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Preface

What Froude says of history is true also of astronomy: it is the mostimpressive where it transcends explanation. It is not the mathematicsof astronomy, but the wonder and the mystery that seize upon theimagination. The calculation of an eclipse owes all its prestige tothe sublimity of its data; the operation, in itself, requires no moremental effort than the preparation of a railway time-table. 

The dominion which astronomy has always held over the minds of men isakin to that of poetry; when the former becomes merely instructive andthe latter purely didactic, both lose their power over theimagination. Astronomy is known as the oldest of the sciences, and itwill be the longest-lived because it will always have arcana that havenot been penetrated. 

Some of the things described in this book are little known to theaverage reader, while others are well known; but all possess thefascination of whatever is strange, marvelous, obscure, or mysterious-- magnified, in this case, by the portentous scale of the phenomena. 

The idea of the author is to tell about these things in plainlanguage, but with as much scientific accuracy as plain language willpermit, showing the wonder that is in them without getting away fromthe facts. Most of them have hitherto been discussed only in technicalform, and in treatises that the general public seldom sees and neverreads. 

Among the topics touched upon are: * The strange unfixedness of the ``fixed stars, '' the vast migrations of the suns and worlds constituting the universe. * The slow passing out of existence of those collocations of stars which for thousands of years have formed famous ``constellations, '' preserving the memory of mythological heroes and heroines, and perhaps of otherwise unrecorded history. * The tendency of stars to assemble in immense clouds, swarms, and clusters. * The existence in some of the richest regions of the universe of absolutely black, starless gaps, deeps, or holes, as if one were looking out of a window into the murkiest night. * The marvelous phenomena of new, or temporary, stars, which appear as suddenly as conflagrations, and often turn into something else as eccentric as themselves. * The amazing forms of the ``whirlpool, '' ``spiral, '' ``pinwheel, '' and ``lace, '' or ``tress, '' nebulæ. * The strange surroundings of the sun, only seen in particular circumstances, but evidently playing a constant part in the daily phenomena of the solar system. * The mystery of the Zodiacal Light and the Gegenschein. * The extraordinary transformations undergone by comets and their tails. * The prodigies of meteorites and masses of stone and metal fallen from the sky. * The cataclysms that have wrecked the moon. * The problem of life and intelligence on the planet Mars. * The problematical origin and fate of the asteroids. * The strange phenomena of the auroral lights. 

An attempt has been made to develop these topics in an orderly way, showing their connection, so that the reader may obtain a broadgeneral view of the chief mysteries and problems of astronomy, and anidea of the immense field of discovery which still lies, almostunexplored, before it. 

The Windows of Absolute Night

To most minds mystery is more fascinating than science. But whenscience itself leads straight up to the borders of mystery and therecomes to a dead stop, saying, ``At present I can no longer see myway, '' the force of the charm is redoubled. On the other hand, theillimitable is no less potent in mystery than the invisible, whencethe dramatic effect of Keats' ``stout Cortez'' staring at theboundless Pacific while all his men look at each other with a wildsurmise, ``silent upon a peak in Darien. '' It is with similar feelingsthat the astronomer regards certain places where from the peaks of theuniverse his vision seems to range out into endless empty space. Hesees there the shore of his little isthmus, and, beyond, unexploredimmensity. 

The name, ``coal-sacks, '' given to these strange voids is hardlydescriptive. Rather they produce upon the mind the effect of blankwindows in a lonely house on a pitch-dark night, which, when looked atfrom the brilliant interior, become appalling in their rayless murk. Infinity seems to acquire a new meaning in the presence of these blackopenings in the sky, for as one continues to gaze it loses its purelymetaphysical quality and becomes a kind of entity, like the ocean. Theobserver is conscious that he can actually see the beginning of itsebon depths, in which the visible universe appears to float like anenchanted island, resplendent within with lights and life and gorgeousspectacles, and encircled with screens of crowded stars, but with itsdazzling vistas ending at the fathomless sea of pure darkness whichencloses all. 

The Galaxy, or Milky Way, surrounds the borders of our island in spacelike a stellar garland, and when openings appear in it they are, bycontrast, far more impressive than the general darkness of theinterstellar expanse seen in other directions. Yet even that expanseis not everywhere equally dark, for it contains gloomy deepsdiscernable with careful watching. Here, too, contrast plays animportant part, though less striking than within the galactic region. Some of Sir William Herschel's observations appear to indicate anassociation between these tenebrious spots and neighboring star cloudsand nebulæ. It is an illuminating bit of astronomical history thatwhen he was sweeping the then virgin heavens with his great telescopeshe was accustomed to say to his sister who, note-book in hand, waitedat his side to take down his words, fresh with the inspiration ofdiscovery: ``Prepare to write; the nebulæ are coming; here space isvacant. ''

The most famous of the ``coal-sacks, '' and the first to be brought togeneral attention before astronomers had awakened to the significanceof such things, lies adjacent to the ``Southern Cross, '' and is trulyan amazing phenomenon. It is not alone the conspicuousness of thiscelestial vacancy, opening suddenly in the midst of one of the richestparts of the Galaxy, that has given it its fame, but quite as much thesuperstitious awe with which it was regarded by the early explorers ofthe South Seas. To them, as well as to those who listened in raptwonder to their tales, the ``Coal-sack'' seemed to possess some occultconnection with the mystic ``Cross. '' In the eyes of the sailors itwas not a vacancy so much as a sable reality in the sky, and as, shuddering, they stared at it, they piously crossed themselves. It wasanother of the magical wonders of the unknown South, and as such itformed the basis of many a ``wild surmise'' and many a sea-dog's yarn. Scientific investigation has not diminished its prestige, and today notraveler in the southern hemisphere is indifferent to its fascinatingstrangeness, while some find it the most impressive spectacle of theantarctic heavens. 

All around, up to the very edge of the yawning gap, the sheen of theMilky Way is surpassingly glorious; but there, as if in obedience toan almighty edict, everything vanishes. A single faint star is visiblewithin the opening, producing a curious effect upon the sensitivespectator, like the sight of a tiny islet in the midst of a black, motionless, waveless tarn. The dimensions of the lagoon of darkness, which is oval or pear-shaped, are eight degrees by five, so that itoccupies a space in the sky about one hundred and thirty times greaterthan the area of the full moon. It attracts attention as soon as theeye is directed toward the quarter where it exists, and by virtue ofthe rarity of such phenomena it appears a far greater wonder than thedrifts of stars that are heaped around it. Now that observatories aremultiplying in the southern hemisphere, the great austral``Coal-sack'' will, no doubt, receive attention proportioned to itsimportance as one of the most significant features of the sky. Alreadyat the Sydney Observatory photographs have shown that the southernportion of this Dead Sea of Space is not quite ``bottomless, ''although its northern part defies the longest sounding lines of theastronomer. 

There is a similar, but less perfect, ``coal-sack'' in the northernhemisphere, in the constellation of ``The Swan, '' which, strange tosay, also contains a well-marked figure of a cross outlined by stars. This gap lies near the top of the cross-shaped figure. It is best seenby averted vision, which brings out the contrast with the Milky Way, which is quite brilliant around it. It does not, however, exercise thesame weird attraction upon the eye as the southern ``Coal-sack, '' forinstead of looking like an absolute void in the sky, it rather appearsas if a canopy of dark gauze had been drawn over the stars. We shallsee the possible significance of this appearance later. 

Just above the southern horizon of our northern middle latitudes, insummer, where the Milky Way breaks up into vast sheets of nebulousluminosity, lying over and between the constellations Scorpio andSagittarius, there is a remarkable assemblage of ``coal-sacks, ''though none is of great size. One of them, near a conspicuousstar-cluster in Scorpio, M80, is interesting for having been the firstof these strange objects noted by Herschel. Probably it was itsnearness to M80 which suggested to his mind the apparent connection ofsuch vacancies with star-clusters which we have already mentioned. 

But the most marvelous of the ``coal-sacks'' are those that have beenfound by photography in Sagittarius. One of Barnard's earliest andmost excellent photographs includes two of them, both in thestar-cluster M8. The larger, which is roughly rectangular in outline, contains one little star, and its smaller neighbor is lune-shaped --surely a most singular form for such an object. Both are associatedwith curious dark lanes running through the clustered stars liketrails in the woods. Along the borders of these lanes the stars areranked in parallel rows, and what may be called the bottoms of thelanes are not entirely dark, but pebbled with faint stellar points. One of them which skirts the two dark gaps and traverses the clusteralong its greatest diameter is edged with lines of stars, recallingthe alignment of the trees bordering a French highway. This road ofstars cannot be less than many billions of miles in length!

All about the cluster the bed of the Galaxy is strangely disturbed, and in places nearly denuded, as if its contents had been raked awayto form the immense stack and the smaller accumulations of starsaround it. The well-known ``Trifid Nebula'' is also included in thefield of the photograph, which covers a truly marvelous region, sointricate in its mingling of nebulæ, star-clusters, star-swarms, star-streams, and dark vacancies that no description can do itjustice. Yet, chaotic as it appears, there is an unmistakablesuggestion of unity about it, impressing the beholder with the ideathat all the different parts are in some way connected, and have notbeen fortuitously thrown together. Miss Agnes M. Clerke made thestriking remark that the dusky lanes in M8 are exemplified on thelargest scale in the great rift dividing the Milky Way, from Cygnus inthe northern hemisphere all the way to the ``Cross'' in the southern. Similar lanes are found in many other clusters, and they are generallyassociated with flanking rows of stars, resembling in theirarrangement the thick-set houses and villas along the roadways thattraverse the approaches to a great city. 

But to return to the black gaps. Are they really windows in thestar-walls of the universe? Some of them look rather as if they hadbeen made by a shell fired through a luminous target, allowing the eyeto range through the hole into the void space beyond. If science isdiscretely silent about these things, what can the more venturesomeand less responsible imagination suggest? Would a huge ``runawaysun, '' like Arcturus, for instance, make such an opening if it shouldpass like a projectile through the Milky Way? It is at least astimulating inquiry. Being probably many thousands of times moremassive than the galactic stars, such a stellar missile would not bestopped by them, though its direction of flight might be altered. Itwould drag the small stars lying close to its course out of theirspheres, but the ultimate tendency of its attraction would be to sweepthem round in its wake, thus producing rather a star-swarm than avacancy. Those that were very close to it might be swept away in itsrush and become its satellites, careering away with it in its flightinto outer space; but those that were farther off, and they would, ofcourse, greatly outnumber the nearer ones, would tend inward from allsides toward the line of flight, as dust and leaves collect behind aspeeding motor (though the forces operating would be different), andwould fill up the hole, if hole it were. A swarm thus collected shouldbe rounded in outline and bordered with a relatively barren ring fromwhich the stars had been ``sucked'' away. In a general sense the M8cluster answers to this description, but even if we undertook toaccount for its existence by a supposition like the above, the blackgaps would remain unexplained, unless one could make a further drafton the imagination and suggest that the stars had been thrown into avast eddy, or system of eddies, whose vortices appear as dark holes. Only a maelstrom-like motion could keep such a funnel open, forwithout regard to the impulse derived from the projectile, the propermotions of the stars themselves would tend to fill it. Perhaps someother cause of the whirling motion may be found. As we shall see whenwe come to the spiral nebulæ, gyratory movements are exceedinglyprevalent throughout the universe, and the structure of the Milky Wayis everywhere suggestive of them. But this is hazardous sport even forthe imagination -- to play with suns as if they were but thistle-downin the wind or corks in a mill-race. 

Another question arises: What is the thickness of the hedge of starsthrough which the holes penetrate? Is the depth of the openingsproportionate to their width? In other words, is the Milky Way roundin section like a rope, or flat and thin like a ribbon? The answer isnot obvious, for we have little or no information concerning therelative distances of the faint galactic stars. It would be easier, certainly, to conceive of openings in a thin belt than in a massivering, for in the first case they would resemble mere rifts and breaks, while in the second they would be like wells or bore-holes. Then, too, the fact that the Milky Way is not a continuous body but is made up ofstars whose actual distances apart is great, offers another quandary;persistent and sharply bordered apertures in such an assemblage are apriori as improbable, if not impossible, as straight, narrow holesrunning through a swarm of bees. 

The difficulty of these questions indicates one of the reasons why ithas been suggested that the seeming gaps, or many of them, are notopenings at all, but opaque screens cutting off the light from starsbehind them. That this is quite possible in some cases is shown byBarnard's later photographs, particularly those of the singular regionaround the star Rho Ophiuchi. Here are to be seen somber lanes andpatches, apparently forming a connected system which covers an immensespace, and which their discoverer thinks may constitute a ``darknebula. '' This seems at first a startling suggestion; but, after all, why should their not be dark nebulæ as well as visible ones? In truth, it has troubled some astronomers to explain the luminosity of thebright nebulæ, since it is not to be supposed that matter in sodiffuse a state can be incandescent through heat, and phosphorescentlight is in itself a mystery. The supposition is also in accord withwhat we know of the existence of dark solid bodies in space. Manybright stars are accompanied by obscure companions, sometimes asmassive as themselves; the planets are non-luminous; the same is trueof meteors before they plunge into the atmosphere and become heated byfriction; and many plausible reasons have been found for believingthat space contains as many obscure as shining bodies of great size. It is not so difficult, after all, then, to believe that there areimmense collections of shadowy gases and meteoric dust whose presenceis only manifested when they intercept the light coming from shiningbodies behind them. 

This would account for the apparent extinguishment of light in openspace, which is indicated by the falling off in relative number oftelescopic stars below the tenth magnitude. Even as things are, theamount of light coming to us from stars too faint to be seen with thenaked eye is so great that the statement of it generally surprisespersons who are unfamiliar with the inner facts of astronomy. It hasbeen calculated that on a clear night the total starlight from theentire celestial sphere amounts to one-sixtieth of the light of thefull moon; but of this less than one-twenty-fifth is due to starsseparately distinguished by the eye. If there were no obscuring mediumin space, it is probable that the amount of starlight would benoticeably and perhaps enormously increased. 

But while it seems certain that some of the obscure spots in the MilkyWay are due to the presence of ``dark nebulæ, '' or concealing veils ofone kind or another, it is equally certain that there are many whichare true apertures, however they may have been formed, and by whateverforces they may be maintained. These, then, are veritable windows ofthe Galaxy, and when looking out of them one is face to face with thegreat mystery of infinite space. There the known universe visiblyends, but manifestly space itself does not end there. It is not withinthe power of thought to conceive an end to space, for the instant wethink of a terminal point or line the mind leaps forward to thebeyond. There must be space outside as well as inside. Eternity oftime and infinity of space are ideas that the intellect cannot fullygrasp, but neither can it grasp the idea of a limitation to eitherspace or time. The metaphysical conceptions of hypergeometry, orfourth-dimensional space, do not aid us. 

Having, then, discovered that the universe is a thing contained insomething indefinitely greater than itself; having looked out of itswindows and found only the gloom of starless night outside -- whatconclusions are we to draw concerning the beyond? It seems as empty asa vacuum, but is it really so? If it be, then our universe is a singleatom astray in the infinite; it is the only island in an ocean withoutshores; it is the one oasis in an illimitable desert. Then the MilkyWay, with its wide-flung garland of stars, is afloat like a tinysmoke-wreath amid a horror of immeasurable vacancy, or it is anevanescent and solitary ring of sparkling froth cast up for a momenton the viewless billows of immensity. From such conclusions the mindinstinctively shrinks. It prefers to think that there is somethingbeyond, though we cannot see it. Even the universe could not bear tobe alone -- a Crusoe lost in the Cosmos! As the inhabitants of themost elegant château, with its gardens, parks, and crowds ofattendants, would die of loneliness if they did not know that theyhave neighbors, though not seen, and that a living world of indefiniteextent surrounds them, so we, when we perceive that the universe haslimits, wish to feel that it is not solitary; that beyond the hedgesand the hills there are other centers of life and activity. Couldanything be more terrible than the thought of an isolated universe?The greater the being, the greater the aversion to seclusion. Only theinfinite satisfies; in that alone the mind finds rest. 

We are driven, then, to believe that the universal night whichenvelopes us is not tenantless; that as we stare out of thestar-framed windows of the Galaxy and see nothing but uniformblackness, the fault is with our eyes or is due to an obscuringmedium. Since our universe is limited in extent, there must be otheruniverses beyond it on all sides. Perhaps if we could carry ourtelescopes to the verge of the great ``Coal-sack'' near the ``Cross, ''being then on the frontier of our starry system, we could discern, sparkling afar off in the vast night, some of the outer galaxies. Theymay be grander than ours, just as many of the suns surrounding us areimmensely greater than ours. If we could take our stand somewhere inthe midst of immensity and, with vision of infinite reach, look aboutus, we should perhaps see a countless number of stellar systems, amidwhich ours would be unnoticeable, like a single star among themultitude glittering in the terrestial sky on a clear night. Somemight be in the form of a wreath, like our own; some might beglobular, like the great star-clusters in Hercules and Centaurus; somemight be glittering circles, or disks, or rings within rings. If wecould enter them we should probably find a vast variety ofcomposition, including elements unknown to terrestrial chemistry; forwhile the visible universe appears to contain few if any substancesnot existing on the earth or in the sun, we have no warrant to assumethat others may not exist in infinite space. 

And how as to gravitation? We do not know that gravitation acts beyondthe visible universe, but it is reasonable to suppose that it does. Atany rate, if we let go its sustaining hand we are lost, and can onlywander hopelessly in our speculations, like children astray. If theempire of gravitation is infinite, then the various outer systems musthave some, though measuring by our standards an imperceptible, attractive influence upon each other, for gravitation never lets goits hold, however great the space over which it is required to act. Just as the stars about us are all in motion, so the starry systemsbeyond our sight may be in motion, and our system as a whole may bemoving in concert with them. If this be so, then after interminableages the aspect of the entire system of systems must change, itsvarious members assuming new positions with respect to one another. Inthe course of time we may even suppose that our universe will approachrelatively close to one of the others; and then, if men are yet livingon the earth, they may glimpse through the openings which revealnothing to us now, the lights of another nearing star system, like thesignals of a strange squadron, bringing them the assurance (which canbe but an inference at present) that the ocean of space has otherargosies venturing on its limitless expanse. 

There remains the question of the luminiferous ether by whose agencythe waves of light are borne through space. The ether is as mysteriousas gravitation. With regard to ether we only infer its existence fromthe effects which we ascribe to it. Evidently the ether must extend asfar as the most distant visible stars. But does it continue onindefinitely in outer space? If it does, then the invisibility of theother systems must be due to their distance diminishing the quantityof light that comes from them below the limit of perceptibility, or tothe interposition of absorbing media; if it does not, then the reasonwhy we cannot see them is owing to the absence of a means ofconveyance for the light waves, as the lack of an interplanetaryatmosphere prevents us from hearing the thunder of sun-spots. (It isinteresting to recall that Mr Edison was once credited with theintention to construct a gigantic microphone which should render theroar of sun-spots audible by transforming the electric vibrations intosound-waves). On this supposition each starry system would beenveloped in its own globule of ether, and no light could cross fromone to another. But the probability is that both the ether andgravitation are ubiquitous, and that all the stellar systems areimmersed in the former like clouds of phosphorescent organisms in thesea. 

So astronomy carries the mind from height to greater height. Men werelong in accepting the proofs of the relative insignificance of theearth; they were more quickly convinced of the comparative littlenessof the solar system; and now the evidence assails their reason thatwhat they had regarded as the universe is only one mote gleaming inthe sunbeams of Infinity. 

Star-Clouds, Star-Clusters, and Star-Streams

In the preceding chapter we have seen something of the strangelycomplicated structure of the Galaxy, or Milky Way. We now proceed tostudy more comprehensively that garlanded ``Pathway of the Gods. ''

Judged by the eye alone, the Milky Way is one of the most delicatelybeautiful phenomena in the entire realm of nature -- a shimmer ofsilvery gauze stretched across the sky; but studied in the light ofits revelations, it is the most stupendous object presented to humanken. Let us consider, first, its appearance to ordinary vision. Itsapparent position in the sky shifts according to the season. On aserene, cloudless summer evening, in the absence of the moon, whoselight obscures it, one sees the Galaxy spanning the heavens from northto southeast of the zenith like a phosphorescent arch. In early springit forms a similar but, upon the whole, less brilliant arch west ofthe zenith. Between spring and summer it lies like a long, faint, twilight band along the northern horizon. At the beginning of winterit again forms an arch, this time spanning the sky from east to west, a little north of the zenith. These are its positions as viewed fromthe mean latitude of the United States. Even the beginner instar-gazing does not have to watch it throughout the year in order tobe convinced that it is, in reality, a great circle, extendingentirely around the celestial sphere. We appear to be situated nearits center, but its periphery is evidently far away in the depths ofspace. 

Although to the casual observer it seems but a delicate scarf oflight, brighter in some places than in others, but hazy and indefiniteat the best, such is not its appearance to those who study it withcare. They perceive that it is an organic whole, though marvelouslycomplex in detail. The telescope shows that it consists of stars toofaint and small through excess of distance to be separately visible. Of the hundred million suns which some estimates have fixed as theprobable population of the starry universe, the vast majority (atleast thirty to one) are included in this strange belt of misty light. But they are not uniformly distributed in it; on the contrary, theyare arrayed in clusters, knots, bunches, clouds, and streams. Theappearance is somewhat as if the Galaxy consisted of innumerableswarms of silver-winged bees, more or less intermixed, some massedtogether, some crossing the paths of others, but all governed by asingle purpose which leads them to encircle the region of space inwhich we are situated. 

From the beginning of the systematic study of the heavens, the facthas been recognized that the form of the Milky Way denotes the schemeof the sidereal system. At first it was thought that the shape of thesystem was that of a vast round disk, flat like a cheese, and filledwith stars, our sun and his relatively few neighbors being placed nearthe center. According to this view, the galactic belt was an effect ofperspective; for when looking in the direction of the plane of thedisk, the eye ranged through an immense extension of stars whichblended into a glimmering blur, surrounding us like a ring; while whenlooking out from the sides of the disk we saw but few stars, and inthose directions the heavens appeared relatively blank. Finally it wasrecognized that this theory did not correspond with the observedappearances, and it became evident that the Milky Way was not a mereeffect of perspective, but an actual band of enormously distant stars, forming a circle about the sphere, the central opening of the ring(containing many scattered stars) being many times broader than thewidth of the ring itself. Our sun is one of the scattered stars in thecentral opening. 

As already remarked, the ring of the Galaxy is very irregular, and inplaces it is partly broken. With its sinuous outline, its pendantsprays, its graceful and accordant curves, its bunching of masses, itsoccasional interstices, and the manifest order of a general plangoverning the jumble of its details, it bears a remarkable resemblanceto a garland -- a fact which appears the more wonderful when we recallits composition. That an elm-tree should trace the lines of beautywith its leafy and pendulous branches does not surprise us; but we canonly gaze with growing amazement when we behold a hundred million sunsimitating the form of a chaplet! And then we have to remember thatthis form furnishes the ground-plan of the universe. 

As an indication of the extraordinary speculations to which themystery of the Milky Way has given rise, a theory recently (1909)proposed by Prof. George C. Comstock may be mentioned. Starting withthe data (first) that the number of stars increases as the Milky Wayis approached, and reaches a maximum in its plane, while on the otherhand the number of nebulæ is greatest outside the Milky Way andincreases with distance from it, and (second) that the Milky Way, although a complete ring, is broad and diffuse on one side throughone-half its course -- that half alone containing nebulæ -- andrelatively narrow and well defined on the opposite side, the author ofthis singular speculation avers that these facts can best be explainedby supposing that the invisible universe consists of twointerpenetrating parts, one of which is a chaos of indefinite extent, strewn with stars and nebulous dust, and the other a long, broad butcomparatively thin cluster of stars, including the sun as one of itscentral members. This flat star-cluster is conceived to be movingedgewise through the chaos, and, according to Professor Comstock, itacts after the manner of a snow-plough sweeping away the cosmic dustand piling it on either hand above and below the plane of the movingcluster. It thus forms a transparent rift, through which we seefarther and command a view of more stars than through the intensifieddust-clouds on either hand. This rift is the Milky Way. The dustthrown aside toward the poles of the Milky Way is the substance of thenebulæ which abound there. Ahead, where the front of the star-ploughis clearing the way, the chaos is nearer at hand, and consequentlythere the rift subtends a broader angle, and is filled with primordialdust, which, having been annexed by the vanguard of the star-swarm, forms the nebulæ seen only in that part of the Milky Way. But behind, the rift appears narrow because there we look farther away betweendust-clouds produced ages ago by the front of the plough, and noscattered dust remains in that part of the rift. 

In quoting an outline of this strikingly original theory the presentwriter should not be understood as assenting to it. That it appearsbizarre is not, in itself, a reason for rejecting it, when we aredealing with so problematical and enigmatical a subject as the MilkyWay; but the serious objection is that the theory does notsufficiently accord with the observed phenomena. There is too muchevidence that the Milky Way is an organic system, however fantasticits form, to permit the belief that it can only be a rift in chaoticclouds. As with every organism, we find that its parts are more orless clearly repeated in its ensemble. Among all the strange thingsthat the Milky Way contains there is nothing so extraordinary asitself. Every astronomer must many times have found himself marvelingat it in those comparatively rare nights when it shows all its beautyand all its strangeness. In its great broken rifts, divisions, andspirals are found the gigantic prototypes of similar forms in itsstar-clouds and clusters. As we have said, it determines the generalshape of the whole sidereal system. Some of the brightest stars in thesky appear to hang like jewels suspended at the ends of tasselsdropped from the Galaxy. Among these pendants are the Pleiades and theHyades. Orion, too, the ``Mighty Hunter, '' is caught in ``a loop oflight'' thrown out from it. The majority of the great first-magnitudestars seem related to it, as if they formed an inner ring inclined atan angle of some twenty degrees to its plane. Many of the long curvesthat set off from it on both sides are accompanied by correspondingcurves of lucid stars. In a word, it offers every appearance ofstructural connection with the entire starry system. That the universeshould have assumed the form of a wreath is certainly a matter forastonishment; but it would have been still more astonishing if it hadbeen a cube, a rhomboid, or a dodecahedron, for then we should havehad to suppose that something resembling the forces that shapecrystals had acted upon the stars, and the difficulty of explainingthe universe by the laws of gravitation would have been increased. 

From the Milky Way as a whole we pass to the vast clouds, swarms, andclusters of stars of which it is made up. It may be, as someastronomers hold, that most of the galactic stars are much smallerthan the sun, so that their faintness is not due entirely to theeffect of distance. Still, their intrinsic brilliance attests theirsolar character, and considering their remoteness, which has beenestimated at not less than ten thousand to twenty thousand light-years(a light-year is equal to nearly six thousand thousand million miles)their actual masses cannot be extremely small. The minutest of themare entitled to be regarded as real suns, and they vary enormously inmagnitude. The effects of their attractions upon one another can onlybe inferred from their clustering, because their relative movementsare not apparent on account of the brevity of the observations that wecan make. But imagine a being for whom a million years would be but asa flitting moment; to him the Milky Way would appear in a state ofceaseless agitation -- swirling with ``a fury of whirlpool motion. ''

The cloud-like aspect of large parts of the Galaxy must always haveattracted attention, even from naked-eye observers, but the truestar-clouds were first satisfactorily represented in Barnard'sphotographs. The resemblance to actual clouds is often startling. Someare close-packed and dense, like cumuli; some are wispy or mottled, like cirri. The rifts and modulations, as well as the generaloutlines, are the same as those of clouds of vapor or dust, and onenotices also the characteristic thinning out at the edges. But we mustbeware of supposing that the component suns are thickly crowded as theparticles forming an ordinary cloud. They look, indeed, as if theywere matted together, because of the irradiation of light, but inreality millions and billions of miles separate each star from itsneighbors. Nevertheless they form real assemblages, whose members arefar more closely related to one another than is our sun to the starsaround him, and if we were in the Milky Way the aspect of thenocturnal sky would be marvelously different from its presentappearance. 

Stellar clouds are characteristic of the Galaxy and are not foundbeyond its borders, except in the ``Magellanic Clouds'' of thesouthern hemisphere, which resemble detached portions of the MilkyWay. These singular objects form as striking a peculiarity of theaustral heavens as does the great ``Coal-sack'' described in Chapter1. But it is their isolation that makes them so remarkable, for theircomposition is essentially galactic, and if they were included withinits boundaries they would not appear more wonderful than many otherparts of the Milky Way. Placed where they are, they look like massesfallen from the great stellar arch. They are full of nebulæ andstar-clusters, and show striking evidences of spiral movement. 

Star-swarms, which are also characteristic features of the Galaxy, differ from star-clouds very much in the way that their name wouldimply -- i. E. , their component stars are so arranged, even when theyare countless in number, that the idea of an exceedingly numerousassemblage rather than that of a cloud is impressed on the observer'smind. In a star-swarm the separate members are distinguishable becausethey are either larger or nearer than the stars composing a ``cloud. ''A splendid example of a true star-swarm is furnished by Chi Persei, inthat part of the Milky Way which runs between the constellationsPerseus and Cassiopeia. This swarm is much coarser than many others, and can be seen by the naked eye. In a small telescope it appearsdouble, as if the suns composing it had divided into two parties whichkeep on their way side by side, with some commingling of their memberswhere the skirts of the two companies come in contact. 

Smaller than either star-clouds or star-swarms, and differing fromboth in their organization, are star-clusters. These, unlike theothers, are found outside as well as inside the Milky Way, althoughthey are more numerous inside its boundaries than elsewhere. The termstar-cluster is sometimes applied, though improperly, to assemblageswhich are rather groups, such, for instance, as the Pleiades. In theirmost characteristic aspect star-clusters are of a globular shape --globes of suns! A famous example of a globular star-cluster, but onenot included in the Milky Way, is the ``Great Cluster in Hercules. ''This is barely visible to the naked eye, but a small telescope showsits character, and in a large one it presents a marvelous spectacle. Photographs of such clusters are, perhaps, less effective than thoseof star-clouds, because the central condensation of stars in them isso great that their light becomes blended in an indistinguishableblur. The beautiful effect of the incessant play of infinitesimal raysover the apparently compact surface of the cluster, as if it were aglobe of the finest frosted silver shining in an electric beam, isalso lost in a photograph. Still, even to the eye looking directly atthe cluster through a powerful telescope, the central part of thewonderful congregation seems almost a solid mass in which the starsare packed like the ice crystals in a snowball. 

