomes Spread out along its Path--Absorption of Meteors by the Earth--The Discovery of the Relation between Meteors and Comets--The Remarkable Investigations concerning the November Meteors--Two Showers in Successive Years--No Particles have ever been Identified from the Great Shooting Star Showers--Meteoric Stones--Chladni's Researches--Early Cases of Stone-falls--The Meteorite at Ensisheim--Collections of Meteorites--The Rowton Siderite--Relative Frequency of Iron and Stony Meteorites--Fragmentary Character of Meteorites--Tschermak's Hypothesis--Effects of Gravitation on a Missile ejected from a Volcano--Can they have come from the Moon?--The Claims of the Minor Planets to the Parentage of Meteorites--Possible Terrestrial Origin--The Ovifak Iron. 

In the preceding chapters we have dealt with the gigantic bodies whichform the chief objects in what we know as the solar system. We havestudied mighty planets measuring thousands of miles in diameter, and wehave followed the movements of comets whose dimensions are often to betold by millions of miles. Once, indeed, in a previous chapter we havemade a descent to objects much lower in the scale of magnitude, and wehave examined that numerous class of small bodies which we call theminor planets. It is now, however, our duty to make a still further, andthis time a very long step, downwards in the scale of magnitude. Eventhe minor planets must be regarded as colossal objects when comparedwith those little bodies whose presence is revealed to us in aninteresting and sometimes in a striking manner. 

These small bodies compensate in some degree for their minute size bythe profusion in which they exist. No attempt, indeed, could be made totell in figures the myriads in which they swarm throughout space. Theyare probably of very varied dimensions, some of them being many poundsor perhaps tons in weight, while others seem to be not larger thanpebbles, or even than grains of sand. Yet, insignificant as these bodiesmay seem, the sun does not disdain to undertake their control. Eachparticle, whether it be as small as the mote in a sunbeam or as mightyas the planet Jupiter, must perforce trace out its path around the sunin conformity with the laws of Kepler. 

Who does not know that beautiful occurrence which we call a shootingstar, or which, in its more splendid forms, is sometimes called a meteoror fireball? It is to objects of this class that we are now to directour attention. 

A small body is moving round the sun. Just as a mighty planet revolvesin an ellipse, so even a small object will be guided round and round inan ellipse with the sun in the focus. There are, at the present moment, inconceivable myriads of such meteors moving in this manner. They aretoo small and too distant for our telescopes, and we never see themexcept under extraordinary circumstances. 

When the meteor flashes into view it is moving with such enormousvelocity that it often traverses more than twenty miles in a second oftime. Such a velocity is almost impossible near the earth's surface: theresistance of the air would prevent it. Aloft, in the emptiness ofspace, there is no air to impede its flight. It may have been movinground and round the sun for thousands, perhaps for millions of years, without suffering any interference; but the supreme moment arrives, andthe meteor perishes in a streak of splendour. 

In the course of its wanderings the body comes near the earth, andwithin a few hundred miles of its surface begins to encounter the uppersurface of the atmosphere with which the earth is enclosed. To a bodymoving with the appalling velocity of a meteor, a plunge into theatmosphere is usually fatal. Even though the upper layers of air areexcessively attenuated, yet they suddenly check the velocity almost asa rifle bullet would be checked when fired into water. As the meteorrushes through the atmosphere the friction of the air warms its surface;gradually it becomes red-hot, then white-hot, and is finally driven offinto vapour with a brilliant light, while we on the earth, one or twohundred miles below, exclaim: "Oh, look, there is a shooting star!"

We have here an experiment illustrating the mechanical theory of heat. It may seem incredible that mere friction should be sufficient togenerate heat enough to produce so brilliant a display, but we mustrecollect two facts: first, that the velocity of the meteor is, perhaps, one hundred times that of a rifle bullet; and, second, that theefficiency of friction in developing heat is proportional to the squareof the velocity. The meteor in passing through the air may thereforedevelop by the friction of the air about ten thousand times as much heatas the rifle bullet. We do not make an exaggerated estimate in supposingthat the latter missile becomes heated ten degrees by friction; yet ifthis be admitted, we must grant that there is such an enormousdevelopment of heat attending the flight of the meteor that even afraction of it would be sufficient to drive the object into vapour. 

Let us first consider the circumstances in which these external bodiesare manifested to us, and, for the sake of illustration, we may take aremarkable fireball which occurred on November 6th, 1869. This body wasseen from many different places in England; and by combining andcomparing these observations, we obtain accurate information as to theheight of the object and the velocity with which it travelled. 

It appears that this meteor commenced to be visible at a point ninetymiles above Frome, in Somersetshire, and that it vanished twenty-sevenmiles over the sea, near St. Ives, in Cornwall. The path of the body, and the principal localities from which it was observed, are shown inthe map (Fig. 75). The whole length of its visible course was about 170miles, which was performed in a period of five seconds, thus giving anaverage velocity of thirty-four miles per second. A remarkable featurein the appearance which this fireball presented was the long persistentstreak of luminous cloud, about fifty miles long and four miles wide, which remained in sight for fully fifty minutes. We have in this examplean illustration of the chief features of the phenomena of a shootingstar presented on a very grand scale. It is, however, to be observedthat the persistent luminous streak is not a universal, nor, indeed, avery common characteristic of a shooting star. 

[Illustration: Fig. 75. --The Path of the Fireball of November 6th, 1869. ]

The small objects which occasionally flash across the field of thetelescope show us that there are innumerable telescopic shooting stars, too small and too faint to be visible to the unaided eye. These objectsare all dissipated in the way we have described; it is, in fact, only atthe moment, and during the process of their dissolution, that we becomeaware of their existence. Small as these missiles probably are, theirvelocity is so prodigious that they would render the earth uninhabitablewere they permitted to rain down unimpeded on its surface. We must, therefore, among the other good qualities of our atmosphere, not forgetthat it constitutes a kindly screen, which shields us from a tempest ofmissiles, the velocity of which no artillery could equal. It is, infact, the very fury of these missiles which is the cause of their utterdestruction. Their anxiety to strike us is so great, that frictiondissolves them into harmless vapour. 

Next to a grand meteor such as that we have just described, the moststriking display in connection with shooting stars is what is known as ashower. These phenomena have attracted a great deal of attention withinthe last century, and they have abundantly rewarded the labour devotedto them by affording some of the most interesting astronomicaldiscoveries of modern times. 

The showers of shooting stars do not occur very frequently. No doubt thequickened perception of those who especially attend to meteors willdetect a shower when others see only a few straggling shooting stars;but, speaking generally, we may say that the present generation canhardly have witnessed more than two or three such occurrences. I havemyself seen two great showers, one of which, in November, 1866, hasimpressed itself on my memory as a glorious spectacle. 

To commence the history of the November meteors it is necessary to lookback for nearly a thousand years. On the 12th of October, in the year902, occurred the death of a Moorish king, and in connection with thisevent an old chronicler relates how "that night there were seen, as itwere lances, an infinite number of stars, which scattered themselveslike rain to right and left, and that year was called the Year of theStars. "

No one now believes that the heavens intended to commemorate the deathof the king by that display. The record is, however, of considerableimportance, for it indicates the year 902 as one in which a great showerof shooting stars occurred. It was with the greatest interestastronomers perceived that this was the first recorded instance of thatperiodical shower, the last of whose regular returns were seen in 1799, 1833, and 1866. Further diligent literary research has revealed here andthere records of startling appearances in the heavens, which fit in withsuccessive returns of the November meteors. From the first instance, in902, to the present day there have been twenty-nine visits of theshower; and it is not unlikely that these may have all been seen in someparts of the earth. Sometimes they may have been witnessed by savages, who had neither the inclination nor the means to place on record anapparition which to them was a source of terror. Sometimes, however, these showers were observed by civilised communities. Their nature wasnot understood, but the records were made; and in some cases, at allevents, these records have withstood the corrosion of time, and have nowbeen brought together to illustrate this curious subject. We havealtogether historical notices of twelve of these showers, collectedmainly by the industry of Professor H. A. Newton whose labours havecontributed so much to the advancement of our knowledge of shootingstars. 

Let us imagine a swarm of small objects roaming through space. Think ofa shoal of herrings in the ocean, extending over many square miles, andcontaining countless myriads of individuals; or think of those enormousflocks of wild pigeons in the United States of which Audubon has toldus. The shoal of shooting stars is perhaps much more numerous than theherrings or the pigeons. The shooting stars are, however, not very closetogether; they are, on an average, probably some few miles apart. Theactual bulk of the shoal is therefore prodigious; and its dimensions areto be measured by hundreds of thousands of miles. 

[Illustration: Fig. 76. --The Orbit of a Shoal of Meteors. ]

The meteors cannot choose their own track, like the shoal of herrings, for they are compelled to follow the route which is prescribed to themby the sun. Each one pursues its own ellipse in complete independence ofits neighbours, and accomplishes its journey, thousands of millions ofmiles in length, every thirty-three years. We cannot observe the meteorsduring the greater part of their flight. There are countless myriads ofthese bodies at this very moment coursing round their path. We never seethem till the earth catches them. Every thirty-three years the earthmakes a haul of these meteors just as successfully as the fishermanamong the herrings, and in much the same way, for while the fishermanspreads his net in which the fishes meet their doom, so the earth has anatmosphere wherein the meteors perish. We are told that there is no fearof the herrings becoming exhausted, for those the fishermen catch are asnothing compared to the profusion in which they abound in ocean. We maysay the same with regard to the meteors. They exist in such myriads, that though the earth swallows up millions every thirty-three years, plenty are left for future showers. The diagram (Fig. 76) will explainthe way in which the earth makes her captures. We there see the orbitin which our globe moves around the sun, as well as the elliptic path ofthe meteors, though it should be remarked that it is not convenient todraw the figure exactly to scale, so that the path of the meteors isrelatively much larger than here represented. Once each year the earthcompletes its revolution, and between the 13th and the 16th of Novembercrosses the track in which the meteors move. It will usually happen thatthe great shoal is not at this point when the earth is passing. Thereare, however, some stragglers all along the path, and the earthgenerally catches a few of these at this date. They dart into ouratmosphere as shooting stars, and form what we usually speak of as theNovember meteors. 

It will occasionally happen that when the earth is in the act ofcrossing the track it encounters the bulk of the meteors. Through theshoal our globe then plunges, enveloped, of course, with the surroundingcoat of air. Into this net the meteors dash in countless myriads, neveragain to emerge. In a few hours' time, the earth, moving at the rate ofeighteen miles a second, has crossed the track and emerges on the otherside, bearing with it the spoils of the encounter. Some few meteors, which have only narrowly escaped capture, will henceforth bear evidenceof the fray by moving in slightly different orbits, but the remainingmeteors of the shoal continue their journey without interruption;perhaps millions have been taken, but probably hundreds of millions havebeen left. 

Such was the occurrence which astonished the world on the night betweenNovember 13th and 14th, 1866. We then plunged into the middle of theshoal. The night was fine; the moon was absent. The meteors weredistinguished not only by their enormous multitude, but by theirintrinsic magnificence. I shall never forget that night. On thememorable evening I was engaged in my usual duty at that time ofobserving nebulæ with Lord Rosse's great reflecting telescope. I was ofcourse aware that a shower of meteors had been predicted, but nothingthat I had heard prepared me for the splendid spectacle so soon to beunfolded. It was about ten o'clock at night when an exclamation from anattendant by my side made me look up from the telescope, just in time tosee a fine meteor dash across the sky. It was presently followed byanother, and then again by more in twos and in threes, which showed thatthe prediction of a great shower was likely to be verified. At this timethe Earl of Rosse (then Lord Oxmantown) joined me at the telescope, and, after a brief interval, we decided to cease our observations of thenebulæ and ascend to the top of the wall of the great telescope (Fig. 7, p. 18), whence a clear view of the whole hemisphere of the heavens couldbe obtained. There, for the next two or three hours, we witnessed aspectacle which can never fade from my memory. The shooting starsgradually increased in number until sometimes several were seen at once. Sometimes they swept over our heads, sometimes to the right, sometimesto the left, but they all diverged from the east. As the night wore on, the constellation Leo ascended above the horizon, and then theremarkable character of the shower was disclosed. All the tracks of themeteors radiated from Leo. (_See_ Fig. 74, p. 368. ) Sometimes a meteorappeared to come almost directly towards us, and then its path was soforeshortened that it had hardly any appreciable length, and looked likean ordinary fixed star swelling into brilliancy and then as rapidlyvanishing. Occasionally luminous trains would linger on for many minutesafter the meteor had flashed across, but the great majority of thetrains in this shower were evanescent. It would be impossible to say howmany thousands of meteors were seen, each one of which was bright enoughto have elicited a note of admiration on any ordinary night. 

The adjoining figure (Fig. 77) shows the remarkable manner in which theshooting stars of this shower diverged from a point. It is not to besupposed that all these objects were in view at the same moment. Theobserver of a shower is provided with a map of that part of the heavensin which the shooting stars appear. He then fixes his attention on oneparticular shooting star, and observes carefully its track with respectto the fixed stars in its vicinity. He then draws a line upon his mapin the direction in which the shooting star moved. Repeating the sameobservation for several other shooting stars belonging to the shower, his map will hardly fail to show that their different tracks almost alltend from one point or region of the figure. There are, it is true, afew erratic ones, but the majority observe this law. It certainly looks, at first sight, as if all the shooting stars did actually dart from thispoint; but a little reflection will show that this is a case in whichthe real motion is different from the apparent. If there actually were apoint from which these meteors diverged, then from different parts ofthe earth the point would be seen in different positions with respect tothe fixed stars; but this is not the case. The radiant, as this point iscalled, is seen in the same part of the heavens from whatever stationthe shower is visible. 

[Illustration: Fig. 77. --The Radiant Point of Shooting Stars. ]

We are, therefore, led to accept the simple explanation afforded by thetheory of perspective. Those who are acquainted with the principles ofthis science know that when a number of parallel lines in an object haveto be represented in a drawing, they must all be made to pass throughthe same point in the plane of the picture. When we are looking at theshooting stars, we see the projections of their paths upon the surfaceof the heavens. From the fact that those paths pass through the samepoint, we are to infer that the shooting stars belonging to the sameshower are moving in parallel lines. 

We are now able to ascertain the actual direction in which the shootingstars are moving, because a line drawn from the eye of the observer tothe radiant point must be parallel to that direction. Of course, it isnot intended to convey the idea that throughout all space the shootingstars of one shower are moving in parallel lines; all we mean is thatduring the short time in which we see them the motion of each of theshooting stars is sensibly a straight line, and that all these straightlines are parallel. 

In the year 1883 the great meteor shoal of the Leonids (for so thisshower is called) attained its greatest distance from the sun, and thencommenced to return. Each year the earth crossed the orbit of themeteors; but the shoal was not met with, and no noteworthy shower ofstars was perceived. Every succeeding year found the meteors approachingthe critical point, and the year 1899 brought the shoal to the earth'strack. In that year a brilliant meteoric shower was expected, but theresult fell far short of expectation. The shoal of meteors is of suchenormous length that it takes more than a year for the mighty processionto pass through the critical portion of its orbit which lies across thetrack of the earth. We thus see that the meteors cannot escape theearth. It may be that when the shoal begins to reach this neighbourhoodthe earth will have just left this part of its path, and a year willhave elapsed before the earth gets round again. Those meteors that havethe good fortune to be in the front of the shoal will thus escape thenet, but some of those behind will not be so fortunate, and the earthwill again devour an incredible host. It has sometimes happened thatcasts into the shoal have been obtained in two consecutive years. If theearth happened to pass through the front part in one year, then theshoal is so long that the earth will have moved right round its orbitof 600, 000, 000 miles, and will again dash through the critical spotbefore the entire number have passed. History contains records of caseswhen, in two consecutive Novembers, brilliant showers of Leonids havebeen seen. 

As the earth consumes such myriads of Leonids each thirty-three years, it follows that the total number must be decreasing. The splendour ofthe showers in future ages will, no doubt, be affected by thiscircumstance. They cannot be always so bright as they have been. It isalso of interest to notice that the shape of the shoal is graduallychanging. Each meteor of the shoal moves in its own ellipse round thesun, and is quite independent of the rest of these bodies. Each one hasthus a special period of revolution which depends upon the length of theellipse in which it happens to revolve. Two meteors will move around thesun in the same time if the lengths of their ellipses are exactly equal, but not otherwise. The lengths of these ellipses are many hundreds ofmillions of miles, and it is impossible that they can be all absolutelyequal. In this may be detected the origin of a gradual change in thecharacter of the shower. Suppose two meteors A and B be such that Atravels completely round in thirty-three years, while B takesthirty-four years. If the two start together, then when A has finishedthe first round B will be a year behind; the next time B will be twoyears behind, and so on. The case is exactly parallel to that of anumber of boys who start for a long race, in which they have to runseveral times round the course before the distance has beenaccomplished. At first they all start in a cluster, and perhaps for thefirst round or two they may remain in comparative proximity; gradually, however, the faster runners get ahead and the slower ones lag behind, sothe cluster becomes elongated. As the race continues, the clusterbecomes dispersed around the entire course, and perhaps the first boywill even overtake the last. Such seems the destiny of the Novembermeteors in future ages. The cluster will in time come to be spread outaround the whole of this mighty track, and no longer will a superbdisplay have to be recorded every thirty-three years. 

It was in connection with the shower of November meteors in 1866 that avery interesting and beautiful discovery in mathematical astronomy wasmade by Professor Adams. We have seen that the Leonids must move in anelliptic path, and that they return every thirty-three years, but thetelescope cannot follow them during their wanderings. All that we knowby observation is the date of their occurrence, the point of the heavensfrom which they radiate, and the great return every thirty-three years. Putting these various facts together, it is possible to determine theellipse in which the meteors move--not exactly: the facts do not go sofar--they only tell us that the ellipse must be one of five possibleorbits. These five possible orbits are--firstly, the immense ellipse inwhich we now know the meteorites do revolve, and for which they requirethe whole thirty-three years to complete a revolution; secondly, anearly circular orbit, very little larger than the earth's path, whichthe meteors would traverse in a few days more than a year; anothersimilar orbit, in which the time would be a few days short of the year;and two other small elliptical orbits lying inside the earth's orbit. Itwas clearly demonstrated by Professor Newton, of New Haven, U. S. A. , thatthe observed facts would be explained if the meteors moved in any one ofthese paths, but that they could not be explained by any otherhypothesis. It remained to see which of these orbits was the true one. Professor Newton himself made the suggestion of a possible method ofsolving the problem. The test he proposed was one of some difficulty, for it involved certain intricate calculations in the theory ofperturbations. Fortunately, however, Professor Adams undertook theinquiry, and by his successful labours the path of the Leonids has beencompletely ascertained. 

[Illustration: Fig. 78. --The History of the Leonids. ]

When the ancient records of the appearance of great Leonid showers wereexamined, it was found that the date of their occurrence undergoes agradual and continuous change, which Professor Newton fixed at one dayin seventy years. It follows as a necessary consequence that the pointwhere the path of the meteors crosses the earth's track is not fixed, but that at each successive return they cross at a point about half adegree further on in the direction in which the earth is travelling. Itfollows that the orbit in which the meteors are revolving is undergoingchange; the path they follow in one revolution varies slightly from thatpursued in the next. As, however, these changes proceed in the samedirection, they may gradually attain considerable dimensions; and theamount of change which is produced in the path of the meteors in thelapse of centuries may be estimated by the two ellipses shown in Fig. 78. The continuous line represents the orbit in A. D. 126; the dottedline represents it at present. 

This unmistakable change in the orbit is one that astronomers attributeto what we have already spoken of as perturbation. It is certain thatthe elliptic motion of these bodies is due to the sun, and that if theywere only acted on by the sun the ellipse would remain absolutelyunaltered. We see, then, in this gradual change of the ellipse theinfluence of the attractions of the planets. It was shown that if themeteors moved in the large orbit, this shifting of the path must be dueto the attraction of the planets Jupiter, Saturn, Uranus, and the Earth;while if the meteors followed one of the smaller orbits, the planetsthat would be near enough and massive enough to act sensibly on themwould be the Earth, Venus, and Jupiter. Here, then, we see how thequestion may be answered by calculation. It is difficult, but it ispossible, to calculate what the attraction of the planets would becapable of producing for each of the five different suppositions as tothe orbit. This is what Adams did. He found that if the meteors moved inthe great orbit, then the attraction of Jupiter would account fortwo-thirds of the observed change, while the remaining third was due tothe influence of Saturn, supplemented by a small addition on account ofUranus. In this way the calculation showed that the large orbit was apossible one. Professor Adams also computed the amount of displacementin the path that could be produced if the meteors revolved in any of thefour smaller ellipses. This investigation was one of an arduouscharacter, but the results amply repaid the labour. It was shown thatwith the smaller ellipses it would be impossible to obtain adisplacement even one-half of that which was observed. These four orbitsmust, therefore, be rejected. Thus the demonstration was complete thatit is in the large path that the meteors revolve. 

The movements in each revolution are guided by Kepler's laws. When atthe part of its path most distant from the sun the velocity of a meteoris at its lowest, being then but little more than a mile a second; as itdraws in, the speed gradually increases, until, when the meteor crossesthe earth's track, its velocity is no less than twenty-six miles asecond. The earth is moving very nearly in the opposite direction atthe rate of eighteen miles a second, so that, if the meteor happen tostrike the earth's atmosphere, it does so with the enormous velocity ofnearly forty-four miles a second. If a collision is escaped, then themeteor resumes its onward journey with gradually declining velocity, andby the time it has completed its circuit a period of thirty-three yearsand a quarter will have elapsed. 

The innumerable meteors which form the Leonids are arranged in anenormous stream, of a breadth very small in comparison with its length. If we represent the orbit by an ellipse whose length is seven feet, thenthe meteor stream will be represented by a thread of the finestsewing-silk, about a foot and a half or two feet long, creeping alongthe orbit. [34] The size of this stream may be estimated from theconsideration that even its width cannot be less than 100, 000 miles. Itslength may be estimated from the circumstance that, although itsvelocity is about twenty-six miles a second, yet the stream takes abouttwo years to pass the point where its orbit crosses the earth's track. On the memorable night between the 13th and 14th of November, 1866, theearth plunged into this stream near its head, and did not emerge on theother side until five hours later. During that time it happened that thehemisphere of the earth which was in front contained the continents ofEurope, Asia, and Africa, and consequently it was in the Old World thatthe great shower was seen. On that day twelvemonth, when the earth hadregained the same spot, the shoal had not entirely passed, and the earthmade another plunge. This time the American continent was in the van, and consequently it was there that the shower of 1867 was seen. Even inthe following year the great shoal had not entirely passed, and sincethen a few stragglers along the route have been encountered at eachannual transit of the earth across this meteoric highway. 

The diagram is also designed to indicate a remarkable speculation whichwas put forward on the high authority of Le Verrier, with the view ofexplaining how the shoal came to be introduced into the solar system. The orbit in which the meteors revolve does not intersect the paths ofJupiter, Saturn, or Mars, but it does intersect the orbit of Uranus. Itmust sometimes happen that Uranus is passing through this point of itspath just as the shoal arrives there. Le Verrier has demonstrated thatsuch an event took place in the year A. D. 126, but that it has nothappened since. We thus seem to have a clue to a very wonderful historyby which the meteors are shown to have come into our system in the yearnamed. The expectations or a repetition of the great shower in 1899which had been widely entertained, and on good grounds, were notrealised. Hardly more than a few meteors of the ordinary type wereobserved. 

Assuming that the orbit of the August meteors was a parabola, Schiaparelli computed the dimensions and position in space of thisorbit, and when he had worked this out, he noticed that the orbitcorresponded in every particular with the orbit of a fine comet whichhad appeared in the summer of 1862. This could not be a mere matter ofaccident. The plane in which the comet moved coincided exactly with thatin which the meteors moved; so did the directions of the axes of theirorbits, while the direction of the motion is the same, and the shortestdistance from the sun to the orbit is also in the two cases identical. This proved to demonstration that there must be some profound physicalconnection between comets and swarms of meteors. And a further proof ofthis was shortly afterwards furnished, when Le Verrier had computed theorbit of the November meteors, for this was at once noticed to beprecisely the same as the orbit of a comet which had passed itsperihelion in January, 1866, and for which the period of revolution hadbeen found to be thirty-three years and two months. 

Among the Leonids we see occasionally fireballs brighter than Venus, andeven half the apparent size of the moon, bursting out withlightning-like flashes, and leaving streaks which last from a minute toan hour or more. But the great majority are only as bright as stars ofthe second, third, or fourth magnitude. As the amount of light given bya meteor depends on its mass and velocity, we can form some idea as tothe actual weight of one of these meteors, and it appears that most ofthem do not weigh nearly as much as a quarter of an ounce; indeed, it isprobable that many do not weigh a single grain. But we have seen that acomet in all probability is nothing but a very loose swarm of smallparticles surrounded by gas of very slight density, and we have alsoseen that the material of a comet must by degrees be more or lessdissipated through space. We have still to tell a wonderful story of thebreaking up of a comet and what appears to have become of the particlesthereof. 

A copious meteoric shower took place on the night of the 27th November, 1872. On this occasion the shooting stars diverged from a radiant pointin the constellation of Andromeda. As a spectacle, it was unquestionablyinferior to the magnificent display of 1866, but it is difficult to saywhich of the two showers has been of greater scientific importance. 

It surely is a remarkable coincidence that the earth should encounterthe Andromedes (for so this shower is called) at the very moment when itis crossing the track of Biela's comet. We have observed the directionfrom which the Andromedes come when they plunge into the atmosphere; wecan ascertain also the direction in which Biela's comet is moving whenit passes the earth's track, and we find that the direction in which thecomet moves and the direction in which the meteors move are identical. This is, in itself, a strong and almost overwhelming presumption thatthe comet and the shooting stars are connected; but it is not all. Wehave observations of this swarm dating back to the eighteenth century, and we find that the date of its appearance has changed from the 6th or7th of December to the end of November in perfect accordance with theretrograde motion of the crossing-point of the earth's orbit and theorbit of Biela's comet. This comet was observed in 1772, and again in1805-6, before its periodic return every seven years was discovered. Itwas discovered by Biela in 1826, and was observed again in 1832. In 1846the astronomical world was startled to find that there were now twocomets in place of one, and the two fragments were again perceived atthe return in 1852. In 1859 Biela's comet could not be seen, owing toits unfavourable situation with regard to the earth. No trace of Biela'scomet was seen in 1865-66, when its return was also due, nor has it everbeen seen since. It therefore appears that in the autumn of 1872 thetime had arrived for the return of Biela's comet, and thus theoccurrence of the great shower of the Andromedes took place about thetime when Biela's comet was actually due. The inference is irresistiblethat the shooting stars, if not actually a part of the comet itself, areat all events most intimately connected therewith. This shower is alsomemorable for the telegram sent from Professor Klinkerfues to Mr. Pogsonat Madras. The telegram ran as follows:--"Biela touched earth on 27th. Search near Theta Centauri. " Pogson did search and did find a comet, but, unfortunately, owing to bad weather he only secured observations ofit on two nights. As we require three observations to determine theorbit of a planet or comet, it is not possible to compute the orbit ofPogson's, but it seems almost certain that the latter cannot beidentical with either of the two components of Biela's comet. It is, however, likely that it really was a comet moving along the same trackas Biela and the meteors. 

Another display of the Biela meteors took place in 1885, just givingtime for two complete revolutions of the swarm since 1872. The displayon the 27th November, 1885, was magnificent; Professor Newton estimatedthat at the time of maximum the meteors came on at the rate of 75, 000per hour. In 1892 the comet ought again to have returned to perihelion, but in that year no meteors were seen on the 27th November, while manywere seen on the 23rd from the same radiant. The change in the point ofintersection between the orbit of the meteors and the orbit of theearth indicated by this difference of four days was found by Bredichinto be due to the perturbing action of Jupiter on the motion of theswarm. 

It is a noticeable circumstance that the great meteoric showers seemnever yet to have projected a missile which has reached the earth'ssurface. Out of the myriads of Leonids, of Perseids, or of Andromedes, not one particle has ever been seized and identified. [35] Those bodieswhich fall from the sky to the earth, and which we call meteorites, donot seem to come from the great showers, so far as we know. They may, indeed, have quite a different origin from that of the periodic meteors. 

It is somewhat curious that the belief in the celestial origin ofmeteorites is of modern growth. In ancient times there were, no doubt, rumours of wonderful stones which had fallen down from the heavens tothe earth, but these reports seem to have obtained but little credit. They were a century ago regarded as perfectly fabulous, though there wasabundant testimony on the subject. Eye-witnesses averred that they hadseen the stones fall. The bodies themselves were unlike other objects inthe neighbourhood, and cases were even authenticated where men had beenkilled by these celestial visitors. 

No doubt the observations were generally made by ignorant and illiteratepersons. The true parts of the record were so mixed up with imaginaryadditions, that cautious men refused to credit the statements that suchobjects really fell from the sky. Even at the present day it is oftenextremely difficult to obtain accurate testimony on such matters. Forinstance, the fall of a meteorite was observed by a Hindoo in thejungle. The stone was there, its meteoric character was undoubted, andthe witness was duly examined as to the details of the occurrence; buthe was so frightened by the noise and by the danger he believed himselfto have narrowly escaped, that he could tell little or nothing. He feltcertain, however, that the meteorite had hunted him for two hoursthrough the jungle before it fell to the earth!

In the year 1794 Chladni published an account of the remarkable mass ofiron which the traveller Pallas had discovered in Siberia. It was thenfor the first time recognised that this object and others similar to itmust have had a celestial origin. But even Chladni's reputation and thearguments he brought forward failed to procure universal assent. Shortlyafterwards a stone of fifty-six pounds was exhibited in London, whichseveral witnesses declared they had seen fall at Wold Cottage, inYorkshire, in 1795. This body was subsequently deposited in our nationalcollection, and is now to be seen in the Natural History Museum at SouthKensington. The evidence then began to pour in from other quarters;portions of stone from Italy and from Benares were found to be ofidentical composition with the Yorkshire stone. The incredulity of thosewho had doubted the celestial origin of these objects began to give way. A careful memoir on the Benares meteorite, by Howard, was published inthe "Philosophical Transactions" for 1802, while, as if to complete thedemonstration, a great shower of stones took place in the following yearat L'Aigle, in Normandy. The French Academy deputed the physicist Biotto visit the locality and make a detailed examination of thecircumstances attending this memorable shower. His enquiry removed everytrace of doubt, and the meteoric stones have accordingly beentransferred from the dominions of geology to those of astronomy. It maybe noted that the recognition of the celestial origin of meteoriteshappens to be simultaneous with the discovery of the first of the minorplanets. In each case our knowledge of the solar system has beenextended by the addition of numerous minute bodies, which, notwithstanding their insignificant dimensions, are pregnant withinformation. 

When the possibility of stone-falls has been admitted, we can turn tothe ancient records, and assign to them the credit they merit, whichwas withheld for so many centuries. Perhaps the earliest of all thesestone-falls which can be said to have much pretension to historicalaccuracy is that of the shower which Livy describes as having fallen, about the year 654 B. C. , on the Alban Mount, near Rome. Among the moremodern instances, we may mention one which was authenticated in a veryemphatic manner. It occurred in the year 1492 at Ensisheim, in Alsace. The Emperor Maximilian ordered a minute narrative of the circumstancesto be drawn up and deposited with the stone in the church. The stone wassuspended in the church for three centuries, until in the FrenchRevolution it was carried off to Colmar, and pieces were broken from it, one of which is now in our national collection. Fortunately, thisinteresting object has been restored to its ancient position in thechurch at Ensisheim, where it remains an attraction to sight-seers atthis day. The account is as follows:--"In the year of the Lord 1492, onthe Wednesday before St. Martin's Day, November 7th, a singular miracleoccurred, for between eleven o'clock and noon there was a loud clap ofthunder and a prolonged confused noise, which was heard at a greatdistance, and a stone fell from the air in the jurisdiction of Ensisheimwhich weighed 260 pounds, and the confused noise was at other placesmuch louder than here. Then a boy saw it strike on ploughed ground inthe upper field towards the Rhine and the Ill, near the district ofGisgang, which was sown with wheat, and it did no harm, except that itmade a hole there; and then they conveyed it from the spot, and manypieces were broken from it, which the Land Vogt forbade. They thereforecaused it to be placed in the church, with the intention of suspendingit as a miracle, and there came here many people to see this stone, sothere were many remarkable conversations about this stone; the learnedsaid they knew not what it was, for it was beyond the ordinary course ofnature that such a large stone should smite from the height of the air, but that it was really a miracle from God, for before that time neverwas anything heard like it, nor seen, nor written. When they found thatstone, it had entered into the earth to half the depth of a man'sstature, which everybody explained to be the will of God that it shouldbe found, and the noise of it was heard at Lucerne, at Villingen, and atmany other places, so loud that the people thought that the houses hadbeen overturned; and as the King Maximilian was here, the Monday afterSt. Catherine's Day of the same year, his Royal Excellency ordered thestone which had fallen to be brought to the castle, and after havingconversed a long time about it with the noblemen, he said that thepeople of Ensisheim should take it and order it to be hung up in thechurch, and not to allow anybody to take anything from it. HisExcellency, however, took two pieces of it, of which he kept one, andsent the other to Duke Sigismund of Austria, and there was a great dealof talk about the stone, which was suspended in the choir, where itstill is, and a great many people came to see it. "

Admitting the celestial origin of the meteorites, they surely claim ourclosest attention. They afford the only direct method we possess ofobtaining a knowledge of the materials of bodies exterior to our planet. We can take a meteorite in our hands, we can analyse it, and find theelements of which it is composed. We shall not attempt to enter into anyvery detailed account of the structure of meteorites; it is rather amatter for the consideration of chemists and mineralogists than forastronomers. A few of the more obvious features will be all that werequire. They will serve as a preliminary to the discussion of theprobable origin of these bodies. 

In the Natural History Museum at South Kensington we may examine asuperb collection of meteorites. They have been brought together fromall parts of the earth, and vary in size from bodies not much largerthan a pin's head up to vast masses weighing many hundredweights. Thereare also models of celebrated meteorites, of which the originals aredispersed through various other museums. 

Many meteorites have nothing very remarkable in their externalappearance. If they were met with on the sea beach, they would be passedby without more notice than would be given to any other stone. Yet, whata history a meteorite might tell us if we could only manage to obtainit! It fell; it was seen to fall from the sky; but what was its courseanterior to that movement? Where was it 100 years ago, 1, 000 years ago?Through what regions of space has it wandered? Why did it never fallbefore? Why has it actually now fallen? Such are some of the questionswhich crowd upon us as we ponder over these most interesting bodies. Some of these objects are composed of very characteristic materials;take, for example, one of the more recent arrivals, known as the Rowtonsiderite. This body differs very much from the more ordinary kind ofstony meteorite. It is an object which even a casual passer-by wouldhardly pass without notice. Its great weight would also attractattention, while if it be scratched or rubbed with a file, it wouldappear to be a mass of nearly pure iron. We know the circumstances inwhich that piece of iron fell to the earth. It was on the 20th of April, 1876, about 3. 40 p. M. , that a strange rumbling noise, followed by astartling explosion, was heard over an area of several miles in extentamong the villages in Shropshire, eight or ten miles north of theWrekin. About an hour after this occurrence a farmer noticed that theground in one of his grass-fields had been disturbed, and he probed thehole which the meteorite had made, and found it, still warm, abouteighteen inches below the surface. Some men working at no great distancehad heard the noise made in its descent. This remarkable object, weighs7-3/4 lbs. It is an irregular angular mass of iron, though all its edgesseem to have been rounded by fusion in its transit through the air. Itis covered with a thick black pellicle of the magnetic oxide of iron, except at the point where it first struck the ground. The Duke ofCleveland, on whose property it fell, afterwards presented it to ournational institution already referred to, where, as the Rowton siderite, it attracts the attention of everyone who is interested in thesewonderful bodies. 

This siderite is specially interesting on account of its distinctlymetallic character. Falls of objects of this particular type are not sofrequent as are those of the stony meteorites; in fact, there are only afew known instances of meteoric irons having been actually seen tofall, while the observed falls of stony meteorites are to be counted inscores or in hundreds. The inference is that the iron meteorites aremuch less frequent than the stony ones. This is, however, not theimpression that the visitor to the Museum would be likely to receive. Inthat extensive collection the meteoric irons are by far the moststriking objects. The explanation is not difficult. Those giganticmasses of iron are unquestionably meteoric: no one doubts that this isthe case. Yet the vast majority of them have never been seen to fall;they have simply been found, in circumstances which point unmistakablyto their meteoric nature. Suppose, for instance, that a traveller on oneof the plains of Siberia or of Central America finds a mass of metalliciron lying on the surface of the ground, what explanation can berendered of such an occurrence? No one has brought the iron there, andthere is no iron within hundreds of miles. Man never fashioned thatobject, and the iron is found to be alloyed with nickel in a manner thatis always observed in known meteorites, and is generally regarded as asure indication of a meteoric origin. Observe also, that as ironperishes by corrosion in our atmosphere, that great mass of iron cannothave lain where it is for indefinite ages; it must have been placedthere at some finite time. Only one source for such an object isconceivable; it must have fallen from the sky. On the same plains thestony meteorites have also fallen in hundreds and in thousands, but theycrumble away in the course of time, and in any case would not arrest theattention of the traveller as the irons are likely to do. Hence itfollows, that although the stony meteorites seem to fall much morefrequently, yet, unless they are actually observed at the moment ofdescent, they are much more liable to be overlooked than the meteoricirons. Hence it is that the more prominent objects of the Britishcollection are the meteoric irons. 

We have said that a noise accompanied the descent of the Rowtonsiderite, and it is on record that a loud explosion took place when themeteorite fell at Ensisheim. In this we have a characteristic feature ofthe phenomenon. Nearly all the descents of meteorites that have beenobserved seem to have been ushered in by a detonation. We do not, however, assert that this is quite an invariable feature; and it is alsothe case that meteors often detonate without throwing down any solidfragments that have been collected. The violence associated with thephenomenon is forcibly illustrated by the Butsura meteorite. This objectfell in India in 1861. A loud explosion was heard, several fragments ofstone were collected from distances three or four miles apart; and whenbrought together, they were found to fit, so as to enable the primitiveform of the meteorite to be reconstructed. A few of the pieces arewanting (they were, no doubt, lost by falling unobserved into localitiesfrom which they could not be recovered), but we have obtained piecesquite numerous enough to permit us to form a good idea of the irregularshape of the object before the explosion occurred which shattered itinto fragments. This is one of the ordinary stony meteorites, and isthus contrasted with the Rowton siderite which we have just beenconsidering. There are also other types of meteorites. The Breitenbachiron, as it is called, is a good representative of a class of thesebodies which lie intermediate between the meteoric irons and the stones. It consists of a coarsely cellular mass of iron, the cavities beingfilled with mineral substances. In the Museum, sections of intermediateforms are shown in which this structure is exhibited. 

Look first at the most obvious characteristic of these meteorites. We donot now allude to their chemical composition, but to their externalappearance. What is the most remarkable feature in the shape of theseobjects?--surely it is that they are fragments. They are evidentlypieces that are _broken_ from some larger object. This is apparent bymerely looking at their form; it is still more manifest when we examinetheir mechanical structure. It is often found that meteorites arethemselves composed of smaller fragments. Such a structure may beillustrated by a section of an aërolite found on the Sierra of Chaco, weighing about 30 lbs. (Fig. 79). 

The section here represented shows the composite structure of thisobject, which belongs to the class of stony meteorites. Its shape showsthat it was really a fragment with angular edges and corners. No doubtit may have been much more considerable when it first dashed into theatmosphere. The angular edges now seen on the exterior may be due to anexplosion which then occurred; but this will not account for thestructure of the interior. We there see irregular pieces of varied formand material agglomerated into a single mass. If we would seek foranalogous objects on the earth, we must look to some of the volcanicrocks, where we have multitudes of irregular angular fragments cementedtogether by a matrix in which they are imbedded. The evidence presentedby this meteorite is conclusive as to one circumstance with regard tothe origin of these objects. They must have come as fragments, from somebody of considerable, if not of vast, dimensions. In this meteoritethere are numerous small grains of iron mingled with mineral substances. The iron in many meteorites has, indeed, characters resembling thoseproduced by the actual blasting of iron by dynamite. Thus, a largemeteoric iron from Brazil has been found to have been actually shiveredinto fragments at some time anterior to its fall on the earth. Thesefragments have been cemented together again by irregular veins ofmineral substances. 

[Illustration: Fig. 79. --Section of the Chaco Meteorite. ]

For an aërolite of a very different type we may refer to thecarbonaceous meteorite of Orgueil, which fell in France on the 14th May, 1864. On the occasion of its descent a splendid meteor was seen, rivalling the full moon in size. The actual diameter of this globe offire must have been some hundreds of yards. Nearly a hundred fragmentsof the body were found scattered over a tract of country fifteen mileslong. This object is of particular interest, inasmuch as it belongs to arare group of aërolites, from which metallic iron is absent. It containsmany of the same minerals which are met with in other meteorites, but inthese fragments they are _associated with carbon_, and with substancesof a white or yellowish crystallisable material, soluble in ether, andresembling some of the hydrocarbons. Such a substance, if it had notbeen seen falling to the earth, would probably be deemed a productresulting from animal or vegetable life!

We have pointed out how a body moving with great velocity and impingingupon the air may become red-hot and white-hot, or even be driven offinto vapour. How, then, does it happen that meteorites escape this fieryordeal, and fall down to the earth, with a great velocity, no doubt, butstill, with very much less than that which would have sufficed to drivethem off into vapour? Had the Rowton siderite, for instance, struck ouratmosphere with a velocity of twenty miles a second, it seemsunquestionable that it would have been dissipated by heat, though, nodoubt, the particles would ultimately coalesce so as to descend slowlyto the earth in microscopic beads of iron. How has the meteorite escapedthis fate? It must be remembered that our earth is also moving with avelocity of about eighteen miles per second, and that the _relative_velocity with which the meteorite plunges into the air is that whichwill determine the degree to which friction is operating. If themeteorite come into direct collision with the earth, the velocity of thecollision will be extremely great; but it may happen that though theactual velocities of the two bodies are both enormous, yet the relativevelocity may be comparatively small. This is, at all events, oneconceivable explanation of the arrival of a meteorite on the surface ofthe earth. 

