A CATECHISM OF THE STEAM ENGINE

IN ITS VARIOUS APPLICATIONS TO MINES, MILLS, STEAM NAVIGATION, RAILWAYS, AND AGRICULTURE. 

WITH

PRACTICAL INSTRUCTIONS FOR THE MANUFACTUREAND MANAGEMENT OF ENGINES OF EVERY CLASS. 

BY

JOHN BOURNE, C. E. 

_NEW AND REVISED EDITION. _

[Transcriber's Note: Inconsistencies in chapter headings and numberingof paragraphs and illustrations have been retained in this edition. ]

PREFACE

TO THE FOURTH EDITION. 

For some years past a new edition of this work has been called for, but Iwas unwilling to allow a new edition to go forth with all the originalfaults of the work upon its head, and I have been too much engaged in thepractical construction of steam ships and steam engines to find time forthe thorough revision which I knew the work required. At length, however, Ihave sufficiently disengaged myself from these onerous pursuits toaccomplish this necessary revision; and I now offer the work to the public, with the confidence that it will be found better deserving of the favorableacceptation and high praise it has already received. There are very fewerrors, either of fact or of inference, in the early editions, which I havehad to correct; but there are many omissions which I have had to supply, and faults of arrangement and classification which I have had to rectify. Ihave also had to bring the information, which the work professes to afford, up to the present time, so as to comprehend the latest improvements. 

For the sake of greater distinctness the work is now divided into chapters. Some of these chapters are altogether new, and the rest have received suchextensive additions and improvements as to make the book almost a new one. One purpose of my emendations has been to render my remarks intelligible toa tyro, as well as instructive to an advanced student. With this view, Ihave devoted the first chapter to a popular description of the SteamEngine--which all may understand who can understand anything--and in thesubsequent gradations of progress I have been careful to set no objectbefore the reader for the first time, of which the nature and functions arenot simultaneously explained. The design I have proposed to myself, in thecomposition of this work, is to take a young lad who knows nothing of steamengines, and to lead him by easy advances up to the highest point ofinformation I have myself attained; and it has been a pleasing duty to meto smooth for others the path which I myself found so rugged, and toimpart, for the general good of mankind, the secrets which others haveguarded with so much jealousy. I believe I am the first author who hascommunicated that practical information respecting the steam engine, whichpersons proposing to follow the business of an engineer desire to possess. My business has, therefore, been the rough business of a pioneer; and whilehewing a road through the trackless forest, along which all might hereaftertravel with ease, I had no time to attend to those minute graces ofcomposition and petty perfection of arrangement and collocation, which arethe attribute of the academic grove, or the literary parterre. I am, nevertheless, not insensible to the advantages of method and cleararrangement in any work professing to instruct mankind in the principlesand practice of any art; and many of the changes introduced into thepresent edition of this work are designed to render it less exceptionablein this respect. The woodcuts now introduced into the work for the firsttime will, I believe, much increase its interest and utility; and upon thewhole I am content to dismiss it into circulation, in the belief that thosewho peruse it attentively will obtain a more rapid and more practicalacquaintance with the steam engine in its various applications, than theywould be likely otherwise to acquire. 

I have only to add that I have prepared a sequel to the present work, inthe shape of a Hand-Book of the Steam Engine, containing the whole of therules given in the present work, illustrated by examples worked out atlength, and also containing such useful tables and other data, as theengineer requires to refer to constantly in the course of his practice. This work may be bound up with the "Catechism, " if desired, to which it isin fact a Key. 

I shall thankfully receive from engineers, either abroad or at home, accounts of any engines or other machinery, with which they may becomefamiliar in their several localities; and I shall be happy, in my turn, toanswer any inquiries on engineering subjects which fall within the compassof my information. If young engineers meet with any difficulty in theirstudies, I shall be happy to resolve it if I can; and they may communicatewith me upon any such point without hesitation, in whatever quarter of theworld they may happen to be. 

JOHN BOURNE. 9 BILLITER STREET, LONDON, _March 1st, 1856_. 

PREFACE

TO THE FIFTH EDITION. 

The last edition of the present work, consisting of 3, 500 copies, havingbeen all sold off in about ten months, I now issue another edition, thedemand for the work being still unabated. It affords, certainly, somepresumption that a work in some measure supplies an ascertained want, when, though addressing only a limited circle--discoursing only of technicalquestions, and without any accident to stimulate it into notoriety, --itattains so large a circulation as the present work has reached. Besidesbeing reprinted in America, it has been translated into German, French, Dutch, and I believe, into some other languages, so that there is, perhaps, not too much vanity in the inference that it has been found serviceable tothose perusing it. I can with truth say, that the hope of rendering someservice to mankind, in my day and generation, has been my chief inducementin writing it, and if this end is fulfilled, I have nothing further todesire. 

I regret that circumstances have prevented me from yet issuing the"Hand-Book" which I have had for some time in preparation, and to which, inmy Preface of the last year, I referred. I hope to have sufficient leisureshortly, to give that and some other of my literary designs the necessaryattention. Whatever may have been the other impediments to a more prolificauthorship, certainly one of them has not been the coldness of theapprobation with which my efforts have been received, since my pastperformances seem to me to have met with an appreciation far exceedingtheir deserts. 

JOHN BOURNE. 

_February 2d, 1857_. 

PUBLISHERS' NOTICE. 

In offering to the American public a reprint of a work on the Steam Engineso deservedly successful, and so long considered standard, the publishershave not thought it necessary that it should be an exact copy of theEnglish edition; there were some details in which they thought it could beimproved, and better adapted to the use of American engineers. On thisaccount, the size of the page has been increased to a full 12mo, to admitof larger illustrations, which in the English edition are often on toosmall a scale; and some of the illustrations themselves have been suppliedby others equally applicable, more recent, and to us more familiarexamples. The first part of Chapter XI, devoted in the English edition toEnglish portable and fixed agricultural engines, in this edition givesplace entirely to illustrations from American practice, of steam engines asapplied to different purposes, and of appliances and machines necessary tothem. But with the exception of some of the illustrations and thedescription of them, and the correction of a few typographical errors, thisedition is a faithful transcript of the latest English edition. 

CONTENTS. 

Classification of Engines. 

Nature and uses of a Vacuum. 

Velocity of falling Bodies and Momentum of moving Bodies. 

Central Forces. 

Centres of Gravity, Gyration, and Oscillation. 

The Pendulum and Governor. 

The Mechanical Powers. 

Friction. 

Strength of materials and Strains subsisting in Machines. 

CHAP. I. --GENERAL DESCRIPTION OF THE STEAM ENGINE. 

The Boiler. 

The Engine. 

The Marine Engine. 

Screw Engines. 

The Locomotive Engine. 

CHAP. II. --HEAT, COMBUSTION, AND STEAM. 

Heat. 

Combustion. 

Steam. 

CHAP. III. --EXPANSION OF STEAM AND ACTION OF THE VALVES. 

CHAP. IV. --MODES OF ESTIMATING THE POWER AND PERFORMANCE OFENGINES AND BOILERS. 

Horses Power. 

Duty of Engines and Boilers. 

The Indicator. 

Dynamometer, Gauges, and Cataract. 

CHAP. V. --PROPORTIONS OF BOILERS. 

Heating and Fire Grate Surface. 

Calorimeter and Vent. 

Evaporative Power of Boilers. 

Modern Marine and Locomotive Boilers. 

The Blast in Locomotives. 

Boiler Chimneys. 

Steam Room and Priming. 

Strength of Boilers. 

Boiler Explosions. 

CHAP. VI. --PROPORTIONS OF ENGINES. 

Steam Passages. 

Air Pump, Condenser, and Hot and Cold Water Pumps. 

Fly Wheel. 

Strengths of Land Engines. 

Strengths of Marine and Locomotive Engines. 

CHAP. VII. --CONSTRUCTIVE DETAILS OF BOILERS. 

Land and Marine Boilers. 

Incrustation and Corrosion of Boilers. 

Locomotive Boilers. 

CHAP. VIII. --CONSTRUCTIVE DETAILS OF ENGINES. 

Pumping Engines. 

Various forms of Marine Engines. 

Cylinders, Pistons, and Valves. 

Air Pump and Condenser. 

Pumps, Cocks, and Pipes. 

Details of the Screw and Screw Shaft. 

Details of the Paddles and Paddle Shaft. 

The Locomotive Engine. 

CHAP. IX. --STEAM NAVIGATION. 

Resistance of Vessels in Water. 

Experiments on the Resistance of Vessels. 

Influence of the size of Vessels upon their Speed. 

Structure and Operation of Paddle Wheels. 

Configuration and Action of the Screw. 

Comparative Advantages of Paddle and Screw Vessels. 

Comparative Advantages of different kinds of Screws. 

Proportions of Screws. 

Screw Vessels with full and auxiliary Power. 

Screw and Paddles combined. 

CHAP. X. --EXAMPLES OF ENGINES OF RECENT CONSTRUCTION. 

Oscillating Paddle Engines. 

Direct acting Screw Engine. 

Locomotive Engine. 

CHAP. XI. --ON VARIOUS FORMS AND APPLICATIONS OF THE STEAM ENGINE. 

Governor. 

Donkey Pumps. 

Portable Steam Engines. 

Stationary Engines. 

Steam Fire Engines. 

Steam Excavator. 

CHAP. XII. --MANUFACTURE AND MANAGEMENT OF STEAM ENGINES. 

Construction of Engines. 

Erection of Engines. 

Management of Marine Boilers. 

Management of Marine Engines. 

Management of Locomotives. 

MECHANICAL PRINCIPLES OF THE STEAM ENGINE. 

CLASSIFICATION OF ENGINES. 

1. _Q. _--What is meant by a vacuum?

_A. _--A vacuum means an empty space; a space in which there is neitherwater nor air, nor anything else that we know of. 

2. _Q. _--Wherein does a high pressure differ from a low pressure engine?

_A. _--In a high pressure engine the steam, after having pushed the pistonto the end of the stroke, escapes into the atmosphere, and the impellingforce is therefore that due to the difference between the pressure of thesteam and the pressure of the atmosphere. In the condensing engine thesteam, after having pressed the piston to the end of the stroke, passesinto the condenser, in which a vacuum is maintained, and the impellingforce is that due to the difference between the pressure of the steam abovethe piston, and the pressure of the vacuum beneath it, which is nothing;or, in other words, you have then the whole pressure of the steam urgingthe piston, consisting of the pressure shown by the safety-valve on theboiler, and the pressure of the atmosphere besides. 

3. _Q. _--In what way would you class the various kinds of condensingengines?

_A. _--Into single acting, rotative, and rotatory engines. Single actingengines are engines without a crank, such as are used for pumping water. Rotative engines are engines provided with a crank, by means of which arotative motion is produced; and in this important class stand marine andmill engines, and all engines, indeed, in which the rectilinear motion ofthe piston is changed into a circular motion. In rotatory engines the steamacts at once in the production of circular motion, either upon a revolvingpiston or otherwise, but without the use of any intermediate mechanism, such as the crank, for deriving a circular from a rectilinear motion. Rotatory engines have not hitherto been very successful, so that only thesingle acting or pumping engine, and the double acting or rotative enginecan be said to be in actual use. For some purposes, such, for example, asforcing air into furnaces for smelting iron, double acting engines areemployed, which are nevertheless unfurnished with a crank; but engines ofthis kind are not sufficiently numerous to justify their classification asa distinct species, and, in general, those engines may be considered to besingle acting, by which no rotatory motion is imparted. 

4. _Q. _--Is not the circular motion derived from a cylinder engine veryirregular, in consequence of the unequal leverage of the crank at thedifferent parts of its revolution?

_A. _--No; rotative engines are generally provided with a fly-wheel tocorrect such irregularities by its momentum; but where two engines withtheir respective cranks set at right angles are employed, the irregularityof one engine corrects that of the other with sufficient exactitude formany purposes. In the case of marine and locomotive engines, a fly-wheel isnot employed; but for cotton spinning, and other purposes requiring greatregularity of motion, its use with common engines is indispensable, thoughit is not impossible to supersede the necessity by new contrivances. 

5. _Q. _--You implied that there is some other difference between singleacting and double acting engines, than that which lies in the use orexclusion of the crank?

_A. _--Yes; single acting engines act only in one way by the force of thesteam, and are returned by a counter-weight; whereas double acting enginesare urged by the steam in both directions. Engines, as I have already said, are sometimes made double acting, though unprovided with a crank; and therewould be no difficulty in so arranging the valves of all ordinary pumpingengines, as to admit of this action; for the pumps might be contrived toraise water both by the upward and downward stroke, as indeed in some minesis already done. But engines without a crank are almost always made singleacting, perhaps from the effect of custom, as much as from any otherreason, and are usually spoken of as such, though it is necessary to knowthat there are some deviations from the usual practice. 

NATURE AND USES OF A VACUUM. 

6. _Q. _--The pressure of a vacuum you have stated is nothing; but how canthe pressure of a vacuum be said to be nothing, when a vacuum occasions apressure of 15 lbs. On the square inch?

_A. _--Because it is not the vacuum which exerts this pressure, but theatmosphere, which, like a head of water, presses on everything immergedbeneath it. A head of water, however, would not press down a piston, if thewater were admitted on both of its sides; for an equilibrium would then beestablished, just as in the case of a balance which retains its equilibriumwhen an equal weight is added to each scale; but take the weight out of onescale, or empty the water from one side of the piston, and motion orpressure is produced; and in like manner pressure is produced on a pistonby admitting steam or air upon the one side, and withdrawing the steam orair from the other side. It is not, therefore, to a vacuum, but rather tothe existence of an unbalanced plenum, that the pressure made manifest byexhaustion is due, and it is obvious therefore that a vacuum of itselfwould not work an engine. 

7. _Q. _--How is the vacuum maintained in a condensing engine?

_A. _--The steam, after having performed its office in the cylinder, ispermitted to pass into a vessel called the condenser, where a shower ofcold water is discharged upon it. The steam is condensed by the cold water, and falls in the form of hot water to the bottom of the condenser. Thewater, which would else be accumulated in the condenser, is continuallybeing pumped out by a pump worked by the engine. This pump is called theair pump, because it also discharges any air which may have entered withthe water. 

8. _Q. _--If a vacuum be an empty space, and there be water in thecondenser, how can there be a vacuum there?

_A. _--There is a vacuum above the water, the water being only like so muchiron or lead lying at the bottom. 

9. _Q. _--Is the vacuum in the condenser a perfect vacuum?

_A. _--Not quite perfect; for the cold water entering for the purpose ofcondensation is heated by the steam, and emits a vapor of a tensionrepresented by about three inches of mercury; that is, when the commonbarometer stands at 30 inches, a barometer with the space above the mercurycommunicating with the condenser, will stand at about 27 inches. 

10. _Q. _--Is this imperfection of the vacuum wholly attributable to thevapor in the condenser?

_A. _--No; it is partly attributable to the presence of a small quantity ofair which enters with the water, and which would accumulate until itdestroyed the vacuum altogether but for the action of the air pump, whichexpels it with the water, as already explained. All common water contains acertain quantity of air in solution, and this air recovers its elasticitywhen the pressure of the atmosphere is taken off, just as the gas in sodawater flies up so soon as the cork of the bottle is withdrawn. 

11. _Q. _--Is a barometer sometimes applied to the condensers of steamengines?

_A. _--Yes; and it is called the vacuum gauge, because it shows the degreeof perfection the vacuum has attained. Another gauge, called the steamgauge, is applied to the boiler, which indicates the pressure of the steamby the height to which the steam forces mercury up a tube. Gauges are alsoapplied to the boiler to indicate the height of the water within it so thatit may not be burned out by the water becoming accidentally too low. Insome cases a succession of cocks placed a short distance above one anotherare employed for this purpose, and in other cases a glass tube is placedperpendicularly in the front of the boiler and communicating at each endwith its interior. The water rises in this tube to the same height as inthe boiler itself, and thus shows the actual water level. In most of themodern boilers both of these contrivances are adopted. 

12. _Q. _--Can a condensing engine be worked with a pressure less than thatof the atmosphere?

_A. _--Yes, if once it be started; but it will be a difficult thing to startan engine, if the pressure of the steam be not greater than that of theatmosphere. Before an engine can be started, it has to be blown throughwith steam to displace the air within it, and this cannot be effectuallydone if the pressure of the steam be very low. After the engine is started, however, the pressure in the boiler may be lowered, if the engine belightly loaded, until there is a partial vacuum in the boiler. Such apractice, however, is not to be commended, as the gauge cocks becomeuseless when there is a partial vacuum in the boiler; inasmuch as, whenthey are opened, the water will not rush out, but air will rush in. It isimpossible, also, under such circumstances, to blow out any of the sedimentcollected within the boiler, which, in the case of the boilers of steamvessels, requires to be done every two hours or oftener. This isaccomplished by opening a large cock which permits some of the supersaltedwater to be forced overboard by the pressure of the steam. In some cases, in which the boiler applied to an engine is of inadequate size, thepressure within the boiler will fall spontaneously to a point considerablybeneath the pressure of the atmosphere; but it is preferable, in suchcases, partially to close the throttle valve in the steam pipe, whereby theissue of steam to the engine is diminished; and the pressure in the boileris thus maintained, while the cylinder receives its former supply. 

13. _Q. _--If a hole be opened into a condenser of a steam engine, will airrush into it?

_A. _--If the hole communicates with the atmosphere, the air will be drawnin. 

14. _Q. _--With what Velocity does air rush into a vacuum?

_A. _--With the velocity which a body would acquire by falling from theheight of a homogeneous atmosphere, which is an atmosphere of the samedensity throughout as at the earth's surface; and although such anatmosphere does not exist in nature, its existence is supposed, in order tofacilitate the computation. It is well known that the velocity with whichwater issues from a cistern is the same that would be acquired by a bodyfalling from the level of the head to the level of the issuing point; whichindeed is an obvious law, since every particle of water descends and issuesby virtue of its gravity, and is in its descent subject to the ordinarylaws of falling bodies. Air rushing into a vacuum is only another exampleof the same general principle: the velocity of each particle will be thatdue to the height of the column of air which would produce the pressuresustained; and the weight of air being known, as well as the pressure itexerts on the earth's surface, it becomes easy to tell what height a columnof air, an inch square, and of the atmospheric density, would require tobe, to weigh 15 lbs. The height would be 27, 818 feet, and the velocitywhich the fall of a body from such a height produces would be 1, 338 feetper second. 

VELOCITY OF FALLING BODIES AND MOMENTUM OF MOVING BODIES. 

15. _Q. _--How do you determine the velocity of falling bodies of differentkinds?

_A. _--All bodies fall with the same velocity, when there is no resistancefrom the atmosphere, as is shown by the experiment of letting fall, fromthe top of a tall exhausted receiver, a feather and a guinea, which reachthe bottom at the same time. The velocity of falling bodies is one that isaccelerated uniformly, according to a known law. When the height from whicha body falls is given, the velocity acquired at the end of the descent canbe easily computed. It has been found by experiment that the square root ofthe height in feet multiplied by 8. 021 will give the velocity. 

16. _Q. _--But the velocity in what terms?

_A. _--In feet per second. The distance through which a body falls bygravity in one second is 16-1/12 feet; in two seconds, 64-4/12 feet; inthree seconds, 144-9/12 feet; in four seconds, 257-4/12 feet, and so on. Ifthe number of feet fallen through in one second be taken as unity, then therelation of the times to the spaces will be as follows:--

Number of seconds | 1| 2| 3| 4| 5| 6|Units of space passed through | 1| 4| 9|16|25|36| &c. 

so that it appears that the spaces passed through by a falling body are asthe squares of the times of falling. 

17. _Q. _--Is not the urging force which causes bodies to fall the force ofgravity?

_A. _--Yes; the force of gravity or the attraction of the earth. 

18. _Q. _--And is not that a uniform force, or a force acting with a uniformpressure?

_A. _--It is. 

19. _Q. _--Therefore during the first second of falling as much impellingpower will be given by the force of gravity as during every succeedingsecond?

_A. _--Undoubtedly. 

20. _Q. _--How comes it, then, that while the body falls 64-4/12 feet in twoseconds, it falls only 16-1/12 feet in one second; or why, since it fallsonly 16-1/12 feet in one second, should it fall more than twice 16-1/12feet in two?

_A. _--Because 16-1/12 feet is the average and not the maximum velocityduring the first second. The velocity acquired _at the end_ of the 1stsecond is not 16-1/12, but 32-1/6 feet per second, and at the end of the 2dsecond a velocity of 32-1/6 feet has to be added; so that the totalvelocity at the end of the 2d second becomes 64-2/6 feet; at the end of the3d, the velocity becomes 96-3/6 feet, at the end of the 4th, 128-4/6 feet, and so on. These numbers proceed in the progression 1, 2, 3, 4, &c. , sothat it appears that the velocities acquired by a falling body at differentpoints, are simply as the times of falling. But if the velocities be as thetimes, and the total space passed through be as the squares of the times, then the total space passed through must be as the squares of the velocity;and as the _vis viva_ or mechanical power inherent in a falling body, ofany given weight, is measurable by the height through which it descends, itfollows that the _vis viva_ is proportionate to the square of the velocity. Of two balls therefore, of equal weight, but one moving twice as fast asthe other, the faster ball has four times the energy or mechanical forceaccumulated in it that the slower ball has. If the speed of a fly-wheel bedoubled, it has four times the _vis viva_ it possessed before--_vis viva_being measurable by a reference to the height through which a body musthave fallen, to acquire the velocity given. 

21. _Q. _--By what considerations is the _vis viva_ or mechanical energyproper for the fly-wheel of an engine determined?

_A. _--By a reference to the power produced every half-stroke of the engine, joined to the consideration of what relation the energy of the fly-wheelrim must have thereto, to keep the irregularities of motion within thelimits which are admissible. It is found in practice, that when the powerresident in the fly-wheel rim, when the engine moves at its average speed, is from two and a half to four times greater than the power generated bythe engine in one half-stroke--the variation, depending on the energyinherent in the machinery the engine has to drive and the equability ofmotion required--the engine will work with sufficient regularity for mostordinary purposes, but where great equability of motion is required, itwill be advisable to make the power resident in the fly-wheel equal to sixtimes the power generated by the engine in one half-stroke. 

22. _Q. _---Can you give a practical rule for determining the properquantity of cast iron for the rim of a fly-wheel in ordinary land engines?

_A. _--One rule frequently adopted is as follows:--Multiply the meandiameter of the rim by the number of its revolutions per minute, and squarethe product for a divisor; divide the number of actual horse power of theengine by the number of strokes the piston makes per minute, multiply thequotient by the constant number 2, 760, 000, and divide the product by thedivisor found as above; the quotient is the requisite quantity of cast ironin cubic feet to form the fly-wheel rim. 

23. _Q. _--What is Boulton and Watt's rule for finding the dimensions of thefly-wheel?

_A. _--Boulton and Watt's rule for finding the dimensions of the fly-wheelis as follows:--Multiply 44, 000 times the length of the stroke in feet bythe square of the diameter of the cylinder in inches, and divide theproduct by the square of the number of revolutions per minute multiplied bythe cube of the diameter of the fly-wheel in feet. The resulting numberwill be the sectional area of the rim of the fly-wheel in square inches. 

CENTRAL FORCES. 

24. _Q. _--What do you understand by centrifugal and centripetal forces?

_A. _--By centrifugal force, I understand the force with which a revolvingbody tends to fly from the centre; and by centripetal force, I understandany force which draws it to the centre, or counteracts the centrifugaltendency. In the conical pendulum, or steam engine governor, which consistsof two metal balls suspended on rods hung from the end of a verticalrevolving shaft, the centrifugal force is manifested by the divergence ofthe balls, when the shaft is put into revolution; and the centripetalforce, which in this instance is gravity, predominates so soon as thevelocity is arrested; for the arms then collapse and hang by the side ofthe shaft. 

25. _Q. _--What measures are there of the centrifugal force of bodiesrevolving in a circle?

_A. _--The centrifugal force of bodies revolving in a circle increases asthe diameter of the circle, if the number of revolutions remain the same. If there be two fly-wheels of the same weight, and making the same numberof revolutions per minute, but the diameter of one be double that of theother, the larger will have double the amount of centrifugal force. Thecentrifugal force of the _same wheel_, however, increases as the square ofthe velocity; so that if the velocity of a fly-wheel be doubled, it willhave four times the amount of centrifugal force. 

26. _Q. _--Can you give a rule for determining the centrifugal force of abody of a given weight moving with a given velocity in a circle of a givendiameter?

_A. _--Yes. If the velocity in feet per second be divided by 4. 01, thesquare of the quotient will be four times the height in feet from which abody must have fallen to have acquired that velocity. Divide this quadrupleheight by the diameter of the circle, and the quotient is the centrifugalforce in terms of the weight of the body, so that, multiplying the quotientby the actual weight of the body, we have the centrifugal force in poundsor tons. Another rule is to multiply the square of the number ofrevolutions per minute by the diameter of the circle in feet, and to dividethe product by 5, 870. The quotient is the centrifugal force in terms of theweight of the body. 

27. _Q. _--How do you find the velocity of the body when its centrifugalforce and the diameter of the circle in which it moves are given?

_A. _--Multiply the centrifugal force in terms of the weight of the body bythe diameter of the circle in feet, and multiply the square root of theproduct by 4. 01; the result will be the velocity of the body in feet persecond. 

28. _Q. _--Will you illustrate this by finding the velocity at which thecast iron rim of a fly-wheel 10 feet in diameter would burst asunder by itscentrifugal force?

_A. _--If we take the tensile strength of cast iron at 15, 000 lbs. Persquare inch, a fly-wheel rim of one square inch of sectional area wouldsustain 30, 000 lbs. If we suppose one half of the rim to be so fixed to theshaft as to be incapable of detachment, then the centrifugal force of theother half of the rim at the moment of rupture must be equal to 30, 000 lbs. Now 30, 000 lbs. Divided by 49. 48 (the weight of the half rim) is equal to606. 3, which is the centrifugal force in terms of the weight. Then by therule given in the last answer 606. 3 x 10 = 6063, the square root of whichis 78 nearly, and 78 x 4. 01 = 312. 78, the velocity of the rim in feet persecond at the moment of rupture. 

29. _Q. _--What is the greatest velocity at which it is safe to drive a castiron fly-wheel?

_A. _--If we take 2, 000 lbs. As the utmost strain per square inch to whichcast iron can be permanently subjected with safety; then, by a similarprocess to that just explained, we have 4, 000 lbs. /49. 48 = 80. 8 whichmultiplied by 10 = 808, the square root of which is 28. 4, and 28. 4 x 4. 01 =113. 884, the velocity of the rim in feet per second, which may beconsidered as the highest consistent with safety. Indeed, this limit shouldnot be approached in practice on account of the risks of fracture fromweakness or imperfections in the metal. 

30. _Q. _--What is the velocity at which the wheels of railway trains mayrun if we take 4, 000 lbs. Per square inch as the greatest strain to whichmalleable iron should be subjected?

_A. _--The weight of a malleable iron rim of one square inch sectional areaand 7 feet diameter is 21. 991 feet x 3. 4 lbs. = 74. 76, one half of which is37. 4 lbs. Then by the same process as before, 8, 000/37. 4 = 213. 9, thecentrifugal force in terms of the weight: 213. 9 x 7, the diameter of thewheel = 1497. 3, the square root of which, 38. 3 x 4. 01 = 155. 187 feet persecond, the highest velocity of the rims of railway carriage wheels that isconsistent with safety. 155. 187 feet per second is equivalent to 105. 8miles an hour. As 4, 000 lbs. Per square inch of sectional area is theutmost strain to which iron should be exposed in machinery, railway wheelscan scarcely be considered safe at speed even considerably under 100 milesan hour, unless so constructed that the centrifugal force of the rim willbe counteracted, to a material extent, by the centripetal action of thearms. Hooped wheels are very unsafe, unless the hoops are, by some processor other, firmly attached to the arms. It is of no use to increase thedimensions of the rim of a wheel with the view of giving increased strengthto counteract the centrifugal force, as every increase in the weight of therim will increase the centrifugal force in the same proportion. 

CENTRES OF GRAVITY, GYRATION, AND OSCILLATION. 

31. _Q. _--What do you understand by the centre of gravity of a body?

_A. _--That point within it, in which the whole of the weight may besupposed to be concentrated, and which continually endeavors to gain thelowest possible position. A body hung in the centre of gravity will remainat rest in any position. 

32. _Q. _--What is meant by the centre of gyration?

_A. _--The centre of gyration is that point in a revolving body in which thewhole momentum may be conceived to be concentrated, or in which the wholeeffect of the momentum resides. If the ball of a governor were to be movedin a straight line, the momentum might be said to be concentrated at thecentre of gravity of the ball; but inasmuch as, by its revolution round anaxis, the part of the ball furthest removed from the axis moves morequickly than the part nearest to it, the momentum cannot be supposed to beconcentrated at the centre of gravity, but at a point further removed fromthe central shaft, and that point is what is called the centre of gyration. 

33. _Q. _--What is the centre of oscillation?

_A. _--The centre of oscillation is a point in a pendulum or any swingingbody, such, that if all the matter of the body were to be collected intothat point, the velocity of its vibration would remain unaffected. It is infact the mean distance from the centre of suspension of every atom, in aratio which happens not to be an arithmetical one. The centre ofoscillation is always in a line passing through the centre of suspensionand the centre of gravity. 

THE PENDULUM AND GOVERNOR. 

34. _Q. _--By what circumstance is the velocity of vibration of a pendulousbody determined?

_A. _--By the length of the suspending rod only, or, more correctly, by thedistance between the centre of suspension and the centre of oscillation. The length of the arc described does not signify, as the times of vibrationwill be the same, whether the arc be the fourth or the four hundredth of acircle, or at least they will be nearly so, and would be so exactly, if thecurve described were a portion of a cycloid. In the pendulum of clocks, therefore, a small arc is preferred, as there is, in that case, no sensibledeviation from the cycloidal curve, but in other respects the size of thearc does not signify. 

35. _Q. _--If then the length of a pendulum be given, can the number ofvibrations in a given time be determined?

_A. _--Yes; the time of vibration bears the same relation to the time inwhich a body would fall through a space equal to half the length of thependulum, that the circumference of a circle bears to its diameter. Thenumber of vibrations made in a given time by pendulums of differentlengths, is inversely as the square roots of their lengths. 

36. _Q. _--Then when the length of the second's pendulum is known the properlength of a pendulum to make any given number of vibrations in the minutecan readily be computed?

_A. _--Yes; the length of the second's pendulum being known, the length ofanother pendulum, required to perform any given number of vibrations in theminute, may be obtained by the following rule: multiply the square root ofthe given length by 60, and divide the product by the given number ofvibrations per minute; the square of the quotient is the length of pendulumrequired. Thus if the length of a pendulum were required that would make 70vibrations per minute in the latitude of London, then SQRT(39. 1393) x 60/70= (5. 363)^2 = 28. 75 in. Which is the length required. 

37. _Q. _--Can you explain how it comes that the length of a pendulumdetermines the number of vibrations it makes in a given time?

_A. _--Because the length of the pendulum determines the steepness of thecircle in which the body moves, and it is obvious, that a body will descendmore rapidly over a steep inclined plane, or a steep arc of a circle, thanover one in which there is but a slight inclination. The impelling force isgravity, which urges the body with a force proportionate to the distancedescended, and if the velocity due to the descent of a body through a givenheight be spread over a great horizontal distance, the speed of the bodymust be slow in proportion to the greatness of that distance. It is clear, therefore, that as the length of the pendulum determines the steepness ofthe arc, it must also determine the velocity of vibration. 

38. _Q. _--If the motions of a pendulum be dependent on the speed with whicha body falls, then a certain ratio must subsist between the distancethrough which a body falls in a second, and the length of the second'spendulum?

_A. _--And so there is; the length of the second's pendulum at the level ofthe sea in London, is 39. 1393 inches, and it is from the length of thesecond's pendulum that the space through which a body falls in a second hasbeen determined. As the time in which a pendulum vibrates is to the time inwhich a heavy body falls through half the length of the pendulum, as thecircumference of a circle is to its diameter, and as the height throughwhich a body falls is as the square of the time of falling, it is clearthat the height through which a body will fall, during the vibration of apendulum, is to half the length of the pendulum as the square of thecircumference of a circle is to the square of its diameter; namely, as9. 8696 is to 1, or it is to the whole length of the pendulum as the half ofthis, namely, 4. 9348 is to 1; and 4. 9348 times 39. 1393 in. Is 16-1/12 ft. Very nearly, which is the space through which a body falls by gravity in asecond. 

39. _Q. _--Are the motions of the conical pendulum or governor reducible tothe same laws which apply to the common pendulum?

_A. _--Yes; the motion of the conical pendulum may be supposed to becompounded of the motions of two common pendulums, vibrating at rightangles to one another, and one revolution of a conical pendulum will beperformed in the same time as two vibrations of a common pendulum, of whichthe length is equal to the vertical height of the point of suspension abovethe plane of revolution of the balls. 

40. _Q. _--Is not the conical pendulum or governor of a steam engine drivenby the engine?

_A. _--Yes. 

41. _Q. _--Then will it not be driven round as any other mechanism would beat a speed proportional to that of the engine?

_A. _--It will. 

42. _Q. _--Then how can the length of the arms affect the time ofrevolution?

[Illustration: Fig. 1. ]

_A. _--By flying out until they assume a vertical height answering to thevelocity with which they rotate round the central axis. As the speed isincreased the balls expand, and the height of the cone described by thearms is diminished, until its vertical height is such that a pendulum ofthat length would perform two vibrations for every revolution of thegovernor. By the outward motion of the arms, they partially shut off thesteam from the engine. If, therefore, a certain expansion of the balls bedesired, and a certain length be fixed upon for the arms, so that thevertical height of the cone is fixed, then the speed of the governor mustbe such, that it will make half the number of revolutions in a given timethat a pendulum equal in length to the height of the cone would make ofvibrations. The rule is, multiply the square root of the height of the conein inches by 0. 31986, and the product will be the right time of revolutionin seconds. If the number of revolutions and the length of the arms befixed, and it is wanted to know what is the diameter of the circledescribed by the balls, you must divide the constant number 187. 58 by thenumber of revolutions per minute, and the square of the quotient will bethe vertical height in inches of the centre of suspension above the planeof the balls' revolution. Deduct the square of the vertical height ininches from the square of the length of the arm in inches, and twice thesquare root of the remainder is the diameter of the circle in which thecentres of the balls revolve. 

43. _Q. _ Cannot the operation of a governor be deduced merely from theconsideration of centrifugal and centripetal forces?

_A. _--It can; and by a very simple process. The horizontal distance of thearm from the spindle divided by the vertical height, will give the amountof centripetal force, and the velocity of revolution requisite to producean equivalent centrifugal force may be found by multiplying the centripetalforce of the ball in terms of its own weight by 70, 440, and dividing theproduct by the diameter of the circle made by the centre of the ball ininches; the square root of the quotient is the number of revolutions perminute. By this rule you fix the length of the arms, and the diameter ofthe base of the cone, or, what is the same thing, the angle at which it isdesired the arms shall revolve, and you then make the speed or number ofrevolutions such, that the centrifugal force will keep the balls in thedesired position. 

44. _Q. _--Does not the weight of the balls affect the question?

_A. _--Not in the least; each ball may be supposed to be made up of a numberof small balls or particles, and each particle of matter will act foritself. Heavy balls attached to a governor are only requisite to overcomethe friction of the throttle valve which shuts off the steam, and of theconnections leading thereto. Though the weight of a ball increases itscentripetal force, it increases its centrifugal force in the sameproportion. 

THE MECHANICAL POWERS. 

45. _Q. _--What do you understand by the mechanical powers?

_A. _--The mechanical powers are certain contrivances, such as the wedge, the screw, the inclined plane, and other elementary machines, which converta small force acting through a great space into a great force actingthrough a small space. In the school treatises on mechanics, a certainnumber of these devices are set forth as the mechanical powers, and eachseparate device is treated as if it involved a separate principle; but nota tithe of the contrivances which accomplish the stipulated end arerepresented in these learned works, and there is no very obvious necessityfor considering the principle of each contrivance separately when theprinciples of all are one and the same. Every pressure acting with acertain velocity, or through a certain space, is convertible into a greaterpressure acting with a less velocity, or through a smaller space; but thequantity of mechanical force remains unchanged by its transformation, andall that the implements called mechanical powers accomplish is to effectthis transformation. 

46. _Q. _--Is there no power gained by the lever?

_A. _--Not any: the power is merely put into another shape, just as thecontents of a hogshead of porter are the same, whether they be let off byan inch tap or by a hole a foot in diameter. There is a greater gush in theone case than the other, but it will last a shorter time; when a lever isused there is a greater force exerted, but it acts through a shorterdistance. It requires just the same expenditure of mechanical power to lift1 lb. Through 100 ft. , as to lift 100 lbs. Through 1 foot. A cylinder of agiven cubical capacity will exert the same power by each stroke, whetherthe cylinder be made tall and narrow, or short and wide; but in the onecase it will raise a small weight through a great height, and in the othercase, a great weight through a small height. 

47. _Q. _--Is there no loss of power by the use of the crank?

_A. _--Not any. Many persons have supposed that there was a loss of power bythe use of the crank, because at the top and bottom centres it is capableof exerting little or no power; but at those times there is little or nosteam consumed, so that no waste of power is occasioned by the peculiarity. Those who imagine that there is a loss of power caused by the crank perplexthemselves by confounding the vertical with the circumferential velocity. If the circle of the crank be divided by any number of equidistanthorizontal lines, it will be obvious that there must be the same steamconsumed, and the same power expended, when the crank pin passes from thelevel of one line to the level of the other, in whatever part of the circleit may be, those lines being indicative of equal ascents or descents of thepiston. But it will be seen that the circumferential velocity is greaterwith the same expenditure of steam when the crank pin approaches the topand bottom centres; and this increased velocity exactly compensates for thediminished leverage, so that there is the same power given out by the crankin each of the divisions. 

48. _Q. _--Have no plans been projected for gaining power by means of alever?

_A. _--Yes, many plans, --some of them displaying much ingenuity, but alldisplaying a complete ignorance of the first principles of mechanics, whichteach that power cannot be gained by any multiplication of levers andwheels. I have occasionally heard persons say: "You gain a great deal ofpower by the use of a capstan; why not apply the same resource in the caseof a steam vessel, and increase the power of your engine by placing acapstan motion between the engine and paddle wheels?" Others I have heardsay: "By the hydraulic press you can obtain unlimited power; why not theninterpose a hydraulic press between the engines and the paddles?" To thesequestions the reply is sufficiently obvious. Whatever you gain in force youlose in velocity; and it would benefit you little to make the paddlesrevolve with ten times the force, if you at the same time caused them tomake only a tenth of the number of revolutions. You cannot, by anycombination of mechanism, get increased force and increased speed at thesame time, or increased force without diminished speed; and it is from theignorance of this inexorable condition, that such myriads of schemes forthe realization of perpetual motion, by combinations of levers, weights, wheels, quicksilver, cranks, and other mere pieces of inert matter, havebeen propounded. 

49. _Q. _--Then a force once called into existence cannot be destroyed?

_A. _--No; force is eternal, if by force you mean power, or in other wordspressure acting though space. But if by force you mean mere pressure, thenit furnishes no measure of power. Power is not measurable by force but byforce and velocity combined. 

50. _Q. _--Is not power lost when two moving bodies strike one other andcome to a state of rest?

_A. _--No, not even then. The bodies if elastic will rebound from oneanother with their original velocity; if not elastic they will sustain analteration of form, and heat or electricity will be generated of equivalentvalue to the power which has disappeared. 

51. _Q. _--Then if mechanical power cannot be lost, and is being dailycalled into existence, must not there be a daily increase in the powerexisting in the world?

_A. _--That appears probable unless it flows back in the shape of heat orelectricity to the celestial spaces. The source of mechanical power is thesun which exhales vapors that descend in rain, to turn mills, or whichcauses winds to blow by the unequal rarefaction of the atmosphere. It isfrom the sun too that the power comes which is liberated in a steam engine. The solar rays enable plants to decompose carbonic acid gas, the product ofcombustion, and the vegetation thus rendered possible is the source of coaland other combustible bodies. The combustion of coal under a steam boilertherefore merely liberates the power which the sun gave out thousands ofyears before. 

FRICTION. 

52. _Q. _--What is friction?

_A. _--Friction is the resistance experienced when one body is rubbed uponanother body, and is supposed to be the result of the natural attractionwhich bodies have for one another, and of the interlocking of theimpalpable asperities upon the surfaces of all bodies, however smooth. There is, no doubt, some electrical action involved in its production, notyet recognized, nor understood; and it is perhaps traceable to thedisturbance of the electrical equilibrium of the particles of the bodyowing to the condensation or change of figure which all bodies mustexperience when subjected to a strain. When motion in opposite directionsis given to smooth surfaces, the minute asperities of one surface mustmount upon those of the other, and both will be abraded and worn away, inwhich act power must be expended. The friction of smooth rubbing substancesis less when the composition of those substances is different, than when itis the same, the particles being supposed to interlock less when theopposite prominences or asperities are not coincident. 

53. _Q. _--Does friction increase with the extent of rubbing surface?

_A. _--No; the friction, so long as there is no violent heating or abrasion, is simply in the proportion of the pressure keeping the surfaces together, or nearly so. It is, therefore, an obvious advantage to have the bearingsurfaces of steam engines as large as possible, as there is no increase offriction by extending the surface, while there is a great increase in thedurability. When the bearings of an engine are made too small, they verysoon wear out. 

54. _Q. _--Does friction increase in the same ratio as velocity?

_A. _--No; friction does not increase with the velocity at all, if thefriction over a given amount of surface be considered; but it increases asthe velocity, if the comparison be made with the time during which thefriction acts. Thus the friction of each stroke of a piston is the same, whether it makes 20 strokes in the minute, or 40: in the latter case, however, there are twice the number of strokes made, so that, though thefriction per stroke is the same, the friction per minute is doubled. Thefriction, therefore, of any machine per hour varies as the velocity, thoughthe friction per revolution remains, at all ordinary velocities, the same. Of excessive velocities we have not sufficient experience to enable us tostate with confidence whether the same law continues to operate among them. 

55. _Q. _--Can you give any approximate statement of the force expended inovercoming friction?

_A. _--It varies with the nature of the rubbing bodies. The friction of ironsliding upon iron, has generally been taken at about one tenth of thepressure, when the surfaces are oiled and then wiped again, so that no filmof oil is interposed. The friction of iron rubbing upon brass has generallybeen taken at about one eleventh of the pressure under the samecircumstances; but in machines in actual operation, where a film of somelubricating material is interposed between the rubbing surfaces, it is notmore than one third of this amount or 1/33d of the weight. While this, however, is the average result, the friction is a good deal less in somecases. Mr. Southern, in some experiments upon the friction of the axle of agrindstone--an account of which may be found in the 65th volume of thePhilosophical Transactions--found the friction to amount to less than1/40th of the weight; and Mr. Wood, in some experiments upon the frictionof locomotive axles, found that by ample lubrication the friction may bemade as little as 1/60th of the weight. In some experiments upon thefriction of shafts by Mr. G. Rennie, he found that with a pressure of from1 to 5 cwt. The friction did not exceed 1/39th of the pressure when tallowwas the unguent employed; with soft soap it became 1/34th. The fact appearsto be that the amount of the resistance denominated friction depends, in agreat measure, upon the nature of the unguent employed, and in certaincases the viscidity of the unguent may occasion a greater retardation thanthe resistance caused by the attrition. In watchwork therefore, and otherfine mechanism, it is necessary both to keep the bearing surfaces small, and to employ a thin and limpid oil for the purpose of lubrication, for theresistance caused by the viscidity of the unguent increases with the amountof surface, and the amount of surface is relatively greater in the smallerclass of works. 

56. _Q. _--Is a very thin unguent preferable also for the larger class ofbearings?

_A. _--The nature of the unguent, proper for different bearings, appears todepend in a great measure upon the amount of the pressure to which thebearings are subjected, --the hardest unguents being best where the pressureis greatest. The function of lubricating substances is to prevent therubbing surfaces from coming into contact, whereby abrasion would beproduced, and unguents are effectual in this respect in the proportion oftheir viscidity; but if the viscidity of the unguent be greater than whatsuffices to keep the surfaces asunder, an additional resistance will beoccasioned; and the nature of the unguent selected should always havereference, therefore, to the size of the rubbing surfaces, or to thepressure per square inch upon them. With oil the friction appears to be aminimum when the pressure on the surface of a bearing is about 90 lbs. Persquare inch. The friction from too small a surface increases twice asrapidly as the friction from too large a surface, added to which, thebearing, when the surface is too small, wears rapidly away. 

57. _Q. _--Has not M. Morin, in France, made some very complete experimentsto determine the friction of surfaces of different kinds sliding upon oneanother?

_A. _--He has; but the result does not differ materially from what is statedabove, though, upon the whole, M. Morin, found the resistance due tofriction to be somewhat greater than it has been found to be by variousother engineers. When the surfaces were merely wiped with a greasy cloth, but had no film of lubricating material interposed, the friction of brassupon cast iron he found to be . 107, or about 1/10th of the load, which wasalso the friction of cast iron upon oak. But when a film of lubricatingmaterial was interposed, he found that the friction was the same whetherthe surfaces were wood on metal, wood on wood, metal on wood, or metal onmetal; and the amount of the friction in such case depended chiefly on thenature of the unguent. With a mixture of hog's lard and olive oilinterposed between the surfaces, the friction was usually from 1/12th to1/14th of the load, but in some cases it was only 1/20th of the load. 

58. _Q. _--May water be made to serve for purposes of lubrication?

_A. _--Yes, water will answer very well if the surface be very largerelatively with the pressure; and in screw vessels where the propellershaft passes through a long pipe at the stern, the stuffing box ispurposely made a little leaky. The small leakage of water into the vesselwhich is thus occasioned, keeps the screw shaft in this situation alwayswet, and this is all the lubrication which this bearing requires orobtains. 

59. _Q. _--What is the utmost pressure which may be employed without heatingwhen oil is the lubricating material?

_A. _--That will depend upon the velocity. When the pressure exceeds 800lbs. Per square inch, however, upon the section of the bearing in adirection parallel with the axis, then the oil will be forced out and thebearing will necessarily heat. 

60. _Q. _--But, with, a given velocity, can you tell the limit of pressurewhich will be safe in practice; or with a given pressure, can you tell thelimit of velocity?