The same question rises to the lips of every observer: How can theypossibly have been brought into such a situation? The marvel does notgrow less when we know that, instead of being closely compacted, thestars of the cluster are probably separated by millions of miles; forwe know that their distances apart are slight as compared with theirremoteness from the Earth. Sir William Herschel estimated their numberto be about fourteen thousand, but in fact they are uncountable. If wecould view them from a point just within the edge of the assemblage, they would offer the appearance of a hollow hemisphere emblazoned withstars of astonishing brilliancy; the near-by ones unparalleled insplendor by any celestial object known to us, while the more distantones would resemble ordinary stars. An inhabitant of the cluster wouldnot know, except by a process of ratiocination, that he was dwellingin a globular assemblage of suns; only from a point far outside wouldtheir spherical arrangement become evident to the eye. Imaginefourteen-thousand fire-balloons with an approach to regularity in aspherical space -- say, ten miles in diameter; there would be anaverage of less than thirty in every cubic mile, and it would benecessary to go to a considerable distance in order to see them as aglobular aggregation; yet from a point sufficiently far away theywould blend into a glowing ball. 

Photographs show even better than the best telescopic views that thegreat cluster is surrounded with a multitude of dispersed stars, suggestively arrayed in more or less curving lines, which radiate fromthe principle mass, with which their connection is manifest. Thesestars, situated outside the central sphere, look somewhat like vagrantbees buzzing round a dense swarm where the queen bee is sitting. Yetwhile there is so much to suggest the operation of central forces, bringing and keeping the members of the cluster together, theattentive observer is also impressed with the idea that the wholewonderful phenomenon may be the result of explosion. As soon as thisthought seizes the mind, confirmation of it seems to be found in theappearance of the outlying stars, which could be as readily explainedby the supposition that they have been blown apart as that they haveflocked together toward a center. The probable fact that the starsconstituting the cluster are very much smaller than our sun might beregarded as favoring the hypothesis of an explosion. Of their realsize we know nothing, but, on the basis of an uncertain estimate oftheir parallax, it has been calculated that they may averageforty-five thousand miles in diameter -- something more than half thediameter of the planet Jupiter. Assuming the same mean density, fourteen thousand such stars might have been formed by the explosionof a body about twice the size of the sun. This recalls the theory ofOlbers, which has never been altogether abandoned or disproved, thatthe Asteroids were formed by the explosion of a planet circulatingbetween the orbits of Mars and Jupiter. The Asteroids, whatever theirmanner of origin, form a ring around the sun; but, of course, theexplosion of a great independent body, not originally revolving abouta superior center of gravitational force, would not result in theformation of a ring of small bodies, but rather of a dispersed mass ofthem. But back of any speculation of this kind lies the problem, atpresent insoluble: How could the explosion be produced? (See thequestion of explosions in Chapters 6 and 14). 

Then, on the other hand, we have the observation of Herschel, sinceabundantly confirmed, that space is unusually vacant in the immediateneighborhood of condensed star-clusters and nebulæ, which, as far asit goes, might be taken as an indication that the assembled stars hadbeen drawn together by their mutual attractions, and that the tendencyto aggregation is still bringing new members toward the cluster. Butin that case there must have been an original condensation of stars atthat point in space. This could probably have been produced by thecoagulation of a great nebula into stellar nuclei, a process whichseems now to be taking place in the Orion Nebula. 

A yet more remarkable globular star-cluster exists in the southernhemisphere, Omega Centauri. In this case the central condensation ofstars presents an almost uniform blaze of light. Like the Herculescluster, that in Centaurus is surrounded with stars scattered over abroad field and showing an appearance of radial arrangement. In fact, except for its greater richness, Omega Centauri is an exact duplicateof its northern rival. Each appears to an imaginative spectator as averitable ``city of suns. '' Mathematics shrinks from the task ofdisentangling the maze of motions in such an assemblage. It would seemthat the chance of collisions is not to be neglected, and this ideafinds a certain degree of confirmation in the appearance of``temporary stars'' which have more than once blazed out in, or closeby, globular star-clusters. 

This leads up to the notable fact, first established by ProfessorBailey a few years ago, that such clusters are populous with variablestars. Omega Centauri and the Hercules cluster are especiallyremarkable in this respect. The variables found in them are all ofshort period and the changes of light show a noteworthy tendency touniformity. The first thought is that these phenomena must be due tocollisions among the crowded stars, but, if so, the encounters cannotbe between the stars themselves, but probably between stars and meteorswarms revolving around them. Such periodic collisions might go on forages without the meteors being exhausted by incorporation with thestars. This explanation appears all the more probable because onewould naturally expect that flocks of meteors would abound in a closeaggregation of stars. It is also consistent with Perrine's discovery-- that the globular star clusters are powdered with minute starsstrewn thickly among the brighter ones. 

In speaking of Professor Comstock's extraordinary theory of the MilkyWay, the fact was mentioned that, broadly speaking, the nebulæ areless numerous in the galactic belt than in the comparatively openspaces on either side of it, but that they are, nevertheless, abundantin the broader half of the Milky Way which he designates as the frontof the gigantic ``plough'' supposed to be forcing its way through theenveloping chaos. In and around the Sagittarius region theintermingling of nebulæ and galactic star clouds and clusters isparticularly remarkable. That there is a causal connection nothoughtful person can doubt. We are unable to get away from theevidence that a nebula is like a seed-ground from which stars springforth; or we may say that nebulæ resemble clouds in whose bosomraindrops are forming. The wonderful aspect of the admixtures ofnebulæ and star-clusters in Sagittarius has been described in Chapter1. We now come to a still more extraordinary phenomenon of this kind-- the Pleiades nebulæ. 

The group of the Pleiades, although lying outside the main course ofthe Galaxy, is connected with it by a faint loop, and is the scene ofthe most remarkable association of stars and nebulous matter known inthe visible universe. The naked eye is unaware of the existence ofnebulæ in the Pleiades, or, at the best, merely suspects that there issomething of the kind there; and even the most powerful telescopes arefar from revealing the full wonder of the spectacle; but inphotographs which have been exposed for many hours consecutively, inorder to accumulate the impression of the actinic rays, the revelationis stunning. The principle stars are seen surrounded by, and, as itwere, drowned in, dense nebulous clouds of an unparalleled kind. Theforms assumed by these clouds seem at first sight inexplicable. Theylook like fleeces, or perhaps more like splashes and daubs of luminouspaint dashed carelessly from a brush. But closer inspection shows thatthey are, to a large extent, woven out of innumerable threads of filmytexture, and there are many indications of spiral tendencies. Each ofthe bright stars of the group -- Alcyone, Merope, Maia, Electra, Taygeta, Atlas -- is the focus of a dense fog (totally invisible, remember, alike to the naked eye and to the telescope), and theseparticular stars are veiled from sight behind the strange mists. Running in all directions across the relatively open spaces arenebulous wisps and streaks of the most curious forms. On some of thenebular lines, which are either straight throughout, or if they changedirection do so at an angle, little stars are strung like beads. Inone case seven or eight stars are thus aligned, and, as if toemphasize their dependence upon the chain which connects them, when itmakes a slight bend the file of stars turns the same way. Many otherstar rows in the group suggest by their arrangement that they, too, were once strung upon similar threads which have now disappeared, leaving the stars spaced along their ancient tracks. We seem forced tothe conclusion that there was a time when the Pleiades were embeddedin a vast nebula resembling that of Orion, and that the cloud has nowbecome so rare by gradual condensation into stars that the meresttrace of it remains, and this would probably have escaped detectionbut for the remarkable actinic power of the radiant matter of which itconsists. The richness of many of these faint nebulous masses inultra-violet radiations, which are those that specifically affect thephotographic plate, is the cause of the marvelous revelatory power ofcelestial photography. So the veritable unseen universe, asdistinguished from the ``unseen universe'' of metaphysicalspeculation, is shown to us. 

A different kind of association between stars and nebulæ is shown insome surprising photographic objects in the constellation Cygnus, where long, wispy nebulæ, billions of miles in length, some of themlooking like tresses streaming in a breeze, lie amid fields of starswhich seem related to them. But the relation is of a most singularkind, for notwithstanding the delicate structure of the long nebulæthey appear to act as barriers, causing the stars to heap themselveson one side. The stars are two, three, or four times as numerous onone side of the nebulæ as on the other. These nebulæ, as far asappearance goes, might be likened to rail fences, or thin hedges, against which the wind is driving drifts of powdery snow, which, whilescattered plentifully all around, tends to bank itself on the leewardside of the obstruction. The imagination is at a loss to account forthese extraordinary phenomena; yet there they are, faithfully givingus their images whenever the photographic plate is exposed to theirradiations. 

Thus the more we see of the universe with improved methods ofobservation, and the more we invent aids to human senses, eachenabling us to penetrate a little deeper into the unseen, the greaterbecomes the mystery. The telescope carried us far, photography iscarrying us still farther; but what as yet unimagined instrument willtake us to the bottom, the top, and the end? And then, what hithertountried power of thought will enable us to comprehend the meaning ofit all?

Stellar Migrations

To the untrained eye the stars and the planets are notdistinguishable. It is customary to call them all alike ``stars. '' Butsince the planets more or less rapidly change their places in the sky, in consequence of their revolution about the sun, while the starsproper seem to remain always in the same relative positions, thelatter are spoken of as ``fixed stars. '' In the beginnings ofastronomy it was not known that the ``fixed stars'' had any motionindependent of their apparent annual revolution with the whole skyabout the earth as a seeming center. Now, however, we know that theterm ``fixed stars'' is paradoxical, for there is not a single reallyfixed object in the whole celestial sphere. The apparent fixity in thepositions of the stars is due to their immense distance, combined withthe shortness of the time during which we are able to observe them. Itis like viewing the plume of smoke issuing from a steamer, hull down, at sea: if one does not continue to watch it for a long time itappears to be motionless, although in reality it may be traveling atgreat speed across the line of sight. Even the planets seem fixed inposition if one watches them for a single night only, and the moredistant ones do not sensibly change their places, except after manynights of observation. Neptune, for instance, moves but little morethan two degrees in the course of an entire year, and in a month itschange of place is only about one-third of the diameter of the fullmoon. 

Yet, fixed as they seem, the stars are actually moving with a speed incomparison with which, in some cases, the planets might almost be saidto stand fast in their tracks. Jupiter's speed in his orbit is abouteight miles per second, Neptune's is less than three and one-halfmiles, and the earth's is about eighteen and one-half miles; whilethere are ``fixed stars'' which move two hundred or three hundredmiles per second. They do not all, however, move with so great avelocity, for some appear to travel no faster than the planets. But inall cases, notwithstanding their real speed, long-continued andexceedingly careful observations are required to demonstrate that theyare moving at all. No more overwhelming impression of the frightfuldepths of space in which the stars are buried can be obtained than byreflecting upon the fact that a star whose actual motion across theline of sight amounts to two hundred miles per second does not changeits apparent place in the sky, in the course of a thousand years, sufficiently to be noticed by the casual observer of the heavens!

There is one vast difference between the motions of the stars andthose of the planets to which attention should be at once called: theplanets, being under the control of a central force emanating fromtheir immediate master, the sun, all move in the same direction and inorbits concentric about the sun; the stars, on the other hand, move inevery conceivable direction and have no apparent center of motion, forall efforts to discover such a center have failed. At one time, whentheology had finally to accept the facts of science, a grandioseconception arose in some pious minds, according to which the Throne ofGod was situated at the exact center of His Creation, and, seatedthere, He watched the magnificent spectacle of the starry systemsobediently revolving around Him. Astronomical discoveries andspeculations seemed for a time to afford some warrant for this view, which was, moreover, an acceptable substitute for the abandonedgeocentric theory in minds that could only conceive of God as asuperhuman artificer, constantly admiring his own work. No longer agothan the middle of the nineteenth century a German astronomer, Maedler, believed that he had actually found the location of thecenter about which the stellar universe revolved. He placed it in thegroup of the Pleiades, and upon his authority an extraordinaryimaginative picture was sometimes drawn of the star Alcyone, thebrightest of the Pleiades, as the very seat of the Almighty. This ideaeven seemed to gain a kind of traditional support from the mysticsignificance, without known historical origin, which has for manyages, and among widely separated peoples, been attached to theremarkable group of which Alcyone is the chief. But since Maedler'stime it has been demonstrated that the Pleiades cannot be the centerof revolution of the universe, and, as already remarked, all attemptsto find or fix such a center have proved abortive. Yet so powerful wasthe hold that the theory took upon the popular imagination, that eventoday astronomers are often asked if Alcyone is not the probable siteof ``Jerusalem the Golden. ''

If there were a discoverable center of predominant gravitative power, to which the motions of all the stars could be referred, those motionswould appear less mysterious, and we should then be able to concludethat the universe was, as a whole, a prototype of the subsidiarysystems of which it is composed. We should look simply to the law ofgravitation for an explanation, and, naturally, the center would beplaced within the opening enclosed by the Milky Way. If it were therethe Milky Way itself should exhibit signs of revolution about it, likea wheel turning upon its hub. No theory of the star motions as a wholecould stand which failed to take account of the Milky Way as the basisof all. But the very form of that divided wreath of stars forbids theassumption of its revolution about a center. Even if it could beconceived as a wheel having no material center it would not have theform which it actually presents. As was shown in Chapter 2, there isabundant evidence of motion in the Milky Way; but it is not motion ofthe system as a whole, but motion affecting its separate parts. Instead of all moving one way, the galactic stars, as far as theirmovements can be inferred, are governed by local influences andconditions. They appear to travel crosswise and in contrarydirections, and perhaps they eddy around foci where great numbers haveassembled; but of a universal revolution involving the entire mass wehave no evidence. 

Most of our knowledge of star motions, called ``proper motions, ''relates to individual stars and to a few groups which happen to be sonear that the effects of their movements are measurable. In some casesthe motion is so rapid (not in appearance, but in reality) that thechief difficulty is to imagine how it can have been imparted, and whatwill eventually become of the ``runaways. '' Without a collision, or aseries of very close approaches to great gravitational centers, a startraveling through space at the rate of two hundred or three hundredmiles per second could not be arrested or turned into an orbit whichwould keep it forever flying within the limits of the visibleuniverse. A famous example of these speeding stars is ``1830Groombridge, '' a star of only the sixth magnitude, and consequentlyjust visible to the naked eye, whose motion across the line of sightis so rapid that it moves upon the face of the sky a distance equal tothe apparent diameter of the moon every 280 years. The distance ofthis star is at least 200, 000, 000, 000, 000 miles, and may be two orthree times greater, so that its actual speed cannot be less than twohundred, and may be as much as four hundred, miles per second. Itcould be turned into a new course by a close approach to a great sun, but it could only be stopped by collision, head-on, with a body ofenormous mass. Barring such accidents it must, as far as we can see, keep on until it has traversed our stellar system, whence in mayescape and pass out into space beyond, to join, perhaps, one of thoseother universes of which we have spoken. Arcturus, one of the greatestsuns in the universe, is also a runaway, whose speed of flight hasbeen estimated all the way from fifty to two hundred miles per second. Arcturus, we have every reason to believe, possesses hundreds of timesthe mass of our sun -- think, then, of the prodigious momentum thatits motion implies! Sirius moves more moderately, its motion acrossthe line of sight amounting to only ten miles per second, but it is atthe same time approaching the sun at about the same speed, its actualvelocity in space being the resultant of the two displacements. 

What has been said about the motion of Sirius brings us to anotheraspect of this subject. The fact is, that in every case of stellarmotion the displacement that we observe represents only a part of theactual movement of the star concerned. There are stars whose motioncarries them straight toward or straight away from the earth, and suchstars, of course, show no cross motion. But the vast majority aretraveling in paths inclined from a perpendicular to our line of sight. Taken as a whole, the stars may be said to be flying about like themolecules in a mass of gas. The discovery of the radial component inthe movements of the stars is due to the spectroscope. If a star isapproaching, its spectral lines are shifted toward the violet end ofthe spectrum by an amount depending upon the velocity of approach; ifit is receding, the lines are correspondingly shifted toward the redend. Spectroscopic observation, then, combined with micrometricmeasurements of the cross motion, enables us to detect the realmovement of the star in space. Sometimes it happens that a star'sradial movement is periodically reversed; first it approaches, andthen it recedes. This indicates that it is revolving around a near-bycompanion, which is often invisible, and superposed upon this motionis that of the two stars concerned, which together may be approachingor receding or traveling across the line of sight. Thus thecomplications involved in the stellar motions are often exceedinglygreat and puzzling. 

Yet another source of complication exists in the movement of our ownstar, the sun. There is no more difficult problem in astronomy thanthat of disentangling the effects of the solar motion from those ofthe motions of the other stars. But the problem, difficult as it is, has been solved, and upon its solution depends our knowledge of thespeed and direction of the movement of the solar system through space, for of course the sun carries its planets with it. One element of thesolution is found in the fact that, as a result of perspective, thestars toward which we are going appear to move apart toward all pointsof the compass, while those behind appear to close up together. Thenthe spectroscopic principle already mentioned is invoked for studyingthe shift of the lines, which is toward the violet in the stars aheadof us and toward the red in those that we are leaving behind. Ofcourse the effects of the independent motions of the stars must becarefully excluded. The result of the studies devoted to this subjectis to show that we are traveling at a speed of twelve to fifteen milesper second in a northerly direction, toward the border of theconstellations Hercules and Lyra. A curious fact is that the morerecent estimates show that the direction is not very much out of astraight line drawn from the sun to the star Vega, one of the mostmagnificent suns in the heavens. But it should not be inferred fromthis that Vega is drawing us on; it is too distant for its gravitationto have such an effect. 

Many unaccustomed thoughts are suggested by this mighty voyage of thesolar system. Whence have we come, and whither do we go? Every year ofour lives we advance at least 375, 000, 000 miles. Since the traditionaltime of Adam the sun has led his planets through the wastes of spaceno less than 225, 000, 000, 000 miles, or more than 2400 times thedistance that separates him from the earth. Go back in imagination tothe geologic ages, and try to comprehend the distance over which theearth has flown. Where was our little planet when it emerged out ofthe clouds of chaos? Where was the sun when his ``thunder march''began? What strange constellations shone down upon our globe when itsmasters of life were the monstrous beasts of the ``Age of Reptiles''?A million years is not much of a span of time in geologic reckoning, yet a million years ago the earth was farther from its present placein space than any of the stars with a measurable parallax are now. Itwas more than seven times as far as Sirius, nearly fourteen times asfar as Alpha Centauri, three times as far as Vega, and twice as far asArcturus. But some geologists demand two hundred, three hundred, evenone thousand million years to enable them to account for theevolutionary development of the earth and its inhabitants. In athousand million years the earth would have traveled farther than fromthe remotest conceivable depths of the Milky Way!

Other curious reflections arise when we think of the form of theearth's track as it follows the lead of the sun, in a journey whichhas neither known beginning nor conceivable end. There are probablymany minds which have found a kind of consolation in the thought thatevery year the globe returns to the same place, on the same side ofthe sun. This idea may have an occult connection with our traditionalregard for anniversaries. When that period of the year returns atwhich any great event in our lives has occurred we have the feelingthat the earth, in its annual round, has, in a manner, brought us backto the scene of that event. We think of the earth's orbit as awell-worn path which we traverse many times in the course of alifetime. It seems familiar to us, and we grow to have a sort ofattachment to it. The sun we are accustomed to regard as a fixedcenter in space, like the mill or pump around which the harnessedpatient mule makes his endless circuits. But the real fact is that theearth never returns to the place in space where it has once quitted. In consequence of the motion of the sun carrying the earth and theother planets along, the track pursued by our globe is a vast spiralin space continually developing and never returning upon its course. It is probable that the tracks of the sun and the others stars arealso irregular, and possibly spiral, although, as far as can be atpresent determined, they appear to be practically straight. Everystar, wherever it may be situated, is attracted by its fellow-starsfrom many sides at once, and although the force is minimized bydistance, yet in the course of many ages its effects must becomemanifest. 

Looked at from another side, is there not something immenselystimulating and pleasing to the imagination in the idea of sostupendous a journey, which makes all of us the greatest of travelers?In the course of a long life a man is transported through space thirtythousand million miles; Halley's Comet does not travel one-quarter asfar in making one of its immense circuits. And there are adventures onthis voyage of which we are just beginning to learn to take account. Space is full of strange things, and the earth must encounter some ofthem as it advances through the unknown. Many singular speculationshave been indulged in by astronomers concerning the possible effectsupon the earth of the varying state of the space that it traverses. Even the alternation of hot and glacial periods has sometimes beenascribed to this source. When tropical life flourished around thepoles, as the remains in the rocks assure us, the needed hightemperature may, it has been thought, have been derived from thepresence of the earth in a warm region of space. Then, too, there is acertain interest for us in the thought of what our familiar planet haspassed through. We cannot but admire it for its long journeying as weadmire the traveler who comes to us from remote and unexplored lands, or as we gaze with a glow of interest upon the first locomotive thathas crossed a continent, or a ship that has visited the Arctic orAntarctic regions. If we may trust the indications of the presentcourse, the earth, piloted by the sun, has come from the Milky Way inthe far south and may eventually rejoin that mighty band of stars inthe far north. 

While the stars in general appear to travel independently of oneanother, except when they are combined in binary or trinary systems, there are notable exceptions to this rule. In some quarters of the skywe behold veritable migrations of entire groups of stars whose membersare too widely separated to show any indications of revolution about acommon center of gravity. This leads us back again to the wonderfulgroup of the Pleiades. All of the principle stars composing that groupare traveling in virtually parallel lines. Whatever force set themgoing evidently acted upon all alike. This might be explained by theassumption that when the original projective force acted upon themthey were more closely united than they are at present, and that indrifting apart they have not lost the impulse of the primal motion. Orit may be supposed that they are carried along by some current inspace, although it would be exceedingly difficult, in the presentstate of our knowledge, to explain the nature of such a current. Yetthe theory of a current has been proposed. As to an attractive centeraround which they might revolve, none has been found. Another instanceof similar ``star-drift'' is furnished by five of the seven starsconstituting the figure of the ``Great Dipper. '' In this case thestars concerned are separated very widely, the two extreme ones by notless than fifteen degrees, so that the idea of a common motion wouldnever have been suggested by their aspect in the sky; and the casebecomes the more remarkable from the fact that among and between themthere are other stars, some of the same magnitude, which do not sharetheir motion, but are traveling in other directions. Still otherexamples of the same phenomenon are found in other parts of the sky. Of course, in the case of compact star-clusters, it is assumed thatall the members share a like motion of translation through space, andthe same is probably true of dense star-swarms and star-clouds. 

The whole question of star-drift has lately assumed a new phase, inconsequence of the investigations of Kapteyn, Dyson, and Eddington onthe ``systematic motions of the stars. '' This research will, it ishoped, lead to an understanding of the general law governing themovements of the whole body of stars constituting the visibleuniverse. Taking about eleven hundred stars whose proper motions havebeen ascertained with an approach to certainty, and which aredistributed in all parts of the sky, it has been shown that thereexists an apparent double drift, in two independent streams, moving indifferent and nearly opposed directions. The apex of the motion ofwhat is called ``Stream I'' is situated, according to ProfessorKapteyn, in right ascension 85°, declination south 11°, which placesit just south of the constellation Orion; while the apex of ``StreamII'' is in right ascension 260°, declination south 48°, placing it inthe constellation Ara, south of Scorpio. The two apices differ verynearly 180° in right ascension and about 120° in declination. Thediscovery of these vast star-streams, if they really exist, is one ofthe most extraordinary in modern astronomy. It offers the correlationof stellar movements needed as the basis of a theory of thosemovements, but it seems far from revealing a physical cause for them. As projected against the celestial sphere the stars forming the twoopposite streams appear intermingled, some obeying one tendency andsome the other. As Professor Dyson has said, the hypothesis of thisdouble movement is of a revolutionary character, and calls for furtherinvestigation. Indeed, it seems at first glance not less surprisingthan would be the observation that in a snow-storm the flakes over ourheads were divided into two parties and driving across each other'scourse in nearly opposite directions, as if urged by interpenetratingwinds. 

But whatever explanation may eventually be found for the motions ofthe stars, the knowledge of the existence of those motions must alwaysafford a new charm to the contemplative observer of the heavens, forthey impart a sense of life to the starry system that would otherwisebe lacking. A stagnant universe, with every star fixed immovably inits place, would not content the imagination or satisfy our longingfor ceaseless activity. The majestic grandeur of the evolutions of thecelestial hosts, the inconceivable vastness of the fields of space inwhich they are executed, the countless numbers, the immeasurabledistances, the involved convolutions, the flocking and the scattering, the interpenetrating marches and countermarches, the strange communityof impulsion affecting stars that are wide apart in space and causingthem to traverse the general movement about them like aides anddespatch-bearers on a battle-field -- all these arouse an intensity ofinterest which is heightened by the mystery behind them. 

The Passing of the Constellations

From a historical and picturesque point of view, one of the moststriking results of the motions of the stars described in the lastchapter is their effect upon the forms of the constellations, whichhave been watched and admired by mankind from a period so early thatthe date of their invention is now unknown. The constellations areformed by chance combinations of conspicuous stars, like figures in akaleidoscope, and if our lives were commensurate with the æons ofcosmic existence we should perceive that the kaleidoscope of theheavens was ceaselessly turning and throwing the stars into newsymmetries. Even if the stars stood fast, the motion of the solarsystem would gradually alter the configurations, as the elements of alandscape dissolve and recombine in fresh groupings with thetraveler's progress amid them. But with the stars themselves all inmotion at various speeds and in many directions, the changes occurmore rapidly. Of course, ``rapid'' is here understood in a relativesense; the wheel of human history to an eye accustomed to the majesticprogression of the universe would appear to revolve with the velocityof a whirling dynamo. Only the deliberation of geological movementscan be contrasted with the evolution and devolution of theconstellations. 

And yet this secular fluctuation of the constellation figures is notwithout keen interest for the meditative observer. It is anotherreminder of the swift mutability of terrestial affairs. To the passingglance, which is all that we can bestow upon these figures, theyappear so immutable that they have been called into service to formthe most lasting records of ancient thought and imagination that wepossess. In the forms of the constellations, the most beautiful, and, in imaginative quality, the finest, mythology that the world has everknown has been perpetuated. Yet, in a broad sense, this scroll ofhuman thought imprinted on the heavens is as evanescent as the summerclouds. Although more enduring than parchment, tombs, pyramids, andtemples, it is as far as they from truly eternizing the memory of whatman has fancied and done. 

Before studying the effects that the motions of the stars have had andwill have upon the constellations, it is worth while to consider alittle further the importance of the stellar pictures as archives ofhistory. To emphasize the importance of these effects it is onlynecessary to recall that the constellations register the oldesttraditions of our race. In the history of primeval religions they arethe most valuable of documents. Leaving out of account for the momentthe more familiar mythology of the Greeks, based on something olderyet, we may refer for illustration to that of the mysterious Maya raceof America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a groupof ruins perched, after the Mexican and Central-American plan, on thesummits of pyramidal mounds which mark the site of an ancienttheogonic center of the Mayas. Here the temples all evidently refer toa cult based upon the constellations as symbols. The figures and thenames, of course, were not the same as those that we have derived fromour Aryan ancestors, but the star groups were the same or nearly so. For instance, the loftiest of the temples at Izamal was connected withthe sign of the constellation known to us as Cancer, marking the placeof the sun at the summer solstice, at which period the sun wassupposed to descend at noon like a great bird of fire and consume theofferings left upon the altar. Our Scorpio was known to the Mayas as asign of the ``Death God. '' Our Libra, the ``Balance, '' with which theidea of a divine weighing out of justice has always been connected, seems to be identical with the Mayan constellation Teoyaotlatohua, with which was associated a temple where dwelt the priests whosespecial business it was to administer justice and to foretell thefuture by means of information obtained from the spirits of the dead. Orion, the ``Hunter'' of our celestial mythology, was among the Mayasa ``Warrior, '' while Sagittarius and others of our constellations wereknown to them (under different names, of course), and all were endowedwith a religious symbolism. And the same star figures, having the samesignificance, were familiar to the Peruvians, as shown by the templesat Cuzco. Thus the imagination of ancient America sought in theconstellations symbols of the unchanging gods. 

But, in fact, there is no nation and no people that has not recognizedthe constellations, and at one period or another in its historyemployed them in some symbolic or representative capacity. As handledby the Greeks from prehistoric times, the constellation myths becamethe very soul of poetry. The imagination of that wonderful raceidealized the principal star groups so effectively that the figuresand traditions thus attached to them have, for civilized mankind, displaced all others, just as Greek art in its highest forms standswithout parallel and eclipses every rival. The Romans translated noheroes and heroines of the mythical period of their history to thesky, and the deified Cæsars never entered that lofty company, but theheavens are filled with the early myths of the Greeks. Heraklesnightly resumes his mighty labors in the stars; Zeus, in the form ofthe white ``Bull, '' Taurus, bears the fair Europa on his back throughthe celestial waves; Andromeda stretches forth her shackled arms inthe star-gemmed ether, beseeching aid; and Perseus, in a blaze ofdiamond armor, revives his heroic deeds amid sparkling clouds ofstellar dust. There, too, sits Queen Cassiopeia in her dazzling chair, while the Great King, Cepheus, towers gigantic over the pole. Professor Young has significantly remarked that a great number of theconstellations are connected in some way or other with the ArgonauticExpedition -- that strangely fascinating legend of earliest Greekstory which has never lost its charm for mankind. In view of all this, we may well congratulate ourselves that the constellations willoutlast our time and the time of countless generations to follow us;and yet they are very far from being eternal. Let us now study some ofthe effects of the stellar motions upon them. 