We have shown in the earlier parts of the chapter that the well-knownstar showers are intimately connected with comets. In fact, each starshower revolves in the path pursued by a comet, and the shooting starparticles have, in all probability, been themselves derived from thecomet. Showers of shooting stars have, therefore, an intimate connectionwith comets, but it is doubtful whether meteorites have any connectionwith comets. It has already been remarked that meteorites have neverbeen known to fall in the great star showers. No particle of a meteoriteis known to have dropped from the countless host of the Leonids or ofthe Perseids; as far as we know, the Lyrids never dropped a meteorite, nor did the Quadrantids, the Geminids, or the many other showers withwhich every astronomer is familiar. There is no reason to connectmeteorites with these showers, and it is, therefore, doubtful whether weshould connect meteorites with comets. 

With reference to the origin of meteorites it is difficult to speak withany great degree of confidence. Every theory of meteorites presentsdifficulties, so it seems that the only course open to us is to choosethat view of their origin which seems least improbable. It appears to methat this condition is fulfilled in the theory entertained by theAustrian mineralogist, Tschermak. He has made a study of the meteoritesin the rich collection at Vienna, and he has come to the conclusion thatthe "meteorites have had a volcanic source on some celestial body. " Letus attempt to pursue this reasoning and discuss the problem, which maybe thus stated:--Assuming that at least some of the meteorites have beenejected from volcanoes, on what body or bodies in the universe mustthese volcanoes be situated? This is really a question for astronomersand mathematicians. Once the mineralogists assure us that these bodiesare volcanic, the question becomes one of calculation and of the balanceof probabilities. 

The first step in the enquiry is to realise distinctly the dynamicalconditions of the problem. Conceive a volcano to be located on a planet. The volcano is supposed to be in a state of eruption, and in one of itsmighty throes projects a missile aloft: this missile will ascend, itwill stop, and fall down again. Such is the case at present in theeruptions of terrestrial volcanoes. Cotopaxi has been known to hurlprodigious stones to a vast height, but these stones assuredly return toearth. The gravitation of the earth has gradually overcome the velocityproduced by the explosion, and down the body falls. But let us supposethat the eruption is still more violent, and that the stones areprojected from the planet to a still greater height above its surface. Suppose, for instance, that the stone should be shot up to a heightequal to the planet's radius, the attraction of gravitation will then bereduced to one-fourth of what it was at the surface, and hence theplanet will find greater difficulty in pulling back the stone. Not onlyis the distance through which the stone has to be pulled back increasedas the height increases, but the efficiency of gravitation is weakened, so that in a twofold way the difficulty of recalling the stone isincreased. We have already more than once alluded to this subject, andwe have shown that there is a certain critical velocity appropriate toeach planet, and depending on its mass and its radius. If the missile beprojected upwards with a velocity equal to or greater than this, then itwill ascend never to return. We all recollect Jules Verne's voyage tothe moon, in which he described the Columbiad, an imaginary cannon, capable of shooting out a projectile with a velocity of six or sevenmiles a second. This is the critical velocity for the earth. If we couldimagine the air removed, then a cannon of seven-mile power would projecta body upwards which would never fall down. 

The great difficulty about Tschermak's view of the volcanic origin ofthe meteorites lies in the tremendous initial velocity which isrequired. The Columbiad is a myth, and we know no agent, natural orartificial, at the present time on the earth, adequate to the productionof a velocity so appalling. The thunders of Krakatoa were heardthousands of miles away, but in its mightiest throes it discharged nomissiles with a velocity of six miles a second. We are therefore led toenquire whether any of the other celestial bodies are entitled to theparentage of the meteorites. We cannot see volcanoes on any other bodyexcept the moon; all the other bodies are too remote for an inspectionso minute. Does it seem likely that volcanoes on the moon can everlaunch forth missiles which fall upon the earth?

This belief was once sustained by eminent authority. The mass of themoon is about one-eightieth of the mass of the earth. It would not betrue to assert that the critical velocity of projection varies directlyas the mass of the planet. The correct law is, that it varies directlyas the square root of the mass, and inversely as the square root of theradius. It is hence shown that the velocity required to project amissile away from the moon is only about one-sixth of that which wouldbe required to project a missile away from the earth. If the moon had onits surface volcanoes of one-mile power, it is quite conceivable thatthese might be the source of meteorites. We have seen how the wholesurface of the moon shows traces of intense volcanic activity. A missilethus projected from the moon could undoubtedly fall on the earth, and itis not impossible that some of the meteorites may really have come fromthis source. There is, however, one great difficulty about the volcanoeson the moon. Suppose an object were so projected, it would, under theattraction of the earth, in accordance with Kepler's laws, move aroundthe earth as a focus. If we set aside the disturbances produced by allother bodies, as well as the disturbance produced by the moon itself, wesee that the meteorite if it once misses the earth can never fallthereon. It would be necessary that the shortest distance of the earth'scentre from the orbit of the projectile should be less than the radiusof the earth, so that if a lunar meteorite is to fall on the earth, itmust do so the first time it goes round. The journey of a meteorite fromthe moon to the earth is only a matter of days, and therefore, asmeteorites are still falling, it would follow that they must still beconstantly ejected from the moon. The volcanoes on the moon are, however, not now active; observers have long studied its surface, andthey find no reliable traces of volcanic activity at the present day. It is utterly out of the question, whatever the moon may once have beenable to do, that at the present date she could still continue to launchforth meteorites. It is just possible that a meteorite expelled from themoon in remote antiquity, when its volcanoes were active, may, under theinfluence of the disturbances of the other bodies of the system, haveits orbit so altered, that at length it comes within reach of theatmosphere and falls to the earth, but in no circumstances could themoon send us a meteorite at present. It is therefore reasonable to lookelsewhere in our search for volcanoes fulfilling the conditions of theproblem. 

Let us now direct our attention to the planets, and examine thecircumstances in which volcanoes located thereon could eject a meteoritewhich should ultimately tumble on the earth. We cannot see the planetswell enough to tell whether they have or ever had any volcanoes; but thealmost universal presence of heat in the large celestial masses seems toleave us in little doubt that some form of volcanic action might befound in the planets. We may at once dismiss the giant planets, such asJupiter or Saturn: their appearance is very unlike a volcanic surface;while their great mass would render it necessary to suppose that themeteorites were expelled with terrific velocity if they should succeedin escaping from the gravitation of the planet. Applying the rulealready given, a volcano on Jupiter would have to be five or six timesas powerful as the volcano on the earth. To avoid this difficulty, wenaturally turn to the smaller planets of the system; take, for instance, one of that innumerable host of minor planets, and let us enquire howfar this body is likely to have ejected a missile which should fall uponthe earth. Some of these globes are only a few miles in diameter. Thereare bodies in the solar system so small that a very moderate velocitywould be sufficient to project a missile away from them altogether. Wehave, indeed, already illustrated this point in discussing the minorplanets. It has been suggested that a volcano placed on one of the minorplanets might be quite powerful enough to start the meteorites on along ramble through space until the chapter of accidents brought theminto collision with the earth. There is but little difficulty ingranting that there might be such volcanoes, and that they might besufficiently powerful to drive bodies from the surface of the planet;but we must remember that the missiles are to fall on the earth, anddynamical considerations are involved which merit our close attention. To concentrate our ideas, we shall consider one of the minor planets, and for this purpose let us take Ceres. If a meteorite is to fall uponthe earth, it must pass through the narrow ring, some 8, 000 miles wide, which marks the earth's path; it will not suffice for the missile topass through the ecliptic on the inside or on the outside of the ring, it must be actually through this narrow strip, and then if the earthhappens to be there at the same moment the meteorite will fall. Thefirst condition to be secured is, therefore, that the path of themeteorite shall traverse this narrow ring. This is to be effected byprojection from some point in the orbit of Ceres. But it can be shown onpurely dynamical grounds that although the volcanic energy sufficient toremove the projectile from Ceres may be of no great account, yet if thatprojectile is to cross the earth's track, the dynamical requirements ofthe case demand a volcano on Ceres at the very least of three-milepower. We have thus gained but little by the suggestion of a minorplanet, for we have not found that a moderate volcanic power would beadequate. But there is another difficulty in the case of Ceres, inasmuchas the ring on the ecliptic is very narrow in comparison with the otherdimensions of the problem. Ceres is a long way off, and it would requirevery great accuracy in volcanic practice on Ceres to project a missileso that it should just traverse this ring and fall neither inside noroutside, neither above nor below. There must be a great many misses forevery hit. We have attempted to make the calculation by the aid of thetheory of probabilities, and we find that the chances against thisoccurrence are about 50, 000 to 1, so that out of every 50, 000projectiles hurled from a point in the orbit of Ceres only a single onecan be expected to satisfy even the first of the conditions necessary ifit is ever to tumble on our globe. It is thus evident that there are twoobjections to Ceres (and the same may be said of the other minorplanets) as a possible source of the meteorites. Firstly, thatnotwithstanding the small mass of the planet a very powerful volcanowould still be required; and secondly, that we are obliged to assumethat for every one which ever reached the earth at least 50, 000 musthave been ejected. It is thus plain that if the meteorites have reallybeen driven from some planet of the solar system, large or small, thevolcano must, from one cause or another, have been a very powerful one. We are thus led to enquire which planet possesses on other grounds thegreatest probability in its favour. 

We admit of course that at the present time the volcanoes on the earthare utterly devoid of the necessary power; but were the terrestrialvolcanoes always so feeble as they are in these later days? Grounds arenot wanting for the belief that in the very early days of geologicaltime the volcanic energy on the earth was much greater than at present. We admit fully the difficulties of the view that the meteorites havereally come from the earth; but they must have some origin, and it isreasonable to indicate the source which seems to have most probabilityin its favour. Grant for a moment that in the primæval days of volcanicactivity there were some mighty throes which hurled forth missiles withthe adequate velocity: these missiles would ascend, they would pass fromthe gravitation of the earth, they would be seized by the gravitation ofthe sun, and they would be compelled to revolve around the sun for everafter. No doubt the resistance of the air would be a very greatdifficulty, but this resistance would be greatly lessened were thecrater at a very high elevation above the sea level, while, if a vastvolume of ejected gases or vapours accompanied the more solid material, the effect of the resistance of the air would be still further reduced. Some of these objects might perhaps revolve in hyperbolic orbits, andretreat never to return; while others would be driven into ellipticpaths. Round the sun these objects would revolve for ages, but at eachrevolution--and here is the important point--they would traverse thepoint from which they were originally launched. In other words, everyobject so projected from the earth would at each revolution cross thetrack of the earth. We have in this fact an enormous probability infavour of the earth as contrasted with Ceres. Only one Ceres-ejectedmeteorite out of every 50, 000 would probably cross the earth's track, while every earth-projected meteorite would necessarily do so. 

If this view be true, then there must be hosts of meteorites traversingspace in elliptic orbits around the sun. These orbits have one featurein common: they all intersect the track of the earth. It will sometimeshappen that the earth is found at this point at the moment the meteoriteis crossing; when this is the case the long travels of the little bodyare at an end, and it tumbles back on the earth from which it parted somany ages ago. 

It is well to emphasise the contrast between the lunar theory ofmeteorites (which we think improbable) and the terrestrial theory (whichappears to be probable). For the lunar theory it would, as we have seen, be necessary that some of the lunar volcanoes should be still active. Inthe terrestrial theory it is only necessary to suppose that thevolcanoes on the earth once possessed sufficient explosive power. No onesupposes that the volcanoes at present on the earth eject now thefragments which are to form future meteorites; but it seems possiblethat the earth may be now slowly gathering back, in these quiet times, the fragments she ejected in an early stage of her history. Assuming, therefore, with Tschermak, that many meteorites have had a volcanicorigin on some considerable celestial body, we are led to agree withthose who think that most probably that body is the earth. 

It is interesting to notice a few circumstances which seem tocorroborate the view that many meteorites are of ancient terrestrialorigin. The most characteristic constituent of these bodies is the alloyof iron and nickel, which is almost universally present. Sometimes, asin the Rowton siderite, the whole object consists of little else;sometimes this alloy is in grains distributed through the mass. WhenNordenskjöld discovered in Greenland a mass of native iron containingnickel, this was at once regarded as a celestial visitor. It was calledthe Ovifak meteorite, and large pieces of the iron were conveyed to ourmuseums. There is, for instance, in the national collection a mostinteresting exhibit of the Ovifak substance. Close examination showsthat this so-called meteorite lies in a bed of basalt which has beenvomited from the interior of the earth. Those who believe in themeteoric origin of the Ovifak iron are constrained to admit that shortlyafter the eruption of the basalt, and while it was still soft, thisstupendous iron meteorite of gigantic mass and bulk happened to fallinto this particular soft bed. The view is, however, steadily gainingground that this great iron mass was no celestial visitor at all, butthat it simply came forth from the interior of the earth with the basaltitself. The beautiful specimens in the British Museum show how the irongraduates into the basalt in such a way as to make it highly probablethat the source of the iron is really to be sought in the earth and notexternal thereto. Should further research establish this, as now seemsprobable, a most important step will have been taken in proving theterrestrial origin of meteorites. If the Ovifak iron be reallyassociated with the basalt, we have a proof that the iron-nickel alloyis indeed a terrestrial substance, found deep in the interior of theearth, and associated with volcanic phenomena. This being so, it will beno longer difficult to account for the iron in undoubted meteorites. When the vast volcanoes were in activity they ejected masses of thisiron-alloy, which, having circulated round the sun for ages, have atlast come back again. As if to confirm this view, Professor Andrewsdiscovered particles of native iron in the basalt of the Giant'sCauseway, while the probability that large masses of iron are thereassociated with the basaltic formation was proved by the researches onmagnetism of the late Provost Lloyd. 

Besides the more solid meteorites there can be no doubt that the_débris_ of the ordinary shooting stars must rain down upon the earth ingentle showers of celestial dust. The snow in the Arctic regions hasoften been found stained with traces of dust which contains particles ofiron. Similar particles have been found on the towers of cathedrals andin many other situations where it could only have been deposited fromthe air. There can be hardly a doubt that some of the motes in thesunbeam, and many of the particles which good housekeepers abhor asdust, have indeed a cosmical origin. In the famous cruise of the_Challenger_ the dredges brought up from the depths of the Atlantic no"wedges of gold, great anchors, heaps of pearl, " but among the mud whichthey raised are to be found numerous magnetic particles which there isevery reason to believe fell from the sky, and thence subsided to thedepths of the ocean. Sand from the deserts of Africa, when examinedunder the microscope, yield traces of minute iron particles which bearthe marks of having experienced a high temperature. 

The earth draws in this cosmic dust continuously, but the earth nownever parts with a particle of its mass. The consequence is inevitable;the mass of the earth must be growing, and though the change may be asmall one, yet to those who have studied Darwin's treatise on"Earth-worms, " or to those who are acquainted with the modern theory ofevolution, it will be manifest that stupendous results can be achievedby slight causes which tend in one direction. It is quite probable thatan appreciable part of the solid substance of our globe may have beenderived from meteoric matter which descends in perennial showers uponits surface. 

CHAPTER XVIII. 

THE STARRY HEAVENS. 

 The Constellations--The Great Bear and the Pointers--The Pole Star--Cassiopeia--Andromeda, Pegasus, and Perseus--The Pleiades: Auriga, Capella, Aldebaran--Taurus, Orion, Sirius; Castor and Pollux--The Lion--Boötes, Corona, and Hercules--Virgo and Spica--Vega and Lyra--The Swan. 

The student of astronomy should make himself acquainted with theprincipal constellations in the heavens. This is a pleasing acquirement, and might well form a part of the education of every child in thekingdom. We shall commence our discussion of the sidereal system with abrief account of the principal constellations visible in the northernhemisphere, and we accompany our description with such outline maps ofthe stars as will enable the beginner to identify the chief features ofthe starry heavens. 

In an earlier chapter we directed the attention of the student to theremarkable constellation of stars which is known to astronomers as UrsaMajor, or the Great Bear. It forms the most conspicuous group in thenorthern skies, and in northern latitudes it never sets. At eleven p. M. In the month of April the Great Bear is directly overhead (for anobserver in the United Kingdom); at the same hour in September it is lowdown in the north; at the same hour July it is in the west; by Christmasit is at the east. From the remotest antiquity this group of stars hasattracted attention. The stars in the Great Bear were comprised in agreat catalogue of stars, made two thousand years ago, which has beenhanded down to us. From the positions of the stars given in thiscatalogue it is possible to reconstruct the Great Bear as it appeared inthose early days. This has been done, and it appears that the sevenprincipal stars have not changed in this lapse of time to any largeextent, so that the configuration of the Great Bear remains practicallythe same now as it was then. The beginner must first obtain anacquaintance with this group of seven stars, and then his furtherprogress in this branch of astronomy will be greatly facilitated. TheGreat Bear is, indeed, a splendid constellation, and its only rival isto be found in Orion, which contains more brilliant stars, though itdoes not occupy so large a region in the heavens. 

[Illustration: Fig. 80. --The Great Bear and Pole Star. ]

[Illustration: Fig. 81. --The Great Bear and Cassiopeia. ]

In the first place, we observe how the Great Bear enables the Pole Star, which is the most important object in the northern heavens, to bereadily found. The Pole Star is very conveniently indicated by thedirection of the two stars, b and a, of the Great Bear, whichare, accordingly, generally known as the "pointers. " This use of theGreat Bear is shown on the diagram in Fig. 80, in which the line ba, produced onwards and slightly curved, will conduct to the PoleStar. There is no likelihood of making any mistake in this star, as itis the only bright one in the neighbourhood. Once it has been seen itwill be readily identified on future occasions, and the observer willnot fail to notice how constant is the position which it preserves inthe heavens. The other stars either rise or set, or, like the GreatBear, they dip down low in the north without actually setting, but thePole Star exhibits no considerable changes. In summer or winter, bynight or by day, the Pole Star is ever found in the same place--atleast, so far as ordinary observation is concerned. No doubt, when weuse the accurate instruments of the observatory the notion of the fixityof the Pole Star is abandoned; we then see that it has a slow motion, and that it describes a small circle every twenty-four hours around thetrue pole of the heavens, which is not coincident with the Pole Star, though closely adjacent thereto. The distance is at present a littlemore than a degree, and it is gradually lessening, until, in the yearA. D. 2095, the distance will be under half a degree. 

The Pole Star itself belongs to another inconsiderable group of starsknown as the Little Bear. The two principal members of this group, nextin brightness to the Pole Star, are sometimes called the "Guards. " TheGreat Bear and the Little Bear, with the Pole Star, form a group in thenorthern sky not paralleled by any similarly situated constellation inthe southern heavens. At the South Pole there is no conspicuous star toindicate its position approximately--a circumstance disadvantageous toastronomers and navigators in the southern hemisphere. 

It will now be easy to add a third constellation to the two alreadyacquired. On the opposite side of the Pole Star to the Great Bear, andat about the same distance, lies a very pleasing group of five brightstars, forming a W. These are the more conspicuous members of theconstellation Cassiopeia, which contains altogether about sixty starsvisible to the naked eye. When the Great Bear is low down in the north, then Cassiopeia is high overhead. When the Great Bear is high overhead, then Cassiopeia is to be looked for low down in the north. Theconfiguration of the leading stars is so striking that once the eye hasrecognised them future identification will be very easy--the more sowhen it is borne in mind that the Pole Star lies midway betweenCassiopeia and the Great Bear (Fig. 81). These important constellationswill serve as guides to the rest. We shall accordingly show how thelearner may distinguish the various other groups visible from theBritish Islands or similar northern latitudes. 

The next constellation to be recognised is the imposing group whichcontains the Great Square of Pegasus. This is not, like Ursa Major, orlike Cassiopeia, said to be "circumpolar. " The Great Square of Pegasussets and rises daily. It cannot be seen conveniently during the springand the summer, but in autumn and in winter the four stars which markthe corners of the square can be easily recognised. There are certainsmall stars within the region so limited; perhaps about thirty can becounted by an unaided eye of ordinary power in these latitudes. In thesouth of Europe, with its pure and bright skies, the number of visiblestars appears to be greatly increased. An acute observer at Athens hascounted 102 in the same region. 

[Illustration: Fig. 82. --The Great Square of Pegasus. ]

The Great Square of Pegasus can be reached by a line from the Pole Starover the end of Cassiopeia. If it be produced about as far again it willconduct the eye to the centre of the Great Square of Pegasus (Fig. 82). 

The line through b and a in Pegasus continued 45° to thesouth points out the important star Fomalhaut in the mouth of theSouthern Fish. To the right of this line, nearly half-way down, is therather vague constellation of Aquarius, where a small equilateraltriangle with a star in the centre may be noticed. 

The square of Pegasus is not a felicitous illustration of the way inwhich the boundaries of the constellations should be defined. There canbe no more naturally associated group than the four stars of thissquare, and they ought surely to be included in the same constellation. Three of the stars--marked a, b, g--do belong to Pegasus; but that atthe fourth corner--also marked a--is placed in a different figure, knownas Andromeda, whereof it is, indeed, the brightest member. The remainingbright stars of Andromeda are marked b and g, and they are readilyidentified by producing one side of the Square of Pegasus in a curveddirection. We have thus a remarkable array of seven stars, which it isboth easy to identify and easy to remember, notwithstanding that theyare contributed to by three different constellations. They arerespectively a, b, and g of Pegasus; a, b, and g of Andromeda; and a ofPerseus. The three form a sort of handle, as it were, extending from oneside of the square, and are a group both striking in appearance, anduseful in the further identification of celestial objects. B Andromedæ, with two smaller stars, form the girdle of the unfortunate heroine. 

a Persei lies between two other stars (g and d) of thesame constellation. If we draw a curve through these three and prolongit in a bold sweep, we are conducted to one of the gems of the northernheavens--the beautiful star Capella, in Auriga (Fig. 83). Close toCapella are three small stars forming an isosceles triangle--these arethe Hoedi or Kids. Capella and Vega are, with the exception ofArcturus, the two most brilliant stars in the northern heavens; andthough Vega is probably the more lustrous of the two, yet the oppositeopinion has been entertained. Different eyes will frequently formvarious estimates of the relative brilliancy of stars which approacheach other in brightness. The difficulty of making a satisfactorycomparison between Vega and Capella is greatly increased by the widedistance in the heavens at which they are separated, as well as by aslight difference in colour, for Vega is distinctly whiter than Capella. This contrast between the colour of stars is often a source ofuncertainty in the attempt to compare their relative brilliancy; sothat when actual measurements have to be effected by instrumental means, it is necessary to compare the two stars alternately with some object ofintermediate hue. 

[Illustration: Fig. 83. --Perseus and its Neighbouring Stars. ]

On the opposite side of the pole to Capella, but not quite so far away, will be found four small stars in a quadrilateral. They form the head ofthe Dragon, the rest of whose form coils right round the pole. 

If we continue the curve formed by the three stars g, a, andd in Perseus, and if we bend round this curve gracefully into oneof an opposite flexion, in the manner shown in Fig. 83, we are firstconducted to two other principal stars in Perseus, marked e andz. The region of Perseus is one of the richest in the heavens. Wehave here a most splendid portion of the Milky Way, and the field ofthe telescope is crowded with stars beyond number. Even a smalltelescope or an opera-glass directed to this teeming constellationcannot fail to delight the observer, and convey to him a profoundimpression of the extent of the sidereal heavens. We shall give in asubsequent paragraph a brief enumeration of some of the remarkabletelescopic objects in Perseus. Pursuing in the same figure the linee and z, we are conducted to the remarkable little groupknown as the Pleiades. 

[Illustration: Fig. 84. --The Pleiades. ]

The Pleiades form a group so universally known and so easily identifiedthat it hardly seems necessary to give any further specific instructionsfor their discovery. It may, however, be observed that in theselatitudes they cannot be seen before midnight during the summer. Let ussuppose that the search is made at about 11 p. M. At night: on the 1st ofJanuary the Pleiades will be found high up in the sky in the south-west;on the 1st of March, at the same hour, they will be seen to be settingin the west. On the 1st of May they are not visible; on the 1st of Julythey are not visible; on the 1st of September they will be seen low downin the east. On the 1st of November they will be high in the heavens inthe south-east. On the ensuing 1st of January the Pleiades will be inthe same position as they were on the same date in the previous year, and so on from year to year. It need, perhaps, hardly be explained herethat these changes are not really due to movements of theconstellations; they are due, of course, to the apparent annual motionof the sun among the stars. 

[Illustration: Fig. 85. --Orion, Sirius, and the Neighbouring Stars. ]

The Pleiades are shown in the figure (Fig. 84), where a group of tenstars is represented, this being about the number visible with theunaided eye to those who are gifted with very acute vision. The lowesttelescopic power will increase the number of stars to thirty or forty(Galileo saw more than forty with his first telescope), while withtelescopes of greater power the number is largely increased; indeed, nofewer than 625 have been counted with the aid of a powerful telescope. The group is, however, rather too widely scattered to make an effectivetelescopic object, except with a large field and low power. Viewedthrough an opera-glass it forms a very pleasing spectacle. 

[Illustration: Fig. 86. --Castor and Pollux. ]

If we draw a ray from the Pole Star to Capella, and produce itsufficiently far, as shown in Fig. 85, we come to the greatconstellation of our winter sky, the splendid group of Orion. Thebrilliancy of the stars in Orion, the conspicuous belt, and thetelescopic objects which it contains, alike render this groupremarkable, and place it perhaps at the head of the constellations. Theleading star in Orion is known either as a Orionis, or asBetelgeuze, by which name it is here designated. It lies above the threestars, d, e, z, which form the belt. Betelgeuze is astar of the first magnitude, and so also is Rigel, on the opposite sideof the belt. Orion thus enjoys the distinction of containing two starsof the first magnitude in its group, while the five other stars shown inFig. 85 are of the second magnitude. 

The neighbourhood of Orion contains some important stars. If we carry onthe line of the belt upwards to the right, we are conducted to anotherstar of the first magnitude, Aldebaran, which strongly resemblesBetelgeuze in its ruddy colour. Aldebaran is the brightest star in theconstellation of Taurus. It is this constellation which contains thePleiades already referred to, and another more scattered group known asthe Hyades, which can be discovered near Aldebaran. 

[Illustration: Fig. 87. --The Great Bear and the Lion. ]

The line of the belt of Orion continued downwards to the left conductsthe eye to the gem of the sky, the splendid Sirius, which is the mostbrilliant star in the heavens. It has, indeed, been necessary to createa special order of magnitude for the reception of Sirius alone; all theother first magnitude stars, such as Vega and Capella, Betelgeuze andAldebaran, coming a long way behind. Sirius, with a few other stars ofmuch less lustre, form the constellation of Canis Major. 

It is useful for the learner to note the large configuration, of anirregular lozenge shape, of which the four corners are the firstmagnitude stars, Aldebaran, Betelgeuze, Sirius, and Rigel (Fig. 85). The belt of Orion is placed symmetrically in the centre of the group, and the whole figure is so striking that once perceived it is not likelyto be forgotten. 

About half way from the Square of Pegasus to Aldebaran is the chief starin the Ram--a bright orb of the second magnitude; with two others itforms a curve, at the other end of which will be found g of thesame constellation, which was the first double star ever noticed. 

We can again invoke the aid of the Great Bear to point out the stars inthe constellation of Gemini (Fig. 86). If the diagonal joining the starsd and b of the body of the Bear be produced in the directionopposite to the tail, it will lead to Castor and Pollux, two remarkablestars of the second magnitude. This same line carried a little furtheron passes near the star Procyon, of the first magnitude, which is theonly conspicuous object in the constellation of the Little Dog. 

[Illustration: Fig. 88. --Boötes and the Crown. ]

[Illustration: Fig. 89. --Virgo and the neighbouring Constellations. ]

The pointers in the Great Bear marked a b will also serve toindicate the constellation of the Lion. If we produce the line joiningthem in the direction opposite from that used in finding the Pole, weare brought into the body of the Lion. This group will be recognised bythe star of the first magnitude called Regulus. It is one of a series ofstars forming an object somewhat resembling a sickle: three of the groupare of the second magnitude. The Sickle has a special claim on ournotice because it contains the radiant point from which the periodicshooting star shower known as the Leonids diverges. Regulus liesalongside the sun's highway through the stars, at a point which hepasses on the 21st of August every year. 

Between Gemini and Leo the inconspicuous constellation of the Crab maybe found; the most striking object it contains is the misty patch calledPræsepe or the Bee-Hive, which the smallest opera-glass will resolveinto its component stars. 

[Illustration: Fig. 90. --The Constellation of Lyra. ]

The tail of the Great Bear, when prolonged with a continuation of thecurve which it possesses, leads to a brilliant star of the firstmagnitude known as Arcturus, the principal star in the constellation ofBoötes (Fig. 88). A few other stars, marked b, g, d, and e in the same constellation, are also shown in the figure. Among the stars visible in these latitudes Arcturus is to be placed nextto Sirius in point of brightness. Two stars in the southern hemisphere, invisible in these latitudes, termed a Centauri and Canopus, arenearly as bright as Vega and Capella, but not quite as bright asArcturus. 

In the immediate neighbourhood of Boötes is a striking semicirculargroup known as the Crown or Corona Borealis. It will be readily foundfrom its position as indicated in the figure, or it may be identified byfollowing the curved line indicated by b, d, e, and z in the Great Bear. 

[Illustration: Fig. 91. --Vega, the Swan, and the Eagle. ]

The constellation of Virgo is principally characterised by the firstmagnitude star called Spica, or a Virginis. This may be found fromthe Great Bear; for if the line joining the two stars a andg in that constellation be prolonged with a slight curve, it willconduct the eye to Spica. We may here notice another of those largeconfigurations which are of great assistance in the study of the stars. There is a fine equilateral triangle, whereof Arcturus and Spica formtwo of the corners, while the third is indicated by Denebola, the brightstar near the tail of the Lion (Fig. 89). 

In the summer evenings when the Crown is overhead, a line from the PoleStar through its fainter edge, continued nearly to the southern horizon, encounters the brilliant red star Cor Scorpionis, or the Scorpion'sHeart (Antares), which was the first star mentioned as having been seenwith the telescope in the daytime. 

The first magnitude star, Vega, in the constellation of the Lyre, canbe readily found at the corner of a bold triangle, of which the PoleStar and Arcturus form the base (Fig. 90). The brilliant whiteness ofVega will arrest the attention, while the small group of neighbouringstars which form the Lyre produces one of the best definedconstellations. 

Near Vega is another important constellation, known as the Swan orCygnus. The brightest star will be identified as the vertex of aright-angled triangle, of which the line from Vega to the Pole Star isthe base, as shown in Fig. 91. There are in Cygnus five principal stars, which form a constellation of rather remarkable form. 

The last constellation which we shall here describe is that of Aquila orthe Eagle, which contains a star of the first magnitude, known asAltair; this group can be readily found by a line from Vega over bCygni, which passes near the line of three stars, forming thecharacteristic part of the Eagle. 

We have taken the opportunity to indicate in these sketches of theconstellations the positions of some other remarkable telescopicobjects, the description of which we must postpone to the followingchapters. 

CHAPTER XIX. 

THE DISTANT SUNS. 

 Sirius Contrasted with the Sun--Stars can be Weighed, but not in general Measured--The Companion of Sirius--Determination of the Weights of Sirius and his Companion--Dark Stars--Variable and Temporary Stars--Enormous Number of Stars. 

The splendid pre-eminence of Sirius has caused it to be observed withminute care from the earliest times in the history of astronomy. Eachgeneration of astronomers devoted time and labour to determine the exactplaces of the brightest stars in the heavens. A vast mass ofobservations as to the place of Sirius among the stars had thus beenaccumulated, and it was found that, like many other stars, Sirius hadwhat astronomers call _proper motion_. Comparing the place of Siriuswith regard to the other stars now with the place which it occupied onehundred years ago, there is a difference of two minutes (127") in itssituation. This is a small quantity: it is so small that the unaided eyecould not see it. Could we now see the sky as it appeared one centuryago, we should still see this star in its well-known place to the leftof Orion. Careful alignment by the eye would hardly detect that Siriuswas moving in two, or even in three or in four centuries. But theaccuracy of the meridian circle renders these minute quantities evident, and gives to them their true significance. To the eye of the astronomer, Sirius, instead of creeping along with a movement which centuries willnot show, is pursuing its majestic course with a velocity appropriate toits dimensions. 

Though the velocity of Sirius is _about_ 1, 000 miles a minute, yet itis sometimes a little more and sometimes a little less than its meanvalue. To the astronomer this fact is pregnant with information. WereSirius an isolated star, attended only by planets of comparativeinsignificance, there could be no irregularity in its motion. If it wereonce started with a velocity of 1, 000 miles a minute, then it mustpreserve that velocity. Neither the lapse of centuries nor the mightylength of the journey could alter it. The path of Sirius would beinflexible in its direction; and it would be traversed with unalterablevelocity. 

[Illustration: Fig. 92. --The Orbit of Sirius (Professor Burnham). ]

The fact that Sirius had not been moving uniformly was of such interestthat it arrested the attention of Bessel when he discovered theirregularities in 1844. Believing, as Bessel did, that there must besome adequate cause for these disturbances, it was hardly possible todoubt what the cause must be. When motion is disturbed there must beforce in action, and the only force that we recognise in such cases isthat known as gravitation. But gravity can only act from one body toanother body; so that when we seek for the derangement of Sirius bygravitation, we are obliged to suppose that there must be some mightyand massive body near Sirius. The question was taken up again by Petersand by Auwers, who were able to discover, from the irregularities ofSirius, the nature of the path of the disturbing body. They were able toshow that it must revolve around Sirius in a period of about fiftyyears, and although they could not tell its distance from Sirius, yetthey were able to point out the direction in which it must lie. Fig. 92shows the orbit of Sirius as given by Mr. Burnham, of YerkesObservatory. 

The detection of the attendant of Sirius, and the measures which havebeen made thereon, enable us to determine the weight of this famousstar. Let us attempt to illustrate this subject. It must, no doubt, beadmitted that the numerical estimates we employ have to be received witha certain degree of caution. The companion of Sirius is a difficultobject to observe, and previous to 1896 it had only been followedthrough an arc of 90°. We are, therefore, hardly as yet in a position tospeak with absolute accuracy as to the periodic time in which thecompanion completes its revolution. We may, however, take this time tobe fifty-two years. We also know the distance from Sirius to hiscompanion, and we may take it to be about twenty-one times the distancefrom the earth to the sun. It is useful, in the first place, to comparethe revolution of the companion around Sirius with the revolution of theplanet Uranus around the sun. Taking the earth's distance as unity, theradius of the orbit of Uranus is about nineteen, and Uranus takeseighty-four years to accomplish a complete revolution. We have no planetin the solar system at a distance of twenty-one; but from Kepler's thirdlaw it may be shown that, if there were such a planet, its periodic timewould be about ninety-nine years. We have now the necessary materialsfor making the comparison between the mass of Sirius and the mass ofthe sun. A body revolving around Sirius at a certain distance completesits journey in fifty-two years. To revolve around the sun at the samedistance a body should complete its journey in ninety-nine years. Thequicker the body is moving the greater must be the centrifugal force, and the greater must be the attractive power of the central body. It canbe shown from the principles of dynamics that the attractive power isinversely proportional to the square of the periodic time. Hence, then, the attractive power of Sirius must bear to the attractive power of thesun the proportion which the square of ninety-nine has to the square offifty-two. As the distances are in each case supposed to be equal, theattractive powers will be proportional to the masses, and hence weconclude that the mass of Sirius, together with that of his companion, is to the mass of the sun, together with that of his planet, in theratio of three and a half to one. We had already learned that Sirius wasmuch brighter than the sun; now we have learned that it is also muchmore massive. 

Before we leave the consideration of Sirius, there is one additionalpoint of very great interest which it is necessary to consider. There isa remarkable contrast between the brilliancy of Sirius and hiscompanion. Sirius is a star far transcending all other stars of thefirst magnitude, while his companion is extremely faint. Even if it werecompletely withdrawn from the dazzling proximity of Sirius, thecompanion would be only a small star of the eighth or ninth magnitude, far below the limits of visibility to the unaided eye. To put the matterin numerical language, Sirius is 5, 000 times as bright as its companion, but only about twice as heavy! Here is a very great contrast; and thispoint will appear even more forcible if we contrast the companion ofSirius with our sun. The companion is slightly heavier than our sun; butin spite of its slightly inferior bulk, our sun is much more powerful asa light-giver. One hundred of the companions of Sirius would not give asmuch light as our sun! This is a result of very considerablesignificance. It teaches us that besides the great bodies in theuniverse which attract attention by their brilliancy, there are alsoother bodies of stupendous mass which have but littlebrilliancy--probably some of them possess none at all. This suggests agreatly enhanced conception of the majestic scale of the universe. Italso invites us to the belief that the universe which we behold bearsbut a small ratio to the far larger part which is invisible in thesombre shades of night. In the wide extent of the material universe wehave here or there a star or a mass of gaseous matter sufficientlyheated to be luminous, and thus to become visible from the earth; butour observation of these luminous points can tell us little of theremaining contents of the universe. 

The most celebrated of all the variable stars is that known as Algol, whose position in the constellation of Perseus is shown in Fig. 83. Thisstar is conveniently placed for observation, being visible every nightin our latitude, and its interesting changes can be observed without anytelescopic aid. Everyone who desires to become acquainted with the greattruths of astronomy should be able to recognise this star, and shouldhave also followed it during one of its periods of change. Algol isusually a star of the second magnitude; but in a period between two andthree days, or, more accurately, in an interval of 2 days 20 hours 48minutes and 55 seconds, its brilliancy goes through a most remarkablecycle of variations. The series commences with a gradual decline of thestar's brightness, which in the course of four and a half hours fallsfrom the second magnitude down to the fourth. At this lowest stage ofbrightness Algol remains for about twenty minutes, and then begins toincrease, until in three and a half hours it regains the secondmagnitude, at which it continues for about 2 days 12 hours, when thesame series commences anew. It seems that the period required by Algolto go through its changes is itself subject to a slow but certainvariation. We shall see in a following chapter how it has been provedthat the variability of Algol is due to the occasional interposition ofa dark companion which cuts off a part of the lustre of the star. Allthe circumstances can thus be accounted for, and even the weight and thesize of Algol and its dark companion be determined. 

There are, however, other classes of variable stars, the fluctuation ofwhose light can hardly be due to occasional obscuration by dark bodies. This is particularly the case with those variables which are generallyfaint, but now and then flare up for a short time, after which temporaryexaltation they again sink down to their original condition. The periodsof such changes are usually from six months to two years. The best knownexample of a star of this class was discovered more than three hundredyears ago. It is situated in the constellation Cetus, a little south ofthe equator. This object was the earliest known case of a variable star, except the so-called temporary stars, to which we shall presently refer. The variable in Cetus received the name of Mira, or the wonderful. Theperiod of the fluctuations of Mira Ceti is about eleven months, duringthe greater part of which time the star is of the ninth magnitude, andconsequently invisible to the naked eye. When the proper time hasarrived, its brightness begins to increase rather suddenly. It soonbecomes a conspicuous object of the second or third magnitude. In thiscondition it remains for eight or ten days, and then declines moreslowly than it rose until it is reduced to its original faintness, aboutthree hundred days after the rise commenced. 

More striking to the general observer than the ordinary variable starsare the _temporary stars_ which on rare occasions suddenly make theirappearance in the heavens. The most famous object of this kind was thatwhich blazed out in the beginning of November, 1572, and which whenfirst seen was as bright as Venus at its maximum brightness. It could, indeed, be seen in full daylight by sharp-sighted people. As far ashistory can tell us, no other temporary star has ever been as bright asthis one. It is specially associated with the name of Tycho Brahe, foralthough he was not the discoverer, he made the best observations of theobject, and he proved that it was at a distance comparable with that ofthe ordinary fixed stars. Tycho described carefully the gradual declineof the wonderful star until it disappeared from his view about the endof March, 1574, for the telescope, by which it could doubtless have beenfollowed further, had not yet been invented. During the decline thecolour of the object gradually changed; at first it was white, and bydegrees became yellow, and in the spring of 1573 reddish, likeAldebaran. About May, 1573, we are told somewhat enigmatically that it"became like lead, or somewhat like Saturn, " and so it remained as longas it was visible. What a fund of information our modern spectroscopesand other instruments would supply us with if so magnificent a star wereto burst out in these modern days!

But though we have not in our own times been favoured with a view of atemporary star as splendid as the one seen by Tycho Brahe and hiscontemporaries, it has been our privilege to witness several minoroutbursts of this kind. It seems likely that we should possess morerecords of temporary stars from former times if a better watch had beenkept for them. That is at any rate the impression we get when we see howseveral of the modern stars of this kind have nearly escaped usaltogether, notwithstanding the great number of telescopes which are nowpointed to the sky on every clear night. 

In 1866 a star of the second magnitude suddenly appeared in theconstellation of the crown (Corona Borealis). It was first seen on the12th May, and a few days afterwards it began to fade away. Argelander'smaps of the northern heavens had been published some years previously, and when the position of the new star had been accurately determined, itwas found that it was identical with an insignificant looking starmarked on one of the maps as of the 9-1/2 magnitude. The star exists inthe same spot to this day, and it is of the same magnitude as it wasprior to its spasmodic outburst in 1866. This was the first new starwhich was spectroscopically examined. We shall give in Chapter XXIII. Ashort account of the features of its spectrum. 

The next of these temporary bright stars, Nova Cygni, was first seen byJulius Schmidt at Athens on the 24th November, 1876, when it was betweenthe third and fourth magnitudes, and he maintains that it cannot havebeen conspicuous four days earlier, when he was looking at the sameconstellation. By some inadvertence the news of the discovery was notproperly circulated, and the star was not observed elsewhere for aboutten days, when it had already become considerably fainter. The decreaseof brightness went on very slowly; in October, 1877, the star was onlyof the tenth magnitude, and it continued getting fainter until itreached the fifteenth magnitude; in other words, it became a minutetelescopic star, and it is so still in the very same spot. As this stardid not reach the first or second magnitude it would probably haveescaped notice altogether if Schmidt had not happened to look at theSwan on that particular evening. 