_A. _--Yes; that may be done by the following empirical rule, which has beenderived from observations made upon bearings of different sizes and movingwith different velocities. Divide the number 70, 000 by the velocity of thesurface of the bearing in feet per minute. The quotient will be the numberof pounds per square inch of section in the line of the axis that may beput upon the bearing. Or, if we divide 70, 000 by the number of pounds persquare inch of section, then the quotient will be the velocity in feet perminute at which the circumference of the bearing may work. 

61. _Q. _--The number of square inches upon which the pressure is reckoned, is not the circumference of the bearing multiplied by its length, but thediameter of the bearing multiplied by its length?

_A. _--Precisely so, it will be the diameter multiplied by the length of thebearing. 

62. _Q. _--What is the amount of friction in the case of surfaces slidingupon one another in sandy or muddy water--such surfaces, for example, asare to be found in the sluices of valves for water?

_A. _--Various experiments have been made by Mr. Summers of Southampton toascertain the friction of brass surfaces sliding upon each other in saltwater, with the view of finding the power required for moving sluice doorsfor lock gates and for other similar purposes. The surfaces were planed astrue and smooth as the planing machine would make them, but were _not_filed or scraped, and the result was as follows:

Area of Slide Weight or Pressure on Power required to move therubbing rubbing Surface. Slide _slowly_ in muddySurface. Salt Water, kept stirred up. 

Sq. In. Lb. Lb. 8 56 21. 5" 112 44. " 168 65. 5" 224 88. 5" 336 140. 5" 448 170. 75

[Illustration: Fig. 2. Sketch of Slide. The facing on which the slide movedwas similar, but three or four times as long. ]

These results were the average of eight fair trials; in each case, thesliding surfaces were totally immersed in muddy salt water, and althoughthe apparatus used for drawing the slide along was not very delicatelyfitted up, the power required may be considered as a sufficientapproximation for practical purposes. 

It appears from these experiments, that rough surfaces follow the same lawas regards friction that is followed by smooth, for in each case thefriction increases directly as the pressure. 

STRENGTH OF MATERIALS AND STRAINS SUBSISTING IN MACHINES. 

63. _Q. _--In what way are the strengths of the different parts of a steamengine determined?

_A. _--By reference to the amount of the strain or pressure to which theyare subjected, and to the cohesive strength of the iron or other materialof which they are composed. The strains subsisting in engines are usuallycharacterized as tensile, crushing, twisting, breaking, and shearingstrains; but they may be all resolved into strains of extension and strainsof compression; and by the power of the materials to resist these twostrains, will their practical strength be measurable. 

64. _Q. _--What are the ultimate strengths of the malleable and cast iron, brass, and other materials employed in the construction of engines?

_A. _--The tensile and crushing strengths of any given material are by nomeans the same. The tensile strength, or strength when extended, of goodbar iron is about 60, 000 lbs. , or nearly 27 tons per square inch ofsection; and the tensile strength of cast iron is about 15, 000 lbs. , or say6 3/4 to 7 tons per square inch of section. These are the weights which arerequired to break them. The crushing strain of cast iron, however, is about100, 000 lbs. , or 44 1/2 tons; whereas the crushing strength of malleableiron is not more than 27, 000 lbs. , or 12 tons, per square inch of section, and indeed it is generally less than this. The ultimate tensile strength, therefore, of malleable iron is four times greater than that of cast iron, but the crushing strength of cast iron is between three and four timesgreater than that of wrought iron. It may be stated, in round numbers, thatthe tensile strength of malleable iron is twice greater than its crushingstrength; or, in other words, that it will take twice the strain to break abar of malleable iron by drawing it asunder endways, than will cripple itby forcing it together endways like a pillar; whereas a bar of cast ironwill be drawn asunder with one sixth of the force that will be required tobreak or cripple it when forced together endways like a pillar. 

65. _Q. _--What is the cohesive strength of steel?

_A. _--The ultimate tensile strength of good cast or blistered steel isabout twice as great as that of wrought iron, being about 130, 000 lbs. Persquare inch of section. The tensile strength of gun metal, such as is usedin engines, is about 36, 000 lbs. Per square inch of section; of wroughtcopper about 33, 000 lbs. ; and of cast copper about 19, 000 lbs. Per squareInch of section. 

66. _Q. _--Is the crushing strength of steel greater or less than itstensile strength?

_A. _--It is about twice greater. A good steel punch will punch through aplate of wrought iron of a thickness equal to the diameter of the punch. Apunch therefore of an inch diameter will pierce a plate an inch thick. Nowit is well known, that the strain required to punch a piece of metal out ofa plate, is just the same as that required to tear asunder a bar of iron ofthe same area of cross section as the area of the surface cut. The area ofthe surface cut in this case will be the circumference of the punch, 3. 1416inches, multiplied by the thickness of the plate, 1 inch, which makes thearea of the cut surface 3. 1416 square inches. The area of the point of thepunch subjected to the pressure is . 7854 square inches, so that the areacut to the area crushed is as four to one. In other words, it will requirefour times the strain to crush steel that is required to tear asundermalleable iron, or it will take about twice the strain to crush steel thatit will require to break it by extension. 

67. _Q. _--What strain may be applied to malleable iron in practice?

_A. _--A bar of wrought iron to which a tensile or compressing strain isapplied, is elongated or contracted like a very stiff spiral spring, nearlyin the proportion of the amount of strain applied up to the limit at whichthe strength begins to give way, and within this limit it will recover itsoriginal dimensions when the strain is removed. If, however, the strain becarried beyond this limit, the bar will not recover its originaldimensions, but will be permanently pulled out or pushed in, just as wouldhappen to a spring to which an undue strain had been applied. This limit iswhat is called the limit of elasticity; and whenever it is exceeded, thebar, though it may not break immediately, will undergo a progressivedeterioration, and will break in the course of time. The limit ofelasticity of malleable iron when extended, or, in other words, the tensilestrain to which a bar of malleable iron an inch square may be subjectedwithout permanently deranging its structure, is usually taken at 17, 800lbs. , or from that to 10 tons, depending on the quality of the iron. It hasalso been found that malleable iron is extended about one ten-thousandthpart of its length for every ton of direct strain applied to it. 

68. _Q. _--What is the limit of elasticity of cast iron?

_A. _--It is commonly taken at 15, 300 lbs. Per square inch of section; butthis is certainly much too high, as it exceeds the tensile strength ofirons of medium quality. A bar of cast iron if compressed by weights willbe contracted in length twice as much as a bar of malleable iron undersimilar circumstances; but malleable iron, when subjected to a greaterstrain than 12 tons per square inch of section, gradually crumples up bythe mere continuance of the weight. A cast-iron bar one inch square and tenfeet long, is shortened about one tenth of an inch by a compressing forceof 10, 000 lbs. , whereas a malleable iron bar of the same dimensions wouldrequire to shorten it equally a compressing force of 20, 000 lbs. As theload, however, approaches 12 tons, the compressions become nearly equal, and above that point the rate of the compression of the malleable ironrapidly increases. A bar of cast iron, when at its breaking point by theapplication of a tensile strain, is stretched about one six-hundredth partof its length; and an equal strain employed to compress it, would shortenit about one eight-hundredth part of its length. 

69. _Q. _--But to what strain may the iron used in the construction ofengines be safely subjected?

_A. _--The most of the working parts of modern engines are made of malleableiron, and the utmost strain to which wrought iron should be subjected inmachinery is 4000 lbs. Per square inch of section. Cast iron should not besubjected to more than half of this. In locomotive boilers the strain of4000 lbs. Per square inch of section is sometimes exceeded by nearly onehalf; but such an excess of strain approaches the limits of danger. 

70. _Q. _--Will you explain in what way the various strains subsisting in asteam engine may be resolved into tensile and crushing strains; also inwhat way the magnitude of those strains may be determined?

_A. _--To take the case of a beam subjected to a transverse strain, such asthe great beam of an engine, it is clear, if we suppose the beam brokenthrough the middle, that the amount of strain at the upper and lower edgesof the beam, where the whole strain may be supposed to be collected, will, with any given pressure on the piston, depend upon the proportion of thelength to the depth of the beam. One edge of the beam breaks by extension, and the other edge by compression; and the upper and lower edges may beregarded as pillars, one of which is extended by the strain, and the otheris compressed. If, to make an extreme supposition, the depth of the beam istaken as equal to its length, then the pillars answering to the edges ofthe beam will be compressed, and extended by what is virtually a bellcranklever with equal arms; the horizontal distance from the main centre to theend of the beam being one of the arms, and the vertical height from themain centre to the top edge of the beam being the other arm. The distance, therefore, passed through by the fractured edge of the beam during a strokeof the engine, will be equal to the length of the stroke; and the strain itwill have to sustain will consequently be equal to the pressure on thepiston. If its motion were only half that of the piston, as would be thecase if its depth were made one half less, the strain the beam would haveto bear would be twice as great; and it may be set down as an axiom, thatthe strain upon any part of a steam engine or other machine is inverselyequal to the strain produced by the prime mover, multiplied by thecomparative velocity with which the part in question moves. If any part ofan engine moves with a less velocity than the piston, it will have agreater strain on it, if resisted, than is thrown upon the piston. If itmoves with a greater velocity than the piston, it will have a less strainupon it, and the difference of strain will in every case be in the inverseproportion of the difference of the velocity. 

71. _Q. _--Then, in computing the amount of metal necessary to give duestrength to a beam, the first point is to determine the velocity with whichthe edge of the beam moves at that point were the strain is greatest?

_A. _--The web of a cast-iron beam or girder serves merely to connect theupper and lower edges or flanges rigidly together, so as to enable theextending and compressing strains to be counteracted in an effectual mannerby the metal of those flanges. It is only necessary, therefore, to make theflanges of sufficient strength to resist effectually the crushing andtensile strains to which they are exposed, and to make the web of the beamof sufficient strength to prevent a distortion of its shape from takingplace. 

72. _Q. _--Is the strain greater from being movable or intermittent than ifit was stationary?

_A. _--Yes it is nearly twice as great from being movable. Engineers are inthe habit of making girders intended to sustain a stationary load, aboutthree times stronger than the breaking weight; but if the load be a movableone, as is the case in the girders of railway bridges, they make thestrength equal to six times the breaking weight. 

73. _Q. _--Then the strain is increased by the suddenness with which it isapplied?

_A. _--If a weight be placed on a long and slender beam propped up in themiddle, and the prop be suddenly withdrawn, so as to allow deflection totake place, it is clear that the deflection must be greater than if theload had been gradually applied. The momentum of the weight and also of thebeam itself falling through the space through which it has been deflected, has necessarily to be counteracted by the elasticity of the beam; and thebeam will, therefore, be momentarily bent to a greater extent than what isdue to the load, and after a few vibrations up and down it will finallysettle at that point of deflection which the load properly occasions. It isobvious that a beam must be strong enough, not merely to sustain thepressure due to the load, but also that accession of pressure due to thecounteracted momentum of the weight and of the beam itself. Although insteam engines the beam is not loaded by a weight, but by the pressure ofthe steam, yet the momentum of the beam itself must in every case becounteracted, and the momentum will be considerable in every case in whicha large and rapid deflection takes place. A rapid deflection increases theamount of the deflection as well as the amount of the strain, as is seen inthe cylinder cover of a Cornish pumping engine, into which the steam issuddenly admitted, and in which the momentum of the particles of the metalput into motion increases the deflection to an extent such as the merepressure of the steam could not produce. 

74. _Q. _--What will be the amount of increased strain consequent upondeflection?

_A. _--The momentum of any moving body being proportional to the square ofits velocity, it follows that the strain will be proportional to the squareof the amount of deflection produced in a specified time. 

75. _Q. _--But will not the inertia of a beam resist deflection, as well asthe momentum increase deflection?

_A. _--No doubt that will be so; but whether in practical cases increase ofmass without reference to strength or load will, upon the whole, increaseor diminish deflection, will depend very much upon the magnitude of themass relatively with the magnitude of the deflecting pressure, and therapidity with which that pressure is applied and removed. Thus if a forceor weight be very suddenly applied to the middle of a ponderous beam, andbe as suddenly withdrawn, the inertia of the beam will, as in the case ofthe collision of bodies, tend to resist the force, and thus obviatedeflection to a considerable extent; but if the pressure be so longcontinued as to produce the amount of deflection due to the pressure, theeffect of the inertia in that case will be to increase the deflection. 

76. _Q. _--Will the pressure given to the beam of an engine in differentdirections facilitate its fracture?

_A. _--Iron beams bent alternately in opposite directions, or alternatelydeflected and released, will be broken in the course of time with a muchless strain than is necessary to produce immediate fracture. It has beenfound, experimentally, that a cast-iron bar, deflected by a revolving camto only half the extent due to its breaking weight, will in no casewithstand 900 successive deflections; but, if bent by the cam to only onethird of its ultimate deflection, it will withstand 100, 000 deflectionswithout visible injury. Looking, however, to the jolts and vibrations towhich engines are subject, and the sudden strains sometimes thrown uponthem, either from water getting into the cylinder or otherwise, it does notappear that a strength answering to six times the breaking weight will givesufficient margin for safety in the case of cast-iron beams. 

77. _Q. _--Does the same law hold in the case of the deflection of malleableiron bars?

_A. _--In the case of malleable iron bars it has been found that no veryperceptible damage was caused by 10, 000 deflections, each deflection beingsuch as was due to half the load that produced a large permanentdeflection. 

78. _Q. _--The power of a rod or pillar to resist compression becomes verylittle when the diameter is small and the length great?

_A. _--The power of a rod or pillar to resist compression, varies nearly asthe fourth power of the diameter divided by the square of the length. Inthe case of hollow cylindrical columns of cast iron, it has been found, experimentally, that the 3. 55th power of the internal diameter, subtractedfrom the 3. 55th power of the external diameter, and divided by the 1. 7thpower of the length, will represent the strength very nearly. In the caseof hollow cylindrical columns of malleable iron, experiment shows that the3. 59th power of the internal diameter, subtracted from the 3. 59th power ofthe external diameter, and divided by the square of the length, gives aproper expression for the strength; but this rule only holds where thestrain does not exceed 8 or 9 tons on the square inch of section. Beyond 12or 13 tons per square inch of section, the metal cannot be depended upon towithstand the strain, though hollow pillars will sometimes bear 15 or 16tons per square inch of section. 

79. _Q. _--Does not the thickness of the metal of the pillars or tubesaffect the question?

_A. _--It manifestly does; for a tube of very thin metal, such as gold leafor tin foil, would not stand on end at all, being crushed down by its ownweight. It is found, experimentally, that in malleable iron tubes of therespective thicknesses of . 525, . 272, and . 124 inches, the resistances persquare inch of section are 19. 17, 14. 47, and 7. 47 tons respectively. Thepower of plates to resist compression varies nearly as the cube, or morenearly as the 2. 878th power of their thickness; but this law only holds solong as the pressure applied does not exceed from 9 to 12 tons per squareinch of section. When the pressure is greater than this the metal iscrushed, and a new law supervenes, according to which it is necessary toemploy plates of twice or three times the thickness, to obtain twice theresisting power. 

80. _Q. _--In a riveted tube, will the riveting be much, damaged by heavystrains?

_A. _--It will be most affected by percussion. Long-continued impact on theside of a tube, producing a deflection of only one fifth of that whichwould be required to injure it by pressure, is found to be destructive ofthe riveting; but in large riveted structures, such as a ship or a railwaybridge, the inertia of the mass will, by resisting the effect of impact, prevent any injurious action from this cause from taking place. 

81. _Q. _--Will the power of iron to resist shocks be in all casesproportional to its power to resist strains?

_A. _--By no means. Some cast iron is very hard and brittle; and although itwill in this state resist compression very strongly, it, will be easilybroken by a blow. Iron which has been remelted many times generally fallsinto this category, as it will also do if run into very small castings. Ithas been found, by experiment, that iron of which the crushing weight persquare inch is about 42 tons, will, if remelted twelve times, bear acrushing weight of 70 tons, and if remelted eighteen times it will bear acrushing weight of 83 tons; but taking its power to resist impact in itsfirst state at 706, this power will be raised at the twelfth remelting to1153, and will be sunk at the eighteenth remelting to 149. 

82. _Q. _--From all this it appears that a combination of cast iron andmalleable iron is the best for the beams of engines?

_A. _--Yes, and for all beams. Engine beams should be made deeper at themiddle than they are now made; the web should be lightened by holes piercedin it, and round the edge of the beam there should be a malleable iron hoopor strap securely attached to the flanges by riveting or otherwise. Theflanges at the edges of engine beams are invariably made too small. It isin them that the strength of the beam chiefly resides. 

CHAPTER I. 

GENERAL DESCRIPTION OF THE STEAM ENGINE. 

 * * * * *

THE BOILER. 

83. _Q. _--What are the chief varieties of the steam engine in actualpractical use?

_A. _--There is first the single-acting engine, which is used for pumpingwater; the rotative land engine, which is employed to drive mills andmanufactories; the rotative marine engine, which is used to propel steamvessels; and the locomotive engine, which is employed on railways. The lastis always a high-pressure engine; the others are, for the most part, condensing engines. 

84. _Q. _--Will you explain the construction and action of the single-actingengine, used for draining mines?

_A. _--Permit me then to begin with the boiler, which is common andnecessary to all engines; and I will take the example of a wagon boiler, such as was employed by Boulton and Watt universally in their earlyengines, and which is still in extensive use. This boiler is a longrectangular vessel, with a rounded top, like that of a carrier's wagon, from its resemblance to which it derives its name. A fire is set beneathit, and flues constructed of brickwork encircle it, so as to keep the flameand smoke in contact with the boiler for a sufficient time to absorb theheat. 

[Illustration: Fig. 3]

85. _Q. _--This species of boiler has not an internal furnace, but is set inbrickwork, in which the furnace is formed?

_A. _--Precisely so. The general arrangement and configuration will be atonce understood by a reference to the annexed figure (fig. 3), which is atransverse section of a wagon boiler. The line b represents the top of thegrate or fire bars, which slope downward from the front at an angle ofabout 25°, giving the fuel a tendency to move toward the back of the grate. The supply of air ascends from the ash pit through the grate bars, and theflame passes over a low wall or bridge, and traverses the bottom of theboiler. The smoke rises up at the back of the boiler, and proceeds throughthe flue F along one side to the front, and returns along the other side ofthe boiler, and then ascends the chimney. The performance of this course bythe smoke is what is termed a wheel draught, as the smoke wheels once roundthe boiler, and then ascends the chimney. 

86. _Q. _--Is the performance of this course by the smoke universal in wagonboilers?

_A. _--No; such boilers sometimes have what is termed a split draught. Thesmoke and flame, when they reach the end of the boiler, pass in this casethrough an iron flue or tube, reaching from end to end of the boiler; andon arriving at the front of the boiler, the smoke splits or separates--onehalf passing through a flue on the one side of the boiler, and the otherhalf passing through a flue on the other side of the boiler--both of theseflues having their debouch in the chimney. 

87. _Q. _--What are the appliances usually connected with a wagon boiler?

_A. _--On the top of the boiler, near the front, is a short cylinder, with alid secured by bolts. This is the manhole door, the purpose of which is toenable a man to get into the inside of the boiler when necessary forinspection and repair. On the top of this door is a small valve openingdownward, called the atmospheric valve. The intention of this valve is toprevent a vacuum from being formed accidentally in the boiler, which mightcollapse it; for if the pressure in the boiler subsides to a pointmaterially below the pressure of the atmosphere, the valve will open andallow air to get in. A bent pipe, which rises up from the top of theboiler, immediately behind the position of the manhole, is the steam pipefor conducting the steam to the engine; and a bent pipe which ascends fromthe top of the boiler, at the back end, is the waste-steam pipe forconducting away the steam, which escapes through the safety valve. Thisvalve is set in a chest, standing on the top of the boiler, at the foot ofthe waste-steam pipe, and it is loaded with iron or leaden weights to apoint answerable to the intended pressure of the steam. 

88. _Q. _--How is the proper level of the water in the boiler maintained?

_A. _--By means of a balanced buoy or float. This float is attached to arod, which in its turn is attached to a lever set on the top of a largeupright pipe. The upper part of the pipe is widened out into a smallcistern, through a short pipe in the middle of which a chain passes to thedamper; but any water emptied into this small cistern cannot pass into thepipe, except through a small valve fixed to the lever to which the rod isattached. The water for replenishing the boiler is pumped into the smallcistern on the top of the pipe; and it follows from these arrangements thatwhen the buoy falls, the rod opens the small valve and allows the feedwater to enter the pipe, which communicates with the water in the boiler;whereas, when the buoy rises, the feed cannot enter the pipe, and it has, therefore, to run to waste through an overflow pipe provided for thepurpose. 

89. _Q. _--How is the strength of the fire regulated?

_A. _--The draught through the furnaces of land boilers is regulated by aplate of metal or a damper, as it is called, which slides like a sluice upand down in the flue, and this damper is closed more or less when theintensity of the fire has to be moderated. In wagon boilers this isgenerally accomplished by self-acting mechanism. In the small cistern pipe, which is called a stand pipe, the water rises up to a height proportionalto the pressure of the steam, and the surface of the water in this pipewill rise or fall with the fluctuations in the pressure of the steam. Inthis pipe a float is placed, which communicates by means of a chain withthe damper. If the pressure of the steam rises, the float will be raisedand the damper closed, whereas, if the pressure in the boiler falls, thereverse of this action will take place. 

[Illustration: Fig. 4. ]

[Illustration: Fig. 5. ]

90. _Q. _--Are all land boilers of the same construction as that which youhave just described?

_A. _--No; many land boilers are now made of a cylindrical form, with one ortwo internal flues in which the furnace is placed. A boiler of this kind isrepresented in Figs. 4 and 5, and which is the species of boilerprincipally used in Cornwall. In this boiler a large internal cylinder orflue runs from end to end. In the fore part of this cylinder the furnace isplaced, and behind the furnace a large tube filled with water extends tothe end of the boiler. This internal tube is connected to the bottom partof the boiler by a copper pipe standing vertically immediately behind thefurnace bridge, and to the top part of the boiler by a bent copper pipewhich stands in a vertical position near the end of the boiler. The smoke, after passing through the central flue, circulates round the sides andbeneath the bottom of the boiler before its final escape into the chimney. The boiler is carefully covered over to prevent the dispersion of the heat. 

[Illustration: Fig. 6]

91. _Q. _--Will you describe the construction of the boilers used in steamvessels?

_A. _--These are of two classes, flue boilers and tubular boilers, but thelatter are now most used. In the flue boiler the furnaces are set withinthe boiler, and the flues proceeding from them wind backwards and forwardswithin the boiler until finally they meet and enter the chimney. Figs. 6, 7, and 8 are different views of the flue boilers of the steamer Forth. There are 4 boilers (as shown in plan, Fig. 6), with 3 furnaces in each, or12 furnaces in all. Fig. 7 is an elevation of 2 boilers, the one to theright being the front view, and that to the left a transverse section. Fig. 8 is a longitudinal section through 2 boilers. The direction of the arrowsin plan and longitudinal section, will explain the direction of the smokecurrent. 

[Illustration: Fig. 7. ]

[Illustration: Fig. 8. ]

92. _Q. _--Is this arrangement different from that obtaining in tubularboilers?

_A. _--In tubular boilers, the smoke after leaving the furnace just passesonce through a number of small tubes and then enters the chimney. Thesetubes are sometimes of brass, and they are usually about 3 inches indiameter, and 6 or 7 feet long. 

[Illustration: Fig. 9. ]

[Illustration: Fig. 10. ]

[Illustration: Fig. 11. ]

Figs. 9, 10, and 11 represent a marine tubular boiler; fig. 9 being avertical longitudinal section, fig. 10 half a front elevation and half atransverse section, and fig. 11 half a back elevation and half a transversesection near the end. There is a projecting part on the top of the boilercalled the "steam chest, " of which the purpose is to retain for the use ofthe cylinder a certain supply of steam in a quiescent state, in order thatit may have time to clear itself of foam or spray. A steam chest is a usualpart of all marine boilers. In fig. 9 A is the furnace, B the steam chest, and C the smoke box which opens into the chimney. The front of the smokebox is usually closed by doors which may be opened when necessary to sweepthe soot out of the tubes. 

The following are some forms of American boilers:

Figs. 12 and 13 are the transverse and longitudinal sections of a commonform of American marine boiler. 

Figs. 14 and 15 are the front and sectional elevation of one of the boilersof the U. S. Steamer Water Witch. 

[Illustration: Fig. 12. ]

[Illustration: Fig. 13. ]

[Illustration: Fig. 14. ]

[Illustration: Fig. 15. ]

Fig. 16 is a longitudinal section of a boiler of the drop flue variety. Forland purposes the lowest range of tubes is generally omitted, and the smokemakes a last return beneath the bottom of the boiler. 

Figs. 17 and 18 are the transverse and longitudinal sections of a tubularboiler, built in 1837 by R. L. Stevens for the steamboat Independence. 

[Illustration: Fig. 16. ]

[Illustration: Fig. 17. ]

[Illustration: Fig. 18. ]

Fig. 19 is a longitudinal section of a common wood-burning locomotive. 

[Illustration: Fig. 19. ]

THE ENGINE. 

93. _Q. _--The steam passes from the boiler through, the steam pipe into thecylinder of the engine?

_A. _--And presses up and down the piston alternately, being admittedalternately above and below the piston by suitable valves provided for thatpurpose. 

94. _Q. _--This reciprocating motion is all that is required in a pumpingengine?

_A. _--The prevailing form of the pumping engine consists of a great beamvibrating on a centre like the beam of a pair of scales, and the cylinderis in connection with one end of the beam and the pump stands at the otherend. The pump end of the beam is usually loaded, so as to cause it topreponderate when the engine is at rest; and the whole effort of the steamis employed in overcoming this preponderance until a stroke is performed, when, the steam being shut off, the heavy end of the beam again falls andthe operation is repeated. 

95. _Q. _--in the double-acting engine the piston is pushed by the steamboth ways, whereas in the single-acting engine it is only pushed one way?

_A. _--The structure and action of a double-acting land engine of the kindintroduced by Mr. Watt, will be understood by a reference to the annexedfigure (fig. 20), where an engine of this kind is shown in section. A isthe cylinder in which a movable piston, T, is forced alternately up anddown by the alternate admission, to each side, of the steam from theboiler. The piston, by means of a rod called the piston rod, gives motionto the beam V W, which by means of a heavy bar, P, called the connectingrod, moves the crank, Q, and with it the fly wheel, X, from which themachinery to be driven derives its motion. 

96. _Q. _--Where does the steam enter from the boiler?

[Illustration: Fig. 20. ]

_A. _--At the steam pipe, B. The throttle valve in that pipe is anelliptical plate of metal swivelling on a spindle passing through its edgefrom side to side, and by turning which more or less the opening throughthe pipe will be more or less closed. The extent to which this valve isopened or closed is determined by the governor, D, the balls of which, asthey collapse or expand, move up or down a collar on the governor spindle, which motion is communicated to the throttle valve by suitable rods andbell-cranks. The governor, it will be seen, consists substantially of twoheavy balls attached to arms fixed upon an upright shaft, which is kept inrevolution by means of a cord driven by a pulley on the fly wheel shaft. The velocity with which the balls of the governor revolve beingproportional to that of the fly wheel, it will follow, that if by reason oftoo rapid a supply of steam, an undue speed be given to the fly wheel, andtherefore to the balls, a divergence of the balls will take place to anextent corresponding to the excess of velocity, and this movement beingcommunicated to the throttle valve it will be partly closed (see fig. 1), the supply of steam to the engine will be diminished, and the velocity ofits motion will be reduced. If, on the other hand, the motion of the engineis slower than is requisite, owing to a deficient supply of steam throughB, then the balls, not being sufficiently affected by centrifugal force, will fall towards the vertical spindle, and the throttle valve, C, will bemore fully opened, whereby a more ample supply of steam will be admitted tothe cylinder, and the speed of the engine will be increased to therequisite extent. 

97. _Q. _--The piston must be made to fit the cylinder accurately so as toprevent the passage of steam?

_A. _--The piston is accurately fitted to the cylinder, and made to move init steam tight by a packing of hemp driven tightly into a groove or recessround the edge of the piston, and which is squeezed down by an iron ringheld by screws. The piston divides the cylinder into two compartments, between which there is no communication by which steam or any other elasticfluid can pass. A casing set beside the cylinder contains the valves, bymeans of which the steam which impels the piston is admitted and withdrawn, as the piston commences its motion in each direction. The upper steam boxB, is divided into three compartments by two valves. Above the upper steamvalve V, is a compartment communicating with the steam pipe B. Below thelower valve E is another compartment communicating with a pipe called theeduction pipe, which leads downwards from the cylinder to the condenser, inwhich vessel the steam is condensed by a jet of cold water. By the valve V, a communication may be opened or closed between the boiler and the top ofthe cylinder, so as to permit or prevent a supply of steam from the one topass to the other. By the valve E a communication may be open or closedbetween the top of the cylinder and the condenser, so that the steam in thetop compartment of the cylinder may either be permitted to escape into thecondenser, or may be confined to the cylinder. The continuation of thesteam pipe leads to the lower steam box B', which, like the upper, isdivided into three compartments by two valves V' and E', and the action ofthe lower valves is in all respects the same as that of the upper. 

98. _Q. _--Are all these valves connected together so that they actsimultaneously?

_A. _--The four valves V, E, V', E' are connected by rods to a single handleH, which handle is moved alternately up and down by means of pins ortappets, placed on the rod which works the air pump. When the handle H ispressed down, the levers in connexion with it open the upper exhaustingvalve E, and the lower steam valve V', and close the upper steam valve Vand the lower exhausting valve E'. On the other hand, when the handle H ispressed up it opens the upper steam valve V and the lower exhausting valveE', and at the same time closes the upper exhausting valve E, and the lowersteam valve V'. 

99. _Q. _--Where is the condenser situated?

_A. _--The condenser K is immerged in a cistern of cold water. At its sidethere is a tube I, for the admission of water to condense the steam, andwhich is governed by a cock, by opening which to any required extent, a jetof cold water may be made to play in the condenser. From the bottom of thecondenser a short pipe leads to the air pump J, and in this pipe there is aflap valve, called the foot valve, opening towards the air pump. The airpump is a pump set in the same cistern of cold water that holds thecondenser, and it is fitted with a piston or bucket worked by the rod L, attached to the great beam, and fitted with a valve opening upwards in themanner of a common sucking pump. The upper part of the air pumpcommunicates with a small cistern S, called the hot well, through a valveopening outwards and called the delivery valve. A pump M, called the hotwater pump, lifts hot water out of the hot well to feed the boiler, andanother pump N lifts cold water from a well or other source of supply, tomaintain the supply of water to the cold water cistern, in which thecondenser and air pump are placed. 

100. Q. --Will you explain now the manner in which the engine acts?

A. --The piston being supposed to be at the top of the cylinder, the handleH will be raised by the lower pin or tappet on the air pump rod, and thevalves V and E' will be opened, and at the same time the other pair ofvalves V' and E will be closed. Steam will therefore be admitted above thepiston and the steam or air which had previously filled the cylinder belowthe piston will be drawn off to the condenser. It will there encounter thejet of cold water, which is kept constantly playing there by keeping thecock I sufficiently open. It will thus be immediately condensed or reducedto water, and the cylinder below the piston will have a vacuum in it. Thesteam therefore admitted from the steam pipe through the open valve V tothe top of the cylinder, not being resisted by pressure below, will pressthe piston to the bottom of the cylinder. As it approaches that position, the handle H will be struck down by the upper pin or tappet on the air pumprod, and the valves V and E', previously open, will be closed, while thevalves V' and E, previously closed, will be opened. The steam which hasjust pressed down the piston, and which now fills the cylinder above thepiston, will then flow off, through the open valve E, to the condenser, where it will be immediately condensed by the jet of cold water; and steamfrom the boiler, admitted through the open valve V', will fill the cylinderbelow the piston, and press the piston upwards. When the piston has reachedthe top of the cylinder, the lower pin on the air pump rod will have struckthe handle upwards, and will thereby have closed the valves V' and E, andopened the valves V and E'. The piston will then be in the same situationas in the commencement, and will again descend, and so will continue to bedriven up and down by the steam. 

101. Q. --But what becomes of the cold water which is let into the condenserto condense the steam?

A. --It is pumped out by the air pump in the shape of hot water, itstemperature having been raised considerably by the admixture of the steamin it. When the air pump piston ascends it leaves behind it a vacuum; andthe foot valve being relieved from all pressure, the weight of the water inthe condenser forces it open, and the warm water flows from the condenserinto the lower part of the air pump, from which its return to the condenseris prevented by the intervening valve. When the air pump piston descends, its pressure on the liquid under it will force open the valve in it, through which the hot water will ascend; and when the bucket descends tothe bottom of the pump barrel, the warm water which was below it will allhave passed above it, and cannot return. When the bucket next ascends, thewater above it, not being able to return through the bucket valve, will beforced into the hot well through the delivery valve S. The hot water pumpM, pumps a small quantity of this hot water into the boiler, to compensatefor the abstraction of the water that has passed off in the form of steam. The residue of the hot water runs to waste. 

102. _Q. _--By what expedient is the piston rod enabled to pass through thecylinder cover without leaking steam out of the cylinder or air into it?

_A. _--The hole in the cylinder lid, through which the piston rod passes, isfurnished with a recess called a stuffing box, into which a stuffing orpacking of plaited hemp is forced, which, pressing on the one side againstthe interior of the stuffing box, and on the other side against the pistonrod, which is smooth and polished, prevents any leakage in this situation. The packing of this stuffing box is forced down by a ring of metaltightened by screws. This ring, which accurately fits the piston rod, has aprojecting flange, through which bolts pass for tightening the ring downupon the packing; and a similar expedient is employed in nearly every casein which packing is employed. 

103. _Q. _--In what way is the piston rod connected to the great beam?

_A. _--The piston rod is connected to the great beam by means of two links, one at each side of the beam shown at _f g_, (fig. 21. ) These links areusually made of the same length as the crank, and their purpose is toenable the end of the great beam to move in the arc of a circle while thepiston rod maintains the vertical position. The point of junction, therefore, of the links and the piston rod is of the form of a knuckle orbend at some parts of the stroke. 

104. _Q. _--But what compels the top of the piston rod to maintain thevertical position?

_A. _--Some engines have guide rods set on each side of the piston rod, andeyes on the top of the piston rod engage these guide rods, and maintain thepiston rod in a vertical position in every part of the stroke. Morecommonly, however, the desired end is attained by means of a contrivancecalled the parallel motion. 

105. _Q. _--What is the parallel motion?

_A. _--The parallel motion is an arrangement of jointed rods, so connectedtogether that the divergence from the vertical line at any point in the arcdescribed by the beam is corrected by an equal and opposite divergence dueto the arc performed by the jointed rods during the stroke; and as theseopposite deviations mutually correct one another, the result is that thepiston rod moves in a vertical direction. 

106. _Q. _--Will you explain the action more in detail?

_A. _--The pin, fig 21, which passes through the end of the beam at _f_ hasa link _f g_ hung on each side of the beam, and a short cross bar, called across head, extends from the bottom of one of these links to the bottom ofthe other, which cross head is perforated with a hole in the middle for thereception of the piston rod. There are similar links _b d_ at the point ofthe main beam, where the air pump rod is attached. There are two rods _d g_connecting the links _b d_ with the links _f g_, and these rods, as theyalways continue parallel to the main beam throughout the stroke, are called_parallel bars_. Attached to the end of these two rods at _d_ are two otherrods _c d_, of which the ends at _c_ are attached to stationary pins, whilethe ends at _d_ follow the motion of the lower ends of the links _b d_. These rods are called the _radius bars_. Now it is obvious that the arcdescribed by the point _d_, with _c_ as a centre, is opposite to the arcdescribed by the point _g_ with _d_ as a centre. The rod _d g_ is, therefore, drawn back horizontally by the arc described at _d_ to an extentequal to the versed sine of the arc described at _g_, or, in other words, the line described by the point _g_ becomes a straight line instead of acurve. 

[Illustration: Fig. 21. ]

107. _Q. _--Does the air pump rod move vertically as well as the piston rod?

_A. _--It does. The air pump rod is suspended from a cross head, passingfrom the centre of one of the links _b d_ to the centre of the other link, on the opposite side of the beam. Now, as the distance from the centralaxis of the great beam to the point _b_ is equal to the length of the rod_c d_, it will follow that the upper end of the link will follow one arc, and the lower end an equal and opposite arc. A point in the centre of thelink, therefore, where these opposite motions meet, will follow no arc atall, but will move up and down vertically in a straight line. 

108. _Q. _--The use of the crank is to obtain a circular motion from areciprocating motion?

_A. _--That is the object of it, and it accomplishes its object in a veryperfect manner, as it gradually arrests the velocity of the piston towardsthe end of the stroke, and thus obviates what would otherwise be aninjurious shock upon the machine. When the crank approaches the lowest partof its throw, and at the same time the piston is approaching the top of thecylinder, the motion of the crank becomes nearly horizontal, or, in otherwords, the piston is only advanced through a very short distance, for anygiven distance measured on the circle described by the crank pin. Since, then, the velocity of rotation of the crank is nearly uniform, it willfollow that the piston will move very slowly as it approaches the end ofthe stroke; and the piston is brought to a state of rest by this graduallyretarded motion, both at the top and the bottom of the stroke. 

109. _Q. _--What causes the crank to revolve at a uniform velocity?

_A. _--The momentum of the machinery moved by the piston, but moreespecially of the fly wheel, which by its operation redresses the unequalpressures communicated by the crank, and compels the crank shaft to revolveat a nearly uniform velocity. Everyone knows that a heavy wheel if put intorapid rotation cannot be immediately stopped. At the beginning and end ofthe stroke when the crank is vertical, no force of torsion can be exertedon the crank shaft by the crank, but this force is at its maximum when thecrank is horizontal. From the vertical point, where this force is nothing, to the horizontal point, where it is at its maximum, the force of torsionexerted on the crank shaft is constantly varying; and the fly wheel by itsmomentum redresses these irregularities, and carries the crank through that"dead point, " as it is termed, where the piston cannot impart any rotativeforce. 

110. _Q. _--Are the configuration and structure of the steam engine, as itleft the hand of Watt, materially different from those of modern engines?

_A. _--There is not much difference. In modern rotative land engines, thevalves for admitting the steam to the cylinder or condenser, instead ofbeing clack or pot-lid valves moved by tappets on the air pump rod, areusually sluice or sliding valves, moved by an eccentric wheel on the crankshaft. Sometimes the beam is discarded altogether, and malleable iron ismore largely used in the construction of engines instead of the cast iron, which formerly so largely prevailed. But upon the whole the steam engine ofthe present day is substantially the engine of Watt; and he who perfectlyunderstands the operation of Watt's engine, will have no difficulty inunderstanding the operation of any of the numerous varieties of enginessince introduced. 

THE MARINE ENGINE. 

111. _Q. _--Will you describe the principal features of the kind of steamengine employed for the propulsion of vessels?

_A. _--Marine engines are of two kinds, --paddle engines and screw engines. In the one case the propelling instrument is paddle wheels kept in rotationat each side of the ship: in the other case, the propelling instrument is ascrew, consisting of two or more twisted vanes, revolving beneath the waterat the stern. Of each class of engines there are many distinct varieties. 

112. _Q. _--What are the principal varieties of the paddle engine?

[Illustration: Fig. 22. ]

[Illustration: Fig. 23. ]

_A. _--There is the side lever engine (fig. 26), and the oscillating engine(fig. 27), besides numerous other forms of engine which are less known oremployed, such as the trunk (fig. 22), double cylinder (fig. 23), annular, Gorgon (fig. 24), steeple (fig. 25), and many others. The side leverengine, however, and the oscillating engine, are the only kinds of paddleengines which have been received with wide or general favor. 

[Illustration: Fig. 24. ]

113. _Q. _--Will you explain the main distinctive features of the side leverengine?

_A. _--In all paddle vessels, whatever be their subordinate characteristics, a great shaft of wrought iron, s, turned round by the engine, has to becarried from side to side of the vessel, on which shaft are fixed thepaddle wheels. The paddle wheels may either be formed with fixed floatboards for engaging the water, like the boards of a common undershot waterwheel, or they may be formed with _feathering_ float boards as they aretermed, which is float boards movable on a centre, and so governed byappropriate mechanism that they enter and leave the water in a nearlyvertical position. The common fixed or radial floats, however, are the kindmost widely employed, and they are attached to the arms of two or morerings of malleable iron which are fixed by appropriate centres on thepaddle shaft. It is usual in steam vessels to employ two engines, thecranks of which are set at right angles with one another. When the paddlewheels are turned by the engines, the float boards engaging the water causea forward thrust to be imparted to the shaft, which propels forward thevessel on the same principle that a boat is propelled by the action ofoars. 

[Illustration: Fig. 25. ]

114. _Q. _--These remarks apply to all paddle vessels?

_A. _--They do. With respect to the side lever engine, it may be describedto be such a modification of the land beam engine already described, aswill enable it to be got below the deck of a vessel. With this view, instead of a single beam being placed overhead, two beams are used, one ofwhich is set on each side of the engine as low down as possible. The crosshead which engages the piston rod is made somewhat longer than the diameterof the cylinder, and two great links or rods proceed one from each end ofthe cross head to one of the side levers or beams. A similar cross bar atthe other end of the beams serves to connect them together and to theconnecting rod which, proceeding from thence upwards, engages the crank, and thereby turns round the paddle wheels. 

115. _Q. _--Will you further illustrate this general description by anexample?

[Illustration: Fig. 26. ]

_Q. _--Fig. 26 is a side elevation of a side lever engine; x x represent thebeams or keelsons to which the engines are attached, and on which theboilers rest. The engines are tied down by strong bolts passing through thebottom of the vessel, but the boiler keeps its position by its weightalone. The condenser and air pump are worked off the side levers by meansof side rods and a cross head. A strong gudgeon, called the _main centre_, passes through the condenser at K, the projecting ends of which serve tosupport the side levers or beams. L is the piston rod, which, by means ofthe cross head and side rods, is connected to the side levers or beams, oneof which is shown at H H. The line M represents the connecting rod, towhich motion is imparted by the beams, through the medium of the cross tailextending between the beams, and which by means of the crank turns thepaddle shaft S. The eccentric which works the slide valve is placed uponthe paddle shaft. It consists of a disc of metal encircled by a hoop, towhich a rod is attached, and the disc is perforated with a hole for theshaft, not in the centre, but near one edge. When, therefore, the shaftrevolves, carrying the eccentric with it, the rod attached to theencircling hoop receives a reciprocating motion, just as it would do ifattached to a crank in the shaft. 

116. _Q. _--Will you describe the mode of starting the engine?

_A. _--I may first mention that when the engine is at rest, the connectionbetween the eccentric and the slide valve is broken, by lifting the end ofthe eccentric rod out of a notch which engages a pin on the valve shaft, and the valve is at such times free to be moved by hand by a bar of iron, applied to a proper part of the valve gear for that purpose. This being so, the engineer, when he wishes to start the engine, first opens a small valvecalled the _blow through valve_, which permits steam from the boiler toenter the engine both above and below the piston, and also to fill thecondenser and air pump. This steam expels the air from the interior of theengine, and also any water which may have accumulated there; and when thishas been done, the blow through valve is shut, and a vacuum very soon formswithin the engine, by the condensation of the steam. If now the slide valvebe moved by hand, the steam from the boiler will be admitted on one side ofthe piston, while there is a vacuum on the other side, and the piston will, therefore, be moved in the desired direction. When the piston reaches theend of the stroke, the valve has to be moved in the reverse direction, whenthe piston will return, and after being moved thus by hand, once or twice, the connection of the valve with the eccentric is to be restored byallowing the notch on the end of the eccentric rod to engage the pin on thevalve lever, when the valve will be thereafter moved by the engine in theproper manner. It will, of course, be necessary, when the engine begins tomove, to open the injection cock a little, to enable water to enter for thecondensation of the steam. In the most recent marine engines, a somewhatdifferent mechanism from this is used for giving motion to the valves, butthat mechanism will be afterwards described. 

117. _Q. _--Are all marine engines condensing engines?

_A. _--Nearly all of them are so; but recently a number of gunboats havebeen constructed, with high pressure engines. In general, however, marineengines are low pressure or condensing engines. 

118. _Q. _--Will you now describe the chief features of the oscillatingpaddle marine engine?

_A. _--In the oscillating paddle marine engine, the arrangement of thepaddle shaft and paddle wheels is the same as in the case alreadydescribed, but the whole of the side levers, side rods, cross head, crosstail, and connecting rod are discarded. The cylinder is set immediatelyunder the crank; the top of the piston rod is connected immediately to thecrank pin; and, to enable the piston rod to accommodate itself to themovement of the crank, the cylinder is so constructed as to be susceptibleof vibrating or oscillating upon two external axes or trunnions. Thesetrunnions are generally placed about half way up on the sides of thecylinder; and through one of them steam is received from the boiler, whilethrough the other the steam escapes to the condenser. The air pump isusually worked by means of a crank in the shaft, which crank moves the airpump bucket up and down as the shaft revolves. 

119. _Q. _--Will you give an example of a paddle oscillating engine?

_A. _--I will take as an example the oscillating engines constructed byMessrs. Ravenhill & Salked, for the Holyhead Packets. Fig. 27 is alongitudinal section of this vessel, showing an engine and boiler; and fig. 28 is a transverse section of one of the engines, showing also one of thewheels. There are two cylinders in this vessel, and one air pump, whichlies in an inclined position, and is worked by a crank in the shaft whichstretches between the cylinders, and which is called the _intermediateshaft_. A A, is one of the cylinders, B B the piston rod, and C C thecrank. D is the crank in the intermediate shaft, which works the air pumpE. There are double eccentrics fixed on the shaft, whereby the movement ofthe slide valves is regulated. The purpose of the double eccentrics is toenable an improved arrangement of valve gear to be employed, which isdenominated the _link motion_, and which will be described hereafter. I Iare the steam pipes leading to the steam trunnions K K, on which, and onthe eduction trunnions connected with the pipe M, the cylinders oscillate. 

120. _Q. _--By what species of mechanism are the positions of the paddlefloats of feathering wheels governed?

_A. _--The floats are supported by spurs projecting from the rim of thewheel, and they may be moved upon the points of the spurs, to which theyare attached by pins, by means of short levers proceeding from the backs ofthe floats, and connected to rods which proceed towards the centre of thewheel. The centre, however, to which these rods proceed is not concentricwith the wheel, and the rods, therefore, are moved in and out as the wheelrevolves, and impart a corresponding motion to the floats. In somefeathering wheels the proper motion is given to the rods by means of aneccentric on the ship's side. The action of paddle wheels, whether radialor feathering, will be more fully described in the chapter on SteamNavigation. 

SCREW ENGINES. 

121. _Q. _--What are the principal varieties of screw engines?