We begin with the familiar figure of the ``Great Dipper. '' He who hasnot drunk inspiration from its celestial bowl is not yet admitted tothe circle of Olympus. This figure is made up of seven conspicuousstars in the constellation Ursa Major, the ``Greater Bear. '' Thehandle of the ``Dipper'' corresponds to the tail of the imaginary``Bear, '' and the bowl lies upon his flank. In fact, the figure of adipper is so evident and that of a bear so unevident, that to mostpersons the ``Great Dipper'' is the only part of the constellationthat is recognizable. Of the seven stars mentioned, six are of nearlyequal brightness, ranking as of the second magnitude, while theseventh is of only the third magnitude. The difference is verystriking, since every increase of one magnitude involves an increaseof two-and-a-half times in brightness. There appears to be littledoubt that the faint star, which is situated at the junction of thebowl and the handle, is a variable of long period, since three hundredyears ago it was as bright as its companions. But however that may be, its relative faintness at the present time interferes but little withthe perfection of the ``Dipper's'' figure. In order the more readilyto understand the changes which are taking place, it will be well tomention both the names and the Greek letters which are attached to theseven stars. Beginning at the star in the upper outer edge of the rimof the bowl and running in regular order round the bottom and then outto the end of the handle, the names and letters are as follows: Dubhe({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}), Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrezis the faint star already mentioned at the junction of the bowl andhandle, and Mizar, in the middle of the handle, has a close, naked-eyecompanion which is named Alcor. The Arabs called this singular pair ofstars ``The Horse and Rider. '' Merak and Duhbe are called ``ThePointers, '' because an imaginary line drawn northward through themindicates the Pole Star. 

Now it has been found that five of these stars -- viz. , Merak, Phaed, Megrez, Alioth, and Mizar (with its comrade) -- are moving withpractically the same speed in an easterly direction, while the othertwo, Dubhe and Benetnasch, are simultaneously moving westward, themotions of Benetnasch being apparently more rapid. The consequence ofthese opposed motions is, of course, that the figure of the ``Dipper''cannot always have existed and will not continue to exist. In theaccompanying diagrams it has been thought interesting to show therelative positions of these seven stars, as seen from the point whichthe earth now occupies, both in the past and in the future. Arrowsattached to the stars in the figure representing the presentappearance of the ``Dipper'' indicate the directions of the motionsand the distances over which they will carry the stars in a period ofabout five hundred centuries. The time, no doubt, seems long, butremember the vast stretch of ages through which the earth has passed, and then reflect that no reason is apparent why our globe should notcontinue to be a scene of animation for ten thousand centuries yet tocome. The fact that the little star Alcor placed so close to Mizarshould accompany the latter in its flight is not surprising, but thattwo of the principal stars of the group should be found moving in adirection directly opposed to that pursued by the other five issurprising in the highest degree; and it recalls the strange theory ofa double drift affecting all the stars, to which attention was calledin the preceding chapter. It would appear that Benetnasch and Dubhebelong to one ``current, '' and Merak, Phaed, Megrez, Alioth, and Mizarto the other. As far as is known, the motion of the seven stars arenot shared by the smaller stars scattered about them, but on thetheory of currents there should be such a community of motion, andfurther investigation may reveal it. 

From the ``Great Dipper'' we turn to a constellation hardly lessconspicuous and situated at an equal distance from the pole on theother side -- Cassiopeia. This famous star-group commemorating theromantic Queen of Ethiopia whose vain boasting of her beauty waspunished by the exposure of her daughter Andromeda to the ``SeaMonster, '' is well-marked by five stars which form an irregular letter``W'' with its open side toward the pole. Three of these stars areusually ranked as of the second magnitude, and two of the third; butto ordinary observation they appear of nearly equal brightness, andpresent a very striking picture. They mark out the chair and a part ofthe figure of the beautiful queen. Beginning at the right-hand, orwestern, end of the ``W, '' their Greek letter designations are: Beta({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), andEpsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilonare traveling eastwardly at various speeds, while the fifth, Gamma, moves in a westerly direction. The motion of Beta is more rapid thanthat of any of the others. It should be said, however, that no littleuncertainty attaches to the estimates of the rate of motion of starswhich are not going very rapidly, and different observers often varyconsiderably in their results. 

In the beautiful ``Northern Crown, '' one of the most perfect andcharming of all the figures to be found in the stars, the alternatecombining and scattering effects of the stellar motions are shown bycomparing the appearance which the constellation must have had fivehundred centuries ago with that which it has at present and that whichit will have in the future. The seven principle stars of the asterism, forming a surprisingly perfect coronet, have movements in threedirections at right angles to one another. That in these circumstancesthey should ever have arrived at positions giving them so striking anappearance of definite association is certainly surprising; from itsaspect one would have expected to find a community of movementgoverning the brilliants of the ``Crown, '' but instead of that we findevidence that they will inevitably drift apart and the beautifulfigure will dissolve. 

A similar fate awaits such asterisms as the ``Northern Cross'' inCygnus; the ``Crow'' (Corvus), which stands on the back of the great``Sea Serpent, '' Hydra, and pecks at his scales; ``Job's Coffin''(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini);the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyadesin Taurus. In the case of the Hyades, two controlling movements aremanifest: one, affecting five of the stars which form the well-knownfigure of a letter ``V, '' is directed northerly; the other, whichcontrols the direction of two stars, has an easterly trend. The chiefstar of the group, Aldebaran, one of the finest of all stars both forits brilliance and its color, is the most affected by the easterlymotion. In time it will drift entirely out of connection with itspresent neighbors. Although the Hyades do not form so compact a groupas the Pleiades in the same constellation, yet their appearance ofrelationship is sufficient to awaken a feeling of surprise over thefact that, as with the stars of the ``Dipper, '' their association isonly temporary or apparent. 

The great figure of Orion appears to be more lasting, not because itsstars are physically connected, but because of their great distance, which renders their movements too deliberate to be exactlyascertained. Two of the greatest of its stars, Betelgeuse and Rigel, possess, as far as has been ascertained, no perceptible motion acrossthe line of sight, but there is a little movement perceptible in the``Belt. '' At the present time this consists of an almost perfectstraight line, a row of second-magnitude stars about equally spacedand of the most striking beauty. In the course of time, however, thetwo right-hand stars, Mintaka and Alnilam (how fine are these Arabicstar names!) will approach each other and form a naked-eye double, butthe third, Alnita, will drift away eastward, so that the ``Belt'' willno longer exist. 

For one more example, let us go to the southern hemisphere, whose mostcelebrated constellation, the ``Southern Cross, '' has found a place inall modern literatures, although it has no claim to consideration onaccount of association with ancient legends. This most attractiveasterism, which has never ceased to fascinate the imagination ofChristendom since it was first devoutly described by the earlyexplorers of the South, is but a passing collocation of brilliantstars. Yet even in its transfigurations it has been for hundreds ofcenturies, and will continue to be for hundreds of centuries to come, a most striking object in the sky. Our figures show its appearance inthree successive phases: first, as it was fifty thousand years ago(viewed from the earth's present location); second, as it is in ourday; and, third, as it will be an equal time in the future. Thenearness of these bright stars to one another -- the length of thelonger beam of the ``Cross'' is only six degrees -- makes this groupvery noticeable, whatever the arrangement of its components may be. The largest star, at the base of the ``Cross, '' is of the firstmagnitude, two of the others are of the second magnitude, and thefourth is of the third. Other stars, not represented in the figures, increase the effect of a celestial blazonry, although they do not helpthe resemblance to a cross. 

But since the motion of the solar system itself will, in the course ofso long a period as fifty thousand years, produce a great change inthe perspective of the heavens as seen from the earth, by carrying usnearly nineteen trillion miles from our present place, why, it may beasked, seek to represent future appearances of the constellationswhich we could not hope to see, even if we could survive so long? Theanswer is: Because these things aid the mind to form a picture of theeffects of the mobility of the starry universe. Only by showing thechanges from some definite point of view can we arrive at a duecomprehension of them. The constellations are more or less familiar toeverybody, so that impending changes of their forms must at oncestrike the eye and the imagination, and make clearer the significanceof the movements of the stars. If the future history of mankind is toresemble its past and if our race is destined to survive yet a millionyears, then our remote descendents will see a ``new heavens'' if not a``new earth, '' and will have to invent novel constellations toperpetuate their legends and mythologies. 

If our knowledge of the relative distances of the stars were morecomplete, it would be an interesting exercise in celestial geometry toproject the constellations probably visible to the inhabitants ofworlds revolving around some of the other suns of space. Our sun istoo insignificant for us to think that he can make a conspicuousappearance among them, except, perhaps, in a few cases. As seen, forinstance, from the nearest known star, Alpha Centauri, the sun wouldappear of the average first magnitude, and consequently from thatstandpoint he might be the gem of some little constellation which hadno Sirius, or Arcturus, or Vega to eclipse him with its superiorsplendor. But from the distance of the vast majority of the stars thesun would probably be invisible to the naked eye, and as seen fromnearer systems could only rank as a fifth or sixth magnitude star, unnoticed and unknown except by the star-charting astronomer. 

Conflagrations in the Heavens

Suppose it were possible for the world to take fire and burn up -- assome pessimists think that it will do when the Divine wrath shall havesufficiently accumulated against it -- nobody out of our own littlecorner of space would ever be aware of the catastrophe! With all theirtelescopes, the astronomers living in the golden light of Arcturus orthe diamond blaze of Canopus would be unable to detect the leastglimmer of the conflagration that had destroyed the seat of Adam andhis descendents, just as now they are totally ignorant of itsexistence. 

But at least fifteen times in the course of recorded history menlooking out from the earth have beheld in the remote depths of spacegreat outbursts of fiery light, some of them more splendidly luminousthan anything else in the firmament except the sun! If they wereconflagrations, how many million worlds like ours were required tofeed their blaze?

It is probable that ``temporary'' or ``new'' stars, as these wonderfulapparitions are called, really are conflagrations; not in the sense ofa bonfire or a burning house or city, but in that of a sudden eruptionof inconceivable heat and light, such as would result from thestripping off the shell of an encrusted sun or the crashing togetherof two mighty orbs flying through space with a hundred times thevelocity of the swiftest cannon-shot. 

Temporary stars are the rarest and most erratic of astronomicalphenomena. The earliest records relating to them are not very clear, and we cannot in every instance be certain that it was one of theseappearances that the ignorant and superstitious old chroniclers aretrying to describe. The first temporary star that we are absolutelysure of appeared in 1572, and is known as ``Tycho's Star, '' becausethe celebrated Danish astronomer (whose remains, with hisgold-and-silver artificial nose -- made necessary by a duel -- stillintact, were disinterred and reburied in 1901) was the first toperceive it in the sky, and the most assiduous and successful in hisstudies of it. As the first fully accredited representative of itsclass, this new star made its entry upon the scene with becomingéclat. It is characteristic of these phenomena that they burst intoview with amazing suddenness, and, of course, entirely unexpectedly. Tycho's star appeared in the constellation Cassiopeia, near a nowwell-known and much-watched little star named Kappa, on the evening ofNovember 11, 1572. The story has often been repeated, but it neverloses interest, how Tycho, going home that evening, saw people in thestreet pointing and staring at the sky directly over their heads, andfollowing the direction of their hands and eyes he was astonished tosee, near the zenith, an unknown star of surpassing brilliance. Itoutshone the planet Jupiter, and was therefore far brighter than thefirst magnitude. There was not another star in the heavens that couldbe compared with it in splendor. Tycho was not in all respects freefrom the superstitions of his time -- and who is? -- but he had thetrue scientific instinct, and immediately he began to study thestranger, and to record with the greatest care every change in itsaspect. First he determined as well as he could with the imperfectinstruments of his day, many of which he himself had invented, theprecise location of the phenomena in the sky. Then he followed thechanges that it underwent. At first it brightened until its lightequaled or exceeded that of the planet Venus at her brightest, astatement which will be appreciated at its full value by anyone whohas ever watched Venus when she plays her dazzling rôle of ``EveningStar, '' flaring like an arc light in the sunset sky. It even became sobrilliant as to be visible in full daylight, since, its position beingcircumpolar, it never set in the latitude of Northern Europe. Finallyit began to fade, turning red as it did so, and in March, 1574, itdisappeared from Tycho's searching gaze, and has never been seen againfrom that day to this. None of the astronomers of the time could makeanything of it. They had not yet as many bases of speculation as wepossess today. 

Tycho's star has achieved a romantic reputation by being fancifullyidentified with the ``Star of Bethlehem, '' said to have led thewondering Magi from their eastern deserts to the cradle-manger of theSavior in Palestine. Many attempts have been made to connect thistraditional ``star'' with some known phenomenon of the heavens, andnone seems more idle than this. Yet it persistently survives, and noastronomer is free from eager questions about it addressed by peoplewhose imagination has been excited by the legend. It is only necessaryto say that the supposition of a connection between the phenomenon ofthe Magi and Tycho's star is without any scientific foundation. It wasoriginally based on an unwarranted assumption that the star of Tychowas a variable of long period, appearing once every three hundred andfifteen years, or thereabout. If that were true there would have beenan apparition somewhere near the traditional date of the birth ofChrist, a date which is itself uncertain. But even the data on whichthe assumption was based are inconsistent with the theory. Certainmonkish records speak of something wonderful appearing in the sky inthe years 1264 and 945, and these were taken to have been outbursts ofTycho's star. Investigation shows that the records more probably referto comets, but even if the objects seen were temporary stars, theirdates do not suit the hypothesis; from 945 to 1264 there is a gap of319 years, and from 1264 to 1572 one of only 308 years; moreover 337years have now (1909) elapsed since Tycho saw the last glimmer of hisstar. Upon a variability so irregular and uncertain as that, even ifwe felt sure that it existed, no conclusion could be found concerningan apparition occurring 2000 years ago. 

In the year 1600 (the year in which Giordano Bruno was burned at thestake for teaching that there is more than one physical world), atemporary star of the third magnitude broke out in the constellationCygnus, and curiously enough, considering the rarity of suchphenomena, only four years later another surprisingly brilliant oneappeared in the constellation Ophiuchus. This is often called``Kepler's star, '' because the great German astronomer devoted to itthe same attention that Tycho had given to the earlier phenomenon. It, too, like Tycho's, was at first the brightest object in the stellarheavens, although it seems never to have quite equaled its famouspredecessor in splendor. It disappeared after a year, also turning ofa red color as it became more faint. We shall see the significance ofthis as we go on. Some of Kepler's contemporaries suggested that theoutburst of this star was due to a meeting of atoms in space, and ideabearing a striking resemblance to the modern theory of ``astronomicalcollisions. ''

In 1670, 1848, and 1860 temporary stars made their appearance, butnone of them was of great brilliance. In 1866 one of the secondmagnitude broke forth in the ``Northern Crown'' and awoke muchinterest, because by that time the spectroscope had begun to beemployed in studying the composition of the stars, and Hugginsdemonstrated that the new star consisted largely of incandescenthydrogen. But this star, apparently unlike the others mentioned, wasnot absolutely new. Before its outburst it had shown as a star of theninth magnitude (entirely invisible, of course, to the naked eye), andafter about six weeks it faded to its original condition in which ithas ever since remained. In 1876 a temporary star appeared in theconstellation Cygnus, and attained at one time the brightness of thesecond magnitude. Its spectrum and its behavior resembled those of itsimmediate predecessor. In 1885, astronomers were surprised to see asixth-magnitude star glimmering in the midst of the hazy cloud of thegreat Andromeda Nebula. It soon absolutely disappeared. Its spectrumwas remarkable for being ``continuous, '' like that of the nebulaitself. A continuous spectrum is supposed to represent a body, or amass, which is either solid or liquid, or composed of gas under greatpressure. In January, 1892, a new star was suddenly seen in theconstellation Auriga. It never rose much above the fourth magnitude, but it showed a peculiar spectrum containing both bright and darklines of hydrogen. 

But a bewildering surprise was now in store; the world was to beholdat the opening of the twentieth century such a celestial spectacle ashad not been on view since the times of Tycho and Kepler. Beforedaylight on the morning of February 22, 1901, the Rev. DoctorAnderson, of Edinburgh, an amateur astronomer, who had also been thefirst to see the new star in Auriga, beheld a strange object in theconstellation Perseus not far from the celebrated variable star Algol. He recognized its character at once, and immediately telegraphed thenews, which awoke the startled attention of astronomers all over theworld. When first seen the new star was no brighter than Algol (lessthan the second magnitude), but within twenty-four hours it wasablaze, outshining even the brilliant Capella, and far surpassing thefirst magnitude. At the spot in the sky where it appeared nothingwhatever was visible on the night before its coming. This is knownwith certainty because a photograph had been made of that very regionon February 21, and this photograph showed everything down to thetwelfth magnitude, but not a trace of the stranger which burst intoview between the 21st and the 22nd like the explosion of a rocket. 

Upon one who knew the stars the apparition of this intruder in awell-known constellation had the effect of a sudden invasion. The newstar was not far west of the zenith in the early evening, and in thatposition showed to the best advantage. To see Capella, the hithertounchallenged ruler of that quarter of the sky, abased by comparisonwith this stranger of alien aspect, for there was always an unfamiliarlook about the ``nova, '' was decidedly disconcerting. It seemed toportend the beginning of a revolution in the heavens. One couldunderstand what the effect of such an apparition must have been in thesuperstitious times of Tycho. The star of Tycho had burst forth on thenorthern border of the Milky Way; this one was on its southern border, some forty-five degrees farther east. 

Astronomers were well-prepared this time for the scientific study ofthe new star, both astronomical photography and spectroscopy havingbeen perfected, and the results of their investigations werecalculated to increase the wonder with which the phenomenon wasregarded. The star remained at its brightest only a few days; then, like a veritable conflagration, it began to languish; and, like thereflection of a dying fire, as it sank it began to glow with the redcolor of embers. But its changes were spasmodic; once about everythree days it flared up only to die away again. During thesefluctuations its light varied alternately in the ratio of one to six. Finally it took a permanent downward course, and after a few monthsthe naked eye could no longer perceive it; but it remained visiblewith telescopes, gradually fading until it had sunk to the ninthmagnitude. Then another astonishing change happened: in Augustphotographs taken at the Yerkes Observatory and at Heidelberg showedthat the ``nova'' was surrounded by a spiral nebula! The nebula hadnot been there before, and no one could doubt that it represented aphase of the same catastrophe that had produced the outburst of thenew star. At one time the star seemed virtually to have disappeared, as if all its substance had been expanded into the nebulous cloud, butalways there remained a stellar nucleus about which the misty spiralspread wider and ever wider, like a wave expanding around a center ofdisturbance. The nebula too showed a variability of brightness, andfour condensations which formed in it seemed to have a motion ofrevolution about the star. As time went on the nebula continued toexpand at a rate which was computed to be not less than twentythousand miles per second! And now the star itself, showingindications of having turned into a nebula, behaved in a most erraticmanner, giving rise to the suspicion that it was about to burst outagain. But this did not occur, and at length it sunk into a state oflethargy from which it has to the present time not recovered. But thenebulous spiral has disappeared, and the entire phenomena as it now(1909) exists consists of a faint nebulous star of less than the ninthmagnitude. 

The wonderful transformations just described had been forecast inadvance of the discovery of the nebulous spiral encircling the star bythe spectroscopic study of the latter. At first there was nosuggestion of a nebular constitution, but within a month or twocharacteristic nebular lines began to appear, and in less than sixmonths the whole spectrum had been transformed to the nebular type. Inthe mean time the shifting of the spectral lines indicated acomplication of rapid motions in several directions simultaneously. These motions were estimated to amount to from one hundred to fivehundred miles per second. 

The human mind is so constituted that it feels forced to seek anexplanation of so marvelous a phenomenon as this, even in the absenceof the data needed for a sound conclusion. The most naturalhypothesis, perhaps, is that of a collision. Such a catastrophe couldcertainly happen. It has been shown, for instance, that in infinity oftime the earth is sure to be hit by a comet; in the same way it may beasserted that, if no time limit is fixed, the sun is certain to runagainst some obstacle in space, either another star, or a dense meteorswarm, or one of the dark bodies which there is every reason tobelieve abound around us. The consequences of such a collision areeasy to foretell, provided that we know the masses and the velocitiesof the colliding bodies. In a preceding chapter we have discussed themotions of the sun and stars, and have seen that they are so swiftthat an encounter between any two of them could not but be disastrous. But this is not all; for as soon as two stars approached within a fewmillion miles their speed would be enormously increased by theirreciprocal attractions and, if their motion was directed radially withrespect to their centers, they would come together with a crash thatwould reduce them both to nebulous clouds. It is true that the chancesof such a ``head-on'' collision are relatively very small; two starsapproaching each other would most probably fall into closed orbitsaround their common center of gravity. If there were a collision itwould most likely be a grazing one instead of a direct front-to-frontencounter. But even a close approach, without any actual collision, would probably prove disastrous, owing to the tidal influence of eachof the bodies on the other. Suns, in consequence of their enormousmasses and dimensions and the peculiarities of their constitution, areexceedingly dangerous to one another at close quarters. Propinquityawakes in them a mutually destructive tendency. Consisting of matterin the gaseous, or perhaps, in some cases, liquid, state, their tidalpull upon each other if brought close together might burst themasunder, and the photospheric envelope being destroyed the internalincandescent mass would gush out, bringing fiery death to any planetsthat were revolving near. Without regard to the resulting disturbanceof the earth's orbit, the close approach of a great star to the sunwould be in the highest degree perilous to us. But this is a dangerwhich may properly be regarded as indefinitely remote, since, at ourpresent location in space, we are certainly far from every star exceptthe sun, and we may feel confident that no great invisible body isnear, for if there were one we should be aware of its presence fromthe effects of its attraction. As to dark nebulæ which may possiblylie in the track that the solar system is pursuing at the rate of375, 000, 000 miles per year, that is another question -- and they, too, could be dangerous!

This brings us directly back to ``Nova Persei, '' for among the manysuggestions offered to explain its outburst, as well as those of othertemporary stars, one of the most fruitful is that of a collisionbetween a star and a vast invisible nebula. Professor Seeliger, ofMunich, first proposed this theory, but it afterward underwent somemodifications from others. Stated in a general form, the idea is thata huge dark body, perhaps an extinguished sun, encountered in itsprogress through space a widespread flock of small meteors forming adark nebula. As it plunged into the swarm the friction of theinnumerable collisions with the meteors heated its surface toincandescence, and being of vast size it then became visible to us asa new star. Meanwhile the motion of the body through the nebula, andits rotation upon itself, set up a gyration in the blazing atmosphereformed around it by the vaporized meteors; and as this atmospherespread wider, under the laws of gyratory motion a rotation in theopposite direction began in the inflamed meteoric cloud outside thecentral part of the vortex. Thus the spectral lines were caused toshow motion in opposite directions, a part of the incandescent massapproaching the earth simultaneously with the retreat of another part. So the curious spectroscopic observations before mentioned wereexplained. This theory might also account for the appearance of thenebulous spiral first seen some six months after the originaloutburst. The sequent changes in the spectrum of the ``nova'' areaccounted for by this theory on the assumption, reasonable enough initself, that at first the invading body would be enveloped in avaporized atmosphere of relatively slight depth, producing by itsabsorption the fine dark lines first observed; but that as time wenton and the incessant collisions continued, the blazing atmospherewould become very deep and extensive, whereupon the appearance of thespectral lines would change, and bright lines due to the light of theincandescent meteors surrounding the nucleus at a great distance wouldtake the place of the original dark ones. The vortex of meteors onceformed would protect the flying body within from further immediatecollisions, the latter now occurring mainly among the meteorsthemselves, and then the central blaze would die down, and theoriginal splendor of the phenomenon would fade. 

But the theories about Nova Persei have been almost as numerous as theastronomers who have speculated about it. One of the most startling ofthem assumed that the outburst was caused by the running amuck of adark star which had encountered another star surrounded with planets, the renewed outbreaks of light after the principal one had faded beingdue to the successive running down of the unfortunate planets! Yetanother hypothesis is based on what we have already said of the tidalinfluence that two close approaching suns would have upon each other. Supposing two such bodies which had become encrusted, but remainedincandescent and fluid within, to approach within almost strikingdistance; they would whirl each other about their common center ofgravity, and at the same time their shells would burst under the tidalstrain, and their glowing nuclei being disclosed would produce a greatoutburst of light. Applying this theory to a ``nova, '' like that of1866 in the ``Northern Crown, '' which had been visible as a small starbefore the outbreak, and which afterward resumed its former aspect, weshould have to assume that a yet shining sun had been approached by adark body whose attraction temporarily burst open its photosphere. Itmight be supposed that in this case the dark body was too far advancedin cooling to suffer the same fate from the tidal pull of its victim. But a close approach of that kind would be expected to result in theformation of a binary system, with orbits of great eccentricity, perhaps, and after the lapse of a certain time the outburst should berenewed by another approximation of the two bodies. A temporary starof that kind would rather be ranked as a variable. 

The celebrated French astronomer, Janssen, had a different theory ofNova Persei, and of temporary stars in general. According to his idea, such phenomena might be the result of chemical changes taking place ina sun without interference by, or collision with, another body. Janssen was engaged for many years in trying to discover evidence ofthe existence of oxygen in the sun, and he constructed his observatoryon the summit of Mount Blanc specially to pursue that research. Hebelieved that oxygen must surely exist in the sun since we find somany other familiar elements included in the constitution of the solarglobe, and as he was unable to discover satisfactory evidence of itspresence he assumed that it existed in a form unknown on the earth. Ifit were normally in the sun's chromosphere, or coronal atmosphere, hesaid, it would combine with the hydrogen which we know is there andform an obscuring envelope of water vapor. It exists, then, in aspecial state, uncombined with hydrogen; but let the temperature ofthe sun sink to a critical point and the oxygen will assume its normalproperties and combine with the hydrogen, producing a mighty outburstof light and heat. This, Janssen thought, might explain the phenomenaof the temporary stars. It would also, he suggested, account for theirbrief career, because the combination of the elements would be quicklyaccomplished, and then the resulting water vapor would form anatmosphere cutting off the radiation from the star within. 

This theory may be said to have a livelier human interest than some ofthe others, since, according to it, the sun may carry in its veryconstitution a menace to mankind; one does not like to think of itbeing suddenly transformed into a gigantic laboratory for theexplosive combination of oxygen and hydrogen! But while Janssen'stheory might do for some temporary stars, it is inadequate to explainall the phenomena of Nova Persei, and particularly the appearance ofthe great spiral nebula that seemed to exhale from the heart of thestar. Upon the whole, the theory of an encounter between a star and adark nebula seems best to fit the observations. By that hypothesis theexpanding billow of light surrounding the core of the conflagration isvery well accounted for, and the spectroscopic peculiarities are alsoexplained. 

Dr Gustov Le Bon offers a yet more alarming theory, suggesting thattemporary stars are the result of atomic explosion; but we shall touchupon this more fully in Chapter 14. 

Twice in the course of this discussion we have called attention to thechange of color invariably undergone by temporary stars in the laterstages of their career. This was conspicuous with Nova Persei whichglowed more and more redly as it faded, until the nebulous light beganto overpower that of the stellar nucleus. Nothing could be moresuggestive of the dying out of a great fire. Moreover, change of colorfrom white to red is characteristic of all variable stars of longperiod, such as ``Mira'' in Cetus. It is also characteristic of starsbelieved to be in the later stages of evolution, and consequentlyapproaching extinction, like Antares and Betelgeuse, and still morenotably certain small stars which ``gleam like rubies in the field ofthe telescope. '' These last appear to be suns in the closing period ofexistence as self-luminous bodies. Between the white stars, such asSirius and Rigel, and the red stars, such as Aldebaran and AlphaHerculis, there is a progressive series of colors from golden yellowthrough orange to deep red. The change is believed to be due to theincrease of absorbing vapors in the stellar atmosphere as the bodycools down. In the case of ordinary stars these changes no doubtoccupy many millions of years, which represent the average duration ofsolar life; but the temporary stars run through similar changes in afew months: they resemble ephemeral insects -- born in the morning anddoomed to perish with the going down of the sun. 

Explosive and Whirling Nebulæ

One of the most surprising triumphs of celestial photography wasProfessor Keeler's discovery, in 1899, that the great majority of thenebulæ have a distinctly spiral form. This form, previously known inLord Rosse's great ``Whirlpool Nebula, '' had been supposed to beexceptional; now the photographs, far excelling telescopic views inthe revelation of nebular forms, showed the spiral to be the typicalshape. Indeed, it is a question whether all nebulæ are not to someextent spiral. The extreme importance of this discovery is shown inthe effect that it has had upon hitherto prevailing views of solar andplanetary evolution. For more than three-quarters of a centuryLaplace's celebrated hypothesis of the manner of origin of the solarsystem from a rotating and contracting nebula surrounding the sun hadguided speculation on that subject, and had been tentatively extendedto cover the evolution of systems in general. The apparent forms ofsome of the nebulæ which the telescope had revealed were regarded, andby some are still regarded, as giving visual evidence in favor of thistheory. There is a ``ring nebula'' in Lyra with a central star, and a``planetary nebula'' in Gemini bearing no little resemblance to theplanet Saturn with its rings, both of which appear to be practicalrealizations of Laplace's idea, and the elliptical rings surroundingthe central condensation of the Andromeda Nebula may be cited for thesame kind of proof. 