We are not so likely to miss seeing a new star since astronomers havepressed the photographic camera into their service. This became evidentin 1892, when the last conspicuous temporary star appeared in Auriga. Onthe 24th January, Dr. Anderson, an astronomer in Edinburgh, noticed ayellowish star of the fifth magnitude in the constellation Auriga, and aweek later, when he had compared a star-map with the heavens and madesure that the object was really a new star, he made his discoverypublic. In the case of this star we are able to fix fairly closely themoment when it first blazed out. In the course of the regularphotographic survey of the heavens undertaken at the Harvard CollegeObservatory (Cambridge, Massachusetts) the region of the sky where thenew star appeared had been photographed on thirteen nights from October21st to December 1st, 1891, and on twelve nights from December 10th toJanuary 20th, 1892. On the first series of plates there was no trace ofthe Nova, while it was visible on the very first plate of the secondseries as a star of the fifth magnitude. Fortunately it turned out thatProfessor Max Wolf of Heidelberg, a most successful celestialphotographer, had photographed the same region on the 8th December, andthis photograph does not show the star, so that it cannot on that nighthave been as bright as the ninth magnitude. Nova Auriga must thereforehave flared up suddenly between the 8th and the 10th of December. According to the Harvard photographs, the first maximum of brightnessoccurred about the 20th of December, when the magnitude was 4-1/2. Thedecrease of the brightness was very irregular; the star fluctuated forthe five weeks following the first of February between the fourth andthe sixth magnitude, but after the beginning of March, 1892, thebrightness declined very rapidly, and at the end of April the star wasseen as an exceedingly faint one (sixteenth magnitude) with the greatLick Refractor. When this mighty instrument was again pointed to theNova in the following August, it had risen nearly to the tenthmagnitude, after which it gradually became extremely faint again, and isso still. 

The temporary and the variable stars form but a very small section ofthe vast number of stars with which the vault of the heavens is studded. That the sun is no more than a star, and the stars are no less thansuns, is a cardinal doctrine of astronomy. The imposing magnificence ofthis truth is only realised when we attempt to estimate the countlessmyriads of stars. This is a problem on which our calculations arenecessarily vain. Let us, therefore, invoke the aid of the poet toattempt to express the innumerable, and conclude this chapter with thefollowing lines of Mr. Allingham:--

 "But number every grain of sand, Wherever salt wave touches land; Number in single drops the sea; Number the leaves on every tree, Number earth's living creatures, all That run, that fly, that swim, that crawl; Of sands, drops, leaves, and lives, the count Add up into one vast amount, And then for every separate one Of all those, let a flaming SUN Whirl in the boundless skies, with each Its massy planets, to outreach All sight, all thought: for all we see Encircled with infinity, Is but an island. "

CHAPTER XX. 

DOUBLE STARS. 

 Interesting Stellar Objects--Stars Optically Double--The Great Discovery of the Binary Stars made by Herschel--The Binary Stars describe Elliptic Paths--Why is this so important?--The Law of Gravitation--Special Double Stars--Castor--Mizar--The Coloured Double Stars--b Cygni. 

The sidereal heavens contain few more interesting objects for thetelescope than can be found in the numerous class of double stars. Theyare to be counted in thousands; indeed, _many_ thousands can be found inthe catalogues devoted to this special branch of astronomy. Many ofthese objects are, no doubt, small and comparatively uninteresting, butsome of them are among the most conspicuous stars in the heavens, suchas Sirius, whose system we have already described. We shall in thisbrief account select for special discussion and illustration a few ofthe more remarkable double stars. We shall particularly notice some ofthose that can be readily observed with a small telescope, and we haveindicated on the sketches of the constellations in a previous chapterhow the positions of these objects in the heavens can be ascertained. 

It had been shown by Cassini in 1678 that certain stars, which appearedto the unaided eye as single points of light, really consisted of two ormore stars, so close together that the telescope was required for theirseparation. [36] The number of these objects was gradually increased byfresh discoveries, until in 1781 (the same year in which Herscheldiscovered Uranus) a list containing eighty double stars was publishedby the astronomer Bode. These interesting objects claimed the attentionof Herschel during his memorable researches. The list of known doublesrapidly swelled. Herschel's discoveries are to be enumerated byhundreds, while he also commenced systematic measurements of thedistance by which the stars were separated, and the direction in whichthe line joining them pointed. It was these measurements whichultimately led to one of the most important and instructive of allHerschel's discoveries. When, in the course of years, his observationswere repeated, Herschel found that in some cases the relative positionof the stars had changed. He was thus led to the discovery that in manyof the double stars the components are so related that they revolvearound each other. Mark the importance of this result. We must rememberthat the stars are suns, comparable, it may be, with our sun inmagnitude; so that here we have the astonishing spectacle of pairs ofsuns in mutual revolution. There is nothing very surprising in the factthat movements should be observed, for in all probability every body inthe universe is in motion. It is the particular character of themovement which is specially interesting and instructive. 

It had been imagined that the proximity of the two stars forming adouble must be only accidental. It was thought that amid the vast hostof stars in the heavens it not unfrequently happened that one star wasso nearly behind another (as seen from the earth) that when the two wereviewed in the telescope they produced the effect of a double star. Nodoubt many of the so-called double stars are produced in this way. Herschel's discovery shows that this explanation will not always answer, but that in many cases we really have two stars close together, and inmotion round their common centre of gravity. 

When the measurements of the distances and the positions of double starshad been accumulated during many years, they were taken over by themathematicians to be treated by their methods. There is one peculiarityabout double star observations: they have not--they cannot have--theaccuracy which the computer of an orbit demands. If the distance betweenthe pair of stars forming a binary be four seconds, the orbit we have toscrutinise is only as large as the apparent size of a penny-piece at thedistance of one mile. It would require very careful measurement to makeout the form of a penny a mile off, even with good telescopes. If thepenny were tilted a little, it would appear, not circular, but oval; andit would be possible, by measuring this oval, to determine how much thepenny was tilted. All this requires skilful work: the errors, viewedintrinsically, may not be great, but viewed with reference to the wholesize of the quantities under consideration, they are very appreciable. We therefore find the errors of observation far more prominent inobservations of this class than is generally the case when themathematician assumes the task of discussing the labours of theobserver. 

The interpretation of Herschel's discovery was not accomplished byhimself; the light of mathematics was turned on his observations of thebinary stars by Savary, and afterwards by other mathematicians. Undertheir searching enquiries the errors of the measurements were disclosed, and the observations were purified from the grosser part of theirinaccuracy. Mathematicians could then apply to their corrected materialsthe methods of enquiry with which they were familiar; they could deducewith fair precision the actual shape of the orbit of the binary stars, and the position of the plane in which that orbit is contained. Theresult is not a little remarkable. It has been proved that the motion ofeach of the stars is performed in an ellipse which contains the centreof gravity of the two stars in its focus. This has been actually shownto be true in many binary stars; it is believed to be true in all. Butwhy is this so important? Is not motion in an ellipse common enough?Does not the earth revolve in an ellipse round the sun? And do not theplanets also revolve in ellipses?

It is this very fact that elliptic motion is so common in the planets ofthe solar system which renders its discovery in binary stars of suchimportance. From what does the elliptic motion in the solar systemarise? Is it not due to the law of attraction, discovered by Newton, which states that every mass attracts every other mass with a forcewhich varies inversely as the square of the distance? That law ofattraction had been found to pervade the whole solar system, and itexplained the movements of the bodies of our system with marvellousfidelity. But the solar system, consisting of the sun, and the planets, with their satellites, the comets, and a host of smaller bodies, formedmerely a little island group in the universe. In the economy of thistiny cosmical island the law of gravitation reigns supreme; beforeHerschel's discovery we never could have known whether that law was notmerely a piece of local legislation, specially contrived for theexigencies of our particular system. This discovery gave us theknowledge which we could have gained from no other source. From thebinary stars came a whisper across the vast abyss of space. That whispertold us that the law of gravitation was not peculiar to the solarsystem. It told us the law extended to the distant shores of the abyssin which our island is situated. It gives us grounds for believing thatthe law of gravitation is obeyed throughout the length, breadth, anddepth of the entire visible universe. 

One of the finest binary stars is that known as Castor, the brighter ofthe two principal stars in the constellation of Gemini. The position ofCastor on the heavens is indicated in Fig. 86, page 418. Viewed by theunaided eye, Castor resembles a single star; but with a moderately goodtelescope it is found that what seems to be one star is really twoseparate stars, one of which is of the third magnitude, while the otheris somewhat less. The angular distance of these two stars in the heavensis not so great as the angle subtended by a line an inch long viewed ata distance of half a mile. Castor is one of the double stars in whichthe components have been observed to possess a motion of revolution. Themovement is, however, extremely slow, and the lapse of centuries will berequired before a revolution is completely effected. 

A beautiful double star can be readily identified in the constellationof Ursa Major (_see_ Fig. 80, page 410). It is known as Mizar, and isthe middle star (z) of the three which form the tail. In the closeneighbourhood of Mizar is the small star Alcor, which can be readilyseen with the unaided eye; but when we speak of Mizar as a double star, it is not to be understood that Alcor is one of the components of thedouble. Under the magnifying power of the telescope Alcor is seen to betransferred a long way from Mizar, while Mizar itself is split up intotwo suns close together. These components are of the second and thefourth magnitudes respectively, and as the apparent distance is nearlythree times as great as in Castor, they are observed with facility evenin a small telescope. This is, indeed, the best double star in theheavens for the beginner to commence his observations upon. We cannot, however, assert that Mizar is a binary, inasmuch as observations havenot yet established the existence of a motion of revolution. Still lessare we able to say whether Alcor is also a member of the same group, orwhether it may not merely be a star which happens to fall nearly in theline of vision. Recent spectroscopic observations have shown that thelarger component of Mizar is itself a double, consisting of a pair ofsuns so close together that there is not the slightest possibility oftheir ever being seen separately by the most powerful telescope in theworld. 

A pleasing class of double stars is that in which we have the remarkablephenomenon of colours, differing in a striking degree from the coloursof ordinary stars. Among the latter we find, in the great majority ofcases, no very characteristic hue; some are, however, more or lesstinged with red, some are decidedly ruddy, and some are intensely red. Stars of a bluish or greenish colour are much more rare, [37] and when astar of this character does occur, it is almost invariably as one of apair which form a double. The other star of the double is sometimes ofthe same hue, but more usually it is yellow or ruddy. 

One of the loveliest of these objects, which lies within reach oftelescopes of very moderate pretensions, is that found in theconstellation of the Swan, and known as b Cygni (Fig. 91). Thisexquisite object is composed of two stars. The larger, about the thirdmagnitude, is of a golden-yellow, or topaz, colour; the smaller, of thesixth magnitude, is of a light blue. These colours are nearlycomplementary, but still there can be no doubt that the effect is notmerely one of contrast. That these two stars are both tinged with thehues we have stated can be shown by hiding each in succession behind abar placed in the field of view. It has also been confirmed in a verystriking manner by spectroscopic investigation; for we see that the bluestar has experienced a special absorption of the red rays, while themore ruddy light of the other star has arisen from the absorption of theblue rays. The contrast of the colours in this object can often be veryeffectively seen by putting the eye-piece out of focus. The discs thusproduced show the contrast of colours better than when the telescopeexhibits merely two stellar points. 

Such are a few of these double and multiple stars. Their numbers arebeing annually augmented; indeed, one observer--Mr. Burnham, formerly onthe staff of the Lick Observatory, and now an observer in the YerkesObservatory--has added by his own researches more than 1, 000 new doublesto the list of those previously known. 

The interest in this class of objects must necessarily be increased whenwe reflect that, small as the stars appear to be in our telescopes, theyare in reality suns of great size and splendour, in many cases rivallingour own sun, or, perhaps, even surpassing him. Whether these suns haveplanets attending upon them we cannot tell; the light reflected from theplanet would be utterly inadequate to the penetration of the vast extentof space which separates us from the stars. If there be planetssurrounding these objects, then, instead of a single sun, such planetswill be illuminated by two, or, perhaps, even more suns. What wondrouseffects of light and shade must be the result! Sometimes both suns willbe above the horizon together, sometimes only one sun, and sometimesboth will be absent. Especially remarkable would be the condition of aplanet whose suns were of the coloured type. To-day we have a red sunilluminating the heavens, to-morrow it would be a blue sun, and, perhaps, the day after both the red sun and the blue sun will be in thefirmament together. What endless variety of scenery such a thoughtsuggests! There are, however, grave dynamical reasons for doubtingwhether the conditions under which such a planet would exist could bemade compatible with life in any degree resembling the life with whichwe are familiar. The problem of the movement of a planet under theinfluence of two suns is one of the most difficult that has ever beenproposed to mathematicians, and it is, indeed, impossible in the presentstate of analysis to solve with accuracy all the questions which itimplies. It seems not at all unlikely that the disturbances of theplanet's orbit would be so great that it would be exposed tovicissitudes of light and of temperature far transcending thoseexperienced by a planet moving, like the earth, under the supremecontrol of a single sun. 

CHAPTER XXI. 

THE DISTANCES OF THE STARS. 

 Sounding-line for Space--The Labours of Bessel--Meaning of Annual Parallax--Minuteness of the Parallactic Ellipse Illustrated--The Case of 61 Cygni--Different Comparison Stars used--The Proper Motion of the Star--Struve's Investigations--Can they be Reconciled?--Researches at Dunsink--Conclusion obtained--Accuracy which such Observations admit Examined--The Proper Motion of 61 Cygni--The Permanence of the Sidereal Heavens--The New Star in Cygnus--Its History--No Appreciable Parallax--A Mighty Outburst of Light--The Movement of the Solar System through Space--Herschel's Discovery--Journey towards Lyra--Probabilities. 

We have long known the dimensions of the solar system with more or lessaccuracy. Our knowledge includes the distances of the planets and thecomets from the sun, as well as their movements. We have alsoconsiderable knowledge of the diameters and the masses of many of thedifferent bodies which belong to the solar system. We have long known, in fact, many details of the isolated group nestled together under theprotection of the sun. The problem for consideration in the presentchapter involves a still grander survey than is required for measures ofour solar system. We propose to carry the sounding-line across the vastabyss which separates the group of bodies closely associated about oursun from the other stars which are scattered through the realms ofspace. For centuries the great problem of star distance has engaged theattention of those who have studied the heavens. It would be impossibleto attempt here even an outline of the various researches which havebeen made on the subject. In the limited survey which we can make, wemust glance first at the remarkable speculative efforts which have beendirected to the problem, and then we shall refer to those labours whichhave introduced the problem into the region of accurate astronomy. 

No attempt to solve the problem of the absolute distances of the starswas successful until many years after Herschel's labours were closed. Fresh generations of astronomers, armed with fresh appliances, have formany years pursued the subject with unremitting diligence, but for along time the effort seemed hopeless. The distances of the stars were sogreat that they could not be ascertained until the utmost refinements ofmechanical skill and the most elaborate methods of mathematicalcalculation were brought to converge on the difficulty. At last it wasfound that the problem was beginning to yield. A few stars have beeninduced to disclose the secret of their distance. We are able to givesome answer to the question--How far are the stars? though it must beconfessed that our reply up to the present moment is both hesitating andimperfect. Even the little knowledge which has been gained possessesinterest and importance. As often happens in similar cases, thediscovery of the distance of a star was made independently about thesame time by two or three astronomers. The name of Bessel stands outconspicuously in this memorable chapter of astronomy. Bessel proved(1840) that the distance of the star known as 61 Cygni was a measurablequantity. His demonstration possessed such unanswerable logic thatuniversal assent could not be withheld. Almost simultaneously with theclassical labours of Bessel we have Struve's measurement of the distanceof Vega, and Henderson's determination of the distance of the southernstar a Centauri. Great interest was excited in the astronomicalworld by these discoveries, and the Royal Astronomical Society awardedits gold medal to Bessel. It appropriately devolved on Sir John Herschelto deliver the address on the occasion of the presentation of the medal:that address is a most eloquent tribute to the labours of the threeastronomers. We cannot resist quoting the few lines in which Sir Johnsaid:--

 "Gentlemen of the Royal Astronomical Society, --I congratulate you and myself that we have lived to see the great and hitherto impassable barrier to our excursion into the sidereal universe, that barrier against which we have chafed so long and so vainly--_æstuantes angusto limite mundi_--almost simultaneously overleaped at three different points. It is the greatest and most glorious triumph which practical astronomy has ever witnessed. Perhaps I ought not to speak so strongly; perhaps I should hold some reserve in favour of the bare possibility that it may be all an illusion, and that future researches, as they have repeatedly before, so may now fail to substantiate this noble result. But I confess myself unequal to such prudence under such excitement. Let us rather accept the joyful omens of the time, and trust that, as the barrier has begun to yield, it will speedily be effectually prostrated. "

Before proceeding further, it will be convenient to explain briefly howthe distance of a star can be measured. The problem is one of a whollydifferent character from that of the sun's distance, which we havealready discussed in these pages. The observations for the determinationof stellar parallax are founded on the familiar truth that the earthrevolves around the sun. We may for our present purpose assume that theearth revolves in a circular path. The centre of that path is at thecentre of the sun, and the radius of the path is 92, 900, 000 miles. Owingto our position on the earth, we observe the stars from a point of viewwhich is constantly changing. In summer the earth is 185, 800, 000 milesdistant from the position which it occupied in winter. It follows thatthe apparent positions of the stars, as projected on the background ofthe sky, must present corresponding changes. We do not now mean that theactual positions of the stars are really displaced. The changes are onlyapparent, and while oblivious of our own motion, which produces thedisplacements, we attribute the changes to the stars. 

On the diagram in Fig. 93 is an ellipse with certain months--viz. , January, April, July, October--marked upon its circumference. Thisellipse may be regarded as a miniature picture of the earth's orbitaround the sun. In January the earth is at the spot so marked; in Aprilit has moved a quarter of the whole journey; and so on round the wholecircle, returning to its original position in the course of one year. When we look from the position of the earth in January, we see the starA projected against the point of the sky marked 1. Three months laterthe observer with his telescope is carried round to April; but he nowsees the star projected to the position marked 2. Thus, as the observermoves around the whole orbit in the annual revolution of the earth, sothe star appears to move round in an ellipse on the background of thesky. In the technical language of astronomers, we speak of this as theparallactic ellipse, and it is by measuring the major axis of thisellipse that we determine the distance of the star from the sun. Half ofthis major axis, or, what comes to the same thing, the angle which theradius of the earth's orbit subtends as seen from the star, is calledthe star's "annual parallax. "

[Illustration: Fig. 93. --The Parallactic Ellipse. ]

The figure shows another star, B, more distant from the earth and thesolar system generally than the star previously considered. This staralso describes an elliptic path. We cannot, however, fail to notice thatthe parallactic ellipse belonging to B is much smaller than that of A. The difference in the sizes of the ellipses arises from the differentdistances of the stars from the earth. The nearer the star is to theearth the greater is the ellipse, so that the nearest star in theheavens will describe the largest ellipse, while the most distant starwill describe the smallest ellipse. We thus see that the distance of thestar is inversely proportional to the size of the ellipse, and if wemeasure the angular value of the major axis of the ellipse, then, by anexceedingly simple mathematical manipulation, the distance of the starcan be expressed as a multiple of a radius of the earth's orbit. Assuming that radius to be 92, 900, 000 miles, the distance of the star isobtained by simple arithmetic. The difficulty in the process arises fromthe fact that these ellipses are so small that our micrometers oftenfail to detect them. 

How shall we adequately describe the extreme minuteness of theparallactic ellipses in the case of even the nearest stars? In thetechnical language of astronomers, we may state that the longestdiameter of the ellipse never subtends an angle of more than one and ahalf seconds. In a somewhat more popular manner, we would say that onethousand times the major axis of the very largest parallactic ellipsewould not be as great as the diameter of the full moon. For a still moresimple illustration, let us endeavour to think of a penny-piece placedat a distance of two miles. If looked at edgeways it will be linear, iftilted a little it would be elliptic; but the ellipse would, even atthat distance, be greater than the greatest parallactic ellipse of anystar in the sky. Suppose a sphere described around an observer, with aradius of two miles. If a penny-piece were placed on this sphere, infront of each of the stars, every parallactic ellipse would be totallyconcealed. 

The star in the Swan known as 61 Cygni is not remarkable either for itssize or for its brightness. It is barely visible to the unaided eye, andthere are some thousands of stars which are apparently larger andbrighter. It is, however, a very interesting example of that remarkableclass of objects known as double stars. It consists of two nearly equalstars close together, and evidently connected by a bond of mutualattraction. The attention of astronomers is also specially directedtowards the star by its large proper motion. In virtue of that propermotion, the two components are carried together over the sky at the rateof five seconds annually. A proper motion of this magnitude is extremelyrare, yet we do not say it is unparalleled, for there are some few starswhich have a proper motion even more rapid; but the remarkable duplexcharacter of 61 Cygni, combined with the large proper motion, render itan unique object, at all events, in the northern hemisphere. 

When Bessel proposed to undertake the great research with which his namewill be for ever connected, he determined to devote one, or two, orthree years to the continuous observations of one star, with the view ofmeasuring carefully its parallactic ellipse. How was he to select theobject on which so much labour was to be expended? It was all-importantto choose a star which should prove sufficiently near to reward hisefforts by exhibiting a measurable parallax. Yet he could have butlittle more than surmise and analogy as a guide. It occurred to him thatthe exceptional features of 61 Cygni afforded the necessary presumption, and he determined to apply the process of observation to this star. Hedevoted the greater part of three years to the work, and succeeded indiscovering its distance from the earth. 

Since the date of Sir John Herschel's address, 61 Cygni has received thedevoted and scarcely remitted attention of astronomers. In fact, wemight say that each succeeding generation undertakes a new discussion ofthe distance of this star, with the view of confirming or of criticisingthe original discovery of Bessel. The diagram here given (Fig. 94) isintended to illustrate the recent history of 61 Cygni. 

When Bessel engaged in his labours, the pair of stars forming the doublewere at the point indicated on the diagram by the date 1838. The nextepoch occurred fifteen years later, when Otto Struve undertook hisresearches, and the pair of stars had by that time moved to theposition marked 1853. Finally, when the same object was more recentlyobserved at Dunsink Observatory, the pair had made still anotheradvance, to the position indicated by the date 1878. Thus, in fortyyears this double star had moved over an arc of the heavens upwards ofthree minutes in length. The actual path is, indeed, more complicatedthan a simple rectilinear movement. The two stars which form the doublehave a certain relative velocity, in consequence of their mutualattraction. It will not, however, be necessary to take this intoaccount, as the displacement thus arising in the lapse of a single yearis far too minute to produce any inconvenient effect on the parallacticellipse. 

[Illustration: Fig. 94. --61 Cygni and the Comparison Stars. ]

The case of 61 Cygni is, however, exceptional. It is one of our nearestneighbours in the heavens. We can never find its distance accurately toone or two billions of miles; but still we have a consciousness that anuncertainty amounting to twenty billions is too large a percentage ofthe whole. We shall presently show that we believe Struve was right, yet it does not necessarily follow that Bessel was wrong. The apparentparadox can be easily explained. It would not be easily explained ifStruve had used the _same comparison star_ as Bessel had done; butStruve's comparison star was different from either of Bessel's, and thisis probably the cause of the discrepancy. It will be recollected thatthe essence of the process consists of the comparison of the smallellipse made by the distant star with the larger ellipse made by thenearer star. If the two stars were at the same distance, the processwould be wholly inapplicable. In such a case, no matter how near thestars were to the earth, no parallax could be detected. For the methodto be completely successful, the comparison star should be at leasteight times as far as the principal star. Bearing this in mind, it isquite possible to reconcile the measures of Bessel with those of Struve. We need only assume that Bessel's comparison stars are about three timesas far as 61 Cygni, while Struve's comparison star is at least eight orten times as far. We may add that, as the comparison stars used byBessel are brighter than that of Struve, there really is a presumptionthat the latter is the most distant of the three. 

We have here a characteristic feature of this method of determiningparallax. Even if all the observations and the reductions of a parallaxseries were mathematically correct, we could not with strict proprietydescribe the final result as the parallax of one star. It is only the_difference_ between the parallax of the star and that of the comparisonstar. We can therefore only assert that the parallax sought cannot beless than the quantity determined. Viewed in this manner, thediscrepancy between Struve and Bessel vanishes. Bessel asserted that thedistance of 61 Cygni could not be _more_ than sixty billions of miles. Struve did not contradict this--nay, he certainly confirmed it--when heshowed that the distance could not be more than forty billions. 

Nearly half a century has elapsed since Struve made his observations. Those observations have certainly been challenged; but they are, on thewhole, confirmed by other investigations. In a critical review of thesubject Auwers showed that Struve's determination is worthy ofconsiderable confidence. Yet, notwithstanding this authoritativeannouncement, the study of 61 Cygni has been repeatedly resumed. Dr. Brünnow, when Astronomer Royal of Ireland, commenced a series ofobservations on the parallax of 61 Cygni, which were continued andcompleted by the present writer, his successor. Brünnow chose a fourthcomparison star (marked on the diagram), different from any of thosewhich had been used by the earlier observers. The method of observingwhich Brünnow employed was quite different from that of Struve, thoughthe filar micrometer was used in both cases. Brünnow sought to determinethe parallactic ellipse by measuring the difference in declinationbetween 61 Cygni and the comparison star. [38] In the course of a year itis found that the difference in declination undergoes a periodic change, and from that change the parallactic ellipse can be computed. In thefirst series of observations I measured the difference of declinationbetween the preceding star of 61 Cygni and the comparison star; in thesecond series I took the other component of 61 Cygni and the samecomparison star. We had thus two completely independent determinationsof the parallax resulting from two years' work. The first of these makesthe distance forty billions of miles, and the second makes it almostexactly the same. There can be no doubt that this work supports Struve'sdetermination in correction of Bessel's, and therefore we may perhapssum up the present state of our knowledge of this question by sayingthat the distance of 61 Cygni is much nearer to the forty billions ofmiles which Struve found than to the sixty billions which Besselfound. [39]

It is desirable to give the reader the means of forming his own opinionas to the quality of the evidence which is available in such researches. The diagram in Fig. 95 here shown has been constructed with this object. It is intended to illustrate the second series of observations ofdifference of declination which I made at Dunsink. Each of the dotsrepresents one night's observations. The height of the dot is theobserved difference of declination between 61 (B) Cygni and thecomparison star. The distance along the horizontal line--or theabscissa, as a mathematician would call it--represents the date. Theseobservations are grouped more or less regularly in the vicinity of acertain curve. That curve expresses where the observations should havebeen, had they been absolutely perfect. The distances between the dotsand the curve may be regarded as the errors which have been committed inmaking the observations. 

[Illustration: Fig. 95. --Parallax in Declination of 61 Cygni. ]

Perhaps it will be thought that in many cases these errors appear tohave attained very undesirable dimensions. Let us, therefore, hasten tosay that it was precisely for the purpose of setting forth these errorsthat this diagram has been shown; we have to exhibit the weakness of thecase no less than its strength. The errors of the observations are not, however, intrinsically so great as might at first sight be imagined. Toperceive this, it is only necessary to interpret the scale on which thisdiagram has been drawn by comparison with familiar standards. Thedistance from the very top of the curve to the horizontal line denotesan angle of only four-tenths of a second. This is about the apparentdiameter of a penny-piece at a distance of _ten miles_! We can nowappraise the true magnitude of the errors which have been made. It willbe noticed that no one of the dots is distant from the curve by muchmore than half of the height of the curve. It thus appears that thegreatest error in the whole series of observations amounts to but two orthree tenths of a second. This is equivalent to our having pointed thetelescope to the upper edge of a penny-piece fifteen or twenty milesoff, instead of to the lower edge. This is not a great blunder. A rifleteam whose errors in pointing were more than a hundred times as greatmight still easily win every prize at Bisley. 

We have entered into the history of 61 Cygni with some detail, becauseit is the star whose distance has been most studied. We do not say that61 Cygni is the nearest of all the stars; it would, indeed, be very rashto assert that any particular star was the nearest of all the countlessmillions in the heavenly host. We certainly know one star which seemsnearer than 61 Cygni; it lies in one of the southern constellations, andits name is a Centauri. This star is, indeed, of memorableinterest in the history of the subject. Its parallax was firstdetermined at the Cape of Good Hope by Henderson; subsequent researcheshave confirmed his observations, and the elaborate investigations of Dr. Gill have proved that the parallax of this star is about three-quartersof a second, so that it is only two-thirds of the distance of 61 Cygni. 

61 Cygni arrested our attention, in the first instance, by thecircumstance that it had the large proper motion of five secondsannually. We have also ascertained that the annual parallax is abouthalf a second. The combination of these two statements leads to a resultof considerable interest. It teaches us that 61 Cygni must each yeartraverse a distance of not less than ten times the radius of the earth'sorbit. Translating this into ordinary figures, we learn that this starmust travel nine hundred and twenty million miles per annum. It mustmove between two and three million miles each day, but this can only beaccomplished by maintaining the prodigious velocity of thirty miles persecond. There seems to be no escape from this conclusion. The factswhich we have described, and which are now sufficiently wellestablished, are inconsistent with the supposition that the velocity of61 Cygni is less than thirty miles per second; the velocity may begreater, but less it cannot be. 

For the last hundred and fifty years we know that 61 Cygni has beenmoving in the same direction and with the same velocity. Prior to theexistence of the telescope we have no observation to guide us; wecannot, therefore, be absolutely certain as to the earlier history ofthis star, yet it is only reasonable to suppose that 61 Cygni has beenmoving from remote antiquity with a velocity comparable with that it hasat present. If disturbing influences were entirely absent, there couldbe no trace of doubt about the matter. _Some_ disturbing influence, however, there must be; the only question is whether that disturbinginfluence is sufficient to modify seriously the assumption we have made. A powerful disturbing influence might greatly alter the velocity of thestar; it might deflect the star from its rectilinear course; it mighteven force the star to move around a closed orbit. We do not, however, believe that any disturbing influence of this magnitude need becontemplated, and there can be no reasonable doubt that 61 Cygni movesat present in a path very nearly straight, and with a velocity verynearly uniform. 

As the distance of 61 Cygni from the sun is forty billions of miles, andits velocity is thirty miles a second, it is easy to find how long thestar would take to accomplish a journey equal to its distance from thesun. The time required will be about 40, 000 years. In the last 400, 000years 61 Cygni will have moved over a distance ten times as great asits present distance from the sun, whatever be the direction of motion. This star must therefore have been about ten times as far from the earth400, 000 years ago as it is at present. Though this epoch is incrediblymore remote than any historical record, it is perhaps not incomparablewith the duration of the human race; while compared with the vast lapseof geological time, such periods seem trivial and insignificant. Geologists have long ago repudiated mere thousands of years; they nowclaim millions, and many millions of years, for the performance ofgeological phenomena. If the earth has existed for the millions of yearswhich geologists assert, it becomes reasonable for astronomers tospeculate on the phenomena which have transpired in the heavens in thelapse of similar ages. By the aid of our knowledge of star distances, combined with an assumed velocity of thirty miles per second, we canmake the attempt to peer back into the remote past, and show how greatare the changes which our universe seems to have undergone. 

In a million years 61 Cygni will apparently have moved through adistance which is twenty-five times as great as its present distancefrom the sun. Whatever be the direction in which 61 Cygni ismoving--whether it be towards the earth or from the earth, to the rightor to the left, it must have been about twenty-five times as far off amillion years ago as it is at present; but even at its present distance61 Cygni is a small star; were it ten times as far it could only be seenwith a good telescope; were it twenty-five times as far it would barelybe a visible point in our greatest telescopes. 

The conclusions arrived at with regard to 61 Cygni may be applied withvarying degrees of emphasis to other stars. We are thus led to theconclusion that many of the stars with which the heavens are strewn areapparently in slow motion. But this motion though apparently slow mayreally be very rapid. When standing on the sea-shore, and looking at asteamer on the distant horizon, we can hardly notice that the steamer ismoving. It is true that by looking again in a few minutes we can detecta change in its place; but the motion of the steamer seems slow. Yet ifwe were near the steamer we would find that it was rushing along at therate of many miles an hour. It is the distance which causes theillusion. So it is with the stars: they seem to move slowly because theyare very distant, but were we near them, we could see that in themajority of cases their motions are a thousand times as fast as thequickest steamer that ever ploughed the ocean. 

It thus appears that the permanence of the sidereal heavens, and thefixity of the constellations in their relative positions, are onlyephemeral. When we rise to the contemplation of such vast periods oftime as the researches of geology disclose, the durability of theconstellations vanishes! In the lapse of those stupendous ages stars andconstellations gradually dissolve from view, to be replaced by others ofno greater permanence. 

It not unfrequently happens that a parallax research proves abortive. The labour has been finished, the observations are reduced anddiscussed, and yet no value of the parallax can be obtained. Thedistance of the star is so vast that our base-line, although it isnearly two hundred millions of miles long, is too short to bear anyappreciable ratio to the distance of the star. Even from such failures, however, information may often be drawn. 

Let me illustrate this by an account derived from my own experience atDunsink. We have already mentioned that on the 24th November, 1876, awell-known astronomer--Dr. Schmidt, of Athens--noticed a new bright starof the third magnitude in the constellation Cygnus. On the 20th ofNovember Nova Cygni was invisible. Whether it first burst forth on the21st, 22nd, or 23rd no one can tell; but on the 24th it was discovered. Its brilliancy even then seemed to be waning; so, presumably, it wasbrightest at some moment between the 20th and 24th of November. Theoutbreak must thus have been comparatively sudden, and we know of nocause which would account for such a phenomenon more simply than agigantic collision. The decline in the brilliancy was much more tardythan its growth, and more than a fortnight passed before the starrelapsed into insignificance--two or three days (or less) for the rise, two or three weeks for the fall. Yet even two or three weeks was a shorttime in which to extinguish so mighty a conflagration. It iscomparatively easy to suggest an explanation of the sudden outbreak; itis not equally easy to understand how it can have been subdued in a fewweeks. A good-sized iron casting in one of our foundries takes nearly asmuch time to cool as sufficed to abate the celestial fires in NovaCygni!

On this ground it seemed not unreasonable to suppose that perhaps NovaCygni was not really a very extensive conflagration. But, if such werethe case, the star must have been comparatively _near_ to the earth, since it presented so brilliant a spectacle and attracted so muchattention. It therefore appeared a plausible object for a parallaxresearch; and consequently a series of observations were made some yearsago at Dunsink. I was at the time too much engaged with other work todevote very much labour to a research which might, after all, only proveillusory. I simply made a sufficient number of micrometric measurementsto test whether a _large_ parallax existed. It has been already pointedout how each star appears to describe a minute parallactic ellipse, inconsequence of the annual motion of the earth, and by measurement ofthis ellipse the parallax--and therefore the distance--of the star canbe determined. In ordinary circumstances, when the parallax of a star isbeing investigated, it is necessary to measure the position of the starin its ellipse on many different occasions, distributed over a period ofat least an entire year. The method we adopted was much less laborious. It was sufficiently accurate to test whether or not Nova Cygni had a_large_ parallax, though it might not have been delicate enough todisclose a small parallax. At a certain date, which can be readilycomputed, the star is at one end of the parallactic ellipse, and sixmonths later the star is at the other end. By choosing suitable times inthe year for our observations, we can measure the star in those twopositions when it is most deranged by parallax. It was by observationsof this kind that I sought to detect the parallax of Nova Cygni. Itsdistance from a neighbouring star was carefully measured by themicrometer at the two seasons when, if parallax existed, those distancesshould show their greatest discrepancy; but no certain differencebetween these distances could be detected. The observations, therefore, failed to reveal the existence of a parallactic ellipse--or, in otherwords, the distance of Nova Cygni was too great to be measured byobservations of this kind. 

It is certain that if Nova Cygni had been one of the nearest stars theseobservations would not have been abortive. We are therefore entitled tobelieve that Nova Cygni must be at least 20, 000, 000, 000, 000 miles fromthe solar system; and the suggestion that the brilliant outburst was ofsmall dimensions must, it seems, be abandoned. The intrinsic brightnessof Nova Cygni, when at its best, cannot have been greatly if at allinferior to the brilliancy of our sun himself. If the sun were withdrawnfrom us to the distance of Nova Cygni, it would seemingly have dwindleddown to an object not more brilliant than the variable star. How thelustre of such a stupendous object declined so rapidly remains, therefore, a mystery not easy to explain. Have we not said that theoutbreak of brilliancy in this star occurred between the 20th and the24th of November, 1876? It would be more correct to say that the tidingsof that outbreak reached our system at the time referred to. The realoutbreak must have taken place at least three years previously. Indeed, at the time that the star excited such commotion in the astronomicalworld here, it had already relapsed again into insignificance. 

In connection with the subject of the present chapter we have toconsider a great problem which was proposed by Sir William Herschel. Hesaw that the stars were animated by proper motion; he saw also that thesun is a star, one of the countless host of heaven, and he was thereforeled to propound the stupendous question as to whether the sun, like theother stars which are its peers, was also in motion. Consider all thatthis great question involves. The sun has around it a retinue ofplanets and their attendant satellites, the comets, and a host ofsmaller bodies. The question is, whether all this superb system isrevolving around the sun _at rest_ in the middle, or whether the wholesystem--sun, planets, and all--is not moving on bodily through space. 

Herschel was the first to solve this noble problem; he discovered thatour sun and the splendid retinue by which it is attended are moving inspace. He not only discovered this, but he ascertained the direction inwhich the system was moving, as well as the approximate velocity withwhich that movement was probably performed. It has been shown that thesun and his system is now hastening towards a point of the heavens nearthe constellation Lyra. The velocity with which the motion is performedcorresponds to the magnitude of the system; quicker than the swiftestrifle-bullet that was ever fired, the sun, bearing with it the earth andall the other planets, is now sweeping onwards. We on the earthparticipate in that motion. Every half hour we are something like tenthousand miles nearer to the constellation of Lyra than we should havebeen if the solar system were not animated by this motion. As we areproceeding at this stupendous rate towards Lyra, it might at first besupposed that we ought soon to get there; but the distances of the starsin that neighbourhood seem not less than those of the stars elsewhere, and we may be certain that the sun and his system must travel at thepresent rate for far more than a million years before we have crossedthe abyss between our present position and the frontiers of Lyra. Itmust, however, be acknowledged that our estimate of the actual _speed_with which our solar system is travelling is exceedingly uncertain, butthis does not in the least affect the fact that we are moving in thedirection first approximately indicated by Herschel (_see_ ChapterXXIII. ). 

It remains to explain the method of reasoning which Herschel adopted, bywhich he was able to make this great discovery. It may sound strange tohear that the detection of the motion of the sun was not made by lookingat the sun; all the observations of the luminary itself with all thetelescopes in the world would never tell us of that motion, for thesimple reason that the earth, whence our observations must be made, participates in it. A passenger in the cabin of a ship usually becomesaware that the ship is moving by the roughness of the sea; but if thesea be perfectly calm, then, though the tables and chairs in the cabinare moving as rapidly as the ship, yet we do not see them moving, because we are also travelling with the ship. If we could not go out ofthe cabin, nor look through the windows, we would never know whether theship was moving or at rest; nor could we have any idea as to thedirection in which the ship was going, or as to the velocity with whichthat motion was performed. 

The sun, with his attendant host of planets and satellites, may belikened to the ship. The planets may revolve around the sun just as thepassengers may move about in the cabin, but as the passengers, bylooking at objects on board, can never tell whither the ship is going, so we, by merely looking at the sun, or at the other planets or membersof the solar system, can never tell if our system as a whole is inmotion. 

The conditions of a perfectly uniform movement along a perfectly calmsea are not often fulfilled on the waters with which we are acquainted, but the course of the sun and his system is untroubled by anydisturbance, so that the majestic progress is conducted with absoluteuniformity. We do not feel the motion; and as all the planets aretravelling with us, we can get no information from them as to the commonmotion by which the whole system is animated. 

The passengers are, however, at once apprised of the ship's motion whenthey go on deck, and when they look at the sea surrounding them. Let ussuppose that their voyage is nearly accomplished, that the distant landappears in sight, and, as evening approaches, the harbour is discernedinto which the ship is to enter. Let us suppose that the harbour has, asis often the case, a narrow entrance, and that its mouth is indicated bya lighthouse on each side. When the harbour is still a long way off, near the horizon, the two lights are seen close together, and now thatthe evening has closed in, and the night has become quite dark, thesetwo lights are all that remain visible. While the ship is still somemiles from its destination the two lights seem close together, but asthe distance decreases the two lights seem to open out; gradually theship gets nearer, while the lights are still opening, till finally, whenthe ship enters the harbour, instead of the two lights being directly infront, as at the commencement, one of the lights is passed by on theright hand, while the other is similarly found on the left. If, then, weare to discover the motion of the solar system, we must, like thepassenger, look at objects unconnected with our system, and learn ourown motion by their apparent movements. But are there any objects in theheavens unconnected with our system? If all the stars were like theearth, merely the appendages of our sun, then we never could discoverwhether we were at rest or whether we were in motion: our system mightbe in a condition of absolute rest, or it might be hurrying on with aninconceivably great velocity, for anything we could tell to thecontrary. But the stars do not belong to the system of our sun; theyare, rather, suns themselves, and do not recognise the sway of our sun, as this earth is obliged to do. The stars will, therefore, act as theexternal objects by which we can test whether our system is voyagingthrough space. 

With the stars as our beacons, what ought we to expect if our system bereally in motion? Remember that when the ship was approaching theharbour the lights gradually opened out to the right and left. But theastronomer has also lights by which he can observe the navigation ofthat vast craft, our solar system, and these lights will indicate thepath along which he is borne. If our solar system be in motion, weshould expect to find that the stars were gradually spreading away fromthat point in the heavens towards which our motion tends. This isprecisely what we do find. The stars in the constellations are graduallyspreading away from a central point near the constellation of Lyra, andhence we infer that it is towards Lyra that the motion of the solarsystem is directed. 

There is one great difficulty in the discussion of this question. Havewe not had occasion to observe that the stars themselves are in actualmotion? It seems certain that every star, including the sun himself as astar, has each an individual motion of its own. The motions of the starsas we see them are partly apparent as well as partly real; they partlyarise from the actual motion of each star and partly from the motion ofthe sun, in which we partake, and which produces an apparent motion ofthe star. How are these to be discriminated? Our telescopes and ourobservations can never effect this decomposition directly. To accomplishthe analysis, Herschel resorted to certain geometrical methods. Hismaterials at that time were but scanty, but in his hands they provedadequate, and he boldly announced his discovery of the movement of thesolar system. 