[Illustration: Fig. 27. ]

[Illustration: Fig. 28. ]

_A. _--The engines employed for the propulsion of screw vessels are dividedinto two great classes, --geared engines and direct acting engines; and eachof these classes again has many varieties. In screw vessels, the shaft onwhich the screw is set requires to revolve at a much greater velocity thanis required in the case of the paddle shaft of a paddle vessel; and ingeared engines this necessary velocity of rotation is obtained by theintervention of toothed wheels, --the engines themselves moving with theusual velocity of paddle engines; whereas in direct acting engines therequired velocity of rotation is obtained by accelerating the speed of theengines, and which are connected immediately to the screw shaft. 

122. _Q. _--Will you describe some of the principal varieties of gearedengines?

_A. _--A good many of the geared engines for screw vessels are made in thesame manner as land engines, with a beam overhead, which by means of aconnecting rod extending downwards, gives motion to the crank shaft, onwhich are set the cog wheels which give motion to pinions on the screwshaft, --the teeth of the wheels being generally of wood and the teeth ofthe pinions of iron. There are usually several wheels on the crank shaftand several pinions on the screw shaft; but the teeth of each do not run inthe same line, but are set a little in advance of one another, so as todivide the thickness of the tooth into as many parts as there areindependent wheels or pinions. By this arrangement the wheels work moresmoothly than they would otherwise do. 

123. _Q. _--What other forms are there of geared screw engines?

_A. _--In some cases the cylinders lie on their sides in the manner of thecylinders of a locomotive engine. In other cases vertical trunk engines areemployed; and in other cases vertical oscillating engines. 

124. _Q. _--Will you give an example of a geared vertical oscillatingengine?

_A. _--The engines of a geared oscillating engine are similar to the paddlewheel engines (figs. 27 and 28), but the engines are placed lengthways ofthe ship, and instead of a paddle wheel on the main shaft, there is ageared wheel which connects with a pinion on the screw shaft. The enginesof the Great Britain are made off the same patterns as the paddle enginesconstructed by Messrs. John Penn & Son, for H. M. S. Sphinx. The diameter ofeach cylinder is 82-1/2 inches, the length of travel or stroke of thepiston is 6 feet, and the nominal power is 500 horses. The Great Britain isof 3, 500 tons burden, and her displacement at 16 feet draught of water is2, 970 tons. The diameter of the screw is 15-1/2 feet, length of screw inthe line of the shaft, 3 feet 2 inches, and the pitch of the screw, 19feet. 

125. _Q. _--What do you mean by the pitch of the screw?

_A. _--A screw propeller may be supposed to be a short piece cut off a screwof large diameter like a spiral stair, and the pitch of a spiral stair isthe vertical height from any given step to the step immediately overhead. 

126. _Q. _--What is the usual number of arms?

_A. _--Generally a screw has two arms, but sometimes it has three or more. The Great Britain had three arms or twisted blades resembling the vanes ofa windmill. The multiple of the gearing in the Great Britain is 3 to 1, andthere are 17-1/2 square feet of heating surface in the boiler for eachnominal horse power. The crank shaft being put into motion by the engine, carries round with it the great cog wheel, or aggregation of cog wheels, affixed to its extremity; and these wheels acting on suitable pinions onthe screw shaft, cause the screw to make three revolutions for everyrevolution made by the engine. 

127. _Q. _--What are the principal varieties of direct acting screw engines?

_A. _--In some cases four engines have been employed instead of two, and thecylinders have been laid on their sides on each side of the screw shaft. This multiplication of engines, however, introduces needless complication, and is now but little used. In other cases two inverted cylinders are setabove the screw shaft on appropriate framing; and connecting rods attachedto the ends of the piston rods turn round cranks in the screw shaft. 

128. _Q. _--What is the kind of direct acting screw engine employed byMessrs. Penn. 

_A. _--It is a horizontal trunk engine. In this engine a round pipe called atrunk penetrates the piston, to which it is fixed, being in fact cast inone piece with it; and the trunk also penetrates the top and bottom of thecylinder, through which it moves, and is made tight therein by means ofstuffing boxes. The connecting rod is attached at one end to a pin fixed inthe middle of the trunk, while the other end engages the crank in the usualmanner. The air pump is set within the condenser, and is wrought by a rodwhich is fixed to the piston and derives its motion therefrom. The air pumpis of that species which is called double-acting. The piston or bucket isformed without valves in it, but an inlet and outlet valve is fixed to eachend of the pump, through the one of which the water is drawn into the pumpbarrel, and through the other of which it is expelled into the hot well. 

THE LOCOMOTIVE ENGINE. 

129. _Q. _--Will you describe the more important features of the locomotiveengine?

_A. _--The locomotive employed to draw carriages upon railways, consists ofa cylindrical boiler filled with brass tubes, through which the hot airpasses on its progress from the furnace to the chimney, and attached to theboiler are two horizontal cylinders fitted with pistons, valves, connectingrods, and other necessary apparatus to enable the power exerted by thepistons to turn round the cranked axle to which the driving wheels areattached. There are, therefore, two independent engines entering into thecomposition of a locomotive, the cranks of which are set at right angleswith one another, so that when one crank is at its dead point, the othercrank is in a position to act with its maximum efficacy. The drivingwheels, which are fixed on the crank shaft and turn round with it, propelthe locomotive forward on the rails by the mere adhesion of friction, andthis is found sufficient not merely to move the locomotive, but to draw along train of carriages behind it. 

130. _Q. _--Are locomotive engines condensing or high pressure engines. 

_A. _--They are invariably high pressure engines, and it would be impossibleor at least highly inconvenient, to carry the water necessary for thepurpose of condensation. The steam, therefore, after it has urged thepiston to the end of the stroke, escapes into the atmosphere. In locomotiveengines the waste steam is always discharged into the chimney through avertical pipe, and by its rapid passage it greatly increases the intensityof the draught in the chimney, whereby a smaller fire grate suffices forthe combustion of the fuel, and the evaporative power of the boiler is muchincreased. 

131. _Q. _--Can you give an example of a good locomotive engine of the usualform?

_A. _--To do this I will take the example of one of Hawthorn's locomotiveengines with six wheels represented in fig. 29; not one of the most modernconstruction now in use, nor yet one of the most antiquated. M is thecylinder, R the connecting rod, C C the eccentrics by which the slide valveis moved; J J is the steam pipe by which the steam is conducted from thesteam dome of the boiler to the cylinder. Near the smoke stack end of thispipe is a valve K or regulator moved by a handle _p_ at the front of theboiler, and of which the purpose is to regulate the admission of the steamto the cylinder; _f_ is a safety valve kept closed by springs; N is theeduction pipe, or, as it is commonly termed in locomotives, the _blastpipe_, by which the steam, escaping from the cylinder after the stroke hasbeen performed, is projected up the chimney H. The water in the boiler ofcourse covers the tubes and also the top of the furnace or fire box. Itwill be understood that there are two engines in each locomotive, though, from the figure being given in section, only one engine can be shown. Thecylinders of this engine are each 14 inches diameter; the length of thestroke of the piston is 21 inches. There are two sets of driving wheels, 5feet diameter, with outside connections. 

[Illustration: Fig. 29. ]

132. _Q. _--What is the tender of a locomotive?

_A. _--It is a carriage attached to the locomotive, of which the purpose isto contain coke for feeding the furnace, and water for replenishing theboiler. 

133. _Q. _--Can you give examples of modern locomotives?

[Illustration: Fig. 30. ]

[Illustration: Fig. 31. ]

_A. _--The most recent locomotives resemble in their material features thelocomotive represented in fig. 29. I can, however, give examples of some ofthe most powerful engines of recent construction. Fig. 30 representsGooch's express engine, adapted for the wide gauge of the Great WesternRailway; and fig. 31 represents Crampton's express engine, adapted for theordinary or narrow gauge railways. The cylinders of Gooch's engine are each18 inches diameter, and 24 inches stroke; the driving wheels are 8 feet indiameter; the fire grate contains 21 square feet of area; and the heatingsurface of the fire box is 153 square feet. There are in all 305 tubes inthe boiler, each of 2 inches diameter, giving a heating surface in thetubes of 1799 square feet. The total heating surface, therefore, is 1952square feet. Mr. Gooch states that an engine of this class will evaporatefrom 300 to 360 cubic feet of water in the hour, and will convey a load of236 tons at a speed of 40 miles an hour, or a load of 181 tons at a speedof 60 miles an hour. The weight of this engine empty is 31 tons; of thetender 8-1/2 tons; and the total weight of the engine when loaded is 50tons. In one of Crampton's locomotives, the Liverpool, with one set more ofcarrying wheels than the fig. , the cylinders are of 24 inches diameter and18 inches stroke; the driving wheels are 8 feet in diameter; the fire gratecontains 21-1/2 square feet of area; and the heating surface of the firebox is 154 square feet. There are in all 300 tubes in the boiler of 2-3/16inches external diameter, giving a surface in the tubes of 2136 squarefeet, and a total heating surface of 2290 square feet. The weight of thisengine is stated to be 35 tons when ready to proceed on a journey. Bothengines were displayed at the Great Exhibition in 1851, as examples of themost powerful locomotive engines then made. The weight of such engines isvery injurious to the railway; bending, crushing, and disturbing the rails, and trying very severely the whole of the railway works. No doubt theweight may be distributed upon a greater number of wheels, but if theweight resting on the driving wheels be much reduced, they will not havesufficient bite upon the rails to propel the train without slipping. This, however, is only one of the evils which the demand for high rates of speedhas produced. The width of the railway, or, as it is termed, the _gauge_ ofthe rails, being in most of the railways in this kingdom limited to 4 feet8-1/2 inches, a corresponding limitation is imposed on the diameter of theboiler; which in its turn restricts the number of the tubes which can beemployed. As, however, the attainment of a high rate of speed requires muchpower, and consequently much heating surface in the boiler, and as thenumber of tubes cannot be increased without reducing their diameter, it hasbecome necessary, in the case of powerful engines, to employ tubes of asmall diameter, and of a great length, to obtain the necessary quantity ofheating surface; and such tubes require a very strong draught in thechimney to make them effective. With a draught of the usual intensity thewhole of the heat will be absorbed in the portion of the tube nearest thefire box, leaving that portion nearest the smoke box nothing to do but totransmit the smoke; and with long tubes of small diameter, therefore, avery strong draught is indispensable. To obtain such a draught inlocomotives, it is necessary to contract the mouth of the blast pipe, whereby the waste steam will be projected into the chimney with greaterforce; but this contraction involves an increase of the pressure on theeduction side of the piston, and consequently causes a diminution in thepower of the engine. Locomotives with small and long tubes, therefore, willrequire more coke to do the same work than locomotives in which larger andshorter tubes may be employed. 

CHAPTER II. 

HEAT, COMBUSTION, AND STEAM. 

HEAT. 

134. _Q. _--What is meant by latent heat?

_A. _--By latent heat is meant the heat existing in bodies which is notdiscoverable by the touch or by the thermometer, but which manifests itsexistence by producing a change of state. Heat is absorbed in theliquefaction of ice, and in the vaporization of water, yet the temperaturedoes not rise during either process, and the heat absorbed is thereforesaid to become latent. The term is somewhat objectionable, as the effectproper to the absorption of heat has in each case been made visible; and itwould be as reasonable to call hot water latent steam. Latent heat, in thepresent acceptation of the term, means sensible liquefaction orvaporization; but to produce these changes heat is as necessary as toproduce the expansion of mercury in a thermometer tube, which is taken asthe measure of temperature; and it is hard to see on what ground heat canbe said to be latent when its presence is made manifest by changes whichonly heat can effect. It is the _temperature_ only that is latent, andlatent temperature means sensible vaporization or liquefaction. 

135. _Q. _--But when you talk of the latent heat of steam, what do you meanto express?

_A. _--I mean to express the heat consumed in accomplishing the vaporizationcompared with that necessary for producing the temperature. The latent heatof steam is usually reckoned at about 1000 degrees, by which it is meantthat there is as much heat in any given weight of steam as would raise itsconstituent water 1000 degrees if the expansion of the water could beprevented, or as would raise 1000 times that quantity of water one degree. The boiling point of water, being 212 degrees, is 180 degrees above thefreezing point of water--the freezing point being 32 degrees; so that itrequires 1180 times as much heat to raise 1 lb. Of water into steam, as toraise 1180 lbs. Of water one degree; or it requires about as much heat toraise a pound of boiling water into steam, as would raise 5-1/2 lbs. Ofwater from the freezing to the boiling point; 5-1/2 multiplied by 180 being990 or 1000 nearly. 

136. _Q. _--When it is stated that the latent heat of steam is 1000 degrees, it is only meant that this is a rough approximation to the truth?

_A. _--Precisely so. The latent heat, in point of fact, is not uniform atall temperatures, neither is the total amount of heat the same at alltemperatures. M. Regnault has shown, by a very elaborate series ofexperiments on steam, which he has lately concluded, that the total heat insteam increases somewhat with the pressure, and that the latent heatdiminishes somewhat with the pressure. This will be made obvious by thefollowing numbers:

Pressure. Temperature. Total Heat. Latent Heat. 15 lbs. 213. 1° 1178. 9° 965. 8° 50 281. 0 1199. 6 918. 6 100 327. 8 1213. 9 886. 1

If, then, steam of 100 lbs. Be expanded down to steam of 15 lbs. , it willhave 35 degrees of heat over that which is required for the maintenance ofthe vaporous state, or, in other words, it will be surcharged with heat. 

137. _Q. _--What do you understand by specific heat?

_A. _--By specific heat, I understand the relative quantities of heat inbodies at the same temperature, just as by specific gravity I understandthe relative quantities of matter in bodies of the same bulk. Equal weightsof quicksilver and water at the same temperature do not contain the samequantities of heat, any more than equal bulks of those liquids contain thesame quantity of matter. The absolute quantity of heat in any body is notknown; but the relative heat of bodies at the same temperature, or in otherwords their specific heats, have been ascertained and arranged in tables, --the specific heat of water being taken as unity. 

138. _Q. _--In what way does the specific heat of a body enable the quantityof heat in it to be determined?

_A. _--If any body has only half the specific heat of water, then a pound ofthat body will, at any given temperature, have only half the heat in itthat is in a pound of water at the same temperature. The specific heat ofair is . 2669, that of water being 1; or it is 3. 75 times less than that ofwater. An amount of heat, therefore, which would raise a pound of water 1degree would raise a pound of air 3. 75 degrees. 

COMBUSTION. 

139. _Q. _--What is the nature of combustion?

_A. _--Combustion is nothing more than an energetic chemical combination, or, in other words, it is the mutual neutralization of opposingelectricities. When coal is brought to a high temperature it acquires astrong affinity for oxygen, and combination with oxygen will produce morethan sufficient heat to maintain the original temperature; so that part ofthe heat is rendered applicable to other purposes. 

140. _Q. _--Does air consist of oxygen?

_A. _--Air consists of oxygen and nitrogen mixed together in the proportionof 3. 29 lbs. Of nitrogen to 1 lb. Of oxygen. Every pound of coal requiresabout 2. 66 lbs. Of oxygen for its saturation, and therefore for every poundof coal burned, 8. 75 pounds of nitrogen must pass through the fire, supposing all the oxygen to enter into combination. In practice, however, this perfection of combination does not exist; from one-third to one-halfof the oxygen will pass through the fire without entering into combinationat all; so that from 16 to 18 lbs. Of air are required for every pound ofcoal burned. 18 lbs. Of air are about 240 cubic feet, which may be taken asthe quantity of air required for the combustion of a pound of coal inpractice. 

141. _Q. _--What are the constituents of coal?

_A. _--The chief constituent of coal is carbon or pure charcoal, which isassociated in various proportions with volatile and earthy matters. Englishcoal contains 80 to 90 per cent. Of carbon, and from 8 to 18 per cent. Ofvolatile and earthy matters, but sometimes more than this. The volatilematters are hydrogen, nitrogen, oxygen, and sulphur. 

142. _Q. _--What is the difference between anthracite and bituminous coal?

_A. _--Anthracite consists almost entirely of carbon, having 91 per cent. Ofcarbon, with about 7 per cent. Of volatile matter and 2 per cent. Of ashes. Newcastle coal contains about 83 per cent. Of carbon, 14 per cent. Ofvolatile matter, and 3 per cent. Of ashes. 

143. _Q. _--Will you recapitulate the steps by which you determine thequantity of air required for the combustion of coal?

_A. _--Looking to the quantity of oxygen required to unite chemically withthe various constituents of the coal, we find for example that in 100 lbs. Of anthracite coal, consisting of 91. 44 lbs. Of carbon, and 3. 46 lbs. Ofhydrogen, we shall for the 91. 44 lbs. Of carbon require 243. 84 lbs. Ofoxygen--since to saturate a pound of carbon by the formation of carbonicacid, requires 2-2/3 lbs. Of oxygen. To saturate a pound of hydrogen in theformation of water, requires 8 lbs. Of oxygen; hence 3. 46 Fibs. Of hydrogenwill take 27. 68 lbs. Of oxygen for its saturation. If then we add 243. 84lbs. To 27. 68 lbs. We have 271. 52 lbs. Of oxygen required for thecombustion of 100 lbs. Of coal. A given weight of air contains nearly 23. 32per cent of oxygen; hence to obtain 271. 52 lbs. Of oxygen, we must haveabout four times that quantity of atmospheric air, or more accurately, 1164lbs. Of air for the combustion of 100 lbs. Of coal. A cubic foot of air atordinary temperature weighs about . 075 lbs. ; so that 100 lbs. Of coalrequire 15, 524 cubic feet of air, or 1 lb. Of coal requires about 155 cubicfeet of air, supposing every atom of the oxygen to enter into combination. If, then, from one-third to one-half of the air passes unconsumed throughthe fire, an allowance of 240 cubic feet of air for each pound of coal willbe a small enough allowance to answer the requirements of practice, and insome cases as much as 300 cubic feet will be required, --the differencedepending mainly on the peculiar configuration of the furnace. 

144. _Q. _--Can you state the evaporative efficacy of a pound of coal?

_A. _--The evaporative efficacy of a pound of carbon has been foundexperimentally to be equivalent to that necessary to raise 14, 000 lbs. Ofwater through 1 degree, or 14 lbs. Of water through 1000 degrees, supposingthe whole heat generated to be absorbed by the water. Now, if the water beraised into steam from a temperature of 60°, then 1118. 9° of heat will haveto be imparted to it to convert it into steam of 15 lbs. Pressure persquare inch. 14, 000 / 1118. 9 = 12. 512 Lbs. Will be the number of pounds ofwater, therefore, which a pound of carbon can raise into steam of 15 lbs. Pressure from a temperature of 60°. This, however, is a considerably largerresult than can be expected in practice. 

145. _Q. _--Then what is the result that may be expected in practice?

_A. _--The evaporative powers of different coals appear to be nearlyproportional to the quantity of carbon in them; and bituminous coal is, therefore, less efficacious than coal consisting chiefly of pure carbon. Apound of the best Welsh or anthracite coal is capable of raising from 9-1/2to 10 lbs. Of water from 212° into steam, whereas a pound of the bestNewcastle is not capable of raising more than about 8-1/2 lbs. Of waterfrom 212° into steam; and inferior coals will not raise more than 6-1/2lbs. Of water into steam. In America it has been found that 1 lb. Of thebest coal is equal to 2-1/2 lbs. Of pine wood, or, in some cases to 3 lbs. ;and a pound of pine wood will not usually evaporate more than about 2 1/2lbs. Of water, though, by careful management, it may be made to evaporate 41/2 lbs. Turf will generate rather more steam than wood. Coke is equal orsomewhat superior to the best coal in evaporative effect. 

146. _Q. _--How much water will a pound of coal raise into steam in ordinaryboilers?

_A. _--From 6 to 8 lbs. Of water in the generality of land boilers of mediumquality, the difference depending on the kind of boiler, the kind of coal, and other circumstances. Mr. Watt reckoned his boilers as capable ofevaporating 10. 08 cubic feet of water with a bushel or 84 lbs. Of Newcastlecoal, which is equivalent to 7 1/2 lbs. Of water evaporated by 1 lb. Ofcoal, and this may be taken as the performance of common land boilers atthe present time. In some of the Cornish boilers, however, a pound of coalraises 11. 8 lbs. Of boiling water into steam, or a cwt. Of coal evaporatesabout 21 cubic feet of water from 212°. 

147. _Q. _--What method of firing ordinary furnaces is the best?

_A. _--The coals should be broken up into small pieces, and sprinkled thinlyand evenly over the fire a little at a time. The thickness of the stratumof coal upon the grate should depend upon the intensity of the draught: inordinary land or marine boilers it should be thin, whereas in locomotiveboilers it requires to be much thicker. If the stratum of coal be thickwhile the draught is sluggish, the carbonic acid resulting from combustioncombines with an additional atom of carbon in passing through the fire, andis converted into carbonic oxide, which may be defined to be invisiblesmoke, as it carries off a portion of the fuel: if, on the contrary, thestratum of coal be thin while the draught is very rapid, an injuriousrefrigeration is occasioned by the excess of air passing through thefurnace. The fire should always be spread of uniform thickness over thebars of the grate, and should be without any holes or uncovered places, which greatly diminish the effect of the fuel by the refrigeratory actionof the stream of cold air which enters thereby. A wood fire requires to beabout 6 inches thicker than a coal one, and a turf fire requires to be 3 or4 inches thicker than a wood one, so that the furnace bars must be placedlower where wood or turf is burned, to enable the surface of the fire to beat the same distance from the bottom of the boiler. 

148. _Q. _--Is a slow or a rapid combustion the most beneficial?

_A. _--A slow combustion is found by experiment to give the best results asregards economy of fuel, and theory tells us that the largest advantagewill necessarily be obtained where adequate time has been afforded for acomplete combination of the constituent atoms of the combustible, and thesupporter of combustion. In many of the cases, however, which occur inpractice, a slow combustion is not attainable; but the tendencies of slowcombustion are both to save the fuel, and to burn the smoke. 

149. _Q. _--Is not the combustion in the furnaces of the Cornish boilersvery slow?

A. --Yes, very slow; and there is in consequence very little smoke evolved. The coal used in Cornwall is Welsh coal, which evolves but little smoke, and is therefore more favorable for the success of a smokeless furnace; butin the manufacturing districts, where the coal is more bituminous, it isfound that smoke may be almost wholly prevented by careful firing and bythe use of a large capacity of furnace. 

150. _Q. _--Do you consider slow combustion to be an advisable thing topractise in steam vessels?

_A. _--No, I do not. When the combustion is slow, the heat in the furnacesand flues is less intense, and a larger amount of heating surfaceconsequently becomes necessary to absorb the heat. In locomotives, wherethe heat of the furnace is very intense, there will be the same economy offuel with an allowance of 5 or 6 square feet of surface to evaporate acubic foot of water as in common marine boilers with 10 or 12. 

151. _Q. _--What is the method of consuming smoke pursued in themanufacturing districts?

_A. _--In Manchester, where some stringent regulations for the prevention ofsmoke have for some time been in force, it is found that the readiest wayof burning the smoke is to have a very large proportion of furnace room, whereby slow combustion may be carried on. In some cases, too, a favourableresult is arrived at by raising a ridge of coal across the furnace lyingagainst the bridge, and of the same height: this ridge speedily becomes amass of incandescent coke, which promotes the combustion of the smokepassing over it. 

152. _Q. _--Is the method of admitting a stream of air into the flues toburn the smoke regarded favorably?

_A. _--No; it is found to be productive of injury to the boiler by theviolent alternations of temperature it occasions, as at some times cold airimpinges on the iron of the boiler, and at other times flame, --just asthere happens to be smoke or no smoke emitted by the furnace. Boilers, therefore, operating upon this principle, speedily become leaky, and aremuch worn by oxidation, so that, if the pressure is considerable, they areliable to explode. It is very difficult to apportion the quantity of airadmitted, to the varying wants of the fire; and as air may at some times berushing in when there is no smoke to consume, a loss of heat, and anincreased consumption of fuel may be the result of the arrangement; and, indeed, such is the result in practice, though a carefully performedexperiment usually demonstrates a saving in fuel of 10 or 12 per cent. 

153. _Q. _--What other plans have been contrived for obviating the nuisanceof smoke?

_A. _--They are too various for enumeration, but most of them either operateupon the principle of admitting air into the flues to accomplish thecombustion of the uninflammable parts of the smoke, or seek to attain thesame object by passing the smoke over or through the fire or otherincandescent material. Some of the plans, indeed, profess to burn theinflammable gases as they are evolved from the coal, without permitting theadmixture of any of the uninflammable products of combustion which enterinto the composition of smoke; but this object has been very imperfectlyfulfilled in any of the contrivances yet brought under the notice of thepublic, and in some cases these contrivances have been found to createweightier evils than they professed to relieve. 

154. _Q. _--You refer, I suppose, to Mr. Charles Wye Williams' Argandfurnace?

_A. _--I chiefly refer to it, though I also comprehend all other schemes inwhich there is a continuous admission of air into the flues, with anintermittent generation of smoke. 

155. _Q. _--This is not so in Prideaux's furnace?

_A. _--No; in that furnace the air is admitted only during a certaininterval, or for so long, in fact, as there is smoke to be consumed. 

156. _Q. _--Will you explain the chief peculiarities of that furnace?

_A. _--The whole peculiarity is in the furnace door. The front of the doorconsists of metal Venetians, which are opened when the top lever is liftedup, and shut when that lever descends to its lowest position. When thefurnace door is opened to replenish the fire with coals, the top lever israised up, and with it the piston of the small cylinder attached to theside of the furnace. The Venetians are thereby opened, and a stream of airenters the furnace, which, being heated in its passage among the numerousheated plates attached to the back of the furnace door, is in a favorablecondition for effecting the combustion of the inflammable parts of thesmoke. The piston in the small cylinder gradually subsides and closes theVenetians; and the rate of the subsidence of the piston may obviously beregulated by a cock, or, as in this case, a small screw valve, so that theVenetians shall just close when there is no more smoke to be consumed;--theair or other fluid within the cylinder being forced out by the piston inits descent. 

157. _Q. _--Had Mr. Watt any method of consuming smoke?

_A. _--He tried various methods, but eventually fixed upon the method ofcoking the coal on a dead plate at the furnace door, before pushing it intothe fire. That method is perfectly effectual where the combustion is soslow that the requisite time for coking is allowed, and it is muchpreferable to any of the methods of admitting air at the bridge orelsewhere, to accomplish the combustion of the inflammable parts of thesmoke. 

158. _Q. _--What are the details of Mr. Watt's arrangement as now employed?

_A. _--The fire bars and the dead plate are both set at a considerableinclination, to facilitate the advance of the fuel into the furnace. InBoulton and Watt's 30 horse power land boiler, the dead plate and thefurnace bars are both about 4 feet long, and they are set at the angle of30 degrees with the horizon. 

159. _Q. _--Is the use of the dead plate universally adopted in Boulton andWatt's land boilers?

_A. _--It is generally adopted, but in some cases Boulton and Watt havesubstituted the plan of a revolving grate for consuming the smoke, and thedead plate then becomes both superfluous and inapplicable. In thiscontrivance the fire is replenished with coals by a self-acting mechanism. 

160. _Q. _--Will you explain the arrangement of the revolving grate?

_A. _--The fire grate is made like a round table capable of turninghorizontally upon a centre; a shower of coal is precipitated upon the gratethrough a slit in the boiler near the furnace mouth, and the smoke evolvedfrom the coal dropped at the front part of the fire is consumed by passingover the incandescent fuel at the back part, from which all the smoke musthave been expelled in the revolution of the grate before it can havereached that position. 

161. _Q. _--Is a furnace with a revolving grate applicable to a steamvessel?

_A. _--I see nothing to prevent its application. But the arrangement of theboiler would perhaps require to be changed, and it might be preferable tocombine its use with the employment of vertical tubes, for the transmissionof the smoke. The introduction of any effectual automatic contrivance forfeeding the fire in steam vessels, would bring about an important economy, at the same time that it would give the assurance of the work being betterdone. It is very difficult to fire furnaces by hand effectually at sea, especially in rough weather and in tropical climates; whereas machinerywould be unaffected by any such disturbing causes, and would perform withlittle expense the work of many men. 

162. _Q. _--The introduction of some mechanical method of feeding the firewith coals would enable a double tier of furnaces to be adopted in steamvessels without inconvenience?

_A. _--Yes, it would have at least that tendency; and as the space availablefor area of grate is limited in a steam vessel by the width of the vessel, it would be a great convenience if a double tier of furnaces could beemployed without a diminished effect. It appears to me, however, that theobjection would still remain of the steam raised by the lower furnace beingcooled and deadened by the air entering the ash-pit of the upper fire, forit would strike upon the metal of the ash-pit bottom. 

163. _Q. _--Have any other plans been devised for feeding the fire byself-acting means besides that of a revolving grate?

_A. _--Yes, many plans, but none of them, perhaps, are free from anobjectionable complication. In some arrangements the bars are made likescrews, which being turned round slowly, gradually carry forward the coal;while in other arrangements the same object is sought to be attained byalternately lifting and depressing every second bar at the end nearest themouth of the furnace. In Juckes' furnace, the fire bars are arranged in themanner of rows of endless chains working over a roller at the mouth of thefurnace, and another roller at the farther end of the furnace. Theserollers are put into slow revolution, and the coal which is deposited atthe mouth of the furnace is gradually carried forward by the motion of thechains, which act like an endless web. The clinkers and ashes left afterthe combustion of the coal, are precipitated into the ash-pit, where thechain turns down over the roller at the extremity of the furnace. InMessrs. Maudslays' plan of a self-feeding furnace the fire bars are formedof round tubes, and are placed transversely across the furnace. The ends ofthe bars gear into endless screws running the whole length of the furnace, whereby motion is given to the bars, and the coal is thus carried graduallyforward. It is very doubtful whether any of these contrivances satisfy allthe conditions required in a plan for feeding furnaces of the ordinary formby self-acting means, but the problem of providing a suitable contrivance, does not seem difficult of accomplishment, and will no doubt be effectedunder adequate temptation. 

164. _Q. _--Have not many plans been already contrived which consume thesmoke of furnaces very effectually?

_A. _--Yes, many plans; and besides those already mentioned there areHall's, Coupland's, Godson's, Robinson's, Stevens's, Hazeldine's, Indie's, Bristow and Attwood's, and a great number of others. One plan, whichpromises well, consists in making the flame descend through the fire bars, and the fire bars are formed of tubes set on an incline and filled withwater, which water will circulate with a rapidity proportionate to theintensity of the heat. After all, however, the best remedy for smokeappears to consist in removing from it those portions which form the smokebefore the coal is brought into use. Many valuable products may be got fromthe coal by subjecting it to this treatment; and the residuum will be morevaluable than before for the production of steam. 

STEAM. 

165. _Q. _--Have experiments been made to determine the elasticity of steamat different temperatures?

_A. _--Yes; very careful experiments. The following rule expresses theresults obtained by Mr. Southern:--To the given temperature in degrees ofFahrenheit add 51. 3 degrees; from the logarithm of the sum, subtract thelogarithm of 135. 767, which is 2. 1327940; multiply the remainder by 5. 13, and to the natural number answering to the sum, add the constantfraction . 1, which will give the elastic force in inches of mercury. If theelastic force be known, and it is wanted to determine the correspondingtemperature, the rule must be modified thus:--From the elastic force, ininches of mercury, subtract the decimal . 1, divide the logarithm of theremainder by 5. 13, and to the quotient add the logarithm 2. 1327940; findthe natural number answering to the sum, and subtract therefrom theconstant 51. 3; the remainder will be the temperature sought. The FrenchAcademy, and the Franklin Institute, have repeated Mr. Southern'sexperiments on a larger scale; the results obtained by them are not widelydifferent, and are perhaps nearer the truth, but Mr. Southern's results aregenerally adopted by engineers, as sufficiently accurate for practicalpurposes. 

166. _Q. _--Have not some superior experiments upon this subject been latelymade in France?

_A. _--Yes, the experiments of M. Regnault upon this subject have been veryelaborate and very carefully conducted, and the results are probably moreaccurate than have been heretofore obtained. Nevertheless, it isquestionable how far it is advisable to disturb the rules of Watt andSouthern, with which the practice of engineers is very much identified, forthe sake of emendations which are not of such magnitude as to influencematerially the practical result. M. Regnault has shown that the totalamount of heat, existing in a given weight of steam, increases slightlywith the pressure, so that the sum of the latent and sensible heats do notform a constant quantity. Thus, in steam of the atmospheric pressure, orwith 14. 7 Lbs. Upon the square inch, the sensible heat of the steam is 212degrees, the latent heat 966. 6 degrees, and the sum of the latent andsensible heats 1178. 6 degrees; whereas in steam of 90 pounds upon thesquare inch the sensible heat is 320. 2 degrees, the latent heat 891. 4degrees, and the sum of the latent and sensible heats 1211. 0 degrees. Thereis, therefore, 33 degrees less of heat in any given weight of water, raisedinto steam of the atmospheric pressure, than if raised into steam of 90Lbs. [1] pressure. 

167. _Q. _--What expansion does water undergo in its conversion into steam?

_A. _--A cubic inch of water makes about a cubic foot of steam of theatmospheric pressure. 

168. _Q. _--And how much at a higher pressure?

_A. _--That depends upon what the pressure is. But the proportion is easilyascertained, for the pressure and the bulk of a given quantity of steam, asof air or any other elastic fluid, are always inversely proportional to oneanother. Thus if a cubic inch of water makes a cubic foot of steam, withthe pressure of one atmosphere, it will make half a cubic foot with thepressure of two atmospheres, a third of a cubic foot with the pressure ofthree atmospheres, and so on in all other proportions. High pressure steamindeed is just low pressure steam forced into a less space, and thepressure will always be great in the proportion in which the space iscontracted. 

169. _Q. _--If this be so, the quantity of heat in a given weight of steammust be nearly the same, whether the steam is high or low pressure?

_A. _--Yes; the heat in steam is nearly a constant quantity, at allpressures, but not so precisely. Steam to which an additional quantity ofheat has been imparted after leaving the boiler, or as it is called"surcharged steam, " comes under a different law, for the elasticity of suchsteam may be increased without any addition being made to its weight; butsurcharged steam is not at present employed for working engines, and it maytherefore be considered in practice that a pound of steam contains verynearly the same quantity of heat at all pressures. 

170. _Q. _--Does not the quantity of heat in any body vary with thetemperature?

_A. _--Other circumstances remaining the same the quantity of heat in a bodyincreases with the temperatures. 

171. _Q. _--And is not high pressure steam hotter than low pressure steam?

_A. _--Yes, the temperature of steam rises with the pressure. 

172. _Q. _--How then comes it, that there is the same quantity of heat inthe same weight of high and low pressure steam, when the high pressuresteam has the highest temperature?

_A. _--Because although the temperature or sensible heat rises with thepressure, the latent heat becomes less in about the same proportion. And ashas been already explained, the latent and sensible heats taken togethermake up nearly the same amount at all temperatures; but the amount issomewhat greater at the higher temperatures. As a damp sponge becomes wetwhen subjected to pressure, so warm vapor becomes hot when forced into lessbulk, but in neither case does the quantity of moisture or the quantity ofheat sustain any alteration. Common air becomes so hot by compression thattinder may be inflamed by it, as is seen in the instrument for producinginstantaneous light by suddenly forcing air into a syringe. 

173. _Q. _--What law is followed by surcharged steam on the application ofheat?

_A. _--The same as that followed by air, in which the increments in volumeare very nearly in the same proportion as the increments in temperature;and the increment in volume for each degree of increased temperature is1/490th part of the volume at 32°. A volume of air which, at thetemperature of 32°, occupies 100 cubic feet, will at 212° fill a space of136. 73 cubic feet. The volume which air or steam--out of contact withwater--of a given temperature acquires by being heated to a highertemperature, the pressure remaining the same, may be found by the followingrule:--To each of the temperatures before and after expansion, add theconstant number 458: divide the greater sum by the less, and multiply thequotient by the volume at the lower temperature; the product will give theexpanded volume. 

174. _Q. _--If the relative volumes of steam and water are known, is itpossible to tell the quantity of water which should be supplied to aboiler, when the quantity of steam expended is specified?

_A. _--Yes; at the atmospheric pressure, about a cubic inch of water has tobe supplied to the boiler for every cubic foot of steam abstracted; atother pressures, the relative bulk of water and steam may be determined asfollows:--To the temperature of steam in degrees of Fahrenheit, add theconstant number 458, multiply the sum by 37. 3, and divide the product bythe elastic force of the steam in pounds per square inch; the quotient willgive the volume required. 

175. _Q. _--Will this rule give the proper dimensions of the pump forfeeding the boiler with water?

_A. _--No; it is necessary in practice thatthe feed pump should be able to supply the boiler with a much largerquantity of water than what is indicated by these proportions, from therisk of leaks, priming, or other disarrangements, and the feed pump isusually made capable of raising 3-1/2 times the water evaporated by theboiler. About 1/240th of the capacity of the cylinder answers very well forthe capacity of the feed pump in the case of low pressure engines, supposing the cylinder to be double acting, and the pump single acting; butit is better to exceed this size. 

176. _Q. _--Is this rule for the size of the feed pump applicable to thecase of high pressure engines?

_A. _--Clearly not; for since a cylinder full of high pressure steam, contains more water than the same cylinder full of low pressure steam, thesize of the feed must vary in the same proportion as the density of thesteam. In all pumps a good deal of the effect is lost from the imperfectaction of the valves; and in engines travelling at a high rate of speed, inparticular, a large part of the water is apt to return, through the suctionvalve of the pump, especially if much lift be permitted to that valve. Insteam vessels moreover, where the boiler is fed with salt water, and wherea certain quantity of supersalted water has to be blown out of the boilerfrom time to time, to prevent the water from reaching too high a degree ofconcentration, the feed pump requires to be of additional size to supplythe extra quantity of water thus rendered necessary. When the feed water isboiling or very hot, as in some engines is the case, the feed pump will notdraw from a depth, and will altogether act less efficiently, so that anextra size of pump has to be provided in consequence. These and otherconsiderations which might be mentioned, show the propriety of making thefeed pump very much larger than theory requires. The proper proportions ofpumps, however, forms part of a subsequent chapter. 

[1] A table containing the results arrived at by M. Regnault is given inthe Key. 

CHAPTER III. 

EXPANSION OF STEAM AND ACTION OF THE VALVES. 

177. _Q. _--What is meant by working engines expansively?

_A. _--Adjusting the valves, so that the steam is shut off from the cylinderbefore the end of the stroke, whereby the residue of the stroke is left tobe completed by the expanding steam. 

178. _Q. _--And what is the benefit of that practice?

_A. _--It accomplishes an important saving of steam, or, what is the samething, of fuel; but it diminishes the power of the engine, while increasingthe power of the steam. A larger engine will be required to do the samework, but the work will be done with a smaller consumption of fuel. If, forexample, the steam be shut off when only half the stroke is completed, there will only be half the quantity of steam used. But there will be morethan half the power exerted; for although the pressure of the steamdecreases after the supply entering from the boiler is shut off, yet itimparts, during its expansion, _some_ power, and that power, it is clear, is obtained without any expenditure of steam or fuel whatever. 

179. _Q. _--What will be the pressure of the steam, under suchcircumstances, at the end of the stroke?

_A. _--If the steam be shut off at half stroke, the pressure of the steam, reckoning the total pressure both below and above the atmosphere, will justbe one-half of what it was at the beginning of the stroke. It is a wellknown law of pneumatics, that the pressure of elastic fluids variesinversely as the spaces into which they are expanded or compressed. Forexample, if a cubic foot of air of the atmospheric density be compressedinto the compass of half a cubic foot, its elasticity will be increasedfrom 15 lbs. On the square inch to 30 lbs. On the square inch; whereas, ifits volume be enlarged to two cubic feet, its elasticity will be reduced to7-1/2 lbs. On the square inch, being just half its original pressure. Thesame law holds in all other proportions, and with all other gases andvapors, provided their temperature remains unchanged; and if the steamvalve of an engine be closed, when the piston has descended throughone- fourth of the stroke, the steam within the cylinder will, at the endof the stroke, just exert one-fourth of its initial pressure. 

180. _Q. _--Then by computing the varying pressure at a number of stages, the average or mean pressure throughout the stroke may be approximatelydetermined?

[Illustration: Fig. 32. Diagram showing law of expansion of steam in acylinder. ]

_A. _--Precisely so. Thus in the accompanying figure, (fig. 32), let E be acylinder, J the piston, _a_ the steam pipe, _c_ the upper port, _f_ thelower port, _d_ the steam pipe, prolonged to _e_ the equilibrium valve, _g_the eduction valve, M the steam jacket, N the cylinder cover, O stuffingbox, _n_ piston rod, P cylinder bottom; let the cylinder be supposed to bedivided in the direction of its length into any number of equal parts, saytwenty, and let the diameter of the cylinder represent the pressure of thesteam, which, for the sake of simplicity, we may take at 10 lbs. , so thatwe may divide the cylinder, in the direction of its diameter, into tenequal parts. If now the piston be supposed to descend through five of thedivisions, and the steam valve then be shut, the pressure at eachsubsequent position of the piston will be represented by a series, computedaccording to the laws of pneumatics, and which, if the initial pressure berepresented by 1, will give a pressure of . 5 at the middle of the stroke, and . 25 at the end of it. 

If this series be set off on the horizontal lines, it will mark out ahyperbolic curve--the area of the part exterior to which represents thetotal efficacy of the stroke, and the interior area, therefore, representsthe diminution in the power of a stroke, when the steam is cut off atone- fourth of the descent. If the squares above the point, where the steamis cut off, be counted, they will be found to amount to 50; and if thosebeneath that point be counted or estimated, they will be found to amount toabout 69. These squares are representative of the power exerted; so thatwhile an amount of power represented by 50 has been obtained by theexpenditure of a quarter of a cylinder full of steam, we get an amount ofpower represented by 69, without any expenditure of steam at all, merely bypermitting the steam first used to expand into four times its originalvolume. 

181. _Q. _--Then by working an engine expansively, the power of the steam isincreased, but the power of the engine is diminished?

_A. _--Yes. The efficacy of a given quantity of steam is more than doubledby expanding the steam four times, while the efficacy of each stroke ismade nearly one-half less. And, therefore, to carry out the expansiveprinciple in practice, the cylinder requires to be larger than usual, orthe piston faster than usual, in the proportion in which the expansion iscarried out. Every one who is acquainted with simple arithmetic, cancompute the terminal pressure of steam in a cylinder, when he knows theinitial pressure and the point at which the steam is cut off; and he canalso find, by the same process, any pressure intermediate between the firstand the last. By setting down these pressures in a table, and taking theirmean, he can determine the effect, with tolerable accuracy, of anyparticular measure of expansion. It is necessary to remark, that it is thetotal pressure of the steam that he must take; not the pressure above theatmosphere, but the pressure above a perfect vacuum. 

182. _Q. _--Can you give any rule for ascertaining at one operation theamount of benefit derivable from expansion?

_A. _--Divide the length of stroke through which the steam expands, by thelength of stroke performed with full pressure, which last call 1; thehyperbolic logarithm of the quotient is the increase of efficiency due toexpansion. According to this rule it will be found, that if a givenquantity of steam, the power of which working at full pressure isrepresented by 1, be admitted into a cylinder of such a size that itsingress is concluded when one-half the stroke has been performed, itsefficacy will be raised by expansion to 1. 69; if the admission of the steambe stopped at one-third of the stroke, the efficacy will be 2. 10; atone- fourth, 2. 39; at one-fifth, 2. 61; at one-sixth, 2. 79; at one-seventh, 2. 95; at one-eighth, 3. 08. The expansion, however, cannot be carriedbeneficially so far as one-eighth, unless the pressure of the steam in theboiler be very considerable, on account of the inconvenient size ofcylinder or speed of piston which would require to be adopted, thefriction of the engine, and the resistance of vapor in the condenser, whichall become relatively greater with a smaller urging force. 

183. _Q. _--Is this amount of benefit actually realized in practice?

_A. _--Only in some cases. It appears to be indispensable to the realizationof any large amount of benefit by expansion, that the cylinder should beenclosed in a steam jacket, or should in some other way be effectuallyprotected from refrigeration. In some engines not so protected, it has beenfound experimentally that less benefit was obtained from the fuel byworking expansively than by working without expansion--the whole benefitdue to expansion being more than counteracted by the increasedrefrigeration due to the larger surface of the cylinder required to developthe power. In locomotive engines, with outside cylinders, this condition ofthe advantageous use of expansion has been made very conspicuous, as hasalso been the case in screw steamers with four cylinders, and in which therefrigerating surface of the cylinders was consequently large. 

184. _Q. _--The steam is admitted to and from the cylinder by means of aslide or sluice valve?

[Illustration: Fig. 33. ]

_A. _--Yes; and of the slide valve there are many varieties; but the kindsmost in use are the D valve, --so called from its resemblance to a halfcylinder or D in its cross section--and the three ported valve, shown infig. 33, which consists of a brass or iron box set over the two ports oropenings into the cylinder, and a central port which conducts away thesteam to the atmosphere or condenser; but the length of the box is soadjusted that it can only cover one of the cylinder ports and the centralor eduction port at the same time. The effect, therefore, of moving thevalve up and down, as is done by the eccentric, is to establish aconnection alternately between each cylinder port and the central passagewhereby the steam escapes; and while the steam is escaping from beneath thepiston, the position of the valve is such, that a free communication existsbetween the space above the piston and the steam in the boiler. The pistonis thus urged alternately up and down--the valve so changing its positionbefore the piston arrives at the end of the stroke, that the pressure is bythat time thrown on the reverse side of the piston, so as to urge it intomotion in the opposite direction. 

185. _Q. _--Is the motion of the valve, then, the reverse of that of thepiston?

_A. _--No. The valve does not move down when the piston moves down, nor doesit move down when the piston moves up; but it moves from its mid position, to the extremity of its throw, and back again to its mid position, whilethe piston makes an upward or downward movement, so that the motion is asit were at right angles to the motion of the piston; or it is the samemotion that the piston of another engine, the crank of which is set atright angles with that of the first engine, would acquire. 

186. _Q. _--Then in a steam vessel the valve of one engine may be workedfrom the piston of the other?

_A. _--Yes, it may; or it may be worked from its own connecting rod; and inthe case of locomotive engines, this has sometimes been done. 

187. _Q. _--What is meant by the lead of the valve?

_A. _--The amount of opening which the valve presents for the admission ofthe steam, when the piston is just beginning its stroke. It is foundexpedient that the valve should have opened a little to admit steam on thereverse side of the piston before the stroke terminates; and the amount ofthis opening, which is given by turning the eccentric more or less roundupon the shaft, is what is termed the lead. 