But since Keeler's discovery there has been a decided turning away ofspeculation another way. The form of the spiral nebulæ seems to beentirely inconsistent with the theory of an originally globular ordisk-shaped nebula condensing around a sun and throwing or leaving offrings, to be subsequently shaped into planets. Some astronomers, indeed, now reject Laplace's hypothesis in toto, preferring to thinkthat even our solar system originated from a spiral nebula. Since thespiral type prevails among the existing nebulæ, we must make anymechanical theory of the development of stars and planetary systemsfrom them accord with the requirements which that form imposes. Aglance at the extraordinary variations upon the spiral which ProfessorKeeler's photographs reveal is sufficient to convince one of thedifficulty of the task of basing a general theory upon them. In truth, it is much easier to criticize Laplace's hypothesis than to invent asatisfactory substitute for it. If the spiral nebulæ seem to oppose itthere are other nebulæ which appear to support it, and it may be thatno one fixed theory can account for all the forms of stellar evolutionin the universe. Our particular planetary system may have originatedvery much as the great French mathematician supposed, while othershave undergone, or are now undergoing, a different process ofdevelopment. There is always a too strong tendency to regard animportant new discovery and the theories and speculations based uponit as revolutionizing knowledge, and displacing or overthrowingeverything that went before. Upon the plea that ``Laplace only made aguess'' more recent guesses have been driven to extremes and treatedby injudicious exponents as ``the solid facts at last. ''

Before considering more recent theories than Laplace's, let us seewhat the nature of the photographic revelations is. The vast celestialmaelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may betaken as the leading type of the spiral nebulæ, although there areless conspicuous objects of the kind which, perhaps, better illustratesome of their peculiarities. Lord Rosse's nebula appears far morewonderful in the photographs than in his drawings made with the aid ofhis giant reflecting telescope at Parsonstown, for the photographicplate records details that no telescope is capable of showing. Supposewe look at the photograph of this object as any person of common sensewould look at any great and strange natural phenomenon. What is thefirst thing that strikes the mind? It is certainly the appearance ofviolent whirling motion. One would say that the whole glowing mass hadbeen spun about with tremendous velocity, or that it had been setrotating so rapidly that it had become the victim of ``centrifugalforce, '' one huge fragment having broken loose and started to gyrateoff into space. Closer inspection shows that in addition to theprincipal focus there are various smaller condensations scatteredthrough the mass. These are conspicuous in the spirals. Some of themare stellar points, and but for the significance of their location wemight suppose them to be stars which happen to lie in a line betweenus and the nebula. But when we observe how many of them follow mostfaithfully the curves of the spirals we cannot but conclude that theyform an essential part of the phenomenon; it is not possible tobelieve that their presence in such situations is merely fortuitous. One of the outer spirals has at least a dozen of these star-likepoints strung upon it; some of them sharp, small, and distinct, othersmore blurred and nebulous, suggesting different stages ofcondensation. Even the part which seems to have been flung loose fromthe main mass has, in addition to its central condensation, at leastone stellar point gleaming in the half-vanished spire attached to it. Some of the more distant stars scattered around the ``whirlpool'' lookas if they too had been shot out of the mighty vortex, afterwardcondensing into unmistakable solar bodies. There are at least twocurved rows of minute stars a little beyond the periphery of theluminous whirl which clearly follow lines concentric with those of thenebulous spirals. Such facts are simply dumbfounding for anyone whowill bestow sufficient thought upon them, for these are suns, thoughthey may be small ones; and what a birth is that for a sun!

Look now again at the glowing spirals. We observe that hardly havethey left the central mass before they begin to coagulate. In someplaces they have a ``ropy'' aspect; or they are like peascods filledwith growing seeds, which eventually will become stars. The greatfocus itself shows a similar tendency, especially around itscircumference. The sense that it imparts of a tremendous shatteringforce at work is overwhelming. There is probably more matter in thatwhirling and bursting nebula than would suffice to make a hundredsolar systems! It must be confessed at once that there is noconfirmation of the Laplacean hypothesis here; but what hypothesiswill fit the facts? There is one which it has been claimed does so, but we shall come to that later. In the meanwhile, as a preparation, fix in the memory the appearance of that second spiral mass spinningbeside its master which seems to have spurned it away. 

For a second example of the spiral nebulæ look at the one in theconstellation Triangulum. God, how hath the imagination of puny manfailed to comprehend Thee! Here is creation through destruction with avengeance! The spiral form of the nebula is unmistakable, but it ishalf obliterated amid the turmoil of flying masses hurled away on allsides with tornadic fury. The focus itself is splitting asunder underthe intolerable strain, and in a little while, as time is reckoned inthe Cosmos, it will be gyrating into stars. And then look at thecyclonic rain of already finished stars whirling round the outskirtsof the storm. Observe how scores of them are yet involved in thefading streams of the nebulous spirals; see how they have been throwninto vast loops and curves, of a beauty that half redeems the terrorof the spectacle enclosed within their lines -- like iridescent cirrihovering about the edges of a hurricane. And so again are suns born!

Let us turn to the exquisite spiral in Ursa Major; how different itsaspect from that of the other! One would say that if the terrific coilin Triangulum has all but destroyed itself in its fury, this one onthe contrary has just begun its self-demolition. As one gazes oneseems to see in it the smooth, swift, accelerating motion thatprecedes catastrophe. The central part is still intact, dense, anduniform in texture. How graceful are the spirals that smoothly risefrom its oval rim and, gemmed with little stars, wind off into thedarkness until they have become as delicate as threads of gossamer!But at bottom the story told here is the same -- creation by gyration!

Compare with the above the curious mass in Cetus. Here the plane ofthe whirling nebula nearly coincides with our line of sight and we seethe object at a low angle. It is far advanced and torn to shreds, andif we could look at it perpendicularly to its plane it is evident thatit would closely resemble the spectacle in Triangulum. 

Then take the famous Andromeda Nebula (see Frontispiece), which is sovast that notwithstanding its immense distance even the naked eyeperceives it as an enigmatical wisp in the sky. Its image on thesensitive plate is the masterpiece of astronomical photography; forwild, incomprehensible beauty there is nothing that can be comparedwith it. Here, if anywhere, we look upon the spectacle of creation inone of its earliest stages. The Andromeda Nebula is apparently lessadvanced toward transformation into stellar bodies than is that inTriangulum. The immense crowd of stars sprinkled over it and itsneighborhood seem in the main to lie this side of the nebula, andconsequently to have no connection with it. But incipient stars (insome places clusters of them) are seen in the nebulous rings, whileone or two huge masses seem to give promise of transformation intostellar bodies of unusual magnitude. I say ``rings'' because althoughthe loops encompassing the Andromeda Nebula have been called spiralsby those who wish utterly to demolish Laplace's hypothesis, yet theyare not manifestly such, as can be seen on comparing them with theundoubted spirals of the Lord Rosse Nebula. They look quite as muchlike circles or ellipses seen at an angle of, say, fifteen or twentydegrees to their plane. If they are truly elliptical they accordfairly well with Laplace's idea, except that the scale of magnitude isstupendous, and if the Andromeda Nebula is to become a solar system itwill surpass ours in grandeur beyond all possibility of comparison. 

There is one circumstance connected with the spiral nebulæ, andconspicuous in the Andromeda Nebula on account of its brightness, which makes the question of their origin still more puzzling; they allshow continuous spectra, which, as we have before remarked, indicatethat the mass from which the light comes is either solid or liquid, ora gas under heavy pressure. Thus nebulæ fall into two classes: the``white'' nebulæ, giving a continuous spectrum; and the ``green''nebulæ whose spectra are distinctly gaseous. The Andromeda Nebula isthe great representative of the former class and the Orion Nebula ofthe latter. The spectrum of the Andromeda Nebula has been interpretedto mean that it consists not of luminous gas, but of a flock of starsso distant that they are separately indistinguishable even withpowerful telescopes, just as the component stars of the Milky Way areindistinguishable with the naked eye; and upon this has been based thesuggestion that what we see in Andromeda is an outer universe whosestars form a series of elliptical garlands surrounding a central massof amazing richness. But this idea is unacceptable if for no otherreason than that, as just said, all the spiral nebulæ possess the samekind of spectrum, and probably no one would be disposed to regard themall as outer universes. As we shall see later, the peculiarity of thespectra of the spiral nebulæ is appealed to in support of a modernsubstitute for Laplace's hypothesis. 

Finally, without having by any means exhausted the variety exhibitedby the spiral nebulæ, let us turn to the great representative of theother species, the Orion Nebula. In some ways this is even moremarvelous than the others. The early drawings with the telescopefailed to convey an adequate conception either of its sublimity or ofits complication of structure. It exists in a nebulous region ofspace, since photographs show that nearly the whole constellation isinterwoven with faintly luminous coils. To behold the entry of thegreat nebula into the field even of a small telescope is a startlingexperience which never loses its novelty. As shown by the photographs, it is an inscrutable chaos of perfectly amazing extent, where spiralbands, radiating streaks, dense masses, and dark yawning gaps arestrangely intermingled without apparent order. In one place fourconspicuous little stars, better seen in a telescope than in thephotograph on account of the blurring produced by over-exposure, aresuggestively situated in the midst of a dark opening, and no observerhas ever felt any doubt that these stars have been formed from thesubstance of the surrounding nebula. There are many other starsscattered over its expanse which manifestly owe their origin to thesame source. But compare the general appearance of this nebula withthe others that we have studied, and remark the difference. If theunmistakably spiral nebulæ resemble bursting fly-wheels or grindstonesfrom whose perimeters torrents of sparks are flying, the Orion Nebularather recalls the aspect of a cloud of smoke and fragments producedby the explosion of a shell. This idea is enforced by the look of theouter portion farthest from the bright half of the nebula, wheresharply edged clouds with dark spaces behind seem to be billowing awayas if driven by a wind blowing from the center. 

Next let us consider what scientific speculation has done in theeffort to explain these mysteries. Laplace's hypothesis can certainlyfind no standing ground either in the Orion Nebula or in those of aspiral configuration, whatever may be its situation with respect tothe grand Nebula of Andromeda, or the ``ring'' and ``planetary''nebulæ. Some other hypothesis more consonant with the appearances mustbe found. Among the many that have been proposed the most elaborate isthe ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton. It is to be remarked that it applies to the spiral nebulædistinctively, and not to an apparently chaotic mass of gas like thevast luminous cloud in Orion. The gist of the theory is that thesecurious objects are probably the result of close approaches to eachother of two independent suns, reminding us of what was said on thissubject when we were dealing with temporary stars. Of the previoushistory of these appulsing suns the theory gives us no account; theyare simply supposed to arrive within what may be called an effectivetide-producing distance, and then the drama begins. Some of theprobable consequences of such an approach have been noticed in Chapter5; let us now consider them a little more in detail. 

Tides always go in couples; if there is a tide on one side of a globethere will be a corresponding tide on the other side. The cause is tobe found in the law that the force of gravitation varies inversely asthe square of the distance; the attraction on the nearest surface ofthe body exercised by another body is greater than on its center, andgreater yet than on its opposite surface. If two great globes attracteach other, each tends to draw the other out into an ellipsoidalfigure; they must be more rigid than steel to resist this -- and eventhen they cannot altogether resist. If they are liquid or gaseous theywill yield readily to the force of distortion, the amount of whichwill depend upon their distance apart, for the nearer they are thegreater becomes the tidal strain. If they are encrusted without andliquid or gaseous in the interior, the internal mass will strive toassume the figure demanded by the tidal force, and will, if it can, burst the restraining envelope. Now this is virtually the predicamentof the body we call a sun when in the immediate presence of anotherbody of similarly great mass. Such a body is presumably gaseousthroughout, the component gases being held in a state of rigidity bythe compression produced by the tremendous gravitational force oftheir own aggregate mass. At the surface such a body is enveloped in ashell of relatively cool matter. Now suppose a great attracting body, such as another sun, to approach near enough for the difference in itsattraction on the two opposite sides of the body and on its center tobecome very great; the consequence will be a tidal deformation of thewhole body, and it will lengthen out along the line of thegravitational pull and draw in at the sides, and if its shell offersconsiderable resistance, but not enough to exercise a completerestraint, it will be violently burst apart, or blown to atoms, andthe internal mass will leap out on the two opposite sides in greatfiery spouts. In the case of a sun further advanced in cooling thanours the interior might be composed of molten matter while theexterior crust had become rigid like the shell of an egg; then theforce of the ``tidal explosion'' produced by the appulse of anothersun would be more violent in consequence of the greater resistanceovercome. Such, then, is the mechanism of the first phase in thehistory of a spiral nebula according to the Planetesimal Hypothesis. Two suns, perhaps extinguished ones, have drawn near together, and anexplosive outburst has occured in one or both. The second phase callsfor a more agile exercise of the imagination. 

To simplify the case, let us suppose that only one of the tugging sunsis seriously affected by the strain. Its vast wings produced by theoutburst are twisted into spirals by their rotation and the contendingattractions exercised upon them, as the two suns, like battleships indesperate conflict, curve round each other, concentrating theirdestructive energies. Then immense quantities of débris are scatteredabout in which eddies are created, and finally, as the sun that causedthe damage goes on its way, leaving its victim to repair its injuriesas it may, the dispersed matter cools, condenses, and turns intostreams of solid particles circling in elliptical paths about theirparent sun. These particles, or fragments, are the ``planetesimals''of the theory. In consequence of the inevitable intersection of theorbits of the planetesimals, nodes are formed where the flyingparticles meet, and at these nodes large masses are graduallyaccumulated. The larger the mass the greater its attraction, and atlast the nodal points become the nuclei of great aggregations fromwhich planets are shaped. 

This, in very brief form, is the Planetesimal Hypothesis which we areasked to substitute for that based on Laplace's suggestion as anexplanation of the mode of origin of the solar system; and thephenomena of the spiral nebulæ are appealed to as offering evidentsupport to the new hypothesis. We are reminded that they areelliptical in outline, which accords with the hypothesis; that theirspectra are not gaseous, which shows that they may be composed ofsolid particles like the planetesimals; and that their central massespresent an oval form, which is what would result from the tidaleffects, as just described. We also remember that some of them, likethe Lord Rosse and the Andromeda nebulæ, are visually double, and inthese cases we might suppose that the two masses represent thetide-burst suns that ventured into too close proximity. It may beadded that the authors of the theory do not insist upon the appulse oftwo suns as the only way in which the planetesimals may haveoriginated, but it is the only supposition that has been worked out. 

But serious questions remain. It needs, for instance, but a glance atthe Triangulum monster to convince the observer that it cannot be asolar system which is being evolved there, but rather a swarm ofstars. Many of the detached masses are too vast to admit of thesupposition that they are to be transformed into planets, in our senseof planets, and the distances of the stars which appear to have beenoriginally ejected from the focal masses are too great to allow us toliken the assemblage that they form to a solar system. Then, too, nonodes such as the hypothesis calls for are visible. Moreover, in mostof the spiral nebulæ the appearances favor the view that thesupposititious encountering suns have not separated and gone eachrejoicing on its way, after having inflicted the maximum possibledamage on its opponent, but that, on the contrary, they remain inclose association like two wrestlers who cannot escape from eachother's grasp. And this is exactly what the law of gravitationdemands; stars cannot approach one another with impunity, with regardeither to their physical make-up or their future independence ofmovement. The theory undertakes to avoid this difficulty by assumingthat in the case of our system the approach of the foreign body to thesun was not a close one -- just close enough to produce the tidalextrusion of the relatively insignificant quantity of matter needed toform the planets. But even then the effect of the appulse would be tochange the direction of flight, both of the sun and of its visitor, and there is no known star in the sky which can be selected as thesun's probable partner in their ancient pas deux. That there areunconquered difficulties in Laplace's hypothesis no one would deny, but in simplicity of conception it is incomparably more satisfactory, and with proper modifications could probably be made more consonantwith existing facts in our solar system than that which is offered toreplace it. Even as an explanation of the spiral nebulæ, not as solarsystems in process of formation, but as the birthplaces of stellarclusters, the Planetesimal Hypothesis would be open to manyobjections. Granting its assumptions, it has undoubtedly a strongmathematical framework, but the trouble is not with the mathematicsbut with the assumptions. Laplace was one of the ablest mathematiciansthat ever lived, but he had never seen a spiral nebula; if he had, hemight have invented a hypothesis to suit its phenomena. His actualhypothesis was intended only for our solar system, and he left it inthe form of a ``note'' for the consideration of his successors, withthe hope that they might be able to discover the full truth, which heconfessed was hidden from him. It cannot be said that that truth hasyet been found, and when it is found the chances are that intuitionand not logic will have led to it. 

The spiral nebulæ, then, remain among the greatest riddles of theuniverse, while the gaseous nebulæ, like that of Orion, are no lessmysterious, although it seems impossible to doubt that both forms givebirth to stars. It is but natural to look to them for light on thequestion of the origin of our planetary system; but we should notforget that the scale of the phenomena in the two cases is vastlydifferent, and the forces in operation may be equally different. Ahill may have been built up by a glacier, while a mountain may be theproduct of volcanic forces or of the upheaval of the strata of theplanet. 

The Banners of the Sun

As all the world knows, the sun, a blinding globe pouring forth aninconceivable quantity of light and heat, whose daily passage throughthe sky is caused by the earth's rotation on its axis, constitutes themost important phenomenon of terrestial existence. Viewed with a darkglass to take off the glare, or with a telescope, its rim is seen tobe a sharp and smooth circle, and nothing but dark sky is visiblearound it. Except for the interference of the moon, we should probablynever have known that there is any more of the sun than our eyesordinarily see. 

But when an eclipse of the sun occurs, caused by the interposition ofthe opaque globe of the moon, we see its immediate surroundings, whichin some respects are more wonderful than the glowing central orb. These surroundings, although not in the sense in which we apply theterm to the gaseous envelope of the earth, may be called the sun'satmosphere. They consist of two very different parts -- first, the red``prominences, '' which resemble tongues of flame ascending thousandsof miles above the sun's surface; and, second, the ``corona, '' whichextends to distances of millions of miles from the sun, and shineswith a soft, glowing light. The two combined, when well seen, make aspectacle without parallel among the marvels of the sky. Although manyattempts have been made to render the corona visible when there is noeclipse, all have failed, and it is to the moon alone that we owe itsrevelation. To cover the sun's disk with a circular screen will notanswer the purpose because of the illumination of the air all aboutthe observer. When the moon hides the sun, on the other hand, thesunlight is withdrawn from a great cylinder of air extending to thetop of the atmosphere and spreading many miles around the observer. There is then no glare to interfere with the spectacle, and the coronaappears in all its surprising beauty. The prominences, however, although they were discovered during an eclipse, can now, with the aidof the spectroscope, be seen at any time. But the prominences arerarely large enough to be noticed by the naked eye, while thestreamers of the corona, stretching far away in space, like ghostlybanners blown out from the black circle of the obscuring moon, attractevery eye, and to this weird apparition much of the fear inspired byeclipses has been due. But if the corona has been a cause of terror inthe past it has become a source of growing knowledge in our time. 

The story of the first scientific observation of the corona and theprominences is thrillingly interesting, and in fact dramatic. Theobservation was made during the eclipse of 1842, which fortunately wasvisible all over Central and Southern Europe so that scores ofastronomers saw it. The interest centers in what happened at Pavia inNorthern Italy, where the English astronomer Francis Baily had set uphis telescope. The eclipse had begun and Bailey was busy at histelescope when, to quote his own words in the account which he wrotefor the Memoirs of the Royal Astronomical Society:

 I was astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified by the sight of one of the most brilliant and splendid phenomena that can well be imagined; for at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory, similar in shape and magnitude to that which painters draw round the heads of saints... 

 Pavia contains many thousand inhabitants, the major part of whom were at this early hour walking about the streets and squares or looking out of windows in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was instantaneous, there was a universal shout from every observer which ``made the welkin ring, '' and for the moment withdrew my attention from the object with which I was immediately occupied. I had, indeed, expected the appearance of a luminous circle round the moon during the time of total obscurity; but I did not expect, from any of the accounts of preceding eclipses that I had read, to witness so magnificent an exhibition as that which took place... 

 Splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet I must confess that there was at the same time something in its singular and wonderful appearance that was appalling... 

 But the most remarkable circumstance attending the phenomenon was the appearance of three large protuberances apparently emanating from the circumference of the moon, but evidently forming a portion of the corona. They had the appearance of mountains of a prodigious elevation; their color was red tinged with lilac or purple; perhaps the color of the peach-blossom would more nearly represent it. They somewhat resembled the tops of the snowy Alpine mountains when colored by the rising or the setting sun. They resembled the Alpine mountains in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona... 

 The whole of these protuberances were visible even to the last moment of total obscuration, and when the first ray of light was admitted from the sun they vanished, with the corona, altogether, and daylight was instantly restored. 

I have quoted nearly all of this remarkable description not alone forits intrinsic interest, but because it is the best depiction that canbe found of the general phenomena of a total solar eclipse. Still, notevery such eclipse offers an equally magnificent spectacle. Theeclipses of 1900 and 1905, for instance, which were seen by thewriter, the first in South Carolina and the second in Spain, fell farshort of that described by Bailey in splendor and impressiveness. Ofcourse, something must be allowed for the effect of surprise; Baileyhad not expected to see what was so suddenly disclosed to him. Butboth in 1900 and 1905 the amount of scattered light in the sky wassufficient in itself to make the corona appear faint, and there wereno very conspicuous prominences visible. Yet on both occasions therewas manifest among the spectators that mingling of admiration and aweof which Bailey speaks. The South Carolinians gave a cheer and theladies waved their handkerchiefs when the corona, ineffably delicateof form and texture, melted into sight and then in two minutes meltedaway again. The Spaniards, crowded on the citadel hill of Burgos, withtheir king and his royal retinue in their midst, broke out with agreat clapping of hands as the awaited spectacle unfolded itself inthe sky; and on both occasions, before the applause began, after anawed silence a low murmur ran through the crowds. At Burgos it is saidmany made the sign of the cross. 

It was not long before Bailey's idea that the prominences were a partof the corona was abandoned, and it was perceived that the twophenomena were to a great extent independent. At the eclipse of 1868, which the astronomers, aroused by the wonderful scene of 1842, andeager to test the powers of the newly invented spectroscope, flockedto India to witness, Janssen conceived the idea of employing thespectroscope to render the prominences visible when there was noeclipse. He succeeded the very next day, and these phenomena have beenstudied in that way ever since. 

There are recognized two kinds of prominences -- the ``erruptive'' andthe ``quiescent. '' The latter, which are cloud-like in form, may beseen almost anywhere along the edge of the sun; but the former, whichoften shoot up as if hurled from mighty volcanoes, appear to beassociated with sun-spots, and appear only above the zones where spotsabound. Either of them, when seen in projection against the brilliantsolar disk, appears white, not red, as against a background of sky. The quiescent prominences, whose elevation is often from fortythousand to sixty thousand miles, consist, as the spectroscope shows, mainly of hydrogen and helium. The latter, it will be remembered, isan element which was known to be in the sun many years before thediscovery that it also exists in small quantities on the earth. A factwhich may have a significance which we cannot at present see is thatthe emanation from radium gradually and spontaneously changes intohelium, an alchemistical feat of nature that has opened many curiousvistas to speculative thinkers. The eruptive prominences, which do notspread horizontally like the others, but ascend with marvelousvelocity to elevations of half a million miles or more, are apparentlycomposed largely of metallic vapors -- i. E. Metals which are usuallysolid on the earth, but which at solar temperatures are kept in avolatilized state. The velocity of their ascent occasionally amountsto three hundred or four hundred miles per second. It is known frommathematical considerations that the gravitation of the sun would notbe able to bring back any body that started from its surface with avelocity exceeding three hundred and eighty-three miles per second; soit is evident that some of the matter hurled forth in eruptiveprominences may escape from solar control and go speeding out intospace, cooling and condensing into solid masses. There seems to be noreason why some of the projectiles from the sun might not reach theplanets. Here, then, we have on a relatively small scale, explosionsrecalling those which it has been imagined may be the originatingcause of some of the sudden phenomena of the stellar heavens. 

Of the sun-spots it is not our intention here specifically to speak, but they evidently have an intimate connection with eruptiveprominences, as well as some relation, not yet fully understood, withthe corona. Of the real cause of sun-spots we know virtually nothing, but recent studies by Professor Hale and others have revealed astrange state of things in the clouds of metallic vapors floatingabove them and their surroundings. Evidences of a cyclonic tendencyhave been found, and Professor Hale has proved that sun-spots arestrong magnetic fields, and consist of columns of ionized vaporsrotating in opposite directions in the two hemispheres. A fact whichmay have the greatest significance is that titanium and vanadium havebeen found both in sun-spots and in the remarkable variable Mira Ceti, a star which every eleven months, or thereabout, flames up with greatbrilliancy and then sinks back to invisibility with the naked eye. Ithas been suggested that sun-spots are indications of the beginning ofa process in the sun which will be intensified until it falls into thestate of such a star as Mira. Stars very far advanced in evolution, without showing variability, also exhibit similar spectra; so thatthere is much reason for regarding sunspots as emblems of advancingage. 

The association of the corona with sun-spots is less evident than thatof the eruptive prominences; still such an association exists, for theform and extent of the corona vary with the sun-spot period of whichwe shall presently speak. The constitution of the corona remains to bediscovered. It is evidently in part gaseous, but it also probablycontains matter in the form of dust and small meteors. It includes onesubstance altogether mysterious -- ``coronium. '' There are reasons forthinking that this may be the lightest of all the elements, andProfessor Young, its discoverer, said that it was ``absolutely uniquein nature; utterly distinct from any other known form of matter, terrestial, solar, or cosmical. '' The enormous extent of the corona isone of its riddles. Since the development of the curious subject ofthe ``pressure of light'' it has been proposed to account for thesustentation of the corona by supposing that it is borne upon thebillows of light continually poured out from the sun. Experiment hasproved, what mathematical considerations had previously pointed out asprobable, that the waves of light exert a pressure or driving force, which becomes evident in its effects if the body acted upon issufficiently small. In that case the light pressure will prevail overthe attraction of gravitation, and propel the attenuated matter awayfrom the sun in the teeth of its attraction. The earth itself would bedriven away if, instead of consisting of a solid globe of immenseaggregate mass, it were a cloud of microscopic particles. The reasonis that the pressure varies in proportion to the surface of the bodyacted upon, while the gravitational attraction is proportional to thevolume, or the total amount of matter in the body. But the surface ofany body depends upon the square of its diameter, while the volumedepends upon the cube of the diameter. If, for instance, the diameteris represented by 4, the surface will be proportional to 4 × 4, or 16, and the volume to 4 × 4 × 4, or 64; but if the diameter is taken as 2, the surface will be 2 × 2, or 4, and the volume 2 × 2 × 2, or 8. Now, the ratio of 4 to 8 is twice as great as that of 16 to 64. If thediameter is still further decreased, the ratio of the surface to thevolume will proportionally grow larger; in other words, the pressurewill gain upon the attraction, and whatever their original ratio mayhave been, a time will come, if the diminution of size continues, whenthe pressure will become more effective than the attraction, and thebody will be driven away. Supposing the particles of the corona to bebelow the critical size for the attraction of a mass like that of thesun to control them, they would be driven off into the surroundingspace and appear around the sun like the clouds of dust around a mill. We shall return to this subject in connection with the Zodiacal Light, the Aurora, and Comets. 

On the other hand, there are parts of the corona which suggest bytheir forms the play of electric or magnetic forces. This isbeautifully shown in some of the photographs that have been made ofthe corona during recent eclipses. Take, for instance, that of theeclipse of 1900. The sheaves of light emanating from the poles lookprecisely like the ``lines of force'' surrounding the poles of amagnet. It will be noticed in this photograph that the corona appearsto consist of two portions: one comprising the polar rays just spokenof, and the other consisting of the broader, longer, and less-definedmasses of light extending out from the equatorial and middle-latitudezones. Yet even in this more diffuse part of the phenomenon one candetect the presence of submerged curves bearing more or lessresemblance to those about the poles. Just what part electricity orelectro-magnetism plays in the mechanism of the solar radiation it isimpossible to say, but on the assumption that it is a very importantpart is based the hypothesis that there exists a direct solarinfluence not only upon the magnetism, but upon the weather of theearth. This hypothesis has been under discussion for half a century, and still we do not know just how much truth it represents. It iscertain that the outbreak of great disturbances on the sun, accompanied by the formation of sun-spots and the upshooting oferuptive prominences (phenomena which we should naturally expect to beattended by action), have been instantly followed by corresponding``magnetic storms'' on the earth and brilliant displays of the aurorallights. There have been occasions when the influence has manifesteditself in the most startling ways, a great solar outburst beingfollowed by a mysterious gripping of the cable and telegraph systemsof the world, as if an invisible and irresistible hand had seizedthem. Messages are abruptly cut off, sparks leap from the telegraphinstruments, and the entire earth seems to have been thrown into amagnetic flurry. These occurrences affect the mind with a deepimpression of the dependence of our planet on the sun, such as we donot derive from the more familiar action of the sunlight on the growthof plants and other phenomena of life depending on solar influences. 

Perhaps the theory of solar magnetic influence upon the weather isbest known in connection with the ``sun-spot cycle. '' This, at anyrate, is, as already remarked, closely associated with the corona. Itsexistence was discovered in 1843 by the German astronomer Schwabe. Itis a period of variable length, averaging about eleven years, duringwhich the number of spots visible on the sun first increases to amaximum, then diminishes to a minimum, and finally increases again toa maximum. For unknown reasons the period is sometimes two or threeyears longer than the average and sometimes as much shorter. Nevertheless, the phenomena always recur in the same order. Starting, for instance, with a time when the observer can find few or no spots, they gradually increase in number and size until a maximum, in bothsenses, is reached, during which the spots are often of enormous sizeand exceedingly active. After two or three years they begin todiminish in number, magnitude, and activity until they almost or quitedisappear. A strange fact is that when a new period opens, the spotsappear first in high northern and southern latitudes, far from thesolar equator, and as the period advances they not only increase innumber and size, but break out nearer and nearer to the equator, thelast spots of a vanishing period sometimes lingering in the equatorialregion after the advance-guard of its successor has made itsappearance in the high latitudes. Spots are never seen on the equatornor near the poles. It was not very long after the discovery of thesun-spot cycle that the curious observation was made that a strikingcoincidence existed between the period of the sun-spots and anotherperiod affecting the general magnetic condition of the earth. When acurved line representing the varying number of sun-spots was comparedwith another curve showing the variations in the magnetic state of theearth the two were seen to be in almost exact accord, a rise in onecurve corresponding to a rise in the other, and a fall to a fall. Continued observation has proved that this is a real coincidence andnot an accidental one, so that the connection, although as yetunexplained, is accepted as established. But does the influence extendfurther, and directly affect the weather and the seasons as well asthe magnetic elements of the earth? A final answer to this questioncannot yet be given, for the evidence is contradictory, and theinterpretations put upon it depend largely on the predilections of thejudges. 