So astounding an announcement demanded the severest test which the mostrefined astronomical resources could suggest. There is a certainpowerful and subtle method which astronomers use in the effort tointerpret nature. Bishop Butler has said that probability is the guideof life. The proper motion of a star has to be decomposed into twoparts, one real and the other apparent. When several stars are taken, wemay conceive an infinite number of ways into which the movements of eachstar can be so decomposed. Each one of these conceivable divisions willhave a certain element of probability in its favour. It is the businessof the mathematician to determine the amount of that probability. Thecase, then, is as follows:--Among all the various systems one must betrue. We cannot lay our finger for certain on the true one, but we cantake that which has the highest degree of probability in its favour, andthus follow the precept of Butler to which we have already referred. Amathematician would describe his process by calling it the method ofleast squares. Since Herschel's discovery, one hundred years ago, manyan astronomer using observations of hundreds of stars has attacked thesame problem. Mathematicians have exhausted every refinement which thetheory of probabilities can afford, but only to confirm the truth ofthat splendid theory which seems to have been one of the flashes ofHerschel's genius. 

CHAPTER XXII. 

STAR CLUSTERS AND NEBULÆ. 

 Interesting Sidereal Objects--Stars not Scattered uniformly--Star Clusters--Their Varieties--The Cluster in Perseus--The Globular Cluster in Hercules--The Milky Way--A Cluster of Minute Stars--The Magellanic Clouds--Nebulæ distinct from Clouds--Number of known Nebulæ--The Constellation of Orion--The Position of the Great Nebula--The Wonderful Star th Orionis--The Drawing of the Great Nebula in Lord Rosse's Telescope--Photographs of this Wonderful Object--The Great Nebula in Andromeda--The Annular Nebula in Lyra--Resemblance to Vortex Rings--Planetary Nebulæ--Drawings of Several Remarkable Nebulæ--Nature of Nebulæ--Spectra of Nebulæ--Their Distribution; the Milky Way. 

We have already mentioned Saturn as one of the most glorious telescopicspectacles in the heavens. Setting aside the obvious claims of the sunand of the moon, there are, perhaps, two other objects visible fromthese latitudes which rival Saturn in the splendour and the interest oftheir telescopic picture. One of these objects is the star cluster inHercules; the other is the great nebula in Orion. We take these objectsas typical of the two great classes of bodies to be discussed in thischapter, under the head of Star Clusters and Nebulæ. 

The stars, which to the number of several millions bespangle the sky, are not scattered uniformly. We can see that while some regions arecomparatively barren, others contain stars in profusion. Sometimes wehave a small group, like the Pleiades; sometimes we have a stupendousregion of the heavens strewn over with stars, as in the Milky Way. Suchobjects are called star clusters. We find every variety in the clusters;sometimes the stars are remarkable for their brilliancy, sometimes fortheir enormous numbers, and sometimes for the remarkable form in whichthey are grouped. Sometimes a star cluster is adorned withbrilliantly-coloured stars; sometimes the luminous points are so closetogether that their separate rays cannot he disentangled; sometimes thestars are so minute or so distant that the cluster is barelydistinguishable from a nebula. 

Of the clusters remarkable at once both for richness and brilliancy ofthe individual stars, we may mention the cluster in the Sword-handle ofPerseus. The position of this object is marked on Fig. 83, page 415. Tothe unaided eye a hazy spot is visible, which in the telescope expandsinto two clusters separated by a short distance. In each of them we haveinnumerable stars, crowded together so as to fill the field of view ofthe telescope. The splendour of this object may be appreciated when wereflect that each one of these stars is itself a brilliant sun, perhapsrivalling our own sun in lustre. There are, however, regions in theheavens near the Southern Cross, of course invisible from northernlatitudes, in which parts of the Milky Way present a richer appearanceeven than the cluster in Perseus. 

The most striking type of star cluster is well exhibited in theconstellation of Hercules. In this case we have a group of minute starsapparently in a roughly globular form. Fig. 96 represents this object asseen in Lord Rosse's great telescope, and it shows three radiatingstreaks, in which the stars seem less numerous than elsewhere. It isestimated that this cluster must contain from 1, 000 to 2, 000 stars, allconcentrated into an extremely small part of the heavens. Viewed in avery small telescope, this object resembles a nebula. The position ofthe cluster in Hercules is shown in a diagram previously given (Fig. 88, page 420). We have already referred to this glorious aggregation ofstars as one of the three especially interesting objects in the heavens. 

[Illustration: PLATE D. 

MILKY WAY NEAR MESSIER II. 

_Photographed by E. E. Barnard, 29th June, 1892. _]

The Milky Way forms a girdle which, with more or less regularity, sweeps completely around the heavens; and when viewed with thetelescope, is seen to consist of myriads of minute stars. In some placesthe stars are much more numerous than elsewhere. All these stars areincomparably more distant than the sun, which they surround, so it isevident that our sun and, of course, the system which attends him lieactually inside the Milky Way. It seems tempting to pursue the thoughthere suggested, and to reflect that the whole Milky Way may, after all, be merely a star cluster, comparable in size with some of the other starclusters which we see, and that, viewed from a remote point in space, the Milky Way would seem to be but one of the many clusters of starscontaining our sun as an indistinguishable unit. 

[Illustration: Fig. 96. --The Globular Cluster in Hercules. ]

In the southern hemisphere there are two immense masses which areconspicuously visible to the naked eye, and resemble detached portionsof the Milky Way. They cannot be seen by observers in our latitude, andare known as the Magellanic clouds or the two nubeculæ. Their structure, as revealed to an observer using a powerful telescope, is of greatcomplexity. Sir John Herschel, who made a special study of theseremarkable objects, gives the following description of them: "Thegeneral ground of both consists of large tracts and patches ofnebulosity in every stage of resolution, from light irresolvable, in areflector of eighteen inches aperture, up to perfectly separated starslike the Milky Way, and clustering groups sufficiently insulated andcondensed to come under the designation of irregular and in some casespretty rich clusters. But besides these there are also nebulæ inabundance and globular clusters in every state of condensation. " It canhardly be doubted that the two nubeculæ, which are, roughly speaking, round, or, rather, oval, are not formed accidentally by a vast number ofvery different objects being ranged at various distances along the sameline of sight, but that they really represent two great systems ofobjects, widely different in constitution, which here are congregated ineach other's neighbourhood, whereas they generally do not co-exist closeto each other in the Milky Way, with which the mere naked-eye view wouldotherwise lead us to associate the Magellanic clouds. 

When we direct a good telescope to the heavens, we shall occasionallymeet with one of the remarkable celestial objects which are known asnebulæ. They are faint cloudy spots, or stains of light on the blackbackground of the sky. They are nearly all invisible to the naked eye. These celestial objects must not for a moment be confounded with clouds, in the ordinary meaning of the word. The latter exist only suspended inthe atmosphere, while nebulæ are immersed in the depths of space. Cloudsshine by the light of the sun, which they reflect to us; nebulæ shinewith no borrowed light; they are self-luminous. Clouds change from hourto hour; nebulæ do not change even from year to year. Clouds are farsmaller than the earth; while the smallest nebula known to us isincomparably greater than the sun. Clouds are within a few miles of theearth; the nebulæ are almost inconceivably remote. 

Immediately after Herschel and his sister had settled at Slough hecommenced his review of the northern heavens in a systematic manner. Forobservations of this kind it is essential that the sky be free fromcloud, while even the light of the moon is sufficient to obliterate thefainter and more interesting objects. It was in the long and finewinter nights, when the stars were shining brilliantly and the pale pathof the Milky Way extended across the heavens, that the labour was to bedone. The telescope being directed to the heavens, the ordinary diurnalmotion by which the sun and stars appear to rise and set carries thestars across the field of view in a majestic panorama. The stars enterslowly into the field of view, slowly move across it, and slowly leaveit, to be again replaced by others. Thus the observer, by merelyremaining passive at the eye-piece, sees one field after another passbefore him, and is enabled to examine their contents. It follows, thateven without moving the telescope a long narrow strip of the heavens isbrought under review, and by moving the telescope slightly up and downthe width of this strip can be suitably increased. On another night thetelescope is brought into a different position, and another strip of thesky is examined; so that in the course of time the whole heavens can becarefully scrutinised. 

Herschel stands at the eye-piece to watch the glorious procession ofcelestial objects. Close by, his sister Caroline sits at her desk, penin hand, to take down the observations as they fall from her brother'slips. In front of her is a chronometer from which she can note the time, and a contrivance which indicates the altitude of the telescope, so thatshe can record the exact position of the object in connection with thedescription which her brother dictated. Such was the splendid schemewhich this brother and sister had arranged to carry out as the object oftheir life-long devotion. The discoveries which Herschel was destined tomake were to be reckoned not by tens or by hundreds, but by thousands. The records of these discoveries are to be found in the "PhilosophicalTransactions of the Royal Society, " and they are among the richesttreasures of those volumes. It was left to Sir John Herschel, the onlyson of Sir William, to complete his father's labour by repeating thesurvey of the northern heavens and extending it to the southernhemisphere. He undertook with this object a journey to the Cape of GoodHope, and sojourned there for the years necessary to complete the greatwork. 

[Illustration: Fig. 97. --The Constellation of Orion, showing thePosition of the Great Nebula. ]

As the result of the gigantic labours thus inaugurated and continued byother observers, there are now about eight thousand nebulæ known to us, and with every improvement of the telescope fresh additions are beingmade to the list. They differ from one another as eight thousand pebblesselected at random on a sea-beach might differ--namely, in form, size, colour, and material--but yet, like the pebbles, bear a certain genericresemblance to each other. To describe this class of bodies in anydetail would altogether exceed the limits of this chapter; we shallmerely select a few of the nebulæ, choosing naturally those of the mostremarkable character, and also those which are representatives of thedifferent groups into which nebulæ may be divided. 

[Illustration: PLATE XIV. 

THE GREAT NEBULA IN ORION. ]

We have already stated that the great nebula in the constellation ofOrion is one of the most interesting objects in the heavens. It is alikeremarkable whether we consider its size or its brilliancy, the care withwhich it has been studied, or the success which has attended the effortsto learn something of its character. To find this object, we refer toFig. 97 for the sketch of the chief stars in this constellation, wherethe letter A indicates the middle one of the three stars which form thesword-handle of Orion. Above the handle will be seen the three starswhich form the well-known belt so conspicuous in the wintry sky. Thestar A, when viewed attentively with the unaided eye, presents asomewhat misty appearance. In the year 1618 Cysat directed a telescopeto this star, and saw surrounding it a curious luminous haze, whichproved to be the great nebula. Ever since his time this object has beendiligently studied by many astronomers, so that very many observationshave been made of the great nebula, and even whole volumes have beenwritten which treat of nothing else. Any ordinary telescope will showthe object to some extent, but the more powerful the telescope the moreare the curious details revealed. 

[Illustration: Fig. 98. --The Multiple star (th Orionis) in theGreat Nebula of Orion. ]

In the first place, the object which we have denoted by A (thOrionis, also called the trapezium of Orion) is in itself the moststriking multiple star in the whole heavens. It consists really of sixstars, represented in the next diagram (Fig. 98). These points are soclose together that their commingled rays cannot be distinguishedwithout a telescope. Four of them are, however, easily seen in quitesmall instruments, but the two smaller stars require telescopes ofconsiderable power. And yet these stars are suns, comparable, it may be, with our sun in magnitude. 

It is not a little remarkable that this unrivalled group of six sunsshould be surrounded by the renowned nebula; the nebula or the multiplestar would, either of them alone, be of exceptional interest, and herewe have a combination of the two. It seems impossible to resist drawingthe conclusion that the multiple star really lies in the nebula, and notmerely along the same line of vision. It would, indeed, seem to be atvariance with all probability to suppose that the presentation of thesetwo exceptional objects in the same field of view was merely accidental. If the multiple star be really in the nebula, then this object affordsevidence that in one case at all events the distance of a nebula is aquantity of the same magnitude as the distance of a star. This isunhappily almost the entire extent of our knowledge of the distances ofthe nebulæ from the earth. 

The great nebula of Orion surrounds the multiple star, and extends outto a vast distance into the neighbouring space. The dotted circle drawnaround the star marked A in Fig. 97 represents approximately the extentof the nebula, as seen in a moderately good telescope. The nebula is ofa faint bluish colour, impossible to represent in a drawing. Itsbrightness is much greater in some places than in others; the centralparts are, generally speaking, the most brilliant, and the luminositygradually fades away as the edge of the nebula is approached. In fact, we can hardly say that the nebula has any definite boundary, for witheach increase of telescopic power faint new branches can be seen. Thereseems to be an empty space in the nebula immediately surrounding themultiple star, but this is merely an illusion, produced by the contrastof the brilliant light of the stars, as the spectroscopic examination ofthe nebula shows that the nebulous matter is continuous between thestars. 

The plate of the great nebula in Orion which is here shown (Plate XIV. )represents, in a reduced form, the elaborate drawing of this object, which has been made with the Earl of Rosse's great reflecting telescopeat Parsonstown. [40] A telescopic view of the nebula shows two hundredstars or more, scattered over its surface. It is not necessary tosuppose that these stars are immersed in the substance of the nebula asthe multiple star appears to be; they may be either in front of it, or, less probably, behind it, so as to be projected on the same part of thesky. 

[Illustration: PLATE XV. 

PHOTOGRAPH OF THE NEBULA 31 M ANDROMEDÆ

EXPOSURE 4 HOURS, ENLARGED 3 TIMES. 

TAKEN BY MR. ISAAC ROBERTS, 29 DECEMBER, 1882. ]

A considerable number of drawings of this unique object have been madeby other astronomers. Among these we must mention that executed byProfessor Bond, in Cambridge, Mass. , which possesses a faithfulness indetail that every student of this object is bound to acknowledge. Oflate years also successful attempts have been made to photograph thegreat nebula. The late Professor Draper was fortunate enough to obtainsome admirable photographs. In England Mr. Common was the first to takemost excellent photographs of the nebula, and superb photographs of thesame object have also been obtained by Dr. Roberts and Mr. W. E. Wilson, which show a vast extension of the nebula into regions which it was notpreviously known to occupy. 

The great nebula in Andromeda, which is faintly visible to the unaidedeye, is shown in Plate XV. , which has been copied with permission fromone of the astonishing photographs that Dr. Isaac Roberts has obtained. Two dark channels in the nebula cannot fail to be noticed, and thenumber of faint stars scattered over its surface is also a point towhich attention may be drawn. To find this object we must look out forCassiopeia and the Great Square of Pegasus, and then the nebula will beeasily perceived in the position shown on p. 413. In the year 1885 a newstar of the seventh magnitude suddenly appeared close to the brightestpart of the nebula, and declined again to invisibility after the lapseof a few months. 

The nebula in Lyra is the most conspicuous ring nebula in the heavens, but it is not to be supposed that it is the only member of this class. Altogether, there are about a dozen of these objects. It seems difficultto form any adequate conception of the nature of such a body. It is, however, impossible to view the annular nebulæ without being, at allevents, reminded of those elegant objects known as vortex rings. Who hasnot noticed a graceful ring of steam which occasionally escapes from thefunnel of a locomotive, and ascends high into the air, only dissolvingsome time after the steam not so specialised has disappeared? Suchvortex rings can be produced artificially by a cubical box, one openside of which is covered with canvas, while on the opposite side of thebox is a circular hole. A tap on the canvas will cause a vortex ring tostart from the hole; and if the box be filled with smoke, this ring willbe visible for many feet of its path. It would certainly be far too muchto assert that the annular nebulæ have any real analogy to vortex rings;but there is, at all events, no other object known to us with which theycan be compared. 

The heavens contain a number of minute but brilliant objects known asthe planetary nebulæ. They can only be described as globes of glowingbluish-coloured gas, often small enough to be mistaken for a star whenviewed through a telescope. One of the most remarkable of these objectslies in the constellation Draco, and can be found half-way between thePole Star and the star g Draconis. Some of the more recentlydiscovered planetary nebulæ are extremely small, and they have indeedonly been distinguished from small stars by the spectroscope. It is alsoto be noticed that such objects are a little out of the stellar focus inthe refracting telescope in consequence of their blue colour. Thisremark does not apply to a reflecting telescope, as this instrumentconducts all the rays to a common focus. 

There are many other forms of nebulæ: there are long nebulous rays;there are the wondrous spirals which have been disclosed in Lord Rosse'sgreat reflector; there are the double nebulæ. But all these variousobjects we must merely dismiss with this passing reference. There is agreat difficulty in making pictorial representations of such nebulæ. Most of them are very faint--so faint, indeed, that they can only beseen with close attention even in powerful instruments. In makingdrawings of these objects, therefore, it is impossible to avoidintensifying the fainter features if an intelligible picture is to bemade. With this caution, however, we present Plate XVI. , which exhibitsseveral of the more remarkable nebulæ as seen through Lord Rosse's greattelescope. 

[Illustration: Fig. 99. --The Nebula N. G. C. , 1, 499. 

(_By E. E. Barnard, Lick Observatory, September 21, 1895. _)]

The actual nature of the nebulæ offers a problem of the greatestinterest, which naturally occupied the mind of the first assiduousobserver of nebulæ, William Herschel, for many years. At first heassumed all nebulæ to be nothing but dense aggregations of stars--a verynatural conclusion for one who had so greatly advanced the optical powerof telescopes, and was accustomed to see many objects which in a smalltelescope looked nebulous become "resolved" into stars when scrutinisedwith a telescope of large aperture. But in 1864, when Sir WilliamHuggins first directed a telescope armed with a spectroscope to one ofthe planetary nebulæ, it became evident that at least some nebulæ werereally clouds of fiery mist and not star clusters. 

We shall in our next chapter deal with the spectra of the fixed stars, but we may here in anticipation remark that these spectra arecontinuous, generally showing the whole length of spectrum, from red toviolet, as in the sun's spectrum, though with many and importantdifferences as to the presence of dark and bright lines. A star clustermust, of course, give a similar spectrum, resulting from thesuperposition of the spectra of the single stars in the cluster. Manynebulæ give a spectrum of this kind; for instance, the great nebula inAndromeda. But it does not by any means follow from this that theseobjects are only clusters of ordinary stars, as a continuous spectrummay be produced not only by matter in the liquid or solid state, or bygases at high pressure, but also by gases at lower pressure but hightemperature under certain conditions. A continuous spectrum in the caseof a nebula, therefore, need not indicate that the nebula is a clusterof bodies comparable in size and general constitution with our sun. Butif a spectrum of bright lines is given by a nebula, we can be certainthat gases at low pressure are present in the object under examination. And this was precisely what Sir William Huggins discovered to be thecase in many nebulæ. When he first decided to study the spectra ofnebulæ, he selected for observation those objects known as planetarynebulæ--small, round, or slightly oval discs, generally without centralcondensation, and looking like ill-defined planets. The colour of theirlight, which often is blue tinted with green, is remarkable, since thisis a colour very rare among single stars. The spectrum was found to betotally different to that of any star, consisting merely of three orfour bright lines. The brightest one is situated in the bluish-greenpart of the spectrum, and was at first thought to be identical with aline of the spectrum of nitrogen, but subsequent more accurate measureshave shown that neither this nor the second nebular line correspond toany dark line in the solar spectrum, nor can they be producedexperimentally in the laboratory, and we are therefore unable to ascribethem to any known element. The third and fourth lines were at once seento be identical with the two hydrogen lines which in the solar spectrumare named F and g. 

[Illustration: PLATE E. 

NEBULÆ IN THE PLEIADES. 

_From a Photograph by Dr. Isaac Roberts. _]

Spectrum analysis has here, as on so many other occasions, renderedservices which no telescope could ever have done. The spectra of nebulæhave, after Huggins, been studied, both visually and photographically, by Vogel, Copeland, Campbell, Keeler, and others, and a great many veryfaint lines have been detected in addition to those four which aninstrument of moderate dimensions shows. It is remarkable that the redC-line of hydrogen, ordinarily so bright, is either absent orexcessively faint in the spectra of nebulæ, but experiments by Franklandand Lockyer have shown that under certain conditions of temperature andpressure the complicated spectrum of hydrogen is reduced to one greenline, the F-line. It is, therefore, not surprising that the spectra ofgaseous nebulæ are comparatively simple, as the probably low density ofthe gases in them and the faintness of these bodies would tend to reducethe spectra to a small number of lines. Some gaseous nebulæ also showfaint continuous spectra, the place of maximum brightness of which isnot in the yellow (as in the solar spectrum), but about the green. It isprobable that these continuous spectra are really an aggregate of veryfaint luminous lines. 

A list of all the nebulæ known to have a gaseous spectrum would nowcontain about eighty members. In addition to the planetary nebulæ, manylarge and more diffused nebulæ belong to this class, and this is alsothe case with the annular nebula in Lyra and the great nebula of Orion. It is needless to say that it is of special interest to find this grandobject enrolled among the nebulæ of a gaseous nature. In this nebulaCopeland detected the wonderful D3 line of helium at a time when"helium" was a mere name, a hypothetical something, but which we nowknow to be an element very widely distributed through the universe. Ithas since been found in several other nebulæ. The ease with which thecharacteristic gaseous spectrum is recognised has suggested the idea ofsweeping the sky with a spectroscope in order to pick up new planetarynebulæ, and a number of objects have actually been discovered byPickering and Copeland in this manner, as also more recently byPickering by examining spectrum photographs of various regions of thesky. Most of these new objects when seen through a telescope look likeordinary stars, and their real nature could never have been detectedwithout the spectroscope. 

When we look up at the starry sky on a clear night, the stars seem atfirst sight to be very irregularly distributed over the heavens. Hereand there a few bright stars form characteristic groups, like Orion orthe Great Bear, while other equally large tracts are almost devoid ofbright stars and only contain a few insignificant ones. If we take abinocular, or other small telescope, and sweep the sky with it, theresult seems to be the same--now we come across spaces rich in stars;now we meet with comparatively empty places. But when we approach thezone of the Milky Way, we are struck with the rapid increase of thenumber of stars which fill the field of the telescope; and when we reachthe Milky Way itself, the eye is almost unable to separate the singlepoints of light, which are packed so closely together that they producethe appearance to the naked eye of a broad, but very irregular, band ofdim light, which even a powerful telescope in some places can hardlyresolve into stars. How are we to account for this remarkablearrangement of the stars? What is the reason of our seeing so few at theparts of the heavens farthest from the Milky Way, and so very many in ornear that wonderful belt? The first attempt to give an answer to thesequestions was made by Thomas Wright, an instrument maker in London, in abook published in 1750. He supposed the stars of our sidereal system tobe distributed in a vast stratum of inconsiderable thickness comparedwith its length and breadth. If we had a big grindstone made of glass, in which had become uniformly imbedded a vast quantity of grains of sandor similar minute particles, and if we were able to place our eyesomewhere near the centre of this grindstone, it is easy to see that weshould see very few particles near the direction of the axle of thegrindstone, but a great many if we looked towards any point of thecircumference. This was Wright's idea of the structure of the Milky Way, and he supposed the sun to be situated not very far from the centre ofthis stellar stratum. 

[Illustration: PLATE F. 

ô CENTAURI. 

_From a Drawing in the Publications of Harvard College Observatory. _]

If the Milky Way itself did not exist--and we had simply the fact tobuild on that the stars appeared to increase rapidly in number towards acertain circle (almost a great circle) spanning the heavens--then thedisc theory might have a good deal in its favour. But the telescopicstudy of the Milky Way, and even more the marvellous photographs of itscomplicated structure produced by Professor Barnard, have given thedeath blow to the old theory, and have made it most reasonable toconclude that the Milky Way is really, and not only apparently, a mightystream of stars encircling the heavens. We shall shortly mention a fewfacts which point in this direction. A mere glance is sufficient to showthat the Milky Way is not a single belt of light; near the constellationAquila it separates into two branches with a fairly broad intervalbetween them, and these branches do not meet again until they haveproceeded far into the southern hemisphere. The disc theory had, inorder to explain this, to assume that the stellar stratum was cleft intwo nearly to the centre. But even if we grant this, how can we accountfor the numerous more or less dark holes in the Milky Way, the largestand most remarkable of which is the so-called "coal sack" in thesouthern hemisphere? Obviously we should have to assume the existence ofa number of tunnels, drilled through the disc-like stratum, and by somestrange sympathy all directed towards the spot where our solar system issituated. And the many small arms which stretch out from the Milky Waywould have to be either planes seen edgeways or the convexities ofcurved surfaces viewed tangentially. The improbability of these variousassumptions is very great. But evidence is not wanting that therelatively bright stars are crowded together along the same zone wherethe excessively faint ones are so closely packed. The late Mr. Proctorplotted all the stars which occur in Argelander's great atlas of thenorthern hemisphere, 324, 198 in number, on a single chart, and thoughthese stars are all above the tenth magnitude, and thus superior inbrightness to that innumerable host of stars of which the individualmembers are more or less lost in the galactic zone, and on thehypothesis of uniform distribution ought to be relatively near to us, the chart shows distinctly the whole course of the Milky Way by theclustering of these stars. This disposes sufficiently of the idea thatthe Milky Way is nothing but a disc-like stratum seen projected on theheavenly sphere; after this it is hardly necessary to examine ProfessorBarnard's photographs and see how fairly bright and very faint regionsalternate without any attempt at regularity, in order to becomeconvinced that the Milky Way is more probably a stream of starsclustered together, a stream or ring of incredibly enormous dimensions, inside which our solar system happens to be situated. But it must beadmitted that it is premature to attempt to find the actual figure ofthis stream or to determine the relative distance of the variousportions of it. 

[Illustration: PLATE XVI. 

NEBULÆ

OBSERVED WITH LORD ROSSE'S GREAT TELESCOPE. ]

CHAPTER XXIII. 

THE PHYSICAL NATURE OF THE STARS. 

 Star Spectroscopes--Classification of Stellar Spectra--Type I. , with very Few Absorption Lines--Type II. , like the Sun--Type III. , with Strongly Marked Dark Bands--Distribution of these Classes over the Heavens--Motion in the Line of Sight--Orbital Motion Discovered with the Spectroscope: New Class of Binaries--Spectra of Temporary Stars--Nature of these Bodies. 

We have frequently in the previous chapters had occasion to refer to therevelations of the spectroscope, which form an important chapter in thehistory of modern science. By its aid a mighty stride has been taken inour attempt to comprehend the physical constitution of the sun. In thepresent chapter we propose to give an account of what the spectroscopetells us about the physical constitution of the fixed stars. 

Quite a new phase of astronomy is here opened up. Every improvement intelescopes revealed fainter and fainter objects, but all the telescopesin the world could not answer the question as to whether iron and otherelements are to be found in the stars. The ordinary star is a mightyglowing globe, hotter than a Bessemer converter or a Siemens furnace; ifiron is in the star, it must be not only white-hot and molten, butactually converted into vapour. But the vapour of iron is not visible inthe telescope. How would you recognise it? How would you know if itcommingled with the vapour of many other metals or other substances? Itis, in truth, a delicate piece of analysis to discriminate iron in theglowing atmosphere of a star. But the spectroscope is adequate to thetask, and it renders its analysis with an amount of evidence that isabsolutely convincing. 

That the spectra of the moon and planets are practically nothing butfaint reproductions of the spectrum of the sun was discovered by thegreat German optician Fraunhofer about the year 1816. By placing a prismin front of the object glass of a small theodolite (an instrument usedfor geodetic measurements) he was able to ascertain that Venus and Marsshowed the same spectrum as the sun, while Sirius gave a very differentone. This important observation encouraged him to procure betterinstrumental means with which to continue the work, and he succeeded indistinguishing the chief characteristics of the various types of stellarspectra. The form of instrument which Fraunhofer adopted for this work, in which the prism was placed outside the object glass of the telescope, has not been much used until within the last few years, owing to thedifficulty of obtaining prisms of large dimensions (for it is obviousthat the prism ought to be as large as the object glass if the fullpower of the latter is to be made use of), but this is the simplest formof spectroscope for observing spectra of objects of no sensible angulardiameter, like the fixed stars. The parallel rays from the stars aredispersed by the prism into a spectrum, and this is viewed by means ofthe telescope. But as the image of the star in the telescope is nothingbut a luminous point, its spectrum will be merely a line in which itwould not be possible to distinguish any lines crossing it laterallysuch as those we see in the spectrum of the sun. A cylindrical lens is, therefore, placed before the eye-piece of the telescope, and as this hasthe effect of turning a point into a line and a line into a band, thenarrow spectrum of the star is thereby broadened out into a luminousband in which we can distinguish any details that exist. In other formsof stellar spectroscope we require a slit which must be placed in thefocus of the object glass, and the general arrangement is similar tothat which we have described in the chapter on the sun, except that acylindrical lens is required. 

The study of the spectra of the fixed stars made hardly any progressuntil the principles of spectrum analysis had been established byKirchhoff in 1859. When the dark lines in the solar spectrum had beenproperly interpreted, it was at once evident that science had openedwide the gates of a new territory for human exploration, of the veryexistence of which hardly anyone had been aware up to that time. We haveseen to what splendid triumphs the study of the sun has led theinvestigators in this field, and we have seen how very valuable resultshave been obtained by the new method when applied to observations ofcomets and nebulæ. We shall now give some account of what has beenlearned with regard to the constitution of the fixed stars by theresearches which were inaugurated by Sir William Huggins and continuedand developed by him, as well as by Secchi, Vogel, Pickering, Lockyer, Dunér, Scheiner and others. Here, as in the other modern branches ofastronomy, photography has played a most important part, not onlybecause photographed spectra of stars extend much farther at the violetend than the observer can follow them with his eye, but also because thepositions of the lines can be very accurately measured on thephotographs. 

The first observer who reduced the apparently chaotic diversity ofstellar spectra to order was Secchi, who showed that they might all begrouped according to four types. Within the last thirty years, however, so many modifications of the various types have been found that it hasbecome necessary to subdivide Secchi's types, and most observers nowmake use of Vogel's classification, which we shall also for convenienceadopt in this chapter. 

_Type I. _--In the spectra of stars of this class the metallic lines, which are so very numerous and conspicuous in the sun's violet spectrum, are very faint and thin, or quite invisible, and the blue and whiteparts are very intensely bright. Vogel subdivides the class into threegroups. In the first (I. A) the hydrogen lines are present, and areremarkably broad and intense; Sirius, Vega, and Regulus are examples ofthis group. The great breadth of the lines probably indicates that thesestars are surrounded by hydrogen atmospheres of great dimensions. It isgenerally acknowledged that stars of this group must be the hottest ofall, and support is lent to this view by the appearance in theirspectra of a certain magnesium line, which, as Sir Norman Lockyer showedmany years ago, by laboratory experiments, does not appear in theordinary spectrum of magnesium, but is indicative of the presence of thesubstance at a very high temperature. In the spectra of stars of GroupI. B the hydrogen lines and the few metallic lines are of equal breadth, and the magnesium line just mentioned is the strongest of all. Rigel andseveral other bright stars in Orion belong to this group, and it isremarkable that helium is present at least in some of these stars, sothat (as Professor Keeler remarks) the spectrum of Rigel may almost beregarded as the nebular spectrum reversed (lines dark instead ofbright), except that the two chief nebular lines are not reversed in thestar. This fact will doubtless eventually be of great importance to ourunderstanding the successive development of a star from a nebula; and astar like Rigel is no doubt also of very high temperature. This isprobably not the case with stars of the third subdivision of Type I. (I. C), the spectra of which are distinguished by the presence of brighthydrogen lines and the bright helium line D3. Among the stars havingthis very remarkable kind of spectrum is a very interesting variablestar in the constellation Lyra (b) and the star known as gCassiopeiæ, both of which have been assiduously observed, their spectrapossessing numerous peculiarities which render an explanation of thephysical constitution of the stars of this subdivision a very difficultmatter. 

Passing to _Type II. _, we find spectra in which the metallic lines arestrong. The more refrangible end of the spectrum is fainter than in theprevious Class, and absorption bands are sometimes found towards the redend. In its first subdivision (II. A) are contained spectra with a largenumber of strong and well-defined lines due to metals, the hydrogenlines being also well seen, though they are not specially conspicuous. Among the very numerous stars of this group are Capella, Aldebaran, Arcturus, Pollux, etc. The spectra of these stars are in factpractically identical with the spectrum of our own sun, as shown, forinstance, by Dr. Scheiner, of the Potsdam Astrophysical Observatory, whohas measured several hundred lines on photographs of the spectrum ofCapella, and found a very close agreement between these lines andcorresponding ones in the solar spectrum. We can hardly doubt that thephysical constitution of these stars is very similar to that of our sun. This cannot be the case with the stars of the second subdivision (II. B), the spectra of which are very complex, each consisting of a continuousspectrum crossed by numerous dark lines, on which is superposed a secondspectrum of bright lines. Upwards of seventy stars are known to possessthis extraordinary spectrum, the only bright one among them being a starof the third magnitude in the southern constellation Argus. Here againwe have hydrogen and helium represented by bright lines, while theorigin of the remaining bright lines is doubtful. With regard to thephysical constitution of the stars of this group it is very difficult tocome to a definite conclusion, but it would seem not unlikely that wehave here to do with stars which are not only surrounded by anatmosphere of lower temperature, causing the dark lines, but which, outside of that, have an enormous envelope of hydrogen and other gases. In one star at least of this group Professor Campbell, of the LickObservatory, has seen the F line as a long line extending a veryappreciable distance on each side of the continuous spectrum, and withan open slit it was seen as a large circular disc about six seconds indiameter; two other principal hydrogen lines showed the same appearance. As far as this observation goes, the existence of an extensive gaseousenvelope surrounding the star seems to be indicated. 

_Type III. _ contains comparatively few stars, and the spectra arecharacterised by numerous dark bands in addition to dark lines, whilethe more refrangible parts are very faint, for which reason the starsare more or less red in colour. This class has two strongly markedsubdivisions. In the first (III. A) the principal absorption linescoincide with similar ones in the solar spectrum, but with greatdifferences as to intensity, many lines being much stronger in thesestars than in the sun, while many new lines also appear. Thesedissimilarities are, however, of less importance than the peculiarabsorption bands in the red, yellow, and green parts of the spectrum, overlying the metallic lines, and being sharply defined on the sidetowards the violet and shading off gradually towards the red end of thespectrum. Bands of this kind belong to chemical combinations, and thisappears to show that somewhere in the atmospheres of these distant sunsthe temperature is low enough to allow stable chemical combinations tobe formed. The most important star of this kind is Betelgeuze or aOrionis, the red star of the first magnitude in the shoulder of Orion;but it is of special importance to note that many variable stars of longperiod have spectra of Type III. A. Sir Norman Lockyer predicted in 1887that bright lines, probably of hydrogen, would eventually be found toappear at the maximum of brightness, when the smaller swarm is supposedto pass through the larger one, and this was soon afterwards confirmedby the announcement that Professor Pickering had found a number ofhydrogen lines bright on photographs, obtained at Harvard CollegeObservatory, of the spectrum of the remarkable variable, Mira Ceti, atthe time of maximum. Professor Pickering has since then reported thatbright lines have been found on the plates of forty-one previously knownvariables of this class, and that more than twenty other stars have beendetected as variables by this peculiarity of their spectrum; that is, bright lines being seen in them suggested that the stars were variable, and further photometric investigations corroborated the fact. 

The second subdivision (III. B) contains only comparatively faint stars, of which none exceed the fifth magnitude, and is limited to a smallnumber of red stars. The strongly marked bands in their spectra aresharply defined and dark on the red side, while they fade away graduallytowards the violet, exactly the reverse of what we see in the spectra ofIII. A. These bands appear to arise from the absorption due tohydrocarbon vapours present in the atmospheres of these stars; but thereare also some lines visible which indicate the presence of metallicvapours, sodium being certainly among these. There can be little doubtthat these stars represent the last stage in the life of a sun, when ithas cooled down considerably and is not very far from actual extinction, owing to the increasing absorption of its remaining light in theatmosphere surrounding it. 

The method employed for the spectroscopic determination of the motion ofa star in the line of sight is the same as the method we have describedin the chapter on the sun. The position of a certain line in thespectrum of a star is compared with the position of the correspondingbright line of an element in an artificially produced spectrum, and inthis manner a displacement of the stellar line either towards the violet(indicating that the star is approaching us) or towards the red(indicating that it is receding) may be detected. The earliest attemptof this sort was made in 1867 by Sir William Huggins, who compared the Fline in the spectrum of Sirius with the same line of the spectrum ofhydrogen contained in a vacuum tube reflected into the field of hisastronomical spectroscope, so that the two spectra appeared side byside. The work thus commenced and continued by him was afterwards takenup at the Greenwich Observatory; but the results obtained by thesedirect observations were never satisfactory, as remarkable discrepanciesappeared between the values obtained by different observers, and even bythe same observer on different nights. This is not to be wondered atwhen we bear in mind that the velocity of light is so enormous comparedwith any velocity with which a heavenly body may travel, that the changeof wave length resulting from the latter motion can only be a veryminute one, difficult to perceive, and still more difficult to measure. But since photography was first made use of for these investigations byDr. Vogel, of Potsdam, much more accordant and reliable results havebeen obtained, though even now extreme care is required to avoidsystematic errors. To give some idea of the results obtainable, wepresent in the following table the values of the velocity per second ofa number of stars observed in 1896 and 1897 by Mr. H. F. Newall with theBruce spectrograph attached to the great 25-inch Newall refractor of theCambridge Observatory, and we have added the values found at Potsdam byVogel and Scheiner. The results are expressed in kilometres (1 km. =0·62 English mile). The sign + means that the star is receding fromus, -that it is approaching. 

 Newall. Vogel. Scheiner. Aldebaran + 49·2 + 47·6 + 49·4 Betelgeuze + 10·6 + 15·6 + 18·8 Procyon - 4·2 - 7·2 - 10·5 Pollux - 0·7 + 1·9 + 0·4 g Leonis - 39·9 - 36·5 - 40·5 Arcturus - 6·4 - 7·0 - 8·3

These results have been corrected for the earth's orbital motion roundthe sun, but not for the sun's motion through space, as the amount ofthe latter is practically unknown, or at least very uncertain; so thatthe above figures really represent the velocity per second of thevarious stars relative to the sun. We may add that the direction andvelocity of the sun's motion may eventually be ascertained fromspectroscopic measures of a great number of stars, and it seems likelythat the sun's velocity will be much more accurately found in this waythan by the older method of combining proper motions of stars withspeculations as to the average distances of the various classes ofstars. This has already been attempted by Dr. Kempf, who from thePotsdam spectrographic observations found the sun's velocity to be 18·6kilometres, or 11·5 miles per second, a result which is probably not farfrom the truth. 

But the spectra of the fixed stars can also tell us something aboutorbital motion in these extremely distant systems. If one star revolvedround another in a plane passing through the sun, it must on one side ofthe orbit move straight towards us and on the other side move straightaway from us, while it will not alter its distance from us while it ispassing in front of, or behind, the central body. If we therefore findfrom the spectroscopic observations that a star is alternately movingtowards and away from the earth in a certain period, there can be nodoubt that this star is travelling round some unseen body (or, rather, round the centre of gravity of both) in the period indicated by theshifting of the spectral lines. In Chapter XIX. We mentioned thevariable star Algol in the constellation Perseus, which is one of aclass of variable stars distinguished by the fact that for the greaterpart of the period they remain of unaltered brightness, while for a veryshort time they become considerably fainter. That this was caused bysome sort of an eclipse--or, in other words, by the periodic passage ofa dark body in front of the star, hiding more or less of the latter fromus--was the simplest possible hypothesis, and it had already years agobeen generally accepted. But it was not possible to prove that this wasthe true explanation of the periodicity of stars like Algol untilProfessor Vogel, from the spectroscopic observations made at Potsdam, found that before every minimum Algol is receding from the sun, while itis approaching us after the minimum. Assuming the orbit to be circular, the velocity of Algol was found to be twenty-six miles per second. Fromthis and the length of the period (2d. 22h. 48m. 55s. ) and the time ofobscuration it was easy to compute the size of the orbit and the actualdimensions of the two bodies. It was even possible to go a step furtherand to calculate from the orbital velocities the masses of the twobodies, [41] assuming them to be of equal density--an assumption which isno doubt very uncertain. The following are the approximate elements ofthe Algol system found by Vogel:--

 Diameter of Algol 1, 054, 000 miles. Diameter of companion 825, 000 miles. Distance between their centres 3, 220, 000 miles. Orbital velocity of Algol 26 miles per sec. Orbital velocity of companion 55 miles per sec. Mass of Algol 4/9 of sun's mass. Mass of companion 2/9 of sun's mass. 

The period of Algol has been gradually decreasing during the lastcentury (by six or seven seconds), but whether this is caused by themotion of the pair round a third and very much more distant body, assuggested by Mr. Chandler, has still to be found out. 

We have already mentioned that in order to produce eclipses, and therebyvariations of light, it is necessary that the line of sight should lienearly in the plane of the orbit. It is also essential that there shouldbe a considerable difference of brightness between the two bodies. Theseconditions must be fulfilled in the fifteen variable stars of the Algolclass now known; but according to the theory of probability, there mustbe many more binary systems like that of Algol where these conditionsare not fulfilled, and in those cases no variations will occur in thestars' brightness. Of course, we know many cases of a luminous startravelling round another, but there must also be cases of a largecompanion travelling round another at so small a distance that ourtelescopes are unable to "divide" the double star. This has actuallybeen discovered by means of the spectroscope. If we suppose an extremelyclose double star to be examined with the spectroscope, the spectra ofthe two components will be superposed, and we shall not be aware that wereally see two different spectra. But during the revolution of the twobodies round their common centre of gravity there must periodically comea time when one body is moving towards us and the other moving from us, and consequently the lines in the spectrum of the former will be subjectto a minute, relative shift towards the violet end of the spectrum, andthose of the other to a minute shift towards the red. Those lines whichare common to the two spectra will therefore periodically become double. A discovery of this sort was first made in 1889 by Professor Pickeringfrom photographs of the spectrum of Mizar, or z Ursa Majoris, thelarger component of the well-known double star in the tail of the GreatBear. Certain of the lines were found to be double at intervals offifty-two days. The maximum separation of the two components of eachline corresponds to a relative velocity of one star as compared with theother of about a hundred miles per second, but subsequent observationshave shown the case to be very complicated, either with a very eccentricelliptic orbit or possibly owing to the presence of a third body. TheHarvard College photographs also showed periodic duplicity of lines inthe star b Aurigæ, the period being remarkably short, only threedays and twenty-three hours and thirty-seven minutes. In 1891 Vogelfound, from photographs of the spectrum of Spica, the first magnitudestar in Virgo, that this star alternately recedes from and approaches tothe solar system, the period being four days. Certain other"spectroscopic binaries" have since then been found, notably onecomponent of Castor, with a period of three days, found by M. Belopolsky, and a star in the constellation Scorpio, with a period ofonly thirty-four hours, detected on the Harvard spectrograms. 