188. _Q. _--And what is meant by the lap of the valve?

_A. _--It is an elongation of the valve face to a certain extent over theport, whereby the port is closed sooner than would otherwise be the case. This extension is chiefly effected at that part of the valve where thesteam is admitted, or upon the _steam side_ of the valve, as the technicalphrase is; and the intent of the extension is to close the steam passagebefore the end of the stroke, whereby the engine is made to operate to acertain extent expansively. In some cases, however, there is also a certainamount of lap given to the escape or eduction side, to prevent the eductionfrom being performed too soon when the lead is great; but in all casesthere is far less lap on the eduction than on the steam side, very oftenthere is none, and sometimes less than none, so that the valve is incapableof covering both the ports at once. 

189. _Q. _--What is the usual proportional length of stroke of the valve?

_A. _--The common stroke of the valve in rotative engines is twice thebreadth or depth of the port, and the length of the valve face will then bejust the breadth of the port when there is lap on neither the steam noreduction side. Whatever lap is given, therefore, makes the valve face justso much longer. In some engines, however, the stroke of the valve is a gooddeal more than twice the breadth of the port; and it is to the stroke ofthe valve that the amount of lap should properly be referred. 

190. _Q. _--Can you tell what amount of lap will accomplish any given amountof expansion?

_A. _--Yes, when the stroke of the valve is known. From the length of thestroke of the piston subtract that part of the stroke which is intended tobe accomplished before the steam is cut off; divide the remainder by thelength of the stroke of the piston, and extract the square root of thequotient, which multiply by half the stroke of the valve, and from theproduct take half the lead; the remainder will be the lap required. 

191. _Q. _--Can you state how we may discover at what point of the strokethe eduction passage will be closed?

_A. _--To find how much before the end of the stroke the eduction passagewill be closed:--to the lap on the steam side add the lead, and divide thesum by half the stroke of the valve; find the arc whose sine is equal tothe quotient, and add 90° to it. ; divide the lap on the eduction side byhalf the stroke of the valve, and find the arc whose cosine is equal to thequotient; subtract this arc from the one last obtained, and find the cosineof the remainder; subtract this cosine from 2, and multiply the remainderby half the stroke of the piston; the product is the distance of the pistonfrom the end of the stroke when the eduction passage is closed. 

192. _Q. _--Can you explain how we may determine the distance of the pistonfrom the end of the stroke, before the steam urging it onward is allowed toescape?

_A. _--To find how far the piston is from the end of its stroke when thesteam that is propelling it by expansion is allowed to escape to theatmosphere or condenser--to the lap on the steam side add the lead; dividethe sum by half the stroke of the valve, and find the arc whose sine isequal to the quotient; find the arc whose sine is equal to the lap on theeduction side, divided by half the stroke of the valve; add these two arcstogether and subtract 90°; find the cosine of the residue, subtract it from1, and multiply the remainder by half the stroke of the piston; the productis the distance of the piston from the end of its stroke when the steamthat is propelling it is allowed to escape into the atmosphere orcondenser. In using these rules, all the dimensions are to be taken ininches, and the answers will be found in inches also. 

193. _Q. _--Is it a benefit or a detriment to open the eduction passagebefore the end of the stroke?

_A. _--In engines working at a high rate of speed, such as locomotiveengines, it is very important to open the exhaust passage for the escape ofthe steam before the end of the stroke, as an injurious amount of backpressure is thus prevented. In the earlier locomotives a great loss ofeffect was produced from inattention to this condition; and when lap wasapplied to the valves to enable the steam to be worked expansively, it wasfound that a still greater benefit was collaterally obtained by the earlierescape of the steam from the eduction passages, and which was incidental tothe application of lap to the valves. The average consumption of coke permile was reduced by Mr. Woods from 40 lbs. Per mile to 15 lbs. Per mile, chiefly by giving a free outlet to the escaping steam. 

194. _Q. _--To what extent can expansion be carried beneficially by means oflap upon the valve?

_A. _--To about one-third of the stroke; that is, the valve may be made withso much lap, that the steam will be cut off when two thirds of the strokehave been performed, leaving the residue to be accomplished by the agencyof the expanding steam; but if more lap be put on than answers to thisamount of expansion, a very distorted action of the valve will be produced, which may impair the efficiency of the engine. If a further amount ofexpansion than this is wanted, it may be accomplished by wire drawing thesteam, or by so contracting the steam passage that the pressure within thecylinder must decline when the speed of the piston is accelerated, as it isabout the middle of the stroke. 

195. _Q. _--Will you explain how this result ensues?

_A. _--If the valve be so made as to shut off the steam by the time twothirds of the stroke have been performed, and the steam be at the same timethrottled in the steam pipe, the full pressure of the steam within thecylinder cannot be maintained except near the beginning of the stroke wherethe piston travels slowly; for, as the speed of the piston increases, thepressure necessarily subsides, until the piston approaches the other end ofthe cylinder, where the pressure would rise again but that the operation ofthe lap on the valve by this time has had the effect of closing thecommunication between the cylinder and steam pipe, so as to prevent moresteam from entering. By throttling the steam, therefore, in the manner hereindicated, the amount of expansion due to the lap may be doubled, so thatan engine with lap enough upon the valve to cut off the steam at two-thirdsof the stroke, may, by the aid of wire drawing, be virtually renderedcapable of cutting off the steam at one-third of the stroke. 

196. _Q. _--Is this the usual way of cutting off the steam?

_A. _--No; the usual way of cutting off the steam is by means of a separatevalve, termed an expansion valve; but such a device appears to be hardlynecessary in ordinary engines. In the Cornish engines, where the steam iscut off in some cases at one-twelfth of the stroke, a separate valve forthe admission of steam, other than that which permits its escape, is ofcourse indispensable; but in common rotative engines, which may realizeexpansive efficacy by throttling, a separate expansion valve does notappear to be required. 

197. _Q. _--That is, where much expansion is required, an expansion valve isa proper appendage, but where not much is required, a separate expansionvalve may be dispensed with?

_A. _--Precisely so. The wire drawing of the steam causes a loss of part ofits power, and the result will not be quite so advantageous by throttlingas by cutting off. But for moderate amounts of expansion it will suffice, provided there be lap upon the slide valve. 

198. _Q. _--Will you explain the structure or configuration of expansionapparatus of the usual construction?

[Illustration: Fig 34. ]

_A. _--The structure of expansion apparatus is very various; but all thekinds operate either on the principle of giving such a motion to the slidevalve as will enable it to cut off the steam, at the desired point, or onthe principle of shutting off the steam by a separate valve in the steampipe or valve casing. The first class of apparatus has not been found somanageable, and is not in extensive use, except in that form known as thelink motion. Of the second class, the most simple probably is theapplication of a cam giving motion to the throttle valve, or to a valve ofthe same construction, which either accurately fits the steam pipe, orwhich comes round to a face, which, however, it is restrained from touchingby a suitable construction of the cam. A kind of expansion valve, oftenemployed in marine engines of low speed, is the kind used in the Cornishengines, and known as the equilibrium valve. This valve is represented infig. 34. It consists substantially of an annulus or bulging cylinder ofbrass, with a steam-tight face both at its upper and lower edges, at whichpoints it fits accurately upon a stationary seat. This annulus may beraised or lowered without being resisted by the pressure of the steam, andin rotative engines it is usually worked by a cam on the shaft. Theexpansion cam is put on the shaft in two pieces, which are fastened to eachother by means of four bolts passing through lugs, and is fixed to theshaft by keys. A roller at one end of a bell-crank lever, which isconnected with the expansion valve, presses against the cam, so that themotion of the lever will work the valve. The roller is kept against the camby a weight on a lever attached to the same shaft, but a spring isnecessary for high speeds. If the cam were concentric with the shaft, thelever which presses upon it would remain stationary, and also the expansionvalve; but by the projection of the cam, the end of the lever receives areciprocating motion, which is communicated to the valve. 

199. _Q. _--The cam then works the valve?

_A. _--Yes. The position of the projection of the cam determines the pointin relation to the stroke at which the valve is opened, and itscircumferential length determines the length of the time during which thevalve continues open. The time at which the valve should begin to open isthe same under all circumstances, but the duration of its opening varieswith the amount of expansion desired. In order to obtain this variableextent of expansion, there are several projections made upon the cam, eachof which gives a different degree, or _grade_ as it is usually called, ofexpansion. These grades all begin at the same point on the cam, but are ofdifferent lengths, so that they begin to move the lever at the same time, but differ in the time of returning it to its original position. 

200. _Q. _--How is the degree of expansion changed?

_A. _--The change of expansion is effected by moving the roller on to thedesired grade; which is done by slipping the lever carrying the rollerendways on the shaft or pin sustaining it. 

201. _Q. _--Are such cams applicable in all cases?

_A. _--In engines moving at a high rate of speed the roller will be thrownback from the cam by its momentum, unless it be kept against it by means ofsprings. In some cases I have employed a spring formed of a great number ofdiscs of India rubber to keep the roller against the cam, but a few brassdiscs require to be interposed to prevent the India rubber discs from beingworn in the central hole. 

202. _Q. _--May not the percussion incident to the action of a cam at a highspeed, when the roller is not kept up to the face by springs, be obviatedby giving a suitable configuration to the cam itself?

_A. _--It may at all events be reduced. The outline of the cam should be aparabola, so that the valve may be set in motion precisely as a fallingbody would be; but it will, nevertheless, be necessary that the roller onwhich the cam presses should be forced upward by a spring rather than by acounterweight, as there will thus be less inertia or momentum in the massthat has to be moved. 

203. _Q. _--An additional slide valve is sometimes used for cutting off thesteam?

_A. _--Yes, very frequently; and the slide valve is sometimes on the side orback of the valve casing, and sometimes on the back of the main ordistributing valve, and moving with it. 

204. _Q. _--Are cams used in locomotive engines?

_A. _--In locomotive engines the use of cams is inadmissible, and otherexpedients are employed, of which those contrived by Stephenson and byCabrey operate on the principle of accomplishing the requisite variationsof expansion by altering the throw of the slide valve. 

205. _Q. _--What is Stephenson's arrangement?

[Illustration: Fig. 35. ]

_A. _--Stephenson connects the ends of the forward and backward eccentricrods by a link with a curved slot in which a pin upon the end of the valverod works. By moving this link so as to bring the forward eccentric rod inthe same line with the valve rod, the valve receives the motion due to thateccentric; whereas if the backward eccentric rod is brought in a line withthe valve rod, the valve gets the motion proper for reversing, and if thelink be so placed that the valve rod is midway between the two eccentricrods, the valve will remain nearly stationary. This arrangement, which isnow employed extensively, is what is termed "the link motion. " It isrepresented in the annexed figure, fig. 35, where _e_ is the valve rod, which is attached by a pin to an open curved link susceptible of beingmoved up and down by the bell-crank lever _f''_ _f''_, supported on thecentre _g_, and acting on the links _f_, while the valve rod _e_ remains inthe same horizontal plane; _d d'_ are the eccentric rods, and the link isrepresented in its lowest position. The dotted lines _h' h''_ show theposition of the eccentric rods when the link is in its highest position, and _l l'_ when in mid position. 

206. _Q. _--What is Cabrey's arrangement?

_A. _--Mr. Cabrey makes his eccentric rod terminate in a pin which worksinto a straight slotted lever, furnished with jaws similar to the jaws onthe eccentric rods of locomotives. By raising the pin of the eccentric rodin this slot, the travel of the valve will be varied, and expansive actionwill be the result. 

207. _Q. _--What other forms of apparatus are there for working steamexpansively?

_A. _--They are too numerous for description here, but a few of them may beenumerated. Fenton seeks to accomplish the desired object by introducing aspiral feather on the crank axle, by moving the eccentric laterally againstwhich the eccentric is partially turned round so as to cut off the steam ata different part of the stroke. Dodds seeks to attain the same end bycorresponding mechanical arrangements. Farcot, Edwards, and Lavagrian cutoff the steam by the application of a supplementary valve at the back ofthe ordinary valve, which supplementary valve is moved by tappets fixed tothe valve casing. Bodmer, in 1841, and Meyer, in 1842, employed two slidesor blocks fitted over apertures in the ordinary slide valve, and whichblocks were approximated or set apart by a right and left handed screwpassing through both. [1] Hawthorn, in 1843, employed as an expansion valvea species of frame lying on the ordinary cylinder face upon the outside ofthe valve, and working up against the steam side of the valve at each endso as to cut off the steam. In the same year Gonzenbach patented anarrangement which consists of an additional slide valve and valve casingplaced on the back of the ordinary slide valve casing, and through thissupplementary valve the steam must first pass. This supplementary valve isworked by a double ended lever, slotted at one end for the reception of apin on the valve link, the position of which in the slot determines thethrow of the supplementary valve, and the consequent degree of expansion. 

208. _Q. _--What is the arrangement of expansion valve used in the mostapproved modern engines?

_A. _--In modern engines, either marine or locomotive, it is found that ifthey are fitted with the link motion, as they nearly all are, a very goodexpansive action can be obtained by giving a suitable adjustment to it, without employing an expansion valve at all. Diagrams taken from enginesworked in this manner show a very excellent result, and most of the modernengines trust for their expansive working to the link motion and thethrottle valve. 

[1] In 1838 I patented an arrangement of expansion valve, consisting of twomovable plates set upon the ordinary slide valve, and which might be drawntogether or asunder by means of a right and left handed screw passingthrough both plates. The valve spindle was hollow, and a prolongation ofthe screw passed up through it, and was armed on the top with a smallwheel, by means of which the plates might be adjusted while the engine wasat work. In 1839 I fitted an expansion valve in a steam vessel, consistingof two plates, connected by a rod, and moved by tappets up against thesteam edges of the valve. In another steam vessel I fitted the same speciesof valve, but the motion was not derived from tappets, but from a movingpart of the engine, though at the moderate speed at which these enginesworked I found tappets to operate well and make little noise. In 1837 Iemployed, as an expansion valve, a rectangular throttle valve, accuratelyfitting a bored out seat, in which it might be made to revolve, though itdid not revolve in working. This valve was moved by a pin in a pinion, making two revolutions for every revolution of the engine, and theconfiguration of the seat determined the amount of the expansion. In 1855 Ihave again used expansion valves of this construction in engines making onehundred revolutions per minute, and with perfectly satisfactory results. --J. B. 

CHAPTER IV. 

MODES OF ESTIMATING THE POWER AND PERFORMANCE OF ENGINES AND BOILERS. 

HORSES POWER. 

209. _Q. _--What do you understand by a horse power?

_A. _--An amount of mechanical force that will raise 33, 000 lbs. One foothigh in a minute. This standard was adopted by Mr. Watt, as the averageforce exerted by the strongest London horses; the object of hisinvestigation being to enable him to determine the relation between thepower of a certain size of engine and the power of a horse, so that when itwas desired to supersede the use of horses by the erection of an engine, hemight, from the number of horses employed, determine the size of enginethat would be suitable for the work. 

210. _Q. _--Then when we talk of an engine of 200 horse power, it is meantthat the impelling efficacy is equal to that of 200 horses, each lifting33, 000 lbs. One foot high in a minute?

_A. _--No, not now; such was the case in Watt's engines, but the capacity ofcylinder answerable to a horse power has been increased by most engineerssince his time, and the pressure on the piston has been increased also, sothat what is now called a 200 horse power engine exerts, almost in everycase, a greater power than was exerted in Watt's time, and a horse power, in the popular sense of the term, has become a mere conventional unit forexpressing a certain size of engine, without reference to the powerexerted. 

211. _Q. _--Then, each nominal horse power of a modern engine may raise muchmore than 33, 000 lbs. One foot high in a minute?

_A. _--Yes; some raise 52, 000 lbs. , others 60, 000 lbs. , and others 66, 000lbs. , one foot high in a minute by each nominal horse power. Some enginesindeed work as high as five times above the nominal power, and therefore nocomparison can be made between the performances of different engines, unless the power actually exerted be first discovered. 

212. _Q. _--How is the power actually exerted by engines ascertained?

_A. _--By means of an instrument called the indicator, which is a miniaturecylinder and piston attached to the cylinder cover of the main engine, andwhich indicates, by the pressure exerted on a spring, the amount ofpressure or vacuum existing within the cylinder. From this pressure, expressed in pounds per square inch, deduct a pound and a half of pressurefor friction, the loss of power in working the air pump, &c. ; multiply thearea of the piston in square inches by this residual pressure, and by themotion of the piston, in feet per minute, and divide by 33, 000; thequotient is the actual number of horses power of the engine. The sameresult is attained by squaring the diameter of the cylinder, multiplying bythe pressure per square inch, as shown by the indicator, less a pound and ahalf, and by the motion of the piston, in feet per minute, and dividing by42, 017. 

213. _Q. _ How is the nominal power of an engine ascertained?

_A. _--Since the nominal power is a mere conventional expression, it isclear that it must be determined by a merely conventional process. Thenominal power of ordinary condensing engines may be ascertained by thefollowing rule: multiply the square of the diameter of the cylinder ininches, by the velocity of the piston in feet per minute, and divide theproduct by 6, 000; the quotient is the number of nominal horses power. Inusing this rule, however, it is necessary to adopt the speed of pistonprescribed by Mr. Watt, which varies with the length of the stroke. Thespeed of piston with a 2 feet stroke is, according to his system, 160 perminute; with a 2 ft. 6 in. Stroke, 170; 3 ft. , 180; 3 ft. 6 in. , 189; 4ft. , 200; 5 ft. , 215; 6 ft. , 228; 7 ft. , 245; 8 ft. , 256 ft. 

214. _Q. _--Does not the speed of the piston increase with the length of thestroke?

_A. _--It does: the speed of the piston varies nearly as the cube root ofthe length of the stroke. 

215. _Q. _--And may not therefore some multiple of the cube root of thelength of the stroke be substituted for the velocity of the piston indetermining the nominal power?

_A. _--The substitution is quite practicable, and will accomplish somesimplification, as the speed of piston proper for the different lengths ofstroke cannot always be remembered. The rule for the nominal power ofcondensing engines when thus arranged, will be as follows: multiply thesquare of the diameter of the cylinder in inches by the cube root of thestroke in feet, and divide the product by 47; the quotient is the number ofnominal horses power of the engine, supposing it to be of the ordinarycondensing description. This rule assumes the existence of a uniformeffective pressure upon the piston of 7 lbs. Per square inch; Mr. Wattestimated the effective pressure upon the piston of his 4 horse powerengines at 6-8 lbs. Per square inch, and the pressure increased slightlywith the power, and became 6. 94 lbs. Per square inch in engines of 100horse power; but it appears to be more convenient to take a uniformpressure of 7 lbs. For all powers. Small engines, indeed, are somewhat lesseffective in proportion than large ones, but the difference can be made upby slightly increasing the pressure in the boiler; and small boilers willbear such an increase without inconvenience. 

216. _Q. _--How do you ascertain the power of high pressure engines?

_A. _--The actual power is readily ascertained by the indicator, by the sameprocess by which the actual power of low pressure engines is ascertained. The friction of a locomotive engine when unloaded is found by experiment tobe about 1 lb. Per square inch on the surface of the pistons, and theadditional friction caused by any additional resistance is estimated atabout . 14 of that resistance; but it will be a sufficiently nearapproximation to the power consumed by friction in high pressure engines, if we make a deduction of a pound and a half from the pressure on thataccount, as in the case of low pressure engines. High pressure engines, itis true, have no air pump to work; but the deduction of a pound and a halfof pressure is relatively a much smaller one where the pressure is high, than where it does not much exceed the pressure of the atmosphere. Therule, therefore, for the actual horse power of a high pressure engine willstand thus: square the diameter of the cylinder in inches, multiply by thepressure of the steam in the cylinder per square inch less 1-1/2 lb. , andby the speed of the piston in feet per minute, and divide by 42, 017; thequotient is the actual horse power. 

217. _Q. _--But how do you ascertain the nominal horse power of highpressure engines?

_A. _--The nominal horse power of a high pressure engine has never beendefined; but it should obviously hold the same relation to the actual poweras that which obtains in the case of condensing engines, so that an engineof a given nominal power may be capable of performing the same work, whether high pressure or condensing. This relation is maintained in thefollowing rule, which expresses the nominal horse power of high pressureengines: multiply the square of the diameter of the cylinder in inches bythe cube root of the length of stroke in feet, and divide the product by15. 6. This rule gives the nominal power of a high pressure engine threetimes greater than that of a low pressure engine of the same dimensions;the average effective pressure being taken at 21 lbs. Per square inchinstead of 7 lbs. , and the speed of the piston in feet per minute being inboth rules 128 times the cube root of the length of stroke. [1]

218. _Q. _--Is 128 times the cube root of the stroke in feet per minute theordinary speed of all engines?

_A. _--Locomotive engines travel at a quicker speed--an innovation broughtabout not by any process of scientific deduction, but by the accidents andexigencies of railway transit. Most other engines, however, travel at aboutthe speed of 128 times the cube root of the stroke in feet; but some marinecondensing engines of recent construction travel at as high a rate as 700feet per minute. To mitigate the shock of the air pump valves in cases inwhich a high speed has been desirable, as in the case of marine enginesemployed to drive the screw propeller without intermediate gearing, Indiarubber discs, resting on a perforated metal plate, are now generallyadopted; but the India rubber should be very thick, and the guards employedto keep the discs down should be of the same diameter as the discsthemselves. 

219. _Q. _--Can you suggest any eligible method of enabling condensingengines to work satisfactorily at a high rate of speed?

_A. _--The most feasible way of enabling condensing engines to worksatisfactorily at a high speed, appears to lie in the application ofbalance weights to the engine, so as to balance the momentum of its movingparts, and the engine must also be made very strong and rigid. It appearsto be advisable to perform the condensation partly in the air pump, insteadof altogether in the condenser, as a better vacuum and a superior action ofthe air pump valves will thus be obtained. Engines constructed upon thisplan may be driven at four times the speed of common engines, whereby anengine of large power may be purchased for a very moderate price, and becapable of being put into a very small compass; while the motion, frombeing more equable, will be better adapted for most purposes for which arotary motion is required. Even for pumping mines and blowing ironfurnaces, engines of this kind appear likely to come into use, for they aremore suitable than other engines for driving the centrifugal pump, which inmany cases appears likely to supersede other kinds of pumps for liftingwater; and they are also conveniently applicable to the driving of fans, which, when so arranged that the air condensed by one fan is employed tofeed another, and so on through a series of 4 or 5, have succeeded inforcing air into a furnace with a pressure of 2-1/2 lbs. On the squareinch, and with a far steadier flow than can be obtained by a blast enginewith any conceivable kind of compensating apparatus. They are equallyapplicable if blast cylinders be employed. 

220. _Q. _--Then, if by this modification of the engine you enable it towork at four times the speed, you also enable it to exert four times thepower?

_A. _--Yes; always supposing it to be fully supplied with steam. The nominalpower of this new species of engine can readily be ascertained by takinginto account the speed of the piston, and this is taken into account by theAdmiralty rule for power. 

221. _Q. _--What is the Admiralty rule for determining the power of anengine?

_A. _--Square the diameter of the cylinder in inches, which multiply by thespeed of the piston in feet per minute, and divide by 6, 000; the quotientis the power of the engine by the Admiralty rule. [2]

222. _Q. _--The high speed engine does not require so heavy a fly wheel ascommon engines?

_A. _--No; the fly wheel will be lighter, both by virtue of its greatervelocity of rotation, and because the impulse communicated by the piston isless in amount and more frequently repeated, so as to approach more nearlyto the condition of a uniform pressure. 

223. _Q. _--Can nominal be transformed into actual horse power?

_A. _--No; that is not possible in the case of common condensing engines. The actual power exerted by an engine cannot be deduced from its nominalpower, neither can the nominal power be deduced from the power actuallyexerted, or from anything else than the dimensions of the cylinder. Theactual horse power being a dynamical unit, and the nominal horse power ameasure of capacity of the cylinder, are obviously incomparable things. 

224. _Q. _--That is, the _nominal_ power is a commercial unit by whichengines are bought and sold, and the _actual_ power a scientific unit bywhich the quality of their performance is determined?

_A. _--Yes; the nominal power is as much a commercial measure as a yard or abushel, and is not a thing to be ascertained by any process of science, butto be fixed by authority in the same manner as other measures. The actualpower, on the contrary, is a mechanical force or dynamical effort capableof raising a given weight through a given distance in a given time, and ofwhich the amount is ascertainable by scientific investigation. 

225. _Q. _--Is there any other measure of an actual horse power than 33, 000lbs. Raised one foot high in the minute?

_A. _--There cannot be any _different_ measure, but there are severalequivalent measures. Thus the evaporation of a cubic foot of water in thehour, or the expenditure of 33 cubic feet of low pressure steam per minute, is reckoned equivalent to an actual horse power, or 528 cubic feet of waterraised one foot high in the minute involves the same result. 

[1] Tables of the horse power of both high and low pressure engines are given in the Key. 

[2] Example. --What is the power of an engine of 42 inches diameter, 3-1/2 feet stroke, and making 85 strokes per minute? The speed of the piston will be 7 (the length of a double stroke) x 85 = 595 feet per minute. Now 42 x 42 = 1, 764 x 595 = 1, 049, 580 / 6, 000 = 175 horses power. 

DUTY OF ENGINES AND BOILERS. 

226. _Q. _--What is meant by the duty of a engine?

_A. _--The work done in relation to the fuel consumed. 

227. _Q. _--And how is the duty ascertained?

_A. _--In ordinary mill or marine engines it can only be ascertained by theindicator, as the load upon such engines is variable, and cannot readily bedetermined; but in the case of engines pumping water, where the load isconstant, the number of strokes performed by the engine will represent thework done, and the amount of work done by a given quantity of coal

represents the duty. In Cornwall the duty of an engine is expressed by thenumber of millions of pounds raised one foot high by a bushel, or 94 lbs. Of Welsh coal. A bushel of Newcastle coal will only weigh 84 Lbs. ; and incomparing the duty of a Cornish engine with the performance of an engine insome locality where a different kind of coal is used, it is necessary topay regard to such variations. 

228. _Q. _--Can you tell the duty of an engine when you know its consumptionof coal per horse power per hour?

_A. _--Yes, if the power given be the actual, and not the nominal, power. Divide 166. 32 by the number of pounds of coal consumed per actual horsepower per hour; the quotient is the duty in millions of pounds. If youalready have the duty in millions of pounds, and wish to know theequivalent consumption in pounds per actual horse power per hour, divide166. 32 by the duty in millions of pounds; the quotient is the consumptionper actual horse power per hour. The duty of a locomotive engine isexpressed by the weight of coke it consumes in transporting a ton throughthe distance of one mile upon a railway; but this is a very imperfectmethod of representing the duty, as the tractive efficacy of a pound ofcoke becomes less as the speed of the locomotive becomes greater; and thelaw of variation is not accurately known. 

229. _Q. _--What amount of power is generated in good engines of theordinary kind by a given weight of coal?

_A. _--The duty of different kinds of engines varies very much, and thereare also great differences in the performance of different engines of thesame class. In ordinary rotative condensing engines of good construction, 10 lbs. Of coal per nominal horse power per hour is a common consumption;but such engines exert nearly twice their nominal power, so that theconsumption per actual horse power per hour may be taken at from 5 to 6lbs. Engines working very expansively, however, attain an economy muchsuperior to this. The average duty of the pumping engines in Cornwall isabout 60, 000, 000 lbs. Raised 1 ft. High by a bushel of Welsh coals, whichweighs 94 lbs. This is equivalent to a consumption of 3. 1 lbs. Of coal peractual horse power per hour; but some engines reach a duty of above100, 000, 000, or 1. 74 lbs. Of coal per actual horse power per hour. Locomotives consume from 8 to 10 lbs. Of coke in evaporating a cubic footof water, and the evaporation of a cubic foot of water per hour may be setdown as representing an actual horse power in locomotives as well as incondensing engines, if expansion be not employed. When the locomotive isworked expansively, however, there is of course a less consumption of waterand fuel per horse power, or per ton per mile, than when the full pressureis used throughout the stroke; and most locomotives now operate with asmuch expansion as can be conveniently given by the slide valves. 

230. _Q. _--But is not the evaporative power of locomotives affectedmaterially by the proportions of the boiler?

_A. _--Yes, but this may be said of all boilers; but in locomotive boilers, perhaps, the effect of any misproportion becomes more speedily manifest. Ahigh temperature of the fire box is found to be conducive to economy offuel; and this condition, in its turn, involves a small area of grate bars. The heating surface of locomotive boilers should be about 80 square feetfor each square foot of grate bars, and upon each foot of grate bars about1 cwt. Of coke should be burnt in the hour. 

231. _Q. _--Probably the heat is more rapidly absorbed when the temperatureof the furnace is high?

_A. _--That seems to be the explanation. The rapidity with which a hot bodyimparts heat to a colder, varies as the square of the difference oftemperature; so that if the temperature of the furnace be very high, thelarger part of the heat passes into the water at the furnace, therebyleaving little to be transmitted by the tubes. If, on the contrary, thetemperature of the furnace be low, a large part of the heat will pass intothe tubes, and more tube surface will be required to absorb it. About 16cubic feet of water should be evaporated by a locomotive boiler for each, square foot of fire grate, which, with the proportion of heating surfacealready mentioned, leaves 5 square feet of heating surface to evaporate acubic foot of water in the hour. This is only about half the amount ofsurface usual in land and marine boilers per cubic foot evaporated, and itssmall amount is due altogether to the high temperature of the furnace, which, by the rapidity of transmission it causes, is tantamount to anadditional amount of heating surface. 

232. _Q. _--You have stated that the steam and vacuum gauges are generallyglass tubes, up which mercury is forced by the steam or sucked by thevacuum?

_A. _--Vacuum gauges are very often of this construction, but steamgauges more frequently consist of a small iron tube, bent like the letterU, and into which mercury is poured. The one end of this tube communicateswith the boiler, and the other end with the atmosphere; and when thepressure of the steam rises in the boiler, the mercury is forced down inthe leg communicating with the boiler and rises in the other leg, and thedifference of level in the legs denotes the pressure of the steam. In thisgauge a rise of the mercury one inch in the one leg involves a differenceof the level between the two legs of two inches, and an inch of rise is, therefore, equivalent to two inches of mercury, or a pound of pressure. Asmall float of wood is placed in the open leg to show the rise or fall ofthe mercury, and this leg is surmounted by a brass scale, graduated ininches, to the marks of which the float points. 

233. _Q. _--What other kinds of steam and vacuum gauges are there?

_A. _--There are many other kinds; but probably Bourdon's gauges are now inmore extended use than, any other, and their operation has been found to besatisfactory in practice. The principle of their action may be explained tobe, that a thin elliptical metal tube, if bent into a ring, will seek tocoil or uncoil itself if subjected to external or internal pressure, and toan extent proportional to the pressure applied. The end of the tube issharpened into an index, and moves to an extent corresponding to thepressure applied to the tube; but in the more recent forms of thisapparatus, a dial and a hand, like those of a clock, are employed, and thehand is moved round by a toothed sector connected to the tube, and whichsector acts on a pinion attached to the hand. Mr. Shank, of Paisley, haslately introduced a form of steam gauge like a thermometer, with aflattened bulb; and the pressure of the steam, by compressing the bulb, causes the mercury to rise to a point proportional to the pressure applied. 

THE INDICATOR. 

234. _Q. _--You have already stated that the actual power of an engine isascertained by an instrument called the indicator, which consists of asmall cylinder with a piston moving against a spring, and compressing it toan extent answerable to the pressure of the steam. Will you explain furtherthe structure and mode of using that instrument?

[Illustration: Fig. 36]

_A. _--The structure of the common form of indicator will be most readilyapprehended by a reference to fig. 36, which is a McNaught's indicator. Upon a movable barrel A, a piece of paper is wound, the ends of which aresecured by the slight brass clamps shown in the drawing. The barrel issupported by the bracket _b_, proceeding from the body of the indicator, and at the bottom of the barrel a watch spring is coiled with one endattached to the barrel and the other end to the bracket, so that when thebarrel is drawn round by a string wound upon its lower end like a rollerblind, the spring returns the barrel to its original position, when thestring is relaxed. The string is attached to some suitable part of theengine, and at every stroke the string is drawn out, turning round thebarrel, and the barrel is returned again by the spring on the returnstroke. 

235. _Q_--But in what way can these reciprocations of the barrel determinethe power of the engine?

_A. _--They do not determine it of themselves, but are only part of theoperation. In the inside of the cylinder _c_ there is a small piston movingsteam tight in a cylinder of which _d_ is the piston rod, and _e_ a spiralspring of steel, which the piston, when forced upwards by the steam orsucked downwards by the vacuum, either compresses or extends; _f_ is a cockattached to the cylinder of the indicator, and which is screwed into thecylinder cover. It is obvious that, so soon as this cock is opened, thepiston will be forced up when the space above the piston of the engine isopened to the boiler, and sucked down when that space is opened to thecondenser--in each case to an extent proportionate to the pressure of thesteam or the perfection of the vacuum, the top of the piston _c_ being opento the atmosphere. A pencil, _p_, with a knife hinge, is inserted into thepiston rod, at _e_, and the point of the pencil bears upon the surface ofthe paper wound upon the drum A. If the drum A did not revolve, this pencilwould merely trace on the paper a vertical line; but as the drum A movesround and back again every stroke of the engine, and as the pencil moves upand down again every stroke of the engine, the combined movements traceupon the paper a species of rectangle, which is called an indicatordiagram; and the nature of this diagram determines the nature of theengine's performance. 

236. _Q. _--How does it do this?

_A. _--It is clear that if the pencil was moved up instantaneously to thetop of its stroke, and was also moved down instantaneously to the bottom ofits stroke, and if it remained without fluctuation while at the top andbottom, the figure described by the pencil would be a perfect rectangle, ofwhich the vertical height would represent the total pressure of the steamand vacuum, and therefore the total pressure urging the piston of theengine. But in practice the pencil will neither rise nor fallinstantaneously, nor will it remain at a uniform height throughout thestroke. If the steam be worked expansively the pressure will begin to fallso soon as the steam is cut off; and at the end of the stroke, when thesteam comes to be discharged, the subsidence of pressure will not beinstantaneous, but will occupy an appreciable time. It is clear, therefore, that in no engine can the diagram described by an indicator be a completerectangle; but the more nearly it approaches to a rectangle, the largerwill be the power produced at every stroke with any given pressure, and thearea of the space included within the diagram will in every case accuratelyrepresent the power exerted by the engine during that stroke. 

237. _Q. _--And how is this area ascertained?

_A. _--It may be ascertained in various ways; but the usual mode is to takethe vertical height of the diagram at a number of equidistant points on abase line, and then to take the mean of these several heights asrepresentative of the mean pressure actually urging the piston. Now if youhave the pressure on the piston per square inch, and if you know the numberof square inches in its area, and the velocity with which it moves in feetper minute, you have obviously the dynamical effort of the engine, or, inother words, its actual power. 

238. _Q. _--How is the base line you have referred to obtained?

_A. _--In proceeding to take an indicator diagram, the first thing to bedone is to allow the barrel to make two or three reciprocations with thepencil resting against it, before opening the cock attached to thecylinder. There will thus be traced a horizontal line, which is called the_atmospheric line_, and in condensing engines, a part of the diagram willbe above and a part of it below this line; whereas, in high pressureengines the whole of the diagram will be above this line. Upon this linethe vertical ordinates may be set off at equal distances, or upon any baseline parallel to it; but the usual course is to erect the ordinates on theatmospheric line. 

239. _Q. _--Will you give an example of an indicator diagram?

[Illustration: Fig. 37]

_A. _--Fig. 37 is an indicator diagram taken from a low pressure engine, andthe waving line _a b c_, forming a sort of irregular parallelogram, is thatwhich is described by the pencil. The atmospheric line is represented bythe line o o. The scale at the side shows the pressure of the steam, whichin this engine rose to about 9 lbs. Per square inch, and the vacuum fell to11 lbs. The steam begins to be cut off when, about one-fourth of the strokehas been performed, and the pressure consequently falls. 

240. _Q. _--Is this species of indicator which you have just describedapplicable to locomotive engines?

_A. _--It is no doubt applicable under suitable conditions; but anotherspecies of indicator has been applied by Mr. Gooch to locomotive engines, which presents several features of superiority for such a purpose. 

This indicator has its cylinder placed horizontally; and its pistoncompresses two elliptical springs; a slide valve is substituted for a cock, to open or close the communication with the engine. The top of the pistonrod of this indicator is connected to the short arm of a smaller lever, tothe longer arm of which the pencil is attached, and the pencil has thus aconsiderably larger amount of motion than the piston; but it moves in thearc of a circle instead of in a straight line. The pencil marks on a web ofpaper, which is unwound from one drum and wound on to another, so that asuccession of diagrams are taken without the necessity of any intermediatemanipulation. 

241. _Q. _--These diagrams being taken with a pencil moving in an arc, willbe of a distorted form?

_A. _--They will not be of the usual form, but they may be easily translatedinto the usual form. It is undoubtedly preferable that the indicator shouldact immediately in the production of the final form of diagram. 

DYNAMOMETER, GAUGES, AND CATARACT. 

242. _Q. _--What other gauges or instruments are there for telling thestate, or regulating the power of an engine?

_A. _--There is the counter for telling the number of strokes the enginemakes, and the dynamometer for ascertaining the tractive power of steamvessels or locomotives; then there are the gauge cocks, and glass tubes, orfloats, for telling the height of water in the boiler; and in pumpingengines there is the cataract for regulating the speed of the engine. 

243. _Q. _--Will you describe the mechanism of the counter?

_A. _--The counter consists of a train of wheel work, so contrived that byevery stroke of the engine an index hand is moved forward a certain space, whereby the number of strokes made by the engine in any given time isaccurately recorded. In most cases the motion is communicated by means of adetent, --attached to some reciprocating part of the engine, --to a ratchetwheel which gives motion to the other wheels in its slow revolution; but itis preferable to derive the motion from some revolving part of the engineby means of an endless screw, as where the ratchet is used the detent willsometimes fail to carry it round the proper distance. In the countercontrived by Mr. Adie, an endless screw works into the rim of two smallwheels situated on the same axis, but one wheel having a tooth more thanthe other, whereby a differential motion is obtained; and the difference inthe velocity of the two wheels, or their motion upon one another, expressesthe number of strokes performed. The endless screw is attached to somerevolving part of the engine, whereby a rotatory motion is imparted to it;and the wheels into which the screws work hang down from it like apendulum, and are kept stationary by the action of gravity. 

244. _Q. _--What is the nature of the dynamometer?

_A. _--The dynamometer employed for ascertaining the traction upon railwaysconsists of two flat springs joined together at the ends by links, and theamount of separation of the springs at the centre indicates, by means of asuitable hand and dial, the force of traction. A cylinder of oil, with asmall hole through its piston, is sometimes added to this instrument toprevent sudden fluctuations. In screw vessels the forward thrust of thescrew is measured by a dynamometer constructed on the principle of aweighing machine, in which a small spring pressure at the index willbalance a very great pressure where the thrust is employed; and in eachcase the variations of pressure are recorded by a pencil on a sheet ofpaper, carried forward by suitable mechanism, whereby the mean thrust iseasily ascertained. The tractive force of paddle wheel steamers isascertained by a dynamometer fixed on shore, to which the floating vesselis attached by a rope. Sometimes the power of an engine is ascertained by afriction break dynamometer applied to the shaft. 

345. _Q. _--What will determine the amount of thrust shown by thedynamometer?

_A. _--In locomotives and in paddle steamers it will be determined by theforce turning the wheels, and by the smallness of the diameter of thewheels; for with small wheels the thrust will be greater than with largewheels. In screw vessels the thrust will be determined by the force turninground the screw, and by the smallness of the screw's pitch; for with anygiven force of torsion a fine pitch of screw will give a greater thrustthan a coarse pitch of screw, just as is the case when a screw works in asolid nut. 

246. _Q. _--Will you explain the use of the glass gauges affixed to theboiler?

_A. _--The glass gauges are tubes affixed to the fronts of boilers, by theaid of which the height of the water within the boilers is readilyascertainable, for the water will stand at the same height in the tube asin the boiler, with which there is a communication maintained both at thetop and bottom of the tube by suitable stopcocks. The cocks connecting theglass tube with the boiler should always be so constructed that the tubemay be blown through with the steam, to clear it of any internal concretionthat may impair its transparency; and the construction of the sockets inwhich the tube is inserted should be such, that, even when there is steamin the boiler, a broken tube may be replaced with facility. 

247. _Q. _--What then are the gauge cocks?

_A. _--The gauge cocks are cocks penetrating the boiler at differentheights, and which, when opened, tell whether it is water or steam thatexists at the level at which they are respectively inserted. It is unsafeto trust to the glass gauges altogether as a means of ascertaining thewater level, as sometimes they become choked, and it is necessary, therefore, to have gauge cocks in addition; but if the boiler be short ofsteam, and a partial vacuum be produced within it, the glass gauges becomeof essential service, as the gauge cocks will not operate in such a case, for though opened, instead of steam and water escaping from them, the airwill rush into the boiler. It is expedient to carry a pipe from the lowerend of the glass tube downward into the water of the boiler, and a pipefrom the upper end upward into the steam in the boiler, so as to preventthe water from boiling down through the tube, as it might otherwise do, andprevent the level of the water from being ascertainable. The average levelof water in the boiler should be above the centre of the tube; and thelowest of the gauge cocks should always run water, and the highest shouldalways blow steam. 

248. _Q. _--Is not a float sometimes employed to indicate the level of thewater in the boiler?

_A. _--A float for telling the height of water in the boiler is employedonly in the case of land boilers, and its action is like that of a buoyfloating on the surface, which, by means of a light rod passing verticallythrough the boiler, shows at what height the water stands. The float isusually formed of stone or iron, and is so counterbalanced as to make itsoperation the same as if it were a buoy of timber; and it is generally putin connection with the feed valve, so that in proportion as the floatrises, the supply of feed water is diminished. The feed water in landboilers is admitted from a small open cistern, situated at the top of anupright or stand pipe set upon the boiler, and in which there is a columnof water sufficiently high to balance the pressure of the steam. 

249. _Q. _--What is the cataract which is employed to regulate the speed ofpumping engines?

[Illustration: Fig. 38. ]

_A. _--The cataract consists of a small pump-plunger _b_ and barrel, set ina cistern of water, the barrel being furnished on the one side with avalve, _c_, opening inwards, through which the water obtains admission tothe pump chamber from the cistern, and on the other by a plug, _d_, throughwhich, if the plunger be forced down, the water must pass out of the pumpchamber. The engine in the upward stroke of the piston, which isaccomplished by the preponderance of weight at the pump end of the beam, raises up the plunger of the cataract by means of a small rod, --the waterentering readily through the valve already referred to; and when the enginereaches the top of the stroke, it liberates the rod by which the plungerhas been drawn up, and the plunger then descends by gravity, forcing outthe water through the cock, the orifice of which has previously beenadjusted, and the plunger in its descent opens the injection valve, whichcauses the engine to make a stroke. 

250. _Q. _--Suppose the cock of the cataract be shut?

_A. _--If the cock of the cataract be shut, it is clear that the plungercannot descend at all, and as in that case the injection valve cannot beopened, the engine must stand still; but if the cock be slightly opened, the plunger will descend slowly, the injection valve will slowly open, andthe engine will make a gradual stroke as it obtains the water necessary forcondensation. The extent to which the cock is open, therefore, willregulate the speed with which the engine works; so that, by the use of thecataract, the speed of the engine may be varied to suit the variations inthe quantity of water requiring to be lifted from the mine. In some casesan air cylinder, and in other cases an oil cylinder, is employed instead ofthe apparatus just described; but the principle on which the whole of thesecontrivances operate is identical, and the only difference is in thedetail. 

251. _Q. _--You have now shown that the performance of an engine isdeterminable by the indicator; but how do you determine the power of theboiler?

_A. _--By the quantity of water it evaporates. There is, however, no veryconvenient instrument for determining the quantity of water supplied to aboiler, and the consequence is that this element is seldom ascertained. 

CHAPTER V. 

PROPORTION OF BOILERS. 

HEATING AND FIRE GRATE SURFACE. 

252. _Q. _--What are the considerations which must chiefly be attended to insettling the proportions of boilers?

_A. _--In the first place there must be sufficient grate surface to enablethe quantity of coal requisite for the production of the steam to beconveniently burnt, taking into account the intensity of the draught; andin the next place there must be a sufficient flue surface readily to absorbthe heat thus produced, so that there may be no needless waste of heat bythe chimney. The flues, moreover, must have such an area, and the chimneymust be of such dimensions, as will enable a suitable draught through thefire to be maintained; and finally the boiler must be made capable ofcontaining such supplies of water and steam as will obviate inconvenientfluctuations in the water level, and abate the risk of water being carriedover into the engine with the steam. With all these conditions the boilermust be as light and compact as possible, and must be so contrived as to becapable of being cleaned and repaired with facility. 

253. _Q. _--Supposing, then, that you had to proportion a boiler, whichshould be capable of supplying steam sufficient to propel a steam vessel orrailway train at a given speed, or to perform any other given work, howwould you proceed?

_A. _--I would first ascertain the resistance which had to be overcome, andthe velocity with which it was necessary to overcome it. I should then bein a position to know what pressure and volume of steam were required toovercome the resistance at the prescribed rate of motion; and, finally, Ishould allow a sufficient heating and fire grate surface in the boileraccording to the kind of boiler it was, to furnish the requisite quantityof steam, or, in other words, to evaporate the requisite quantity of water. 

254. _Q. _--will you state the amount of heating surface and grate surfacenecessary to evaporate a given quantity of water?

_A. _--The number of square feet of heating or flue surface, required toevaporate a cubic foot of water per hour, is about 70 square feet inCornish boilers, 8 to 11 square feet in land and marine boilers, and 5 or 6square feet in locomotive boilers. The number of square feet of heatingsurface per square foot of fire grate, is from 13 to 15 square feet inwagon boilers; about 40 square feet in Cornish boilers; and from 50 to 90square feet in locomotive boilers. About 80 square feet in locomotives is avery good proportion. 

255. _Q. _--What is the heating surface of boilers per horse power?