But, in a broad sense, the sun-spots and the phenomena connected withthem must have a relation to terrestial meteorology, for they provethe sun to be a variable star. Reference was made, a few lines above, to the resemblance of the spectra of sun-spots to those of certainstars which seem to be failing through age. This in itself isextremely suggestive; but if this resemblance had never beendiscovered, we should have been justified in regarding the sun asvariable in its output of energy; and not only variable, but probablyincreasingly so. The very inequalities in the sun-spot cycle aresuspicious. When the sun is most spotted its total light may bereduced by one-thousandth part, although it is by no means certainthat its outgiving of thermal radiations is then reduced. A loss ofone-thousandth of its luminosity would correspond to a decrease of. 0025 of a stellar magnitude, considering the sun as a star viewedfrom distant space. So slight a change would not be perceptible; butit is not alone sun-spots which obscure the solar surface, its entireglobe is enveloped with an obscuring veil. When studied with apowerful telescope the sun's surface is seen to be thickly mottledwith relatively obscure specks, so numerous that it has been estimatedthat they cut off from one-tenth to one-twentieth of the light that weshould receive from it if the whole surface were as brilliant as itsbrightest parts. The condition of other stars warrants the conclusionthat this obscuring envelope is the product of a process ofrefrigeration which will gradually make the sun more and more variableuntil its history ends in extinction. Looking backward, we see a timewhen the sun must have been more brilliant than it is now. At thattime it probably shone with the blinding white splendor of such starsas Sirius, Spica, and Vega; now it resembles the relatively dullProcyon; in time it will turn ruddy and fall into the closing cyclerepresented by Antares. Considering that once it must have been moreradiantly powerful than at present, one is tempted to wonder if thatcould have been the time when tropical life flourished within theearth's polar circles, sustained by a vivific energy in the sun whichit has now lost. 

The corona, as we have said, varies with the sun-spot cycle. When thespots are abundant and active the corona rises strong above thespotted zones, forming immense beams or streamers, which on oneoccasion, at least, had an observed length of ten million miles. Atthe time of a spot minimum the corona is less brilliant and has adifferent outline. It is then that the curved polar rays are mostconspicuous. Thus the vast banners of the sun, shaken out in theeclipse, are signals to tell of its varying state, but it willprobably be long before we can read correctly their messages. 

The Zodiacal Light Mystery

There is a singular phenomenon in the sky -- one of the most puzzlingof all -- which has long arrested the attention of astronomers, defying their efforts at explanation, but which probably not one in ahundred, and possibly not one in a thousand, of the readers of thisbook has ever seen. Yet its name is often spoken, and it is aconspicuous object if one knows when and where to look for it, andwhen well seen it exhibits a mystical beauty which at the same timecharms and awes the beholder. It is called ``The Zodiacal Light, ''because it lies within the broad circle of the Zodiac, marking thesun's apparent annual path through the stars. What it is nobody hasyet been able to find out with certainty, and books on astronomyusually speak of it with singular reserve. But it has given rise tomany remarkable theories, and a true explanation of it would probablythrow light on a great many other celestial mysteries. The Milky Wayis a more wonderful object to look upon, but its nature can becomprehended, while there is a sort of uncanniness about the ZodiacalLight which immediately impresses one upon seeing it, for its part inthe great scheme of extra-terrestrial affairs is not evident. 

If you are out-of-doors soon after sunset -- say, on an evening latein the month of February -- you may perceive, just after the angryflush of the dying winter's day has faded from the sky, a pale ghostlypresence rising above the place where the sun went down. The writerremembers from boyhood the first time it was pointed out to him andthe unearthly impression that it made, so that he afterward avoidedbeing out alone at night, fearful of seeing the spectral thing again. The phenomenon brightens slowly with the fading of the twilight, andsoon distinctly assumes the shape of an elongated pyramid of pearlylight, leaning toward the south if the place of observation is in thenorthern hemisphere. It does not impress the observer at all in thesame manner as the Milky Way; that looks far off and is clearly amongthe stars, but the Zodiacal Light seems closer at hand, as if it weresomething more intimately concerning the earth. To all it immediatelysuggests a connection, also, with the sunken sun. If the night isclear and the moon absent (and if you are in the country, for citylights ruin the spectacles of the sky), you will be able to watch theapparition for a long time. You will observe that the light isbrightest near the horizon, gradually fading as the pyramidal beammounts higher, but in favorable circumstances it may be traced nearlyto the meridian south of the zenith, where its apex at last vanishesin the starlight. It continues visible during the evenings of Marchand part of April, after which, ordinarily, it is seen no more, or ifseen is relatively faint and unimpressive. But when autumn comes itappears again, this time not like a wraith hovering above the westwardtomb of the day-god, but rather like a spirit of the morningannouncing his reincarnation in the east. 

The reason why the Zodiacal Light is best seen in our latitudes at theperiods just mentioned is because at those times the Zodiac is morenearly perpendicular to the horizon, first in the west and then in theeast; and, since the phenomenon is confined within the borders of theZodiac, it cannot be favorably placed for observation when thezodiacal plane is but slightly inclined to the horizon. Its faintlight requires the contrast of a background of dark sky in order to bereadily perceptible. But within the tropics, where the Zodiac isalways at a favorable angle, the mysterious light is more constantlyvisible. Nearly all observant travelers in the equatorial regions havetaken particular note of this phenomenon, for being so much moreconspicuous there than in the temperate zones it at once catches theeye and holds the attention as a novelty. Humboldt mentions it manytimes in his works, for his genius was always attracted by things outof the ordinary and difficult of explanation, and he made many carefulobservations on its shape, its brilliancy, and its variations; forthere can be no doubt that it does vary, and sometimes to anastonishing degree. It is said that it once remained practicallyinvisible in Europe for several years in succession. During a trip toSouth Africa in 1909 an English astronomer, Mr E. W. Maunder, found aremarkable difference between the appearance of the Zodiacal Light onhis going and coming voyages. In fact, when crossing the equator goingsouth he did not see it at all; but on returning he had, on March 6th, when one degree south of the equator, a memorable view of it. 

 It was a bright, clear night, and the Zodiacal Light was extraordinarily brilliant -- brighter than he had ever seen it before. The Milky Way was not to be compared with it. The brightest part extended 75° from the sun. There was a faint and much narrower extension which they could just make out beyond the Pleiades along the ecliptic, but the greater part of the Zodiacal Light showed as a broad truncated column, and it did not appear nearly as conical as he had before seen it. 

When out of the brief twilight of intertropical lands, where the sundrops vertically to the horizon and night rushes on like a wave ofdarkness, the Zodiacal Light shoots to the very zenith, its color isdescribed as a golden tint, entirely different from the silvery sheenof the Milky Way. If I may venture again to refer to personalexperiences and impressions, I will recall a view of the ZodiacalLight from the summit of the cone of Mt Etna in the autumn of the year1896 (more briefly described in Astronomy with the Naked Eye). Thereare few lofty mountains so favorably placed as Etna for observationsof this kind. It was once resorted to by Prof. George E. Hale, in anattempt to see the solar corona without an eclipse. Rising directlyfrom sea-level to an elevation of nearly eleven thousand feet, theobserver on its summit at night finds himself, as it were, lost in themidst of the sky. But for the black flanks of the great cone on whichhe stands he might fancy himself to be in a balloon. On the occasionto which I refer the world beneath was virtually invisible in themoonless night. The blaze of the constellations overhead wasastonishingly brilliant, yet amid all their magnificence my attentionwas immediately drawn to a great tapering light that sprang from theplace on the horizon where the sun would rise later, and that seemedto be blown out over the stars like a long, luminous veil. It was thefinest view of the Zodiacal light that I had ever enjoyed -- thrillingin its strangeness -- but I was almost disheartened by theindifference of my guide, to whom it was only a light and nothingmore. If he had no science, he had less poetry -- rather a remarkablething, I thought, for a child of his clime. The Light appeared to meto be distinctly brighter than the visible part of the Milky Way whichincluded the brilliant stretches in Auriga and Perseus, and its color, if one may speak of color in connection with such an object, seemedricher than that of the galactic band; but I did not think of it asyellow, although Humboldt has described it as resembling a goldencurtain drawn over the stars, and Du Chaillu in Equatorial Africafound it of a bright yellow color. It may vary in color as inconspicuousness. The fascination of that extraordinary sight has neverfaded from my memory. I turned to regard it again and again, althoughI had never seen the stellar heavens so brilliant, and it was one ofthe last things I looked for when the morning glow began softly tomount in the east, and Sicily and the Mediterranean slowly emergedfrom the profound shadow beneath us. 

The Zodiacal Light seems never to have attracted from astronomers ingeneral the amount of careful attention that it deserves; perhapsbecause so little can really be made of it as far as explanation isconcerned. I have referred to the restraint that scientific writersapparently feel in speaking of it. The grounds for speculation that itaffords may be too scanty to lead to long discussions, yet it piquescuriosity, and as we shall see in a moment has finally led to a mostinteresting theory. Once it was the subject of an elaborate series ofstudies which carried the observer all round the world. That was in1845--46, during the United States Exploring Expedition that visitedthe then little known Japan. The chaplain of the fleet, the Rev. MrJones, went out prepared to study the mysterious light in all itsphases. He saw it from many latitudes on both sides of the equator, and the imagination cannot but follow him with keen interest in hisworld-circling tour, keeping his eyes every night fixed upon thephantasm overhead, whose position shifted with that of the hidden sun. He demonstrated that the flow extends at times completely across thecelestial dome, although it is relatively faint directly behind theearth. On his return the government published a large volume of hisobservations, in which he undertook to show that the phenomenon wasdue to the reflection of sunlight from a ring of meteoric bodiesencircling the earth. But, after all, this elaborate investigationsettled nothing. 

Prof. E. E. Barnard has more recently devoted much attention to theZodiacal Light, as well as to a strange attendant phenomenon calledthe ``Gegenschein, '' or Counterglow, because it always appears at thatpoint in the sky which is exactly opposite the sun. The Gegenschein isan extremely elusive phenomenon, suitable only for eyes that have beenspecially trained to see it. Professor Newcomb has cautiously remarkedthat

 it is said that in that point of the heavens directly opposite the sun there is an elliptical patch of light... This phenomenon is so difficult to account for that its existence is sometimes doubted; yet the testimony in its favor is difficult to set aside. 

It certainly cannot be set aside at all since the observations ofBarnard. I recall an attempt to see it under his guidance during avisit to Mount Hamilton, when he was occupied there with the Licktelescope. Of course, both the Gegenschein and the Zodiacal Light aretoo diffuse to be studied with telescopes, which, so to speak, magnifythem out of existence. They can only be successfully studied with thenaked eye, since every faintest glimmer that they afford must beutilized. This is especially true of the Gegenschein. At MountHamilton, Mr Barnard pointed out to me its location with reference tocertain stars, but with all my gazing I could not be sure that I sawit. To him, on the contrary, it was obvious; he had studied it formonths, and was able to indicate its shape, its boundaries, itsdiameter, and the declination of its center with regard to theecliptic. There is not, of course, the shadow of a doubt of theexistence of the Gegenschein, and yet I question if one person in amillion has ever seen or ever will see it. The Zodiacal Light, on theother hand, is plain enough, provided that the time and thecircumstances of the observation are properly chosen. 

In the attempts to explain the Zodiacal Light, the favorite hypothesishas been that it is an appendage of the sun -- perhaps simply anextension of the corona in the plane of the ecliptic, which is notvery far from coinciding with that of the sun's equator. This idea isquite a natural one, because of the evident relation of the light tothe position of the sun. The vast extension of the equatorial wings ofthe corona in 1878 gave apparent support to this hypothesis; if thesubstance of the corona could extend ten million miles from the sun, why might it not extend even one hundred million, gradually fading outbeyond the orbit of the earth? A variation of this hypothesis assumesthat the reflection is due to swarms of meteors circling about thesun, in the plane of its equator, all the way from its immediateneighborhood to a distance exceeding that of the earth. But in neitherform is the hypothesis satisfactory; there is nothing in theappearance of the corona to indicate that it extends even as far asthe planet Mercury, while as to meteors, the orbits of the knownswarms do not accord with the hypothesis, and we have no reason tobelieve that clouds of others exist traveling in the part of spacewhere they would have to be in order to answer the requirements of thetheory. The extension of the corona in 1878 did not resemble in itstexture the Zodiacal Light. 

Now, it has so often happened in the history of science that animportant discovery in one branch has thrown unexpected but mostwelcome light upon some pending problem in some other branch, that astrong argument might be based upon that fact alone against the tooexclusive devotion of many investigators to the narrow lines of theirown particular specialty; and the Zodiacal Light affords a case inpoint, when it is considered in connection with recent discoveries inchemistry and physics. From the fact that atoms are compound bodiesmade up of corpuscles at least a thousand times smaller than thesmallest known atom -- a fact which astounded most men of science whenit was announced a few years ago -- a new hypothesis has beendeveloped concerning the nature of the Zodiacal Light (as well asother astronomical riddles), and this hypothesis comes not from anastronomer, but from a chemist and physicist, the Swede, SvanteArrhenius. In considering an outline of this new hypothesis we needneither accept nor reject it; it is a case rather for suspension ofjudgment. 

To begin with, it carries us back to the ``pressure of light''mentioned in the preceding chapter. The manner in which this pressureis believed generally to act was there sufficiently explained, and itonly remains to see how it is theoretically extended to the particlesof matter supposed to constitute the Zodiacal Light. We know thatcorpuscles, or ``fragments of atoms'' negatively electrified, aredischarged from hot bodies. Streams of these ``ions'' pour from manyflames and from molten metals; and the impact of the cathode andultra-violet rays causes them to gush even from cold bodies. In thevast laboratory of the sun it is but reasonable to suppose thatsimilar processes are taking place. ``As a very hot metal emits thesecorpuscles, '' says Prof. J. J. Thomson, ``it does not seem animprobable hypothesis that they are emitted by that very hot body, thesun. '' Let it be assumed, then, that the sun does emit them; whathappens next? Negatively charged corpuscles, it is known, serve asnuclei to which particles of matter in the ordinary state areattracted, and it is probable that those emitted from the sunimmediately pick up loads in this manner and so grow in bulk. If theygrow large enough the gravitation of the sun draws them back, and theyproduce a negative charge in the solar atmosphere. But it is probablethat many of the particles do not attain the critical size which, according to the principles before explained, would enable thegravitation of the sun to retain them in opposition to the pressure ofthe waves of light, and with these particles the light pressure isdominant. Clouds of them may be supposed to be continually swept awayfrom the sun into surrounding space, moving mostly in or near theplane of the solar equator, where the greatest activity, as indicatedby sunspots and related phenomena, is taking place. As they passoutward into space many of them encounter the earth. If the earth, like the moon, had no atmosphere the particles would impinge directlyon its surface, giving it a negative electric charge. But the presenceof the atmosphere changes all that, for the first of the flyingparticles that encounter it impart to it their negative electricity, and then, since like electric charges repel like, the storm ofparticles following will be sheered off from the earth, and willstream around it in a maze of hyperbolic paths. Those that continue oninto space beyond the earth may be expected to continue picking upwandering particles of matter until their bulk has become so greatthat the solar attraction prevails again over the light pressureacting upon them, and they turn again sunward. Passing the earth ontheir return they will increase the amount of dust-clouds careeringround it; and these will be further increased by the action of theultra-violet rays of the sunlight causing particles to shoot radiallyaway from the earth when the negative charge of the upper atmospherehas reached a certain amount, which particles, although startingsunward, will be swept back to the earth with the oncoming streams. Asthe final result of all this accumulation of flying and gyratingparticles in the earth's neighborhood, we are told that the lattermust be transformed into the semblance of a gigantic solid-headedcomet provided with streaming tails, the longest of them stretchingaway from the direction of the sun, while another shorter one extendstoward the sun. This shorter tail is due to the particles that we havejust spoken of as being driven sunward from the earth by the action ofultra-violet light. No doubt this whole subject is too technical forpopular statement; but at any rate the general reader can understandthe picturesque side of the theory, for its advocates assure us thatif we were on the moon we would doubtless be able to see thecomet-like tails of the earth, and then we could appreciate the partthat they play in producing the phenomenon of the Zodiacal Light. 

That the Light as we see it could be produced by the reflection ofsunlight from swarms of particles careering round the earth in themanner supposed by Arrhenius' hypothesis is evident enough; and itwill be observed that the new theory, after all, is only anothervariant of the older one which attributes the Zodiacal Light to anextension of the solar corona. But it differs from the older theory inoffering an explanation of the manner in which the extension iseffected, and it differentiates between the corona proper and thestreams of negative particles shot away from the sun. In its detailsthe hypothesis of Arrhenius also affords an explanation of manypeculiarities of the Zodiacal Light, such as that it is confined tothe neighborhood of the ecliptic, and that it is stronger on the sideof the earth which is just turning away from a position under the sunthan on the other side; but it would carry us beyond our limits to gointo these particulars. The Gegenschein, according to this theory, isa part of the same phenomenon as the Zodiacal Light, for by the lawsof perspective it is evident that the reflection from the streams ofparticles situated at a point directly opposite to the sun would be ata maximum, and this is the place which the Gegenschein occupies. Apartfrom its geometrical relations to the position of the sun, thevariability of the Zodiacal Light appears to affirm its solardependence, and this too would be accounted for by Arrhenius'hypothesis better than by the old theory of coronal extension. Theamount of corpuscular discharge from the sun must naturally begoverned by the state of relative activity or inactivity of thelatter, and this could not but be reflected in the varying splendor ofthe Zodiacal Light. But much more extended study than has yet beengiven to the subject will be required before we can feel that we knowwith reasonable certainty what this mysterious phenomenon really is. By the hypothesis of Arrhenius every planet that has an atmospheremust have a Zodiacal Light attending it, but the phenomenon is toofaint for us to be able to see it in the case, for instance, of Venus, whose atmosphere is very abundant. The moon has no corresponding``comet's tail'' because, as already explained, of the lack of a lunaratmosphere to repel the streams by becoming itself electrified; but ifthere were a lunar Zodiacal Light, no doubt we could see it because ofthe relative nearness of our satellite. 

Marvels of the Aurora

One of the most vivid recollections of my early boyhood is that ofseeing my father return hastily into the house one evening and callout to the family: ``Come outside and look at the sky!'' Ours was acountry house situated on a commanding site, and as we all emergedfrom the doorway we were dumbfounded to see the heavens filled withpale flames which ran licking and quivering over the stars. Instantlythere sprang into my terrified mind the recollection of an awfuldescription of ``the Day of Judgment'' (the Dies Iræ), which I hadheard with much perturbation of spirit in the Dutch Reformed churchfrom the lips of a tall, dark-browed, dreadfully-in-earnest preacherof the old-fashioned type. My heart literally sank at sight of thespectacle, for it recalled the preacher's very words; it was just ashe had said it would be, and it needed the assured bearing of myelders finally to convince me that

 That Day of Wrath, O dreadful day, When Heaven and Earth shall pass away, As David and the Sibyl say

had not actually come upon us. And even the older members of thehousehold were not untouched with misgivings when menacing spots ofcrimson appeared, breaking out now here, now there, in the shudderingsky. Toward the north the spectacle was appalling. A huge arch spannedan unnaturally dark segment resting on the horizon, and above thisarch sprang up beams and streamers in a state of incessant agitation, sometimes shooting up to the zenith with a velocity that took one'sbreath, and sometimes suddenly falling into long ranks, and marching, marching, marching, like an endless phalanx of fiery specters, andmoving, as I remember, always from east to west. The absolute silencewith which these mysterious evolutions were performed and thequavering reflections which were thrown upon the ground increased theawfulness of the exhibition. Occasionally enormous curtains of lambentflame rolled and unrolled with a majestic motion, or were shaken toand fro as if by a mighty, noiseless wind. At times, too, a suddenbillowing rush would be made toward the zenith, and for a minute thesky overhead would glow so brightly that the stars seemed to have beenconsumed. The spectacle continued with varying intensity for hours. 

This exhibition occurred in Central New York, a latitude in which theAurora Borealis is seldom seen with so much splendor. I rememberanother similar one seen from the city of New York in November, 1882. On this last occasion some observers saw a great upright beam of lightwhich majestically moved across the heavens, stalking like anapparition in the midst of the auroral pageant, of whose generalmovements it seemed to be independent, maintaining always its uprightposture, and following a magnetic parallel from east to west. Thismysterious beam was seen by no less than twenty-six observers indifferent parts of the country, and a comparison of their observationsled to a curious calculation indicating that the apparition was aboutone hundred and thirty-three miles tall and moved at the speed of tenmiles per second!

But, as everybody knows, it is in the Arctic regions that the Aurora, or the ``Northern Lights, '' can best be seen. There, in the long polarnight, when for months together the sun does not rise, the strangecoruscations in the sky often afford a kind of spectral daylight inunison with the weird scenery of the world of ice. The pages in thenarratives of Arctic exploration that are devoted to descriptions ofthe wonderful effects of the Northern Lights are second to none thatman has ever penned in their fascination. The lights, as I havealready intimated, display astonishing colors, particularly shades ofred and green, as they flit from place to place in the sky. Thediscovery that the magnetic needle is affected by the Aurora, quivering and darting about in a state of extraordinary excitementwhen the lights are playing in the sky, only added to the mystery ofthe phenomenon until its electro-magnetic nature had been established. This became evident as soon as it was known that the focus of thedisplays was the magnetic pole; and when the far South was visited theAurora Australis was found, having its center at the South MagneticPole. Then, if not before, it was clear that the earth was a greatglobular magnet, having its poles of opposite magnetism, and that theauroral lights, whatever their precise cause might be, weremanifestations of the magnetic activity of our planet. After theinvention of magnetic telegraphy it was found that whenever a greatAurora occurred the telegraph lines were interrupted in theiroperation, and the ocean cables ceased to work. Such a phenomenon iscalled a ``magnetic storm. ''

The interest excited by the Aurora in scientific circles was greatlystimulated when, in the last half of the nineteenth century, it wasdiscovered that it is a phenomenon intimately associated withdisturbances on the sun. The ancient ``Zurich Chronicles, '' extendingfrom the year 1000 to the year 1800, in which both sun-spots visibleto the naked eye and great displays of the auroral lights wererecorded, first set Rudolf Wolf on the track of this discovery. Thefirst notable proof of the suspected connection was furnished withdramatic emphasis by an occurrence which happened on September 1, 1859. Near noon on that day two intensely brilliant points suddenlybroke out in a group of sun-spots which were under observation by MrR. C. Carrington at his observatory at Redhill, England. The pointsremained visible for not more than five minutes, during which intervalthey moved thirty-five thousand miles across the solar disk. Mr R. Hodgson happened to see the same phenomenon at his observatory atHighgate, and thus all possibility of deception was removed. Butneither of the startled observers could have anticipated what was tofollow, and, indeed, it was an occurrence which has never beenprecisely duplicated. I quote the eloquent account given by MissClerke in her History of Astronomy During the Nineteenth Century. 

 This unique phenomenon seemed as if specially designed to accentuate the inference of a sympathetic relation between the earth and the sun. From August 28 to September 4, 1859, a magnetic storm of unparalleled intensity, extent, and duration was in progress over the entire globe. Telegraphic communication was everywhere interrupted -- except, indeed, that it was in some cases found practicable to work the lines without batteries by the agency of the earth-currents alone; sparks issued from the wires; gorgeous auroras draped the skies in solemn crimson over both hemispheres, and even in the tropics; the magnetic needle lost all trace of continuity in its movements and darted to and fro as if stricken with inexplicable panic. The coincidence was even closer. At the very instant of the solar outburst witnessed by Carrington and Hodgson the photographic apparatus at Kew registered a marked disturbance of all the three magnetic elements; while shortly after the ensuing midnight the electric agitation culminated, thrilling the whole earth with subtle vibrations, and lighting up the atmosphere from pole to pole with coruscating splendors which perhaps dimly recall the times when our ancient planet itself shone as a star. 

If this amazing occurrence stood alone, and as I have already said ithas never been exactly duplicated, doubt might be felt concerning someof the inferences drawn from it; but in varying forms it has beenrepeated many times, so that now hardly anyone questions the realityof the assumed connection between solar outbursts and magnetic stormsaccompanied by auroral displays on the earth. It is true that the lateLord Kelvin raised difficulties in the way of the hypothesis of adirect magnetic action of the sun upon the earth, because it seemed tohim that an inadmissible quantity of energy was demanded to accountfor such action. But no calculation like that which he made is final, since all calculations depend upon the validity of the data; and noauthority is unshakable in science, because no man can possessomniscience. It was Lord Kelvin who, but a few years before the thingwas actually accomplished, declared that aerial navigation was animpracticable dream, and demonstrated its impracticability bycalculation. However the connection may be brought about, it is ascertain as evidence can make it that solar outbursts are coincidentwith terrestial magnetic disturbances, and coincident in such a way asto make the inference of a causal connection irresistible. The sun isonly a little more than a hundred times its own diameter away from theearth. Why, then, with the subtle connection between them afforded bythe ether which conveys to us the blinding solar light and thelife-sustaining solar heat, should it be so difficult to believe thatthe sun's enormous electric energies find a way to us also? No doubtthe impulse coming from the sun acts upon the earth after the mannerof a touch upon a trigger, releasing energies which are already storedup in our planet. 

But besides the evidence afforded by such occurrences as have beenrelated of an intimate connection between solar outbreaks andterrestial magnetic flurries, attended by magnificent auroraldisplays, there is another line of proof pointing in the samedirection. Thus, it is known that the sun-spot period, as remarked ina preceding chapter, coincides in a most remarkable manner with theperiodic fluctuations in the magnetic state of the earth. Thiscoincidence runs into the most astonishing details. For instance, whenthe sun-spot period shortens, the auroral period shortens to preciselythe same extent; as the short sun-spot periods usually bring the mostintense outbreaks of solar activity, so the corresponding shortauroral periods are attended by the most violent magnetic storms; asecular period of about two hundred and twenty-two years affectingsun-spots is said to have its auroral duplicate; a shorter period offifty-five and a half years, which some observers believe that theyhave discovered appears also to be common to the two phenomena; andyet another ``superposed'' period of about thirty-five years, whichsome investigators aver exists, affects sun-spots and aurora alike. Inshort, the coincidences are so numerous and significant that one wouldhave to throw the doctrine of probability to the winds in order to beable to reject the conclusion to which they so plainly lead. 

But still the question recurs: How is the influence transmitted? HereArrhenius comes once more with his hypothesis of negative corpuscles, or ions, driven away from the sun by light-pressure -- a hypothesiswhich seems to explain so many things -- and offers it also as anexplanation of the way in which the sun creates the Aurora. He wouldgive the Aurora the same lineage with the Zodiacal Light. Tounderstand the application of this theory we must first recall thefact that the earth is a great magnet having its two opposite poles ofmagnetism, one near the Arctic and the other near the AntarcticCircle. Like all magnets, the earth is surrounded with ``lines offorce, '' which, after the manner of the curved rays we saw in thephotograph of a solar eclipse, start from a pole, rising at firstnearly vertically, then bend gradually over, passing high above theequator, and finally descending in converging sheaves to the oppositepole. Now the axis of the earth is so placed in space that it lies atnearly a right angle to the direction of the sun, and as the streamsof negatively charged particles come pouring on from the sun (see thelast preceding chapter), they arrive in the greatest numbers over theearth's equatorial regions. There they encounter the lines of magneticforce at the place where the latter have their greatest elevationabove the earth, and where their direction is horizontal to theearth's surface. Obeying a law which has been demonstrated in thelaboratory, the particles then follow the lines of force toward thepoles. While they are above the equatorial regions they do not becomeluminescent, because at the great elevation that they there occupythere is virtually no atmosphere; but as they pass on toward the northand the south they begin to descend with the lines of force, curvingdown to meet at the poles; and, encountering a part of the atmospherecomparable in density with what remains in an exhausted Crookes tube, they produce a glow of cathode rays. This glow is conceived torepresent the Aurora, which may consequently be likened to a giganticexhibition of vacuum-tube lights. Anybody who recalls his student daysin the college laboratory and who has witnessed a display of NorthernLights will at once recognize the resemblance between them in colors, forms, and behavior. This resemblance had often been noted beforeArrhenius elaborated his hypothesis. 

Without intending to treat his interesting theory as more than apossibly correct explanation of the phenomena of the Aurora, we maycall attention to some apparently confirmatory facts. One of the moststriking of these relates to a seasonal variation in the averagenumber of auroræ. It has been observed that there are more in Marchand September than at any other time of the year, and fewer in Juneand December; moreover (and this is a delicate test as applied to thetheory), they are slightly rarer in June than in December. Now allthese facts seem to find a ready explanation in the hypothesis ofArrhenius, thus: (1) The particles issuing from the sun are supposedto come principally from the regions whose excitement is indicated bythe presence of sun-spots (which accords with Hale's observation thatsun-spots are columns of ionized vapors), and these regions have adefinite location on either side of the solar equator, seldomapproaching it nearer than within 5° or 10° north or south, and neverextending much beyond 35° toward either pole; (2) The equator of thesun is inclined about 7° to the plane of the earth's orbit, from whichit results that twice in a year -- viz. , in June and December -- theearth is directly over the solar equator, and twice a year -- viz. , inMarch and September -- when it is farthest north or south of the solarequator, it is over the inner edge of the sun-spot belts. Since thecorpuscles must be supposed to be propelled radially from the sun, fewwill reach the earth when the latter is over the solar equator in Juneand December, but when it is over, or nearly over, the spot belts, inMarch and September, it will be in the line of fire of the more activeparts of the solar surface, and relatively rich streams of particleswill reach it. This, as will be seen from what has been said above, isin strict accord with the observed variations in the frequency ofauroræ. Even the fact that somewhat fewer auroræ are seen in June thanin December also finds its explanation in the known fact that theearth is about three million miles nearer the sun in the winter thanin the summer, and the number of particles reaching it will vary, likethe intensity of light, inversely as the square of the distance. Thesecoincidences are certainly very striking, and they have a cumulativeforce. If we accept the theory, it would appear that we ought tocongratulate ourselves that the inclination of the sun's equator is soslight, for as things stand the earth is never directly over the mostactive regions of the sun-spots, and consequently never suffers fromthe maximum bombardment of charged particles of which the sun iscapable. Incessant auroral displays, with their undulating draperies, flitting colors, and marching columns might not be objectionable fromthe point of view of picturesqueness, but one magnetic storm ofextreme intensity following closely upon the heels of another, formonths on end, crazing the magnetic needle and continually putting thetelegraph and cable lines out of commission, to say nothing of theireffect upon ``wireless telegraphy'', would hardly add to the charms ofterrestrial existence. 