Quite recently Mr. H. F. Newall, at Cambridge, and Mr. Campbell, of theLick Observatory, have shown that a Aurigæ, or Capella, consistsof a sun-like star and a Procyon-like star, revolving in 104 days. 

At first sight there is something very startling in the idea of two sunscircling round each other, separated by an interval which, in comparisonwith their diameters, is only a very small one. In the Algol system, forinstance, we have two bodies, one the size of our own sun and the otherslightly larger, moving round their common centre of gravity in lessthan three days, and at a distance between their surfaces equal to onlytwice the diameter of the larger one. Again, in the system of Spica wehave two great suns swinging round each other in only four days, at adistance equal to that between Saturn and his sixth satellite. Butalthough we have at present nothing analogous to this in our solarsystem, it can be proved mathematically that it is perfectly possiblefor a system of this kind to preserve its stability, if not for ever, atany rate for ages, and we shall see in our last chapter that there wasin all probability a time when the earth and the moon formed a peculiarsystem of two bodies revolving rapidly at a very small distance comparedto the diameters of the bodies. 

It is possible that we have a more complicated system in the star knownas b Lyræ. This is a variable star of great interest, having aperiod of twelve days and twenty-two hours, in which time it rises frommagnitude 4-1/2 to a little above 3-1/2, sinks nearly to the fourthmagnitude, rises again to fully 3-1/2, and finally falls to magnitude4-1/2. In 1891 Professor Pickering discovered that the bright lines inthe spectrum of this star changed their position from time to time, appearing now on one side, now on the other side of corresponding darklines. Obviously these bright lines change their wave length, thelight-giving source alternately receding from and approaching to theearth, and the former appeared to be the case during one-half of theperiod of variation of the star's light, the latter during the otherhalf. The spectrum of this star has been further examined by Belopolskyand others, who have found that the lines are apparently double, butthat one of the components either disappears or becomes very narrow fromtime to time. On the assumption that these lines were really single (theapparent duplicity resulting from the superposition of a dark line), Belopolsky determined the amount of their displacement by measuring thedistances from the two edges of a line of hydrogen (F) to the artificialhydrogen line produced by gas glowing in a tube and photographed alongwith the star-spectrum. Assuming the alternate approach and recession tobe caused by orbital revolution, Belopolsky found that the body emittingthe light of the bright lines moved with an orbital velocity offorty-one miles. He succeeded in 1897 in observing the displacement of adark line due to magnesium, and found that the body emitting it was alsomoving in an orbit, but while the velocities given by the bright F lineare positive after the principal minimum of the star's light, thosegiven by the dark line are negative. Therefore, during the principalminimum it is a star giving the dark line which is eclipsed, and duringthe secondary minimum another star giving the bright line is eclipsed. This wonderful variable will, however, require more observatioêns beforethe problem of its constitution is finally solved, and the same may besaid of several variable stars, _e. G. _ ê Aquilæ and d Cephei, in which a want of harmony has been found between the changes ofvelocity and the fluctuations of the light. 

There are some striking analogies between the complicated spectrum ofb Lyræ and the spectra of temporary stars. The first "new star"which could be spectroscopically examined was that which appeared inCorona Borealis in 1866, and which was studied by Sir W. Huggins. Itshowed a continuous spectrum with dark absorption lines, and also thebright lines of hydrogen; practically the same spectrum as the stars ofType II. B. This was also the case with Schmidt's star of 1876, whichshowed the helium line (D3) and the principal nebula line in additionto the lines of hydrogen; but in the autumn of 1877, when the star hadfallen to the tenth magnitude, Dr. Copeland was surprised to find thatonly one line was visible, the principal nebula line, in which almostthe whole light of the star was concentrated, the continuous spectrumbeing hardly traceable. It seemed, in fact, that the star had beentransformed into a planetary nebula, but later the spectrum seems tohave lost this peculiar monochromatic character, the nebula line havingdisappeared and a faint continuous spectrum alone being visible, whichis also the case with the star of 1866 since it sank down to the tenthmagnitude. A continuous spectrum was all that could be seen of the newstar which broke out in the nebula of Andromeda in 1885, much the sameas the spectrum of the nebula itself. 

When the new star in Auriga was announced, in February, 1892, astronomers were better prepared to observe it spectroscopically, as itwas now possible by means of photography to study the ultra-violet partof the spectrum which to the eye is invisible. The visible spectrum wasvery like that of Nova Cygni of 1876, but when the wave-lengths of allthe bright lines seen and photographed at the Lick Observatory and atPotsdam were measured, a strong resemblance to the bright line spectrumof the chromosphere of the sun became very evident. The hydrogen lineswere very conspicuous, while the iron lines were very numerous, andcalcium and magnesium were also represented. The most remarkablerevelation made by the photographs was, however, that the bright lineswere in many cases accompanied, on the side next the violet, by broaddark bands, while both bright and dark lines were of a compositecharacter. Many of the dark lines had a thin bright line superposed inthe middle, while on the other hand many of the bright lines had two orthree points maxima of brightness. The results of the measures of motionin the line of sight were of special importance. They showed that thesource of light, whence came the thin bright lines within the dark ones, was travelling towards the sun at the enormous rate of 400 miles persecond, and if the bright lines were actual "reversals" of the darkones, then the source of the absorption spectrum must have been endowedwith much the same velocity. On the other hand, if the two or threemaxima of brightness in the bright lines really represent two or threeseparate bodies giving bright lines, the measures indicate that theprincipal one was almost at rest as regards the sun, while the otherswere receding from us at the extraordinary rates of 300 and 600 milesper second. And as if this were not sufficiently puzzling, the star onits revival in August, 1892, as a tenth magnitude star had a totallydifferent spectrum, showing nothing but a number of the bright linesbelonging to planetary nebulæ! It is possible that the principal ones ofthese were really present in the spectrum from the first, but that theirwave lengths had been different owing to change of the motion in theline of sight, so that the nebula lines seen in the autumn wereidentical with others seen in the spring at slightly different places. Subsequent observations of these nebula lines seemed to point to amotion of the Nova towards the solar system (of about 150 miles persecond) which gradually diminished. 

But although we are obliged to confess our inability to say for certainwhy a temporary star blazes up so suddenly, we have every cause to thinkthat these strange bodies will by degrees tell us a great deal about theconstitution of the fixed stars. The great variety of spectra which wesee in the starry universe, nebula spectra with bright lines, stellarspectra of the same general character, others with broad absorptionbands, or numerous dark lines like our sun, or a few absorption linesonly--all this shows us the universe as teeming with bodies in variousstages of evolution. We shall have a few more words to say on thismatter when we come to consider the astronomical significance of heat;but we have reached a point where man's intellect can hardly keep pacewith the development of our instrumental resources, and where ourimagination stands bewildered when we endeavour to systematise theknowledge we have gained. That great caution will have to be exercisedin the interpretation of the observed phenomena is evident from therecent experience of Professor Rowland, of Baltimore, from which welearn that spectral lines are not only widened by increased pressure ofthe light-giving vapour, but that they may be bodily shifted thereby. Dr. Zeeman's discovery, that a line from a source placed in a strongmagnetic field may be both widened, broadened, and doubled, will alsoincrease our difficulties in the interpretation of these obscurephenomena. 

CHAPTER XXIV. 

THE PRECESSION AND NUTATION OF THE EARTH'S AXIS. 

 The Pole is not a Fixed Point--Its Effect on the Apparent Places of the Stars--The Illustration of the Peg-Top--The Disturbing Force which acts on the Earth--Attraction of the Sun on a Globe--The Protuberance at the Equator--The Attraction of the Protuberance by the Sun and by the Moon produces Precession--The Efficiency of the Precessional Agent varies inversely as the Cube of the Distance--The Relative Efficiency of the Sun and the Moon--How the Pole of the Earth's Axis revolves round the Pole of the Ecliptic--Variation of Latitude. 

The position of the pole of the heavens is most conveniently indicatedby the bright star known as the Pole Star, which lies in its immediatevicinity. Around this pole the whole heavens appear to rotate once in asidereal day; and we have hitherto always referred to the pole as thoughit were a fixed point in the heavens. This language is sufficientlycorrect when we embrace only a moderate period of time in our review. Itis no doubt true that the pole lies near the Pole Star at the presenttime. It did so during the lives of the last generation, and it will doso during the lives of the next generation. All this time, however, thepole is steadily moving in the heavens, so that the time will at lengthcome when the pole will have departed a long way from the present PoleStar. This movement is incessant. It can be easily detected and measuredby the instruments in our observatories, and astronomers are familiarwith the fact that in all their calculations it is necessary to holdspecial account of this movement of the pole. It produces an apparentchange in the position of a star, which is known by the term"precession. "

[Illustration: Fig. 100. ]

The movement of the pole is very clearly shown in the accompanyingfigure (Fig. 100), for which I am indebted to the kindness of the lateProfessor C. Piazzi Smyth. The circle shows the track along which thepole moves among the stars. 

The centre of the circle in the constellation of Draco is the pole ofthe ecliptic. A complete journey of the pole occupies the considerableperiod of about 25, 867 years. The drawing shows the position of thepole at the several dates from 4000 B. C. To 2000 A. D. A glance at thismap brings prominently before us how casual is the proximity of the poleto the Pole Star. At present, indeed, the distance of the two isactually lessening, but afterwards the distance will increase until, when half of the revolution has been accomplished, the pole will be at adistance of twice the radius of the circle from the Pole Star. It willthen happen that the pole will be near the bright star Vega or aLyræ, so that our successors some 12, 000 years hence may make use ofVega for many of the purposes for which the Pole Star is at presentemployed! Looking back into past ages, we see that some 2, 000 or 3, 000years B. C. The star a Draconis was suitably placed to serve as thePole Star, when b and d of the Great Bear served as pointers. It need hardly be added, that since the birth of accurate astronomy thecourse of the pole has only been observed over a very small part of themighty circle. We are not, however, entitled to doubt that the motion ofthe pole will continue to pursue the same path. This will be madeabundantly clear when we proceed to render an explanation of this veryinteresting phenomenon. 

The north pole of the heavens is the point of the celestial spheretowards which the northern end of the axis about which the earth rotatesis directed. It therefore follows that this axis must be constantlychanging its position. The character of the movement of the earth, sofar as its rotation is concerned, may be illustrated by a very commontoy with which every boy is familiar. When a peg-top is set spinning, ithas, of course, a very rapid rotation around its axis; but besides thisrotation there is usually another motion, whereby the axis of thepeg-top does not remain in a constant direction, but moves in a conicalpath around the vertical line. The adjoining figure (Fig. 101) gives aview of the peg-top. It is, of course, rotating with great rapidityaround its axis, while the axis itself revolves around the vertical linewith a very deliberate motion. If we could imagine a vast peg-top whichrotated on its axis once a day, and if that axis were inclined at anangle of twenty-three and a half degrees to the vertical, and if theslow conical motion of the axis were such that the revolution of theaxis were completed in about 26, 000 years, then the movements wouldresemble those actually made by the earth. The illustration of thepeg-top comes, indeed, very close to the actual phenomenon ofprecession. In each case the rotation about the axis is far more rapidthan that of the revolution of the axis itself; in each case also theslow movement is due to an external interference. Looking at the figureof the peg-top (Fig. 101) we may ask the question, Why does it not falldown? The obvious effect of gravity would seem to say that it isimpossible for the peg-top to be in the position shown in the figure. Yet everybody knows that this is possible so long as the top isspinning. If the top were not spinning, it would, of course, fall. Ittherefore follows that the effect of the rapid rotation of the top somodifies the effect of gravitation that the latter, instead of producingits apparently obvious consequence, causes the slow conical motion ofthe axis of rotation. This is, no doubt, a dynamical question of somedifficulty, but it is easy to verify experimentally that it is the case. If a top be constructed so that the point about which it is spinningshall coincide with the centre of gravity, then there is no effect ofgravitation on the top, and there is no conical motion perceived. 

[Illustration: Fig. 101. --Illustration of the Motion of Precession. ]

If the earth were subject to no external interference, then thedirection of the axis about which it rotates must remain for everconstant; but as the direction of the axis does not remain constant, itis necessary to seek for a disturbing force adequate to the productionof the phenomena which are observed. We have invariably found that thedynamical phenomena of astronomy can be accounted for by the law ofuniversal gravitation. It is therefore natural to enquire how fargravitation will render an account of the phenomenon of precession; andto put the matter in its simplest form, let us consider the effect whicha distant attracting body can have upon the rotation of the earth. 

To answer this question, it becomes necessary to define precisely whatwe mean by the earth; and as for most purposes of astronomy we regardthe earth as a spherical globe, we shall commence with this assumption. It seems also certain that the interior of the earth is, on the whole, heavier than the outer portions. It is therefore reasonable to assumethat the density increases as we descend; nor is there any sufficientground for thinking that the earth is much heavier in one part than atany other part equally remote from the centre. It is therefore usual insuch calculations to assume that the earth is formed of concentricspherical shells, each one of which is of uniform density; while thedensity decreases from each shell to the one exterior thereto. 

A globe of this constitution being submitted to the attraction of someexternal body, let us examine the effects which that external body canproduce. Suppose, for instance, the sun attracts a globe of thischaracter, what movements will be the result? The first and most obviousresult is that which we have already so frequently discussed, and whichis expressed by Kepler's laws: the attraction will compel the earth torevolve around the sun in an elliptic path, of which the sun is in thefocus. With this movement we are, however, not at this moment concerned. We must enquire how far the sun's attraction can modify the earth'srotation around its axis. It can be demonstrated that the attraction ofthe sun would be powerless to derange the rotation of the earth soconstituted. This is a result which can be formally proved bymathematical calculation. It is, however, sufficiently obvious that theforce of attraction of any distant point on a symmetrical globe mustpass through the centre of that globe: and as the sun is only anenormous aggregate of attracting points, it can only produce acorresponding multitude of attractive forces; each of these forcespasses through the centre of the earth, and consequently the resultantforce which expresses the joint result of all the individual forces mustalso be directed through the centre of the earth. A force of thischaracter, whatever other potent influence it may have, will bepowerless to affect the rotation of the earth. If the earth be rotatingon an axis, the direction of that axis would be invariably preserved; sothat as the earth revolves around the sun, it would still continue torotate around an axis which always remained parallel to itself. Norwould the attraction of the earth by any other body prove moreefficacious than that of the sun. If the earth really were thesymmetrical globe we have supposed, then the attraction of the sun andmoon, and even the influence of all the planets as well, would never becompetent to make the earth's axis of rotation swerve for a singlesecond from its original direction. 

We have thus narrowed very closely the search for the cause of the"precession. " If the earth were a perfect sphere, precession would beinexplicable. We are therefore forced to seek for an explanation ofprecession in the fact that the earth is not a perfect sphere. This wehave already demonstrated to be the case. We have shown that theequatorial axis of the earth is longer than the polar axis, so thatthere is a protuberant zone girdling the equator. The attraction ofexternal bodies is able to grasp this protuberance, and thereby forcethe earth's axis of rotation to change its direction. 

There are only two bodies in the universe which sensibly contribute tothe precessional movement of the earth's axis: these bodies are the sunand the moon. The shares in which the labour is borne by the sun and themoon are not what might have been expected from a hasty view of thesubject. This is a point on which it will be desirable to dwell, as itillustrates a point in the theory of gravitation which is of veryconsiderable importance. 

The law of gravitation asserts that the intensity of the attractionwhich a body can exercise is directly proportional to the mass of thatbody, and inversely proportional to the square of its distance from theattracted point. We can thus compare the attraction exerted upon theearth by the sun and by the moon. The mass of the sun exceeds the massof the moon in the proportion of about 26, 000, 000 to 1. On the otherhand, the moon is at a distance which, on an average, is about one-386thpart of that of the sun. It is thus an easy calculation to show that theefficiency of the sun's attraction on the earth is about 175 times asgreat as the attraction of the moon. Hence it is, of course, that theearth obeys the supremely important attraction of the sun, and pursuesan elliptic path around the sun, bearing the moon as an appendage. 

But when we come to that particular effect of attraction which iscompetent to produce precession, we find that the law by which theefficiency of the attracting body is computed assumes a different form. The measure of efficiency is, in this case, to be found by taking themass of the body and dividing it by the _cube_ of the distance. Thecomplete demonstration of this statement must be sought in the formulæof mathematics, and cannot be introduced into these pages; we may, however, adduce one consideration which will enable the reader in somedegree to understand the principle, though without pretending to be ademonstration of its accuracy. It will be obvious that the nearer thedisturbing body approaches to the earth the greater is the _leverage_(if we may use the expression) which is afforded by the protuberance atthe equator. The efficiency of a given force will, therefore, on thisaccount alone, increase in the inverse proportion of the distance. Theactual intensity of the force itself augments in the inverse square ofthe distance, and hence the capacity of the attracting body forproducing precession will, for a double reason, increase when thedistance decreases. Suppose, for example, that the disturbing body isbrought to half its original distance from the disturbed body, theleverage is by this means doubled, while the actual intensity of theforce is at the same time quadrupled according to the law ofgravitation. It will follow that the effect produced in the latter casemust be eight times as great as in the former case. And this is merelyequivalent to the statement that the precession-producing capacity of abody varies inversely as the cube of the distance. 

It is this consideration which gives to the moon an importance as aprecession-producing agent to which its mere attractive capacity wouldnot have entitled it. Even though the mass of the sun be 26, 000, 000times as great as the mass of the moon, yet when this number is dividedby the cube of the relative value of the distances of the bodies (386), it is seen that the efficiency of the moon is more than twice as greatas that of the sun. In other words, we may say that one-third of themovement of precession is due to the sun, and two-thirds to the moon. 

For the study of the joint precessional effect due to the sun and themoon acting simultaneously, it will be advantageous to consider theeffect produced by the two bodies separately; and as the case of the sunis the simpler of the two, we shall take it first. As the earth travelsin its annual path around the sun, the axis of the earth is directed toa point in the heavens which is 23-1/2° from the pole of the ecliptic. The precessional effect of the sun is to cause this point--the pole ofthe earth--to revolve, always preserving the same angular distance fromthe pole of the ecliptic; and thus we have a motion of the typerepresented in the diagram. As the ecliptic occupies a position whichfor our present purpose we may regard as fixed in space, it follows thatthe pole of the ecliptic is a fixed point on the surface of the heavens;so that the path of the pole of the earth must be a small circle in theheavens, fixed in its position relatively to the surrounding stars. Inthis we find a motion strictly analogous to that of the peg-top. It isthe gravitation of the earth acting upon the peg-top which forces itinto the conical motion. The immediate effect of the gravitation is somodified by the rapid rotation of the top, that, in obedience to aprofound dynamical principle, the axis of the top revolves in a conerather than fall down, as it would do were the top not spinning. In asimilar manner the immediate effect of the sun's attraction on theprotuberance at the equator would be to bring the pole of the earth'saxis towards the pole of the ecliptic, but the rapid rotation of theearth modifies this into the conical movement of precession. 

The circumstances with regard to the moon are much more complicated. The moon describes a certain orbit around the earth; that orbit lies ina certain plane, and that plane has, of course, a certain pole on thecelestial sphere. The precessional effect of the moon would accordinglytend to make the pole of the earth's axis describe a circle around thatpoint in the heavens which is the pole of the moon's orbit. This pointis about 5° from the pole of the ecliptic. The pole of the earth istherefore solicited by two different movements--one a revolution aroundthe pole of the ecliptic, the other a revolution about another point 5°distant, which is the pole of the moon's orbit. It would thus seem thatthe earth's pole should make a certain composite movement due to the twoseparate movements. This is really the case, but there is a point to bevery carefully attended to, which at first seems almost paradoxical. Wehave shown how the potency of the moon as a precessional agent exceedsthat of the sun, and therefore it might be thought that the compositemovement of the earth's pole would conform more nearly to a rotationaround the pole of the plane of the moon's orbit than to a rotationaround the pole of the ecliptic; but this is not the case. Theprecessional movement is represented by a revolution around the pole ofthe ecliptic, as is shown in the figure. Here lies the germ of one ofthose exquisite astronomical discoveries which delight us byillustrating some of the most subtle phenomena of nature. 

The plane in which the moon revolves does not occupy a constantposition. We are not here specially concerned with the causes of thischange in the plane of the moon's orbit, but the character of themovement must be enunciated. The inclination of this plane to theecliptic is about 5°, and this inclination does not vary (except withinvery narrow limits); but the line of intersection of the two planes doesvary, and, in fact, varies so quickly that it completes a revolution inabout 18-2/3 years. This movement of the plane of the moon's orbitnecessitates a corresponding change in the position of its pole. We thussee that the pole of the moon's orbit must be actually revolving aroundthe pole of the ecliptic, always remaining at the same distance of 5°, and completing its revolution in 18-2/3 years. It will, therefore, beobvious that there is a profound difference between the precessionaleffect of the sun and of the moon in their action on the earth. The suninvites the earth's pole to describe a circle around a fixed centre; themoon invites the earth's pole to describe a circle around a centre whichis itself in constant motion. It fortunately happens that thecircumstances of the case are such as to reduce considerably thecomplexity of the problem. The movement of the moon's plane, onlyoccupying about 18-2/3 years, is a very rapid motion compared with thewhole precessional movement, which occupies about 26, 000 years. Itfollows that by the time the earth's axis has completed one circuit ofits majestic cone, the pole of the moon's plane will have gone roundabout 1, 400 times. Now, as this pole really only describes acomparatively small cone of 5° in radius, we may for a firstapproximation take the average position which it occupies; but thisaverage position is, of course, the centre of the circle which itdescribes--that is, the pole of the ecliptic. 

We thus see that the average precessional effect of the moon simplyconspires with that of the sun to produce a revolution around the poleof the ecliptic. The grosser phenomena of the movements of the earth'saxis are to be explained by the uniform revolution of the pole in acircular path; but if we make a minute examination of the track of theearth's axis, we shall find that though it, on the whole, conforms withthe circle, yet that it really traces out a sinuous line, sometimes onthe inside and sometimes on the outside of the circle. This delicatemovement arises from the continuous change in the place of the pole ofthe moon's orbit. The period of these undulations is 18-2/3 years, agreeing exactly with the period of the revolution of the moon's nodes. The amount by which the pole departs from the circle on either side isonly about 9·2 seconds--a quantity rather less than thetwenty-thousandth part of the radius of the sphere. This phenomenon, known as "nutation, " was discovered by the beautiful telescopicresearches of Bradley, in 1747. Whether we look at the theoreticalinterest of the subject or at the refinement of the observationsinvolved, this achievement of the "Vir incomparabilis, " as Bradley hasbeen called by Bessel, is one of the masterpieces of astronomicalgenius. 

The phenomena of precession and nutation depend on movements of theearth itself, and not on movements of the axis of rotation within theearth. Therefore the distance of any particular spot on the earth fromthe north or south pole is not disturbed by either of these phenomena. The latitude of a place is the distance of the place from the earth'sequator, and this quantity remains unaltered in the course of the longprecession cycle of 26, 000 years. But it has been discovered within thelast few years that latitudes are subject to a small periodic change ofa few tenths of a second of arc. This was first pointed out about 1880by Dr. Küstner, of Berlin, and by a masterly analysis of all availableobservations, made in the course of many years past at variousobservatories, Dr. Chandler, of Boston, has shown that the latitude ofevery point on the earth is subject to a double oscillation, the periodof one being 427 days and the other about a year, the mean amplitude ofeach being O"·14. In other words, the spot in the arctic regions, directly in the prolongation of the earth's axis of rotation, is notabsolutely fixed; the end of the imaginary axis moves about in acomplicated manner, but always keeping within a few yards of its averageposition. This remarkable discovery is not only of value as introducinga new refinement in many astronomical researches depending on anaccurate knowledge of the latitude, but theoretical investigations showthat the periods of this variation are incompatible with the assumptionthat the earth is an absolutely rigid body. Though this assumption hasin other ways been found to be untenable, the confirmation of this viewby the discovery of Dr. Chandler is of great importance. 

CHAPTER XXV. 

THE ABERRATION OF LIGHT. 

 The Real and Apparent Movements of the Stars--How they can be Discriminated--Aberration produces Effects dependent on the Position of the Stars--The Pole of the Ecliptic--Aberration makes Stars seem to Move in a Circle, an Ellipse, or a Straight Line according to Position--All the Ellipses have Equal Major Axes--How is this Movement to be Explained?--How to be Distinguished from Annual Parallax--The Apex of the Earth's Way--How this is to be Explained by the Velocity of Light--How the Scale of the Solar System can be Measured by the Aberration of Light. 

We have in this chapter to narrate a discovery of a recondite character, which illustrates in a forcible manner some of the fundamental truths ofAstronomy. Our discussion of it will naturally be divided into twoparts. In the first part we must describe the nature of the phenomenon, and then we must give the extremely elegant explanation afforded by theproperties of light. The telescopic discovery of aberration, as well asits explanation, are both due to the illustrious Bradley. 

The expression _fixed_ star, so often used in astronomy, is to bereceived in a very qualified sense. The stars are, no doubt, well fixedin their places, so far as coarse observation is concerned. Thelineaments of the constellations remain unchanged for centuries, and, incontrast with the ceaseless movements of the planets, the stars are notinappropriately called fixed. We have, however, had more than oneoccasion to show throughout the course of this work that the expression"fixed star" is not an accurate one when minute quantities are held inestimation. With the exact measures of modern instruments, many of thesequantities are so perceptible that they have to be always reckoned within astronomical enquiry. We can divide the movements of the stars intotwo great classes: the real movements and the apparent movements. Theproper motion of the stars and the movements of revolution of the binarystars constitute the real movements of these bodies. These movements arespecial to each star, so that two stars, although close together in theheavens, may differ in the widest degree as to the real movements whichthey possess. It may, indeed, sometimes happen that stars in a certainregion are animated with a common movement. In this phenomenon we havetraces of a real movement shared by a number of stars in a certaingroup. With this exception, however, the real movements of the starsseem to be governed by no systematic law, and the rapidly moving starsare scattered here and there indiscriminately over the heavens. 

The apparent movements of the stars have a different character, inasmuchas we find the movement of each star determined by the place which itoccupies in the heavens. It is by this means that we discriminate thereal movements of the star from its apparent movements, and examine thecharacter of both. 

In the present chapter we are concerned with the apparent movementsonly, and of these there are three, due respectively to precession, tonutation, and to aberration. Each of these apparent movements obeys lawspeculiar to itself, and thus it becomes possible to analyse the totalapparent motion, and to discriminate the proportions in which theprecession, the nutation, and the aberration have severally contributed. We are thus enabled to isolate the effect of aberration as completely asif it were the sole agent of apparent displacement, so that, by analliance between mathematical calculation and astronomical observation, we can study the effects of aberration as clearly as if the stars wereaffected by no other motions. 

Concentrating our attention solely on the phenomena of aberration weshall describe its particular effect upon stars in different regions ofthe sky, and thus ascertain the laws according to which the effects ofaberration are exhibited. When this step has been taken, we shall be ina position to give the beautiful explanation of those laws dependentupon the velocity of light. 

At one particular region of the heavens the effect of aberration has adegree of simplicity which is not manifested anywhere else. This regionlies in the constellation Draco, at the pole of the ecliptic. At thispole, or in its immediate neighbourhood, each star, in virtue ofaberration, describes a circle in the heavens. This circle is veryminute; it would take something like 2, 000 of these circles together toform an area equal to the area of the moon. Expressed in the usualastronomical language, we should say that the diameter of this smallcircle is about 40·9 seconds of arc. This is a quantity which, thoughsmall to the unaided eye, is really of great relative magnitude in thepresent state of telescopic research. It is not only large enough to beperceived, but it can be measured, with an accuracy which actually doesnot admit of a doubt, to the hundredth part of the whole. It is alsoobserved that each star describes its little circle in precisely thesame period of time; and that period is one year, or, in other words, the time of the revolution of the earth around the sun. It is found thatfor all stars in this region, be they large stars or small, single ordouble, white or coloured, the circles appropriate to each have all thesame size, and are all described in the same time. Even from this aloneit would be manifest that the cause of the phenomenon cannot lie in thestar itself. This unanimity in stars of every magnitude and distancerequires some simpler explanation. 

Further examination of stars in different regions sheds new light on thesubject. As we proceed from the pole of the ecliptic, we still find thateach star exhibits an annual movement of the same character as the starsjust considered. In one respect, however, there is a difference. Theapparent path of the star is no longer a circle; it has become anellipse. It is, however, soon perceived that the shape and the positionof this ellipse are governed by the simple law that the further the staris from the pole of the ecliptic the greater is the eccentricity of theellipse. The apparent path of the stars at the same distance from thepole have equal eccentricity, and of the axes of the ellipse the shorteris always directed to the pole, the longer being, of course, perpendicular to it. It is, however, found that no matter how great theeccentricity may become, the major axis always retains its originallength. It is always equal to about 40·9 seconds--that is, to thediameter of the circle of aberration at the pole itself. As we proceedfurther and further from the pole of the ecliptic, we find that eachstar describes a path more and more eccentric, until at length, when weexamine a star on the ecliptic, the ellipse has become so attenuatedthat it has flattened into a line. Each star which happens to lie on theecliptic oscillates to and fro along the ecliptic through an amplitudeof 40·9 seconds. Half a year accomplishes the journey one way, and theother half of the year restores the star to its original position. Whenwe pass to stars on the southern side of the ecliptic, we see the sameseries of changes proceed in an inverse order. The ellipse, from beingactually linear, gradually grows in width, though still preserving thesame length of major axis, until at length the stars near the southernpole of the ecliptic are each found to describe a circle equal to thepaths pursued by the stars at the north pole of the ecliptic. 

The circumstance that the major axes of all those ellipses are of equallength suggests a still further simplification. Let us suppose thatevery star, either at the pole of the ecliptic or elsewhere, pursues anabsolutely circular path, and that all these circles agree not only inmagnitude, but also in being all parallel to the plane of the ecliptic:it is easy to see that this simple supposition will account for theobserved facts. The stars at the pole of the ecliptic will, of course, show their circles turned fairly towards us, and we shall see that theypursue circular paths. The circular paths of the stars remote from thepole of the ecliptic will, however, be only seen somewhat edgewise, andthus the apparent paths will be elliptical, as we actually find them. Wecan even calculate the degree of ellipticity which this surmise wouldrequire, and we find that it coincides with the observed ellipticity. Finally, when we observe stars actually moving in the ecliptic, thecircles they follow would be seen edgewise, and thus the stars wouldhave merely the linear movement which they are seen to possess. All theobserved phenomena are thus found to be completely consistent with thesupposition that every star of all the millions in the heavens describesonce each year a circular path; and that, whether the star be far ornear, this circle has always the same apparent diameter, and lies in aplane always parallel to the plane of the ecliptic. 

We have now wrought the facts of observation into a form which enablesus to examine into the cause of a movement so systematic. Why is it thateach star should seem to describe a small circular path? Why should thatpath be parallel to the ecliptic? Why should it be completed exactly ina twelvemonth? We are at once referred to the motion of the earth aroundthe sun. That movement takes place in the ecliptic. It is completed in ayear. The coincidences are so obvious that we feel almost necessarilycompelled to connect in some way this apparent movement of the starswith the annual movement of the earth around the sun. If there were nosuch connection, it would be in the highest degree improbable that theplanes of the circles should be all parallel to the ecliptic, or thatthe time of revolution of each star in its circle should equal that ofthe revolution of the earth around the sun. As both these conditions arefulfilled, the probability of the connection rises to a value almostinfinite. 

The important question has then arisen as to why the movement of theearth around the sun should be associated in so remarkable a manner withthis universal star movement. There is here one obvious point to benoticed and to be dismissed. We have in a previous chapter discussed theimportant question of the annual parallax of stars, and we have shownhow, in virtue of annual parallax, each star describes an ellipse. Itcan further be demonstrated that these ellipses are really circlesparallel to the ecliptic; so that we might hastily assume that annualparallax was the cause of the phenomenon discovered by Bradley. A singlecircumstance will, however, dispose of this suggestion. The circledescribed by a star in virtue of annual parallax has a magnitudedependent on the distance of the star, so that the circles described byvarious stars are of various dimensions, corresponding to the varieddistances of different stars. The phenomena of aberration, however, distinctly assert that the circular path of each star is of the samesize, quite independently of what its distance may be, and hence annualparallax will not afford an adequate explanation. It should also benoticed that the movements of a star produced by annual parallax aremuch smaller than those due to aberration. There is not any known starwhose circular path due to annual parallax has a diameter one-twentiethpart of that of the circle due to aberration; indeed, in the greatmajority of cases the parallax of the star is an absolutely insensiblequantity. 

There is, however, a still graver and quite insuperable distinctionbetween the parallactic path and the aberrational path. Let us, forsimplicity, think of a star situated near the pole of the ecliptic, andthus appearing to revolve annually in a circle, whether we regard eitherthe phenomenon of parallax or of aberration. As the earth revolves, sodoes the star appear to revolve; and thus to each place of the earth inits orbit corresponds a certain place of the star in its circle. If themovement arise from annual parallax, it is easy to see where the placeof the star will be for any position of the earth. It is, however, foundthat in the movement discovered by Bradley the star never has theposition which parallax assigns to it, but is, in fact, a quarter of thecircumference of its little circle distant therefrom. 

A simple rule will find the position of the star due to aberration. Drawfrom the centre of the ellipse a radius parallel to the direction inwhich the earth is moving at the moment in question, then the extremityof this radius gives the point on its ellipse where the star is to befound. Tested at all seasons, and with all stars, this law is found tobe always verified, and by its means we are conducted to the trueexplanation of the phenomenon. 

We can enunciate the effects of aberration in a somewhat differentmanner, which will show even more forcibly how the phenomenon isconnected with the motion of the earth in its orbit. As the earthpursues its annual course around the sun, its movement at any moment maybe regarded as directed towards a certain point of the ecliptic. Fromday to day, and even from hour to hour, the point gradually moves alongthe ecliptic, so as to complete the circuit in a year. At each moment, however, there is always a certain point in the heavens towards whichthe earth's motion is directed. It is, in fact, the point on thecelestial sphere towards which the earth would travel continuously if, at the moment, the attraction of the sun could be annihilated. It isfound that this point is intimately connected with the phenomenon ofaberration. In fact, the aberration is really equivalent to drawing eachstar from its mean place towards the Apex of the Earth's Way, as thepoint is sometimes termed. It can also be shown by observation that theamount of aberration depends upon the distance from the apex. A starwhich happened to lie on the ecliptic will not be at all deranged byaberration from its mean place when it happens that the apex coincideswith the star. All the stars 10° from the apex will be displaced each bythe same amount, and all directly in towards the apex. A star 20° fromthe apex will undergo a larger degree of displacement, though still inthe same direction, exactly towards the apex; and all stars at the samedistance will be displaced by the same amount. Proceeding thus from theapex, we come to stars at a distance of 90° therefrom. Here the amountof displacement will be a maximum. Each one will be about twenty secondsfrom its average place; but in every case the imperative law will beobeyed, that the displacement of the star from its mean place liestowards the apex of the earth's way. We have thus given two distinctdescriptions of the phenomenon of aberration. In the first we find itconvenient to speak of a star as describing a minute circular path; inthe other we have regarded aberration as merely amounting to aderangement of the star from its mean place in accordance with specifiedlaws. These descriptions are not inconsistent: they are, in fact, geometrically equivalent; but the latter is rather the more perfect, inasmuch as it assigns completely the direction and extent of thederangement caused by aberration in any particular star at anyparticular moment. 

The question has now been narrowed to a very definite form. What is itwhich makes each star seem to close in towards the point towards whichthe earth is travelling? The answer will be found when we make a minuteenquiry into the circumstances in which we view a star in the telescope. 

The beam of rays from a star falls on the object-glass of a telescope;those rays are parallel, and after they pass through the object-glassthey converge to a focus near the eye end of the instrument. Let usfirst suppose that the telescope is at rest; then if the telescope bepointed directly towards the star, the rays will converge to a point atthe centre of the field of view where a pair of cross wires are placed, whose intersection defines the axis of the telescope. The case will, however, be altered if the telescope be moved after the light has passedthrough the objective; the rays of light in the interior of the tubewill pursue a direct path, as before, and will proceed to a focus at thesame precise point as before. As, however, the telescope has moved, itwill, of course, have carried with it the pair of cross wires; they willno longer be at the same point as at first, and consequently the imageof the star will not now coincide with their intersection. 

The movement of the telescope arises from its connection with the earth:for as the earth hurries along at a speed of eighteen miles a second, the telescope is necessarily displaced with this velocity. It might atfirst be thought, that in the incredibly small fraction of timenecessary for light to pass from the object-glass to the eye-piece, thechange in the position of the telescope must be too minute to beappreciable. Let us suppose, for instance, that the star is situatednear the pole of the ecliptic, then the telescope will be conveyed bythe earth's motion in a direction perpendicular to its length. If thetube of the instrument be about twenty feet long, it can be readilydemonstrated that during the time the light travels down the tube themovement of the earth will convey the telescope through a distance ofabout one-fortieth of an inch. [42] This is a quantity very distinctlymeasurable with the magnifying power of the eye-piece, and hence thisderangement of the star's place is very appreciable. It thereforefollows that if we wish the star to be shown at the centre of theinstrument, the telescope is not to be pointed directly at the star, asit would have to be were the earth at rest, but the telescope must bepointed a little in advance of the star's true position; and as wedetermine the apparent place of the star by the direction in which thetelescope is pointed, it follows that the apparent place of the star isaltered by the motion of the earth. 

Every circumstance of the change in the star's place admits of completeexplanation in this manner. Take, for instance, the small circular pathwhich each star appears to describe. We shall, for simplicity, referonly to a star at the pole of the ecliptic. Suppose that the telescopeis pointed truly to the place of the star, then, as we have shown, theimage of the star will be at a distance of one-fortieth of an inch fromthe cross wires. This distance will remain constant, but each night thedirection of the star from the cross wires will change, so that in thecourse of the year it completes a circle, and returns to its originalposition. We shall not pursue the calculations relative to other stars;suffice it here to say that the movement of the earth has been foundadequate to account for the phenomena, and thus the doctrine of theaberration of light is demonstrated. 

It remains to allude to one point of the utmost interest and importance. We have seen that the magnitude of the aberration can be measured byastronomical observation. The amount of this aberration depends upon thevelocity of light, and on the velocity with which the earth's motion isperformed. We can measure the velocity of light by independentmeasurements, in the manner already explained in Chapter XII. We arethus enabled to calculate what the velocity of the earth must be, forthere is only one particular velocity for the earth which, when combinedwith the measured velocity of light, will give the measured value ofaberration. The velocity of the earth being thus ascertained, and thelength of the year being known, it is easy to find the circumference ofthe earth's path, and therefore its radius; that is, the distance fromthe earth to the sun. 

Here is indeed a singular result, and one which shows how profoundly thevarious phenomena of science are interwoven. We make experiments in ourlaboratory, and find the velocity of light. We observe the fixed stars, and measure the aberration. We combine these results, and deducetherefrom the distance from the earth to the sun! Although this methodof finding the sun's distance is one of very great elegance, and admitsof a certain amount of precision, yet it cannot be relied upon as aperfectly unimpeachable method of deducing the great constant. A perfectmethod must be based on the operations of mere surveying, and ought notto involve recondite physical considerations. We cannot, however, failto regard the discovery of aberration by Bradley as a most pleasing andbeautiful achievement, for it not only greatly improves the calculationsof practical astronomy, but links together several physical phenomena ofthe greatest interest. 

CHAPTER XXVI. 

THE ASTRONOMICAL SIGNIFICANCE OF HEAT. 

 Heat and Astronomy--Distribution of Heat--The Presence of Heat in the Earth--Heat in other Celestial Bodies--Varieties of Temperature--The Law of Cooling--The Heat of the Sun--Can its Temperature be Measured?--Radiation connected with the Sun's Bulk--Can the Sun be Exhausting his Resources?--No marked Change has occurred--Geological Evidence as to the Changes of the Sun's Heat Doubtful--The Cooling of the Sun--The Sun cannot be merely an Incandescent Solid Cooling--Combustion will not Explain the Matter--Some Heat is obtained from Meteoric Matter, but this is not Adequate to the Maintenance of the Sun's Heat--The Contraction of a Heated Globe of Gas--An Apparent Paradox--The Doctrine of Energy--The Nebular Theory--Evidence in Support of this Theory--Sidereal Evidence of the Nebular Theory--Herschel's View of Sidereal Aggregation--The Nebulæ do not Exhibit Changes within the Limits of our Observation. 

That a portion of a work on astronomy should bear the title placed atthe head of this chapter will perhaps strike some of our readers asunusual, if not actually inappropriate. Is not heat, it may be said, aquestion merely of experimental physics? and how can it be legitimatelyintroduced into a treatise upon the heavenly bodies and their movements?Whatever weight such objections might have once had need not now beconsidered. The recent researches on heat have shown not only that heathas important bearings on astronomy, but that it has really been one ofthe chief agents by which the universe has been moulded into its actualform. At the present time no work on astronomy could be complete withoutsome account of the remarkable connection between the laws of heat andthe astronomical consequences which follow from those laws. 

In discussing the planetary motions and the laws of Kepler, or indiscussing the movements of the moon, the proper motions of the stars, or the revolutions of the binary stars, we proceed on the suppositionthat the bodies we are dealing with are rigid particles, and thequestion as to whether these particles are hot or cold does not seem tohave any especial bearing. No doubt the ordinary periodic phenomena ofour system, such as the revolution of the planets in conformity withKepler's laws, will be observed for countless ages, whether the planetsbe hot or cold, or whatever may be the heat of the sun. It must, however, be admitted that the laws of heat introduce certainmodifications into the statement of these laws. The effects of heat maynot be immediately perceptible, but they exist--they are constantlyacting; and in the progress of time they are adequate to effecting themightiest changes throughout the universe. 

Let us briefly recapitulate the circumstances of our system which giveto heat its potency. Look first at our earth, which at present seems--onits surface, at all events--to be a body devoid of internal heat; acloser examination will dispel this idea. Have we not the phenomena ofvolcanoes, of geysers, and of hot springs, which show that in theinterior of the earth heat must exist in far greater intensity than wefind on the surface? These phenomena are found in widely differentregions of the earth. Their origin is, no doubt, involved in a good dealof obscurity, but yet no one can deny that they indicate vast reservoirsof heat. It would indeed seem that heat is to be found everywhere in thedeep inner regions of the earth. If we take a thermometer down a deepmine, we find it records a temperature higher than at the surface. Thedeeper we descend the higher is the temperature; and if the same rate ofprogress should be maintained through those depths of the earth which weare not able to penetrate, it can be demonstrated that at twenty orthirty miles below the surface the temperature must be as great as thatof red-hot iron. 