_A. _--About 9 square feet of flue and furnace surface per horse power isthe usual proportion in wagon boilers, reckoning the total surface aseffective surface, if the boilers be of a considerable size; but in thecase of small boilers the proportion is larger. The total heating surfaceof a two horse power wagon boiler is, according to Boulton and Watt'sproportions, 30 square feet, or 15 ft. Per horse power; whereas, in thecase of a 45 horse power boiler the total heating surface is 438 squarefeet, or 9. 6 ft. Per horse power. In marine boilers nearly the sameproportions obtain. The original boilers of the Great Western steamer, byMessrs. Maudslay, were proportioned with about 10 square feet of flue andfurnace surface per horse power, reckoning the total amount as effective;but in the boilers of the Retribution, by the same makers, but of largersize, a somewhat smaller proportion of heating surface was adopted. Boultonand Watt have found that in their marine flue boilers, 9 square feet offlue and furnace surface are requisite to boil off a cubic foot of waterper hour, which is the proportion of heating surface that is allowed intheir land boilers per horse power; but inasmuch as in most modern engines, and especially in marine engines, the nominal considerably exceeds theactual power, they allow 11 or 12 square feet of heating surface pernominal horse power in their marine boilers, and they reckon as effectiveheating surface the tops of the flues, and the whole of the sides of theflues, but hot the bottoms. For their land engines they still retain Mr. Watt's standard of power, which makes the actual and the nominal poweridentical; and an actual horse power is the equivalent of a cubic foot ofwater raised into steam every hour. 

256. _Q. _--What is the proper proportion of fire grate per horse power?

_A. _--Boulton and Watt allow 0. 64 of a square foot area of grate bars pernominal horse power in their marine boilers, and a good effect arises fromthis proportion; but sometimes so large an area of fire grate cannot beconveniently got, and the proportion of half a square foot per horse power, which is the proportion adopted in the original boiler of the GreatWestern, seems to answer very well in engines working with a moderatepressure, and with some expansion; and this proportion is now very widelyadopted. With this allowance, there will be 22 to 24 square feet of heatingsurface per square foot of fire grate; and if the consumption of fuel betaken at 6 lbs. Per nominal horse power per hour, there will be about 12lbs. Of coal consumed per hour on each square foot of grate. The furnacesshould not be more than 6 ft. Long, as, if much longer than this, it willbe impossible to work them properly for any considerable length of time, asthey will become choked with clinker at the back ends. 

257. _Q. _--What quantity of fuel is usually consumed per hour on eachsquare foot of fire grate?

_A. _--The quantity of fuel burned on each square foot of fire grate perhour, varies very much in different boilers; in wagon boilers it is from 10to 13 lbs. ; in Cornish boilers from 3-1/2 to 4 lbs. ; and in locomotiveboilers from 80 to 150 lbs. ; but about 1 cwt. Per hour is a good proportionin locomotives, as has been already explained. 

CALORIMETER AND VENT. 

258. _Q. _--In what manner are the proper sectional area and the propercapacity of the flue of a boiler determined?

_A. _--The proper collective area for the escape of the smoke and flame overthe furnace bridges in marine boilers is 19 square inches per nominal horsepower, according to Boulton and Watt's practice, and for the sectional areaof the flue they allow 18 square inches per horse power. The sectional areaof the flue in square inches is what is termed the _calorimeter_ of theboiler, and the calorimeter divided by the length of the flue in feet iswhat is termed the _vent_. In marine flue boilers of good construction thevent varies between the limits of 20 and 25, according to the size of theboiler and other circumstances--the largest boilers having generally thelargest vents; and the calorimeter divided by the vent will give the lengthof the flue in feet. The flues of all flue boilers diminish in theircalorimeter as they approach the chimney, as the smoke contracts in itsvolume in proportion as it parts with its heat. 

259. _Q. _--Is the method of determining the dimensions of a boiler flue, bya reference to its vent and calorimeter, the method generally pursued?

_A. _--It is Boulton and Watt's method; but some very satisfactory boilershave been made by allowing a proportion of 0. 6 of a square foot of firegrate per nominal horse power, and making the sectional area of the flue atthe largest part 1/7th of the area of fire grate, and at the smallest part, where it enters the chimney, 1/11th of the area of the fire grate. Theseproportions are retained whether the boiler is flue or tubular, and from 14to 16 square feet of tube surface is allowed per nominal horse power. 

260. _Q. _--Are the proportions of vent and calorimeter, taken by Boultonand Watt for marine flue boilers, applicable also to wagon and tubularboilers?

_A. _--No. In wagon and tubular boilers very different proportions prevail, yet the proportions of every kind of boiler are determinable on the samegeneral principle. In wagon boilers the proportion of the perimeter of theflue which is effective as heating surface, is to the total perimeter as 1to 3, or, in some cases as 1 to 2. 5; and with any given area of flue, therefore, the length of the flue must be from 3 to 2. 5 times greater thanwould be necessary if the total surface were effective, else the requisitequantity of heating surface will not be obtained. If, then, the vent be thecalorimeter, divided by the length, and the length be made 3 or 2. 5 timesgreater, the vent must become 3 or 2. 5 times less; and in wagon boilersaccordingly, the vent varies from 8 to 11 instead of from 21 to 25, as inthe case of marine flue boilers. In tubular marine boilers the calorimeteris usually made only about half the amount allowed by Boulton and Watt formarine flue boilers, or, in other words, the collective sectional area ofthe tubes, for the transmission of the smoke, is from 8 to 9 square inchesper nominal horse power. It is better, however, to make the sectional arealarger than this, and to work the boiler with the damper sufficientlyclosed to prevent the smoke and flame from rushing exclusively through afew of the tubes. 

261. _Q. _--What are the ordinary dimensions of the flue in wagon boilers?

_A. _--In Boulton and Watt's 45 horse wagon boiler the area of flue is 18square inches per horse power, but the area per horse power increases veryrapidly as the size of the boiler becomes less, and amounts to about 80square inches per horse power in a boiler of 2 horse power. Some suchincrease is obviously inevitable, if a similar form of flue be retained inthe larger and smaller powers, and at the same time the elongation of theflue in the same proportion as the increase of any other dimension isprevented; but in the smaller class of wagon boilers the consideration offacility of cleaning the flues is also operative in inducing a largeproportion of sectional area. Boulton and Watt's 2 horse power wagon boilerhas 30 square feet of surface, and the flue is 18 inches high above thelevel of the boiler bottom, by 9 inches wide; while their 12 horse wagonboiler has 118 square feet of heating surface, and the dimensions of theflue similarly measured are 36 inches by 13 inches. The width of thesmaller flue, if similarly proportioned to the larger one, would be 6-1/2inches, instead of 9 inches, and, by assuming this dimension, we shouldhave the same proportion of sectional area per square foot of heatingsurface in both boilers. The length of flue in the 2 horse boiler is 19. 5ft. , and in the 12 horse boiler 39 ft. , so that the length and height ofthe flue are increased in the same proportion. 

262. _Q. _--Will you give an example of the proportions of a flue, in thecase of a marine boiler?

_A. _--The Nile steamer, with engines of 110 horse power by Boulton andWatt, is supplied with steam by two boilers, which are, therefore, of 55horses power each. The height of the flue winding within the boiler is 60inches, and its mean width 16-1/2 inches, making a sectional area orcalorimeter of 990 square inches, or 18 square inches per horse power ofthe boiler. The length of the flue is 39 ft. , making the vent 25, which isthe vent proper for large boilers. In the Dee and Solway steamers, by Scottand Sinclair, the calorimeter is only 9. 72 square inches per horse power;in the Eagle, by Caird, 11. 9; in the Thames and Medway, by Maudslay, 11. 34, and in a great number of other cases it does not rise above 12 squareinches per horse power; but the engines of most of these vessels areintended to operate to a certain extent expansively, and the boilers areless powerful in evaporating efficacy on that account. 

263. _Q. _--Then the chief difference in the proportions established byBoulton and Watt, and those followed by the other manufacturers you havementioned is, that Boulton and Watt set a more powerful boiler to do thesame work?

_A. _--That is the main difference. The proportion which one part of theboiler bears to another part is very similar in the cases cited, but theproportion of boiler relatively to the size of the engine varies verymaterially. Thus the calorimeter _of each boiler_ of the Dee and Solway is1296 square inches; of the Eagle, 1548 square inches; and of the Thames andMedway, 1134 square inches; and the length of flue is 57, 60, and 52 ft. Inthe boilers respectively, which makes the respective vents 22-1/2, 25, and21. Taking then the boiler of the Eagle for comparison with the boiler ofthe Nile, as it has the same vent, it will be seen that the proportions ofthe two are almost identical, for 990 is to 1548 as 39 is to 60, nearly;but Messrs. Boulton and Watt would not have set a boiler like that of theEagle to do so much work. 

264. _Q. _--Then the evaporating power of the boiler varies as the sectionalarea of the flue?

_A. _--The evaporating power varies as the square root of the area of theflue, if the length of the flue remain the same; but it varies as the areasimply, if the length of the flue be increased in the same proportion asits other dimensions. The evaporating power of a boiler is referable to theamount of its heating surface, and the amount of heating surface in anyflue or tube is proportional to the product of the length of the tube andthe square root of its sectional area, multiplied by a certain quantitythat is constant for each particular form. But in similar tubes the lengthis proportional to the square root of the sectional area; therefore, insimilar tubes, the amount of heating surface is proportional to thesectional area. On this area also depends the quantity of hot air passingthrough the flue, supposing the intensity of the draught to remainunaffected, and the quantity of hot air or smoke passing through the flueshould vary in the same ratio as the quantity of surface. 

265. _Q. _--A boiler, therefore, to exert four times the power, should havefour times the extent of heating surface, and four times the sectional areaof flue for the transmission of the smoke?

_A. _--Yes; and if the same form of flue is to be retained, it should be oftwice the diameter and twice the length; or twice the height and width ifrectangular, and twice the length. As then the diameter or square root ofthe area increases in the same ratio as the length, the square root of thearea divided by the length ought to be a constant quantity in each type ofboiler, in order that the same proportions of flue may be retained; and inwagon boilers without an internal flue, the height in inches of the flueencircling the boiler divided by the length of the flue in feet will be 1very nearly. Instead of the square root of the area, the effectiveperimeter, or outline of that part of the cross section of the flue whichis effective in generating steam, may be taken; and the effective perimeterdivided by the length ought to be a constant quantity in similar forms offlues and with the same velocity of draught, whatever the size of the fluemay be. 

266. _Q. _--Will this proportion alter if the form of the flue be changed?

_A. _--It is clear, that with any given area of flue, to increase theperimeter by adopting a different shape is tantamount to a diminution ofthe length of the flue; and, if the perimeter be diminished, the length ofthe flue must at the same time be increased, else it will be impossible toobtain the necessary amount of heating surface. In Boulton and Watt's wagonboilers, the sectional area of the flue in square inches per square foot ofheating surface is 5. 4 in the two horse boiler; in the three horse it is4. 74; in the four horse, 4. 35; six horse, 3. 75; eight horse, 4. 33; tenhorse, 3. 96; twelve horse, 3. 63; eighteen horse, 3. 17; thirty horse, 2. 52;and in the forty-five horse boiler, 2. 05 square inches. Taking the amountof heating surface in the 45 horse boiler at 9 square feet per horse power, we obtain 18 square inches of sectional area of flue per horse power, whichis also Boulton and Watt's proportion of sectional area for marine boilerswith internal flues. 

267. _Q. _--If to increase the perimeter of a flue is virtually to diminishthe length, then a tubular boiler where the perimeter is in effect greatlyextended ought to have but a short length of tube?

_A. _--The flue of the Nile steamer if reduced to the cylindrical form wouldbe 35-1/2 inches in diameter to have the same area; but it would thenrequire to be made 47-3/4 feet long, to have the same amount of heatingsurface, excluding the bottom as non-effective. Supposing that with theseproportions the heat is sufficiently extracted from the smoke, then everytube of a tubular boiler in which the same draught existed ought to havevery nearly the same proportions. 

268. _Q. _--But what are the best proportions of the parts of tubularboilers relatively with one another?

_A. _--The proper relative proportions of the parts of tubular boilers mayeasily be ascertained by a reference to the settled proportions of flueboilers; for the same general principles are operative in both cases. Inthe Nile steamer each boiler of 55 horse power has about 497 square feet offlue surface or 9 square feet per horse power, reckoning the total surfaceas effective. The area of the flue, which is rectangular is 990 squareinches, therefore the area is equal to that of a tube 35-1/2 inches indiameter; and such a tube, to have a heating surface of 497 square feet, must be 53. 4 feet or 640. 8 inches in length. The length, therefore, of thetube, will be about 18 times its diameter, and with the same velocity ofdraught these proportions must obtain, whatever the absolute dimensions ofthe tube may be. With a calorimeter, therefore, of 18 square inches perhorse power, the length of a tube 3 inches diameter must not exceed 4 feet6 inches, since the heat will be sufficiently extracted from the smoke inthis length, if the smoke only travels at the velocity due to a calorimeterof 18 square inches per horse power. 

269. _Q. _--Is this, then, the maximum length of flue which can be used intubular boilers with advantage?

_A. _--By no means. The tubes of tubular boilers are almost always more than4 feet 6 inches long, but then the calorimeter is almost always less than18 square inches per horse power--generally about two thirds of this. Indeed, tubular boilers with a large calorimeter are not found to be sosatisfactory as where the calorimeter is small, partly from the propensityof the smoke in such cases to pass through a few of the tubes instead ofthe whole of them, and partly from the deposit of soot which takes placewhen the draught is sluggish. It is a very confusing practice, however, tospeak of nominal horse power in connection with boilers, since that is aquantity quite indeterminate. 

EVAPORATIVE POWER OF BOILERS. 

270. _Q. _--The main thing after all in boilers is their evaporative powers?

_A. _--The proportions of tubular boilers, as of all boilers, shouldobviously have reference to the evaporation required, whereas the demandupon the boiler for steam is very often reckoned contingent upon thenominal horse power of the engine; and as the nominal power of an engine isa conventional quantity by no means in uniform proportion to the actualquantity of steam consumed, perplexing complications as to the properproportions of boilers have in consequence sprung up, to which most of thefailures in that department of engineering may be imputed. It is highlyexpedient, therefore, in planning boilers for any particular engine, toconsider exclusively the actual power required to be produced, and toapportion the capabilities of the boiler accordingly. 

271. _Q. _--In other words you would recommend the inquiry to be restrictedto the mode of evaporating a given number of cubic feet of water in thehour, instead of embracing the problem how an engine of a given nominalpower was to be supplied with steam?

_A. _--I would first, as I have already stated, consider the actual powerrequired to be produced, and then fix the amount of expansion to beadopted. If the engine had to work up to three times its nominal power, asis now common in marine engines, I should either increase correspondinglythe quantity of evaporating surface in the boiler, or adopt such an amountof expansion as would increase threefold the efficacy of the steam, orcombine in a modified manner both of these arrangements. Reckoning theevaporation of a cubic foot of water in the hour as equivalent to an actualhorse power, and allowing a square yard or 9 square feet as the properproportion of flue surface to evaporate a cubic foot of water in the hour, it is clear that I must either give 27 square feet of heating surface inthe boiler to have a trebled power without expansion, or I must cut off thesteam at one seventh of the stroke to obtain a three-fold power withoutincreasing the quantity of heating surface. By cutting off the steam, however, at one third of the stroke, a heating surface of 13-1/2 squarefeet will give a threefold power, and it will usually be the most judiciouscourse to carry the expansion as far as possible, and then to add theproportion of heating surface necessary to make good the deficiency stillfound to exist. 

272. _Q. _--But is it certain that a cubic foot of water evaporated in thehour is equivalent to an actual horse power?

_A. _--An actual horse power as fixed by Watt is 33, 000 lbs. Raised one foothigh in the minute; and in Watt's 40 horse power engine, with a 31-1/2 inchcylinder, 7 feet stroke, and making 17-1/2 strokes a minute, the effectivepressure is 6. 92 lbs. On the square inch clear of all deductions. Now, as ahorse power is 33, 000 lbs. Raised one foot high, and as there are 6. 92 lbs, on the square inch, it is clear that 33, 000 divided by 6. 92, on 4768 squareinches with 6. 92 lbs. On each if lifted 1 foot or 12 inches high, will alsobe equal to a horse power. But 4768 square inches multiplied by 12 inchesin height is 57224. 4 cubic inches, or 33. 1 cubic feet, and this is thequantity of steam which must be expended per minute to produce an actualhorse power. 

273. _Q. _--But are 33 cubic feet of steam expended per minute equivalent toa cubic foot of water expended in the hour?

_A. _. --Not precisely, but nearly so. A cubic foot of water produces 1669cubic feet of steam of the atmospheric density of 15 lbs. Per square inch, whereas a consumption of 33 cubic feet of steam in the minute is 1980 cubicfeet in the hour. In Watt's engines about one tenth was reckoned as loss infilling the waste spaces at the top and bottom of the cylinder, making 1872cubic feet as the quantity consumed per hour without this waste; and inmodern engines the waste at the ends of the cylinder is inconsiderable. 

274. _Q. _--What power was generated by a cubic foot of water in the case ofthe Albion Mill engines when working without expansion?

_A. _--In the Albion Mill engines when working without expansion, it wasfound that 1 lb. Of water in the shape of steam raised 28, 489 lbs. 1 foothigh. A cubic foot of water, therefore, or 62-1/2 lbs. , if consumed in thehour, would raise 1780562. 5 lbs. One foot high in the hour, or would raise29, 676 lbs. One foot high in a minute; and if to this we add one tenth forwaste at the ends of the cylinder, a waste which hardly exists in modernengines, we have 32, 643 lbs. Raised one foot high in the minute, or a horsepower very nearly. In some cases the approximation appears still nearer. Thus, in a 40 horse engine working without expansion, Watt found that . 674feet of water were evaporated from the boiler per minute, which is just acubic foot per horse power per hour; but it is not certain in this casethat the nominal and actual power were precisely identical. It will bequite safe, however, to reckon an actual horse power as producible by theevaporation of a cubic foot of water in the hour in the case of enginesworking without expansion; and for boiling off this quantity in flue orwagon boilers, about 8 lbs. Of coal will be required and 9 square feet offlue surface. 

MODERN MARINE AND LOCOMOTIVE BOILERS. 

275. _Q. _--These proportions appear chiefly to refer to old boilers. I wishyou to state what are the proportions of modern flue and tubular marineboilers. 

_A. _--In modern marine boilers the area of fire grate is less than in Mr. Watt's original boilers, where it was one square foot to nine square feetof heating surface. The heat in the furnace is consequently more intense, and a somewhat less amount of surface suffices to evaporate a cubic foot ofwater. In Boulton and Watt's modern flue boilers they allow for theevaporation of a cubic foot of water 8 square feet of heating surface, 70square inches of fire grate, 13 square inches sectional area of flues, 6square inches sectional area of chimney, 14 square inches area over furnacebridges, ratio of area of flue to area of fire grate 1 to 5. 4. To evaporatea cubic foot of water per hour in tubular boilers, the proportions are--heating surface 9 square feet, fire grate 70 square inches, sectional areaof tubes 10 square inches, sectional area of back uptake 12 square inches, sectional area of front uptake 10 square inches, sectional area of chimney7 square inches, ratio of diameter of tube to length of tube 1/28th to1/30th, cubical content of boiler exclusive of steam chest 6. 5 cubic feet, cubical content of steam chest 1. 5 cubic feet. 

276. _Q. _--These proportions do not apply to locomotive boilers?

_A. _--Not at all. In locomotive boilers the draught is maintained by theprojection of the waste steam which escapes from the cylinders up thechimney, and the draught is much more powerful and the combustion much morerapid than in cases in which the combustion is maintained by the naturaldraught of a chimney, except indeed the chimney be of very unusualtemperature and height. The proportions proper for locomotive boilers willbe seen by the dimensions of a few locomotives of approved construction, which have been found to give satisfactory results in practice, and whichare recorded in the following Table:

 Name of Engine

 Great Britain. Pallas. Snake. Sphinx. Diameter of cylinder 18 in. 15 in. 14-1/4 in. 18 in. Length of stroke 24 in. 20 in. 21 in. 24 in. Diameter of driving wheel 8 ft. 6 ft. 6-1/2 ft. 5 ft. Inside diameter of fire box 53 in. 55 in. 41-1/3 in. 44 in. Inside width of fire box 63 in. 42 in. 43-1/4 in. 39-1/2in. Height of fire box above bars 63 in. 52 in. 48-1/3 in. 55-1/2in. Number of fire bars 29 . .. 32 16Thickness of fire bars 3/4 in. 1-3/4 in. 5/8 in. 1 in. Number of Tubes 305 134 181 142Outside diameter of tubes 2 in. 2 in. 1-7/8 in. 2-1/8 in. Length of tubes 11 ft 3 in 10 ft 6 in 10 ft 3-1/2 in. 14 ft 3-1/4 in. Space between tubes 1/2 in. 3/4 in. 1/2 in. Inside diameter of ferules 1-9/16 in. 1-1/2 in. 1-5/16 in. 1-5/8 in. Diameter of chimney 17 in. 15 in. 13 in. 15-1/2 in. Diameter of blast orifice 5-1/2 in. 4-5/8 in. 4-1/2 in. 4-3/4 in. Area of grate 21 sq. Ft. 16. 04 sqft 12. 4 sq. Ft. 10. 56 sq. FtArea of air space of grate 11. 4 sqft 4. 08 sqft 5. 54 sq. Ft. 5 sq. Ft. Area of tubes 5. 46 sqft 2. 40 sqft 2. 8 sq. Ft. 2. 92 sq. Ft. Area though ferules 4 sq. Ft. 1. 64 sqft 2 sq. Ft. 2. 04 sq. Ft. Area of chimney 1. 77 sqft 1. 23 sqft . 921 sq. Ft. 1. 31 sq. Ft. Area of blast orifice 23. 76 sqin 16. 8 sqin 14. 18 sq. In. 17. 7 sq. In. Heating surface of tubes 1627 sqft 668. 7 sqft 823 sq. Ft. 864 sq. Ft. 

THE BLAST IN LOCOMOTIVES. 

277. _Q. _--What is the amount of draught produced in locomotive boilers incomparison with that existing in other boilers?

_A. _--A good chimney of a land engine will produce a degree of exhaustionequal to from 1-1/2 to 2-1/2 inches of water. In locomotive boilers theexhaustion is in some cases equal to 12 or 13 inches of water, but from 3to 6 inches is a more common proportion. 

278. _Q. _--And what force of blast is necessary to produce this exhaustion?

_A. _--The amount varies in different engines, depending on the sectionalarea of the tubes and other circumstances. But on the average, it may beasserted that such a pressure of blast as will support an inch of mercury, will maintain sufficient exhaustion in the smoke box to support an inch ofwater; and this ratio holds whether the exhaustion is little or great. Toproduce an exhaustion in the smoke box, therefore, of 6 inches of water, the waste steam would require to be of sufficient pressure to support acolumn of 6 inches of mercury, which is equivalent to a pressure of 3 lbs. On the square inch. 

279. _Q. _--How is the force of the blast determined?

_A. _--By the amount of contraction given to the mouth of the blast pipe, which is a pipe which conducts the waste steam from the cylinders anddebouches at the foot of the chimney. If a strong blast be required, themouth of this pipe requires to be correspondingly contracted, but suchcontraction throws a back pressure on the piston, and it is desirable toobtain the necessary draught with as little contraction of the blast pipeas possible. The blast pipe is generally a breeches pipe of which the legsjoin just before reaching the chimney; but it is better to join the twocylinders below, and to let a single pipe ascend to within 12 or 18 inchesof the foot of the chimney. If made with too short a piece of pipe abovethe joining, the steam will be projected against each side of the chimneyalternately, and the draught will be damaged and the chimney worn. Theblast pipe should not be regularly tapered, but should be large in the bodyand gathered in at the mouth. 

280. _Q. _--Is a large and high chimney conducive to strength of draught inlocomotives?

_A. _--It has not been found to be so. A chimney of three or four times itsown diameter in height appears to answer fully as well as a longer one; andit was found that when in an engine with 17 inch cylinders a chimney of15-1/4 inches was substituted for a chimney of 17-1/2 inches, a superiorperformance was the result. The chimney of a locomotive should have halfthe area of the tubes at the ferules, which is the most contracted part, and the blast orifice should have 1/10th of the area of the chimney. Thesectional area of the tubes through the ferules should be as large aspossible. Tubes without ferules it is found pass one fourth more air, andtubes with ferules only at the smoke box end pass one tenth more air thanwhen there are ferules at both ends. 

281. _Q. _--Is the exhaustion produced by the blast as great in the fire boxas in the smoke box?

_A. _--Experiments have been made to determine this, and in few cases has itbeen found to be more than about half as great as ordinary speeds; but muchdepends on the amount of contraction in the tubes. In an experiment madewith an engine having 147 tubes of 1-3/4 inches external diameter, and 13feet 10 inches long, and with a fire grate having an area of 9-1/2 squarefeet, the exhaustion at all speeds was found to be three times greater inthe smoke box than in the fire box. The exhaustion in the smoke box wasgenerally equivalent to 12 inches of water, while in the fire box it wasequivalent to only 4 inches of water; showing that 4 inches were requiredto draw the air through the grate and 8 inches through the tubes. 

282. _Q. _--What will be the increase of evaporation in a locomotive from agiven increase of exhaustion?

_A. _--The rate of evaporation in a locomotive or any other boiler will varyas the quantity of air passing through the fire, and the quantity of airpassing through the fire will vary nearly as the square root of theexhaustion. With four times the exhaustion, therefore, there will be abouttwice the evaporation, and experiment shows that this theoretical law holdswith tolerable accuracy in practice. 

283. _Q. _--But the same exhaustion will not be produced by a given strengthof blast in all engines?

_A. _--No; engines with contracted fire grates and an inadequate sectionalarea of tubes, will require a stronger blast than engines of betterproportions; but in any given engine the relations between the blastexhaustion and evaporation, hold which have been already defined. 

284. _Q. _--Is the intensity of the draught under easy regulation?

_A. _--The intensity of the draught may easily be diminished by partiallyclosing the damper in the chimney, and it may be increased by contractingthe orifice of the blast. A variable blast pipe, the orifice of which maybe enlarged or contracted at pleasure, has been much used. There arevarious devices for this purpose, but the best appears to be that adoptedin Stephenson's engine, where a conical nozzle is moved up or down withinthe blast pipe, which is made somewhat larger in diameter than the base ofthe cone, but with a ring projecting internally, against which the base ofthe cone abuts when the nozzle is pushed up. When the nozzle stands at thetop of the pipe the whole of the steam has to pass through it, and theintensity of the blast is increased by the increased velocity thus given tothe steam; whereas when the nozzle is moved downward the steam escapesthrough the annular opening left between the nozzle and the pipe, as wellas through the nozzle itself, and the intensity of the blast is diminishedby the enlargement of the opening for the escape of the steam thus madeavailable. 

285. _Q. _--What is the best diameter for the tubes of locomotive boilers?

_A. _--Bury's locomotive with 14 inch cylinders contains 92 tubes of 2-1/8thinches external diameter, and 10 feet 6 inches long; whereas Stephenson'slocomotive with 15 inch cylinders contains 150 tubes of 1-5/8ths externaldiameter, 13 feet 6 inches long. In Stephenson's boiler, in order that thepart of the tubes next the chimney may be of any avail for the generationof steam, the draught has to be very intense, which in its turn involves aconsiderable expenditure of power; and it is questionable whether theincreased expenditure of power upon the blast, in Stephenson's long tubedlocomotives, is compensated by the increased generation of steam consequentupon the extension of the heating surface. When the tubes are small indiameter they are apt to become partially choked with pieces of coke; butan internal diameter of 1-5/8ths may be employed without inconvenience ifthe draught be of medium intensity. 

286. _Q. _--Will you illustrate the relation between the length and diameterof locomotive tubes by a comparison with the proportion of flues in flueboilers?

_A. _--In most locomotives the velocity of the draught is such that it wouldrequire very long tubes to extract the heat from the products ofcombustion, if the heat were transmitted through the metal of the tubeswith only the same facility as through the iron of ordinary flue boilers. The Nile steamer, with engines of 110 nominal horses power each, and withtwo boilers having two independent flues in each, of such dimensions as tomake each flue equivalent to 55 nominal horses power, works at 62 per cent. Above the nominal power, so that the actual evaporative efficacy of eachflue would be equivalent to 89 actual horses power, supposing the enginesto operate without expansion; but as the mean pressure in the cylinder issomewhat less than the initial pressure, the evaporative efficacy of eachflue may be reckoned equivalent to 80 actual horses power. With thisevaporative power there is a calorimeter of 990 square inches, or 12. 3square inches per actual horse power; whereas in Stephenson's locomotivewith 150 tubes, if the evaporative power be taken at 200 cubic feet ofwater in the hour, which is a large supposition, the engine will be equalto 200 actual horses power. If the internal diameter of the tubes be takenat thirteen eighths of an inch, the calorimeter per actual horse power willonly be 1. 1136 square inches, or in other words the calorimeter in thelocomotive boiler will be 11. 11 times less than in the flue boiler for thesame power, so that the draught in the locomotive must be 11. 11 timesstronger, and the ratio of the length of the tube to its diameter 11. 11times greater than in the flue boiler, supposing the heat to be transmittedwith only the same facility. The flue of the Nile would require to be 35-1/2 inches in diameter if made of the cylindrical form, and 47-3/4 feetlong; the tubes of a locomotive if 1-3/8ths inch diameter would onlyrequire to be 22. 19 inches long with the same velocity of draught; but asthe draught is 11. 11 times faster than in a flue boiler, the tubes ought tobe 246. 558 inches, or about 20-1/2 feet long according to this proportion. In practice, however, they are one third less than this, which reduces theheating surface from 9 to 6 square feet per actual horse power, and thislength even is found to be inconvenient. It is greatly preferable thereforeto increase the calorimeter, and diminish the intensity of the draught. 

BOILER CHIMNEYS. 

287. _Q. _--By what process do you ascertain the dimensions of the chimneyof a land boiler?

_A. _--By a reference to the volume of air it is necessary in a given timeto supply to the burning fuel, and to the velocity of motion produced bythe rarefaction in the chimney; for the area of the chimney requires to besuch, that with the velocity due to that rarefaction, the quantity of airrequisite for the combustion of the fuel shall pass through the furnace inthe specified time. Thus if 200 cubic feet of air of the atmosphericdensity are required for the combustion of a pound of coal, --though 250lbs. Is nearer the quantity generally required, --and 10 lbs. Of coal perhorse power per hour are consumed by an engine, then 2000 cubic feet of airmust be supplied to the furnace per horse power per hour, and the area ofthe chimney must be such as to deliver this quantity at the increased bulkdue to the high temperature of the chimney when moving with the velocitythe rarefaction within the chimney occasions, and which, in small chimneys, is usually such as to support a column of half an inch of water. Thevelocity with which a denser fluid flows into a rarer one is equal to thevelocity a heavy body acquires in falling through a height equal to thedifference of altitude of two columns of the heavier fluid of such heightsas will produce the respective pressures; and, therefore, when thedifference of pressure or amount of rarefaction in the chimney is known, itis easy to tell the velocity of motion which ought to be produced by it. Inpractice, however, these theoretical results are not to be trusted, untilthey have received such modifications as will make them representative ofthe practice of the most experienced constructors. 

288. _Q. _--What then is the rule followed by the most experiencedconstructors?

_A. _--Boulton and Watt's rule for the dimensions of the chimney of a landengine is as follows:--multiply the number of pounds of coal consumed underthe boiler per hour by 12, and divide the product by the square root of theheight of the chimney in feet; the quotient is the area of the chimney insquare inches in the smallest part. A factory chimney suitable for a 20horse boiler is commonly made about 20 in. Square inside, and 80 ft. High;and these dimensions are those which answer to a consumption of 15 lbs. Ofcoal per horse power per hour, which is a very common consumption infactory engines. If 15 lbs. Of coal be consumed per horse power per hour, the total consumption per hour in a 20 horse boiler will be 300 lbs. , and300 multiplied by 12 = 3600, and divided by 9 (the square root of theheight) = 400, which is the area of the chimney in square inches. It willnot answer well to increase the height of a chimney of this area to morethan 40 or 50 yards, without also increasing the area, nor will it be ofutility to increase the area much without also increasing the height. Thequantity of coal consumed per hour in pounds, multiplied by 5, and dividedby the square root of the height of the chimney, is the proper collectivearea of the openings between the bars of the grate for the admission of airto the fire. 

289. _Q. _--Is this rule applicable to the chimneys of steam vessels?

_A. _--In steam vessels Boulton and Watt have heretofore been in the habitof allowing 8-1/2 square inches of area of chimney per horse power, butthey now allow 6 square inches to 7 square inches. In some steam vessels asteam blast like that of a locomotive, but of a smaller volume, is used inthe chimney, and many of the evils of a boiler deficient in draught may beremedied by this expedient, but a steam blast in a low pressure engineoccasions an obvious waste of steam; it also makes an unpleasant noise, andin steam vessels it frequently produces the inconvenience of carrying thesmaller parts of the coal up the chimney, and scattering it over the deckamong the passengers. It is advisable, therefore, to give a sufficientcalorimeter in all low pressure boilers, and a sufficient height of chimneyto enable the chimney to operate without a steam jet; but it is useful toknow that a steam jet is a resource in the case of a defective boiler, orwhere the boiler has to be urged beyond its power. 

STEAM ROOM AND PRIMING. 

290. _Q. _--What is the capacity of steam room allowed in boilers per horsepower?

_A. _--The capacity of steam room allowed by Boulton and Watt in their landwagon boilers is 8-3/4 cubic feet per horse power in the two horse powerboiler, and 5-3/4 cubic feet in the 20 horse power boiler; and in thelarger class of boilers, such as those suitable for 30 and 45 horse powerengines, the capacity of the steam room does not fall below this amount, and, indeed, is nearer 6 than 5-3/4 cubic feet per horse power. The contentof water is 18-1/2 cubic feet per horse power in the two horse powerboiler, and 15 cubic feet per horse power in the 20 horse power boiler. 

291. _Q. _--Is this the proportion Boulton and Watt allow in their marineboilers?

_A. _--Boulton and Watt in their early steam vessels were in the habit ofallowing for the capacity of the steam, space in marine boilers 16 timesthe content of the cylinder; but as there were two cylinders, this wasequivalent to 8 times the content of both cylinders, which is theproportion commonly followed in land engines, and which agrees very nearlywith the proportion of between 5 and 6 cubic feet of steam room per horsepower already referred to. Taking for example an engine with 23 inchesdiameter of cylinder and 4 feet stroke, which will be 18. 4 horse power--thearea of the cylinder will be 415. 476 square inches, which, multiplied by48, the number of inches in the stroke, will give 19942. 848 for thecapacity of the cylinder in cubic inches; 8 times this is 159542. 784 cubicinches, or 92. 3 cubic feet; 92. 3 divided by 18. 4 is rather more than 5cubic feet per horse power. 

292. _Q. _--Is the production of the steam in the boiler uniform throughoutthe stroke of the engine?

_A. _--It varies with the slight variations in the pressure within theboiler throughout the stroke. Usually the larger part of the steam isproduced during the first part of the stroke of the engine, for there isthen the largest demand for steam, as the steam being commonly cut offsomewhat before the end of the stroke, the pressure rises somewhat in theboiler during that period, and little steam is then produced. There is lessnecessity that the steam space should be large when the flow of steam fromthe boiler is very uniform, as it will be where there are two enginesattached to the boiler at right angles with one another, or where theengines work at a great speed, as in the case of locomotive engines. A highsteam chest too, by rendering boiling over into the steam pipes, or primingas it is called, more difficult, obviates the necessity for so large asteam space; as does also a perforated steam pipe stretching through thelength of the boiler, so as not to take the steam from one place. The useof steam of a high pressure, worked expansively, has the same operation; sothat in modern marine boilers, of the tubular construction, where the wholeor most of these modifying circumstances exist, there is no necessity forso large a proportion of steam room as 5 or 6 cubic feet per nominal horsepower, and about one, 1-1/2, or 2 cubic feet of steam room per cubic footof water evaporated, more nearly represents the general practice. 

293. _Q. _--Is this the proportion of steam room adopted in locomotiveboilers?

_A. _--No; in locomotive boilers the proportion of steam room per cubic footof water evaporated is considerably less even than this. It does notusually exceed 1/5 of a cubic foot per cubic foot of water evaporated; andwith clean water, with a steam dome a few feet high set on the barrel ofthe boiler, or with a perforated pipe stretching from end to end of thebarrel, and with the steam room divided about equally between the barreland the fire box, very little priming is found to occur even with thissmall proportion of total steam room. About 3/4 the depth of the barrel isusually filled with water, and 1/4 with steam. 

294. _Q. _--What is priming?

_A. _--Priming is a violent agitation of the water within the boiler, inconsequence of which a large quantity of water passes off with the steam inthe shape of froth or spray. Such a result is injurious, both as regardsthe efficacy of the engine, and the safety of the engine and boiler; forthe large volume of hot water carried by the steam into the condenserimpairs the vacuum, and throws a great load upon the air pump, whichdiminishes the speed and available power of the engine; and the existenceof water within the cylinder, unless there be safety valves upon thecylinder to permit its escape, will very probably cause some part of themachinery to break, by suddenly arresting the motion of the piston when itmeets the surface of the water, --the slide valve being closed to thecondenser before the termination of the stroke, in all engines with lapupon the valves, so that the water within the cylinder is prevented fromescaping in that direction. At the same time the boiler is emptied of itswater too rapidly for the feed pump to be able to maintain the supply, andthe flues are in danger of being burnt from a deficiency of water abovethem. 

295. _Q. _--What are the causes of priming?

_A. _--The causes of priming are an insufficient amount of steam room, aninadequate area of water level, an insufficient width between the flues ortubes for the ascent of the steam and the descent of water to supply thevacuity the steam occasions, and the use of dirty water in the boiler. Newboilers prime more than old boilers, and steamers entering rivers from thesea are more addicted to priming than if sea or river water had alone beenused in the boilers--probably from the boiling point of salt water beinghigher than that of fresh, whereby the salt water acts like so much moltenmetal in raising the fresh water into steam. Opening the safety valvesuddenly may make a boiler prime, and if the safety valve be situated nearthe mouth of the steam pipe, the spray or foam thus created may be mingledwith the steam passing into the engine, and materially diminish itseffective power; but if the safety valve be situated at a distance from themouth of the steam pipe, the quantity of foam or spray passing into theengine may be diminished by opening the safety valve; and in locomotives, therefore, it is found beneficial to have a safety valve on the barrel ofthe boiler at a point remote from the steam chest, by partially openingwhich, any priming in that part of the boiler adjacent to the steam chestis checked, and a purer steam than before pusses to the engine. 

296. _Q. _--What is the proper remedy for priming?

_A. _--When a boiler primes, the engineer generally closes the throttlevalve partially, turns off the injection water, and opens the furnacedoors, whereby the generation of steam is checked, and a less violentebullition in the boiler suffices. Where the priming arises from aninsufficient amount of steam room, it may be mitigated by putting a higherpressure upon the boiler and working more expansively, or by theinterposition of a perforated plate between the boiler and the steam chest, which breaks the ascending water and liberates the steam. In some cases, however, it may be necessary to set a second steam chest on the top of theexisting one, and it will be preferable to establish a communication withthis new chamber by means of a number of small holes, bored through theiron plate of the boiler, rather than by a single large orifice. Wherepriming arises from the existence of dirty water in the boiler, the evilmay be remedied by the use of collecting vessels, or by blowing off largelyfrom the surface; and where it arises from an insufficient area of waterlevel, or an insufficient width between the flues for the free ascent ofthe steam and the descent of the superincumbent water, the evil may beabated by the addition of circulating pipes in some part of the boiler, which will allow the water to descend freely to the place from whence thesteam rises, the width of the water spaces being virtually increased byrestricting their function to the transmission of a current of steam andwater to the surface. It is desirable to arrange the heating surface insuch a way that the feed water entering the boiler at its lowest point isheated gradually as it ascends, until toward the superior part of the fluesit is raised gradually into steam; but in all cases there will be currentsin the boiler for which it is proper to provide. The steam pipe proceedingto the engine should obviously be attached to the highest point of thesteam chest, in boilers of every construction. 

297. _Q. _--Having now stated the proportions proper to be adopted forevaporating any given quantity of water in steam boilers, will you proceedto show how you would proportion a boiler to do a given amount of work? saya locomotive boiler which will propel a train of 100 tons weight at a speedof 50 miles an hour. 

_A. _--According to experiments on the resistance of railway trains atvarious rates of speed, made by Mr. Gooch, of the Great Western Railway, itappears that a train weighing, with locomotive, tender, and carriages, about 100 tons, experiences, at a speed of 50 miles an hour, a resistanceof about 3, 000 lbs. , or about 30 lbs. Per ton; which resistance includesthe resistance of the engine as well as that of the train. This, therefore, is the force which must be imparted at the circumference of the drivingwheels, except that small part intercepted by the engine itself, and theforce exerted by the pistons must be greater than that at the circumferenceof the driving wheel, in the proportion of their slower motion, or in theproportion of the circumference of the driving wheel to the length of adouble stroke of the engine. If the diameter of the driving wheel be 5-1/2feet, its circumference will be 17. 278 feet, and if the length of thestroke be 18 inches, the length of a double stroke will be 3 feet. Thepressure on the pistons must therefore be greater than the traction at thecircumference of the driving wheel, in the proportion of 17. 278 to 3, or, in other words, the mean pressure on the pistons must be 17, 278 lbs. ; andthe area of cylinders, and pressure of steam, must be such as to produceconjointly this total pressure. It thus becomes easy to tell the volume andpressure of steam required, which steam in its turn represents itsequivalent of water which is to be evaporated from the boiler, and theboiler must be so proportioned, by the rules already given, as to evaporatethis water freely. In the case of a steam vessel, the mode of procedure isthe same, and when the resistance and speed are known, it is easy to tellthe equivalent value of steam. 

STRENGTH OF BOILERS. 

298. _Q. _--What strain should the iron of boilers be subjected to inworking?

_A. _--The iron of boilers, like the iron of machines or structures, iscapable of withstanding a tensile strain of from 50, 000 to 60, 000 lbs. Uponevery square inch of section; but it will only bear a third of this strainwithout permanent derangement of structure, and it does not appearexpedient in any boiler to let the strain exceed 4, 000 lbs. Upon the squareinch of sectional area of metal, especially if it is liable to be weakenedby corrosion. 

299. _Q. _--Have any experiments been made to determine the strength ofboilers?

_A. _--The question of the strength of boilers was investigated veryelaborately a few years ago by a committee of the Franklin Institute, inAmerica, and it was found that the tenacity of boiler plate increased withthe temperature up to 550°, at which point the tenacity began to diminish. At 32°, the cohesive force of a square inch of section was 56, 000 lbs. ; at570°, it was 66, 500 lbs. ; at 720°, 55, 000 lbs. ; at 1, 050°, 32, 000 lbs. ; at1, 240°, 22, 000 lbs. ; and at 1, 317°, 9, 000 lbs. Copper follows a differentlaw, and appears to be diminished in strength by every addition to thetemperature. At 32° the cohesion of copper was found to be 32, 800 lbs. Persquare inch of section, which exceeds the cohesive force at any highertemperature, and the square of the diminution of strength seems to keeppace with the cube of the increased temperature. Strips of iron cut in thedirection of the fibre were found to be about 6 per cent. Stronger thanwhen cut across the grain. Repeated piling and welding was found toincrease the tenacity of the iron, but the result of welding togetherdifferent kinds of iron was not found to be favorable. The accidentaloverheating of a boiler was found to reduce the ultimate or maximumstrength of the plates from 65, 000 to 45, 000 lbs. Per square inch ofsection, and riveting the plates was found to occasion a diminution intheir strength to the extent of one third. These results, however, are notprecisely the same as those obtained by Mr. Fairbairn. 

300. _Q. _--What were the results obtained by him?

_A. _--He found that boiler plate bore a tensile strain of 23 tons persquare inch before rupture, which was reduced to 16 tons per square inchwhen joined together by a double row of rivets, and 13 tons, or about30, 000, when joined together by a single row of rivets. A circular boiler, therefore, with the ends of its plates double riveted, will bear at theutmost about 36, 000 lbs. Per square inch of section, or about 12, 000 lbs. Per square inch of section without permanent derangement of structure. 

301. _Q. _--What pressure do cylindrical boilers sustain in practice?

_A. _--In some locomotive boilers, which are worked with a pressure of 80lbs. Upon the square inch, the thickness of the plates is only 5/16ths ofan inch, while the barrel of the boiler is 39 inches in diameter. It willrequire a length of 3. 2 inches of the boiler when the plates are 5/16thsthick to make up a sectional area of one square inch, and the separatingforce will be 39 times 3. 2 multiplied by 80, which makes the separatingforce 9, 984 lbs. , sustained by two square inches of sectional area--one oneach side; or the strain is 4, 992 lbs. Per square inch of sectional area, which is quite as great strain as is advisable. The accession of strengthderived from the boiler ends is not here taken into account, but neither isthe weakening effect counted that is caused by the rivet holes. Somelocomotives of 4 feet diameter of barrel and of 3/8ths iron have beenworked to as high a pressure as 200 lbs. On the inch; but such feats ofdaring are neither to be imitated nor commended. 

302. _Q. _--Can you give a rule for the proper thickness of cylindricalboilers?

_A. _--The thickness proper for cylindrical boilers of wrought iron, exposedto an internal pressure, may be found by the following rule:--multiply 2. 54times the internal diameter of the cylinder in inches by the greatestpressure within the cylinder per circular inch, and divide by 17, 800; theresult is the thickness in inches. If we apply this rule to the example ofthe locomotive boiler just given, we have 39 x 2. 54 x 62. 832 (the pressureper circular inch corresponding to 80 lbs. Per square inch) = 6224. 1379, and this, divided by 17, 800, gives 0. 349 as the thickness in inches, instead of 0. 3125, or 5/16ths, the actual thickness. If we take thepressure per square inch instead of per circular inch, we obtain thefollowing rule, which is somewhat simpler:--multiply the internal diameterof the cylinder in inches by the pressure in pounds per square inch, anddivide the product by 8, 900; the result is the thickness in inches. Boththese rules give the strain about one fourth of the elastic force, or 4, 450lbs. Per square inch of sectional area of the iron; but 3, 000 lbs. Isenough when the flame impinges directly on the iron, as in some of theordinary cylindrical boilers, and the rule may be adapted for that strainby taking 6, 000 as a divisor instead of 8, 900. 

303. _Q. _--In marine and wagon boilers, which are not of a cylindricalform, how do you procure the requisite strength?

_A. _--Where the sides of the boiler are flat, instead of being cylindrical, a sufficient number of stays must be introduced to withstand the pressure;and it is expedient not to let the strain upon these stays be more than3, 000 lbs. Per square inch of section, as the strength of internal stays inboilers is generally soon diminished by corrosion. Indeed, a strain at allapproaching that upon locomotive boilers would be very unsafe in the caseof marine boilers, on account of the corrosion, both internal and external, to which marine boilers are subject. The stays should be small and numerousrather than large and few in number, as, when large stays are employed, itis difficult to keep them tight at the ends, and oxidation of the shellfollows from leakage at the ends of the stays. All boilers should beproved, when new, to twice or three times the pressure they are intended tobear, and they should be proved occasionally by the hand pump when in use, to detect any weakness which corrosion may have occasioned. 