One or two other curious points in connection with Arrhenius'hypothesis may be mentioned. First, the number of auroræ, according tohis explanation, ought to be greatest in the daytime, when the face ofthe earth on the sunward side is directly exposed to the atomicbombardment. Of course visual observation can give us no informationabout this, since the light of the Aurora is never sufficientlyintense to be visible in the presence of daylight, but the records ofthe magnetic observatories can be, and have been, appealed to forinformation, and they indicate that the facts actually accord with thetheory. Behind the veil of sunlight in the middle of the afternoon, there is good reason to believe, auroral exhibitions often take placewhich would eclipse in magnificence those seen at night if we couldbehold them. Observation shows, too, that auroræ are more frequentbefore than after midnight, which is just what we should expect ifthey originate in the way that Arrhenius supposes. Second, the theoryoffers an explanation of the alleged fact that the formation of cloudsin the upper air is more frequent in years when auroræ are mostabundant, because clouds are the result of the condensation ofmoisture upon floating particles in the atmosphere (in an absolutelydustless atmosphere there would be no clouds), and it has been provedthat negative ions like those supposed to come from the sun play amaster part in the phenomena of cloud formation. 

Yet another singular fact, almost mystical in its suggestions, may bementioned. It seems that the dance of the auroral lights occurs mostfrequently during the absence of the moon from the hemisphere in whichthey appear, and that they flee, in greater part, to the oppositehemisphere when the moon's revolution in an orbit considerablyinclined to the earth's equator brings her into that where they havebeen performing. Arrhenius himself discovered this curious relation ofauroral frequency to the position of the moon north or south of theequator, and he explains it in this way. The moon, like the earth, isexposed to the influx of the ions from the sun; but having noatmosphere, or almost none, to interfere with them, they descenddirectly upon her surface and charge her with an electric negativepotential to a very high degree. In consequence of this she affectsthe electric state of the upper parts of the earth's atmosphere wherethey lie most directly beneath her, and thus prevents, to a largeextent, the negative discharges to which the appearance of the Aurorais due. And so ``the extravagant and erring spirit'' of the Auroraavoids the moon as Hamlet's ghost fled at the voice of the cockannouncing the awakening of the god of day. 

There are even other apparent confirmations of the hypothesis, but weneed not go into them. We shall, however, find one more application ofit in the next chapter, for it appears to be a kind of cure-all forastronomical troubles; at any rate it offers a conceivable solution ofthe question, How does the sun manage to transmit its electricinfluence to the earth? And this solution is so grandiose inconception, and so novel in the mental pictures that it offers, thatits acceptance would not in the least detract from the impression thatthe Aurora makes upon the imagination. 

Strange Adventures of Comets

The fears and legends of ancient times before Science was born, andthe superstitions of the Dark Ages, sedulously cultivated fortheological purposes by monks and priests, have so colored our ideasof the influence that comets have had upon the human mind that manyreaders may be surprised to learn that it was the apparition of awonderful comet, that of 1843, which led to the foundation of ourgreatest astronomical institution, the Harvard College Observatory. Nodoubt the comet superstition existed half a century ago, as, indeed, it exists yet today, but in this case the marvelous spectacle in thesky proved less effective in inspiring terror than in awakening adesire for knowledge. Even in the sixteenth century the views thatenlightened minds took of comets tended powerfully to inspire popularconfidence in science, and Halley's prediction, after seeing andstudying the motion of the comet which appeared in 1682, that it wouldprove to be a regular member of the sun's family and would be seenreturning after a period of about seventy-six years, together with thefulfillment of that prediction, produced a revulsion from thesuperstitious notions which had so long prevailed. 

Then the facts were made plain that comets are subject to the law ofgravitation equally with the planets; that there are many whichregularly return to the neighborhood of the sun (perihelion); and thatthese travel in orbits differing from those of the planets only intheir greater eccentricity, although they have the peculiarity thatthey do not, like the planets, all go round the sun in the samedirection, and do not keep within the general plane of the planetarysystem, but traverse it sometimes from above and sometimes from below. Other comets, including most of the ``great'' ones, appear to travelin parabolic or, in a few cases, hyperbolic orbits, which, not beingclosed curves, never bring them back again. But it is not certain thatthese orbits may not be extremely eccentric ellipses, and that afterthe lapse of hundreds, or thousands, of years the comets that followthem may not reappear. The question is an interesting one, because ifall orbits are really ellipses, then all comets must be permanentmembers of the solar system, while in the contrary case many of themare simply visitors, seen once and never to be seen again. Thehypothesis that comets are originally interlopers might seem to derivesome support from the fact that the certainly periodic ones areassociated, in groups, with the great outer planets, whose attractionappears to have served as a trap for them by turning them intoelliptical orbits and thus making them prisoners in the solar system. Jupiter, owing to his great mass and his commanding situation in thesystem, is the chief ``comet-catcher;'' but he catches them not forhimself, but for the sun. Yet if comets do come originally fromwithout the borders of the planetary system, it does not, by anymeans, follow that they were wanderers at large in space before theyyielded to the overmastering attraction of the sun. Investigation ofthe known cometary orbits, combined with theoretical considerations, has led some astronomers to the conclusion that as the sun travelsonward through space he ``picks up en route'' cometary masses which, without belonging strictly to his empire, are borne along in the samevast ``cosmical current'' that carries the solar system. 

But while no intelligent person any longer thinks that the appearanceof a great comet is a token from the heavenly powers of theapproaching death of a mighty ruler, or the outbreak of a devastatingwar, or the infliction of a terrible plague upon wicked mankind, science itself has discovered mysteries about comets which are notless fascinating because they are more intellectual than theirrational fancies that they have displaced. To bring the subjectproperly before the mind, let us see what the principal phenomenaconnected with a comet are. 

At the present day comets are ordinarily ``picked up'' with thetelescope or the photographic plate before any one except theirdiscoverer is aware of their existence, and usually they remain soinsignificant in appearance that only astronomers ever see them. Yetso great is the prestige of the word ``comet'' that the discovery ofone of these inconspicuous wanderers, and its subsequent movements, become items of the day's news which everybody reads with the feeling, perhaps, that at least he knows what is going on in the universe evenif he doesn't understand it. But a truly great comet presents quite adifferent proposition. It, too, is apt to be detected coming out ofthe depths of space before the world at large can get a glimpse of it, but as it approaches the sun its aspect undergoes a marvelous change. Agitated apparently by solar influence, it throws out a long streamingtail of nebulous light, directed away from the sun and looking as ifblown out like a pennon by a powerful wind. Whatever may be theposition of the comet with regard to the sun, as it circles round himit continually keeps its tail on the off side. This, as we shall soonsee, is a fact of capital importance in relation to the probablenature of comets' tails. Almost at the same time that the formation ofthe tail is observed a remarkable change takes place in the comet'shead, which, by the way, is invariably and not merely occasionally itsmost important part. On approaching the sun the head usuallycontracts. Coincidently with this contraction a nucleus generallymakes its appearance. This is a bright, star-like point in the head, and it probably represents the totality of solid matter that the cometpossesses. But it is regarded as extremely unlikely that even thenucleus consists of a uniformly solid mass. If it were such, cometswould be far more formidable visitors when they pass near the planetsthan they have been found to be. The diameter of the nucleus may varyfrom a few hundred up to several thousand miles; the heads, on theaverage, are from twenty-five thousand to one hundred thousand milesin diameter, although a few have greatly exceeded these dimensions;that of the comet of 1811, one of the most stupendous ever seen, was amillion and a quarter miles in diameter! As to the tails, notwithstanding their enormous length -- some have been more than ahundred million miles long -- there is reason to believe that they areof extreme tenuity, ``as rare as vacuum. '' The smallest stars havebeen seen shining through their most brilliant portions withundiminished luster. 

After the nucleus has been formed it begins to throw out bright jetsdirected toward the sun. A stream, and sometimes several streams, oflight also project sunward from the nucleus, occasionally appearinglike a stunted tail directed oppositely to the real tail. Symmetricalenvelopes which, seen in section, appear as half circles or parabolas, rise sunward from the nucleus, forming a concentric series. The endsof these stream backward into the tail, to which they seem to supplymaterial. Ordinarily the formation of these ejections and envelopes isattended by intense agitation of the nucleus, which twists and turns, swinging and gyrating with an appearance of the greatest violence. Sometimes the nucleus is seen to break up into several parts. Theentire heads of some comets have been split asunder in passing closearound the sun; The comet of 1882 retreated into space after itsperihelion passage with five heads instead of the one that it hadoriginally, and each of these heads had its own tail!

The possession of the spectroscope has enabled astronomers duringlater years to study the chemical composition of comets by analyzingtheir light. At first the only substances thus discovered in them werehydro-carbon compounds, due evidently to the gaseous envelopes inwhich some combination of hydrogen with carbon existed. Behind thisgaseous spectrum was found a faint continuous spectrum ascribed to thenucleus, which apparently both reflects the sunlight and gives forththe light of a glowing solid or liquid. Subsequently sodium and ironlines were found in cometary spectra. The presence of iron would seemto indicate that some of these bodies may be much more massive thanobservations on their attractive effects have indicated. In somerecent comets, such as Morehouse's, in 1908, several lines have beenfound, the origin of which is unknown. 

Without going back of the nineteenth century we may find records ofsome of the most extraordinary comets that man has ever looked upon. In 1811, still spoken of as ``the year of the comet, '' because of thewonderful vintage ascribed to the skyey visitor, a comet shaped like agigantic sword amazed the whole world, and, as it remained visible forseventeen months, was regarded by superstitious persons as a symbol ofthe fearful happenings of Napoleon's Russian campaign. This comet, theextraordinary size of whose head, greatly exceeding that of the sunitself, has already been mentioned, was also remarkable for exhibitingso great a brilliancy without approaching even to the earth's distancefrom the sun. But there was once a comet (and only once -- in the year1729) which never got nearer to the sun than four times the distanceof the earth and yet appeared as a formidable object in the sky. AsProfessor Young has remarked, ``it must have been an enormous comet tobe visible from such a distance. '' And we are to remember that therewere no great telescopes in the year 1729. That comet affects theimagination like a phantom of space peering into the solar system, displaying its enormous train afar off (which, if it had approached asnear as other comets, would probably have become the celestial wonderof all human memory), and then turning away and vanishing in thedepths of immensity. 

In 1843 a comet appeared which was so brilliant that it could be seenin broad day close beside the sun! This was the first authenticatedinstance of that kind, but the occurrence was to be repeated, as weshall see in a moment, less than forty years later. 

The splendid comet of 1858, usually called Donati's, is remembered bymany persons yet living. It was, perhaps, both as seen by the nakedeye and with the telescope, the most beautiful comet of which we haveany record. It too marked a rich vintage year, still remembered in thevineyards of France, where there is a popular belief that a greatcomet ripens the grape and imparts to the wine a flavor not attainableby the mere skill of the cultivator. There are ``comet wines, ''carefully treasured in certain cellars, and brought forth only whentheir owner wishes to treat his guests to a sip from paradise. 

The year 1861 saw another very remarkable comet, of an aspectstrangely vast and diffuse, which is believed to have swept the earthwith its immense tail when it passed between us and the sun on thenight of June 30th, an event which produced no other known effect thanthe appearance of an unwonted amount of scattered light in the sky. 

The next very notable comet was the ``Great Southern Comet'' of 1880, which was not seen from the northern hemisphere. It mimicked theaspect of the famous comet of 1843, and to the great surprise ofastronomers appeared to be traveling in the same path. This proved tobe the rising of the curtain for an astronomical sensationunparalleled in its kind; for two years later another brilliant cometappeared, first in the southern hemisphere, and it too followed thesame track. The startling suggestion was now made that this comet wasidentical with those of 1843 and 1880, its return having been hastenedby the resistance experienced in passing twice through the coronalenvelope, and there were some who thought that it would now swingswiftly round and then plunge straight into the sun, with consequencesthat might be disastrous to us on account of the ``flash of heat''that would be produced by the impact. Nervous people were frightened, but observation soon proved that the danger was imaginary, foralthough the comet almost grazed the sun, and must have rushed throughtwo or three million miles of the coronal region, no retardation ofits immense velocity was perceptible, and it finally passed away in adamaged condition, as before remarked, and has never since appeared. 

Then the probable truth was perceived -- viz. , that the three comets(1843, 1880, and 1882) were not one identical body, but three separateones all traveling in the same orbit. It was found, too, that a cometseen in 1668 bore similar insignia of relationship. The naturalinference was that these four bodies had once formed a single masswhich had been split apart by the disruptive action of the sun. Strength was lent to this hypothesis by the fact that the comet of1882 was apparently torn asunder during its perihelion passage, retreating into space in a dissevered state. But Prof. George Forbeshas a theory that the splitting of the original cometary mass waseffected by an unknown planet, probably greater than Jupiter, situatedat a hundred times the earth's distance from the sun, and revolving ina period of a thousand years. He supposes that the original comet wasnot that of 1668, but one seen in 1556, which has since been``missing, '' and that its disruption occurred from an encounter withthe supposititious planet about the year 1700. Truly from every pointof view comets are the most extraordinary of adventurers!

The comet of 1882 was likewise remarkable for being visible, like itspredecessor of 1843, in full daylight in close proximity to the sun. The story of its detection when almost in contact with the solar diskis dramatic. It had been discovered in the southern hemisphere only acouple of weeks before its perihelion, which occurred on September17th, and on the forenoon of that day it was seen by Doctor Common inEngland, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope, almost touching the sun. It looked like a dazzling white bird withoutspread wings. The southern observers watched it go right into thesun, when it instantly disappeared. What had happened was that thecomet in passing its perihelion point had swung exactly between theearth and the sun. On the following morning it was seen from all partsof the world close by the sun on the opposite side, and it remainedthus visible for three days, gradually receding from the solar disk. It then became visible for northern observers in the morning skybefore sunrise, brandishing a portentous sword-shaped tail which, ifit had been in the evening sky, would have excited the wonder ofhundreds of millions, but situated where it was, comparatively fewever saw it. 

The application of photography to the study of comets has revealedmany curious details which might otherwise have escaped detection, orat best have remained subject to doubt. It has in particular shown notonly the precise form of the tails, but the remarkable vicissitudesthat they undergo. Professor Barnard's photographs of Brooks' comet in1893 suggested, by the extraordinary changes in the form of the tailwhich they revealed, that the comet was encountering a series ofobstructions in space which bent and twisted its tail into fantasticshapes. The reader will observe the strange form into which the tailwas thrown on the night of October 21st. A cloud of meteors throughwhich the comet was passing might have produced such deformations ofits tail. In the photograph of Daniels' comet of 1907, a curiousstriping of the tail will be noticed. The short bright streaks seen inthe photograph, it may be explained, are the images of stars which aredrawn out into lines in consequence of the fact that the photographictelescope was adjusted to follow the motion of the comet while thestars remained at rest. 

But the adventures of comets are not confined to possible encounterswith unknown obstacles. We have referred to the fact that the greatplanets, and especially Jupiter, frequently interfere with the motionsof comets. This interference is not limited to the original alterationof their orbits from possible parabolas to ellipses, but is sometimesexercised again and again, turning the bewildered comets intoelliptical paths of all degrees of eccentricity. A famous example ofthis kind of planetary horse-play is furnished by the story ofLexell's missing comet. This comet was first seen in 1770. Investigation showed that it was moving in an orbit which should bringit back to perihelion every five and a half years; yet it had neverbeen seen before and, although often searched for, has never been seensince. Laplace and Leverrier proved mathematically that in 1767 it hadapproached so close to Jupiter as to be involved among the orbits ofhis satellites. What its track had been before is not known, but onthat occasion the giant planet seized the interloper, threw it into ashort elliptic orbit and sent it, like an arrested vagrant, to receivesentence at the bar of the sun. On this journey it passed within lessthan 1, 500, 000 miles of the earth. The form of orbit which Jupiter hadimpressed required, as we have said, its return in about five and ahalf years; but soon after 1770 it had the misfortune a second time toencounter Jupiter at close range, and he, as if dissatisfied with theleniency of the sun, or indignant at the stranger's familiarity, seized the comet and hurled it out of the system, or at any rate sofar away that it has never since been able to rejoin the family circlethat basks in the immediate rays of the solar hearth. Nor is this theonly instance in which Jupiter has dealt summarily with small cometsthat have approached him with too little deference. 

The function which Jupiter so conspicuously fulfills as master of thehounds to the sun is worth considering a little more in detail. Tochange the figure, imagine the sun in its voyage through space to belike a majestic battleship surrounded by its scouts. Small vessels(the comets, as they are overhauled by the squadron, are taken incharge by the scouts, with Jupiter for their chief, and are forced toaccompany the fleet, but not all are impressed. If a strange cometundertakes to run across Jupiter's bows the latter brings it to, andmakes prize of it by throwing it into a relatively small ellipse withthe sun for its focus. Thenceforth, unless, as happened to the unhappycomet of Lexell, it encounters Jupiter again in such a way as to bediverted by him into a more distant orbit, it can never get away. About thirty comets are now known to have thus been captured by thegreat planet, and they are called ``Jupiter's Comet Family. '' But, onthe other hand, if a wandering comet crosses the wake of the chiefplanetary scout the latter simply drives it away by accelerating itsmotion and compels it to steer off into open space. The transformationof comets into meteors will be considered in the next chapter, buthere, in passing, mention may be made of the strange fate of onemember of Jupiter's family, Biela's comet, which, having become overbold in its advances to its captor, was, after a few revolutions in isimpressed orbit, torn to pieces and turned into a flock of meteors. 

And now let us return to the mystery of comets' tails. That we arefully justified in speaking of the tails of comets as mysterious isproved by the declaration of Sir John Herschel, who averred, in somany words, that ``there is some profound secret and mystery of natureconcerned in this phenomenon, '' and this profound secret and mysteryhas not yet been altogether cleared up. Nevertheless, theall-explaining hypothesis of Arrhenius offers us once more a certainamount of aid. Comets' tails, Arrhenius assures us, are but anotherresult of the pressure of light. The reader will recall theapplications of this theory to the Zodiacal Light and the Aurora. Inthe form in which we now have to deal with it, the supposition is madethat as a comet approaches the sun eruptions of vapor, due to thesolar heat, occur in its nucleus. These are naturally most active onthe side which is directly exposed to the sun, whence the appearanceof the immense glowing envelopes that surround the nucleus on thesunward side. Among the particles of hydro-carbon, and perhaps solidcarbon in the state of fine dust, which are thus set free there willbe many whose size is within the critical limit which enables thelight-waves from the sun to drive them away. Clouds of such particles, then, will stream off behind the advancing comet, producing theappearance of a tail. This accounts for the fact that the tails ofcomets are always directed away from the sun, and it also explains thevarying forms of the tails and the extraordinary changes that theyundergo. The speed of the particles driven before the light-waves mustdepend upon their size and weight, the lightest of a given sizetraveling the most swiftly. By accretion certain particles might grow, thus losing velocity and producing the appearance of bunches in thetail, such as have been observed. The hypothesis also falls in withthe researches of Bredichin, who has divided the tails of comets intothree principal classes -- viz. : (1) Those which appear as long, straight rays; (2) Those which have the form of curved plumes orscimitars; (3) Those which are short, brushy, and curved sharplybackward along the comet's path. In the first type he calculates therepulsive force at from twelve to fifteen times the force of gravity;in the second at from two to four times; and in the third at about oneand a half times. The straight tails he ascribes to hydrogen becausethe hydrogen atom is the lightest known; the sword-shaped tails tohydro-carbons; and the stumpy tails to vaporized iron. It will be seenthat, if the force driving off the tails is that which Arrheniusassumes it to be, the forms of those appendages would accord withthose that Bredichin's theory calls for. At the same time we have anexplanation of the multiple tails with which some comets have adornedthemselves. The comet of 1744, for instance, had at one time no lessthan seven tails spread in a wide curved brush behind it. Donati'scomet of 1858 also had at least two tails, the principal onesword-shaped and the other long, narrow, and as straight as a rule. According to Bredichin, the straight tail must have been composed ofhydrogen, and the other of some form of hydro-carbon whose atoms areheavier than those of hydrogen, and, consequently, when swept away bythe storm of light-waves, followed a curvature depending upon theresultant of the forces operating upon them. The seven tails of thecomet of 1744 presented a kind of diagram graphically exhibiting itscomplex composition, and, if we knew a little more about theconstituents of a comet, we might be able to say from the amount ofcurvature of the different tails just what were the seven substancesof which that comet consisted. 

If these theories seem to the reader fantastic, at any rate they areno more fantastic than the phenomena that they seek to explain. 

Meteors, Fire-Balls, and Meteorites

One of the most terrorizing spectacles with which the heavens haveever caused the hearts of men to quake occurred on the night ofNovember 13, 1833. On that night North America, which faced the storm, was under a continual rain of fire from about ten o'clock in theevening until daybreak. 

The fragments of a comet had struck the earth. 

But the meaning of what had happened was not discovered until longafterward. To the astronomers who, with astonishment not less thanthat of other people, watched the wonderful scene, it was anunparalleled ``shower of meteors. '' They did not then suspect thatthose meteors had once formed the head of a comet. Light dawned when, a year later, Prof. Denison Olmsted, of Yale College, demonstratedthat the meteors had all moved in parallel orbits around the sun, andthat these orbits intersected that of the earth at the point where ourplanet happened to be on the memorable night of November 13th. Professor Olmsted even went so far as to suggest that the cloud ofmeteors that had encountered the earth might form a diffuse comet; butfull recognition of the fact that they were cometary débris camelater, as the result of further investigation. The key to the secretwas plainly displayed in the spectacle itself, and was noticed withoutbeing understood by thousands of the terror-stricken beholders. It wasan umbrella of fire that had opened overhead and covered the heavens;in other words, the meteors all radiated from a particular point inthe constellation Leo, and, being countless as the snowflakes in awinter tempest, they ribbed the sky with fiery streaks. ProfessorOlmsted showed that the radiation of the meteors from a fixed pointwas an effect of perspective, and in itself a proof that they weremoving in parallel paths when they encountered the earth. The fact wasnoted that there had been a similar, but incomparably less brilliant, display of meteors on the same day of November, 1832, and it wasrightly concluded that these had belonged to the same stream, althoughthe true relationship of the phenomena was not immediatelyapprehended. Olmsted ascribed to the meteors a revolution about thesun once in every six months, bringing them to the intersection oftheir orbit with that of the earth every November 13th; but laterinvestigators found that the real period was about thirty-three andone-quarter years, so that the great displays were due three times ina century, and their return was confidently predicted for the year1866. The appearance of the meteors in 1832, a year before the greatdisplay, was ascribed to the great length of the stream which theyformed in space -- so great that they required more than two years tocross the earth's orbit. In 1832 the earth had encountered arelatively rare part of the stream, but in 1833, on returning to thecrossing-place, it found there the richest part of the stream pouringacross its orbit. This explanation also proved to be correct, and thepredicted return in 1866 was duly witnessed, although the display wasmuch less brilliant than in 1833. It was followed by another in 1867. 

In the mean time Olmsted's idea of a cometary relationship of themeteors was demonstrated to be correct by the researches ofSchiaparelli and others, who showed that not only the Novembermeteors, but those of August, which are seen more or less abundantlyevery year, traveled in the tracks of well-known comets, and hadundoubtedly an identical origin with those comets. In other words thecomets and the meteor-swarms were both remnants of original masseswhich had probably been split up by the action of the sun, or of someplanet to which they had made close approaches. The annual periodicityof the August meteors was ascribed to the fact that the separation hadtaken place so long ago that the meteors had become distributed allaround the orbit, in consequence of which the earth encountered someof them every year when it arrived at the crossing-point. ThenLeverrier showed that the original comet associated with the Novembermeteors was probably brought into the system by the influence of theplanet Uranus in the year 126 of the Christian era. AfterwardAlexander Herschel identified the tracks of no less than seventy-sixmeteor-swarms (most of them inconspicuous) with those of comets. Thestill more recent researches of Mr W. F. Denning make it probable thatthere are no meteors which do not belong to a flock or system probablyformed by the disintegration of a cometary mass; even the apparentlysporadic ones which shoot across the sky, ``lost souls in the night, ''being members of flocks which have become so widely scattered that theearth sometimes takes weeks to pass through the region of space wheretheir paths lie. 

The November meteors should have exhibited another pair of spectaclesin 1899 and 1900, and their failure to do so caused at first muchdisappointment, until it was made plain that a good reason existed fortheir absence. It was found that after their last appearance, in 1867, they had been disturbed in their movements by the planets Jupiter andSaturn, whose attractions had so shifted the position of their orbitthat it no longer intersected that of the earth, as it did before. Whether another planetary interference will sometime bring theprincipal mass of the November meteors back to the former point ofintersection with the earth's orbit is a question for the future todecide. It would seem that there may be several parallel streams ofthe November meteors, and that some of them, like those of August, aredistributed entirely around the orbit, so that every mid-November wesee a few of them. 

We come now to a very remarkable example of the disintegration of acomet and the formation of a meteor-stream. In 1826 Biela, ofJosephstadt, Austria, discovered a comet to which his name was given. Calculation showed that it had an orbital period of about six and ahalf years, belonging to Jupiter's ``family. '' On one of its returns, in 1846, it astonished its watchers by suddenly splitting in two. Thetwo comets thus formed out of one separated to a distance of about onehundred and sixty thousand miles, and then raced side by side, sometimes with a curious ligature connecting them, like Siamese twins, until they disappeared together in interplanetary space. In 1852 theycame back, still nearly side by side, but now the distance betweenthem had increased to a million and a quarter of miles. After that, atevery recurrence of their period, astronomers looked for them in vain, until 1872, when an amazing thing happened. On the night of November28th, when the earth was crossing the plane of the orbit of themissing comet, a brilliant shower of meteors burst from the northernsky, traveling nearly in the track which the comet should havepursued. The astronomers were electrified. Klinkerfues, of Göttingen, telegraphed to Pogson, of Madras: ``Biela touched earth; search nearTheta Centauri. '' Pogson searched in the place indicated and saw acometary mass retreating into the southern heavens, where it was soonswallowed from sight!

Since then the Biela meteors have been among the recognized periodicspectacles of the sky, and few if any doubt that they represent aportion of the missing comet whose disintegration began with theseparation into two parts in 1846. The comet itself has never sincebeen seen. The first display of these meteors, sometimes called the``Andromedes, '' because they radiate from the constellation Andromeda, was remarkable for the great brilliancy of many of the fire-balls thatshot among the shower of smaller sparks, some of which were describedas equaling the full moon in size. None of them is known to havereached the earth, but during the display of the same meteors in 1885a meteoric mass fell at Mazapil in Northern Mexico (it is now in theMuseum at Vienna), which many have thought may actually be a piece ofthe original comet of Biela. This brings us to the second branch ofour subject. 

More rare than meteors or falling stars, and more startling, exceptthat they never appear in showers, are the huge balls of fire whichoccasionally dart through the sky, lighting up the landscapes beneathwith their glare, leaving trains of sparks behind them, oftenproducing peals of thunder when they explode, and in many casesfalling upon the earth and burying themselves from a few inches toseveral feet in the soil, from which, more than once, they have beenpicked up while yet hot and fuming. These balls are sometimes calledbolides. They are not really round in shape, although they often lookso while traversing the sky, but their forms are fragmentary, andoccasionally fantastic. It has been supposed that their origin isdifferent from that of the true meteors; it has even been conjecturedthat they may have originated from the giant volcanoes of the moon orhave been shot out from the sun during some of the tremendousexplosions that accompany the formation of eruptive prominences. Bythe same reasoning some of them might be supposed to have come fromsome distant star. Others have conjectured that they are wanderers inspace, of unknown origin, which the earth encounters as it journeyson, and Lord Kelvin made a suggestion which has become classic becauseof its imaginative reach -- viz. , that the first germs of life mayhave been brought to the earth by one of these bodies, ``a fragment ofan exploded world. ''

It is a singular fact that astronomers and scientific men in generalwere among the last to admit the possibility of solid masses fallingfrom the sky. The people had believed in the reality of such phenomenafrom the earliest times, but the savants shook their heads and talkedof superstition. This was the less surprising because noscientifically authenticated instance of such an occurrence was known, and the stones popularly believed to have fallen from the sky hadbecome the objects of worship or superstitious reverence, a fact notcalculated to recommend them to scientific credence. The celebrated``black stone'' suspended in the Kaaba at Mecca is one of thesereputed gifts from heaven; the ``Palladium'' of ancient Troy wasanother; and a stone which fell near Ensisheim, in Germany, was placedin a church as an object to be religiously venerated. Many legends offalling stones existed in antiquity, some of them curiouslytransfigured by the imagination, like the ``Lion of thePeloponnesus, '' which was said to have sprung down from the sky uponthe Isthmus of Corinth. But near the beginning of the nineteenthcentury, in 1803, a veritable shower of falling stones occurred atL'Aigle, in Northern France, and this time astronomers took note ofthe phenomenon and scientifically investigated it. Thousands of thestrange projectiles came from the sky on this occasion, and werescattered over a wide area of country, and some buildings were hit. Four years later another shower of stones occurred at Weston, Conn. , numbering thousands of individuals. The local alarm created in bothcases was great, as well it might be, for what could be moreintimidating than to find the blue vault of heaven suddenly hurlingsolid missiles at the homes of men? After these occurrences it wasimpossible for the most skeptical to doubt any longer, and the regularstudy of ``aerolites, '' or ``meteorites, '' began. 