We find in the other celestial bodies abundant evidence of the presentor the past existence of heat. Our moon, as we have already mentioned, affords a very striking instance of a body which must once have beenvery highly heated. The extraordinary volcanoes on its surface placethis beyond any doubt. It is equally true that those volcanoes havebeen silent for ages, so that, whatever may be the interior condition ofthe moon, the surface has now cooled down. Extending our view further, we see in the great planets Jupiter and Saturn evidence that they arestill endowed with a temperature far in excess of that which the earthhas retained; while, when we look at our sun, we see a body in a stateof brilliant incandescence, and glowing with a fervour to which wecannot approximate in our mightiest furnaces. The various fixed starsare bodies which glow with heat, like our sun; while we have in thenebulæ objects the existence of which is hardly intelligible to us, unless we admit that they are possessed of heat. 

From this rapid survey of the different bodies in our universe oneconclusion is obvious. We may have great doubts as to the actualtemperature of any individual body of the system; but it cannot bedoubted that there is a wide range of temperature among the differentbodies. Some are hotter than others. The stars and suns are perhaps thehottest of all; but it is not improbable that they may be immeasurablyoutnumbered by the cold and dark bodies of the universe, which are to usinvisible, and only manifest their existence in an indirect and casualmanner. 

The law of cooling tells us that every body radiates heat, and that thequantity of heat which it radiates increases when the temperature of thebody increases relatively to the surrounding medium. This law appears tobe universal. It is obeyed on the earth, and it would seem that it mustbe equally obeyed by every other body in space. We thus see that each ofthe planets and each of the stars is continuously pouring forth in alldirections a never-ceasing stream of heat. This radiation of heat isproductive of very momentous consequences. Let us study them, forinstance, in the case of the sun. 

Our great luminary emits an incessant flood of radiant heat in alldirections. A minute fraction of that heat is intercepted by our earth, and is, directly or indirectly, the source of all life, and of nearlyall movement, on our earth. To pour forth heat as the sun does, it isnecessary that his temperature be enormously high. And there are somefacts which permit us to form an estimate of what that temperature mustactually be. 

It is difficult to form any numerical statement of the actualtemperature of the sun. The intensity of that temperature vastlytranscends the greatest artificial heat, and any attempt to clothe suchestimates in figures is necessarily very precarious. But assuming thegreatest artificial temperature to be about 4, 000° Fahr. , we shallprobably be well within the truth if we state the effective temperatureof the sun to be about 14, 000° Fahr. This is the result of a recentinvestigation by Messrs. Wilson and Gray, which seems to be entitled toconsiderable weight. 

The copious outflow of heat from the sun corresponds with its enormoustemperature. We can express the amount of heat in various ways, but itmust be remembered that considerable uncertainty still attaches to suchmeasurements. The old method of measuring heat by the quantity of icemelted may be used as an illustration. It is computed that a shell ofice 43-1/2 feet thick surrounding the whole sun would in one minute bemelted by the sun's heat underneath. A somewhat more elegantillustration was also given by Sir John Herschel, who showed that if acylindrical glacier 45 miles in diameter were to be continually flowinginto the sun with the velocity of light, the end of that glacier wouldbe melted as quickly as it advanced. From each square foot in thesurface of the sun emerges a quantity of heat as great as could beproduced by the daily combustion of sixteen tons of coal. This is, indeed, an amount of heat which, properly transformed into work, wouldkeep an engine of many hundreds of horse-power running from one year'send to the other. The heat radiated from a few acres on the sun would beadequate to drive all the steam engines in the world. When we reflect onthe vast intensity of the radiation from each square foot of the sun'ssurface, and when we combine with this the stupendous dimensions of thesun, imagination fails to realise how vast must be the actualexpenditure of heat. 

In presence of the prodigal expenditure of the sun's heat, we aretempted to ask a question which has the most vital interest for theearth and its inhabitants. We live from hour to hour by the sun'ssplendid generosity; and, therefore, it is important for us to know whatsecurity we possess for the continuance of his favours. When we witnessthe terrific disbursement of the sun's heat each hour, we are compelledto ask whether our great luminary may not be exhausting its resources;and if so, what are the prospects of the future? This question we canpartly answer. The whole subject is indeed of surpassing interest, andredolent with the spirit of modern scientific thought. 

Our first attempt to examine this question must lie in an appeal to thefacts which are attainable. We want to know whether the sun is showingany symptoms of decay. Are the days as warm and as bright now as theywere last year, ten years ago, one hundred years ago? We can find noevidence of any change since the beginning of authentic records. If thesun's heat had perceptibly changed within the last two thousand years, we should expect to find corresponding changes in the distribution ofplants and of animals; but no such changes have been detected. There isno reason to think that the climate of ancient Greece or of ancient Romewas appreciably different from the climates of the Greece and the Romethat we know at this day. The vine and the olive grow now where theygrew two thousand years ago. 

We must not, however, lay too much stress on this argument; for theeffects of slight changes in the sun's heat may have been neutralised bycorresponding adaptations in the pliable organisms of cultivated plants. All we can certainly conclude is that no marked change has taken placein the heat of the sun during historical time. But when we come to lookback into much earlier ages, we find copious evidence that the earth hasundergone great changes in climate. Geological records can on thisquestion hardly be misinterpreted. Yet it is curious to note that thesechanges are hardly such as could arise from the gradual exhaustion ofthe sun's radiation. No doubt, in very early times we have evidencethat the earth's climate must have been much warmer than at present. Wehad the great carboniferous period, when the temperature must almosthave been tropical in Arctic latitudes. Yet it is hardly possible tocite this as evidence that the sun was then much more powerful; for weare immediately reminded of the glacial period, when our temperate zoneswere overlaid by sheets of solid ice, as Northern Greenland is atpresent. If we suppose the sun to have been hotter than it is at presentto account for the vegetation which produced coal, then we ought toassume the sun to be colder than it is now to account for the glacialperiod. It is not reasonable to attribute such phenomena to fluctuationsin the radiation from the sun. The glacial periods prove that we cannotappeal to geology in aid of the doctrine that a secular cooling of thesun is now in progress. The geological variations of climate may havebeen caused by changes in the earth itself, or by changes in its actualorbit; but however they have been caused, they hardly tell us much withregard to the past history of our sun. 

The heat of the sun has lasted countless ages; yet we cannot credit thesun with the power of actually creating heat. We must apply to thetremendous mass of the sun the same laws which we have found by ourexperiments on the earth. We must ask, whence comes the heat sufficientto supply this lavish outgoing? Let us briefly recount the varioussuppositions that have been made. 

Place two red-hot spheres of iron side by side, a large one and a smallone. They have been taken from the same fire; they were both equallyhot; they are both cooling, but the small sphere cools more rapidly. Itspeedily becomes dark, while the large sphere is still glowing, andwould continue to do so for some minutes. The larger the sphere, thelonger it will take to cool; and hence it has been supposed that amighty sphere of the prodigious dimensions of our sun would, if onceheated, cool gradually, but the duration of the cooling would be so longthat for thousands and for millions of years it could continue to be asource of light and heat to the revolving system of planets. Thissuggestion will not bear the test of arithmetic. If the sun had nosource of heat beyond that indicated by its high temperature, we canshow that radiation would cool the sun a few degrees every year. Twothousand years would then witness a very great decrease in the sun'sheat. We are certain that no such decrease can have taken place. Thesource of the sun's radiation cannot be found in the mere cooling of anincandescent mass. 

Can the fires in the sun be maintained by combustion, analogous to thatwhich goes on in our furnaces? Here we would seem to have a source ofgigantic heat; but arithmetic also disposes of this supposition. We knowthat if the sun were made of even solid coal itself, and if that coalwere burning in pure oxygen, the heat that could be produced would onlysuffice for 6, 000 years. If the sun which shone upon the builders of thegreat Pyramid had been solid coal from surface to centre, it must bythis time have been in great part burned away in the attempt to maintainits present rate of expenditure. We are thus forced to look to othersources for the supply of the sun's heat, since neither the heat ofincandescence nor the heat of combustion will suffice. 

There is probably--indeed, we may say certainly--one external sourcefrom which the heat of the sun is recruited. It will be necessary for usto consider this source with some care, though I think we shall find itto be merely an auxiliary of comparatively trifling moment. According tothis view, the solar heat receives occasional accessions from the fallupon the sun's surface of masses of meteoric matter. There can be hardlya doubt that such masses do fall upon the sun; there is certainly nodoubt that if they do, the sun must gain some heat thereby. We haveexperience on the earth of a very interesting kind, which illustratesthe development of heat by meteoric matter. There lies a world ofphilosophy in a shooting star. Some of these myriad objects rush intoour atmosphere and are lost; others, no doubt, rush into the sun withthe same result. We also admit that the descent of a shooting star intothe atmosphere of the sun must be attended with a flash of light and ofheat. The heat acquired by the earth from the flashing of the shootingstars through our air is quite insensible. It has been supposed, however, that the heat accruing to the sun from the same cause may bequite sensible--nay, it has been even supposed that the sun may bere-invigorated from this source. 

Here, again, we must apply the cold principles of weights and measuresto estimate the plausibility of this suggestion. We first calculate theactual weight of meteoric indraught to the sun which would be adequateto sustain the fires of the sun at their present vigour. The mass ofmatter that would be required is so enormous that we cannot usefullyexpress it by imperial weights; we must deal with masses of imposingmagnitude. It fortunately happens that the weight of our moon is aconvenient unit. Conceive that our moon--a huge globe, 2, 000 miles indiameter--were crushed into a myriad of fragments, and that thesefragments were allowed to rain in on the sun; there can be no doubt thatthis tremendous meteoric shower would contribute to the sun rather moreheat than would be required to supply his radiation for a whole year. Ifwe take our earth itself, conceive it comminuted into dust, and allowthat dust to fall on the sun as a mighty shower, each fragment wouldinstantly give out a quantity of heat, and the whole would add to thesun a supply of heat adequate to sustain the present rate of radiationfor nearly one hundred years. The mighty mass of Jupiter treated in thesame way would generate a meteoric display greater in the ratio in whichthe mass of Jupiter exceeds the mass of earth. Were Jupiter to fall intothe sun, enough heat would be thereby produced to scorch the whole solarsystem; while all the planets together would be capable of producingheat which, if properly economised, would supply the radiation of thesun for 45, 000 years. 

It must be remembered that though the moon could supply one year's heat, and Jupiter 30, 000 years' heat, yet the practical question is notwhether the solar system could supply the sun's heat, but whether itdoes. Is it likely that meteors equal in mass to the moon fall into thesun every year? This is the real question, and I think we are bound toreply to it in the negative. It can be shown that the quantity ofmeteors which could be caught by the sun in any one year can be only anexcessively minute fraction of the total amount. If, therefore, amoon-weight of meteors were caught every year, there must be anincredible mass of meteoric matter roaming at large through the system. There must be so many meteors that the earth would be incessantly peltedwith them, and heated to such a degree as to be rendered uninhabitable. There are also other reasons which preclude the supposition that astupendous quantity of meteoric matter exists in the vicinity of thesun. Such matter would produce an appreciable effect on the movement ofthe planet Mercury. There are, no doubt, some irregularities in themovements of Mercury not yet fully explained, but these irregularitiesare very much less than would be the case if meteoric matter existed inquantity adequate to the sustentation of the sun. Astronomers, then, believe that though meteors may provide a rate in aid of the sun'scurrent expenditure, yet that the greater portion of that expendituremust be defrayed from other resources. 

It is one of the achievements of modern science to have effected thesolution of the problem--to have shown how it is that, notwithstandingthe stupendous radiation, the sun still maintains its temperature. Thequestion is not free from difficulty in its exposition, but the matteris one of such very great importance that we are compelled to make theattempt. 

Let us imagine a vast globe of heated gas in space. This is not anentirely gratuitous supposition, inasmuch as there are globes apparentlyof this character; they have been already alluded to as planetarynebulæ. This globe will radiate heat, and we shall suppose that it emitsmore heat than it receives from the radiation of other bodies. The globewill accordingly lose heat, or what is equivalent thereto, but it willbe incorrect to assume that the globe will necessarily fall intemperature. That the contrary is, indeed, the case is a result almostparadoxical at the first glance; but yet it can be shown to be anecessary consequence of the laws of heat and of gases. 

Let us fix our attention on a portion of the gas lying on the surfaceof the globe. This is, of course, attracted by all the rest of theglobe, and thus tends in towards the centre of the globe. If equilibriumsubsists, this tendency must be neutralised by the pressure of the gasbeneath; so that the greater the gravitation, the greater is thepressure. When the globe of gas loses heat by radiation, let us supposethat it grows colder--that its temperature accordingly falls; then, since the pressure of a gas decreases when the temperature falls, thepressure beneath the superficial layer of the gas will decrease, whilethe gravitation is unaltered. The consequence will inevitably be thatthe gravitation will now conquer the pressure, and the globe of gas willaccordingly contract. There is, however, another way in which we canlook at the matter. We know that heat is equivalent to energy, so thatwhen the globe radiates forth heat, it must expend energy. A part of theenergy of the globe will be due to its temperature; but another, and insome respects a more important, part is that due to the separation ofits particles. If we allow the particles to come closer together weshall diminish the energy due to separation, and the energy thus setfree can take the form of heat. But this drawing in of the particlesnecessarily involves a shrinking of the globe. 

And now for the remarkable consequence, which seems to have a veryimportant application in astronomy. As the globe contracts, a part ofits energy of separation is changed into heat; that heat is partlyradiated away, but not so rapidly as it is produced by the contraction. The consequence is, that although the globe is really losing heat andreally contracting, yet that its temperature is actually rising. [43] Asimple case will suffice to demonstrate this result, paradoxical as itmay at first seem. Let us suppose that by contraction of the sphere ithad diminished to one-half its diameter; and let us fix our attention ona cubic inch of the gaseous matter in any point of the mass. After thecontraction has taken place each edge of the cube would be reduced tohalf an inch, and the volume would therefore be reduced to one-eighthpart of its original amount. The law of gases tells us that if thetemperature be unaltered the pressure varies inversely as the volume, and consequently the internal pressure in the cube would in that case beincreased eightfold. As, however, in the case before us, the distancebetween every two particles is reduced to one-half, it will follow thatthe gravitation between every two particles is increased fourfold, andas the area is also reduced to one-fourth, it will follow that thepressure inside the reduced cube is increased sixteenfold; but we havealready seen that with a constant temperature it only increaseseightfold, and hence the temperature cannot be constant, but must risewith the contraction. 

We thus have the somewhat astonishing result that a gaseous globe inspace radiating heat, and thereby growing smaller, is all the timeactually increasing in temperature. But, it may be said, surely thiscannot go on for ever. Are we to suppose that the gaseous mass will goon contracting and contracting with a temperature ever fiercer andfiercer, and actually radiating out more and more heat the more itloses? Where lies the limit to such a prospect? As the body contracts, its density must increase, until it either becomes a liquid, or a solid, or, at any rate, until it ceases to obey the laws of a purely gaseousbody which we have supposed. Once these laws cease to be observed theargument disappears; the loss of heat may then really be attended with aloss of temperature, until in the course of time the body has sunk tothe temperature of space itself. 

It is not assumed that this reasoning can be applied in all itscompleteness to the present state of the sun. The sun's density is nowso great that the laws of gases cannot be there strictly followed. Thereis, however, good reason to believe that the sun was once more gaseousthan at present; possibly at one time he may have been quite gaseousenough to admit of this reasoning in all its fulness. At present the sunappears to be in some intermediate stage of its progress from thegaseous condition to the solid condition. We cannot, therefore, saythat the temperature of the sun is now increasing in correspondence withthe process of contraction. This may be true or it may not be true; wehave no means of deciding the point. We may, however, feel certain thatthe sun is still sufficiently gaseous to experience in some degree therise of temperature associated with the contraction. That rise intemperature may be partly or wholly obscured by the fall in temperaturewhich would be the more obvious consequence of the radiation of heatfrom the partially solid body. It will, however, be manifest that thecooling of the sun may be enormously protracted if the fall oftemperature from the one cause be nearly compensated by the rise oftemperature from the other. It can hardly be doubted that in this wefind the real explanation of the fact that we have no historicalevidence of any appreciable alteration in the radiation of heat from thesun. 

This question is one of such interest that it may be worth while to lookat it from a slightly different point of view. The sun contains acertain store of energy, part of which is continually disappearing inthe form of radiant heat. The energy remaining in the sun is partlytransformed in character; some of it is transformed into heat, whichgoes wholly or partly to supply the loss by radiation. The total energyof the sun must, however, be decreasing; and hence it would seem the sunmust at some time or other have its energy exhausted, and cease to be asource of light and of heat. It is true that the rate at which the suncontracts is very slow. We are, indeed, not able to measure withcertainty the decrease in the sun's bulk. It is a quantity so minute, that the contraction since the birth of accurate astronomy is not largeenough to be perceptible in our telescopes. It is, however, possible tocompute what the contraction of the sun's bulk must be, on thesupposition that the energy lost by that contraction just suffices tosupply the daily radiation of heat. The change is very small when weconsider the present size of the sun. At the present time the sun'sdiameter is about 860, 000 miles. If each year this diameter decreases byabout 300 feet, sufficient energy will be yielded to account for theentire radiation. This gradual decrease is always in progress. 

These considerations are of considerable interest when we apply themretrospectively. If it be true that the sun is at this moment shrinking, then in past times his globe must have been greater than it is atpresent. Assuming the figures already given, it follows that one hundredyears ago the diameter of the sun must have been nearly six milesgreater than it is now; one thousand years ago the diameter wasfifty-seven miles greater; ten thousand years ago the diameter of thesun was five hundred and seventy miles greater than it is to-day. Whenman first trod this earth it would seem that the sun must have been manyhundreds, perhaps many thousands, of miles greater than it is at thistime. 

We must not, however, over-estimate the significance of this statement. The diameter of the sun is so great, that a diminution of 10, 000 mileswould be but little more than the hundredth part of its diameter. If itwere suddenly to shrink to the extent of 10, 000 miles, the change wouldnot be appreciable to ordinary observation, though a much smaller changewould not elude delicate astronomical measurement. It does notnecessarily follow that the climates on our earth in these early timesmust have been very different from those which we find at this day, forthe question of climate depends upon other matters besides sunbeams. 

Yet we need not abruptly stop our retrospect at any epoch, howeverremote. We may go back earlier and earlier, through the long ages whichgeologists claim for the deposition of the stratified rocks; and backagain still further, to those very earliest epochs when life began todawn on the earth. Still we can find no reason to suppose that the lawof the sun's decreasing heat is not maintained; and thus we would seembound by our present knowledge to suppose that the sun grows larger andlarger the further our retrospect extends. We cannot assume that therate of that growth is always the same. No such assumption is required;it is sufficient for our purpose that we find the sun growing largerand larger the further we peer back into the remote abyss of time past. If the present order of things in our universe has lasted long enough, then it would seem that there was a time when the sun must have beentwice as large as it is at present; it must once have been ten times aslarge. How long ago that was no one can venture to say. But we cannotstop at the stage when the sun was even ten times as large as it is atpresent; the arguments will still apply in earlier ages. We see the sunswelling and swelling, with a corresponding decrease in its density, until at length we find, instead of our sun as we know it, a mightynebula filling a gigantic region of space. 

Such is, in fact, the doctrine of the origin of our system which hasbeen advanced in that celebrated speculation known as the nebular theoryof Laplace. Nor can it be ever more than a speculation; it cannot beestablished by observation, nor can it be proved by calculation. It ismerely a conjecture, more or less plausible, but perhaps in some degreenecessarily true, if our present laws of heat, as we understand them, admit of the extreme application here required, and if also the presentorder of things has reigned for sufficient time without the interventionof any influence at present unknown to us. This nebular theory is notconfined to the history of our sun. Precisely similar reasoning may beextended to the individual planets: the farther we look back, the hotterand the hotter does the whole system become. It has been thought that ifwe could look far enough back, we should see the earth too hot for life;back further still, we should find the earth and all the planetsred-hot; and back further still, to an exceedingly remote epoch, whenthe planets would be heated just as much as our sun is now. In a stillearlier stage the whole solar system is thought to have been one vastmass of glowing gas, from which the present forms of the sun, with theplanets and their satellites, have been gradually evolved. We cannot besure that the course of events has been what is here indicated; butthere are sufficient grounds for thinking that this doctrinesubstantially represents what has actually occurred. 

Many of the features in the solar system harmonise with the suppositionthat the origin of the system has been that suggested by the nebulartheory. We have already had occasion in an earlier chapter to allude tothe fact that all the planets perform their revolutions around the sunin the same direction. It is also to be observed that the rotation ofthe planets on their axes, as well as the movements of the satellitesaround their primaries, all follow the same law, with two slightexceptions in the case of the Uranian and Neptunian systems. Acoincidence so remarkable naturally suggests the necessity for somephysical explanation. Such an explanation is offered by the nebulartheory. Suppose that countless ages ago a mighty nebula was slowlyrotating and slowly contracting. In the process of contraction, portionsof the condensed matter of the nebula would be left behind. Theseportions would still revolve around the central mass, and each portionwould rotate on its axis in the same direction. As the process ofcontraction proceeded, it would follow from dynamical principles thatthe velocity of rotation would increase; and thus at length theseportions would consolidate into planets, while the central mass wouldgradually contract to form the sun. By a similar process on a smallerscale the systems of satellites were evolved from the contractingprimary. These satellites would also revolve in the same direction, andthus the characteristic features of the solar system could be accountedfor. 

The nebular origin of the solar system receives considerable countenancefrom the study of the sidereal heavens. We have already dwelt upon theresemblance between the sun and the stars. If, then, our sun has passedthrough such changes as the nebular theory requires, may we notanticipate that similar phenomena should be met with in other stars? Ifthis be so, it is reasonable to suppose that the evolution of some ofthe stars may not have progressed so far as has that of the sun, andthus we may be able actually to witness stars in the earlier phases oftheir development. Let us see how far the telescope responds to theseanticipations. 

The field of view of a large telescope usually discloses a number ofstars scattered over a black background of sky; but the blackness ofthe background is not uniform: the practised eye of the skilled observerwill detect in some parts of the heavens a faint luminosity. This willsometimes be visible over the whole extent of the field, or it may evenoccupy several fields. Years may pass on, and still there is noperceptible change. There can be no illusion, and the conclusion isirresistible that the object is a stupendous mass of faintly luminousglowing gas or vapour. This is the simplest type of nebula; it ischaracterised by extreme faintness, and seems composed of matter of theutmost tenuity. On the other hand we are occasionally presented with thebeautiful and striking phenomenon of a definite and brilliant starsurrounded by a luminous atmosphere. Between these two extreme types ofa faint diffused mass on the one hand, and a bright star with a nebulasurrounding it on the other, a graduated series of various other nebulæcan be arranged. We thus have a series of links passing by imperceptiblegradations from the most faintly diffused nebulæ on the one side, intostars on the other. 

The nebulæ seemed to Herschel to be vast masses of phosphorescentvapour. This vapour gradually cools down, and ultimately condenses intoa star, or a cluster of stars. When the varied forms of nebulæ wereclassified, it almost seemed as if the different links in the processcould be actually witnessed. In the vast faint nebulæ the process ofcondensation had just begun; in the smaller and brighter nebulæ thecondensation had advanced farther; while in others, the star, or stars, arising from the condensation had already become visible. 

But, it may be asked, how did Herschel know this? what is his evidence?Let us answer this question by an illustration. Go into a forest, andlook at a noble old oak which has weathered the storm for centuries;have we any doubt that the oak-tree was once a young small plant, andthat it grew stage by stage until it reached maturity? Yet no one hasever followed an oak-tree through its various stages; the brief span ofhuman life has not been long enough to do so. The reason why we believethe oak-tree to have passed through all these stages is, because we arefamiliar with oak-trees of every gradation in size, from the seedlingup to the noble veteran. Having seen this gradation in a vast multitudeof trees, we are convinced that each individual passes through all thesestages. 

It was by a similar train of reasoning that Herschel was led to adoptthe view of the origin of the stars which we have endeavoured todescribe. The astronomer's life is not long enough, the life of thehuman race might not be long enough, to watch the process by which anebula condenses down so as to form a solid body. But by looking at onenebula after another, the astronomer thinks he is able to detect thevarious stages which connect the nebula in its original form with thefinal form. He is thus led to believe that each of the nebulæ passes, inthe course of ages, through these stages. And thus Herschel adopted theopinion that stars--some, many, or all--have each originated from whatwas once a glowing nebula. 

Such a speculation may captivate the imagination, but it must becarefully distinguished from the truths of astronomy, properly socalled. Remote posterity may perhaps obtain evidence on the subjectwhich to us is inaccessible: our knowledge of nebulæ is too recent. There has not yet been time enough to detect any appreciable changes:for the study of nebulæ can only be said to date from Messier'sCatalogue in 1771. 

Since Herschel's time, no doubt, many careful drawings and observationsof the nebulæ have been obtained; but still the interval has been muchtoo short, and the earlier observations are too imperfect, to enable anychanges in the nebulæ to be investigated with sufficient accuracy. Ifthe human race lasts for very many centuries, and if our presentobservations are preserved during that time for comparison, thenHerschel's theory may perhaps be satisfactorily tested. 

A hundred years have passed since Laplace, with some diffidence, setforth his hypothesis as to the mode of formation of the solar system. Onthe whole it must be said that this "nebular hypothesis" has stood thetest of advancing science well, though some slight modifications havebecome necessary in the light of more recent discoveries. Laplace (andHerschel also) seems to have considered a primitive nebula to consist ofa "fiery mist" or glowing gas at a very high temperature. But this is byno means necessary, as we have seen that the gradual contraction of thevast mass supplies energy which may be converted into heat, and thespectroscopic evidence seems also to point to the existence of amoderate temperature in the gaseous nebulæ, which must be considered tobe representatives of the hypothetical primitive chaos out of which oursun and planets have been evolved. Another point which has beenreconsidered is the formation of the various planets. It was formerlythought that the rotation of the original mass had by degrees caused anumber of rings of different dimensions to be separated from the centralpart, the material of which rings in time collected into single planets. The ring of Saturn was held to be a proof of this process, since we herehave a ring, the condensation of which into one or more satellites hassomehow been arrested. But while it is not impossible that matter in theshape of rings may have been left behind during the contraction of thenebulous mass (indeed, the minor planets between Mars and Jupiter haveperhaps originated in this way), it seems likely that the larger planetswere formed from the agglomeration of matter at a point on the equatorof the rotating nebula. 

The actual steps of the process by which the primeval nebula becametransformed into the solar system seem to lie beyond reach of discovery. 

CHAPTER XXVII. 

THE TIDES. [44]

 Mathematical Astronomy--Lagrange's Theories: how far they are really True--The Solar System not Made of Rigid Bodies--Kepler's Laws True to Observation, but not Absolutely True when the Bodies are not Rigid--The Errors of Observation--The Tides--How the Tides were Observed--Discovery of the Connection between the Tides and the Moon--Solar and Lunar Tides--Work done by the Tides--Whence do the Tides obtain the Power to do the Work?--Tides are Increasing the Length of the Day--Limit to the Shortness of the Day--Early History of the Earth-Moon System--Unstable Equilibrium--Ratio of the Month to the Day--The Future Course of the System--Equality of the Month and the Day--The Future Critical Epoch--The Constant Face of the Moon accounted for--The other Side of the Moon--The Satellites of Mars--Their Remarkable Motions--Have the Tides Possessed Influence in Moulding the Solar System generally?--Moment of Momentum--Tides have had little or no Appreciable Effect on the Orbit of Jupiter--Conclusion. 

That the great discoveries of Lagrange on the stability of the planetarysystem are correct is in one sense strictly true. No one has everventured to impugn the mathematics of Lagrange. Given the planetarysystem in the form which Lagrange assumed and the stability of thatsystem is assured for all time. There is, however, one assumption whichLagrange makes, and on which his whole theory was founded: hisassumption is that the planets are _rigid_ bodies. 

No doubt our earth seems a rigid body. What can be more solid andunyielding than the mass of rocks and metals which form the earth, sofar as it is accessible to us? In the wide realms of space the earth isbut as a particle; it surely was a natural and a legitimate assumptionto suppose that that particle was a rigid body. If the earth wereabsolutely rigid--if every particle of the earth were absolutely at afixed distance from every other particle--if under no stress of forces, and in no conceivable circumstance, the earth experienced even theminutest change of form--if the same could be said of the sun and of allthe other planets--then Lagrange's prediction of the eternal duration ofour system must be fulfilled. 

But what are the facts of the case? Is the earth really rigid? We knowfrom experiment that a rigid body in the mathematical sense of the worddoes not exist. Rocks are not rigid; steel is not rigid; even a diamondis not perfectly rigid. The whole earth is far from being rigid even onthe surface, while part of the interior is still, perhaps, more or lessfluid. The earth cannot be called a perfectly rigid body; still less canthe larger bodies of our system be called rigid. Jupiter and Saturn areperhaps hardly even what could be called solid bodies. The solar systemof Lagrange consisted of a rigid sun and a number of minute rigidplanets; the actual solar system consists of a sun which is in no senserigid, and planets which are only partially so. 

The question then arises as to whether the discoveries of the greatmathematicians of the last century will apply, not only to the idealsolar system which they conceived, but to the actual solar system inwhich our lot has been cast. There can be no doubt that thesediscoveries are approximately true: they are, indeed, so near theabsolute truth, that observation has not yet satisfactorily shown anydeparture from them. 

But in the present state of science we can no longer overlook theimportant questions which arise when we deal with bodies not rigid inthe mathematical sense of the word. Let us, for instance, take thesimplest of the laws to which we have referred, the great law of Kepler, which asserts that a planet will revolve for ever in an elliptic path ofwhich the sun is one focus. This is seen to be verified by actualobservation; indeed, it was established by observation before anytheoretical explanation of that movement was propounded. If, however, westate the matter with a little more precision, we shall find that whatNewton really demonstrated was, that if two _rigid_ particles attracteach other by a law of force which varies with the inverse square of thedistance between the particles, then each of the particles will describean ellipse with the common centre of gravity in the focus. The earth is, to some extent, rigid, and hence it was natural to suppose that therelative behaviour of the earth and the sun would, to a correspondingextent, observe the simple elliptic law of Kepler; as a matter of fact, they do observe it with such fidelity that, if we make allowance forother causes of disturbance, we cannot, even by most carefulobservation, detect the slightest variation in the motion of the eartharising from its want of rigidity. 

There is, however, a subtlety in the investigations of mathematicswhich, in this instance at all events, transcends the most delicateobservations which our instruments enable us to make. The principles ofmathematics tell us that though Kepler's laws may be true for bodieswhich are absolutely and mathematically rigid, yet that if the sun orthe planets be either wholly, or even in their minutest part, devoid ofperfect rigidity, then Kepler's laws can be no longer true. Do we notseem here to be in the presence of a contradiction? Observation tells usthat Kepler's laws are true in the planetary system; theory tells usthat these laws cannot be true in the planetary system, because thebodies in that system are not perfectly rigid. How is this discrepancyto be removed? Or is there really a discrepancy at all? There is not. When we say that Kepler's laws have been proved to be true byobservation, we must reflect on the nature of the proofs which areattainable. We observe the places of the planets with the instruments inour observatories; these places are measured by the help of our clocksand of the graduated circles on the instruments. These observations areno doubt wonderfully accurate; but they do not, they cannot, possessabsolute accuracy in the mathematical sense of the word. We can, forinstance, determine the place of a planet with such precision that itis certainly not one second of arc wrong; and one second is an extremelysmall quantity. A foot-rule placed at a distance of about forty milessubtends an angle of a second, and it is surely a delicate achievementto measure the place of a planet, and feel confident that no errorgreater than this can have intruded into our result. 

When we compare the results of observation with the calculationsconducted on the assumption of the truth of Kepler's laws, and when wepronounce on the agreement of the observations with the calculations, there is always a reference, more or less explicit, to the inevitableerrors of the observations. If the calculations and observations agreeso closely that the differences between the two are minute enough tohave arisen in the errors inseparable from the observations, then we aresatisfied with the accordance; for, in fact, no closer agreement isattainable, or even conceivable. The influence which the want ofrigidity exercises on the fulfilment of the laws of Kepler can beestimated by calculation; it is found, as might be expected, to beextremely small--so small, in fact, as to be contained within thatslender margin of error by which observations are liable to be affected. We are thus not able to discriminate by actual measurement the effectsdue to the absence of rigidity; they are inextricably hid among thesmall errors of observation. 

The argument on which we are to base our researches is really founded ona very familiar phenomenon. There is no one who has ever visited thesea-side who is not familiar with that rise and fall of the sea which wecall the tide. Twice every twenty-four hours the sea advances on thebeach to produce high tide; twice every day the sea again retreats toproduce low tide. These tides are not merely confined to the coasts;they penetrate for miles up the courses of rivers; they periodicallyinundate great estuaries. In a maritime country the tides are of themost profound practical importance; they also possess a significance ofa far less obvious character, which it is our object now to investigate. 

These daily pulses of the ocean have long ceased to be a mystery. Itwas in the earliest times perceived that there was a connection betweenthe tides and the moon. Ancient writers, such as Pliny and Aristotle, have referred to the alliance between the times of high water and theage of the moon. I think we sometimes do not give the ancientastronomers as much credit as their shrewdness really entitles them to. We have all read--we have all been taught--that the moon and the tidesare connected together; but how many of us are in a position to say thatwe have actually noticed that connection by direct personal observation?The first man who studied this matter with sufficient attention toconvince himself and to convince others of its reality must have been agreat philosopher. We know not his name, we know not his nation, we knownot the age in which he lived; but our admiration of his discovery mustbe increased by the reflection that he had not the theory of gravitationto guide him. A philosopher of the present day who had never seen thesea could still predict the necessity of tides as a consequence of thelaw of universal gravitation; but the primitive astronomer, who knew notof the invisible bond by which all bodies in the universe are drawntogether, made a splendid--indeed, a typical--inductive discovery, whenhe ascertained the relation between the moon and the tides. 

We can surmise that this discovery, in all probability, first arose fromthe observations of experienced navigators. In all matters of enteringport or of leaving port, the state of the tide is of the utmost concernto the sailor. Even in the open sea he has sometimes to shape his coursein accordance with the currents produced by the tides; or, in guidinghis course by taking soundings, he has always to bear in mind that thedepth varies with the tide. All matters relating to the tide would thuscome under his daily observation. His daily work, the success of hisoccupation, the security of his life, depend often on the tides; andhence he would be solicitous to learn from his observation all thatwould be useful to him in the future. To the coasting sailor thequestion of the day is the time of high water. That time varies fromday to day; it is an hour or more later to-morrow than to-day, and thereis no very simple rule which can be enunciated. The sailor wouldtherefore welcome gladly any rule which would guide him in a matter ofsuch importance. We can make a conjecture as to the manner in which sucha rule was first discovered. Let us suppose that a sailor at Calais, forexample, is making for harbour. He has a beautiful night--the moon isfull; it guides him on his way; he gets safely into harbour; and thenext morning he finds the tide high between 11 and 12. [45] He oftenrepeats the same voyage, but he finds sometimes a low and inconvenienttide in the morning. At length, however, it occurs to him that _when hehas a moonlight night_ he has a high tide at 11. This occurs once ortwice: he thinks it but a chance coincidence. It occurs again and again. At length he finds it always occurs. He tells the rule to other sailors;they try it too. It is invariably found that when the moon is full, thehigh tide always recurs at the same hour at the same place. Theconnection between the moon and the tide is thus established, and theintelligent sailor will naturally compare other phases of the moon withthe times of high water. He finds, for example, that the moon at thefirst quarter always gives high water at the same hour of the day; andfinally, he obtains a practical rule, by which, from the state of themoon, he can at once tell the time when the tide will be high at theport where his occupation lies. A diligent observer will trace a stillfurther connection between the moon and the tides; he will observe thatsome high tides rise higher than others, that some low tides fall lowerthan others. This is a matter of much practical importance. When adangerous bar has to be crossed, the sailor will feel much additionalsecurity in knowing that he is carried over it on the top of a springtide; or if he has to contend against tidal currents, which in someplaces have enormous force, he will naturally prefer for his voyage theneap tides, in which the strength of these currents is less than usual. The spring tides and the neap tides will become familiar to him, and hewill perceive that the spring tides occur when the moon is full ornew--or, at all events, that the spring tides are within a certainconstant number of days of the full or new moon. It was, no doubt, byreasoning such as this, that in primitive times the connection betweenthe moon and the tides came to be perceived. 

It was not, however, until the great discovery of Newton had disclosedthe law of universal gravitation that it became possible to give aphysical explanation of the tides. It was then seen how the moonattracts the whole earth and every particle of the earth. It was seenhow the fluid particles which form the oceans on the earth were enabledto obey the attraction in a way that the solid parts could not. When themoon is overhead it tends to draw the water up, as it were, into a heapunderneath, and thus to give rise to the high tide. The water on theopposite side of the earth is also affected in a way that might not beat first anticipated. The moon attracts the solid body of the earth withgreater intensity than it attracts the water at the other side whichlies more distant from it. The earth is thus drawn away from the water, and there is therefore a tendency to a high tide as well on the side ofthe earth away from the moon as on that towards the moon. The low tidesoccupy the intermediate positions. 

The sun also excites tides on the earth; but owing to the great distanceof the sun, the difference between its attraction on the sea and on thesolid interior of the earth is not so appreciable. The solar tides arethus smaller than the lunar tides. When the two conspire, they cause aspring tide; when the solar and lunar tides are opposed, we have theneap tide. 

There are, however, a multitude of circumstances to be taken intoaccount when we attempt to apply this general reasoning to theconditions of a particular case. Owing to local peculiarities the tidesvary enormously at the different parts of the coast. In a confined arealike the Mediterranean Sea, the tides have only a comparatively smallrange, varying at different places from one foot to a few feet. Inmid-ocean also the tidal rise and fall is not large, amounting, forinstance, to a range of three feet at St. Helena. Near the greatcontinental masses the tides become very much modified by the coasts. Wefind at London a tide of eighteen or nineteen feet; but the mostremarkable tides in the British Islands are those in the BristolChannel, where, at Chepstow or Cardiff, there is a rise and fall duringspring tides to the height of thirty-seven or thirty-eight feet, and atneap tides to a height of twenty-eight or twenty-nine. These tides aresurpassed in magnitude at other parts of the world. The greatest of alltides are those in the Bay of Fundy, at some parts of which the rise andfall at spring tides is not less than fifty feet. 

The rising and falling of the tide is necessarily attended with theformation of currents. Such currents are, indeed, well known, and insome of our great rivers they are of the utmost consequence. Thesecurrents of water can, like water-streams of any other kind, be made todo useful work. We can, for instance, impound the rising water in areservoir, and as the tide falls we can compel the enclosed water towork a water-wheel before it returns to the sea. We have, indeed, here asource of actual power; but it is only in very unusual circumstancesthat it is found to be economical to use the tides for this purpose. Thequestion can be submitted to calculation, and the area of the reservoircan be computed which would retain sufficient water to work awater-wheel of given horse-power. It can be shown that the area of thereservoir necessary to impound water enough to produce 100 horse-powerwould be 40 acres. The whole question is then reduced to the simple oneof expense: would the construction and the maintenance of this reservoirbe more or less costly than the erection and the maintenance of asteam-engine of equivalent power? In most cases it would seem that thelatter would be by far the cheaper; at all events, we do not practicallyfind tidal engines in use, so that the power of the tides is now runningto waste. The economical aspects of the case may, however, be veryprofoundly altered at some remote epoch, when our stores of fuel, nowso lavishly expended, give appreciable signs of approaching exhaustion. 

The tides are, however, _doing work_ of one kind or another. A tide in ariver estuary will sometimes scour away a bank and carry its materialselsewhere. We have here work done and energy consumed, just as much asif the same task had been accomplished by engineers directing thepowerful arms of navvies. We know that work cannot be done without theconsumption of energy in some of its forms; whence, then, comes theenergy which supplies the power of the tides? At a first glance theanswer to this question seems a very obvious one. Have we not said thatthe tides are caused by the moon? and must not the energy, therefore, bederived from the moon? This seems plain enough, but, unfortunately, itis not true. It is one of those cases by no means infrequent inDynamics, where the truth is widely different from that which seems tobe the case. An illustration will perhaps make the matter clearer. Whena rifle is fired, it is the finger of the rifleman that pulls thetrigger; but are we, then, to say that the energy by which the bullethas been driven off has been supplied by the rifleman? Certainly not;the energy is, of course, due to the gunpowder, and all the rifleman didwas to provide the means by which the energy stored up in the powdercould be liberated. To a certain extent we may compare this with thetidal problem; the tides raised by the moon are the originating causewhereby a certain store of energy is drawn upon and applied to do suchwork as the tides are competent to perform. This store of energy, strange to say, does not lie in the moon; it is in the earth itself. Indeed, it is extremely remarkable that the moon actually gains energyfrom the tides by itself absorbing some of the store which exists in theearth. This is not put forward as an obvious result; it depends upon arefined dynamical theorem. 

We must clearly understand the nature of this mighty store of energyfrom which the tides draw their power, and on which the moon ispermitted to make large and incessant drafts. Let us see in what sensethe earth is said to possess a store of energy. We know that the earthrotates on its axis once every day. It is this rotation which is thesource of the energy. Let us compare the rotation of the earth with therotation of the fly-wheel belonging to a steam-engine. The rotation ofthe fly-wheel is really a reservoir, into which the engine pours energyat each stroke of the piston. The various machines in the mill worked bythe engine merely draw upon the store of energy accumulated in thefly-wheel. The earth may be likened to a gigantic fly-wheel detachedfrom the engine, though still connected with the machines in the mill. From its stupendous dimensions and from its rapid velocity, that greatfly-wheel possesses an enormous store of energy, which must be expendedbefore the fly-wheel comes to rest. Hence it is that, though the tidesare caused by the moon, yet the energy they require is obtained bysimply appropriating some of the vast supply available from the rotationof the earth. 