304. _ Q. _--Will you describe the disposition of the stays in a marineboiler?

_A. _--If the pressure of steam be 20 lbs. On the square inch, which is avery common pressure in tubular boilers, there will be a pressure of 2, 880lbs. On every square foot of flat surface; so that if the strain upon thestays is not to exceed 3, 000 lbs. On the square inch of section, there mustbe nearly a square inch of sectional area of stay for every square foot offlat surface on the top and bottom, sides, and ends of the boiler. Thisvery much exceeds the proportion usually adopted; and in scarcely anyinstance are boilers stayed sufficiently to be safe when the shell iscomposed of flat surfaces. The furnaces should be stayed together withbolts of the best scrap iron, 1-1/4 inch in diameter, tapped through bothplates of the water space with thin nuts in each furnace; and it isexpedient to make the row of stays, running horizontally near the level ofthe bars, sufficiently low to come beneath the top of the bars, so as to beshielded from the action of the fire, with which view they should followthe inclination of the bars. The row of stays between the level of the barsand the top of the furnace should be as near the top of the furnace as willconsist with the functions they have to perform, so as to be removed as faras possible from the action of the heat; and to support the furnace top, cross bars may either be adopted, to which the top is secured with bolts, as in the case of locomotives, or stays tapped into the furnace top, with athin nut beneath, may be carried to the top of the boiler; but very littledependence can be put in such stays as stays for keeping down the top ofthe boiler; and the top of the boiler must, therefore, be stayed nearly asmuch as if the stays connecting it with the furnace crowns did not exist. The large rivets passing through thimbles, sometimes used as stays forwater spaces or boiler shells, are objectionable; as, from the great amountof hammering such rivets have to receive to form the heads, the ironbecomes crystalline, so that the heads are liable to come off, and, indeed, sometimes fly off in the act of being formed. If such a fracture occursbetween the boilers after they are seated in their place, or in anyposition not accessible from the outside, it will in general be necessaryto empty the faulty boiler, and repair the defect from the inside. 

305. _Q. _--What should be the pitch or numerical distribution of the stays?

_A. _--The stays, where the sides of the boiler are flat, and the pressureof the steam is from 20 to 30 lbs. , should be pitched about a foot or 18inches asunder; and in the wake of the tubes, where stays cannot be carriedacross to connect the boiler sides, angle iron ribs, like the ribs of aship, should be riveted to the interior of the boiler, and stays of greaterstrength than the rest should pass across, above, and below the tubes, towhich the angle irons would communicate the strain. The whole of the longstays within a boiler should be firmly riveted to the shell, as if builtwith and forming a part of it; as, by the common method of fixing them inby means of cutters, the decay or accidental detachment of a pin or cuttermay endanger the safety of the boiler. Wherever a large perforation in theshell of any circular boiler occurs, a sufficient number of stays should beput across it to maintain the original strength; and where stays areintercepted by the root of the funnel, short stays in continuation of themshould be placed inside. 

BOILER EXPLOSIONS. 

306. _Q. _--What is the chief cause of boiler explosions?

_A. _--The chief cause of boiler explosions is, undoubtedly, too great apressure of steam, or an insufficient strength of boiler; but manyexplosions have also arisen from the flues having been suffered to becomered hot. If the safety valve of a boiler be accidentally jammed, or if theplates or stays be much worn by corrosion, while a high pressure of steamis nevertheless maintained, the boiler necessarily bursts; and if, from aninsufficiency of water in the boiler, or from any other cause, the fluesbecome highly heated, they may be forced down by the pressure of the steam, and a partial explosion may be the result. The worst explosion is where theshell of the boiler bursts; but the collapse of a furnace or flue is alsovery disastrous generally to the persons in the engine room; and sometimesthe shell bursts and the flues collapse at the same time; for if the fluesget red hot, and water be thrown upon them either by the feed pump orotherwise, the generation of steam may be too rapid for the safety valve topermit its escape with sufficient facility, and the shell of the boilermay, in consequence, be rent asunder. Sometimes the iron of the fluesbecomes highly heated in consequence of the improper configuration of theparts, which, by retaining the steam in contact with the metal, preventsthe access of the water: the bottoms of large flues, upon which the flamebeats down, are very liable to injury from this cause; and the iron offlues thus acted upon may be so softened that the flues will collapseupward with the pressure of the steam. The flues of boilers may also becomered hot in some parts from the attachment of scale, which, from itsimperfect conducting power, will cause the iron to be unduly heated; and ifthe scale be accidentally detached, a partial explosion may occur inconsequence. 

307. _Q. _--Does the contact of water with heated metal occasion aninstantaneous generation of steam?

_A. _--It is found that a sudden disengagement of steam does not immediatelyfollow the contact of water with the hot metal, for water thrown upon redhot iron is not immediately converted into steam, but assumes thespheroidal form and rolls about in globules over the surface. Theseglobules, however high the temperature of the metal may be on which theyare placed, never rise above the temperature of 205°, and give off but verylittle steam; but if the temperature of the metal be lowered, the waterceases to retain the spheroidal form, and comes into intimate contact withthe metal, whereby a rapid disengagement of steam takes place. If water bepoured into a very hot copper flask, the flask may be corked up, as therewill be scarce any steam produced so long as the high temperature ismaintained; but so soon as the temperature is suffered to fall below 350°or 400°, the spheroidal condition being no longer maintainable, steam isgenerated with rapidity, and the cork will be projected from the mouth ofthe flask with great force. 

308. _Q. _--What precautions can be taken to prevent boiler explosions?

_A. _--One useful precaution against the explosion of boilers from too greatan internal pressure, consists in the application of a steam gauge to eachboiler, which will make the existence of any undue pressure in any of theboilers immediately visible; and every boiler should have a safety valve ofits own, the passage leading to which should have no connection with thepassage leading to any of the stop valves used to cut off the connectionbetween the boilers; so that the action of the safety valve may be madeindependent of the action of the stop valve. In some cases stop valves havejammed, or have been carried from their seats into the mouth of the pipecommunicating between them, and the action of the safety valves should berendered independent of all such accidents. Safety valves, themselves, sometimes stick fast from corrosion, from the spindles becoming bent, froma distortion of the boiler top with a high pressure, in consequence ofwhich the spindles become jammed in the guides, and from various othercauses which it would be tedious to enumerate; but the inaction of thesafety valves is at once indicated by the steam gauge, and when discovered, the blow through valves of the engine and blow off cocks of the boilershould at once be opened, and the fires raked out. A cone in the ball ofthe waste steam pipe to send back the water carried upward by the steam, should never be inserted; as in some cases this cone has become loose, andclosed up the mouth of the waste steam pipe, whereby the safety valvesbeing rendered inoperative, the boiler was in danger of bursting. 

309. _Q. _--May not danger arise from excessive priming?

_A. _--If the water be carried out of the boiler so rapidly by priming thatthe level of the water cannot be maintained, and the flues or furnaces arein danger of becoming red hot, the best plan is to open every furnace doorand throw in a few buckets full of water upon the fire, taking care tostand sufficiently to the one side to avoid being scalded by the rush ofsteam from the furnace. There is no time to begin drawing the fires in suchan emergency, and by this treatment the fires, though not altogetherextinguished, will be rendered incapable of doing harm. If the flues bealready red hot, on no account must cold water be suffered to enter theboiler, but the heat should be maintained in the furnaces, and the blow offcocks be opened, or the mud hole doors loosened, so as to let all the waterescape; but at the same time the pressure must be kept quite low in theboiler, so that there will be no danger of the hot flues collapsing withthe pressure of the steam. 

310. _Q. _--Are plugs of fusible metal useful in preventing explosions?

_A. _--Plugs of fusible metal were at one time in much repute as aprecaution against explosion, the metal being so compounded that it meltedwith the heat of high pressure steam; but the device, though ingenious, hasnot been found of any utility in practice. The basis of fusible metal ismercury, and it is found that the compound is not homogeneous, and that themercury is forced by the pressure of the steam out of the interstices ofthe metal combined with it, leaving a porous metal which is not easilyfusible, and which is, therefore, unable to perform its intended function. In locomotives, however, and also in some other boilers, a lead rivet isinserted with advantage in the crown of the fire box, which is melted outif the water becomes too low, and thus gives notice of the danger. 

311. _Q. _--May not explosion occur in marine boilers from the accumulationof salt on the flues?

_A. _--Yes, in marine boilers this is a constant source of danger, which isonly to be met by attention on the part of the engineer. If the water inthe boiler be suffered to become too salt, an incrustation of salt willtake place on the furnaces, which may cause them to become red hot, andthey may then be collapsed even by their own weight aided by a moderatepressure of steam. The expedients which should be adopted for preventingsuch an accumulation of salt from taking place within the boiler as will beinjurious to it, properly fall under the head of the management of steamboilers, and will be explained in a subsequent chapter. 

CHAPTER VI. 

PROPORTIONS OF ENGINES. 

 * * * * *

STEAM PASSAGES. 

312. _Q. _--What size of orifice is commonly allowed for the escape of thesteam through the safety valve in low pressure engines?

_A. _--About 0. 8 of a circular inch per horse power, or a circular inch per1-1/4 horse power. The following rule, however, will give the dimensionssuitable for all kinds of engines, whether high or low pressure:--multiplythe square of the diameter of the cylinder in inches by the speed of thepiston in feet per minute, and divide the product by 375 times the pressureon the boiler per square inch; the quotient is the proper area of thesafety valve in square inches. This rule of course supposes that theevaporating surface has been properly proportioned to the engine power. 

313. _Q. _--Is this rule applicable to locomotives?

_A. _--It is applicable to high pressure engines of every kind. Thedimensions of safety valves, however, in practice are very variable, beingin some cases greater, and in some cases less, than what the rule gives, the consideration being apparently as often what proportions will bestprevent the valve from sticking in its seat, as what proportions willenable the steam to escape freely. In Bury's locomotives, the safety valvewas generally 2-1/2 inches diameter for all sizes of boiler, and the valvewas kept down by a lever formed in the proportion of 5 to 1, fitted at oneend with a Salter's balance. As the area of the valve was 5 square inches, the number of pounds shown on the spring balance denoted the number ofpounds pressure on each square inch of the boiler. 

314. _Q. _--Is there only one safety valve in a locomotive boiler?

_A. _--There are always two. 

315. _Q. _--And are they always pressed down by a spring balance, and neverby weights?

_A. _--They are never pressed down by weights; in fact, weights would notanswer on a locomotive at all, as they would jump up and down with thejerks or jolts of the train, and cause much of the steam to escape. In landand marine boilers, however, the safety valve is always kept down byweights; but in steam vessels a good deal of steam is lost in stormyweather by the opening of the valve, owing to the inertia of the weightswhen the ship sinks suddenly in the deep recess between the waves. 

316. _Q. _--What other sizes of safety valves are used in locomotives?

_A. _--Some are as large as 4 inches diameter, giving 12 square inches ofarea; and others are as small as 1-3/16 inch diameter, giving 1 square inchof area. 

317. _Q. _--And are these valves all pressed down by a Salter's springbalance?

_A. _--In the great majority of cases they are so, and the lever by whichthey are pressed down is generally graduated in the proportion of the areaof the valve to unity; that is, in the case of a valve of 12 inches area, the long end of the lever to which the spring balance is attached is 12times the length of the short end, so that the weight or pressure on thebalance shows the pressure per square inch on the boiler. In some cases, however, a spiral spring, and in other cases a pile of elliptical springs, is placed directly upon the top of the valve, and it appears desirable thatone of the valves at least should be loaded in this manner. It is difficultwhen the lever is divided in such a proportion as 12 to 1, to getsufficient lift of the valve without a large increase of pressure on thespring; and it appears expedient, therefore, to employ a shorter lever, which involves either a reduction in the area of the valve, or an increasedstrength in the spring. 

318. _Q. _--What are the proper dimensions of the steam passages?

_A. _--In slow working engines the common size of the cylinder passages isone twenty-fifth of the area of the cylinder, or one fifth of the diameterof the cylinder, which is the same thing. This proportion corresponds verynearly with one square inch per horse power when the length of the cylinderis about equal to its diameter; and one square inch of area per horse powerfor the cylinder ports and eduction passages answers very well in the caseof engines working at the ordinary speed of 220 feet per minute. The areaof the steam pipe is usually made less than the area of the eduction pipe, especially when the engine is worked expansively, and with a considerablepressure of steam. In the case of ordinary condensing engines, however, working with the usual pressure of from 4 to 8 lbs. Above the atmosphere, the area of the steam pipe is not less than a circular inch per horsepower. In such engines the diameter of the steam pipe may be found by thefollowing rule: divide the number of nominal horse power by 0. 8 and extractthe square root of the quotient, which will be the internal diameter of thesteam pipe. 

319. _Q. _--Will you explain by what process of computation theseproportions are arrived at?

_A. _--The size of the steam pipe is so regulated that there will be nomaterial disparity of pressure between the cylinder and boiler; and infixing the size of the eduction passage the same object is kept in view. When the diameter of the cylinder and the velocity with which the pistontravels are known, it is easy to tell what the velocity of the steam in thesteam pipe will be; for if the area of the cylinder be 25 times greaterthan that of the steam pipe, the steam in the steam pipe must travel 25times faster than the piston, and the difference of pressure requisite toproduce this velocity of the steam can easily be ascertained, by findingwhat height a column of steam must be to give that velocity, and what theweight or pressure is of such a column. In practice, however, thisproportion is always exceeded from the condensation of steam in the pipe. 

320. _Q. _--If the relation you have mentioned subsist between the area ofthe steam passages and the velocity of the piston, then the passages mustbe larger when the piston travels very rapidly?

_A. _--And they are so made. The area of the ports of locomotive engines isusually so proportioned as to be from 1/10th to 1/8th the area of thecylinder--in some cases even as much as 1/6th; and in all high speedengines the ports should be very large, and the valve should have a gooddeal of travel so as to open the port very quickly. The area of port whichit appears advisable to give to modern engines of every description, isexpressed by the following rule:--multiply the area of the cylinder insquare inches by the speed of the piston in feet per minute, and divide theproduct by 4, 000; the quotient is the area of each cylinder port in squareinches. This rule gives rather more than a square inch of port per nominalhorse power to condensing engines working at the ordinary speed; but theexcess is but small, and is upon the right side. For engines travellingvery fast it gives a good deal more area than the common proportion, whichis too small in nearly every case. In locomotive engines the eduction pipepasses into the chimney and the force of the issuing steam has the effectof maintaining a rapid draught through the furnace as before explained. Theorifice of the waste steam pipe, or the blast pipe as it is termed, is muchcontracted in some engines with the view of producing a fiercer draught, and an area of 1/22d of the cylinder is a common proportion; but this is asmuch contraction as should be allowed, and is greater than is advisable. 

321. _Q. _--In engines moving at a high rate of speed, you have stated thatit is important to give the valve lead, or in other words to allow thesteam to escape before the end of the stroke?

_A. _--Yes, this is very important, else the piston will have to force outthe steam from the cylinder, and will be much resisted. Near the end of thestroke the piston begins to travel slowly, and if the steam be thenpermitted to escape, very little of the effective stroke is lost, and timeis afforded to the steam, before the motion of the piston is againaccelerated, to make its escape by the port. In some locomotives, frominattention to this adjustment, and from a contracted area of tube section, which involved a strong blast, about half the power of the engine has beenlost; but in more recent engines, by using enlarged ports and by givingsufficient lead, this loss has been greatly diminished. 

322. _Q. _--What do you call sufficient lead?

_A. _--In fast going engines I would call it sufficient lead, when theeduction port was nearly open at the end of the stroke. 

323. _Q. _--Can you give any example of the benefit of increasing the lead?

_A. _--The early locomotives were made with very little lead, and theproportions were in fact very much the same as those previously existing inland engines. About 1832, the benefits of lap upon the valve, which hadbeen employed by Boulton and Watt more than twenty years before, werebeginning to be pretty generally apprehended; and, in the following year, this expedient of economy was applied to the steamer Manchester, in theClyde, and to some other vessels, with very marked success. Shortly afterthis time, lap began to be applied to the valves of locomotives, and it wasfound that not only was there a benefit from the operation of expansion, but that there was a still greater benefit from the superior facility ofescape given to the steam, inasmuch as the application of lap involved thenecessity of turning the eccentric round upon the shaft, which caused theeduction to take place before the end of the stroke. In 1840, one of theengines of the Liverpool and Manchester Railway was altered so as to have 1inch lap on the valve, and 1 inch opening on the eduction side at the endof the stroke, the valve having a total travel of 4-1/4 inches. Theconsumption of fuel per mile fell from 36. 3 lbs. To 28. 6 lbs, or about 25per cent. , and a softer blast sufficed. By using larger exhaust passages, larger tubes, and closer fire bars, the consumption was subsequentlybrought down to 15 lbs. Per mile. 

AIR PUMP, CONDENSER, AND HOT AND COLD WATER PUMPS. 

324. _Q. _--Will you state the proper dimensions of the air pump andcondenser in laud and marine engines?

_A_--Mr. Watt made the air pump of his engine half the diameter of thecylinder and half the stroke, or one eighth of the capacity, and thecondenser was usually made about the same size as the air pump; but as thepressure of the steam has been increased in all modern engines, it isbetter to make the air pump a little larger than this proportion. 0. 6 ofthe diameter of the cylinder and half the stroke answers very well, and thecondenser may be made as large as it can be got with convenience, thoughthe same size as the air pump will suffice. 

325. _Q. _--Are air pumps now sometimes made double acting?

_A. _--Most of the recent direct acting marine engines for driving the screware fitted with a double acting air pump, and when the air pump is doubleacting, it need only be about half the size that is necessary when it issingle acting. It is single acting in nearly every case, except the case ofdirect acting screw engines of recent construction. 

326. _Q. _--What is the difference between a single and a double acting airpump?

_A. _--The single acting air pump expels the air and water from thecondenser only in the upward stroke of the pump, whereas a double actingair pump expels the air and water both in the upward and downward stroke. It has, therefore, to be provided with inlet and outlet valves at bothends, whereas the single acting pump has only to be provided with an inletor foot valve, as it is termed, at the bottom, and with an outlet ordelivery valve, as it is termed, at the top. The single acting air pumprequires to be provided with a valve or valves in the piston or bucket ofthe pump, to enable the air and water lying below the bucket when it beginsto descend, and which have entered from the condenser during the upwardstroke, to pass through the bucket into the space above it during thedownward stroke, from whence they are expelled into the atmosphere on theupward stroke succeeding. But in the double acting air pump no valve isrequired in the piston or bucket of the pump, and all that is necessary isan inlet and outlet valve at each end. 

337. _Q_--What are the dimensions of the foot and discharge valves of theair pump?

_A. _--The area through the foot and discharge valves is usually made equalto one fourth of the area of the air pump, and the diameter of the wastewater pipe is made one fourth of the diameter of the cylinder, which givesan area somewhat less than that of the foot and discharge valve passages. But this proportion only applies in slow engines. In fast engines, with theair pump bucket moving as fast as the piston, the area through the foot anddischarge valves should be equal to the area of the pump itself, and thewaste water pipe should be of about the same dimensions. 

328. _Q. _--You have stated that double acting air pumps need only be ofhalf the size of single acting ones. Does that relation hold at all speeds?

_A. _--It holds at all speeds if the velocity of the pump buckets are ineach case the same; but it does not hold if the engine with the singleacting pump works slowly, and the engine with the double acting pump movesrapidly, as in the case of direct acting screw engines. All pumps moving ata high rate of speed lose part of their efficiency, and such pumps shouldtherefore be of extra size. 

329. _Q. _--How do you estimate the quantity of water requisite forcondensation?

_A. _--Mr. Watt found that the most beneficial temperature of the hot wellof his engines was 100 degrees. If, therefore, the temperature of the steambe 212°, and the latent heat 1, 000°, then 1, 212° may be taken to representthe heat contained in the steam, or 1, 112° if we deduct the temperature ofthe hot well. If the temperature of the injection water be 50°, then 50degrees of cold are available for the abstraction of heat; and as the totalquantity of heat to be abstracted is that requisite to raise the quantityof water in the steam 1, 112 degrees, or 1, 112 times that quantity onedegree, it would raise one fiftieth of this, or 22. 24 times the quantity ofwater in the steam, 50 degrees. A cubic inch of water therefore raised intosteam will require 22. 24 cubic inches of water at 50 degrees for itscondensation, and will form therewith 23. 24 cubic inches of hot water at100 degrees. Mr. Watt's practice was to allow about a wine pint (28. 9 cubicinches) of injection water, for every cubic inch of water evaporated fromthe boiler. 

330. _Q. _--Is not a good vacuum in an engine conducive to increased power?

_A. _--It is. 

331. _Q. _--And is not the vacuum good in the proportion in which thetemperature is low, supposing there to be no air leaks?

_A. _--Yes. 

332. _Q. _--Then how could Mr. Watt find a temperature of 100° in the waterdrawn from the condenser, to be more beneficial than a temperature of 70°or 80°, supposing there to be an abundant supply of cold water?

333. _A. _--Because the superior vacuum due to a temperature of 70° or 80°involves the admission of so much cold water into the condenser, which hasafterward to be pumped out in opposition to the pressure of the atmosphere, that the gain in the vacuum does not equal the loss of power occasioned bythe additional load upon the pump, and there is therefore a clear loss bythe reduction of the temperature below 100°, if such reduction be caused bythe admission of an additional quantity of water. If the reduction oftemperature, however, be caused by the use of colder water, there is a gainproduced by it, though the gain will within certain limits be greater ifadvantage be taken of the lowness of the temperature to diminish thequantity of injection. 

334. _Q. _--How do you determine the proper area of the injection orifice?

_A. _--The area of the injection orifice proper for any engine can easily betold when the quantity of water requisite to condense the steam is known, and the pressure is specified under which the water enters the condenser. The vacuum in the condenser may be taken at 26 inches of mercury, which isequivalent to a column of water 29. 4 ft. High, and the square root of 29. 4multiplied by 8. 021 is 43. 15, which is the velocity in feet per second thata heavy body would acquire in falling 29. 4 ft. , or with which the waterwould enter the condenser. Now, if a cubic foot of water evaporated perhour be equivalent to an actual horse power, and 28. 9 cubic inches of waterbe requisite for the condensation of a cubic inch of water in the form ofsteam, 28. 9 cubic feet of condensing water per horse power per hour, or13. 905 cubic inches per second, will be necessary for the engine, and thesize of the injection orifice must be such that this quantity of waterflowing with the velocity of 43. 15 ft. Per second, or 517. 8 inches persecond, will gain admission to the condenser. Dividing, therefore, 13. 905, the number of cubic inches to be injected, by 517. 8, the velocity of influxin inches per second, we get 0. 02685 for the area of the orifice in squareinches; but inasmuch as it has been found by experiment that the actualdischarge of water through a hole in a thin plate is only six tenths of thetheoretical discharge on account of the contracted vein, the area of theorifice must be increased in the proportion of such diminution of effect, or be made 0. 04475, or 1/22d of a square inch per horse power. This, itwill be remarked, is the theoretical area required per actual horse power;but as the friction and contractions in the pipe further reduce thedischarge, the area is made 1/15th of a square inch per actual horse power, or rather per cubic foot of water evaporated from the boiler. 

335. _Q. _--Cannot the condensation of the steam be accomplished by anyother means than by the admission of cold water into the condenser?

_A. _--It may be accomplished by the method of external cold, as it iscalled, which consists in the application of a large number of thinmetallic surfaces to the condenser, on the one side of which the steamcirculates, while on the other side there is a constant current of coldwater, and the steam is condensed by coming into contact with the coldsurfaces, without mingling with the water used for the purpose ofrefrigeration. The first kind of condenser employed by Mr. Watt wasconstructed after this fashion, but he found it in practice to beinconvenient from its size, and to become furred up or incrusted when thewater was bad, whereby the conducting power of the metal was impaired. Hetherefore reverted to the use of the jet of cold water, as being upon thewhole preferable. The jet entered the condenser instead of the cylinder aswas the previous practice, and this method is now the one in common use. Some few years ago, a good number of steam vessels were fitted with Hall'scondensers, which operated on the principle of external cold, and whichconsisted of a faggot of small copper tubes surrounded by water; but theuse of those condensers has not been persisted in, and most of the vesselsfitted with them have returned to the ordinary plan. 

336. _Q. _--You stated that the capacity of the feed pump was 1/240th of thecapacity of the cylinder in the case of condensing engines, --the enginebeing double acting and the pump single acting, --and that in high pressureengines the capacity of the pump should be greater in proportion to thepressure of the steam. Can you give any rule that will express the propercapacity for the feed pump at all pressures?

_A. _--That will not be difficult. In low pressure engines the pressure inthe boiler may be taken at 5 lbs. Above the atmospheric pressure, or 20lbs. Altogether; and as high pressure steam is merely low pressure steamcompressed into a smaller compass, the size of the feed pump in relation tothe size of the cylinder must obviously vary in the direct proportion ofthe pressure; and if it be 1/240th of the capacity of the cylinder when thetotal pressure of the steam is 20 lbs. , it must be 1/120th of the capacityof the cylinder when the pressure is 40 lbs. Per square inch, or 25 lbs. Per square inch above the atmospheric pressure. This law of variation isexpressed by the following rule:--multiply the capacity of the cylinder incubic inches by the total pressure of the steam in lbs. Per square inch, orthe pressure per square inch on the safety valve plus 15, and divide theproduct by 4, 800; the quotient is the capacity of the feed pump in cubicinches, when the feed pump is single acting and the engine double acting. If the feed pump be double acting, or the engine single acting, thecapacity of the pump must just be one half of what is given by this rule. 

337. _Q. _--But should not some addition be made to the size of pump thusobtained if the pump works at a high rate of speed?

_A. _--No; this rule makes allowance for defective action. All pumps liftmuch less water than is due to the size of their barrels and the number oftheir strokes. Moderately good pumps lose 50 per cent. Of their theoreticaleffect, and bad pumps 80 per cent. 

338. _Q. _--To what is this loss of effect to be chiefly ascribed?

_A. _--Mainly to the inertia of the water, which, if the pump piston bedrawn up very rapidly, cannot follow it with sufficient rapidity; so thatthere may be a vacant space between the piston and the water; and at thereturn stroke the momentum of the water in the pipe expends itself ingiving a reverse motion to the column of water approaching the pump. Messrs. Kirchweger and Prusman, of Hanover, have investigated this subjectby applying a revolving cock at the end of a pipe leading from an elevatedcistern containing water, and the water escaped at every revolution of thecock in the same manner as if a pump were drawing it. With a column ofwater of 17 feet, they found that at 80 revolutions of the cock per minute, the water delivered per minute by the cock was 9. 45 gallons; but with 140revolutions of the cock per minute, the water delivered per minute by thecock was only 5. 42 gallons. They subsequently applied an air vessel to thepipe beside the cock, when the discharge rose to 12. 9 gallons per minutewith 80 revolutions, and 18. 28 gallons with 140 revolutions. Air vesselsshould therefore be applied to the suction side of fast moving pumps, andthis is now done with good results. 

339. _Q. _--What are the usual dimensions of the cold water pump of landengines?

_A. _--If to condense a cubic inch of water raised into steam 28. 9 cubicinches of condensing water are required, then the cold water pump ought tobe 28. 9 times larger than the feed pump, supposing that its losses wereequally great. The feed pump, however, is made sufficiently large tocompensate for leaks in the boiler and loss of steam through the safetyvalve, so that it will be sufficient if the cold water pump be 24 timeslarger than the feed pump. This ratio is preserved by the following rule:--multiply the capacity of the cylinder in cubic inches by the total pressureof the steam per square inch, or the pressure on the safety valve plus 15, and divide the product by 200. The quotient is the proper capacity of thecold water pump in cubic inches when the engine is double acting, and thepump single acting. 

FLY WHEEL. 

340. _Q. _--By what considerations do you determine the dimensions of thefly wheel of an engine?

_A. _--By a reference to the power generated, each half stroke of theengine, and the number of half strokes that are necessary to give to thefly wheel its standard velocity, supposing the whole power devoted to thatobject. In practice the power resident in the fly varies from 2-1/2 to 6times that generated each half stroke; and if the weight of the wheel beequal to the pressure on the piston, its velocity must be such as it wouldacquire by falling through a height equal to from 2-1/2 to 6 times thestroke, according to the purpose for which the engine is intended. If avery equable motion is required, a heavier or swifter fly wheel must beemployed. 

341. _Q. _--What is Boulton and Watt's rule for fly wheels?

_A. _--Their rule is one which under any given circumstances fixes thesectional area of the fly wheel rim, and it is as follows:--multiply 44, 000times the square of the diameter of the cylinder in inches by the length ofthe stroke in feet, and divide this product by the product of the square ofthe number of revolutions of the fly wheel per minute, multiplied by thecube of its diameter in feet. The quotient is the area of section of thefly wheel rim in square inches. 

STRENGTHS OF LAND ENGINES. 

342. _Q. _--Can you give a rule for telling the proper thickness of thecylinders of steam engines?

_A. _--In low pressure engines the thickness of metal of the cylinder, inengines of a medium size, should be about 1/40th of the diameter of thecylinder, which, with a pressure of steam of 20 lbs. Above the atmosphere, will occasion a strain of only 400 Lbs. Per square inch of section of themetal; the thickness of the metal of the trunnion bearings of oscillatingengines should be 1/32d of the diameter of the cylinder, and the breadth ofthe bearing should be about half its diameter. In high pressure engines thethickness of the cylinder should be about 1/16th its diameter, which, witha pressure of steam of 80 lbs. Upon the square inch, will occasion a strainof 640 lbs. Upon the square inch of section of the metal; and the thicknessof the metal of the trunnion bearings of high pressure oscillating enginesshould be 1/13th of the diameter of the cylinder. The strength, however, isnot the sole consideration in proportioning cylinders, for they must bemade of a certain thickness, however small the pressure is within them, that they may not be too fragile, and will stand boring. While, also, anengine of 40 inches diameter would be about one inch thick, the thicknesswould not be quite two inches in an 80 inch cylinder. In fact there will bea small constant added to the thickness for all diameters, which will berelatively larger the smaller the cylinders become. In the cylinders ofPenn's 12 horse power engines, the diameter of cylinder being 21-1/2inches, the thickness of the metal is 9/16ths: in Penn's 40 inch cylinders, the thickness is 1 inch, and in the engines of the Ripon, Pottinger, andIndus, by Messrs. Miller, Ravenhill and Co. , with cylinders 76 inchesdiameter, the thickness of the metal is 1-11/16. These are all oscillatingengines. 

343. _Q. _--What is the proportion of the piston rod?

_A. _--The diameter of the piston rod is usually made 1/10th of the diameterof the cylinder, or the sectional area of the piston rod is 1/100th of thearea of the cylinder. This proportion, however, is not applicable tolocomotive, or even fast moving marine engines. In locomotive engines thepiston rod is made 1/7th of the diameter of the cylinder, and it is obviousthat where the pressure on the piston is great, the piston rod must belarger than when the pressure on the piston is small. 

344. _Q. _--What are the proper dimensions of the main links of a land beamengine?

_A. _--The sectional area of the main links in land beam engines is 1/113thof the area of the cylinder, and the length of the main links is usuallyhalf the length of the stroke. 

345. _Q. _--What are the dimensions of the connecting rod of a land engine?

_A. _--In land engines the connecting rod is usually of cast iron with acruciform section: the breadth across the arms of the cross is about 1/20thof the length of the rod, the sectional area at the centre 1/28th of thearea of the cylinder, and at the ends 1/35th of the area of the cylinder:the length of the rod is usually 3-1/2 times the length of the stroke. Itis preferable, however, to make the connecting rod of malleable iron, andthen the dimensions will be those proper for marine engines. 

346. _Q. _--What was Mr. Watt's rule for the connecting rod?

_A. _--Some of his connecting rods were of iron and some of wood. Todetermine the thickness when of wood, multiply the square of the diameterof the cylinder in inches by the length of the stroke in feet, and dividethe product by 24. Extract the fourth root of the quotient, which is thethickness in inches. For iron the rule is the same, only the divisor was57. 6 instead of 24. 

347. _Q. _--What are the dimensions of the end studs of a land engine beam?

_A. _--In low pressure engines the diameter of the end studs of the enginebeam are usually made 1/9th of the diameter of the cylinder when of castiron, and 1/10th when of wrought iron, which gives a load with low steam ofabout 500 lbs. Per circular inch of transverse section; but a larger sizeis preferable, as with large bearings the brasses do not wear so rapidlyand the straps are not so likely to be burst by the bearings becoming oval. These sizes, as also those which immediately follow, suppose the pressureon the piston to be 18 lbs. Per circular inch. 

348. _Q. _--How is the strength of a cast iron gudgeon computed?

_A. _--To find the proper size of a cast iron gudgeon adapted to sustain anygiven weight:--multiply the weight in lbs. By the intended length ofbearing expressed in terms of the diameter; divide the product by 500, andextract the square root of the quotient, which is the diameter in inches. 

349. _Q. _--What was Mr. Watt's rule for the strength of gudgeons?

_A. _--Supposing the gudgeon to be square, then, to ascertain the thickness, multiply the weight resting on the gudgeon by the distance between thetrunnions, and divide the product by 333. Extract the cube root of thequotient, which is the thickness in inches. 

350. _Q. _--How do you find the proper strength for the cast iron beam of aland engine?

_A. _--If the force acting at the end of an engine beam be taken at 18 lbs. Per circular inch of the piston, then the force acting at the middle willbe 36 lbs. Per circular inch of the piston, and the proper strength of thebeam at the centre will be found by the following rule:--divide the weightin lbs. Acting at the centre by 250, and multiply the quotient by thedistance between the extreme centres. To find the depth, the breadth beinggiven:--divide this product by the breadth in inches, and extract thesquare root of the quotient, which is the depth. The depth of a land enginebeam at the ends is usually made one third of the depth at the centre (thedepth at the centre being equal to the diameter of the cylinder in the caseof low pressure engines), while the length is made equal to three times thelength of the stroke, and the mean thickness 1/108th of the length--thewidth of the edge bead being about three times the thickness of the web. Inmany modern engines the force acting at the end of the beam is more than 18lbs. Per circular inch of the piston, but the above rules are stillapplicable by taking an imaginary cylinder with an area larger in theproportion of the larger pressure. 

351. _Q. _--What was Mr. Watt's rule for the main beams of his engines?

_A. _--Some of those beams were of wood and some of cast iron. The woodbeams were so proportioned that the thickness was 1/58th of thecircumference, and the depth 1/375. The side of the beam, supposing itsquare, was found by multiplying the diameter of the cylinder by the lengthof the stroke, and extracting the cube root of the quotient, which will bethe depth or thickness of the beam. This rule allows a beam 16 feet long tobend 1/8th of an inch, and a beam 32 feet long to bend 1/4 of an inch. Forcast iron beams the square of the diameter of the cylinder, multiplied bythe length between the centres, is equal to the square of the depth, multiplied by the thickness. 

352. _Q. _--What law does the strength of beams and shafts follow?

_A. _--In the case of beams subjected to a breaking force, the strength withany given cohesion of the material will be proportional to the breadth, multiplied by the square of the depth; and in the case of revolving shaftsexposed to a twisting strain, the strength with any given cohesive power ofthe material will be as the cube of the diameter. 

353. _Q. _--How is the strength of a cast iron shaft to resist torsiondetermined?

_A. _--Experiments upon the force requisite to twist off cast iron necksshow that if the cube of the diameter of neck in inches be multiplied by880, the product will be the force of torsion which will twist them offwhen acting at 6 inches radius; on this fact the following rule is founded:To find the diameter of a cast iron fly wheel shaft:--multiply the squareof the diameter of the cylinder in inches, by the length of the crank ininches, and extract the cube root of the product, which multiply by 0. 3025, and the result will be the proper diameter of the shaft in inches at thesmallest part, when of cast iron. 

354. _Q. _--What was Mr. Watt's rule for the necks of his crank shafts?

_A. _--Taking the pressure on the piston at 12 lbs. Pressure on the squareinch, and supposing this force to be applied at one foot radius, divide thetotal pressure of the piston reduced to 1 foot of radius by 31. 4, andextract the cube root of the quotient, which is the diameter of the shaft:or extract the cube root of 13. 7 times the number of cubic feet of steamrequired to make one revolution, which is also the diameter of the shaft. 

355. _Q. _--Can you give any rule for the strength of the teeth of wheels?

_A. _--To find the proper dimensions for the teeth of a cast iron wheel:--multiply the diameter of the pitch circle in feet by the number ofrevolutions to be made per minute, and reserve the product for a divisor;multiply the number of _actual_ horses power to be transmitted by 240, anddivide the product by the above divisor, which will give the strength. Ifthe pitch be given to find the breadth, divide the above strength by thesquare of the pitch in inches; or if the breadth be given, then to find thepitch divide the strength by the breadth in inches, and extract the squareroot of the quotient, which is the proper pitch in inches. The length ofthe teeth is usually about 5/8ths of the pitch. Pinions to worksatisfactorily should not have less than 30 or 40 teeth, and where thespeed exceeds 220 feet in the minute, the teeth of the larger wheel shouldbe of wood, made a little thicker, to keep the strength unimpaired. 

356. _Q. _--What was Mr. Watt's rule for the pitch of wheels?

_A. _--Multiply five times the diameter of the larger wheel by the diameterof the smaller, and extract the fourth root of the product, which is thepitch. 

STRENGTH OF MARINE AND LOCOMOTIVE ENGINES. 

357. _Q. _--Cannot you give some rules of strength which will be applicablewhatever pressure may be employed?

_A. _--In the rules already given, the effective pressure may be reckoned atfrom 18 to 20 lbs. Upon every square inch of the piston, as is usual inland engines; and if the pressure upon every square inch of the piston bemade twice greater, the dimensions must just be those proper for an engineof twice the area of piston. It will not be difficult, however, tointroduce the pressure into the rules as an element of the computation, whereby the result will be applicable both to high and low pressureengines. 

358. _Q. _--Will you apply this mode of computation to a marine engine, andfirst find the diameter of the piston rod?

_A. _--The diameter of the piston rod may be found by multiplying thediameter of the cylinder in inches, by the square root of the pressure onthe piston in lbs. Per square inch, and dividing by 50, which makes thestrain 1/7th of the elastic force. 

359. _Q. _--What will be the rule for the connecting rod, supposing it to beof malleable iron?

_A. _--The diameter of the connecting rod at the ends, may be found bymultiplying 0. 019 times the square root of the pressure on the piston inlbs. Per square inch by the diameter of the cylinder in inches; and thediameter of the connecting rod in the middle may be found by the followingrule:--to 0. 0035 times the length of the connecting rod in inches, add 1, and multiply the sum by 0. 019 times the square root of the pressure on thepiston in lbs. Per square inch, multiplied by the diameter of the cylinderin inches. The strain is equal to 1/6th of the elastic force. 

360. _Q. _--How will you find the diameter of the cylinder side rods of amarine engine?

_A. _--The diameter of the cylinder side rods at the ends may be found bymultiplying 0. 0129 times the square root of the pressure on the piston inlbs. Per square inch by the diameter of the cylinder; and the diameter ofthe cylinder side rods at the middle is found by the following rule:--to0. 0035 times the length of the rod in inches, add 1, and multiply the sumby 0. 0129 times the square root of the pressure on the piston in lbs. Persquare inch, multiplied by the diameter of the cylinder in inches; theproduct is the diameter of each side rod at the centre in inches. Thestrain upon the side rods is by these rules equal to 1/6th of the elasticforce. 

361. _Q. _--How do you determine the dimensions of the crank?

_A. _--To find the exterior diameter of the large eye of the crank when ofmalleable iron:--to 1. 561 times the pressure of the steam upon the pistonin lbs. Per square inch, multiplied by the square of the length of thecrank in inches, add 0. 00494 times the square of the diameter of thecylinder in inches, multiplied by the square of the number of lbs. Pressureper square inch on the piston; extract the square root of this quantity;divide the result by 75. 59 times the square root of the length of the crankin inches, and multiply the quotient by the diameter of the cylinder ininches; square the product and extract the cube root of the square, towhich add the diameter of the hole for the reception of the shaft, and theresult will be the exterior diameter of the large eye of the crank when ofmalleable iron. The diameter of the small eye of the crank may be found byadding to the diameter of the crank pin 0. 02521 times the square root ofthe pressure on the piston in lbs. Per square inch, multiplied by thediameter of the cylinder in inches. 

362. _Q. _--What will be the thickness of the crank web?

_A. _--The thickness of the web of the crank, supposing it to be continuedto the centre of the shaft, would at that point be represented by thefollowing rule:--to 1. 561 times the square of the length of the crank ininches, add 0. 00494 times the square of the diameter of the cylinder ininches, multiplied by the pressure on the piston in lbs. Per square inch;extract the square root of the sum, which multiply by the diameter of thecylinder squared in inches, and by the pressure on the piston in lbs. Persquare inch; divide the product by 9, 000, and extract the cube root of thequotient, which will be the proper thickness of the web of the crank whenof malleable iron, supposing the web to be continued to the centre of theshaft. The thickness of the web at the crank pin centre, supposing it to becontinued thither, would be 0. 022 times the square root of the pressure onthe piston in lbs. Per square inch, multiplied by the diameter of thecylinder. The breadth of the web of the crank at the shaft centre should betwice the thickness, and at the pin centre 1-1/2 times the thickness of theweb; the length of the large eye of the crank would be equal to thediameter of the shaft, and of the small eye 0. 0375 times the square root ofthe pressure on the piston in lbs. Per square inch, multiplied by thediameter of the cylinder. 

363. _Q. _--Will you apply the same method of computation to find thedimensions of a malleable iron paddle shaft?

_A. _--The method of computation will be as follows:--to find the dimensionsof a malleable iron paddle shaft, so that the strain shall not exceed5/6ths of the elastic force, or 5/6ths of the force iron is capable ofwithstanding without permanent derangement of structure, which in tensilestrains is taken at 17, 800 lbs. Per square inch: multiply the pressure inlbs. Per square inch on the piston by the square of the diameter of thecylinder in inches, and the length of the crank in inches, and extract thecube root of the product, which, multiplied by 0. 08264, will be thediameter of the paddle shaft journal in inches when of malleable iron, whatever the pressure of the steam may be. The length of the paddle shaftjournal should be 1-1/4 times the diameter; and the diameter of the partwhere the crank is put on is often made equal to the diameter over thecollars of the journal or bearing. 

364. _Q. _--How do you find the diameter of the crank pin?

_A. _--The diameter of the crank pin in inches may be found by multiplying0. 02836 times the square root of the pressure on the piston in lbs. Persquare inch, by the diameter of the cylinder in inches. The length of thepin is usually about 9/8th times its diameter, and the strain if all thrownupon the end of the pin will be equal to the elastic force; but in ordinaryworking, the strain will only be equal to 1/3d of the elastic force. 

365. _Q. _--What are the dimensions of the cross head?

_A. _--If the length of the cross head be taken at 1. 4 times the diameter ofthe cylinder, the dimensions of the cross head will be as follows:--theexterior diameter of the eye in the cross head for the reception of thepiston rod, will be equal to the diameter of the hole, plus 0. 02827 timesthe cube root of the pressure on the piston in lbs. Per square inch, multiplied by the diameter of the cylinder in inches; and the depth of theeye will be 0. 0979 times the cube root of the pressure on the piston inlbs. Per square inch, multiplied by the diameter of the cylinder in inches. The diameter of each cross head journal will be 0. 01716 times the squareroot of the pressure on the piston in lbs. Per square inch, multiplied bythe diameter of the cylinder in inches--the length of the journal being9/8ths its diameter. The thickness of the web at centre will be 0. 0245times the cube root of the pressure on the piston in lbs. Per square inch, multiplied by the diameter of the cylinder in inches; and the depth of webat centre will be 0. 09178 times the cube root of the pressure on the pistonin lbs. Per square inch, multiplied by the diameter of the cylinder ininches. The thickness of the web at journal will be 0. 0122 times the squareroot of the pressure on the piston in lbs. Per square inch, multiplied bythe diameter of the cylinder in inches; and the depth of the web at journalwill be 0. 0203 times the square root of the pressure upon the piston inlbs. Per square inch, multiplied by the diameter of the cylinder in inches. In these rules for the cross head, the strain upon the web is 1/2. 225 timesthe elastic force; the strain upon the journal in ordinary working is1/2. 33 times the elastic force; and if the outer ends of the journals arethe only bearing points, the strain is 1/1. 165 times the elastic force, which is very little in excess of the elastic force. 

366. _Q. _--How do you find the diameter of the main centre whenproportioned according to this rule?

_A. _--The diameter of the main centre may be found by multiplying 0. 0367times the square root of the pressure upon the piston in lbs. Per squareinch, by the diameter of the cylinder in inches, which will give thediameter of the main centre journal in inches when of malleable iron, andthe length of the main centre journal should be 1-1/2 times its diameter;the strain upon the main centre journal in ordinary working will be about1/2 the elastic force. 

367. _Q. _--What are the proper dimensions of the gibs and cutters of anengine?

_A. _--The depth of gibs and cutters for attaching the piston rod to thecross head, is 0. 0358 times the cube root of the pressure of the steam onthe piston in lbs. Per square inch, multiplied by the diameter of thecylinder; and the thickness of the gibs and cutters is 0. 007 times the cuberoot of the pressure on the piston in lbs. Per square inch, multiplied bythe diameter of its cylinder. The depth of the cutter through the piston is0. 017 times the square root of the pressure on the piston in lbs. Persquare inch, multiplied by the diameter of the cylinder in inches; and thethickness of the cutter through the piston is 0. 007 times the square rootof the pressure on the piston in lbs. Per square inch, multiplied by thediameter of the cylinder. 

368. _Q. _--Are not some of the parts of an engine constructed according tothese rules too weak, when compared with the other parts?