One of the first things recognized was the fact that fire-balls aresolid meteorites in flight, and not gaseous exhalations in the air, assome had assumed. They burn in the air during their flight, andsometimes, perhaps, are entirely consumed before reaching the ground. Their velocity before entering the earth's atmosphere is equal to thatof the planets in their orbits -- viz. , from twenty to thirty milesper second -- a fact which proves that the sun is the seat of thecentral force governing them. Their burning in the air is notdifficult to explain; it is the heat of friction which so quicklybrings them to incandescence. Calculation shows that a body movingthrough the air at a velocity of about a mile per second will bebrought, superficially, to the temperature of ``red heat'' by frictionwith the atmosphere. If its velocity is twenty miles per second thetemperature will become thousands of degrees. This is the state ofaffairs with a meteorite rushing into the earth's atmosphere; itssurface is liquefied within a few seconds after the friction begins toact, and the melted and vaporized portion of its mass is sweptbackward, forming the train of sparks that follows every greatfire-ball. However, there is one phenomenon connected with the trainsof meteorites which has never been satisfactorily explained: theyoften persist for long periods of time, drifting and turning with thewind, but not ceasing to glow with a phosphorescent luminosity. Thequestion is, Whence comes this light? It must be light without heat, since the fine dust or vapor of which the train can only consist wouldnot retain sufficient heat to render it luminous for so long a time. An extremely remarkable incident of this kind occurred on February 22, 1909, when an immense fire-ball that passed over southern England lefta train that remained visible during two hours, assuming many curiousshapes as it was drifted about by currents in the air. 

But notwithstanding the enormous velocity with which meteorites enterthe air they are soon slowed down to comparatively moderate speed, sothat when they disappear they are usually traveling not faster than amile a second. The courses of many have been traced by observerssituated along their track at various points, and thus a knowledge hasbeen obtained of their height above the ground during their flight andof the length of their visible courses. They generally appear at anelevation of eighty or a hundred miles, and are seldom visible afterhaving descended to within five miles of the ground, unless theobserver happens to be near the striking-point, when he may actuallywitness the fall. Frequently they burst while high in the air andtheir fragments are scattered like shrapnel over the surface of theground, sometimes covering an area of several square miles, but ofcourse not thickly; different fragments of the same meteorite mayreach the ground at points several miles apart. The observed length oftheir courses in the atmosphere varies from fifty to five hundredmiles. If they continued a long time in flight after entering the air, even the largest of them would probably be consumed to the last scrap, but their fiery career is so short on account of their great speedthat the heat does not have time to penetrate very deeply, and somethat have been picked up immediately after their fall have been foundcold as ice within. Their size after reaching the ground is variablewithin wide limits; some are known which weigh several tons, but thegreat majority weigh only a few pounds and many only a few ounces. 

Meteorites are of two kinds: stony meteorites and iron meteorites. Theformer outnumber the latter twenty to one; but many stone meteoritescontain grains of iron. Nickel is commonly found in iron meteorites, so that it might be said that that redoubtable alloy nickel-steel isof cosmical invention. Some twenty-five chemical elements have beenfound in meteorites, including carbon and the ``sun-metal, '' helium. The presence of the latter is certainly highly suggestive inconnection with the question of the origin of meteorites. The ironmeteorites, besides metallic iron and nickel, of which they are almostentirely composed, contain hydrogen, helium, and carbonic oxide, andabout the only imaginable way in which these gases could have becomeabsorbed in the iron would be through the immersion of the latterwhile in a molten or vaporized state in a hot and dense atmospherecomposed of them, a condition which we know to exist only in theenvelopes of the sun and the stars. 

The existence of carbon in the Canyon Diablo iron meteorites isattended by a circumstance of the most singular character -- a very``fairy tale of science. '' In some cases the carbon has becomediamond! These meteoric diamonds are very small; nevertheless, theyare true diamonds, resembling in many ways the little black gemsproduced by Moissan's method with the aid of the electric furnace. Thefact that they are found embedded in these iron meteorites is anotherargument in favor of the hypothesis of the solar or stellar origin ofthe latter. To appreciate this it is necessary to recall the way inwhich Moissan made his diamonds. It was by a combination of theeffects of great heat, great pressure, and sudden or rapid superficialcooling on a mass of iron containing carbon. When he finally brokeopen his iron he found it a pudding stuffed with miniature blackdiamonds. When a fragment of the Canyon Diablo meteoric iron waspolished in Philadelphia over fifteen years ago it cut the emery-wheelto pieces, and examination showed that the damage had been effected bymicroscopic diamonds peppered through the mass. How were thosediamonds formed? If the sun or Sirius was the laboratory that preparedthem, we can get a glimpse at the process of their formation. There isplenty of heat, plenty of pressure, and an abundance of vaporized ironin the sun and the stars. When a great solar eruption takes place, masses of iron which have absorbed carbon may be shot out with avelocity which forbids their return. Plunged into the frightful coldof space, their surfaces are quickly cooled, as Moissan cooled hisprepared iron by throwing it into water, and thus the requisite stressis set up within, and, as the iron solidifies, the included carboncrystallizes into diamonds. Whether this explanation has a germ oftruth in it or not, at any rate it is evident that iron meteoriteswere not created in the form in which they come to us; they must oncehave been parts of immeasurably more massive bodies than themselves. 

The fall of meteorites offers an appreciable, though numericallyinsignificant, peril to the inhabitants of the earth. Historicalrecords show perhaps three or four instances of people being killed bythese bodies. But for the protection afforded by the atmosphere, whichacts as a very effective shield, the danger would doubtless be verymuch greater. In the absence of an atmosphere not only would moremeteorites reach the ground, but their striking force would beincomparably greater, since, as we have seen, the larger part of theiroriginal velocity is destroyed by the resistance of the air. Ameteorite weighing many tons and striking the earth with a velocity oftwenty or thirty miles per second, would probably cause frightfulhavoc. 

It is a singular fact that recent investigations seem to have provedthat an event of this kind actually happened in North America --perhaps not longer than a thousand or two thousand years ago. Thescene of the supposed catastrophe is in northern central Arizona, atCoon Butte, where there is a nearly circular crater in the middle of acircular elevation or small mountain. The crater is somewhat over fourthousand feet in diameter, and the surrounding rim, formed of upturnedstrata and ejected rock fragments, rises at its highest point onehundred and sixty feet above the plain. The crater is about sixhundred feet in depth -- that is, from the rim to the visible floor orbottom of the crater. There is no evidence that volcanic action hasever taken place in the immediate neighborhood of Coon Butte. The rockin which the crater has been made is composed of horizontal sandstoneand limestone strata. Between three hundred and four hundred milliontons of rock fragments have been detached, and a large portion hurledby some cause out of the crater. These fragments lie concentricallydistributed around the crater, and in large measure form the elevationknown as Coon Butte. The region has been famous for nearly twentyyears on account of the masses of meteoric iron found scattered aboutand known as the ``Canyon Diablo'' meteorites. It was one of thesemasses, which consist of nickel-iron containing a small quantity ofplatinum, and of which in all some ten tons have been recovered forsale to the various collectors throughout the world, that as beforementioned destroyed the grinding-tool at Philadelphia through thecutting power of its embedded diamonds. These meteoric irons arescattered about the crater-hill, in concentric distribution, to amaximum distance of about five miles. When the suggestion was firstmade in 1896 that a monster meteorite might have created by its fallthis singular lone crater in stratified rocks, it was greeted withincredulous smiles; but since then the matter has assumed a differentaspect. The Standard Iron Company, formed by Messrs. D. M. Barringer, B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in1903, the owner of this freak of nature, sunk shafts and bored holesto a great depth in the interior of the crater, and also trenched theslopes of the mountain, and the result of their investigations hasproved that the meteoric hypothesis of origin is correct. (See thepapers published in the Proceedings of the Academy of Natural Sciencesof Philadelphia, December, 1905, wherein it is proved that the UnitedStates Geological Survey was wrong in believing this crater to havebeen due to a steam explosion. Since that date there has beendiscovered a great amount of additional confirmatory proof). Materialof unmistakably meteoric origin was found by means of the drills, mixed with crushed rock, to a depth of six hundred to seven hundredfeet below the floor of the crater, and a great deal of it has beenfound admixed with the ejected rock fragments on the outer slopes ofthe mountain, absolutely proving synchronism between the two events, the formation of this great crater and the falling of the meteoriciron out of the sky. The drill located in the bottom of the crater wassent, in a number of cases, much deeper (over one thousand feet) intounaltered horizontal red sandstone strata, but no meteoric materialwas found below this depth (seven hundred feet, or between eleven andtwelve hundred feet below the level of the surrounding plain), whichhas been assumed as being about the limit of penetration. It is notpossible to sink a shaft at present, owing to the water which hasdrained into the crater, and which forms, with the finely pulverizedsandstone, a very troublesome quicksand encountered at about twohundred feet below the visible floor of the crater. As soon as thiswater is removed by pumping it will be easy to explore the depths ofthe crater by means of shafts and drifts. The rock strata (sandstoneand limestone) of which the walls consist present every appearance ofhaving been violently upturned by a huge body penetrating the earthlike a cannon-ball. The general aspect of the crater strikinglyresembles the impression made by a steel projectile shot into anarmor-plate. Mr Tilghman has estimated that a meteorite about fivehundred feet in diameter and moving with a velocity of about fivemiles per second would have made just such a perforation upon strikingrocks of the character of those found at this place. There was somefusion of the colliding masses, and the heat produced some steam fromthe small amount of water in the rocks. As a result there has beenfound at depth a considerable amount of fused quartz (originalsandstone), and with it innumerable particles or sparks of fusednickel-iron (original meteorite). A projectile of that sizepenetrating eleven to twelve hundred feet into the rocky shell of theglobe must have produced a shock which was perceptible several hundredmiles away. 

The great velocity ascribed to the supposed meteorite at the moment ofstriking could be accounted for by the fact that it probably plungednearly vertically downward, for it formed a circular crater in therocky crust of the earth. In that case it would have been lessretarded by the resistance of the atmosphere than are meteorites whichenter the air at a lower angle and shoot ahead hundreds of miles untilfriction has nearly destroyed their original motion when they dropupon the earth. Some meteoric masses of great size, such as Peary'siron meteorite found at Cape York, Greenland, and the almost equallylarge mass discovered at Bacubirito, Mexico, appear to have penetratedbut slightly on striking the earth. This may be explained by supposingthat they pursued a long, horizontal course through the air beforefalling. The result would be that, their original velocity having beenpractically destroyed, they would drop to the ground with a velocitynearly corresponding to that which gravity would impart within theperpendicular distance of their final fall. Asix-hundred-and-sixty-pound meteorite, which fell at Knyahinya, Hungary, striking at an angle of 27° from the vertical, penetrated theground to a depth of eleven feet. 

It has been remarked that the Coon Butte meteorite may have fallen notlonger ago than a few thousand years. This is based upon the fact thatthe geological indications favor the supposition that the event didnot occur more than five thousand years ago, while on the other handthe rings of growth in the cedar-trees growing on the slopes of thecrater show that they have existed there about seven hundred years. Prof. William H. Pickering has recently correlated this with anancient chronicle which states that at Cairo, Egypt, in the year 1029, ``many stars passed with a great noise. '' He remarks that Cairo isabout 100°, by great circle, from Coon Butte, so that if the meteoritethat made the crater was a member of a flock of similar bodies whichencountered the earth moving in parallel lines, some of them mighthave traversed the sky tangent to the earth's surface at Cairo. Thatthe spectacle spoken of in the chronicle was caused by meteorites hedeems exceedingly probable because of what is said about ``a greatnoise;'' meteorites are the only celestial phenomena attended withperceptible sounds. Professor Pickering conjectures that this supposedflock of great meteorites may have formed the nucleus of a comet whichstruck the earth, and he finds confirmation of the idea in the factthat out of the ten largest meteorites known, no less than seven werefound within nine hundred miles of Coon Butte. It would be interestingif we could trace back the history of that comet, and find out whatmalicious planet caught it up in its innocent wanderings and hurled itwith so true an aim at the earth! This remarkable crater is one of themost interesting places in the world, for there is absolutely norecord of such a mass, possibly an iron-headed comet, from outer spacehaving come into collision with our earth. The results of the futureexploration of the depths of the crater will be awaited with muchinterest. 

The Wrecking of the Moon

There are sympathetic moods under whose influence one gazes with acertain poignant tenderness at the worn face of the moon; that little``fossil world'' (the child of our mother earth, too) bears suchterrible scars of its brief convulsive life that a sense of pity isawakened by the sight. The moon is the wonder-land of the telescope. Those towering mountains, whose ``proud aspiring peaks'' castsilhouettes of shadow that seem drawn with india-ink; those vastplains, enchained with gentle winding hills and bordered with giantranges; those oval ``oceans, '' where one looks expectant for the flashof wind-whipped waves; those enchanting ``bays'' and recesses at theseaward feet of the Alps; those broad straits passing between guardianheights incomparably mightier than Gibraltar; those locket-likevalleys as secluded among their mountains as the Vale of Cashmere;those colossal craters that make us smile at the pretensions ofVesuvius, Etna, and Cotopaxi; those strange white ways which pass withthe unconcern of Roman roads across mountain, gorge, and valley -- allthese give the beholder an irresistible impression that it is truly aworld into which he is looking, a world akin to ours, and yet no morelike our world than Pompeii is like Naples. Its air, its waters, itsclouds, its life are gone, and only a skeleton remains -- a mute buteloquent witness to a cosmical tragedy without parallel in the rangeof human knowledge. 

One cannot but regret that the moon, if it ever was the seat ofintelligent life, has not remained so until our time. Think what theconsequences would have been if this other world at our very door hadbeen found to be both habitable and inhabited! We talk rather airilyof communicating with Mars by signals; but Mars never approachesnearer than 35, 000, 000 miles, while the moon when nearest is only alittle more than 220, 000 miles away. Given an effective magnifyingpower of five thousand diameters, which will perhaps be possible atthe mountain observatories as telescopes improve, and we should beable to bring the moon within an apparent distance of about fortymiles, while the corresponding distance for Mars would be more thanseven thousand miles. But even with existing telescopic powers we cansee details on the moon no larger than some artificial constructionson the earth. St Peter's at Rome, with the Vatican palace and thegreat piazza, if existing on the moon, would unquestionably berecognizable as something else than a freak of nature. Large cities, with their radiating lines of communication, would at once betraytheir real character. Cultivated tracts, and the changes produced bythe interference of intelligent beings, would be clearly recognizable. The electric illumination of a large town at night would probably bemarkedly visible. Gleams of reflected sunlight would come to us fromthe surfaces of the lakes and oceans, and a huge ``liner'' traversinga lunar sea could probably be followed by its trail of smoke. As tocommunications by ``wireless'' signals, which certain enthusiasts havethought of in connection with Mars, in the case of the moon theyshould be a relatively simple matter, and the feat might actually beaccomplished. Think what a literature would grow up about the moon ifit were a living world! Its very differences from the earth would onlyaccentuate its interest for us. Night and day on the moon are each twoweeks in length; how interesting it would be to watch the manner inwhich the lunarians dealt with such a situation as that. Lunar andterrestrial history would keep step with each other, and we shouldrecord them both. Truly one might well wish to have a neighbor worldto study; one would feel so much the less alone in space. 

It is not impossible that the moon did at one time have inhabitants ofsome kind. But, if so, they vanished with the disappearance of itsatmosphere and seas, or with the advent of its cataclysmic age. At thebest, its career as a living world must have been brief. If the waterand air were gradually absorbed, as some have conjectured, by itscooling interior rocks, its surface might, nevertheless, have retainedthem for long ages; but if, as others think, their disappearance wasdue to the escape of their gaseous molecules in consequence of theinability of the relatively small lunar gravitation to retain them, then the final catastrophe must have been as swift as it wasinevitable. Accepting Darwin's hypothesis, that the moon was separatedfrom the earth by tidal action while both were yet plastic ornebulous, we may reasonably conclude that it began its career with agood supply of both water and air, but did not possess sufficient massto hold them permanently. Yet it may have retained them long enoughfor life to develop in many forms upon its surface; in fact, there areso many indications that air and water have not always been lacking tothe lunar world that we are driven to invent theories to explain boththeir former presence and their present absence. 

But whatever the former condition of the moon may have been, itsexisting appearance gives it a resistless fascination, and it bears soclearly the story of a vast catastrophe sculptured on its rocky facethat the thoughtful observer cannot look upon it without a feeling ofawe. The gigantic character of the lunar features impresses thebeholder not less than the universality of the play of destructiveforces which they attest. Let us make a few comparisons. Take thelunar crater called ``Tycho'', which is a typical example of its kind. In the telescope Tycho appears as a perfect ring surrounding acircular depression, in the center of which rises a group ofmountains. Its superficial resemblance to some terrestrial volcaniccraters is very striking. Vesuvius, seen from a point verticallyabove, would no doubt look something like that (the resemblance wouldhave been greater when the Monte del Cavallo formed a more completecircuit about the crater cone). But compare the dimensions. Theremains of the outer crater ring of Vesuvius are perhaps half a milein diameter, while the active crater itself is only two or threehundred feet across at the most; Tycho has a diameter of fifty-fourmiles! The group of relatively insignificant peaks in the center ofthe crater floor of Tycho is far more massive than the entire mountainthat we call Vesuvius. The largest known volcanic crater on the earth, Aso San, in Japan, has a diameter of seven miles; it would take sixtycraters like Aso San to equal Tycho in area! And Tycho, though one ofthe most perfect, is by no means the largest crater on the moon. Another, called ``Theophilus, '' has a diameter of sixty-four miles, and is eighteen thousand feet deep. There are hundreds from ten toforty miles in diameter, and thousands from one to ten miles. They areso numerous in many places that they break into one another, like thecells of a crushed honeycomb. 

The lunar craters differ from those of the earth more fundamentallythan in the matter of mere size; they are not situated on the tops ofmountains. If they were, and if all the proportions were the same, acrater like Tycho might crown a conical peak fifty or one hundredmiles high! Instead of being cavities in the summits of mountains, thelunar craters are rather gigantic sink-holes whose bottoms in manycases lie two or three miles below the general surface of the lunarworld. Around their rims the rocks are piled up to a height of from afew hundred to two or three thousand feet, with a comparatively gentleinclination, but on the inner side they fall away in gigantic brokenprecipices which make the dizzy cliffs of the Matterhorn seem but``lover's leaps. '' Down they drop, ridge below ridge, crag under crag, tottering wall beneath wall, until, in a crater named ``Newton, '' nearthe south lunar pole, they attain a depth where the rays of the sunnever reach. Nothing more frightful than the spectacle which many ofthese terrible chasms present can be pictured by the imagination. Asthe lazy lunar day slowly advances, the sunshine, unmitigated byclouds or atmospheric veil of any kind, creeps across their rims andbegins to descend the opposite walls. Presently it strikes the raggedcrest of a ridge which had lain hidden in such darkness as we neverknow on the earth, and runs along it like a line of kindling fire. Rocky pinnacles and needles shoot up into the sunlight out of theblack depths. Down sinks the line of light, mile after mile, andcontinually new precipices and cliffs are brought into view, until atlast the vast floor is attained and begins to be illuminated. In themeanwhile the sun's rays, darting across the gulf, have touched thesummits of the central peaks, twenty or thirty miles from the crater'sinmost edge, and they immediately kindle and blaze like huge starsamid the darkness. So profound are some of these awful craters thatdays pass before the sun has risen high enough above them to chase thelast shadows from their depths. 

Although several long ranges of mountains resembling those of theearth exist on the moon, the great majority of its elevations assumethe crateriform aspect. Sometimes, instead of a crater, we find animmense mountain ring whose form and aspect hardly suggest volcanicaction. But everywhere the true craters are in evidence, even on thesea-beds, although they attain their greatest number and size on thoseparts of the moon -- covering sixty per cent of its visible surface --which are distinctly mountainous in character and which constitute itsmost brilliant portions. Broadly speaking, the southwestern half ofthe moon is the most mountainous and broken, and the northeastern halfthe least so. Right down through the center, from pole to pole, runs awonderful line of craters and crateriform valleys of a magnitudestupendous even for the moon. Another similar line follows the westernedge. Three or four ``seas'' are thrust between these mountainousbelts. By the effects of ``libration'' parts of the oppositehemisphere of the moon which is turned away from the earth are fromtime to time brought into view, and their aspect indicates that thathemisphere resembles in its surface features the one which faces theearth. There are many things about the craters which seem to give somewarrant for the hypothesis which has been particularly urged by Mr G. K. Gilbert, that they were formed by the impact of meteors; but thereare also many things which militate against that idea, and, upon thewhole, the volcanic theory of their origin is to be preferred. 

The enormous size of the lunar volcanoes is not so difficult toaccount for when we remember how slight is the force of lunar gravityas compared with that of the earth. With equal size and density, bodies on the moon weigh only one-sixth as much as on the earth. Impelled by the same force, a projectile that would go ten miles onthe earth would go sixty miles on the moon. A lunar giant thirty-fivefeet tall would weigh no more than an ordinary son of Adam weighs onhis greater planet. To shoot a body from the earth so that it wouldnot drop back again, we should have to start it with a velocity ofseven miles per second; a mile and a half per second would serve onthe moon. It is by no means difficult to believe, then, that a lunarvolcano might form a crater ring eight or ten times broader than thegreatest to be found on the earth, especially when we reflect that inaddition to the relatively slight force of gravity, the materials ofthe lunar crust are probably lighter than those of our terrestrialrocks. 

For similar reasons it seems not impossible that the theory mentionedin a former chapter -- that some of the meteorites that have fallenupon the earth originated from the lunar volcanoes -- is well founded. This would apply especially to the stony meteorites, for it is hardlyto be supposed that the moon, at least in its superficial parts, contains much iron. It is surely a scene most strange that is thuspresented to the mind's eye -- that little attendant of the earth's(the moon has only one-fiftieth of the volume, and only one-eightiethof the mass of the earth) firing great stones back at its parentplanet! And what can have been the cause of this furious outbreak ofvolcanic forces on the moon? Evidently it was but a passing stage inits history; it had enjoyed more quiet times before. As it cooled downfrom the plastic state in which it parted from the earth, it becameincrusted after the normal manner of a planet, and then oceans wereformed, its atmosphere being sufficiently dense to prevent the waterfrom evaporating and the would-be oceans from disappearing continuallyin mist. This, if any, must have been the period of life in the lunarworld. As we look upon the vestiges of that ancient world buried inthe wreck that now covers so much of its surface, it is difficult torestrain the imagination from picturing the scenes which were oncepresented there; and, in such a case, should the imagination befettered? We give it free rein in terrestrial life, and it rewards uswith some of our greatest intellectual pleasures. The wonderfullandscapes of the moon offer it an ideal field with just enoughhalf-hidden suggestions of facts to stimulate its powers. 

The great plains of the Mare Imbrium and the Mare Serenitatis (the``Sea of Showers'' and the ``Sea of Serenity''), bordered in part bylofty mountain ranges precisely like terrestrial mountains, scallopedalong their shores with beautiful bays curving back into the adjoininghighlands, and united by a great strait passing between the nearlyabutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus, ''offer the elements of a scene of world beauty such as it would bedifficult to match upon our planet. Look at the finely modulatedbottom of the ancient sea in Mr Ritchey's exquisite photograph of thewestern part of the Mare Serenitatis, where one seems to see the playof the watery currents heaping the ocean sands in waving lines, makingshallows, bars, and deeps for the mariner to avoid or seek, andaffording a playground for the creatures of the main. What geologistwould not wish to try his hammer on those rocks with their stony pagesof fossilized history? There is in us an instinct which forbids us tothink that there was never any life there. If we could visit the moon, there is not among us a person so prosaic and unimaginative that hewould not, the very first thing, begin to search for traces of itsinhabitants. We would look for them in the deposits on the seabottoms; we would examine the shores wherever the configuration seemedfavorable for harbors and the sites of maritime cities -- forgettingthat it may be a little ridiculous to ascribe to the ancient lunariansthe same ideas that have governed the development of our race; wewould search through the valleys and along the seeming courses ofvanished streams; we would explore the mountains, not the terriblecraters, but the pinnacled chains that recall our own Alps andRockies; seeking everywhere some vestige of the transforming presenceof intelligent life. Perhaps we should find such traces, and perhaps, with all our searching, we should find nothing to suggest that lifehad ever existed amid that universal ruin. 

Look again at the border of the ``Sea of Serenity'' -- what a name forsuch a scene! -- and observe how it has been rent with almostinconceivable violence, the wall of the colossal crater Posidoniusdropping vertically upon the ancient shore and obliterating it, whileits giant neighbor, Le Monnier, opens a yawning mouth as if to swallowthe sea itself. A scene like this makes one question whether, afterall, those may not be right who have imagined that the so-called seabottoms are really vast plains of frozen lava which gushed up infloods so extensive that even the mighty volcanoes were half drownedin the fiery sea. This suggestion becomes even stronger when we turnto another of the photographs of Mr Ritchey's wonderful series, showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!). Notice how near the center of the picture the outline of a huge ringwith radiating ridges shows through the sea bottom; a fossil volcanosubmerged in a petrified ocean! This is by no means the only instancein which a buried world shows itself under the great lunar plains. Yet, as the newer craters in the sea itself prove, the volcanicactivity survived this other catastrophe, or broke out againsubsequently, bringing more ruin to pile upon ruin. 

Yet notwithstanding the evidence which we have just been consideringin support of the hypothesis that the ``seas'' are lava floods, Messrs. Loewy and Puiseux, the selenographers of the ParisObservatory, are convinced that these great plains bear characteristicmarks of the former presence of immense bodies of water. In that casewe should be forced to conclude that the later oceans of the moon layupon vast sheets of solidified lava; and thus the catastrophe of thelunar world assumes a double aspect, the earliest oceans beingswallowed up in molten floods issuing from the interior, while thelands were reduced to chaos by a universal eruption of tremendousvolcanoes; and then a period of comparative quiet followed, duringwhich new seas were formed, and new life perhaps began to flourish inthe lunar world, only to end in another cataclysm, which finally put aterm to the existence of the moon as a life-supporting world. 

Suppose we examine two more of Mr Ritchey's illuminating photographs, and, first, the one showing the crater Theophilus and itssurroundings. We have spoken of Theophilus before, citing the factsthat it is sixty-four miles in diameter and eighteen thousand feetdeep. It will be noticed that it has two brother giants -- Cyrillusthe nearer, and Catharina the more distant; but Theophilus is plainlythe youngest of the trio. Centuries, and perhaps thousands of years, must have elapsed between the periods of their upheaval, for the twoolder craters are partly filled with débris, while it is manifest at aglance that when the south eastern wall of Theophilus was formed, itbroke away and destroyed a part of the more ancient ring of Cyrillus. There is no more tremendous scene on the moon than this; viewed with apowerful telescope, it is absolutely appalling. 

The next photograph shows, if possible, a still wilder region. It isthe part of the moon lying between Tycho and the south pole. Tycho isseen in the lower left-hand part of the picture. To the right, at theedge of the illuminated portion of the moon, are the crater-rings, Longomontanus and Wilhelm I, the former being the larger. Between themare to be seen the ruins of two or three more ancient craters which, together with portions of the walls of Wilhelm I and Longomontanus, have been honeycombed with smaller craters. The vast crateriformdepression above the center of the picture is Clavius, an unrivaledwonder of lunar scenery, a hundred and forty-two miles in its greatestlength, while its whole immense floor has sunk two miles below thegeneral surface of the moon outside the ring. The monstrousshadow-filled cavity above Clavius toward the right is Blancanus, whose aspect here gives a good idea of the appearance of these chasmswhen only their rims are in the sunlight. But observe theindescribable savagery of the entire scene. It looks as though thespirit of destruction had gone mad in this spot. The mighty cratershave broken forth one after another, each rending its predecessor; andwhen their work was finished, a minor but yet tremendous outbreakoccurred, and the face of the moon was gored and punctured withthousands of smaller craters. These relatively small craters (small, however, only in a lunar sense, for many of them would appear giganticon the earth) recall once more the theory of meteoric impact. It doesnot seem impossible that some of them may have been formed by such anagency. 

One would not wish for our planet such a fate as that which hasovertaken the moon, but we cannot be absolutely sure that something ofthe kind may not be in store for it. We really know nothing of theultimate causes of volcanic activity, and some have suggested that theinternal energies of the earth may be accumulating instead of dyingout, and may never yet have exhibited their utmost destructive power. Perhaps the best assurance that we can find that the earth will escapethe catastrophe that has overtaken its satellite is to be found in therelatively great force of its gravitation. The moon has been thevictim of its weakness; given equal forces, and the earth would be thebetter able to withstand them. It is significant, in connection withthese considerations, that the little planet Mercury, which seems alsoto have parted with its air and water, shows to the telescope someindications that it is pitted with craters resembling those that havetorn to pieces the face of the moon. 

Upon the whole, after studying the dreadful lunar landscapes, onecannot feel a very enthusiastic sympathy with those who are seekingindications of the continued existence of some kind of life on themoon; such a world is better without inhabitants. It has met its fate;let it go! Fortunately, it is not so near that it cannot hide itsscars and appear beautiful -- except when curiosity impels us to lookwith the penetrating eyes of the astronomer. 