There is, however, a distinction of a very fundamental character betweenthe earth and the fly-wheel of an engine. As the energy is withdrawnfrom the fly-wheel and consumed by the various machines in the mill, itis continually replaced by fresh energy, which flows in from theexertions of the steam-engine, and thus the velocity of the fly-wheel ismaintained. But the earth is a fly-wheel without the engine. When thetides draw upon the store of energy and expend it in doing work, thatenergy is not replaced. The consequence is irresistible: the energy inthe rotation of the earth must be decreasing. This leads to aconsequence of the utmost significance. If the engine be cut off fromthe fly-wheel, then, as everyone knows, the massive fly-wheel may stillgive a few rotations, but it will speedily come to rest. A similarinference must be made with regard to the earth; but its store of energyis so enormous, in comparison with the demands which are made upon it, that the earth is able to hold out. Ages of countless duration mustelapse before the energy of the earth's rotation can be completelyexhausted by such drafts as the tides are capable of making. Nevertheless, it is necessarily true that the energy is decreasing; andif it be decreasing, then the speed of the earth's rotation must besurely, if slowly, abating. Now we have arrived at a consequence of thetides which admits of being stated in the simplest language. If thespeed of rotation be abating, then the length of the day must beincreasing; and hence we are conducted to the following most importantstatement: that the _tides are increasing the length of the day_. 

To-day is longer than yesterday--to-morrow will be longer than to-day. The difference is so small that even in the course of ages it can hardlybe said to have been distinctly established by observation. We do notpretend to say how many centuries have elapsed since the day was evenone second shorter than it is at present; but centuries are not theunits which we employ in tidal evolution. A million years ago it isquite probable that the divergence of the length of the day from itspresent value may have been very considerable. Let us take a glance backinto the profound depths of times past, and see what the tides have totell us. If the present order of things has lasted, the day must havebeen shorter and shorter the farther we look back into the dim past. Theday is now twenty-four hours; it was once twenty hours, once ten hours;it was once six hours. How much farther can we go? Once the six hours ispast, we begin to approach a limit which must at some point bound ourretrospect. The shorter the day the more is the earth bulged at theequator; the more the earth is bulged at the equator the greater is thestrain put upon the materials of the earth by the centrifugal force ofits rotation. If the earth were to go too fast it would be unable tocohere together; it would separate into pieces, just as a grindstonedriven too rapidly is rent asunder with violence. Here, therefore, wediscern in the remote past a barrier which stops the present argument. There is a certain critical velocity which is the greatest that theearth could bear without risk of rupture, but the exact amount of thatvelocity is a question not very easy to answer. It depends upon thenature of the materials of the earth; it depends upon the temperature;it depends upon the effect of pressure, and on other details notaccurately known to us. An estimate of the critical velocity has, however, been made, and it has been shown mathematically that theshortest period of rotation which the earth could have, without flyinginto pieces, is about three or four hours. The doctrine of tidalevolution has thus conducted us to the conclusion that, at someinconceivably remote epoch, the earth was spinning round its axis in aperiod approximating to three or four hours. 

We thus learn that we are indebted to the moon for the gradualelongation of the day from its primitive value up to twenty-four hours. In obedience to one of the most profound laws of nature, the earth hasreacted on the moon, and the reaction of the earth has taken a tangibleform. It has simply consisted in gradually driving the moon away fromthe earth. You may observe that this driving away of the moon resemblesa piece of retaliation on the part of the earth. The consequence of theretreat of the moon is sufficiently remarkable. The path in which themoon is revolving has at the present time a radius of 240, 000 miles. This radius must be constantly growing larger, in consequence of thetides. Provided with this fact, let us now glance back into the pasthistory of the moon. As the moon's distance is increasing when we lookforwards, so we find it decreasing when we look backwards. The moon musthave been nearer the earth yesterday than it is to-day; the differenceis no doubt inappreciable in years, in centuries, or in thousands ofyears; but when we come to millions of years, the moon must have beensignificantly closer than it is at present, until at length we find thatits distance, instead of 240, 000 miles, has dwindled down to 40, 000, to20, 000, to 10, 000 miles. Nor need we stop--nor can we stop--until wefind the moon actually close to the earth's surface. If the present lawsof nature have operated long enough, and if there has been no externalinterference, then it cannot be doubted that the moon and the earth wereonce in immediate proximity. We can, indeed, calculate the period inwhich the moon must have been revolving round the earth. The nearer themoon is to the earth the quicker it must revolve; and at the criticalepoch when the satellite was in immediate proximity to our earth itmust have completed each revolution in about three or four hours. 

This has led to one of the most daring speculations which has ever beenmade in astronomy. We cannot refrain from enunciating it; but it must beremembered that it is only a speculation, and to be received withcorresponding reserve. The speculation is intended to answer thequestion, What brought the moon into that position, close to the surfaceof the earth? We will only say that there is the gravest reason tobelieve that the moon was, at some very early period, fractured off fromthe earth when the earth was in a soft or plastic condition. 

At the beginning of the history we found the earth and the moon closetogether. We found that the rate of rotation of the earth was only a fewhours, instead of twenty-four hours. We found that the moon completedits journey round the primitive earth in exactly the same time as theprimitive earth rotated on its axis, so that the two bodies were thenconstantly face to face. Such a state of things formed what amathematician would describe as a case of unstable dynamicalequilibrium. It could not last. It may be compared to the case of aneedle balanced on its point; the needle must fall to one side or theother. In the same way, the moon could not continue to preserve thisposition. There were two courses open: the moon must either have fallenback on the earth, and been reabsorbed into the mass of the earth, or itmust have commenced its outward journey. Which of these courses was themoon to adopt? We have no means, perhaps, of knowing exactly what it waswhich determined the moon to one course rather than to another, but asto the course which was actually taken there can be no doubt. The factthat the moon exists shows that it did not return to the earth, butcommenced its outward journey. As the moon recedes from the earth itmust, in conformity with Kepler's laws, require a longer time tocomplete its revolution. It has thus happened that, from the originalperiod of only a few hours, the duration has increased until it hasreached the present number of 656 hours. The rotation of the earth has, of course, also been modified, in accordance with the retreat of themoon. Once the moon had commenced to recede, the earth was released fromthe obligation which required it constantly to direct the same face tothe moon. When the moon had receded to a certain distance, the earthwould complete the rotation in less time than that required by the moonfor one revolution. Still the moon gets further and further away, andthe duration of the revolution increases to a corresponding extent, until three, four, or more days (or rotations of the earth) areidentical with the month (or revolution of the moon). Although thenumber of days in the month increases, yet we are not to suppose thatthe rate of the earth's rotation is increasing; indeed, the contrary isthe fact. The earth's rotation is getting slower, and so is therevolution of the moon, but the retardation of the moon is greater thanthat of the earth. Even though the period of rotation of the earth hasgreatly increased from its primitive value, yet the period of the moonhas increased still more, so that it is several times as large as thatof the rotation of the earth. As ages roll on the moon recedes furtherand further, its orbit increases, the duration of the revolutionaugments, until at length a very noticeable epoch is attained, which is, in one sense, a culminating point in the career of the moon. At thisepoch the revolution periods of the moon, when measured in rotationperiods of the earth, attain their greatest value. It would seem thatthe month was then twenty-nine days. It is not, of course, meant thatthe month and the day at that epoch were the month and the day as ourclocks now measure time. Both were shorter then than now. But what wemean is, that at this epoch the earth rotated twenty-nine times on itsaxis while the moon completed one circuit. 

This epoch has now been passed. No attempt can be made at present toevaluate the date of that epoch in our ordinary units of measurement. Atthe same time, however, no doubt can be entertained as to theimmeasurable antiquity of the event, in comparison with all historicrecords; but whether it is to be reckoned in hundreds of thousands ofyears, in millions of years, or in tens of millions of years, must beleft in great degree to conjecture. 

This remarkable epoch once passed, we find that the course of events inthe earth-moon system begins to shape itself towards that remarkablefinal stage which has points of resemblance to the initial stage. Themoon still continues to revolve in an orbit with a diameter steadily, though very slowly, growing. The length of the month is accordinglyincreasing, and the rotation of the earth being still constantlyretarded, the length of the day is also continually growing. But theratio of the length of the month to the length of the day now exhibits achange. That ratio had gradually increased, from unity at thecommencement, up to the maximum value of somewhere about twenty-nine atthe epoch just referred to. The ratio now begins again to decline, untilwe find the earth makes only twenty-eight rotations, instead oftwenty-nine, in one revolution of the moon. The decrease in the ratiocontinues until the number twenty-seven expresses the days in the month. Here, again, we have an epoch which it is impossible for us to passwithout special comment. In all that has hitherto been said we have beendealing with events in the distant past; and we have at length arrivedat the present state of the earth-moon system. The days at this epochare our well-known days, the month is the well-known period of therevolution of our moon. At the present time the month is abouttwenty-seven of our days, and this relation has remained sensibly truefor thousands of years past. It will continue to remain sensibly truefor thousands of years to come, but it will not remain trueindefinitely. It is merely a stage in this grand transformation; it maypossess the attributes of permanence to our ephemeral view, just as thewings of a gnat seem at rest when illuminated by the electric spark; butwhen we contemplate the history with time conceptions sufficiently amplefor astronomy we realise how the present condition of the earth-moonsystem can have no greater permanence than any other stage in thehistory. 

Our narrative must, however, now assume a different form. We have beenspeaking of the past; we have been conducted to the present; can we sayanything of the future? Here, again, the tides come to our assistance. If we have rightly comprehended the truth of dynamics (and who is therenow that can doubt them?), we shall be enabled to make a forecast of thefurther changes of the earth-moon system. If there be no interruptionfrom any external source at present unknown to us, we can predict--inoutline, at all events--the subsequent career of the moon. We can seehow the moon will still follow its outward course. The path in which itrevolves will grow with extreme slowness, but yet it will always grow;the progress will not be reversed, at all events, before the final stageof our history has been attained. We shall not now delay to dwell on theintervening stages; we will rather attempt to sketch the ultimate typeto which our system tends. In the dim future--countless millions ofyears to come--this final stage will be approached. The ratio of themonth to the day, whose decline we have already referred to, willcontinue to decline. The period of revolution of the moon will growlonger and longer, but the length of the day will increase much morerapidly than the increase in the duration of the moon's period. From themonth of twenty-seven days we shall pass to a month of twenty-six days, and so on, until we shall reach a month of ten days, and, finally, amonth of one day. 

Let us clearly understand what we mean by a month of one day. We meanthat the time in which the moon revolves around the earth will be equalto the time in which the earth rotates around its axis. The length ofthis day will, of course, be vastly greater than our day. The onlyelement of uncertainty in these enquiries arises when we attempt to givenumerical accuracy to the statements. It seems to be as true as the lawsof dynamics that a state of the earth-moon system in which the day andthe month are equal must be ultimately attained; but when we attempt tostate the length of that day we introduce a hazardous element into theenquiry. In giving any estimate of its length, it must be understoodthat the magnitude is stated with great reserve. It may be erroneous tosome extent, though, perhaps, not to any considerable amount. The lengthof this great day would seem to be about equal to fifty-seven of ourdays. In other words, at some critical time in the excessively distantfuture, the earth will take something like 1, 400 hours to perform arotation, while the moon will complete its journey precisely in the sametime. 

We thus see how, in some respects, the first stage of the earth-moonsystem and the last stage resemble each other. In each case we have theday equal to the month. In the first case the day and the month wereonly a small fraction of our day; in the last stage the day and themonth are each a large multiple of our day. There is, however, aprofound contrast between the first critical epoch and the last. We havealready mentioned that the first epoch was one of unstability--it couldnot last; but this second state is one of dynamical stability. Once thatstate has been acquired, it would be permanent, and would endure forever if the earth and the moon could be isolated from all externalinterference. 

There is one special feature which characterises the movement when themonth is equal to the day. A little reflection will show that when thisis the case the earth must constantly direct the same face towards themoon. If the day be equal to the month, then the earth and moon mustrevolve together, as if bound by invisible bands; and whateverhemisphere of the earth be directed to the moon when this state ofthings commences will remain there so long as the day remains equal tothe month. 

At this point it is hardly possible to escape being reminded of thatcharacteristic feature of the moon's motion which has been observed fromall antiquity. We refer, of course, to the fact that the moon at thepresent time constantly turns the same face to the earth. 

It is incumbent upon astronomers to provide a physical explanation ofthis remarkable fact. The moon revolves around our earth once in adefinite number of seconds. If the moon always turns the same face tothe earth, then it is demonstrated that the moon rotates on its axisonce in the same number of seconds also. Now, this would be acoincidence wildly improbable unless there were some physical cause toaccount for it. We have not far to seek for a cause: the tides on themoon have produced the phenomenon. We now find the moon has a ruggedsurface, which testifies to the existence of intense volcanic activityin former times. Those volcanoes are now silent--the internal fires inthe moon seem to have become exhausted; but there was a time when themoon must have been a heated and semi-molten mass. There was a time whenthe materials of the moon were so hot as to be soft and yielding, and inthat soft and yielding mass the attraction of our earth excited greattides. We have no historical record of these tides (they were longanterior to the existence of telescopes, they were probably longanterior to the existence of the human race), but we know that thesetides once existed by the work they have accomplished, and that work isseen to-day in the constant face which the moon turns towards the earth. The gentle rise and fall of the oceans which form our tides present apicture widely different from the tides by which the moon was onceagitated. The tides on the moon were vastly greater than those of theearth. They were greater because the weight of the earth is greater thanthat of the moon, so that the earth was able to produce much morepowerful tides in the moon than the moon has ever been able to raise onthe earth. 

That the moon should bend the same face to the earth depends immediatelyupon the condition that the moon shall rotate on its axis in preciselythe same period as that which it requires to revolve around the earth. The tides are a regulating power of unremitting efficiency to ensurethat this condition shall be observed. If the moon rotated more slowlythan it ought, then the great lava tides would drag the moon roundfaster and faster until it attained the desired velocity; and then, butnot till then, they would give the moon peace. Or if the moon were torotate faster on its axis than in its orbit, again the tides would comefuriously into play; but this time they would be engaged in retardingthe moon's rotation, until they had reduced the speed of the moon to onerotation for each revolution. 

Can the moon ever escape from the thraldom of the tides? This is notvery easy to answer, but it seems perhaps not impossible that the moonmay, at some future time, be freed from tidal control. It is, indeed, obvious that the tides, even at present, have not the extremelystringent control over the moon which they once exercised. We now see noocean on the moon, nor do the volcanoes show any trace of molten lava. There can hardly be tides _on_ the moon, but there may be tides _in_ themoon. It may be that the interior of the moon is still hot enough toretain an appreciable degree of fluidity, and if so, the tidal controlwould still retain the moon in its grip; but the time will probablycome, if it have not come already, when the moon will be cold to thecentre--cold as the temperature of space. If the materials of the moonwere what a mathematician would call absolutely rigid, there can be nodoubt that the tides could no longer exist, and the moon would beemancipated from tidal control. It seems impossible to predicate how farthe moon can ever conform to the circumstances of an actual rigid body, but it may be conceivable that at some future time the tidal controlshall have practically ceased. There would then be no longer anynecessary identity between the period of rotation and that ofrevolution. A gleam of hope is thus projected over the astronomy of thedistant future. We know that the time of revolution of the moon isincreasing, and so long as the tidal governor could act, the time ofrotation must increase sympathetically. We have now surmised a state ofthings in which the control is absent. There will then be nothing toprevent the rotation remaining as at present, while the period ofrevolution is increasing. The privilege of seeing the other side of themoon, which has been withheld from all previous astronomers, may thus inthe distant future be granted to their successors. 

The tides which the moon raises in the earth act as a brake on therotation of the earth. They now constantly tend to bring the period ofrotation of the earth to coincide with the period of revolution of themoon. As the moon revolves once in twenty-seven days, the earth is atpresent going too fast, and consequently the tidal control at thepresent moment endeavours to retard the rotation of the earth. Therotation of the moon long since succumbed to tidal control, but that wasbecause the moon was comparatively small and the tidal power of theearth was enormous. But this is the opposite case. The earth is largeand more massive than the moon, the tides raised by the moon are butsmall and weak, and the earth has not yet completely succumbed to thetidal action. But the tides are constant, they never for an instantrelax the effort to control, and they are gradually tending to renderthe day and the month coincident, though the progress is a very slowone. 

The theory of the tides leads us to look forward to a remote state ofthings, in which the moon revolves around the earth in a period equal tothe day, so that the two bodies shall constantly bend the same face toeach other, provided the tidal control be still able to guide the moon'srotation. So far as the mutual action of the earth and the moon isconcerned, such an arrangement possesses all the attributes ofpermanence. If, however, we venture to project our view to a still moreremote future, we can discern an external cause which must prevent thismutual accommodation between the earth and the moon from being eternal. The tides raised by the moon on the earth are so much greater than thoseraised by the sun, that we have, in the course of our previousreasoning, held little account of the sun-raised tides. This isobviously only an approximate method of dealing with the question. Theinfluence of the solar tide is appreciable, and its importancerelatively to the lunar tide will gradually increase as the earth andmoon approach the final critical stage. The solar tides will have theeffect of constantly applying a further brake to the rotation of theearth. It will therefore follow that, after the day and the month havebecome equal, a still further retardation awaits the length of the day. We thus see that in the remote future we shall find the moon revolvingaround the earth in a shorter time than that in which the earth rotateson its axis. 

A most instructive corroboration of these views is afforded by thediscovery of the satellites of Mars. The planet Mars is one of thesmaller members of our system. It has a mass which is only the eighthpart of the mass of the earth. A small planet like Mars has much lessenergy of rotation to be destroyed than a larger one like the earth. Itmay therefore be expected that the small planet will proceed much morerapidly in its evolution than the large one; we might, therefore, anticipate that Mars and his satellites have attained a more advancedstage of their history than is the case with the earth and hersatellite. 

When the discovery of the satellites of Mars startled the world, in1877, there was no feature which created so much amazement as theperiodic time of the interior satellite. We have already pointed out inChapter X. How Phobos revolves around Mars in a period of 7 hours 39minutes. The period of rotation of Mars himself is 24 hours 37 minutes, and hence we have the fact, unparalleled in the solar system, that thesatellite is actually revolving three times as rapidly as the planet isrotating. There can hardly be a doubt that the solar tides on Mars haveabated its velocity of rotation in the manner just suggested. 

It has always seemed to me that the matter just referred to is one ofthe most interesting and instructive in the whole history of astronomy. We have, first, a very beautiful telescopic discovery of the minutesatellites of Mars, and we have a determination of the anomalousmovement of one of them. We have then found a satisfactory physicalexplanation of the cause of this phenomenon, and we have shown it to bea striking instance of tidal evolution. Finally, we have seen that thesystem of Mars and his satellite is really a forecast of the destinywhich, after the lapse of ages, awaits the earth-moon system. 

It seems natural to enquire how far the influence of tides can havecontributed towards moulding the planetary orbits. The circumstances arehere very different from those we have encountered in the earth-moonsystem. Let us first enunciate the problem in a definite shape. Thesolar system consists of the sun in the centre, and of the planetsrevolving around the sun. These planets rotate on their axes; andcirculating round some of the planets we have their systems ofsatellites. For simplicity, we may suppose all the planets and theirsatellites to revolve in the same plane, and the planets to rotate aboutaxes which are perpendicular to that plane. In the study of the theoryof tidal evolution we must be mainly guided by a profound dynamicalprinciple known as the conservation of the "moment of momentum. " Theproof of this great principle is not here attempted; suffice it to saythat it can be strictly deduced from the laws of motion, and is thusonly second in certainty to the fundamental truths of ordinary geometryor of algebra. Take, for instance, the giant planet, Jupiter. In onesecond he moves around the sun through a certain angle. If we multiplythe mass of Jupiter by that angle, and if we then multiply the productby the square of the distance from Jupiter to the sun, we obtain acertain definite amount. A mathematician calls this quantity the"orbital" moment of momentum of Jupiter. [46] In the same way, if wemultiply the mass of Saturn by the angle through which the planet movesin one second, and this product by the square of the distance betweenthe planet and the sun, then we have the orbital moment of momentum ofSaturn. In a similar manner we ascertain the moment of momentum for eachof the other planets due to revolution around the sun. We have also todefine the moment of momentum of the planets around their axes. In onesecond Jupiter rotates through a certain angle; we multiply that angleby the mass of Jupiter, and by the square of a certain line whichdepends on his internal constitution: the product forms the "rotational"moment of momentum. In a similar manner we find the rotational moment ofmomentum for each of the other planets. Each satellite revolves througha certain angle around its primary in one second; we obtain the momentof momentum of each satellite by multiplying its mass into the angledescribed in one second, and then multiplying the product into thesquare of the distance of the satellite from its primary. Finally, wecompute the moment of momentum of the sun due to its rotation. This weobtain by multiplying the angle through which the sun turns in onesecond by the whole mass of the sun, and then multiplying the product bythe square of a certain line of prodigious length, which depends uponthe details of the sun's internal structure. 

If we have succeeded in explaining what is meant by the moment ofmomentum, then the statement of the great law is comparatively simple. We are, in the first place, to observe that the moment of momentum ofany planet may alter. It would alter if the distance of the planet fromthe sun changed, or if the velocity with which the planet rotates uponits axis changed; so, too, the moment of momentum of the sun may change, and so may those of the satellites. In the beginning a certain totalquantity of moment of momentum was communicated to our system, and notone particle of that total can the solar system, as a whole, squander oralienate. No matter what be the mutual actions of the various bodies ofthe system, no matter what perturbations they may undergo--what tidesmay be produced, or even what mutual collisions may occur--the great lawof the conservation of moment of momentum must be obeyed. If some bodiesin the solar system be losing moment of momentum, then other bodies inthe system must be gaining, so that the total quantity shall remainunaltered. This consideration is one of supreme importance in connectionwith the tides. The distribution of moment of momentum in the system isbeing continually altered by the tides; but, however the tides may ebbor flow, the total moment of momentum can never alter so long asinfluences external to the system are absent. 

We must here point out the contrast between the endowment of our systemwith energy and with moment of momentum. The mutual actions of oursystem, in so far as they produce heat, tend to squander the energy, aconsiderable part of which can be thus dissipated and lost; but themutual actions have no power of dissipating the moment of momentum. 

The total moment of momentum of the solar system being taken to be 100, this is at present distributed as follows:--

 Orbital moment of momentum of Jupiter 60 Orbital moment of momentum of Saturn 24 Orbital moment of momentum of Uranus 6 Orbital moment of momentum of Neptune 8 Rotational moment of momentum of Sun 2 -- 100

The contributions of the other items are excessively minute. The orbitalmoments of momentum of the few interior planets contain but little morethan one thousandth part of the total amount. The rotationalcontributions of all the planets and of their satellites is very muchless, being not more than one sixty-thousandth part of the whole. When, therefore, we are studying the general effects of tides on the planetaryorbits these trifling matters may be overlooked. We shall, however, findit desirable to narrow the question still more, and concentrate ourattention on one splendid illustration. Let us take the sun and theplanet Jupiter, and, supposing all other bodies of our system to beabsent, let us discuss the influence of tides produced in Jupiter by thesun, and of tides in the sun by Jupiter. 

It might be hastily thought that, just as the moon was born of theearth, so the planets were born of the sun, and have gradually recededby tides into their present condition. We have the means of enquiryinto this question by the figures just given, and we shall show that itis impossible that Jupiter, or any of the other planets, can ever havebeen very much closer to the sun than they are at present. In the caseof Jupiter and the sun we have the moment of momentum made up of threeitems. By far the largest of these items is due to the orbitalrevolution of Jupiter, the next is due to the sun, the third is due tothe rotation of Jupiter on its axis. We may put them in round numbers asfollows:--

 Orbital moment of momentum of Jupiter 600, 000 Rotational moment of momentum of Sun 20, 000 Rotational moment of momentum of Jupiter 12

The sun produces tides in Jupiter, those tides retard the rotation ofJupiter. They make Jupiter rotate more and more slowly, therefore themoment of momentum of Jupiter is decreasing, therefore its present valueof 12 must be decreasing. Even the mighty sun himself may be distractedby tides. Jupiter raises tides in the sun, those tides retard the motionof the sun, and therefore the moment of momentum of the sun isdecreasing, and it follows from both causes that the item of 600, 000must be increasing; in other words, the orbital motion of Jupiter mustbe increasing, or Jupiter must be receding from the sun. To this extent, therefore, the sun-Jupiter system is analogous to the earth-moon system. As the tides on the earth are driving away the moon, so the tides inJupiter and the sun are gradually driving the two bodies apart. Butthere is a profound difference between the two cases. It can be provedthat the tides produced in Jupiter by the sun are more effective thanthose produced in the sun by Jupiter. The contribution of the sun may, therefore, be at present omitted; so that, practically, theaugmentations of the orbital moment of momentum of Jupiter are nowachieved at the expense of that stored up by Jupiter's rotation. Butwhat is 12 compared with 600, 000. Even when the whole of Jupiter'srotational moment of momentum and that of his satellites has becomeabsorbed into the orbital motion, there will hardly be an appreciabledifference in the latter. In ancient days we may indeed suppose thatJupiter being hotter was larger than at present, and that he hadconsiderably more rotational moment of momentum. But it is hardlycredible that Jupiter can ever have had one hundred times the moment ofmomentum that he has at present. Yet even if 1, 200 units of rotationalmomentum had been transferred to the orbital motion it would onlycorrespond with the most trivial difference in the distance of Jupiterfrom the sun. We are hence assured that the tides have not appreciablyaltered the dimensions of the orbit of Jupiter, or of the other greatplanets. 

The time will, however, come when the rotation of Jupiter on his axiswill be gradually abated by the influence of the tides. It will then befound that the moment of momentum of the sun's rotation will begradually expended in increasing the orbits of the planets, but as thisreserve only holds about two per cent. Of the whole amount in our systemit cannot produce any considerable effect. 

The theory of tidal evolution, which in the hands of Professor Darwinhas taught us so much with regard to the past history of the systems ofsatellites in the solar system, will doubtless also, as pointed out byDr. See, be found to account for the highly eccentric orbits of doublestar systems. In the earth-moon system we have two bodies exceedinglydifferent in bulk, the mass of the earth being about eighty times asgreat as that of the moon. But in the case of most double stars we haveto do with two bodies not very different as regards mass. It can bedemonstrated that the orbit must have been originally of slighteccentricity, but that tidal friction is capable not only of extending, but also of elongating it. The accelerating force is vastly greater atperiastron (when the two bodies are nearest each other) than at apastron(when their distance is greatest). At periastron the disturbing forcewill, therefore, increase the apastron distance by an enormous amount, while at apastron it increases the periastron distance by a very smallamount. Thus, while the ellipse is being gradually expanded, the orbitgrows more and more eccentric, until the axial rotations have beensufficiently reduced by the transfer of axial to orbital moment ofmomentum. 

And now we must draw this chapter to a close, though there are manyother subjects that might be included. The theory of tidal evolution is, indeed, one of quite exceptional interest. The earlier mathematiciansexpended their labour on the determination of the dynamics of a systemwhich consisted of rigid bodies. We are indebted to contemporarymathematicians for opening up celestial mechanics upon the more realsupposition that the bodies are not rigid; in other words, that they aresubject to tides. The mathematical difficulties are enormously enhanced, but the problem is more true to nature, and has already led to some ofthe most remarkable astronomical discoveries made in modern times. 

 * * * * *

Our Story of the Heavens has now been told. We commenced this work withsome account of the mechanical and optical aids to astronomy; we haveended it with a brief description of an intellectual method of researchwhich reveals some of the celestial phenomena that occurred ages beforethe human race existed. We have spoken of those objects which arecomparatively near to us, and then, step by step, we have advanced tothe distant nebulæ and clusters which seem to lie on the confines of thevisible universe. Yet how little can we see with even our greatesttelescopes, when compared with the whole extent of infinite space! Nomatter how vast may be the depth which our instruments have sounded, there is yet a beyond of infinite extent. Imagine a mighty globedescribed in space, a globe of such stupendous dimensions that it shallinclude the sun and his system, all the stars and nebulæ, and even allthe objects which our finite capacities can imagine. Yet, what ratiomust the volume of this great globe bear to the whole extent of infinitespace? The ratio is infinitely less than that which the water in asingle drop of dew bears to the water in the whole Atlantic Ocean. 

APPENDIX. 

ASTRONOMICAL QUANTITIES. 

THE SUN. 

The sun's mean distance from the earth is 92, 900, 000 miles; his diameteris 866, 000 miles; his mean density, as compared with water, is 1·4; hisellipticity is insensible; he rotates on his axis in a period between 25and 26 days. 

THE MOON. 

The moon's mean distance from the earth is 239, 000 miles. The diameterof the moon is 2, 160 miles; and her mean density, as compared withwater, is 3·5. The time of a revolution around the earth is 27·322 days. 

THE PLANETS. 

___________________________________________________________________________| |Distance from the Sun in | | Mean | |Density || | Millions of Miles. | Periodic |Diameter| Axial |compared|| |-------------------------| Time | in | Rotation. | with || | Mean. | Least. |Greatest. | in Days. | Miles. | | Water. ||-------+-------+-------+---------+----------+--------+----------+--------||Mercury| 36·0| 28·6| 43·3 | 87·969| 3, 030 | (?) | 6·85(?)||Venus | 67·2| 66·6| 67·5 | 224·70 | 7, 700 | (?) | 4·85 ||Earth | 92·9| 91·1| 94·6 | 365·26 | 7, 918 |23 56 4·09| 5·58 ||Mars | 141 | 128 | 155 | 686·98 | 4, 230 |24 37 22·7| 4·01 ||Jupiter| 483 | 459 | 505 | 4, 332·6 | 86, 500 | 9 55 -- | 1·38 ||Saturn | 886 | 834 | 936 |10, 759 | 71, 000 |10 14 -- | 0·72 ||Uranus |1, 782 |1, 700 | 1, 860 |30, 687 | 31, 900 | Unknown | 1·22 ||Neptune|2, 792 |2, 760 | 2, 810 |60, 127 | 34, 800 | Unknown | 1·11 |---------------------------------------------------------------------------

THE SATELLITES OF MARS. 

 Mean Distance from Periodic Time. Name. Centre of Mars. Hrs. Mins. Secs. 

 Phobos 5, 800 miles 7 39 14 Deimos 14, 500 miles 30 17 54

THE SATELLITES OF JUPITER. 

 Mean Distance from Periodic Time. Name. Centre of Jupiter. Days. Hrs. Mins. Secs. 

 New Inner Satellite Barnard 112, 500 miles 0 11 57 22 I. 261, 000 miles 1 18 27 34 II. 415, 000 miles 3 13 13 42 III. 664, 000 miles 7 3 42 33 IV. 1, 167, 000 miles 16 16 32 11

THE SATELLITES OF SATURN. 

 Mean Distance from Periodic Time. Name. Centre of Saturn. Days. Hrs. Mins. Secs. 

 Mimas 115, 000 miles 0 22 37 6 Enceladus 148, 000 miles 1 8 53 7 Tethys 183, 000 miles 1 21 18 26 Dione 235, 000 miles 2 17 41 9 Rhea 329, 000 miles 4 12 25 12 Titan 760, 000 miles 15 22 41 27 Hyperion 921, 000 miles 21 6 38 31 Iapetus 2, 215, 000 miles 79 7 56 40

THE SATELLITES OF URANUS. 

 Mean Distance from Periodic Time. Name. Centre of Uranus. Days. Hrs. Mins. Secs. 

 Ariel 119, 000 miles 2 12 29 21 Umbriel 166, 000 miles 4 3 27 37 Titania 272, 000 miles 8 16 56 30 Oberon 364, 000 miles 13 11 7 6

THE SATELLITE OF NEPTUNE. 

 Mean Distance from Periodic Time. Name. Centre of Neptune. Days. Hrs. Mins. Secs. 

 Satellite 220, 000 miles 5 21 2 44

INDEX. 

A

Aberration of light, 503-512; and the apparent movements of stars, 504, 507; Bradley's discoveries, 503; causes, 507-511; circles of stars, 505-507; dependent upon the velocity of light, 511; effect on Draco, 505; telescopic investigation, 510

Achromatic combination of glasses, 11

Adams, Professor J. C. , and the discovery of Neptune, 324-327, 330-332; and the Ellipse of the Leonids, 386

Aërolite, the Chaco, 398; the Orgueil, 399

Airy, Sir George, 325

Alban Mount Meteorites, the, 393

Alcor, 438

Aldebaran, 209, 418, 419; spectrum of, 480; value of velocity of, 484

Algol, 485, 487

Almagest, the, 7

Alphonsus, 92

Alps, the great valley of the (lunar), 88

Altair, 424

Aluminium in the Sun, 50

Ancients, astronomy of the, 2-7

Andrews, Professor, and basaltic formation at Giant's Causeway, 407

Andromeda, 414; nebula in, 469, 489

Andromedes, The, shooting star shower, and Biela's comet, 390

Antares, 423

Apennines (lunar), 83

Aphelion, 163

Aquarius, 215, 413

Aquila, or the Eagle, 424

Arago, 326

Archimedes, 88

Arcturus, 358, 480; value of velocity of, 484

Argelander's Catalogue of Stars, 431, 476

Argus, 481

Ariel, 309, 559

Aristarchus, 90

Aristillus, 88

Aristotle, lunar crater named after him, 88; credulity respecting his writings, 267; the Moon and the tides and, 535

Asteroids, 229-244

Astrea, 328

Astronomers of Nineveh, 156

Astronomical quantities, 558

Astronomy, ancient, 2-7; Galileo's achievements in, 10; the first phenomenon of, 2

_Athenæum_, the, and Sir John Herschel's letter on Adams's share in the discovery of Neptune, 330

Atmosphere, height of the Earth's, 100

Attraction, between the Moon and the Earth, 75; between the planets, 148; between the Sun and the planets, 144, 148; of Jupiter, 248, 249; producing precession, 498

Auriga, 414, 489

Aurora borealis, 42

Autolycus, 88

Auwers and star distances, 449; and the irregularity in movement of Sirius, 427

Axis, Polar, 196, 497; precession and nutation of the Earth's, 492-502

B

Backlund, and Encke's comet, 349, 351

Barnard, Professor E. E. , and Saturn, 271, 278, 282; and Titan, 294; and the comet of 1892, 355; and the Milky Way, 475

Beehive, the, 422

Belopolsky, M. , and Binaries, 487, 488

Benares meteorite, the, 392

Bessel, and Bradley, 501; and the distance of 61 Cygni, 446, 448, 449; and the distances of stars, 442; and the irregular movements of Sirius, 426; receives gold medal of Royal Astronomical Society, 442

Betelgeuze, 209, 418, 419, 482; value of velocity of, 484

Biela's comet, and Sir John Herschel, 357; and the Andromedes, 390

Binaries, spectroscopic, 487

Binocular glass, 27

Biot and the L'Aigle meteorites, 392

Bode's law, 230; list of double stars, 435

Bond, Professor, and Saturn's satellites, 296; and the nebula in Orion, 469; and the third ring of Saturn, 280

Boötes, 422

Bradley, and nutation, 501; and the aberration of light, 503; his observations of Uranus, 312

Bredichin, Professor, and the tails of comets, 365, 366, 367

Breitenbach iron, the, 397

Bristol Channel, tides in the, 538

Brünnow, Dr. , observations on the parallax of 61 Cygni, 449

_Burial of Sir John Moore_, 72

Burnham, Mr. , and the orbit of Sirius, 427; his additions to the known number of double stars, 439

Butler, Bishop, and probability, 460

Butsura meteorite, 397

C

Cadmium in the Sun, 50

Calais, tides at, 536

Calcium in the Sun, 50

Campbell, Mr. , and Argus, 481; and Mars, 223

Canals on Mars, 220

Cancri 20, 154

Cancri, z, 154

Cancri, th, 154

Canis major, 419

Canopus, 422

Cape Observatory, 27

Capella, 414, 480, 487

Carboniferous period, 518

Cardiff, tides at, 538

Cassini, J. D. , and double stars, 434; and Saturn's satellites, 294; and the rings of Saturn, 278

Cassiopeia, 412

Castor, 420, 487; a binary star, 437; revolution of, 437

Catalogues of stars, 310, 311; Messier's, 529

Catharina, 92

Centauri, a, 422; Dr. Gill's observations of, 451; Henderson's measurement of distance of, 442, 451

Ceres, 231, 232, 238; and meteorites, 404, 405

Chaco meteorite, the, 398

Chacornac, and the lunar crater Schickard, 90

_Challenger_, the cruise of the, and magnetic particles in the Atlantic, 408

Challis, Professor, 326; his search for Neptune, 327, 328, 331, 332

Chandler, Mr. , and Algol, 485

Charles's Wain, 28

Chepstow, tides at, 538

Chéseaux, discoverer of comet of 1744, 367

Chicago, telescope at Yerkes Observatory, 16

Chladni and the meteorite of Siberia, 392

Chromium in the Sun, 50

Chromosphere, the, 54

Chronometers tested by the Moon, 80

Clairaut and the attraction of planets on comets, 342, 343

Clavius, 91; and Jupiter's satellites, 267

Clock, astronomical, 23

Clusters, star, 461-464

Cobalt in the Sun, 50

Coggia's comet, 1874, 337

Colour of light and indication of its source, 46

Colours, the seven primary, 45

Columbiad, the, 401

Columbus, 7

Comets, 112, 149, 250, 336; and the spectroscope, 355; attraction from planets, 342, 360; Biela's, 357; Biela's and the Andromedes, 390; Clairaut's investigations, 342, 343; Coggia's, 337; Common's (1882), 354; connection of, with shooting star showers, 388; constitution of, 336; containing sodium and iron, 356; Donati's (1858), 353, 358, 366; eccentricity of, 360; Encke's, 344-352; existence of carbon in, 356, 367; gravitation and, 343, 348; Halley's investigations about, 341-344; head or nucleus of, 337; Lexell's, 370; mass of, 359; movements of, 336; Newton's explanations of, 338; non-periodic, 353-356; of 1531, 341; of 1607, 341; of 1681, 338, 339; of 1682, 341; of 1744 (Chéseaux's), 367; of 1818, 345; of 1843, 352; of 1866, 388; of 1874, 337; of 1892, 355; origin of, 369; parabolic orbits of, 338-340, 360; periodic return of, 338-341; shape of, 336; size of, 337; tailless, 370; tails of, 337, 361; Bredichin's researches, 365; Chéseaux's, 367; composition of, 365, 369; condensation of, 369; electricity and, 368; gradual growth of, 363; law of direction of, 362; repelled by the Sun, 364; repulsive force of, 364, 368; various types of, 365; Tebbutt's (1881), 353; tenuity of, 357

Common, Dr. , constructor of reflectors, 21; and the comet of 1882, 354; and the nebula in Orion, 469

Cook, Captain, and the transit of Venus, 184

Copeland, Dr. , and Schmidt's star, 489; and the lunar crater, Tycho, 92; and the spectra of nebula, 473; and the transit of Venus, 189

Copernicus and Mercury, 156; confirmation of his theory by the discovery of Jupiter's satellites, 267; his theory of astronomy, 7; lunar crater called after him, 89

Copper in the Sun, 50

Cor scorpionis, 423

Corona Borealis, 423, 488

Corona of Sun, during an eclipse, 62-64, 151

Coronium, 64

Cotopaxi and meteorites, 401

Crab, the, 422

Crabtree, and the transit of Venus, 180

Crape ring of Saturn, 281

Craters in the Moon, 83-85, 87-98

Critical velocity, 103, 104, 237

Crown, the, 423

Cryptograph of Huyghens, the, 277

Cygni, b, 439

Cygni 61, annual parallax of, 450; Bessel's measurement of distance of, 442, 446, 447; Brünnow's observations of, 449; distance from the Sun of, 452; disturbing influence of, 452; double, 446; Professor A. Hall's measurement of, 449; Professor Pritchard's photographic researches concerning, 449; proper motion of, 446; Struve's observations of, 448, 449; velocity of, 452

Cygnus, 424

Cyrillus, 92

Cysat, and the Belt of Orion, 467

D

D line in solar spectrum, 48

Darwin, Professor G. H. , and tidal evolution, 531

Dawes, Professor, and Saturn's third ring, 281

Day, length of, and the Moon, 542; and the tides, 541

Deimos, 226, 558

Denebola, 423

Diffraction, 56

Dione, 559

Dispersion of colours, 47

Distances, astronomical, 558, 559

Doerfel, and comets, 339

Dog star (_see_ Sirius)

Dog, the Little, 420

Donati's comet, 353, 358; tails, 366

Double stars, 434-440

D Q, 236

Draco, nebula in, 470

Dragon, the, 415

Draper, Professor, and the nebula in Orion, 469

Dunsink Observatory, 12, 184, 447, 449

Dynamical stability, 547; theory of Newton, 214

Dynamics and the Earth-Moon system, 546

Dynamics, Galileo the founder of, 10

E

Eagle, the, 424

Earth, The, ancient ideas respecting, 3; annual movement of, and the apparent movement of the stars, 507, 512; attraction of Jupiter, 319; attraction of on Encke's comet, 350; attraction of, on the Leonids, 386; attraction of Saturn, 319; attraction of the Moon, 75, 497; attraction of the Sun, 496; axial rotation of, 558; carboniferous period on, 518; change of climate on, 518; composition of, 496; contact of atmosphere of, with meteors, 377-379; density of, 558; diameter of, 558; distance of, from Mars, 213; distance of, from the Moon, 73, 558; distance of, from the Sun, 31, 114, 184, 240, 265, 351, 512, 558; energy from rotation of, 540; formerly a molten globe, 200, 201; geological records and, 517; glacial period on, 518; gravitation and, 204, 206, 207, 497; heat in the interior of, 94, 197, 198, 251, 514; how it is measured, 193-196; its mass increasing owing to the fall of meteoric matter, 408; its oceans once vapour, 251; once in immediate proximity to the Moon, 542; orbit of, 114; orbit of, its elliptic form, 139; path of deranged by Venus and Mars, 319; periodic time of, 558; plane of orbit of, 309; polar axis of, 196, 492-502; position of, relatively to the Sun and the Moon, 76, 77; precession and nutation of axis of, 492-502; radius of, 193, 512; rotation of, 75, 196, 200, 494, 496; shape of, 192, 195, 197, 201, 207; size of, compared with Jupiter, 119, and with other planets, 119; size and weight of, compared with those of the Sun, 30, and Moon, 74, 75; velocity of, 115, 139, 146, 512, and periodic time, 143; volcanic outbreaks on, 197, and the origin of meteorites, 405; weight of, 202, 248, as compared with Saturn, 271, 272