_A. _--It is obvious, from the varying proportions subsisting in thedifferent parts of the engine between the strain and the elastic force, that in engines proportioned by these rules--which represent neverthelessthe average practice of the best constructors--some of the parts mustpossess a considerable excess of strength over other parts, and it appearsexpedient that this disparity should be diminished, which may best be doneby increasing the strength of the parts which are weakest; inasmuch as thefrequent fracture of some of the parts shows that the dimensions at presentadopted for those parts are scarcely sufficient, unless the iron of whichthey are made is of the best quality. At the same time it is quite certain, that engines proportioned by these rules will work satisfactorily wheregood materials are employed; but it is important to know in what parts goodmaterials and larger dimensions are the most indispensable. In many of theparts, moreover, it is necessary that the dimensions should be proportionedto meet the wear and the tendency to heat, instead of being merelyproportioned to obtain the necessary strength; and the crank pin is one ofthe parts which requires to be large in diameter, and as long as possiblein the bearing, so as to distribute the pressure, and prevent thedisposition to heat which would otherwise exist. The cross head journalsalso should be long and large; for as the tops of the side rods have littletravel, the oil is less drawn into the bearings than if the travel wasgreater, and is being constantly pressed out by the punching strain. Thisstrain should therefore be reduced as far as possible by its distributionover a large surface. In the rules which are contained in the answers tothe ten preceding questions (358 to 367) the pressure on the piston in lbs. Per square inch is taken as the sum of the pressure of steam in the boilerand of the vacuum; the latter being assumed to be 15 lbs. Per square inch. 

CHAPTER VII. 

CONSTRUCTIVE DETAILS OF BOILERS. 

 * * * * *

LAND AND MARINE BOILERS. 

369. _Q. _--Will you explain the course of procedure in the construction andsetting of wagon boilers?

_A. _--Most boilers are made of plates three eighths of an inch thick, andthe rivets are from three eighths to three fourths of an inch in diameter. In the bottom and sides of a wagon boiler the heads of the rivets, or theends formed on the rivets before they are inserted, should be large andplaced next the fire, or on the outside; whereas on the top of the boilerthe heads should be on the inside. The rivets should be placed about twoinches distant from centre to centre, and the centre of the row of rivetsshould be about one inch from the edge of the plate. The edges of theplates should be truly cut, both inside and outside, and after the parts ofthe boiler have been riveted together, the edges of the plates should beset up or caulked with a blunt chisel about a quarter of an inch thick inthe point, and struck by a hammer of about three or four pounds weight, oneman holding the caulking tool while another strikes. 

370. _Q. _--Is this the usual mode of caulking?

_A. _--No, it is not the usual mode; but it is the best mode, and is themode adopted by Mr. Watt. The usual mode now is for one man to caulk theseams with a hammer in one hand and a caulking chisel in the other, and insome of the difficult corners of marine flue boilers it is not easy for twomen to get in. A good deal of the caulking has also sometimes to be donewith the left hand. 

371. _Q. _--Should the boiler be proved after caulking?

_A. _--The boiler should be filled with water and caulked afresh in anyleaky part. When emptied again, all the joints should be painted with asolution of sal ammoniac in urine, and so soon as the seams are well rustedthey should be dried with a gentle fire, and then be painted over with athin putty formed of whiting and linseed oil, the heat being continueduntil the putty becomes so hard that it cannot be readily scratched withthe nail, and care must be taken neither to burn the putty nor todiscontinue the fire until it has become quite dry. 

372. _Q. _--How should the brickwork setting of a wagon boiler be built?

_A. _--In building the brickwork for the setting of the boiler, the partupon which the heat acts with most intensity is to be built with clayinstead of mortar, but mortar is to be used on the outside of the work. Oldbars of flat iron may be laid under the boiler chime to prevent that partof the boiler from being burned out, and bars of iron should also runthrough the brickwork to prevent it from splitting. The top of the boileris to be covered with brickwork laid in the best lime, and if the lime benot of the hydraulic kind, it should be mixed with Dutch terrass, to makeit impenetrable to water. The top of the boiler should be well plasteredwith this lime, which will greatly conduce to the tightness of the seams. Openings into the flues must be left in convenient situations to enable theflues to be swept out when required, and these openings may be closed withcast iron doors jointed with clay or mortar, which may be easily removedwhen required. Adjacent to the chimney a slit must be left in the top ofthe flue with a groove in the brickwork to enable the sliding door ordamper to be fixed in that situation, which by being lowered into the fluewill obstruct the passage of the smoke and moderate the draught, wherebythe chimney will be prevented from drawing the flame into it before theheat has acted sufficiently upon the boiler. 

373. _Q. _--Are marine constructed in the same way as land boilers?

_A. _--There is very little difference in the two cases: the whole of theshells of marine boilers, however, should be double riveted with rivets11/16ths of an inch in diameter, and 2-3/8th inches from centre to centre, the weakening effect of double riveting being much less than that of singleriveting. The furnaces above the line of bars should be of the bestLowmoor, Bowling, or Staffordshire scrap plates, and the portion of eachfurnace above the bars should consist only of three plates, one for the topand one for each side, the lower seam of the side plates being situatedbeneath the level of the bars, so as not to be exposed to the heat of thefurnace. The tube plates of tubular boilers should be of the best Lowmoor, or Bowling iron, seven eighths to one inch thick: the shells should be ofthe best Staffordshire, or Thornycroft S crown iron, 7/16ths of an inchthick. 

374. _Q. _--Of what kind of iron should the angle iron or corner iron becomposed?

_A. _--Angle iron should not be used in the construction of boilers, as inthe manufacture it becomes reedy, and is apt to split up in the directionof its length: it is much the safer practice to bend the plates at thecorners of the boiler; but this must be carefully done, without introducingany more sharp bends than can be avoided, and plates which require to bebent much should be of Lowmoor iron. It will usually be found expedient tointroduce a ring of angle iron around the furnace mouths, though it isdiscarded in the other parts of the boiler; but it should be used assparingly as possible, and any that is used should be of the best quality. 

375. _Q. _--Is it not important to have the holes in the plates opposite toone another?

_A. _--The whole of the plates of a boiler should have the holes for therivets punched, and the edges cut straight, by means of self-actingmachinery, in which a travelling table carries forward the plate with anequal progression every stroke of the punch or shears; and machinery ofthis kind is now extensively employed. The practice of forcing the parts ofboilers together with violence, by means of screw-jacks, and drifts throughthe holes, should not be permitted; as a great strain may thus be thrownupon the rivets, even when there is no steam in the boiler. All rivetsshould be of the best Lowmoor iron. The work should be caulked both withinand without wherever it is accessible, but in the more confined situationswithin the flues the caulking will in many cases have to be done with thehand or chipping hammer, instead of the heavy hammer previously prescribed. 

376. _Q. _--How is the setting of marine boilers with internal furnaceseffected?

_A. _--In the setting of marine boilers care must be taken that no copperbolts or nails project above the wooden platform upon which they rest, andalso that no projecting copper bolts in the sides of the ship touch theboiler, as the galvanic action in such a case would probably soon wear thepoints of contact into holes. The platform may consist of three inchplanking laid across the keelsons nailed with iron, nails, the heads ofwhich are well punched down, and caulked and puttied like a deck. Thesurface may then be painted over with thin putty, and fore and aft boardsof half the thickness may then be laid down and nailed securely with ironnails, having the heads well punched down. This platform must then becovered thinly and evenly with mastic cement and the boiler be set downupon it, and the cement must be caulked beneath the boiler by means ofwooden caulking tools, so as completely to fill every vacuity. Coomings ofwood sloped on the top must next be set round the boiler, and the spacebetween the coomings and the boiler must be caulked full of cement, and besmoothed off on the top to the slope of the coomings, so as to throw offany water that might be disposed to enter between the coomings and theboiler. 

377. _Q. _--How is the cement used for setting marine boilers compounded?

_A. _--Mastic cement proper for the setting of boilers is sold in manyplaces ready made. Hamelin's mastic is compounded as follows:--to any givenweight of sand or pulverized earthenware add two thirds such given weightof powdered Bath, Portland, or other similar stone, and to every 560 lbs. Weight of the mixture add 40 lbs. Weight of litharge, 2 lbs. Of powderedglass or flint, 1 lb. Of minium, and 2 lbs. Of gray oxide of lead; pass themixture through a sieve, and keep it in a powder for use. When wanted foruse, a sufficient quantity of the powder is mixed with some vegetable oilupon a board or in a trough in the manner of mortar, in the proportion of605 lbs. Of the powder to 5 gallons of linseed, walnut, or pink oil, andthe mixture is stirred and trodden upon until it assumes the appearance ofmoistened sand, when it is ready for use. The cement should be used on thesame day as the oil is added, else it will be set into a solid mass. 

378. _Q. _--What is the best length of the furnaces of marine boilers?

_A. _--It has already been stated that furnace bars should not much exceedsix feet in length, as it is difficult to manage long furnaces; but it is afrequent practice to make the furnaces long and narrow, the consequence ofwhich is, that it is impossible to fire them effectually at the after end, especially upon long voyages and in stormy weather, and air escapes intothe flues at the after end of the bars, whereby the efficacy of the boileris diminished. Where the bars are very long it will generally be found thatan increased supply of steam and a diminished consumption of coal will bethe consequence of shortening them, and the bars should always lie with aconsiderable inclination to facilitate the distribution of the fuel overthe after part of the furnace. When there are two lengths of bars in thefurnace, it is expedient to make the central cross bar for bearing up theends double, and to leave a space between the ends of the bars so that theashes may fall through between them. The space thus left enables the barsto expand without injury on the application of heat, whereas without somesuch provision the bars are very liable to get burned out by bending up inthe centre, or at the ends, as they must do if the elongation of the barson the application of heat be prevented; and this must be the effect ofpermitting the spaces at the ends of the bars to be filled up with ashes. At each end of each bed of bars it is expedient to leave a space which theashes cannot fill up so as to cause the bars to jam; and care must be takenthat the heels of the bars do not come against any of the furnace bearers, whereby the room left at the end of the bars to permit the expansion wouldbe rendered of no avail. 

379. _Q. _--Have you any remarks to offer respecting the construction andarrangement of the furnace bridges and dampers of marine boilers?

_A. _--The furnace bridges of marine boilers are walls or partitions builtup at the ends of the furnaces to narrow the opening for the escape of heatinto the flues. They are either made of fire brick or of plate ironcontaining water: in the case of water bridges, the top part of the bridgeshould be made with a large amount of slant so as to enable the steam toescape freely, but notwithstanding this precaution the plates of waterbridges are apt to crack at the bend, so that fire brick bridges appear onthe whole to be preferable. In shallow furnaces the bridges often come toonear the furnace top to enable a man to pass over them; and it will saveexpense if in such bridges the upper portion is constructed of two or threefire blocks, which may be lifted off where a person requires to enter theflues to sweep or repair them, whereby the perpetual demolition andreconstruction of the upper part of the bridge will be prevented. 

380. _Q. _--What is the benefit of bridges?

_A. _--Bridges are found in practice to have a very sensible operation inincreasing the production of steam, and in some boilers in which the brickbridges have been accidentally knocked down by the firemen, a veryconsiderable diminution in the supply of steam has been experienced. Theirchief operation seems to lie in concentrating the heat within the furnaceto a higher temperature, whereby the heat is more rapidly transmitted fromthe furnace to the water, and less heat has consequently to be absorbed bythe flues. In this way the bridges render the heating surface of a boilermore effective, or enable a smaller amount of heating surface to suffice. 

381. _Q. _--Are the bridges behind the furnaces the only bridges used insteam boilers?

_A. _--It is not an uncommon practice to place a hanging bridge, consistingof a plate of iron descending a certain distance into the flue, at thatpart of the flue where it enters the chimney, whereby the stratum of hotair which occupies the highest part of the flue is kept in protractedcontact with the boiler, and the cooler air occupying the lower part of theflue is that which alone escapes. The practice of introducing a hangingbridge is a beneficial one in the case of some boilers, but is notapplicable universally, as boilers with a small calorimeter cannot befurther contracted in the flue without a diminution in their evaporatingpower. In tubular boilers a hanging bridge is not applicable, but in somecases a perforated plate is placed against the ends of the tubes, which bysuitable connections is made to operate as a sliding damper which partiallyor totally closes up the end of every tube, and at other times a damperconstructed in the manner of a venetian blind is employed in the samesituation. These varieties of damper, however, have only yet been used inlocomotive boilers, though applicable to tubular boilers of everydescription. 

382. _Q. _--Is it a benefit to keep the flues or tubes appertaining to eachfurnace distinct?

_A. _--In a flue boiler this cannot be done, but in a tubular boiler it isan advantage that there should be a division between the tubes pertainingto each furnace, so that the smoke of each furnace may be kept apart fromthe smoke of the furnace adjoining it until the smoke of both enters thechimney, as by this arrangement a furnace only will be rendered inoperativein cleaning the fires instead of a boiler, and the tubes belonging to onefurnace may be swept if necessary at sea without interfering injuriouslywith the action of the rest. In a steam vessel it is necessary at intervalsto empty out one or more furnaces every watch to get rid of the clinkerswhich would otherwise accumulate in them; and it is advisable that theconnection between the furnaces should be such that this operation, whenbeing performed on one furnace, shall injure the action of the rest aslittle as possible. 

383. _Q. _--Can any constructive precautions be taken to prevent thefurnaces and tube plates of the boiler from being burned by the intensityof the heat?

_A. _--The sides of the internal furnaces or flues in all boilers should beso constructed that the steam may readily escape from their surfaces, withwhich view it is expedient to make the bottom of the flue somewhat widerthan the top, or slightly conical in the cross section; and the upperplates should always be overlapped by the plates beneath, so that the steamcannot be retained in the overlap, but will escape as soon as it isgenerated. If the sides of the furnace be made high and perfectly vertical, they will speedily be buckled and cracked by the heat, as a film of steamin such a case will remain in contact with the iron which will prevent theaccess of the water, and the iron of the boiler will be injured by the hightemperature it must in that case acquire. To moderate the intensity of theheat acting upon the furnace sides, it is expedient to bring the outsidefire bars into close contact with the sides of the furnace, so as toprevent the entrance of air through the fire in that situation, by whichthe intensity of the heat would be increased. The tube plate nearest thefurnace in tubular boilers should also be so inclined as to facilitate theescape of the steam; and the short bent plate or flange of the tube plate, connecting the tube plate with the top of the furnace, should be made witha gradual bend, as, if the bend be sudden, the iron will be apt to crack orburn away from the concretion of salt. Where the furnace mouths arecontracted by bending in the sides and top of the furnace, as is thegeneral practice, the bends should be gradual, as salt is apt to accumulatein the pockets made by a sudden bend, and the plates will then burn intoholes. 

384. _Q. _--In what manner is the tubing of boilers performed?

_A. _--The tubes of marine boilers are generally iron tubes, three inches indiameter, and between six and seven feet long; but sometimes brass tubes ofsimilar dimensions are employed. When brass tubes are employed, the use offerules driven into the ends of the tubes is sometimes employed to keepthem tight; but when the tubes are of malleable iron, of the thickness ofRussell's boiler tubes, they may be made tight merely by firmly drivingthem into the tube plates, and the same may be done with thick brass tubes. The holes in the tube plate next the front of the boiler are just sensiblylarger in diameter than the holes in the other tube plate, and the holesupon the outer surfaces of both tube plates are very slightly countersunk. The whole of the tubes are driven through both tube plates from the frontof the boiler, --the precaution, however, being taken to drive them ingently at first with a light hand hammer, until the whole of the tubes havebeen inserted to an equal depth, and then they may be driven up by degreeswith a heavy hammer, whereby any distortion of the holes from unequaldriving will be prevented. Finally, the ends of the tubes should be rivetedup so as to fill the countersink; the tubes should be left a little longerthan the distance between the outer surfaces of the tube plates, so thatthe countersink at the ends may be filled by staving up the end of the tuberather than by riveting it over; and the staving will be best accomplishedby means of a mandril with a collar upon it, which is driven into the tubeso that the collar rests upon the end of the tube to be riveted; or a toollike a blunt chisel with a recess in its point may be used, as is the moreusual practice. 

385. _Q. _--Should not stays be introduced in substitution of some of thetubes?

_A. _--It appears expedient in all cases that some of the tubes should bescrewed at the ends, so as to serve as stays if the riveting at the tubeends happens to be burned away, and also to act as abutments to the rivetedtube--or else to introduce very strong rods of about the same diameter as atube, in substitution of some of the tubes; and these stays should havenuts at each end both within and without the tube plates, which nuts shouldbe screwed up, with white lead interposed, before the tubes are inserted. If the tubes are long, their expansion when the boiler is being blown offwill be apt to start them at the ends, unless very securely fixed; and itis difficult to prevent brass tubes of large diameter and proportionatelength from being started at the ends, even when secured by ferules; butthe brass tubes commonly employed are so small as to be susceptible ofsufficient compression endways by the adhesion due to the ferules tocompensate for the expansion, whereby they are prevented from starting atthe ends. In some, of the early marine boilers fitted with brass tubes, agalvanic action at the ends of the tubes was found to take place, and theiron of the tube plates was wasted away in consequence, with rapidity; butfurther experience proved the injury to be attributable chiefly toimperfect fitting, whereby a leakage was caused that induced oxidation, andwhen, the tubes were well fitted any injurious action at the ends of thetubes was found to cease. 

386. _Q. _--What is the best mode of constructing the chimney and the partsin connection therewith?

_A. _--In sea-going steamers the funnel plates are usually about nine feetlong and 3/16ths thick; and where different flues or boilers have theirdebouch in the same chimney, it is expedient to run division plates up thechimney for a considerable distance, to keep the draughts distinct. Thedampers should not be in the chimney but at the end of the boiler flue, sothat they may be available for use if the funnel by accident be carriedaway. The waste steam pipe should be of the same height as the funnel, soas to carry the waste steam clear of it, for if the waste steam strikes thefunnel it will wear the iron into holes; and the waste steam pipes shouldbe made at the bottom with a faucet joint, to prevent the working of thefunnel, when the vessel rolls, from breaking the pipe at the neck. Thereshould be two hoops round the funnel, for the attachment of the funnelshrouds, instead of one, so that the funnel may not be carried overboard ifone hoop breaks, or if the funnel breaks at the upper hoop from thecorrosive action of the waste steam, as sometimes happens. The deck overthe steam chest should be formed of an iron plate supported by angle ironbeams, and there should be a high angle iron cooming round the hole in thedeck through which the chimney ascends, to prevent any water upon the deckfrom leaking down upon the boiler. Around the lower part of the funnelthere should be a sheet iron casing to prevent any inconvenient dispersionof heat in that situation, and another short piece of casing, of a somewhatlarger diameter, and riveted to the chimney, should descend over the firstcasing, so as to prevent the rain or spray which may beat against thechimney from being poured down within the casing upon the top of theboiler. The pipe for conducting away the waste water from the top of thesafety valve should lead overboard, and not into the bilge of the ship, asinconvenience arises from the steam occasionally passing through it, if ithas its termination in the engine room. 

387. _Q. _--Are not the chimneys of some vessels made so that they may belowered when required?

_A. _--The chimneys of small river vessels which have to pass under bridgesare generally formed with a hinge, so that they may be lowered backwardwhen passing under a bridge; and the chimneys of some screw vessels aremade so as to shut up like a spyglass when the fires are put out and thevessel is navigated under sails. In smaller vessels, however, two lengthsof chimney suffice; and in that case there is a standing piece on deck, which, however, does not project above the bulwarks. 

388. _Q. _--Will you explain any further details in the construction ofmarine boilers which occur to you as important?

_A. _--The man-hole and mud-hole doors, unless put on from the outside, likea cylinder cover, with a great number of bolts, should be put on from theinside with cross bars on the outside, and the bolts should be strong, andhave coarse threads and square nuts, so that the threads may not beoverrun, nor the nuts become round, by the unskilful manipulations of thefiremen, by whom these doors are removed or replaced. It is very expedientthat sufficient space should be left between the furnace and the tubes inall tubular boilers to permit a boy to go in to clear away any scale thatmay have formed, and to hold on the rivets in the event of repair beingwanted; and it is also expedient that a vertical row of tubes should beleft out opposite to each water space to allow the ascent of the steam anddescent of the water, as it has been found that the removal of the tubes inthat position, even in a boiler with deficient heating surface, hasincreased the production of steam, and diminished the consumption of fuel. The tubes should all be kept in the same vertical line, so as to permit theintroduction of an instrument to scrape them; but they may be zig-zagged inthe horizontal line, whereby a greater strength of metal will be obtainedaround the holes in the tube plates, and the tubes should not be placed tooclose together, else their heating efficacy will be impaired. 

INCRUSTATION AND CORROSION OF BOILERS. 

389. _Q. _--What is the cause of the formation of scale in marine boilers?

_A. _--Scale is formed in all boilers which contain earthy or salinematters, just in the way in which a scaly deposit, or rock, as it issometimes termed, is formed in a tea kettle. In sea water the chiefingredient is common salt, which exists in solution: the water admitted tothe boiler is taken away in the shape of steam, and the saline matter whichis not vaporizable accumulates in process of time in the boiler, until itsamount is so great that the water is saturated, or unable to hold any morein solution; the salt is then precipitated and forms a deposit whichhardens by heat. The formation of scale, therefore, is similar to theprocess of making salt from sea water by evaporation, the boiler being, infact, a large salt pan. 

390. _Q. _--But is the scale soluble in fresh water like the salt in a saltpan?

_A. _--No, it is not; or if soluble at all, is only so to a very limitedextent. The several ingredients in sea water begin to be precipitated fromsolution at different degrees of concentration; and the sulphate andcarbonate of lime, which begin to be precipitated when a certain state ofconcentration is reached, enter largely into the composition of scale, andgive it its insoluble character. Pieces of waste or other similar objectsleft within a marine boiler appear, when taken out, as if they had beenpetrified; and the scale deposited upon the flues of a marine boilerresembles layers of stone. 

391. _Q/_--Is much inconvenience experienced in marine boilers from theseincrustations upon the flues?

_A. _--Incrustation in boilers at one time caused much more perplexity thanit does at present, as it was supposed that in some seas it was impossibleto prevent the boilers of a steamer from becoming salted up; but it has nowbeen satisfactorily ascertained that there is very little difference in thesaltness of different seas, and that however salt the water may be, theboiler will be preserved from any injurious amount of incrustation byblowing off, as it is called, very frequently, or by permitting aconsiderable portion of the supersalted water to escape at short intervalsinto the sea. If blowing off be sufficiently practised, the scale upon theflues will never be much thicker than a sheet of writing paper, and _noexcuse_ should be accepted from engineers for permitting a boiler to bedamaged by the accumulation of calcareous deposit. 

392. _Q. _--What is the temperature at which sea water boils in a steamboiler?

_A. _--Sea water contains about 1/33rd its weight of salt, and in the openair it boils at the temperature of 213. 2°; if the proportion of salt beincreased to 2/33rds of the weight of the water, the boiling point willrise to 214. 4°; with 3/33rds of salt the boiling point will be 215. 5°;4/33rds, 216. 7°; 5/33rds, 217. 9°; 6/33rds, 219°; 7/33rds, 220. 2°; 8/33rds, 221. 4°; 9/33rds, 222. 5°; 10/33rds, 223. 7°; 11/33rds, 224. 9°; and 12/33rds, which is the point of saturation, 226°. In a steam boiler the boilingpoints of water containing these proportions of salt must be higher, as theelevation of temperature due to the pressure of the steam has to be addedto that due to the saltness of the water; the temperature of steam at theatmospheric pressure being 212°, its temperature, at a pressure of 15 lbs. Per square inch above the atmosphere, will be 250°, and adding to this 4. 7°as the increased temperature due to the saltness of the water when itcontains 4/33rds of salt, we have 254. 7° as the temperature of the water inthe boiler, when it contains 4/33rds of salt and the pressure of the steamis 15 lbs. On the square inch. 

393. _Q. _--What degree of concentration of the salt water may be safelypermitted in a boiler?

_A. _--It is found by experience that when the concentration of the saltwater in a boiler is prevented from exceeding that point at which itcontains 2/33rds its weight of salt, no injurious incrustation will takeplace, and as sea water contains only 1/33rd of its weight of salt, it isclear that it must be reduced by evaporation to one half of its bulk beforeit can contain 2/33rds of salt; or, in other words, a boiler must blow outinto the sea one half of the water it receives as feed, in order to preventthe water from rising above 2/33rds of concentration, or 8 ounces of saltto the gallon. 

394. _Q. _--How do you determine 8 ounces to the gallon to be equivalent totwice the density of salt water, or "two salt waters" as it is sometimescalled?

_A. _--The density of the water of different seas varies somewhat. A gallonof fresh water weighs 10 lbs. ; a gallon of salt water from the Balticweighs 10. 15 lbs. ; a gallon of salt water from the Irish Channel weighs10. 28 lbs. ; and a gallon of salt water from the Mediterranean 10. 29 lbs. Ifwe take an average saltness represented by a weight of 10. 25 lbs. , then agallon of water concentrated to twice this saltness will weigh 10. 5 lbs. , or the salt in it will weigh . 5 lbs or 8 oz. , which is the proportion of 8oz. To the gallon. However, the proportion of 2/33rds gives a greaterproportion than 8 oz. To the gallon, for 2/33 = 1/16 nearly, and 1/16 of 10lbs. = 10 oz. By keeping the density of the water in a marine boiler at theproportion of 8 or 10 oz. To the gallon, no inconvenient amount of scalewill be deposited on the flues or tubes. The bulk of water, it may beremarked, is not increased by putting salt in it up to the point ofsaturation, but only its density is increased. 

395. _Q. _--Is there not a great loss of heat by blowing off so large aproportion of the heated water from the boiler?

_A. _--The loss is not very great. Boilers are sometimes worked at asaltness of 4/33rds, and taking this saltness and supposing the latent heatof steam to be at 1000° at the temperature of 212°, and reckoning the sumof the latent and sensible heats as forming a constant quantity, the latentheat of steam at the temperature of 250° will be 962°, and the total heatof the steam will be 1212° in the case of fresh water; but as the feedwater is sent into the boiler at the temperature of 100°, the accession ofheat it receives from the fuel will be 1112° in the case of fresh water, or1112° increased by 3. 98° in the case of water containing 4/33ds of salt--the 3. 98° being the 4. 7° increase of temperature due to the presence of4/33rds of salt, multiplied by 0. 847 the specific heat of steam. This makesthe total accession of heat received by the steam in the boiler equal to1115. 98°, or say 1116°, which multiplied by 3, as 3 parts of the water areraised into steam, gives us 3348° for the heat in the steam, while theaccession of heat received in the boiler by the 1 part of residual brinewill be 154. 7°, multiplied by 0. 85, the specific heat of the brine, or130. 495°; and 3348° divided by 130. 495° is about 1/26th. It appears, therefore, that by blowing off the boiler to such an extent that thesaltness shall not rise above what answers to 4/33rds of salt, about 1/25thof the heat is blown into the sea; this is but a small proportion, and asthere will be a greater waste of heat, if from the existence of scale uponthe flues the heat can be only imperfectly transmitted to the water, therecannot be even an economy of fuel in niggard blowing off, while it involvesthe introduction of other evils. The proportion of 4/33rds of saltness, however, or 16 oz. To the gallon, is larger than is advisable, especiallyas it is difficult to keep the saltness at a perfectly uniform point, andthe working point should, therefore, be 2/33rds as before prescribed. 

396. _Q. _--Have no means been devised for turning to account the heatcontained in the brine which is expelled from the boiler?

_A. _--To save a part of the heat lost by the operation of blowing off, thehot brine is sometimes passed through a number of small tubes surrounded bythe feed water; but there is no very great gain from the use of suchapparatus, and the tubes are apt to become choked up, whereby the safety ofthe boiler may be endangered by the injurious concentration of itscontents. Pumps, worked by the engine for the extraction of the brine, aregenerally used in connection with the small tubes for the extraction of theheat from the supersalted water; and if the tubes become choked the pumpswill cease to eject the water, while the engineer may consider them to beall the while in operation. 

397. _Q. _--What is the usual mode of blowing off the supersalted water fromthe boiler?

_A. _--The general mode of blowing off the boiler is to allow the water torise gradually for an hour or two above the lowest Working level, and thento open the cock communicating with the sea, and keep it open until thesurface of the water within the boiler has fallen several inches; but insome cases a cock of smaller size is allowed to run water continuously, andin other cases brine pumps are used as already mentioned. In every case inwhich the supersalted water is discharged from the boiler in a continuousstream, a hydrometer or salt gauge of some convenient construction shouldbe applied to the boiler, so that the density of the water may at all timesbe visible, and immediate notice be given of any interruption of theoperation. Various contrivances have been devised for this purpose, themost of which operate on the principle of a hydrometer; but perhaps a moresatisfactory principle would be that of a differential steam gauge, whichwould indicate the difference of pressure between the steam in the boilerand the steam of a small quantity of fresh water enclosed in a suitablevessel, and immerged in the water of the boiler. 

398. _Q. _--What is the advantage of blowing off from the surface of thewater in the boiler?

_A. _--Blowing off from a point near the surface of the water is morebeneficial than blowing off from the bottom of the boiler. Solid particlesof any kind, it is well known, if introduced into boiling water, will lowerthe boiling point in a slight degree, and the steam will chiefly begenerated on the surface of the particles, and indeed will have theappearance of coming out of them; if the particles be small the steamgenerated beneath and around them will balloon them to the surface of thewater, where the steam will be liberated and the particles will descend;and the impalpable particles in a marine boiler, which by their subsidenceupon the flues concrete into scale, are carried in the first instance tothe surface of the water, so that if they be caught there and ejected fromthe boiler, the formation of scale will be prevented. 

399. _Q. _--Are there any plans in operation for taking advantage of thisproperty of particles rising to the surface?

_A. _--Advantage is taken of this property in Lamb's Scale Preventer, whichis substantially a contrivance for blowing off from the surface of thewater that in practice is found to be very effectual; but a float inconnection with a valve at the mouth of the discharging pipe is thereintroduced, so as to regulate the quantity of water blown out by the heightof the water level, or by the extent of opening given to the feed cock. Theoperation, however, of the contrivance would be much the same if the floatwere dispensed with; but the float acts advantageously in hindering thewater from rising too high in the boiler, should too much feed be admitted, and thereby obviates the risk of the water running over into the cylinder. In some boilers sheet iron vessels, called sediment collectors, areemployed, which collect into them the impalpable matter, which in Lamb'sapparatus is ejected from the boiler at once. One of these vessels, ofabout the size and shape of a loaf of sugar, is put into each boiler withthe apex of the cone turned downwards into a pipe leading overboard, forconducting the sediment away from the boiler. The base of the cone standssome distance above the water line, and in its sides conical slits are cut, so as to establish a free communication between the water within theconical vessel and the water outside it. The particles of stony matterwhich are ballooned to the surface by the steam in every other part of theboiler, subside within the cone, where, no steam being generated, the wateris consequently tranquil; and the deposit is discharged overboard by meansof a pipe communicating with the sea. By blowing off from the surface ofthe water, the requisite cleansing action is obtained with less waste ofheat; and where the water is muddy, the foam upon the surface of the wateris ejected from the boiler--thereby removing one of the chief causes ofpriming. 

400. _Q. _--What is the cause of the rapid corrosion of marine boilers?

_A. _--Marine boilers are corroded externally in the region of the steamchest by the dripping of water from the deck; the bottom of the boiler iscorroded by the action of the bilge water, and the ash pits by the practiceof quenching the ashes with, salt water. These sources of injury, however, admit of easy remedy; the top of the boiler may be preserved from externalcorrosion by covering it with felt upon which is laid sheet lead solderedat every joint so as to be impenetrable to water; the ash pits may beshielded by guard plates which are plates fitting into the ash pits andattached to the boiler by a few bolts, so that when worn they may beremoved and new ones substituted, whereby any wear upon the boiler in thatpart will be prevented; and there will be very little wear upon the bottomof a boiler if it be imbedded in mastic cement laid upon a suitableplatform. 

401. _Q. _--Are not marine boilers subject to internal corrosion?

_A. _--Yes; the greatest part of the corrosion of a boiler takes place inthe inside of the steam chest, and the origin of this corrosion is one ofthe obscurest subjects in the whole range of engineering. It cannot be fromthe chemical action of the salt water upon the iron, for the flues andother parts of the boiler beneath the water suffer very little fromcorrosion, and in steam vessels provided with Hall's condensers, whichsupply the boiler with fresh water, not much increased durability of theboiler has been experienced. Nevertheless, marine boilers seldom last morethan for 5 or 6 years, whereas land boilers made of the same quality ofiron often last 18 or 20 years, and it does not appear probable that landboilers would last a very much shorter time if salt water were used inthem. The thin film of scale spread over the parts of a marine boilersituated beneath the water, effectually protect them from corrosion; andwhen the other parts are completely worn out the flues generally remain soperfect, that the hammer marks upon them are as conspicuous as at theirfirst formation. The operation of the steam in corroding the interior ofthe boiler is most capricious--the parts which are most rapidly worn awayin one boiler being untouched in another; and in some cases one side of asteam chest will be very much wasted away while the opposite side remainsuninjured. Sometimes the iron exfoliates in the shape of a black oxidewhich comes away in flakes like the leaves of a book, while in other casesthe iron appears as if eaten away by a strong acid which had a solventaction upon it. The application of felt to the outside of a boiler, has inseveral cases been found to accelerate sensibly its internal corrosion;boilers in which there is a large accumulation of scale appear to be morecorroded than where there is no such deposit; and where the funnel passesthrough the steam chest the iron of the steam chest is invariably much morecorroded than where the funnel does not pass through it. 

402. _Q. _--Can you suggest no reason for the rapid internal corrosion ofmarine boilers?

_A. _--The facts which I have enumerated appear to indicate that theinternal corrosion of marine boilers is attributable chiefly to theexistence of surcharged steam within them, which is steam to which anadditional quantity of heat has been communicated subsequently to itsgeneration, so that its temperature is greater than is due to its elasticforce; and on this hypothesis the observed facts relative to corrosionbecome to some extent explicable. Felt, applied to the outside of a boiler, may accelerate its internal corrosion by keeping the steam in a surchargedstate, when by the dispersion of a part of the heat it would cease to be inthat state; boilers in which there is a large accumulation of scale musthave worked with the water very salt, which necessarily produces surchargedsteam; for the temperature of steam cannot be less than that of the waterfrom which it is generated, and inasmuch as the boiling point of water, under any given pressure, rises with the saltness of the water, thetemperature of the steam must rise with the saltness of the water, thepressure remaining the same; or, in other words, the steam must have ahigher temperature than is due to its elastic force, or be in the state ofsurcharged steam. The circumstance of the chimney flue passing through thesteam will manifestly surcharge the steam with heat, so that all thecircumstances which are found to accelerate corrosion, are it appears suchas would also induce the formation of surcharged steam. 

403. _Q. _--Is it the natural effect of surcharged steam to waste away iron?

_A. _--It is the natural effect of surcharged steam to oxidate the iron withwhich it is in contact, as is illustrated by the familiar process formaking hydrogen gas by sending steam through a red hot tube filled withpieces of iron; and although the action of the surcharged steam in a boileris necessarily very much weaker than where the iron is red hot, itmanifestly must have _some_ oxidizing effect, and the amount of corrosionproduced may be very material where the action is perpetual. Boilers with alarge extent of heating surface, or with descending flues circulatingthrough the cooler water in the bottom of the boiler before ascending thechimney, will be less corroded internally than boilers in which a largequantity of the heat passes away in the smoke; and the corrosion of theboiler will be diminished if the interior of any flue passing through thesteam be coated with fire brick, so as to present the transmission of theheat in that situation. The best practice, however, appears to consist inthe transmission of the smoke through a suitable passage on the outside ofthe boiler, so as to supersede the necessity of carrying any flue throughthe steam at all; or a column of water may be carried round the chimney, into which as much of the feed water may be introduced as the heat of thechimney is capable of raising to the boiling point, as under thislimitation the presence of feed water around the chimney in the steam chestwill fail to condense the steam. 

404. _Q. _--In steam vessels there are usually several boilers?

_A. _--Yes, in steam vessels of considerable power and size. 

405. _Q. _--Are these boilers generally so constructed, that any one of themmay be thrown out of use?

_A. _--Marine boilers are now generally supplied with stop valves, wherebyone boiler may be thrown out of use without impairing the efficacy of theremainder. These stop valves are usually spindle valves of large size, andthey are for the most part set in a pipe which runs across the steamchests, connecting the several boilers together. The spindles of thesevalves should project through stuffing boxes in the covers of the valvechests, and they should be balanced by a weighted lever, and kept incontinual action by the steam. If the valves be lifted up, and be sufferedto remain up, as is the usual practice, they will become fixed by corrosionin that position, and it will be impossible after some time to shut them onan emergency. These valves should always be easily accessible from theengine room; and it ought not to be necessary for the coal boxes to beempty to gain access to them. 

406. _Q. _--Should each boiler have at least one safety valve for itself?

_A. _--Yes; it would be quite unsafe without this provision, as the stopvalve might possibly jam. Sometimes valves jam from a distortion in theshape of the boiler when a considerable pressure is put upon it. 

407. _Q. _--How is the admission of the water into the boiler regulated?

_A. _--The admission of feed water into the boiler is regulated by hand bythe engineer by means of cocks, and sometimes by spindle valves raised andlowered by a screw. Cocks appear to be the preferable expedient, as theyare less liable to accident or derangement than screw valves, and in modernsteam vessels they are generally employed. 

408. _Q. _--At what point of the boiler is the feed introduced?

_A. _--The feed water is usually conducted from the feed cock to a pointnear the bottom of the boiler by means of an internal pipe, the object ofthis arrangement being to prevent the rising steam from being condensed bythe entering water. By being introduced near the bottom of the boiler, thewater comes into contact in the first place with the bottoms of thefurnaces and flues, and extracts heat from them which could not beextracted by water of a higher temperature, whereby a saving of fuel isaccomplished. In some cases the feed water is introduced into a casingaround the chimney, from whence it descends into the boiler. This planappears to be an expedient one when the boiler is short of heating surface, and more than a usual quantity of heat ascends the chimney; but in wellproportioned boilers a water casing round the chimney is superfluous. Whena water casing is used the boiler is generally fed by a head of water, thefeed water being forced up into a small tank, from whence it descends intothe boiler by the force of gravity, while the surplus runs to waste, as inthe feeding apparatus of land engines. 

409. _Q. _--Suppose that the engineer should shut off the feed water fromthe boilers while the engine was working, what would be the result?

_A. _--The result would be to burst the feed pipes, except for a safetyvalve placed on the feed pipe between the engine and the boilers, whichsafety valve opens when any undue pressure comes upon the pipes, and allowsthe water to escape. There is, however, generally a cock on the suctionside of the feed pump, which regulates the quantity of water drawn into thepump. But there must be cocks on the boilers also to determine into whichboiler the water shall be chiefly discharged, and these cocks are sometimesall shut accidentally at the same time. 

410. _Q. _--Is there no expedient in use in steam vessels for enabling theposition of the water level in the boiler to determine the quantity of feedwater admitted?

_A. _--In some steam vessels floats have been introduced to regulate thefeed, but their action cannot be depended on in agitated water, if appliedafter the common fashion. Floats would probably answer if placed in acylinder which communicates with the water in the boiler by means of smallholes; and a disc of metal might be attached to the end of a rod extendingbeneath the water level, so as to resist irregular movements from themotion of the ship at sea, which would otherwise impair the action of theapparatus. 

411. _Q. _--How is the proper level of the water in the boiler of a steamvessel maintained when, the engine is stopped for some time, and the boileris blowing off steam?

_A. _--By means of a separate pump worked sometimes by hand, but usually bya small separate engine called the Donkey engine. This pump, by the aid ofsuitable cocks, will pump from the sea into the boiler; from the sea upondeck either to wash decks or to extinguish fire; and from the bilgeoverboard, through a suitable orifice in the side of the ship. 

LOCOMOTIVE BOILERS. 

412. _Q. _--Will you recapitulate the general features of locomotiveboilers?

_A. _--Locomotive boilers consist of three portions (see fig. 29): thebarrel E, E, containing the tubes, the fire box B, and the smoke box F; ofwhich the barrel smoke box, and external fire box are always of iron, butthe internal fire box is generally made of copper, though sometimes also itis made of iron. The tubes are sometimes of iron, but generally of brassfixed in by ferules. The whole of the iron plates of a locomotive boilerWhich are subjected to the pressure of steam, should be Lowmoor or Bowlingplates of the best quality; and the copper should be coarse grained, ratherthan rich or soft, and be perfectly free from irregularities of structureand lamination. 

413. _Q. _--What are the usual dimensions of the barrel?

_A. _--The thickness of the plates composing the barrel of the boiler variesgenerally from 5/16ths to 3/8ths of an inch, and the plates should run inthe direction of the circumference, so that the fibres of the iron may bein the direction of the strain. The diameter of the barrel commonly variesfrom 3 ft. To 3 ft. 6 inches; the diameter of the rivets should be from11/16ths to 3/4ths of an inch, and the pitch of the rivets or distancebetween their centres should be from 17/8th to 2 inches. 

414. _Q. _--How are the fire boxes of a locomotive constructed?

_A. _--The space between the external and internal fire boxes forms a waterspace, which must be stayed every 4-1/2 or 5 inches by means of copper oriron stay bolts, screwed through the outer fire box into the metal of theinner fire box, and securely riveted within it: iron stay bolts are asdurable as copper, and their superior tenacity gives them an advantage. Sometimes tubes are employed as stays. The internal and external fire boxesare joined together at the bottom by a N-shaped iron, and round the firedoor they are connected by means of a copper ring 1-1/4 in. Thick, and 2in. Broad, --the inner fire box being dished sufficiently outward at thatpoint, and the outer fire box sufficiently inward, to enable a circle ofrivets 3/4 of an inch in diameter passing through the copper ring and thetwo thicknesses of iron, to make a water-tight joint. The thickness of theplates composing the external fire box is in general 3/8ths of an inch ifthe fire box is circular, and from 3/8ths to 1/2 inch if the fire box issquare; and the thickness of the internal fire box is in most cases 7/16thsif copper, and from 3/8ths to 7/16ths of an inch if of iron. Circularinternal fire boxes, if made of iron, should be welded rather than riveted, as the rivet heads are liable to be burnt away by the action of the fire;and when the fire boxes are square each side should consist of a singleplate, turned over at the edges with a radius of 3 inches, for theintroduction of the rivets. 

415. _Q. _--Is there any provision for stiffening the crown of the furnacein a locomotive?

_A. _--The roof of the internal fire box, whether flat as in Stephenson'sengines, or dome shaped as in Bury's, requires to be stiffened with crossstay bars, but the bars require to be stronger and more numerous whenapplied to a flat surface. The ends of these stay bars rest above thevertical sides of the fire box; and to the stay bars thus extending acrossthe crown, the crown is attached at intervals by means of stay bolts. Thereare projecting bosses upon the stay bars encircling the bolts at everypoint where a bolt goes through, but in the other parts they are kept clearof the fire box crown so as to permit the access of water to the metal;and, with the view of facilitating the ascent of the steam, the bottom ofeach stay bar should be sharpened away in those parts where it does nottouch the boiler. 

416. _Q. _--Is any inconvenience experienced from the intense heat in alocomotive furnace?

_A. _--The fire bars in locomotives have always been a source of trouble, asfrom the intensity of the heat in the furnace they become so hot as tothrow off a scale, and to bend under the weight of the fuel. The bestalleviation of these evils lies in making the bars deep and thin: 4 or 5inches deep by five eighths of an inch thick on the upper side, and threeeighths of an inch on the under side, are found in practice to be gooddimensions. In some locomotives a frame carrying a number of fire bars ismade so that it may be dropped suddenly by loosening a catch; but it isfound that any such mechanism can rarely be long kept in working order, asthe molten clinker by running down between the frame and the boiler willgenerally glue the frame into its place. It is therefore found preferableto fix the frame, and to lift up the bars by the dart used by the stoker, when any cause requires the fire to be withdrawn. The furnace bars oflocomotives are always made of malleable iron, and indeed for every speciesof boiler malleable iron bars are to be preferred to bars of cast iron, asthey are more durable, and may if thin be set closer together, whereby thesmall coal or coke is saved that would otherwise fall into the ash pit. Theash box of locomotives is made of plate iron, a quarter thick: it shouldnot be less than 10 in. Deep, and its bottom should be about 9 in. Abovethe level of the rails. The chimney of a locomotive is made of plate ironone eighth of an inch thick: it is usually of the same diameter as thecylinder, but is better smaller, and must not stand more than 14 ft. Highabove the level of the rails. 

417. _Q. _--Are locomotive boilers provided with a steam chest?

_A. _--The upper portion of the external fire box is usually formed into asteam chest, which is sometimes dome shaped, sometimes semicircular, andsometimes of a pyramidical form, and from this steam chest the steam isconducted away by an internal pipe to the cylinders; but in other cases anindependent steam chest is set upon the barrel of the boiler, consisting ofa plate iron cylinder, 20 inches in diameter, 2 feet high, and threeeighths of an inch thick, with a dome shaped top, and with the seam weldedand the edge turned over to form a flange of attachment to the boiler. Thepyramidical dome, of the form employed in Stephenson's locomotives, presents a considerable extent of flat surface to the pressure of thesteam, and this flat surface requires to be very strongly stayed with angleirons and tension rods; whereas the semiglobular dome of the kind employedin Bury's engines requires no staying whatever. Latterly, however, thesedomes over the fire box have been either much reduced in size or abandonedaltogether. 

418. _Q. _--Is any beneficial use made of the surplus steam of a locomotive?

_A. _--To save the steam which is formed when the engine is stationary, apipe is usually fitted to the boiler, which on a cock being turned conductsthe steam into the water in the tender, whereby the feed water is heated, and less fuel is subsequently required. This method of disposing of thesurplus steam may be adopted when the locomotive is descending inclines, oron any occasion where more steam is produced than the engine can consume. 

419. _Q. _--What means are provided to facilitate the inspection and cleaningof locomotive boilers?

_A. _--The man hole, or entrance into the boiler, consists of a circular oroval aperture of about 15 in. Diameter, placed in Bury's locomotive at theapex of the dome, and in Stephenson's upon the front of the boiler, a fewinches below the level of the rounded part; and the cover of the man holein Bury's engine contains the safety valve seats. In whatever situationthis man hole is placed, the surfaces of the ring encircling the hole, andof the internal part of the door or cover, should be accurately fittedtogether by scraping or grinding, so that they need only the interpositionof a little red lead to make them quite tight when screwed together. Leador canvas joints, if of any considerable thickness, will not long withstandthe action of high pressure steam; and the whole of the joints about alocomotive should be such that they require nothing more than a littlepaint or putty, or a ring of wire gauze smeared with white or red lead tomake them perfectly tight. There must be a mud hole opposite the edge ofeach water space, if the fire box be square, to enable the boiler to beeasily cleaned out, and these holes are most conveniently closed by screwedplugs made slightly taper. A cock for emptying the boiler is usually fixedat the bottom of the fire box, and it should be so placed as to beaccessible when the engine is at work, in order that the engine driver mayblow off some water if necessary; but it must not be in such a position asto send the water blown off among the machinery, as it might carry sand orgrit into the bearings, to their manifest injury. 