The Great Mars Problem

Let any thoughtful person who is acquainted with the general facts ofastronomy look up at the heavens some night when they appear in theirgreatest splendor, and ask himself what is the strongest impressionthat they make upon his mind. He may not find it easy to frame ananswer, but when he has succeeded it will probably be to the effectthat the stars give him an impression of the universality ofintelligence; they make him feel, as the sun and the moon cannot do, that his world is not alone; that all this was not made simply to forma gorgeous canopy over the tents of men. If he is of a devout turn ofmind, he thinks, as he gazes into those fathomless deeps and amongthose bewildering hosts, of the infinite multitude of created beingsthat the Almighty has taken under his care. The narrow ideas of theold geocentric theology, which made the earth God's especialfootstool, and man his only rational creature, fall away from him likea veil that had obscured his vision; they are impossible in thepresence of what he sees above. Thus the natural tendency, in thelight of modern progress, is to regard the universe as everywherefilled with life. 

But science, which is responsible for this broadening of men'sthoughts concerning the universality of life, itself proceeds to setlimits. Of spiritual existences it pretends to know nothing, but as tophysical beings, it declares that it can only entertain thesupposition of their existence where it finds evidence of anenvironment suited to their needs, and such environment may noteverywhere exist. Science, though repelled by the antiquatedtheological conception of the supreme isolation of man among createdbeings, regards with complacency the probability that there areregions in the universe where no organic life exists, stars whichshine upon no inhabited worlds, and planets which nourish no animatecreatures. The astronomical view of the universe is that it consistsof matter in every stage of evolution: some nebulous and chaotic; somejust condensing into stars (suns) of every magnitude and order; someshaped into finished solar bodies surrounded by dependent planets;some forming stars that perhaps have no planets, and will have none;some constituting suns that are already aging, and will soon losetheir radiant energy and disappear; and some aggregated into massesthat long ago became inert, cold, and rayless, and that can only berevivified by means about which we can form conjectures, but of whichwe actually know nothing. 

As with the stars, so with the planets, which are the satellites ofstars. All investigations unite to tell us that the planets are notall in the same state of development. As some are large and somesmall, so some are, in an evolutionary sense, young, and some old. Asthey depend upon the suns around which they revolve for their light, heat, and other forms of radiant energy, so their condition varieswith their distance from those suns. Many may never arrive at a statesuitable for the maintenance of life upon their surfaces; some whichare not at present in such a state may attain it later; and the formsof life themselves may vary with the peculiar environment thatdifferent planets afford. Thus we see that we are not scientificallyjustified in affirming that life is ubiquitous, although we are thusjustified in saying that it must be, in a general sense, universal. Wemight liken the universe to a garden known to contain every variety ofplant. If on entering it we see no flowers, we examine the speciesbefore us and find that they are not of those which bloom at thisparticular season, or perhaps they are such as never bear flowers. Yetwe feel no doubt that we shall find flowers somewhere in the garden, because there are species which bloom at this season, and the gardencontains all varieties. 

While it is tacitly assumed that there are planets revolving aroundother stars than the sun, it would be impossible for us to see themwith any telescope yet invented, and no instrument now in thepossession of astronomers could assure us of their existence; so theonly planetary system of which we have visual knowledge is our own. Excluding the asteroids, which could not from any point of view beconsidered as habitable, we have in the solar system eight planets ofvarious sizes and situated at various distances from the sun. Of theseeight we know that one, the earth, is inhabited. The question, then, arises: Are there any of the others which are inhabited or habitable?Since it is our intention to discuss the habitability of only one ofthe seven to which the question applies, the rest may be dismissed ina few words. The smallest of them, and the nearest to the sun, isMercury, which is regarded as uninhabitable because it has noperceptible supply of water and air, and because, owing to theextraordinary eccentricity of its orbit, it is subjected to excessiveand very rapid alterations in the amount of solar heat and lightpoured upon its surface, such alterations being inconsistent with thesupposition that it can support living beings. Even its averagetemperature is more than six and a half times that prevailing on theearth! Another circumstance which militates against its habitabilityis that, according to the results of the best telescopic studies, italways keeps the same face toward the sun, so that one half of theplanet is perpetually exposed to the fierce solar rays, and the otherhalf faces the unmitigated cold of open space. Venus, the next indistance from the sun, is almost the exact twin of the earth in size, and many arguments may be urged in favor of its habitability, althoughit is suspected of possessing the same peculiarity as Mercury, inalways keeping the same side sunward. Unfortunately its atmosphereappears to be so dense that no permanent markings on its surface arecertainly visible, and the question of its actual condition must, forthe present, be left in abeyance. Mars, the first planet more distantfrom the sun than the earth, is the special subject of this chapter, and will be described and discussed a few lines further on. Jupiter, Saturn, Uranus, and Neptune, the four giant planets, all more distantthan Mars, and each more distant than the other in the order named, are all regarded as uninhabitable because none of them appears topossess any degree of solidity. They may have solid or liquid nuclei, but exteriorly they seem to be mere balls of cloud. Of course, one canimagine what he pleases about the existence of creatures suited to thephysical constitution of such planets as these, but they must beexcluded from the category of habitable worlds in the ordinary senseof the term. We go back, then, to Mars. 

It will be best to begin with a description of the planet. Mars is4230 miles in diameter; its surface is not much more than one-quarteras extensive as that of the earth (. 285). Its mean distance from thesun is 141, 500, 000 miles, 48, 500, 000 miles greater than that of theearth. Since radiant energy varies inversely as the square ofdistance, Mars receives less than half as much solar light and heat asthe earth gets. Mars' year (period of revolution round the sun) is 687days. Its mean density is 71 per cent of the earth's, and the force ofgravity on its surface is 38 per cent of that on the surface of theearth; i. E. , a body weighing one hundred pounds on the earth would, iftransported to Mars, weigh but thirty-eight pounds. The inclination ofits equator to the plane of its orbit differs very little from that ofthe earth's equator, and its axial rotation occupies 24 hours 37minutes. So that the length of day and night, and the extent of theseasonal changes on Mars, are almost precisely the same as on theearth. But owing to the greater length of its year, the seasons ofMars, while occurring in the same order, are almost twice as long asours. The surface of the planet is manifestly solid, like that of ourglobe, and the telescope reveals many permanent markings on it, recalling the appearance of a globe on which geographical featureshave been represented in reddish and dusky tints. Around the poles areplainly to be seen rounded white areas, which vary in extent with theMartian seasons, nearly vanishing in summer and extending widely inwinter. The most recent spectroscopic determinations indicate thatMars has an atmosphere perhaps as dense as that to be found on ourloftiest mountain peaks, and there is a perceptible amount of wateryvapor in this atmosphere. The surface of the planet appears to beremarkably level, and it has no mountain ranges. No evidences ofvolcanic action have been discovered on Mars. The dusky and reddishareas were regarded by the early observers as respectively seas andlands, but at present it is not believed that there are any bodies ofwater on the planet. There has never been much doubt expressed thatthe white areas about the poles represent snow. 

It will be seen from this brief description that many remarkableresemblances exist between Mars and the earth, and there is nothingwonderful in the fact that the question of the habitability of theformer has become one of extreme and wide-spread interest, giving riseto the most diverse views, to many extraordinary speculations, andsometimes to regrettably heated controversy. The first champion of thehabitability of Mars was Sir William Herschel, although even beforehis time the idea had been suggested. He was convinced by therevelations of his telescopes, continually increasing in power, thatMars was more like the earth than any other planet. He could notresist the testimony of the polar snows, whose suggestive conduct wasin such striking accord with what occurs upon the earth. Gradually, astelescopes improved and observers increased in number, the principalfeatures of the planet were disclosed and charted, and ``areography, ''as the geography of Mars was called, took its place among therecognized branches of astronomical study. But it was not before 1877that a fundamentally new discovery in areography gave a trulysensational turn to speculation about life on ``the red planet. '' Inthat year Mars made one of its nearest approaches to the earth, andwas so situated in its orbit that it could be observed to greatadvantage from the northern hemisphere of the earth. The celebratedItalian astronomer, Schiaparelli, took advantage of this opportunityto make a trigonometrical survey of the surface of Mars -- as coollyand confidently as if he were not taking his sights across athirty-five-million-mile gulf of empty space -- and in the course ofthis survey he was astonished to perceive that the reddish areas, thencalled continents, were crossed in many directions by narrow, duskylines, to which he gave the suggestive name of ``canals. '' Thus a kindof firebrand was cast into the field of astronomical speculation, which has ever since produced disputes that have sometimes approachedthe violence of political faction. At first the accuracy ofSchiaparelli's observations was contested; it required a powerfultelescope, and the most excellent ``seeing, '' to render theenigmatical lines visible at all, and many searchers were unable todetect them. But Schiaparelli continued his studies in the serene skyof Italy, and produced charts of the gridironed face of Marscontaining so much astonishing detail that one had either to rejectthem in toto or to confess that Schiaparelli was right. As subsequentfavorable oppositions of Mars occurred, other observers began to seethe ``canals'' and to confirm the substantial accuracy of the Italianastronomer's work, and finally few were found who would venture toaffirm that the ``canals'' did not exist, whatever their meaning mightbe. 

When Schiaparelli began his observations it was generally believed, aswe have said, that the dusky areas on Mars were seas, and sinceSchiaparelli thought that the ``canals'' invariably began and ended atthe shores of the ``seas, '' the appropriateness of the title given tothe lines seemed apparent. Their artificial character was immediatelyassumed by many, because they were too straight and too suggestivelygeometrical in their arrangement to permit the conclusion that theywere natural watercourses. A most surprising circumstance noted bySchiaparelli was that the ``canals'' made their appearance after themelting of the polar snow in the corresponding hemisphere had begun, and that they grew darker, longer, and more numerous in proportion asthe polar liquidation proceeded; another very puzzling observation wasthat many of them became double as the season advanced; close besidean already existing ``canal, '' and in perfect parallelism with it, another would gradually make its appearance. That these phenomenaactually existed and were not illusions was proved by laterobservations, and today they are seen whenever Mars is favorablysituated for observation. 

In the closing decade of the nineteenth century, Mr Percival Lowelltook up the work where Schiaparelli had virtually dropped it, and soonadded a great number of ``canals'' to those previously known, so thatin his charts the surface of the wonderful little planet appearscovered as with a spider's web, the dusky lines criss-crossing inevery direction, with conspicuous knots wherever a number of them cometogether. Mr Lowell has demonstrated that the areas originally calledseas, and thus named on the earlier charts, are not bodies of water, whatever else they may be. He has also found that the mysterious linesdo not, as Schiaparelli supposed, begin and end at the edges of thedusky regions, but often continue on across them, reaching in somecases far up into the polar regions. But Schiaparelli was right in hisobservation that the appearance of the ``canals'' is synchronous withthe gradual disappearance of the polar snows, and this fact has becomethe basis of the most extraordinary theory that the subject of life inother worlds has ever given birth to. 

Now, the effect of such discoveries, as we have related, depends uponthe type of mind to whose attention they are called. Many are contentto accept them as strange and inexplicable at present, and to wait forfurther light upon them; others insist upon an immediate inquiryconcerning their probable nature and meaning. Such an inquiry can onlybe based upon inference proceeding from analogy. Mars, say Mr Lowelland those who are of his opinion, is manifestly a solidly incrustedplanet like the earth; it has an atmosphere, though one of greatrarity; it has water vapor, as the snows in themselves prove; it hasthe alternation of day and night, and a succession of seasons closelyresembling those of the earth; its surface is suggestively dividedinto regions of contrasting colors and appearance, and upon thatsurface we see an immense number of lines geometrically arranged, witha system of symmetrical intersections where the lines expand intocircular and oval areas -- and all connected with the annual meltingof the polar snows in a way which irresistibly suggests theinterference of intelligence directed to a definite end. Why, with somany concurrent circumstances to support the hypothesis, should we notregard Mars as an inhabited globe?

But the differences between Mars and the earth are in many ways asstriking as their resemblances. Mars is relatively small; it gets lessthan half as much light and heat as we receive; its atmosphere is sorare that it would be distressing to us, even if we could survive init at all; it has no lakes, rivers, or seas; its surface is an endlessprairie. And its ``canals'' are phenomena utterly unlike anything onthe earth. Yet it is precisely upon these divergences between theearth and Mars, this repudiation of terrestrial standards, that thetheory of ``life on Mars, '' for which Mr Lowell is mainly responsible, is based. Because Mars is smaller than the earth, we are told it mustnecessarily be more advanced in planetary evolution, the underlyingcause of which is the gradual cooling and contraction of the planet'smass. Mars has parted with its internal heat more rapidly than theearth; consequently its waters and its atmosphere have been mostlywithdrawn by chemical combinations, but enough of both yet remain torender life still possible on its surface. As the globe of Mars isevolutionally older than that of the earth, so its forms of organiclife may be proportionally further advanced, and its inhabitants mayhave attained a degree of cultivated intelligence much superior towhat at present exists upon the earth. Understanding the nature andthe causes of the desiccation of their planet, and possessingengineering science and capabilities far in advance of ours, they maybe conceived to have grappled with the stupendous problem of keepingtheir world in a habitable condition as long as possible. Supposingthem to have become accustomed to live in their rarefied atmosphere (athing not inconceivable, since men can live for a time at least in airhardly less rare), the most pressing problem for them is that of awater-supply, without which plant life cannot exist, while animal lifein turn depends for its existence upon vegetation. The only directionin which they can seek water is that of the polar regions, where it isalternately condensed into snow and released in the liquid form by theeffect of the seasonal changes. It is, then, to the annual melting ofthe polar snow-fields that the Martian engineers are supposed to haverecourse in supplying the needs of their planet, and thus providingthe means of prolonging their own existence. It is imagined that theyhave for this purpose constructed a stupendous system of irrigationextending over the temperate and equatorial regions of the planet. The``canals'' represent the lines of irrigation, but the narrow streaksthat we see are not the canals themselves, but the irrigated bandscovered by them. Their dark hue, and their gradual appearance afterthe polar melting has begun, are due to the growth of vegetationstimulated by the water. The rounded areas visible where several``canals'' meet and cross are called by Mr Lowell ``oases. '' These aresupposed to be the principal centers of population and industry. Itmust be confessed that some of them, with their complicated systems ofradiating lines, appear to answer very well to such a theory. Noattempt to explain them by analogy with natural phenomena on the earthhas proved successful. 

But a great difficulty yet remains: How to explain the seeminglymiraculous powers of the supposed engineers? Here recourse is had oncemore to the relative smallness of the planet. We have remarked thatthe force of gravity on Mars is only thirty-eight per cent of that onthe earth. A steam-shovel driven by a certain horse-power would benearly three times as effective there as here. A man of our stature onMars would find his effective strength increased in the sameproportion. But just because of the slight force of gravity there, aMartian might attain to the traditional stature of Goliath withoutfinding his own weight an encumbrance to his activity, while at thesame time his huge muscles would come into unimpeded play, enablinghim single-handed to perform labors that would be impossible to awhole gang of terrestrial workmen. The effective powers of hugemachines would be increased in the same way; and to all this must beadded the fact that the mean density of the materials of which Mars iscomposed is much less than that of the constituents of the earth. Combining all these considerations, it becomes much less difficult toconceive that public works might be successfully undertaken on Marswhich would be hopelessly beyond the limits of human accomplishment. 

Certain other difficulties have also to be met; as, for instance, therelative coldness of the climate of Mars. At its distance it getsconsiderably less than half as much light and heat as we receive. Inaddition to this, the rarity of its atmosphere would naturally beexpected to decrease the effective temperature at the planet'ssurface, since an atmosphere acts somewhat like the glass cover of ahot-house in retaining the solar heat which has penetrated it. It hasbeen calculated that, unless there are mitigating circumstances ofwhich we know nothing, the average temperature at the surface of Marsmust be far below the freezing-point of water. To this it is repliedthat the possible mitigating circumstances spoken of evidently existin fact, because we can see that the watery vapor condenses into snowaround the poles in winter, but melts again when summer comes. Themitigating agent may be supposed to exist in the atmosphere where thepresence of certain gases would completely alter the temperaturegradients. 

It might also be objected that it is inconceivable that the Martianengineers, however great may be their physical powers, and howevergigantic the mechanical energies under their control, could forcewater in large quantities from the poles to the equator. This is anachievement that measures up to the cosmical standard. It is admittedby the champions of the theory that the difficulty is a formidableone; but they call attention to the singular fact that on Mars therecan be found no chains of mountains, and it is even doubtful if rangesof hills exist there. The entire surface of the planet appears to bealmost ``as smooth as a billiard ball, '' and even the broad regionswhich were once supposed to be seas apparently lie at practically thesame level as the other parts, since the ``canals'' in many cases rununinterruptedly across them. Lowell's idea is that these sombre areasmay be expanses of vegetation covering ground of a more or less marshycharacter, for while the largest of them appear to be permanent, thereare some which vary coincidently with the variations of the canals. 

As to the kind of machinery employed to force the water from thepoles, it has been conjectured that it may have taken the form of agigantic system of pumps and conduits; and since the Martians areassumed to be so far in advance of us in their mastery of scientificprinciples, the hypothesis will at least not be harmed by supposingthat they have learned to harness forces of nature whose veryexistence in a manageable form is yet unrecognized on the earth. If wewish to let the imagination loose, we may conjecture that they haveconquered the secret of those intra-atomic forces whose resistlessenergy is beginning to become evident to us, but the possibility ofwhose utilization remains a dream, the fulfillment of which nobodydares to predict. 

Such, in very brief form, is the celebrated theory of Mars as aninhabited world. It certainly captivates the imagination, and if webelieve it to represent the facts, we cannot but watch with thedeepest sympathy this gallant struggle of an intellectual race topreserve its planet from the effects of advancing age and death. Wemay, indeed, wonder whether our own humanity, confronted by such acalamity, could be counted on to meet the emergency with equalstoutness of heart and inexhaustibleness of resource. Up to thepresent time we certainly have shown no capacity to confront Naturetoe to toe, and to seize her by the shoulders and turn her round whenshe refuses to go our way. If we could get into wireless telephoniccommunication with the Martians we might learn from their own lips thesecret of their more than ``Roman recovery. ''

The Riddle of the Asteroids

Between the orbits of Mars and Jupiter revolves the most remarkablesystem of little bodies with which we are acquainted -- the Asteroids, or Minor Planets. Some six hundred are now known, and they mayactually number thousands. They form virtually a ring about the sun. The most striking general fact about them is that they occupy theplace in the sky which should be occupied, according to Bode's Law, bya single large planet. This fact, as we shall see, has led to theinvention of one of the most extraordinary theories in astronomy --viz. , that of the explosion of a world!

Bode's Law, so-called, is only an empiric formula, but until thediscovery of Neptune it accorded so well with the distances of theplanets that astronomers were disposed to look upon it as reallyrepresenting some underlying principle of planetary distribution. Theywere puzzled by the absence of a planet in the space between Mars andJupiter, where the ``law'' demanded that there should be one, and anassociation of astronomers was formed to search for it. There was adecided sensation when, in 1801, Piazzi, of Palermo, announced that hehad found a little planet which apparently occupied the place in thesystem which belonged to the missing body. He named it Ceres, and itwas the first of the Asteroids. The next year Olbers, of Bremen, whilelooking for Ceres with his telescope, stumbled upon another smallplanet which he named Pallas. Immediately he was inspired with theidea that these two planets were fragments of a larger one which hadformerly occupied the vacant place in the planetary ranks, and hepredicted that others would be found by searching in the neighborhoodof the intersection of the orbits of the two already discovered. Thisbold prediction was brilliantly fulfilled by the finding of two more-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been ledto the invention of his hypothesis of a planetary explosion by thefaith which astronomers at that time had in Bode's Law. They appear tohave thought that several planets revolving in the gap where the``law'' called for but one could only be accounted for upon the theorythat the original one had been broken up to form the several. Gravitation demanded that the remnants of a planet blown to pieces, nomatter how their orbits might otherwise differ, should all return atstated periods to the point where the explosion had occurred; henceOlbers' prediction that any asteroids that might subsequently bediscovered would be found to have a common point of orbitalintersection. And curiously enough all of the first asteroids foundpractically answered to this requirement. Olbers' theory seemed to beestablished. 

After the first four, no more asteroids were found until 1845, whenone was discovered; then, in 1847, three more were added to the list;and after that searchers began to pick them up with such rapidity thatby the close of the century hundreds were known, and it had becomealmost impossible to keep track of them. The first four are by far thelargest members of the group, but their actual sizes remained unknownuntil less than twenty years ago. It was long supposed that Vesta wasthe largest, because it shines more brightly than any of the others;but finally, in 1895, Barnard, with the Lick telescope, definitelymeasured their diameters, and proved to everybody's surprise thatCeres is really the chief, and Vesta only the third in rank. Hismeasures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, 239 miles; and Juno, 120 miles. They differ greatly in the reflectivepower of their surfaces, a fact of much significance in connectionwith the question of their origin. Vesta is, surface for surface, rather more than three times as brilliant as Ceres, whence theoriginal mistake about its magnitude. 

Nowadays new asteroids are found frequently by photography, butphysically they are most insignificant bodies, their average diameterprobably not exceeding twenty miles, and some are believed not toexceed ten. On a planet only ten miles in diameter, assuming the samemean density as the earth's, which is undoubtedly too much, the forceof gravity would be so slight that an average man would not weigh morethan three ounces, and could jump off into space whenever he liked. 

Although the asteroids all revolve around the sun in the samedirection as that pursued by the major planets, their orbits areinclined at a great variety of angles to the general plane of theplanetary system, and some of them are very eccentric -- almost asmuch so as the orbits of many of the periodic comets. It has even beenconjectured that the two tiny moons of Mars and the four smallersatellites of Jupiter may be asteroids gone astray and captured bythose planets. Two of the asteroids are exceedingly remarkable for theshapes and positions of their orbits; these are Eros, discovered in1898, and T. G. , 1906, found eight years later. The latter has a meandistance from the sun slightly greater than that of Jupiter, while themean distance of Eros is less than that of Mars. The orbit of Eros isso eccentric that at times it approaches within 15, 000, 000 miles ofthe earth, nearer than any other regular member of the solar systemexcept the moon, thus affording an unrivaled means of measuring thesolar parallax. But for our present purpose the chief interest of Eroslies in its extraordinary changes of light. 

These changes, although irregular, have been observed and photographedmany times, and there seems to be no doubt of their reality. Theirsignificance consists in their possible connection with the form ofthe little planet, whose diameter is generally estimated at not morethan twenty miles. Von Oppolzer found, in 1901, that Eros lostthree-fourths of its brilliancy once in every two hours andthirty-eight minutes. Other observers have found slightly differentperiods of variability, but none as long as three hours. The mostinteresting interpretation that has been offered of this phenomenon isthat it is due to a great irregularity of figure, recalling at onceOlbers' hypothesis. According to some, Eros may be double, the twobodies composing it revolving around each other at very closequarters; but a more striking, and it may be said probable, suggestionis that Eros has a form not unlike that of a dumb-bell, or hour-glass, turning rapidly end over end so that the area of illuminated surfacepresented to our eyes continually changes, reaching at certain times aminimum when the amount of light that it reflects toward the earth isreduced to a quarter of its maximum value. Various other bizarreshapes have been ascribed to Eros, such, for instance, as that of aflat stone revolving about one of its longer axes, so that sometimeswe see its face and sometimes its edge. 

All of these explanations proceed upon the assumption that Eros cannothave a simple globular figure like that of a typical planet, a figurewhich is prescribed by the law of gravitation, but that its shape iswhat may be called accidental; in a word, it is a fragment, for itseems impossible to believe that a body formed in interplanetaryspace, either through nebular condensation or through the aggregationof particles drawn together by their mutual attractions, should not bepractically spherical in shape. Nor is Eros the only asteroid thatgives evidence by variations of brilliancy that there is somethingabnormal in its constitution; several others present the samephenomenon in varying degrees. Even Vesta was regarded by Olbers assufficiently variable in its light to warrant the conclusion that itwas an angular mass instead of a globe. Some of the smaller ones showvery notable variations, and all in short periods, of three or fourhours, suggesting that in turning about one of their axes they presenta surface of variable extent toward the sun and the earth. 

The theory which some have preferred -- that the variability of lightis due to the differences of reflective power on different parts ofthe surface -- would, if accepted, be hardly less suggestive of theorigin of these little bodies by the breaking up of a larger one, because the most natural explanation of such differences would seem tobe that they arose from variations in the roughness or smoothness ofthe reflecting surface, which would be characteristic of fragmentarybodies. In the case of a large planet alternating expanses of land andwater, or of vegetation and desert, would produce a notable variationin the amount of reflection, but on bodies of the size of theasteroids neither water nor vegetation could exist, and an atmospherewould be equally impossible. 

One of the strongest objections to Olbers' hypothesis is that only afew of the first asteroids discovered travel in orbits whichmeasurably satisfy the requirement that they should all intersect atthe point where the explosion occurred. To this it was at firstreplied that the perturbations of the asteroidal orbits, by theattractions of the major planets, would soon displace them in such amanner that they would cease to intersect. One of the firstinvestigations undertaken by the late Prof. Simon Newcomb was directedto the solution of this question, and he arrived at the conclusionthat the planetary perturbations could not explain the actualsituation of the asteroidal orbits. But afterward it was pointed outthat the difficulty could be avoided by supposing that not one but aseries of explosions had produced the asteroids as they now are. Afterthe primary disruption the fragments themselves, according to thissuggestion, may have exploded, and then the resulting orbits would beas ``tangled'' as the heart could wish. This has so far rehabilitatedthe explosion theory that it has never been entirely abandoned, andthe evidence which we have just cited of the probably abnormal shapesof Eros and other asteroids has lately given it renewed life. It is asubject that needs a thorough rediscussion. 

We must not fail to mention, however, that there is a rival hypothesiswhich commends itself to many astronomers -- viz. , that the asteroidswere formed out of a relatively scant ring of matter, situated betweenMars and Jupiter and resembling in composition the immensely moremassive rings from which, according to Laplace's hypothesis, theplanets were born. It is held by the supporters of this theory thatthe attraction of the giant Jupiter was sufficient to prevent thesmall, nebulous ring that gave birth to the asteroids from condensinglike the others into a single planet. 

But if we accept the explosion theory, with its corollary that minorexplosions followed the principal one, we have still an unansweredquestion before us: What caused the explosions? The idea of a worldblowing up is too Titanic to be shocking; it rather amuses theimagination than seriously impresses it; in a word, it seemsessentially chimerical. We can by no appeal to experience form amental picture of such an occurrence. Even the moon did not blow upwhen it was wrecked by volcanoes. The explosive nebulæ and new starsare far away in space, and suggest no connection with such acatastrophe as the bursting of a planet into hundreds of pieces. Wecannot conceive of a great globe thousands of miles in diameterresembling a pellet of gunpowder only awaiting the touch of a match tocause its sudden disruption. Somehow the thought of human agencyobtrudes itself in connection with the word ``explosion, '' and wesmile at the idea that giant powder or nitro-glycerine could blow up aplanet. Yet it would only need enough of them to do it. 

After all, we may deceive ourselves in thinking, as we are apt to do, that explosive energies lock themselves up only in small masses ofmatter. There are many causes producing explosions in nature, everyvolcanic eruption manifests the activity of some of them. Think of thegiant power of confined steam; if enough steam could be suddenlygenerated in the center of the earth by a downpour of all the watersof the oceans, what might not the consequences be for our globe? In asmaller globe, and it has never been estimated that the originalasteroid was even as large as the moon, such a catastrophe would, perhaps, be more easily conceivable; but since we are compelled inthis case to assume that there was a series of successive explosions, steam would hardly answer the purpose; it would be more reasonable tosuppose that the cause of the explosion was some kind of chemicalreaction, or something affecting the atoms composing the explodingbody. Here Dr Gustav Le Bon comes to our aid with a most startlingsuggestion, based on his theory of the dissipation of intra-atomicenergy. It will be best to quote him at some length from his book onThe Evolution of Forces. 

``It does not seem at first sight, '' says Doctor Le Bon, very comprehensible that worlds which appear more and more stable as they cool could become so unstable as to afterward dissociate entirely. To explain this phenomenon, we will inquire whether astronomical observations do not allow us to witness this dissociation. 

 We know that the stability of a body in motion, such as a top or a bicycle, ceases to be possible when its velocity of rotation descends below a certain limit. Once this limit is reached it loses its stability and falls to the ground. Prof. J. J. Thomson even interprets radio-activity in this manner, and points out that when the speed of the elements composing the atoms descends below a certain limit they become unstable and tend to lose their equilibria. There would result from this a commencement of dissociation, with diminution of their potential energy and a corresponding increase of their kinetic energy sufficient to launch into space the products of intra-atomic disintegration. 

 It must not be forgotten that the atom being an enormous reservoir of energy is by this very fact comparable with explosive bodies. These last remain inert so long as their internal equilibria are undisturbed. So soon as some cause or other modifies these, they explode and smash everything around them after being themselves broken to pieces. 

 Atoms, therefore, which grow old in consequence of the diminution of a part of their intra-atomic energy gradually lose their stability. A moment, then, arrives when this stability is so weak that the matter disappears by a sort of explosion more or less rapid. The bodies of the radium group offer an image of this phenomenon -- a rather faint image, however, because the atoms of this body have only reached a period of instability when the dissociation is rather slow. It probably precedes another and more rapid period of dissociation capable of producing their final explosion. Bodies such as radium, thorium, etc. , represent, no doubt, a state of old age at which all bodies must some day arrive, and which they already begin to manifest in our universe, since all matter is slightly radio-active. It would suffice for the dissociation to be fairly general and fairly rapid for an explosion to occur in a world where it was manifested. 

 These theoretical considerations find a solid support in the sudden appearances and disappearances of stars. The explosions of a world which produce them reveal to us, perhaps, how the universes perish when they become old. 

 As astronomical observations show the relative frequency of these rapid destructions, we may ask ourselves whether the end of a universe by a sudden explosion after a long period of old age does not represent its most general ending. 

Here, perhaps, it will be well to stop, since, entrancing as thesubject may be, we know very little about it, and Doctor Le Bon'stheory affords a limitless field for the reader's imagination. _________________________________________________________________

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