Earthquakes, astronomical instruments disturbed by, 24

Eccentricity of planetary ellipses, 136, 211

Eclipse of Jupiter's satellites, 261, 262, 265-267

Ellipse of the Moon, 77-80; of the Sun, 53

Eclipses, ancient explanations of, 6; calculations of the recurrence of, 79, 80

Ecliptic, the, 5, 233; Pole of the, 493, 500, 505

Electric Light, the, 44

Ellipse, the, 136; eccentricity of, 137; focus of, 137; Kepler's discoveries respecting, 136, 138, 142-144, 505; the form which the orbit of a planet takes, 136; the parallactic, 444; variety of form of, 139

Enceladus, 559

Encke, and the distance of the Sun from the Earth, 147, 184; his comet, 344-352

Encke's comet, 344-352; approach to Jupiter of, 349; and Mercury, 349; and the Sun, 346; diminution in periodic time of, 351; distance from Mercury of, 347; disturbed by the Earth, 350, and by Mercury, 348; irregularities of, 347, 351; orbit of, 346; periodical return of, 351; Von Asten's calculations concerning, 349-350

Energy supplying the tides, 539

Ensisheim meteorite, the, 393

Equatorial diameter, 196, 497; telescope, 14

Eratosthenes, 89

Eros, 236

Eruptions, 197

Evening star, 109, 169

Eye, structure of the, 10

F

Faculæ of the Sun, 37

Fire ball of 1869, 375

Fire balls, 374

"Fixed" stars, 503

Flamsteed, first Astronomer-Royal, 311; his _Historia Coelestis_, 311

Focus of planetary ellipse, 137-139

Fomalhaut, 413

Fraunhofer, 478

Fraunhofer lines, 48

Fundy, Bay of, tides in, 538

G

Galileo, achievements of, 10; and Jupiter's satellites, 267; and Saturn's rings, 273, 274; and the Pleiades, 418

Galle, Dr. , and Neptune, 328-330

Gassendi, and the transit of Mercury, 164; and the transit of Venus, 178; lunar crater named after him, 90

Gauss, and the minor planet Ceres, 232

Gemini, constellation of, 303, 420

Geminids, the, 400

Geologists and the lapse of time, 453

Geometers, Oriental, 5

Geometry, cultivation by the ancients of, 6

George III. And Sir W. Herschel, 299, 306

Giant's Causeway, 407

Gill, Dr. D. , 27; and Juno, 243; and the minor planets, 242; and the parallax of a Centauri, 451; and the parallax of Mars, 214

Glacial period, 518

Gravitation, law of, 122-149; and binary stars, 437; and precession, 497; and the Earth's axis, 495, 497, 499; and the parabolic path of comets, 340; and the periodical return of comets, 343; and the weight of the Earth, 203, 204; illustrated by experiments, 123, 124, 127, 129-132; its discovery aided by lunar observations, 108, 125; its influence on the satellites, 149; its influence on stars, 149; its influence on tides, 149; Le Verrier's triumphant proof of, 330; Newton's discoveries, 125, 126, 147; on the Moon, 96; universality amongst the heavenly bodies, 128, 373

Great Bear, 27, 28, 241; configuration, 410; double star in the, 438; positions of, 409, 411

Green, Mr. , and Mars, 220

Greenwich Observatory, 26, 311

Griffiths, Mr. , and Jupiter, 252

Grimaldi, 90

Grubb, Sir Howard, 14

"Guards, " the, 412

_Gulliver's Travels_ and the satellites of Mars, 228

H

Hadley's observations of Saturn, 282

Hall, Professor Asaph, and the satellites of Mars, 225

Halley, and the periodicity of comets, 341-343; and the transit of Venus, 180

Heat, bearings on astronomy, 513; in the interior of the Earth, 197-199, 514; of the Sun, 515-526

Heliometer, the, 243

Helium, 55

Henderson, and the distance of a Centauri, 442, 451

Hercules, star cluster in, 269, 462

Herodotus (lunar crater), 90

Herschel, Caroline, 299, 465

Herschel, Sir John, address to British Association, 328; address on the presentation of gold medal to Bessel, 443; and Biela's comet, 357; and nebulæ, 464; letter to _Athenoeum_ on Adams's share in the discovery of Neptune, 330

Herschel, Sir W. , and double stars, 435, 436; and Saturn, 279; and Saturn's satellites, 295; and the Empress Catherine, 301; and the movement of solar system towards Lyra, 457; discovery of satellite of Uranus by, 308, 309; discovery of Uranus by, 305, 308; early life of, 299; friendship with Sir W. Watson of 302; he makes his own telescopes, 301; "King's Astronomer, " 307; method of making his telescopes, 302; musical talent of, 299; organist of Octagon Chapel, Bath, 300; pardon for desertion from George III. , 299; passion for astronomy of, 300, 301; relinquishes musical profession, 307; sidereal aggregation theory of, 529; study of the nebulæ by, 464-465, 529

Herschelian telescope, 19

_Historia Coelestis_, 311

Hoedi, the, 414

Holmes's, Mr. , comet (1892), 355

Horrocks, and the transit of Venus, 179

Howard, Mr. , and the Benares meteorite, 392

Huggins, Sir W. , 479, 483; and nebulæ, 472

Huyghens, and Saturn's rings, 275-278; discovers first satellite of Saturn, 293

Hyades, the, 419

Hydrogen in Sirius and Vega, 479; in the Sun, 50

Hyginus, 93

Hyperion, 559

I

Iapetus, 559

Iberians, the, 3

Inquisition, the, and Galileo, 10

Iris, 242

Iron, dust in the Arctic regions, 408; in the Sun, 50; of meteorites, the, 396; spectrum of, 50

J

Janssen, M. , 34, 53; and the transit of Venus, 177

Juno, 233, 238

Jupiter, ancient study of, 6; and the Leonids, 386; attraction of, 248; axial rotation of, 558; belts of, 252; brilliancy of, 257; composition of, 250; covered with an atmosphere of clouds, 253, 254; density of, 558; diameter of, 247, 558; distance from the Earth of, 110, 111; distance from the Sun of, 246, 558; habitability of, 257; heat received from the Sun by, 256; internal heat of, 252, 256, 515; lack of permanent features of, 253; lack of solidity of, 248, 253, 254; moment of momentum of, 554, 555; occultation of, 255; orbit of, 114, 115, 246; path of, perturbed by the attraction of Saturn, 316; periodic time of, 558; a planet, or "wanderer, " 111; red spot in 1878, 253; revolution of, 246; rotation of, 201, 202; satellites of, 247, 249, 257-261, 265, 559; satellites of, and gravitation, 266; satellites of, and the Copernican theory, 267; shadow from satellites of, 257; shape of, 201, 202, 247, 252; size of, compared with the Earth, 19, 246, 248, and other planets, 114; and the Sun, 114; storms on, 256; tides on, 555; weight of, 248, 250, and Encke's comet, 350

K

Keeler, Professor, and Saturn's ring, 288

Kempf, Dr. , and the Sun's velocity, 484

Kepler, and comets, 360; and laws of planetary motion, 10; and meteors, 386; and the orbit of Mars, 209; explanation of his laws, 147, 148, 533; his discovery of the shape of the planetary orbits, 136, 138; his first planetary law, 138; lunar crater called after him, 90; prediction of the transit of Venus and Mercury, 163, 178; second law, 141; third law, 142

Kids, the, 414

Kirchhoff, and spectrum analysis, 478

Kirkwood, Professor, and the movements of Saturn's satellites, 296

Klinkerfues, Professor, 390

L

Lagrange, and the theory of planetary perturbation, 320-322; his assumption of planetary rigidity, 531

L'Aigle meteorites, the, 392

Lalande, and Neptune, 332, 333

Landscapes, lunar, 98

Lane, Mr. J. Homer, 522

Laplace, and the nebular theory, 526; and the satellites of Jupiter, 266; and the theory of planetary perturbation, 320

Lassell, Mr. , and Saturn's eighth satellite, 296; discovers Neptune's satellite, 334

Law of gravitation (_see_ Gravitation)

Laws of Planetary Motion (_see_ Planetary Motion)

Lead in the Sun, 50

Ledger, Mr. , and Mercury, 163

Leibnitz, lunar mountains named after him, 93

Lemonnier, and Uranus, 312

Leo, and shooting stars, 380, 420

Leonids, attractions of planets on, 386; breadth of stream of, 387; change of shape of, 383; decrease of, 385; enormous number of, 382; historical records, 383; length of stream of, 387; Le Verrier, and the cause of their introduction into the solar system, 388; meteor shoal of, 382; periodic return of, 382; their connection with comets and Professor, Schiaparelli, 388

Leonis g, value of velocity of, 484

Leverage by equatorial protuberance, 498

Le Verrier, and Mars, 214; and the discovery of Neptune, 324-332; and the introduction of the Leonids into the solar system, 388; and the weight of Mercury, 349

Lexell's comet, 370

Libration, 84

Lick Observatory, 16

Light, aberration of, 503-512; velocity of, 261, 262, 265, 505, 512

Linné, 87, 94

Lion, the, 420, 421

Little Bear, the, 412

Little Dog, the, 420

Livy, and meteorites, 393

Lloyd, Provost, 407

Lockyer, Sir Norman, and Betelgeuze, 482; and solar light, 52

London, tides at, 538

Louvain, F. Terby, and Titan, 295

Lowell, Mr. , and Mercury, 165

Lunar tides, 548, 549

Lyra, motion of solar system towards, 459

Lyre, the, 424; Nebula in, 469

Lyrids, the, 400

M

Mädler, and the lunar craters, 88, 90, 91

Magellanic clouds, 463

Magnesium, colour of flame from, 46; in the Sun, 50

Magnetism, connection with Sun spots, 42

Manganese in the Sun, 50

Maraldi, and the rings of Saturn, 279

Mare crisium, 83; foecunditatis, 83; humorum, 83; imbrium, 83, 98; nectaris 83; nubium, 83; serenitatis, 83; tranquillitatis, 83; vaporum, 83

Mars, ancient study of, 6; appearance of, through the telescope, 218; atmosphere of, 222; axial rotation of, 558; canals on, 220; density of, 558; diameter of, 558; distance, from the Earth of, 213; distance from the Sun of, 213, 558; gravitation on, 225; Le Verrier's discovery of, 214; life improbable on, 224; marking on, 218; movements of, 211-213; opposition of, 209-211; orbit of, 116, 209, 210, 213; orbit of, and the laws of Kepler, 209; parallax (1877), and Dr. D. Gill, 214; periodic time of, 558; a planet or "wanderer, " 111; "Polar Caps" on, 218, 219; proximity to the Earth of, 110; rising and setting of, 209; rotation of, 218; satellites of, 225-228, 558; size of compared with other planets, 116, 216; tides on, 551; water and ice on, 219, 224

Maximilian, Emperor, 393

Mayer, Tobias, and Uranus, 312

Measurement of the Earth, 193-196

Mediterranean, tides in the, 537

Mercury, ancient study of, 6; antiquity of its discovery, 155-157; atmosphere of, 166; attraction on comets of, 347; climate of, 163; comparative proximity to the Earth of, 111; composition of, 160; crescent-shaped, 160; density of, 558; diameter of, 558; distance from the Sun of, 151, 558; habitability of, 163; movement of, 160, 161; its elliptic form, 139, 161; orbit of, 114; period of revolution of, 161; periodic appearances of, 158; periodic time of, 558; perturbations of, 350; a planet or "wanderer, " 111; revolution of, 165; rotation of, and Professor Schiaparelli, 165; size of, compared with other planets, 116; surface of, 162; transit of, 152; transit of, and Gassendi's observations, 164; transit of, predicted by Kepler, 163; velocity of, 162; weight of, 166, 349

Meridian circle, 22, 24

Messier's Catalogue of Stars, 529

Meteors (_see_ Stars, shooting)

Meteorites, 391; Alban Mount, 393; ancient accounts, 392, 393; Benares, 392; Butsura, 397; Chaco, 398; characteristics of, 397; Chladni's account of discovery in Siberia, 392; composition of, 397-399; Ensisheim (1492), 393; Hindoo account of, 391; L'Aigle, 392; not connected with comets, 400; not connected with star showers, 400; Orgueil, 399; origin, 400-408; Ovifak, 407; Rowton, 395-396; Wold Cottage, 392

Micrometer, 86

Milky Way, 462-3, 474-6

Mimas, 559

Minor planets, 229-244

Mira Ceti, 430, 482

Mizar, 438, 486

Moment of momentum, the, 552-554

Month of one day, 547

Moon, The, absence of air on, 85, 99; absence of heat on, 95; agent in causing the tides, 70, 535-537; ancient discoveries respecting, 5; apparent size of, 73; attraction to the Earth of, 75; brightness of, as compared with that of the Sun, 71; changes during the month of, 71, 74; chart of surface of, 81; craters on, 83, 84, 87-98, 514; density of, 558; diameter of, 558; distance from the Earth of, 73, 75, 568; eclipses of, 6, 77-80; illustration of the law of gravitation, 96, 131, 133; landscapes on, 98; life impossible on, 99; measuring heights of mountains, etc. , of, 85, 86; micrometer, 86; motion of, 75; mountains on, 83, 85, 88, 89, 91, 93; phases of, 71, 76; plane of orbit of, 310, 500, 501; poets and artists and, 72; pole, 500; possibility of ejecting meteorites, 402; possibly fractured off from the earth, 543; prehistoric tides on, 548, 549; produces precession, 497-499; proximity to the Earth of, 73, 75; receding from the Earth, 545; relative position of with regard to the Earth and the Sun, 76, 77; revolution of, round the Earth, 75, 76, 558; "seas" on, 82, 83; shadows of, 85; size of, compared with that of the Earth, 74; test for chronometers, a, 80; thraldom of terrestrial tides, 549; waterless, 100; weather not a affected by the phases of, 82; weight of, 74

Motion, laws of planetary, 138, 141, 142, 147, 148

Mountains of the Moon, 83, 85, 93

N

Nasmyth, Mr. , and the formation of lunar craters, 95

Natural History Museum, meteorites, 394

_Nautical Almanack_, 189

Neap Tides, 538

Nebula, in Andromeda, 469; annular, in Lyra, 469; in Orion, 269, 461, 466-469; colour of, 468; magnitude of, 468; nature of, 467; planetary, in Draco, 470; simplest type of a, 528; various grades of, 528

Nebulæ, 464-472; condensation, 528; distances of, 464; double, 470; Herschel's labours respecting, 464-465, 528, 529; number of, 466; planetary, 470; self-luminous, 464; smallest greater than the Sun, 464; spiral, 470

Nebular theory, the, 526

Neptune, 112; Adams's researches, 324-326, 332; Challis's observations of, 326-328; density of, 558; diameter of, 333, 558; disc of, 332; discovery (1846) of, 315; distance from the Sun of, 334, 558; Lalande's observations of, 332, 333; Le Verrier's calculations, 324-332; moment of momentum of, 554; orbit of, 117; periodic time of, 558; revolution of, 334; rotation of uncertain, 333; satellite of, discovered by Mr. Lassell, 559; size of, compared with other planets, 119; vaporous atmosphere of, 333; weight of, 333

Newall, Mr. H. F. , and Capella, 487; and the values of velocity of stars, 483

Newcomb, Professor, 9, 264, 267, 522

Newton, Professor, and meteoric showers, 377, 384

Newton, Sir Isaac, discovery of gravitation verified Kepler's laws, 144; dynamical theory, 214; illustrations of his teaching, 144-147; law of gravitation and, 125, 126, 537; parabolic path of comets and, 338-340; reflecting telescope, 19; weight of the Earth and, 203

Nickel in the Sun, 50

Nineveh, astronomers of, 156

Nordenskjöld, and the Ovifak meteorite, 407

Nova Cygni, 431; brilliancy of, 454; decline of, 455; distance of, 456; parallax of, 455

November meteors, 376, 377, 379

Nutation, and Bradley, 501

O

Oberon, 309, 559

Object-glasses, 11, 12, 14, 16, 19

Observatories, 9-28

Observatory, Cape of Good Hope, 27; Dunsink, 12, 184; Greenwich, 26, 314; Lick, 16; Paris, 22; Uraniborg, 10; Vienna, 14; Washington, 226; Yerkes, 16

Occultation, 102, 215

Oceanus Procellarum, 83

Opera-glass, 27, 28

Opposition of Mars, 209

Orbital moment of momentum, 552

Orbits of planets, 114, 115, 117; dimensions, 139-143; elliptical form, 138-140; minor planets, 232, 234, 239; not exactly circles, 135; of satellites of Uranus, 310; Sun the common focus, 139

Orgueil meteorite, the, 399

Orion, 4, 418

Orion, belt of, 418, 467; brilliancy of, 418; nebula in, 269, 461, 466-469

Orionis, a, 418, 482

Orionis, th, a multiple star, 318, 467

Ovifak meteorite, the, 407

P

Palisa and the minor planets, 234

Pallas, 233, 238

Parabolic path of comets, 338-340

Parallactic ellipse, 444

Parallax, 181, 182, 214, 443; of stars, 507

Paris telescope, 22, 23

Pegasus, great square of, 413, 414

Peg-top, the, and the rotation of the Earth, 494

Pendulum for determining the force of the Earth's attraction, 205

Penumbra of Sun-spot, 51

Perihelion, 163

Periodic times of planets, 139-143, 558

Periodicity of Sun-spots, 41

Perseids, 400

Perseus, 415, 416, 429; sword-handle, 462

Perturbation, planetary, 317-324, 346

Perturbations, theory of, 296

Petavius, 93

Peters, Professor, and charts of minor planets, 234; and the derangement of Sirius, 427

Phases of the Moon, 71, 76

Phobos, 226, 551, 558

Photography, and practical astronomy, 25; and the distance of 61 Cygni, 449; Dr. Roberts and the nebula in Andromeda, 469; Mr. Common and the nebula in Orion, 469; Sir W. Huggins and the spectra of nebulæ, 473

Photosphere, the, 37, 54

Physical nature of the stars, 477

Piazzi, discoverer of the first known minor planet, 203

Pickering, Professor, 218, 220, 255, 265; and Betelgeuze, 482; and planetary nebulæ, 474; and Saturn's satellites, 296; and spectroscopic binaries, 486, 487

Pico, 89

Planetary motion, Kepler's laws of, 138, 141, 142, 147, 148

Planetary nebulæ, 470

Planetary perturbation, 317-324

Planets, ancient ideas respecting, 2, 6; approximate number of, 112; attract each other, 148, 317; attracted by comets, 360; Bode's law, 230; comparative sizes of, 118, 119; distance of, from the Earth, 109-111; distance of, from the Sun, 558; how distinguished from stars, 111; irregularity of motions of, 317-324; Lagrange's theory of rigidity of, 531; light of, derived from the Sun, 113; minor, 229-244; orbits of the four giant, 117; orbits of the four interior, 114; orbits have their focus in the centre of the Sun, 139; orbits not exactly circles, 135; orbits take the form of an ellipse, 136-138; origin of, as suggested by the nebular theory, 526; periodic times of, 139-143, 558; relative distances of, 229; uniformity of direction in their revolution, 120, 322; velocity of, 139-142, 144, 146, 237

Plato (lunar crater), 89

Pleiades, 241, 416; invisible in the summer, 416

Pliny, the tides and the Moon, 535

Plough, the, 28

Pogson, Mr. , 390

Pointers in the Great Bear, 28, 411

Polar axis, 196

Polar caps on Mars, 218, 219

Pole, the, distance of from Pole Star lessening, 494; elevation of, 195; movement of, 492; near a Draconis, 494; near Vega or a Lyra, 494

Pole Star, 194; belongs to the Little Bear, 412; distance of, from the pole of the heavens, 412, 492, 494; position of, 411; slow motion of, 412

Pollux, 420, 480; value of velocity of, 484

Pons, and the comet of 1818, 345

Posidonius, 87

Potassium in the Sun, 50

Præsepe, 422

Precession and nutation of the Earth's axis, 492-502

Proctor, and the stars in Argelander's atlas, 476

Prism, the, 45; its analysing power, 46

Pritchard, Professor, stellar photographic researches of, 449

Procyon, 420; value of velocity of, 484

Prominences on the Sun, 53-59

Ptolemy, his theory of astronomy, 6; lunar crater named after him, 92

Q

Quarantids, the, 400

R

Radius of the Earth, 193, 512

Rainbow, the, 45

Ram, the, 420

Reflectors, 19, 21, 25

Refraction by the prism, 45

Refractors, 11, 14, 16

Regulus, 421, 479

Reservoir formed from tidal water, 538

Retina, the, and the telescope, 10, 11

Rhea, 559

Rigel, 418, 420, 480

Rigidity of the planets, 532, 533

Roberts, Dr. Isaac, and the nebula in Andromeda, 469; and the nebula in Orion, 469

Roemer, and the velocity of light, 261

Romance, planet of, 151-154

Rosse telescope, the, 19, 20, 468, 470

Rotational moment of momentum, 553

Rowland, Professor, and spectral lines, 491

Rowton Siderite, 395

Royal Astronomical Society and Bessel, 442

S

Sappho, 242

Satellites of Jupiter, 249, 250, 257-261, 266, 559; confirmation of the Copernican theory, 267

Satellites of Mars, 209, 225-228, 551, 558

Satellites of Neptune, 334, 559

Satellites of Saturn, 559; Bond's discoveries, 296; Cassini's discoveries, 294; distances, 559; Herschel's discoveries, 295; Huyghens' discovery, 293; Kirkwood's deduction, 296; Lassell's deduction, 296; movements, 296; origin as suggested by the nebular theory, 526

Satellites of Uranus, 308, 309, 310, 559

Saturn, ancient study of, 6; attraction on Uranus, 322; axial rotation of, 558; beauty of, 209; comparative proximity to the Earth of, 110; density of, 558; diameter of, 271, 558; distance of, from the Sun, 268, 271, 558; elliptic path of, 271; gravitation paramount, 283; internal heat of, 272, 515; Leonids and, 386; low density of, 272; moment of momentum of, 554; motion of, 271; orbit of, 117, 118; path of, perturbed by the attraction of Jupiter, 316; periodic time of, 558; period of revolution of, 269; picturesqueness of, 291; position of, in the solar system, 269; rings of, 269; rings, Bonds discovery, 280; rings, Cassini's discovery, 278; rings, consistency, 286; rings, Dawes's discovery, 281; rings, Galileo's discovery, 273, 274; rings, Hadley's observations, 282; rings, Herschel's researches, 279; rings, Huyghens' discovery, 275-278; rings, Keeler's measurement of the rotation, 288; rings, Maraldi's researches, 279; rings, rotation of, 285, 288; rings, spectrum of, 291; rings, Trouvelot's drawing, 278; satellites of, 293, 294, 295, 296, 559; size of, compared with other planets, 119, 269, 272; spectrum of, 291; unequal in appearance to Mars and Venus, 269; velocity of, 271; weight of, compared with the Earth, 272

Savary and binary stars, 436

Schaeberle, Mr. , and Mars, 224

Scheiner, and the values of velocity of stars, 483; observations on Sun-spots, 36

Schiaparelli, Professor, and Mars, 220; and the connection between shooting-star showers and comets, 388; and the rotation of Mercury, 165

Schickard, 90

Schmidt, and Nova Cygni, 454, 489; and the crater Linné, 87; and the Leibnitz Mountains, 93

Schröter, and the crater Posidonius, 87

Schwabe, and Sun-spots, 40

Seas in the Moon, 82

Secchi, and stellar spectra, 479

Shoal of shooting stars, 377; dimensions, 377

Shooting stars (_see_ Stars, shooting)

Sickle, the, 421

Sidereal aggregation theory of Sir W. Herschel, 529

Siderite, Rowton, 395

Sinus Iridum, 83

Sirius, change in position of, 425; companion of, 427, 428; exceptional lustre of, 110; irregularities of movement of, 426; larger than the Sun, 110; most brilliant star, 419; periodical appearances of, 157; proper motion of, 425; spectrum of, 479; velocity of, 426; weight of, 427

Smyth, Professor C. P. , 493

Sodium, colour of flame from, 49; in the Sun, 50

Solar corona, prominences etc. (_see under_ Sun)

Solar system, 107-121; Copernican exposition of the, 7; influence of gravitation on, 149; information respecting, obtained by observing the transit of Venus, 174; island in the universe, 121; minor planets, 229-244; moment of momentum, 554; movement of, towards Lyra, 457; origin of, as suggested by the nebular theory, 526; position of Saturn and Uranus in, 297, 305

South, Sir James, 12

Spectra of stars, 479

Spectro-heliograph, 58

Spectroscope, 43-56; detection of iron in the Sun by the, 50

Spectroscopic binaries, 487

Spectrum analysis, 47; dark lines, 49, 50; gaseous nebulæ, 474; line D, 48, 49

Speculum, the Rosse, 20

Spica, 423, 487

Spider-threads for adjusting the micrometer, 86; for sighting telescopes, 22

Spots on the Sun, 36-43; connection with magnetism, 42; cycles, 41; duration, 41; epochs of maximum, 42; motion, 36; period of revolution, 40; Scheiner's observations, 36; zones in which they occur, 39

Star clusters, 461-464; in Hercules, 462; in Perseus, 462

Stars, apparent movements due to precession, nutation, and aberration, 504; approximate number of, 28; attraction inappreciable, 316; catalogues of, 310, 311, 409, 431; charts of, 325, 328; circular movement of, 505-507

Stars, distances of, 441; Bessel's labours, 442-449; Henderson's labours, 442; method of measuring, 443-445; Struve's work, 442, 448, 449; parallactic ellipse, 444-449

Stars, double, 434; Bode's list, 435; Burnham's additions, 439; Cassini, 434; Herschel, 435, 436; measurement, 435, 436; revolution, 436; Savary, 436; shape of orbit, 436; variation in colour, 438

Stars, elliptic movement of, 506; gravitation and, 149; how distinguished from planets, 111; physical nature of, 477; probability of their possessing a planetary system, 121; real and apparent movements of, 504; really suns, 32, 121

Stars, shooting, attractions of the planets, 386; connection with comets, 388-390; countless in number, 372; dimensions of shoal, 377; features of, 373; length of orbit, 387; orbit, 378; orbit, gradual change, 386; period of revolution, 384; periodic return, 378, 379; shower of November, 1866, 377, 379-380; shower of November, 1866, and Professor Adams, 384, 386; shower of November, 1866, radiation of tracks from Leo, 380; shower of November, 1872, 389; showers, 376; showers and Professor Newton, 377; track, 377; transformed into vapour by friction with the Earth's atmosphere, 374, 376; velocity, 373, 386

Stars, spectra of, 479; teaching of ancients respecting, 3; temperature of, 515; temporary, 430, 488; values of velocity of, 484; variable, 429

Stoney, Dr. G. J. , 387

Strontium, flame from, 46; in the Sun, 50

Struve, Otto, and the distance of Vega, 442, 447; and the distance of 61 Cygni, 448, 449

Sun, The, and the velocity of light, 265; apparent size of, as seen from the planets, 117, 118; as a star, 32; axial rotation of, 558; compared with the Earth, 29; connection of, with the seasons, 4; corona of, during eclipse, 62-64; density of, 65, 558; diameter of, 558; distance of, from Mars, 213; distance of, from Saturn, 271; distance of, from the Earth, 31, 114, 184, 240, 558; eclipse of, 6, 53; ellipticity of, 558; faculæ on surface of, 37; focus of planets' orbits, 138; gradually parting with its heat, 95; granules on surface of, 34; heat of, and its sources, 515-526; heat of, thrown on Jupiter, 256; minor planets and, 240; movement of, towards Lyra, 457; nebular theory of its heat, 526; photographed, 34; precession of the Earth's axis, 497; prominences of, 53-59; relation of, to the Moon, 71; rising and setting of, 2; rotation of, 40, 201; size of, 29; spectrum of, 48; spots on, 36-43; spots, connection with magnetism, 42; storms and convulsions on, 42, 43; surface of, gaseous matter, 34; surface of, mottled, 34; teaching of early astronomers concerning, 3-7; temperature of, 30, 31, 516; texture of, 34; tides on, 530; velocity of, 484; weight of, compared with Jupiter, 250, 350; zodiacal light and, 67; zones on the surface of, 39

Sunbeam, revelations of a, 44

Swan, the, 424, 439, 445

Sword-handle of Perseus, 462

Syrtis major, 222

T

Taurus, constellation of, 231, 419

Tebbutt's comet, 353

Telescope, construction of the first, 10; equatorial (Dunsink), 12-14, 185; Greenwich, 26; Herschelian, 19; Lick, 16, 19; Paris, 22, 23; reflecting, 19, 21; refracting, 11, 14; Rosse, 19, 20, 468, 470; sighting of a, 23; structure of the eye illustrates the principle of the, 10; Vienna, 14-16; Washington, 226; Yerkes, 16

Temporary stars, 430, 488

Tethys, 559

Theophilus, 92

Tides, The, actual energy derived from the Earth, 539; affected by the law of gravitation, 149, 535; affected by the Moon, 70, 535-537; at Bay of Fundy, 538; at Cardiff, 538; at Chepstow, 538; at London, 538; at St. Helena, 538; excited by the Sun, 537; formation of currents, 538; in Bristol Channel, 538; in Mediterranean, 537; in mid-ocean, 538; Jupiter and, 552; length of the day and, 541; lunar, 548, 549; moment of momentum and, 552; neap, 537; rotation of the Earth, and revolution of the Moon, 549; satellites of Mars, 551; solar, 550; spring, 537; variations in, 538; waste of water power, 538; work effected, 539

Tin in the Sun, 50

Titan, 294, 295, 559

Titania, 309, 559

Transit of Mercury, 152, 163, 164

Transit of Venus, 152; Captain Cook, 184; Copeland's observations of, 189; Crabtree's observations of, 180; Gassendi's observations of, 178; Halley's method, 180, 181; Horrocks' observations of, 179, 180; importance of, 173; Kepler's prediction of, 163; observations of, at Dunsink, 184-188

Transit of Vulcan, 152-153

Triesnecker, 84, 93

Trouvelot, Mr. L. , and Saturn's rings, 278

Tschermak, and the origin of meteorites, 400, 401

Tycho (lunar crater), 91

Tycho Brahe, and the Observatory of Uraniborg, 9, 10, 430

U

Umbra of Sun-spot, 51

Umbriel, 309, 559

Unstable dynamical equilibrium, 543

Uraniborg, Observatory of, 10

Uranus, 112; attraction of Saturn, 322; Bradley's observations of, 312; composition of, 308; density of, 558; diameter of, 308, 558; diameter of orbit of, 305; disc of, 308; discovery of, by Herschel, 305, 308; distance from Sun of, 558; ellipse of, 313; first taken for a comet, 304; Flamsteed's observations of, 311, 312; formerly regarded as a star, 311, 312; investigations to discover a planet outside the orbit, 323-324; irregular motion of, 314, 323; Lemonnier's observations of, 312; Leonids and, 386; Mayer's observations of, 312; moment of momentum of, 554; orbit of, 117, 310; periodic time of, 558; period of revolution of, 312; rotation of, 308; satellites of, 559; satellites, discovery by Herschel, 308; satellites, movement nearly circular, 309; satellites, periodical movements, 309; satellites, plane of orbits, 309, 310; size of compared with the Earth, 308; and with other planets, 119; subject to another attraction besides the Sun, 314

Ursa major (_see_ Great Bear)

V

Variable Stars, 429

Vega, 414, 423, 424, 479; Struve's measurement of, 442

Velocity, of light, 261, 262, 265; of light, laws dependent upon, 511; of planets, 140-143, 146, 237; of stars, values of, 483-4

Venus, ancient study of, 6; aspects of, 171; atmosphere of, 189; brilliancy of, 168; density of, 558; diameter of, 191, 558; distance of, from the Sun, 191, 558; habitability of, 173; movement of, 168; neighbour to the Earth, 109; orbit of, 114, 135; orbit form of, 139, 191; periodic time of, 558; a planet or "wanderer, " 111; rotation of, 191; shape of, 169; size of, compared with other planets, 116, 169; surface of, 171; transit of, 152, 176-190; transit, importance of, 173; transit predicted by Kepler, 163; velocity and periodic time of, 142, 143, 191; view of the ancients about, 157

Vesta, 233, 238

Victoria, 242

Vienna telescope, 14-16

Virgo, 423

Vogel and Algol, 485; and Spica, 486, 487; and the spectra of the stars, 479, 483

Volcanic origin of meteorites, 400; outbreaks on the Earth, 197

Von Asten and Encke's comet, 349, 350; and the distance of the Sun, 351; and the weight of Mercury, 166

Vortex rings, 469

Vulcan, 152, 153; and the Sun, 3

W

Wargentin, 90

Watson, Professor, and Mercury, 154

Watson, Sir William, friendship with Herschel, 302

Wave-lengths, 60

Weather, not affected by the Moon, 82

Wilson, Mr. W. E. , and the nebula in Orion, 469

Witt, Herr G. , and Eros, 236

Wold Cottage meteorite, the, 392

Wright, Thomas, and the Milky Way, 474

Y

"Year of Stars, " the, 377

Yerkes Observatory, Chicago, 16

Young, Professor, account of a marvellous Sun-prominence, 42; and Sun-spots, 38; observations on magnetic storms, 39

Z

Zeeman, Dr. , and spectral lines, 491

Zinc in the Sun, 50

Zodiac, the, 5

Zodiacal light, 67

Zone of minor planets, 234

PRINTED BY CASSELL & COMPANY, LIMITED, LA BELLE SAUVAGE, LONDON, E. C. 

FOOTNOTES:

[1] It may, however, be remarked that a star is never _seen_ to set, as, owing to our atmosphere, it ceases to be visible before it reaches thehorizon. 

[2] "Popular Astronomy, " p. 66. 

[3] _Limb_ is the word used by astronomers to denote the _edge_ orcircumference of the apparent disc of a heavenly body. 

[4] "The Sun, " p. 119. 

[5] It has been frequently stated that the outburst in 1859, witnessedby Carrington and Hodgson, was immediately followed by an unusuallyintense magnetic storm, but the records at Kew and Greenwich show thatthe magnetic disturbances on that day were of a very trivial character. 

[6] Some ungainly critic has observed that the poet himself seems tohave felt a doubt on the matter, because he has supplemented the dubiousmoonbeams by the "lantern dimly burning. " The more generous, if somewhata sanguine remark has been also made, that "the time will come when theevidence of this poem will prevail over any astronomical calculations. "

[7] This sketch has been copied by permission from the very beautifulview in Messrs. Nasmyth and Carpenter's book, of which it forms PlateXI. So have also the other illustrations of lunar scenery in PlatesVIII. , IX. The photographs were obtained by Mr. Nasmyth from modelscarefully constructed from his drawings to illustrate the features onthe moon. During the last twenty years photography has completelysuperseded drawing by eye in the delineation of lunar objects. Longseries of magnificent photographs of lunar scenery have been publishedby the Paris and Lick Observatories. 

[8] At the British Association's meeting at Cardiff in 1892, Prof. Copeland exhibited a model of the moon, on which the appearance of thestreaks near full moon was perfectly shown by means of small spheres oftransparent glass attached to the surface. 

[9] The duration of an occultation, or, in other words, the length oftime during which the moon hides the star, would be slightly shorterthan the computed time, if the moon had an atmosphere capable ofsensibly refracting the light from the star. But, so far, ourobservations do not indicate this with certainty. 

[10] I owe my knowledge of this subject to Dr. G. Johnstone Stoney, F. R. S. There has been some controversy as to who originated theingenious and instructive doctrine here sketched. 

[11] The space described by a falling body is proportional to theproduct of the force and the square of the time. The force variesinversely as the square of the distance from the earth, so that thespace will vary as the square of the time, and inversely as the squareof the distance. If, therefore, the distance be increased sixty-fold, the time must also be increased sixty-fold, if the space fallen throughis to remain the same. 

[12] See Newcomb's "Popular Astronomy, " p. 78. 

[13] Recent investigation by Newcomb on the motion of Mercury have ledto the result that the hypothesis of a planet or a ring of very smallplanets between the orbit of Mercury and the sun cannot account for thedifference between theory and observation in the movements of Mercury. Harzer has come to the same result, and has shown that the disturbingelement may possibly be the material of the Solar Corona. 

[14] "The Sun: its Planets, and their Satellites. " London: 1882 (page147). 

[15] James Gregory, in a book on optics written in 1667, had alreadysuggested the use of the transit of Venus for this purpose. 

[16] _See_ "Astronomy and Astrophysics, " No. 128. 

[17] _See_ "Astronomy and Astrophysics, " No. 128. 

[18] This is the curved marking which on Plate XVIII. Appears inlongitude 290° and north of (that is, below) the equator. Here, as inall astronomical drawings, north is at the foot and south at the top. _See_ above, p. 82 (Chapter III. ). 

[19] Now Director of the Lick Observatory. 

[20] The heliometer is a telescope with its object-glass cut in halfalong a diameter. One or both of these halves is movable transversely bya screw. Each half gives a complete image of the object. The measuresare effected by observing how many turns of the screw convey the imageof the star formed by one half of the object-glass to coincide with theimage of the planet formed by the other. 

[21] See "Astronomy and Astrophysics, " No. 109. 

[22] It is only right to add that some observers believe that, inexceptional circumstances, points of Jupiter have shown some slightdegree of intrinsic light. 

[23] Professor Pickering, of Cambridge, Mass. , has, however, effectedthe important improvement of measuring the decline of light of thesatellite undergoing eclipse by the photometer. Much additionalprecision may be anticipated in the results of such observations. 

[24] "Newcomb's Popular Astronomy, " p. 336. 

[25] _See_ Grant, "History of Physical Astronomy, " page 255. 

[26] Now Director of the Lick Observatory. 

[27] We are here neglecting the orbital motion of Saturn, by which thewhole system is moved towards or from the earth, but as this motion iscommon to the ball and the ring, it will not disturb the relativepositions of the three spectra. 

[28] According to Prof. Barnard's recent measures, the diameter of Titanis 2, 700 miles. This is the satellite discovered by Huyghens; it is thesixth in order from the planet. 

[29] Extract from "Three Cities of Russia, " by C. Piazzi Smyth, vol. Ii. , p. 164: "In the year 1796. It then chanced that George III. , ofGreat Britain, was pleased to send as a present to the Empress Catharineof Russia a ten-foot reflecting telescope constructed by Sir WilliamHerschel. Her Majesty immediately desired to try its powers, andRoumovsky was sent for from the Academy to repair to Tsarskoe-Selo, where the Court was at the time residing. The telescope was accordinglyunpacked, and for eight long consecutive evenings the Empress employedherself ardently in observing the moon, planets, and stars; and morethan this, in inquiring into the state of astronomy in her dominions. Then it was that Roumovsky set before the Imperial view the Academy'sidea of removing their observatory, detailing the necessity for, and theadvantages of, such a proceeding. Graciously did the 'Semiramis of theNorth, ' the 'Polar Star, ' enter into all these particulars, and warmlyapprove of the project; but death closed her career within a few weeksafter, and prevented her execution of the design. "

[30] _See_ Professor Holden's "Sir William Herschel, his Life andWorks. "

[31] Arago says that "Lemonnier's records were the image of chaos. "Bouvard showed to Arago one of the observations of Uranus which waswritten on a paper bag that in its time had contained hair-powder. 

[32] The first comet of 1884 also suddenly increased in brightness, while a distinct disc, which hitherto had formed the nucleus, becametransformed into a fine point of light. 

[33] The three numbers 12, 1, and 1/4 are nearly inversely proportionalto the atomic weights of hydrogen, hydrocarbon gas, and iron vapour, andit is for this reason that Bredichin suggested the above-mentionedcomposition of the various types of tail. Spectroscopic evidence of thepresence of hydrogen is yet wanting. 

[34] This illustration, as well as the figure of the path of themeteors, has been derived from Dr. G. J. Stoney's interesting lecture on"The Story of the November Meteors, " at the Royal Institution, in 1879. 

[35] On the 27th November, 1885, a piece of meteoric iron fell atMazapil, in Mexico, during the shower of Andromedes, but whether itformed part of the swarm is not known. It is, however, to be noticedthat meteorites are said to have fallen on several other occasions atthe end of November. 

[36] Hooke had noticed, in 1664, that the star Gamma Arietis was double. 

[37] Perhaps if we could view the stars without the intervention of theatmosphere, blue stars would be more common. The absorption of theatmosphere specially affects the greenish and bluish colours. ProfessorLangley gives us good reason for believing that the sun itself would beblue if it were not for the effect of the air. 

[38] The declination of a star is the arc drawn from the star to theequator at right angles to the latter. 

[39] The distance of 61 Cygni has, however, again been investigated byProfessor Asaph Hall, of Washington, who has obtained a resultconsiderably less than had been previously supposed; on the other hand, Professor Pritchard's photographic researches are in confirmation ofStruve's and those obtained at Dunsink. 

[40] I am indebted for this drawing to the kindness of Messrs. De laRue. 

[41] _See_ Chapter XIX. , on the mass of Sirius and his satellite. 

[42] As the earth carries on the telescope at the rate of 18 miles asecond, and as light moves with the velocity of 180, 000 miles a secondvery nearly, it follows that the velocity of the telescope is about oneten-thousandth part of that of light. While the light moves down thetube 20 feet long, the telescope will therefore have moved theten-thousandth part of 20 feet--_i. E. _, the fortieth of an inch. 

[43] _See_ Newcomb's "Popular Astronomy, " p. 508, where the discovery ofthis law is attributed to Mr. J. Homer Lane, of Washington. Thecontraction theory is due to Helmholtz. 

[44] The theory of Tidal Evolution sketched in this chapter is mainlydue to the researches of Professor G. H. Darwin, F. R. S. 

[45] The hour varies with the locality: it would be 11. 49 at Calais; atLiverpool, 11. 23; at Swansea Bay, 5. 56, etc. 

[46] Having decided upon the units of mass, of angle, and of distancewhich we intend to use for measuring these quantities, then any mass, orangle, or distance is expressed by a certain definite number. Thus if wetake the mass of the earth as the unit of mass, the angle through whichit moves in a second as the unit of angle, and its distance from the sunas the unit of distance, we shall find that the similar quantities forJupiter are expressed by the numbers 316, 0·0843, and 5·2 respectively. Hence its orbital moment of momentum is 316 × 0·0843 × (5·2)².