420. _Q. _--Will you state the dimensions of the tube plate, and the meansof securing the tubes in it?

_A. _--The tube plates are generally made from five eighths to three fourthsof an inch thick, but seven eighths of an inch thick appears to bepreferable, as when the plate is thick the holes will not be so liable tochange their figure during the process of feruling the tubes: the distancebetween the tubes should never be made less than three fourths of an inch, and the holes should be slightly tapered so as to enable the tubes to holdthe tube plates together. The tubes are secured in the tube plates by meansof taper ferules driven into the ends of the tubes. The ferules are for themost part made of steel at the fire box end, and of wrought iron at thesmoke box end, though ferules of malleable cast iron have in some casesbeen used with advantage: malleable cast iron ferules are almost as easilyexpanded when hammered cold upon a mandrel, as the common wrought iron onesare at a working heat. Spring steel, rolled with a feather edge, tofacilitate its conversion into ferules, is supplied by some of thesteel-makers of Sheffield, and it appears expedient to make use of steelthus prepared when steel ferules are employed. In cases where ferules arenot employed, it may be advisable to set out the tube behind the tube plateby means of an expanding mandrel. There are various forms of thisinstrument. One form is that known as Prosser's expanding mandrel, in whichthere are six or eight segments, which are forced out by means of ahexagonal or octagonal wedge, which is forced forward by a screw. When thewedge is withdrawn, the segments collapse sufficiently to enable them toenter the tube, and there is an annular protuberance on the exterior circleof the segments, which protuberance, when the mandrel is put into the tube, just comes behind the inner edge of the tube plate. When the wedge istightened up by the screw, the protuberance on the exterior of the segmentscomposing the mandrel causes a corresponding bulge to take place in thetube, at the back of the tube plate, and the tube is thereby brought intomore intimate contact with the tube plate than would otherwise be the case. There is a steel ring indented into the segments of Prosser's mandrel, tocontract the segments when the central wedge is withdrawn. A moreconvenient form of the instrument, however, is obtained by placing thesegments in a circular box, with one end projecting; and supporting eachsegment in the box by a tenon, which fits into a mortise in the cylindricalbox. To expand the segments, a round tapered piece of steel, like a drift, is forced into a central hole, round which the segments are arranged. Apiece of steel tube, also slit up to enable a central drift to expand it, answers very well; but the thickness of that part of the tube in whichthere requires to be spring enough to let the mandrel expand, requires tobe sufficiently reduced to prevent the pieces from cracking when thecentral drift is driven in by a hammer. The drift is better when made witha globular head, so that it may be struck back by the hammer, as well as bedriven in. An expanding mandrel, with a central drift, is more rapid in itsoperation than when the expansion is produced by means of a screw. 

421. _Q. _--Will you explain the means that are adopted to regulate theadmission of steam to the cylinders?

_A. _--In locomotives, the admission of the steam from the boiler to thecylinders is regulated by a valve called the regulator, which is generallyplaced immediately above the internal fire box, and is connected with twocopper pipes;--one conducting steam from the highest point of the dome downto it, and the other conducting the steam that has passed through it alongthe boiler to the upper part of the smoke box. Regulators may be dividedinto two sorts, viz. , those with, sliding valves and steam ports, and thosewith conical valves and seats, of which the latter kind are the best. Theformer kind have for the most part consisted of a circular valve and face, with radial apertures, the valve resembling the outstretched wings of abutterfly, and being made to revolve on its central pivot by connectinglinks between its outer edges, or by its central spindle. In some ofStephenson's engines the regulator consists of a slide valve covering aport on the top of the valve chests. A rod passes from this valve throughthe smoke box below the boiler, and by means of a lever parallel to thestarting lever, is brought up to the engineer's reach. Cocks were at firstused as regulators, but were given up, as they were found liable to stickfast. A gridiron slide valve has been used by Stephenson, which consists ofa perforated square moving upon a face with an equal number of holes. Thisplan of a valve gives, with a small movement, a large area of opening. InBury's engines a sort of conical plug is used, which is withdrawn byturning the handle in front of the fire box: a spiral grove of a very largepitch is made in the valve spindle, in which fits a pin fixed to theboiler, and by turning the spindle an end motion is given to it, whicheither shuts or opens the steam passage according to the direction in whichit is turned. The best regulator would probably be a valve of theequilibrium description, such as is used in the Cornish engine: there wouldbe no friction in such a regulator, and it could be opened or shut with asmall amount of force. Such valves, indeed, are now sometimes employed forregulators in locomotives. 

CHAPTER VIII. 

CONSTRUCTIVE DETAILS OF ENGINES. 

PUMPING ENGINES. 

422. _Q. _--Will you explain the course of procedure in the erection of apumping engine, such as Boulton and Watt introduced into Cornwall?

_A. _--The best instructions on this subject are those of Mr. Watt himself, which are as follows:--Having fixed on the proper situation of the pump inthe pit, from its centre measure out the distance to the centre of thecylinder, from which set off all the other dimensions of the house, including the thickness of the walls, and dig out the whole of the includedground to the depth of the bottom of the cellar, so that the bottom of thecylinder may stand on a level with the natural ground of the place, orlower, if convenient, for the less the height of the house above theground, the firmer it will be. The foundations of the walls must be laid atleast two feet lower than the bottom of the cellar, unless the foundationbe firm rock; and care must be taken to leave a small drain into the pitquite through the lowest part of the foundation of the lever wall, to letoff any water that may be spilt in the engine house, or may naturally comeinto the cellar. If the foundation at that depth does not prove good, youmust either go down to a better if in your reach, or make it good by aplatform of wood or piles, or both. 

423. _Q. _--These directions refer to the foundations?

_A. _. --Yes; but I will now proceed to the other parts. Within the house, low walls must be built to carry the cylinder beams, so as to leavesufficient room to come at the holding down bolts, and the ends of thesebeams must also be lodged in the wall The lever wall must be built in thefirmest manner, and run solid, course by course, with thin lime mortar, care being taken that the lime has not been long slaked. If the house bebuilt of stone, let the stones be large and long, and let many headers belaid through the wall: it should also be a rule, that every stone be laidon the broadest bed it has, and never set on its edge. A course or twoabove the lintel of the door that leads to the condenser, build into thewall two parallel flat thin bars of iron equally distant from each other, and from the outside and inside of the wall, and reaching the whole breadthof the lever wall. About a foot higher in the wall, lay at every four feetof the breadth of the front, other bars of the same kind at right angles tothe former course, and reaching quite through the thickness of the wall;and at each front corner lay a long bar in the middle of the side walls, and reaching quite through the front wall; if these bars are 10 feet or 12feet long it will be sufficient. When the house is built up nearly to thebottom of the opening under the great beam another double course of bars isto be built in, as has been directed. At the level of the upper cylinderbeams, holes must be left in the walls for their ends, with room to movethem laterally, so that the cylinder may be got in; and smaller holes mustbe left quite through the walls for the introduction of iron bars, whichbeing firmly fastened to the cylinder beams at one end, and screwed at theother or outer end, will serve, by their going through both the front andback walls, to bind the house more firmly together. The spring beams oriron bars fastened to them must reach quite through the back wall, and bekeyed or screwed up tight; and they must be firmly fastened to the leverwall on each side, either by iron bars, firm pieces of wood, or long strongstones, reaching far back into the wall. They must also be bedded solidly, and the residue of the opening must be built up in the firmest manner. 

424. _Q. _--If there be a deficiency of water for the purpose ofcondensation, what course should be pursued?

_A. _--If there be no water in the neighborhood that can be employed for thepurpose of condensation, it will be necessary to make a pond, dug in theearth, for the reception of the water delivered by the air pump, to the endthat it may be cooled and used again for the engine. The pond may be threeor four feet deep, and lined with turf, puddled, or otherwise made watertight. Throwing up the water into the air in the form of a jet to cool it, has been found detrimental; as the water is then charged with air whichvitiates the vacuum. 

425. _Q. _--How is the piston of a pumping engine packed?

_A. _--To pack the piston, take sixty common-sized white or untarredrope-yarns, and with them plait a gasket or flat rope as close and firm aspossible, tapering for eighteen inches at each end, and long enough to goround the piston, and overlapped for that length; coil this rope the thinway as hard as possible, and beat it with a sledge hammer until its breadthanswers the place; put it in and beat it down with a wooden drift and ahand mallet, pour some melted tallow all around, then pack in a layer ofwhite oakum half an inch thick, so that the whole packing may have thedepth of five to six inches, depending on the size of the engine; finally, screw down the junk ring. The packing should be beat solid, but not toohard, otherwise it will create so great a friction as to prevent the easygoing of the engine. Abundance of tallow should be allowed, especially atfirst; the quantity required will be less as the cylinder grows smooth. Insome of the more modern pumping engines, the piston is provided withmetallic packing, consisting for the most part of a single ring with atongue piece to break the joint, and packed behind with hemp. The upperedge of the metallic ring is sharpened away from the inside so as to permitmore conveniently the application of hemp packing behind it; and the junkring is made much the same as if no metallic packing were employed. 

426. _Q. _--Will you explain the mode of putting the engine into operation?

_A. _--To set the engine going, the steam must be raised until the pressurein the steam pipe is at least equal to three pounds on the square inch; andwhen the cylinder jacket is fully warmed, and steam issues freely from thejacket cock, open all the valves or regulators; the steam will thenforcibly blow out the air or water contained in the eduction pipe, and toget rid of the air in the cylinder, shut the steam valve after having blownthrough the engine for a few minutes. The cold water round the condenserwill condense some of the steam contained in the eduction pipe, and itsplace will be supplied by some of the air from the cylinder. The steamvalve must again be opened to blow out that air, and the operation is to berepeated until the air is all drawn out of the cylinder. When that is thecase shut all the valves, and observe if the vacuum gauge shows a vacuum inthe condenser; when there is a vacuum equivalent to three inches ofmercury, open the injection a very little, and shut it again immediately;and if this produces any considerable vacuum, open the exhausting valve avery little way, and the injection at the same time. If the engine does notnow commence its motion, it must be blown through again until it moves. Ifthe engine be lightly loaded, or if there be no water in the pumps, thethrottle valve must be kept nearly closed, and the top and exhaustionregulators must be opened only a very little way, else the engine will makeits stroke with violence, and perhaps do mischief. If there is muchunbalanced weight on the pump end, the plug which opens the steam valvemust be so regulated, that the valve will only be opened very slightly; andif after a few strokes it is found that the engine goes out too slowly, thevalve may be then so adjusted as to open wider. The engine should always bemade to work full stroke, that is, until the catch pins be made to comewithin half an inch of the springs at each end, and the piston should standhigh enough in the cylinder when the engine is at rest, to spill over intothe perpendicular steam pipe any water which may be condensed above it; forif water remain upon the piston, it will increase the consumption of steam. When the engine is to be stopped, shut the injection valve and secure it, and adjust the tappets so as to prevent the exhausting valve from openingand to allow the steam valve to open and remain open, otherwise a partialvacuum may arise in the cylinder, and it may be filled with water from theinjection or from leaks. A single acting engine, when it is in good order, ought to be capable of going as slow as one stroke in ten minutes, and asfast as ten strokes in one minute; and if it does not fulfil theseconditions, there is some fault which should be ascertained and remedied. 

427. _Q. _--Your explanation has reference to the pumping engine asintroduced into Cornwall by Watt: have any modifications been since madeupon it?

_A. _--In the modern Cornish engines the steam is used very expansively, anda high pressure of steam is employed. In some cases a double cylinderengine is used, in which the steam, after having given motion to a smallpiston on the principle of a high pressure engine, passes into a largercylinder, where it operates on the principle of a condensing engine; butthere is no superior effect gained by the use of two cylinders, and thereis greater complexity in the apparatus. Instead of the lever walls, castiron columns are now frequently used for supporting the main beam inpumping engines, and the cylinder end of the main beam is generally madelonger than the pump end in engines made in Cornwall, so as to enable thecylinder to have a long stroke, and the piston to move quickly, withoutcommunicating such a velocity to the pump buckets as will make them workwith such a shock as to wear themselves out quickly. A high pressure ofsteam, too, can be employed where the stroke is long, without involving thenecessity of making the working parts of such large dimensions as wouldotherwise be necessary; for the strength of the parts of a single actingengine will require to be much the same, whatever the length of the strokemay be. 

428. _Q. _--What kind of pump is mostly used in draining deep mines?

_A. _--The pump now universally preferred is the plunger pump, which admitsof being packed or tightened while the engine is at work; but the lowestlift of a mine is generally supplied with a pump on the suction principle, both with the view of enabling the lowest pipe to follow the water withfacility as the shaft is sunk deeper, and to obviate the inconvenience ofthe valves of the pump being rendered inaccessible by any flooding in themine. The pump valves of deep mines are a perpetual source of expense andtrouble, as from the pressure of water upon them it is difficult to preventthem from closing with violence; and many expedients have been contrived tomitigate the evil, of which the valve known as Harvey and West's valve hasperhaps gained the widest acceptation. 

429. _Q. _--Will you describe Harvey and West's pump valve?

_A. _--This valve is a compromise between the equilibrium valve, of the kindemployed for admitting the steam to and from the cylinder in single actingengines, and the common spindle valve formerly used for that purpose; andto comprehend its action, it is necessary that the action of theequilibrium valve, which has been already represented fig. 34, should firstbe understood. This valve consists substantially of a cylinder open at bothends, and capable of sliding upon a stationary piston fixed upon a rod thelength of the cylinder, which proceeds from the centre of the orifice thevalve is intended to close. It is clear, that when the cylinder is presseddown until its edge rests upon the bottom of the box containing it, theorifice of the pipe must be closed, as the steam can neither escape pastthe edge of the cylinder nor between the cylinder and the piston; and it isequally clear, that as the pressure upon the cylinder is equal all aroundit, and the whole of the downward pressure is maintained by the stationarypiston, the cylinder can be raised or lowered without any further exertionof force than is necessary to overcome the friction of the piston and ofthe rod by which the cylinder is raised. Instead of the rubbing surface ofa piston, however, a conical valve face between the cylinder and piston isemployed, which is tight only when the cylinder is in its lowest position;and there is a similar face between, the edge of the cylinder and thebottom of the box in which it is placed. The moving part of the valve, too, instead of being a perfect cylinder, is bulged outward in the middle, so asto permit the steam to escape past the stationary piston when thecylindrical part of the valve is raised. It is clear, that if such a valvewere applied to a pump, no pressure of water within the pump would sufficeto open it, neither would any pressure of water above the valve cause it toshut with violence; and if an equilibrium valve, therefore, be used as apump valve at all, it must be opened and shut by mechanical means. InHarvey and West's valves, however, the equilibrium principle is onlypartially adopted; the lower face is considerably larger in diameter thanthe upper face, and the difference constitutes an annulus of pressure, which will cause the valve to open or shut with the same force as a spindlevalve of the area of the annulus. To deaden the shock still moreeffectually, the lower face of the valve is made to strike upon end wooddriven into an annular recess in the pump bucket; and valves thusconstructed work with very little noise or tremor; but it is found inpractice, that the use of Harvey and West's valve, or any contrivance of asimilar kind, adds materially to the load upon the pump, especially in lowlifts where the addition of a load, to the valve makes a material additionto the total resistance which the engine has to overcome. Instead of endwood driven into a recess for the valve to strike upon, a mixture of tinand lead cast in a recess is now frequently used, and is found to bepreferable to the wood. 

430. _Q. _--Is there any other kind of pump valve which is free from theshocks incidental to the working of common valves?

_A. _--In some cases canvass valves are used for pumps, with the effect ofmaterially mitigating the shock; but they require frequent renewal, and areof inferior eligibility in their action to the slide valve, which might inmany cases be applied to pumps without inconvenience. 

431. _Q. _--Could not a form of pump be devised capable of working withoutvalves at all?

_A. _. --It appears probable, that by working a common reciprocating pump ata high speed, a continuous flow of water might be maintained through thepipes in such a way as to render the existence of any valves superfluousafter once the action was begun, the momentum of the moving water acting infact as valves. The centrifugal pump, however, threatens to supersede pumpsof every other kind; and if the centrifugal pump be employed there will beno necessity for pump valves at all. There is less loss of effect by thecentrifugal pump than by the common pump. 

432. _Q. _--What is the best form of the centrifugal pump?

_A. _--There are two forms in which the centrifugal pump may be applied tomines;--that in which the arms diverge from the bottom, like the letter V;and that in which revolving arms are set in a tight case near the bottom ofthe mine, and are turned by a shaft from the surface. Such pumps both drawand force; and either by arranging them in a succession of lifts in theshaft of the mine, or otherwise, the water may be drawn withoutinconvenience from any depth. The introduction of the centrifugal pumpwould obviously extinguish the single acting engine, as rotative enginesworking at a high speed would be the most appropriate form of engine wherethe centrifugal pump was employed. 

433. _Q. _--This would not be a heavy deprivation?

_A. _--The single acting engine is a remnant of engineering barbarism whichmust now be superseded by more compendious contrivances. The Cornishengines, though rudely manufactured, are very expensive in production, as alarge engine does but little work; whereas by employing a smaller engine, moving with a high speed, the dimensions may be so far diminished that themost refined machinery may be obtained at less than the present cost. 

434. _Q. _--Are not the Cornish engines more economical in fuel than otherengines?

_A. _--It is a mistake to suppose that there is any peculiar virtue in theexisting form of Cornish engine to make it economical in fuel, or that aless lethargic engine would necessarily be less efficient. The large dutyof the engines in Cornwall is traceable to the large employment of theprinciple of expansion, and to a few other causes which may be made ofquite as decisive efficacy in smaller engines working with a quicker speed;and there is therefore no argument in the performance of the presentengines against the proposed substitution. 

VARIOUS FORMS OF MARINE ENGINES. 

435. _Q. _--What species of paddle engine do you consider to be the best?

_A. _--The oscillating engine. 

436. _Q. _--Will you explain the grounds of that preference?

_A. _--The engine occupies little space, consists of few parts, is easilyaccessible for repairs, and may be both light and strong at the same time. In the case of large engines the crank in the intermediate shaft is adisadvantage, as it is difficult to obtain such a forging quite sound. Butby forging it in three cranked flat bars, which are then laid together andwelded into a square shaft, a sound forging will be more probable, and thebars should be rounded a little on the sides which are welded to allow thescoriae to escape during that operation. It is important in so large aforging not to let the fire be too fierce, else the surface of the ironwill be burnt before the heart is brought to a welding heat. In some casesin oscillating engines the air pump has been wrought by an eccentric, andthat may at any time be done where doubt of obtaining a sound intermediateshaft is entertained; but the precaution must be taken to make theeccentric very wide so as to distribute the pressure over a large surface, else the eccentric will be apt to heat. 

437. _Q. _--Have not objections been brought against the oscillating engine?

_A. _--In common with every other improvement, the oscillating engine, atthe time of its introduction, encountered much opposition. The cylinder, itwas said, would become oval, the trunnion bearings would be liable to heatand the trunnion joints to leak, the strain upon the trunnions would be aptto bend in or bend out the sides of the cylinder; and the circumstance ofthe cylinder being fixed across its centre, while the shaft requires toaccommodate itself to the working of the ship, might, it was thought, bethe occasion of such a strain upon the trunnions as would either break themor bend the piston rod. It is a sufficient reply to these objections to saythat they are all hypothetical, and that none of them in practice have beenfound to exist--to such an extent at least as to occasion anyinconvenience; but it is not difficult to show that they are altogetherunsubstantial, even without a recourse to the disproofs afforded byexperience. 

438. _Q. _--Is there not a liability in the cylinder to become oval from thestrain thrown on it by the piston?

_A. _--There is, no doubt, a tendency in oscillating engines for thecylinder and the stuffing box to become oval, but after a number of years'wear it is found that the amount of ellipticity is less than that which isfound to exist in the cylinders of side lever engines after a similartrial. The resistance opposed by friction to the oscillation of thecylinder is so small, that a man is capable of moving a large cylinder withone hand; whereas in the side lever engine, if the parallel motion be inthe least untrue, which is, at some time or other, an almost inevitablecondition, the piston is pushed with great force against the side of thecylinder, whereby a large amount of wear and friction is occasioned. Thetrunnion bearings, instead of being liable to heat like other journals, arekept down to the temperature of the steam by the flow of steam passingthrough them; and the trunnion packings are not liable to leak when thepackings, before being introduced, are squeezed in a cylindrical mould. 

439. _Q. _--Might not the eduction trunnions be immersed in water?

_A. _--In some cases a hollow, or lantern brass, about one third or onefourth the length of the packing space, and supplied with steam or water bya pipe, is introduced in the middle of the packing, so that if there be anyleakage through the trunnion, it will be a leakage of steam or water, whichwill not vitiate the vacuum; but in ordinary cases this device will not benecessary, and it is not commonly employed. It is clear that there can beno buckling of the sides of the cylinder by the strain upon the trunnions, if the cylinder be made strong enough, and in cylinders of the ordinarythickness such an action has never been experienced; nor is it the fact, that the intermediate shaft of steam vessels, to which part alone themotion is communicated by the engine, requires to adapt itself to thealtering forms of the vessel, as the engine and intermediate shaft arerigidly connected, although the paddle shaft requires to be capable of suchan adaptation. Even if this objection existed, however, it could easily bemet by making the crank pin of the ball and socket fashion, which wouldpermit the position of the intermediate shaft, relatively with that of thecylinder, to be slightly changed, without throwing an undue strain upon anyof the working parts. 

440. _Q. _--Is the trunk engine inferior to the oscillating?

_A. _--A very elegant and efficient arrangement of trunk engine suitable forpaddle vessels has latterly been employed by Messrs. Rennie, of which allthe parts resemble those of Penn's oscillating engine except that thecylinders are stationary instead of being movable; and a round trunk orpipe set upon the piston, and moving steam tight through the cylindercover, enables the connecting rod which is fixed to the piston to vibratewithin it to the requisite extent. But the vice of all trunk engines isthat they are necessarily more wasteful of steam, as the large mass ofmetal entering into the composition of the trunk, moving as it doesalternately into the atmosphere and the steam, must cool and condense apart of the steam. The radiation of heat from the interior of the trunkwill have the same operation, though in vertical trunk engines the lossfrom this cause might probably be reduced by filling the trunk with oil, sofar as this could be done without the oil being spilt over the edge. 

441. _Q. _--What species of screw engine do you consider the best?

_A. _--I am inclined to give the preference to a variety of the horizontalsteeple engine, such as was first used in H. M. S. Amphion. In this enginethe cylinders lie on their sides, and they are placed near the side of thevessel with their mouths pointing to the keel. From each cylinder two longpiston rods proceed across the vessel to a cross head working in guides;and from this cross head a connecting rod returns back to the centre of thevessel and gives motion to the crank. The piston rods are so placed in thepiston that one of them passes above the crank shaft, and the other belowthe crank shaft. The cross head lies in the same horizontal plane as thecentre of the cylinder, and a lug projects upwards from the cross head toengage one piston rod, and downwards from the cross head to engage theother piston rod. The air pump is double acting, and its piston or buckethas the same stroke as the piston of the engine. The air pump bucketderives its motion from an arm on the cross head, and a similar arm isusually employed in engines of this class to work the feed and bilge pumps. 

442. _Q. _--Is not inconvenience experienced in direct acting screw enginesfrom the great velocity of their motion?

_A. _--Not if they are properly constructed; but they require to be muchstronger, to be fitted with more care, and to have the bearing surfacesmuch larger than is necessary in engines moving slowly. The momentum of thereciprocating parts should also be balanced by a weight applied to thecrank or crank shaft, as is done in locomotives. A very convenientarrangement for obtaining surface is to form the crank of each engine oftwo cast iron discs cast with heavy sides, the excess of weight upon theheavy sides being nearly equal to that of the piston and its connections. When the piston is travelling in one direction the weights are travellingin the opposite; and the momentum of the piston and its attachments, whichis arrested at each reciprocation, is just balanced by the equal andopposite momentum of the weights. One advantage of the horizontal engineis, that a single engine may be employed, whereby greater simplicity of themachinery and greater economy of fuel will be obtained, since there will beless radiating surface in one cylinder than in two. 

CYLINDERS, PISTONS, AND VALVES, 443. _Q. _--Is it a beneficial practice to make cylinders with steamjackets?

_A. _--In Cornwall, where great attention is paid to economy of fuel, allthe engines are made with steam jackets, and in some cases a flue windsspirally round the cylinder, for keeping the steam hot. Mr. Watt, in hisearly practice, discarded the steam jacket for a time, but resumed itagain, as he found its discontinuance occasioned a perceptible waste offuel; and in modern engines it has been found that where a jacket is usedless coal is consumed than where the use of a jacket is rejected. The causeof this diminished effect is not of very easy perception, for the jacketexposes a larger radiating surface for the escape of the heat than thecylinder; nevertheless, the fact has been established beyond doubt byrepeated trials, that engines provided with a jacket are more economicalthan engines without one. The exterior of the cylinder, or jacket, shouldbe covered with several plies of felt, and then be cased in timber, whichmust be very narrow, the boards being first dried in a stove, and thenbound round the cylinder with hoops, like the staves of a cask. In many ofthe Cornish engines the steam is let into casings formed in the cylindercover and cylinder bottom, for the further economisation of the heat, andthe cylinder stuffing box is made very deep, and a lantern or hollow brassis introduced into the centre of the packing, into which brass the steamgains admission by a pipe provided for the purpose; so that in the event ofthe packing becoming leaky, it will be steam that will be leaked into thecylinder instead of air, which, being incondensable, would impair theefficiency of the engine. A lantern brass, of a similar kind, is sometimesintroduced into the stuffing boxes of oscillating engines, but its usethere is to receive the lateral pressure of the piston rod, and thus takeany strain off the packing. 

444. _Q. _--Will you explain the proper course to pursue in the productionof cylinders?

_A. _--In all engines the valve casing, if made in a separate piece from thecylinder, should be attached by means of a metallic joint, as such abarbarism as a rust joint in such situations is no longer permissible. Inthe case of large engines with valve casings suitable for long slides, anexpansion joint in the valve casing should invariably be inserted, otherwise the steam, by gaining admission to the valve casing before it canenter the cylinder, expands the casing while the cylinder remains unalteredin its dimensions, and the joints are damaged, and in some cases thecylinder is cracked by the great strain thus introduced. The chest of theblow-through valve is very commonly cast upon the valve casing; and inengines where the cylinders are stationary this is the most convenientpractice. All engines, where the valve is not of such a construction as toleave the face when a pressure exceeding that of the steam is created inthe cylinder by priming or otherwise, should be provided with an escapevalve to let out the water, and such valve should be so constructed thatthe water cannot fly out with violence over the attendants; but it shouldbe conducted away by a suitable pipe, to a place where its discharge canoccasion no inconvenience. The stuffing boxes of all engines which cannotbe stopped frequently to be repacked, should be made very deep; metallicpacking in the stuffing box has been used in some engines, consisting inmost instances of one or more rings, cut, sprung, and slipped upon thepiston rod before the cross head is put on, and packed with hemp behind. This species of packing answers very well when the parallel motion is true, and the piston rod free from scratches, and it accomplishes a materialsaving of tallow. In some cases a piece of sheet brass, packed behind withhemp, has been introduced with good effect, a flange being turned over onthe under edge of the brass to prevent it from slipping up or down with themotion of the rod. The sheet brass speedily puts an excellent polish uponthe rod, and such a packing is more easily kept, and requires less tallowthan where hemp alone is employed. In side lever marine engines theattachments of the cylinder to the diagonal stay are generally made of toosmall an area, and the flanges are made too thick. A very thick flange caston any part of a cylinder endangers the soundness of the cylinder, byinducing an unequal contraction of the metal; and it is a preferable courseto make the flange for the attachment or the framing thin, and the surfacelarge--the bolts being turned bolts and nicely fitted. If from malformationin this part the framing works to an inconvenient extent, the bestexpedient appears to be the introduction of a number of steel taperedbolts, the holes having been previously bored out; and if the flanges bethick enough, square keys may also be introduced, half into one flange andhalf into the other, so as to receive the strain. If the jaw cracks orbreaks away, however, it will be best to apply a malleable iron hoop aroundthe cylinder to take the strain, and this will in all cases be thepreferable expedient, where from any peculiarities of structure there is adifficulty in introducing bolts and keys of sufficient strength. 

445. _Q. _--Which is the most eligible species of piston?

_A. _--For large engines, pistons with a metallic packing, consisting of asingle ring, with the ends morticed into one another, and a piece of metallet in flush over the joint and riveted to one end of the ring, appears tobe the best species of piston; and if the cylinder be oscillating, it willbe expedient to chamfer off the upper edge of the ring on the inner side, and to pack it at the back with hemp. If the cylinder be a stationary one, springs may be substituted for the hemp packing, but in any case it will beexpedient to make the vertical joints of the ends of the ring run a littleobliquely, so as to prevent the joint forming a ridge in the cylinder. Forsmall pistons two rings may be employed, made somewhat eccentric internallyto give a greater thickness of metal in the centre of the ring; these ringsmust be set one above the other in the cylinder, and the joints, which areoblique, must be set at right angles with one another, so as to obviate anydisposition of the rings, in their expansion, to wear the cylinder oval. The rings must first be turned a little larger than the diameter of thecylinder, and a piece is then to be cut out, so that when the ends arebrought together the ring will just enter within the cylinder. The ring, while retained in a state of compression, is then to be put in the latheand turned very truly, and finally it is to be hammered on the inside withthe small end of the hammer, to expand the metal, and thus increase theelasticity. 

446. _Q. _--The rings should be carefully fitted to one another laterally?

_A. _--The rings are to be fitted laterally to the piston, and to oneanother, by scraping--a steady pin being fixed upon the flange of thepiston, and fitting into a corresponding hole in the lower ring, to keepthe lower ring from turning round; and a similar pin being fixed into thetop edge of the lower ring to prevent the upper ring from turning round;but the holes into which these pins fit must be made oblong, to enable therings to press outward as the rubbing surfaces wear. In most cases it willbe expedient to press the packing rings out with springs where they are notpacked behind with hemp, and the springs should be made very strong, as theprevailing fault of springs is their weakness. Sometimes short bentsprings, set round at regular intervals between the packing rings and bodyof the piston, are employed, the centre of each spring being secured by asteady pin or bolt screwed into the side of the piston; but it will notsignify much what kind of springs is used, provided they have sufficienttension. When pistons are made of a single ring, or of a succession ofsingle rings, the strength of each ring should be tested previously to itsintroduction into the piston, by means of a lever loaded by a heavy weight. 

447. _Q. _--What kind of piston is employed by Messrs. Penn?

_A. _--Messrs. Penn's piston for oscillating engines has a single packingring, with a tongue piece, or mortice end, made in the manner alreadydescribed. The ring is packed behind with hemp packing, and the piece ofmetal which covers the joint is a piece of thick sheet copper or brass, andis indented into the iron of the ring, so as to offer no obstruction to theapplication of the hemp. The ring is fitted to the piston only on the underedge; the top edge is rounded to a point from the inside, and the junk ringdoes not bear upon it, but the junk ring squeezes down the hemp packingbetween the packing ring and the body of the piston. 

448. _Q. _--How should the piston rod be secured to the piston?

_A. _--The piston rod, where it fits into the piston, should have a gooddeal of taper; for if the taper be too small the rod will be drawn throughthe hole, and the piston will be split asunder. Small grooves are sometimesturned out of the piston rod above and below the cutter hole, and hemp isintroduced in order to make the piston eye tight. Most piston rods arefixed to the piston by means of a gib and cutter, but in some cases theupper portion of the rod within the eye is screwed, and it is fixed intothe piston by means of an indented nut. This nut is in some caseshexagonal, and in other cases the exterior forms a portion of a cone whichcompletely fills a corresponding recess in the piston; but nuts made inthis way become rusted into their seat after some time, and cannot bestarted again without much difficulty. Messrs. Miller, Ravenhill & Co. Fixin their piston rods by means of an indented hexagonal nut, which may bestarted by means of an open box key. The thread of the screw is made flatupon the one side and much slanted on the other, whereby a greater strengthis secured, without creating any disposition to split the nut. In sidelever engines it is a judicious practice to add a nut to the top of thepiston rod, in addition to the cutter for securing the piston rod to thecross head. In a good example of an engine thus provided, the piston rod is7 in. In diameter, and the screw 5 in. ; the part of the rod which fits intothe cross head eye is 1 ft. 5-1/2 in. Long, and tapers from 6-1/2 in. To6-13/16 in. Diameter. This proportion of taper is a good one; if the taperbe less, or if a portion of the piston rod within the cross head eye beleft untapered, as is sometimes the case, it is very difficult to detachthe parts from one another. 

449. _Q. _--Which is the most beneficial construction of slide valve?

_A. _--The best construction of slide valve appears to be that adopted byMessrs. Penn for their larger engines, and which consists of a three portedvalve, to the back of which a ring is applied of an area equal to that ofexhaustion port, and which, by bearing steam tight against the back of thecasing, so that a vacuum may be maintained within the ring, puts the valvein equilibrium, so that it may be moved with an inconsiderable exercise offorce. The back of the valve casing is put on like a door, and its internalsurface is made very true by scraping. There is a hole through the valve soas to conduct away any steam which may enter within the ring by leakage, and the ring is kept tight against the back of the casing by means of aring situated beneath the bearing ring, provided with four lugs, throughwhich bolts pass tapped into bosses on the back of the valve; and, byunscrewing these bolts, --which may be done by means of a box key whichpasses through holes in the casing closed with screwed plugs, --the lowerring is raised upwards, carrying the bearing ring before it. The rings mustobviously be fitted over a boss upon the back of the valve; and between therings, which are of brass, a gasket ring is interposed to compensate by itscompressibility for any irregularity of pressure, and each of the bolts isprovided with a ratchet collar to prevent it from turning back, so that theengineer, in tightening these bolts, will have no difficulty in tighteningthem equally, if he counts the number of clicks made by the ratchet. Wherethis species of valve is used, it is indispensable that large escape valvesbe applied to the cylinder, as a valve on this construction is unable toleave the face. In locomotive engines, the valve universally employed isthe common three ported valve. 

450. _Q. _--Might not an equilibrium valve be so constructed by theinterposition of springs, as to enable it to leave the cylinder face whenan internal force is applied?

_A. _--That can no doubt be done, and in some engines has been done. In thescrew steamer Azof, the valve is of the equilibrium construction, but theplate which carries the packing on which the top ring rests, is an octagon, and fits into an octagonal recess on the back of the valve. Below each sideof the octagon there is a bent flat spring, which lifts up the octagonalplate, and with it the packing ring against the back of the valve casing;and should water get into the cylinder, it escapes by lifting the valve, which is rendered possible by the compressibility of the springs. Anequivalent arrangement is shown in figs. 39 and 40, where the ring islifted by spiral springs. 

[Illustration: Fig. 39. EQUILIBRIUM GRIDIRON SLIDE VALVE. LongitudinalSection. Scale 3/4 inch = 1 foot. ]

451. _Q. _--What species of valve is that shown in figs. 39 and 40?

[Illustration: Fig. 40. EQUILIBRIUM GRIDIRON SLIDE VALVE. Back View withRing removed. Scale 3/4 inch = 1 foot. ]

_A. _--It is an equilibrium gridiron valve; so called because it lets thesteam in and out by more than one port. A A are the ordinary steampassages to the top and bottom of the cylinder; B B is the ring which rubsagainst the back of the valve casing, and D is the eduction passage, S S SS shows the limits of the steam space, for the steam penetrates to thecentral chamber S S by the sides of the valve. When the valve is openedupon the steam side, the cylinder receives steam through both ports at thatend of the cylinder, and both ports at the other end of the cylinder are atthe same time open to the eduction. The benefit of this species of valveis, that it gives the same opening of the valve that is given in ordinaryengines, with half the amount of travel; or if three ports were madeinstead of two, then it would give the same area of opening that is givenin common engines with one third the amount of travel. For direct actingscrew engines this species of valve is now extensively used. 

452. _Q. _--Will you describe the configuration and mode of attachment ofthe eccentric by which the valve is moved?

_A. _--In marine engines, whether paddle or screw, if moving at a slow rateof speed, the eccentric is generally loose upon the shaft, for the purposeof backing, and is furnished with a back balance and catches, so that itmay stand either in the position for going ahead, or in that for goingastern. The body of the eccentric is of cast iron, and it is put on theshaft in two pieces. The halves are put together with rebated joints tokeep them from separating laterally, and they are prevented from slidingout by round steel pins, each ground into both halves; square keys wouldprobably be preferable to round pins in this arrangement, as the pins tendto wedge the jaws of the eccentric asunder. In some cases the halves of theeccentric are bolted together by means of flanges, which is, perhaps, thepreferable practice. The eccentric hoop in marine and land engines isgenerally of brass; it is expedient to cast an oil cup on the eccentrichoop, and, where practicable, a pan should be placed beneath the eccentricfor the reception of the oil droppings. The notch of the eccentric rod forthe reception of the pin of the valve shaft is usually steeled, to preventinconvenient wear; for when the sides of the notch wear, the valve movementis not only disturbed, but it is very difficult to throw the eccentric rodout of gear. It is found to be preferable, however, to fit this notch witha brass bush, for the wear is then less rapid, and it is an easy thing toreplace this bush with another when it becomes worn. The eccentric catchesof the kind usually employed in marine engines, sometimes break off at thefirst bolt hole, and it is preferable to have a bolt in advance of thecatch face, or to have a hoop encircling the shaft with the catches weldedon it, the hoop itself being fixed by bolts or a key. This hoop may eitherbe put on before the cranks in one piece or afterwards in two pieces. 

453. _Q. _--Are such eccentrics used in direct acting screw engines?

_A. _--No; direct acting screw engines are usually fitted with the linkmotion and two fixed eccentrics. 

AIR PUMP AND CONDENSER. 

454. _Q. _--What are the details of the air pump?

_A. _--The air pump bucket and valves are all of brass in modern marineengines, and the chamber of the pump is lined with copper, or made whollyof brass, whereby a single boring suffices. When a copper lining is used, the pump is first bored out, and a bent sheet of copper is introduced, which is made accurately to fill the place, by hammering the copper on theinside. Air pump rods of Muntz's metal or copper are much used. Iron rodscovered with brass are generally wasted away where the bottom cone fitsinto the bucket eye, and if the casing be at all porous, the water willinsinuate itself between the casing and the rod and eat away the iron. Ifiron rods covered with brass be used, the brass casing should come somedistance into the bucket eye; the cutter should be of brass, and a brasswasher should cover the under side of the eye, so as to defend the end ofthe rod from the salt water. Rods of Muntz's metal are probably on thewhole to be preferred. It is a good practice to put a nut on the top of therod, to secure it more firmly in the cross head eye, where that plan can beconveniently adopted. The part of the rod which fits into the cross headeye should have more taper when made of copper or brass, than when made ofiron; as, if the taper be small, the rod may get staved into the eye, whereby its detachment will be difficult. 

455. _Q. _--What species of packing is used in air pumps?

_A. _--Metallic packing has in some instances been employed in air pumpbuckets, but its success has not been such as to lead to its furtheradoption. The packing commonly employed is hemp. A deep solid block ofmetal, however, without any packing, is often employed with a satisfactoryresult; but this block should have circular grooves cut round its edge tohold water. Where ordinary packing is employed, the bucket should always bemade with a junk ring, whereby the packing may be easily screwed down atany time with facility. In slow moving engines the bucket valve isgenerally of the spindle or pot-lid kind, but butterfly valves aresometimes used. The foot and delivery valves are for the most part of theflap or hanging kind. These valves all make a considerable noise inworking, and are objectionable in many ways. Valves on Belidor'sconstruction, which is in effect that of a throttle valve hung off thecentre, were some years ago proposed for the delivery and foot valves; andit appears probable that their operation would be more satisfactory thanthat of the valves usually employed. 

456. _Q. _--Where is the delivery valve usually situated?

_A. _--Some delivery valve seats are bolted into the mouth of the air pump, whereby access to the pump bucket is rendered difficult: but more commonlythe delivery valve is a flap valve exterior to the pump. If delivery valveseats be put in the mouth of the air pump at all, the best mode of fixingthem appears to be that adopted by Messrs. Maudslay. The top of the pumpbarrel is made quite fair across, and upon this flat surface a platecontaining the delivery valve is set, there being a small ledge all roundto keep it steady. Between the bottom of the stuffing box of the pump coverand the eye of the valve seat a short pipe extends encircling the pump rod, its lower end checked into the eye of the valve seat, and its upper endwidening out to form the bottom of the stuffing box of the pump cover. Uponthe top of this pipe some screws press, which are accessible from the topof the stuffing box gland, and the packing also aids in keeping down thepipe, the function of which is to retain the valve seat in its place. Whenthe pump bucket has to be examined the valve seat may be slung with thecover, so as to come up with the same purchase. For the bucket valves ofsuch pumps Messrs. Maudslay employ two or more concentric ring valves witha small lift. These valves have given a good deal of trouble in some cases, in consequence of the frequent fracture of the bolts which guide andconfine the rings; but this is only a fault of detail which is easilyremedied, and the principle appears to be superior to that of any of theother metallic air pump valves at present in common use. 

[Illustration: Fig. 41. TRUNK AIR PUMP. Scale 3/4 inch to 1 foot. ]

457. _Q. _--Are not air pump valves now very generally made of india rubber?

_A. _--They are almost invariably so made if the engines are travellingfast, as in the case of direct acting screw engines, and they are veryoften made of large discs or rings of india rubber, even when the enginestravel slowly. A very usual and eligible arrangement for many purposes isthat shown in fig. 41, where both foot and delivery valves are situated inthe ends of the pump, and they, as well as the valve in the bucket are madeof india rubber rings closing on a grating. The trunk in the air pumpenables guide rods to be dispensed with. 

[Illustration: Fig. 42. PENN'S DISK VALVE FOR AIR PUMP. Section. ]

[Illustration: Fig. 43. PENN'S DISK VALVE FOR AIR PUMP. Ground Plan. ]

[Illustration: Fig. 44. MAUDSLAY'S DISC VALVE FOR AIR PUMP. Section. ]

458. _Q. _--The air pump, when double acting, has of course inlet and outletvalves at each end?

_A. _--Yes; and the general arrangement of the valves of double acting airpumps, such as are usual in direct acting screw engines, is thatrepresented in the figure of Penn's trunk engine already described inChapter I. Each inlet and outlet valve consists of a number of india rubberdiscs set over a perforated brass plate, and each disc is bound down by abolt in the middle, which bolt also secures a brass guard set above thedisc to prevent it from rising too high. The usual configuration of thosevalves is that represented in figs. 42, 43, and 44; figs. 42 and 43 being asection and ground plan of the species of valve used by Messrs. Penn, andfig. 44 being a section of that used by Messrs. Maudslay. It is importantin these valves to have the india rubber thick, --say about an inch thickfor valves eight inches in diameter. It is also advisable to make thecentral bolts with a nut above and a nut below, and to form the bolt with acounter sunk neck, so that it will not fall down when the top nut isremoved. The lower point of the bolt should be riveted over on the nut toprevent it from unscrewing, and the top end should have a split pin throughthe point for the same purpose. The hole through which the bolt passesshould be tapped, though the bolt is not screwed into it, so that if a boltbreaks, a temporary stud may be screwed into the hole without the necessityof taking out the whole plate. The guard should be large, else the disc maystretch in the central hole until it comes over it; but the guard shouldnot permit too much lift of the valve, else a good deal of the water andair will return into the pump at the return stroke before the valve shuts. Penn's guard is rather small, and Maudslay's permits too much lift. 

459. _Q. _--What is the proper area through the valve gratings?

_A. _--The collective area should be at least equal to the area of the pumppiston, and the lower edges of the perforations should be rounded off toafford more free ingress or egress to the water. 

460. _Q. _--Is there much strain thrown on the plates in which the valvesare set?

_A. _--A good deal of strain; and in the earlier direct acting screw enginesthese plates were nearly in every case made too light. They should be madethick, have strong feathers upon them, and be very securely bolted downwith split pins at the points of the bolts, to prevent them fromunscrewing. The plate will be very apt to be broken should some of thebolts become loose. Of course all the bolts and split pins, as well as theplates and guards, must be of brass. 

461. _Q. _--How are the plates to be taken out should that become necessary?

_A. _--They are usually taken out through a door in the top of the hot wellprovided for that purpose, which door should be as large as the platesthemselves; and it is a good precaution to cast upon this door--which willbe of cast iron--six or eight stout projecting feet which will press uponthe top of the outlet or delivery valve plate when the door is screweddown. The upper or delivery valve plate and the lower or foot valve plateshould have similar feet. A large part of the strain will thus betransferred from the plates to the door, which can easily be made strongenough to sustain it. It is advisable that the plates should lie at anangle so that the shock of the water may not come upon the whole surface atonce. 

462. _Q. _--Does the double acting air pump usual in direct acting screwengines, produce as good a vacuum as the single acting air pump usual inpaddle engines?

_A. _--It will do so if properly constructed; but I do not know of any caseof a double acting air pump, with india rubber valves, which has beenproperly constructed. 

463. _Q. _--What is the fault of such pumps?

_A. _--The pump frequently works by starts, as if at times it did not drawat all, and then again on a sudden gorged itself with water, so as to throwa great strain upon the working parts. The vacuum, moreover, is by no meansso good as it should be, and it is a universal vice of direct acting screwengines that the vacuum is defective. I have been at some pains toinvestigate the causes of this imperfection; and in a sugar house enginefitted with pumps like those of a direct acting screw engine to maintain avacuum in the pans, I found that a better vacuum was produced when theengine was going slowly than when it was going fast; which is quite thereverse of what was to have been expected, as the hot water which had to beremoved by the condensation of the steam proceeding from the pan, was aconstant quantity. In this engine, too, which was a high pressure one, theirregularities of the engine consequent upon the fitful catching of thewater by the pump, was more conspicuous, as the working of this vacuum pumpwas the only work that the engine had to perform. 

464. _Q. _--And were you able to discover the cause of these irregularities?

_A. _--The main cause of them I found to be the largeness of the space leftbetween the valve plates in this class of pumps, and out of which there isnothing to press the air or water which may be lying there. It consequentlyhappens, that if there be the slightest leakage of air into the pump, thisair is merely compressed, and not expelled, by the advance of the air pumppiston. It expands again to its former bulk on the return of the pumppiston, and prevents the water from entering until there is such anaccumulation of pressure in the condenser as forces the water into thepump, when the air being expelled by the water, causes a good vacuum to bemomentarily formed in the pump when it gorges itself by taking a suddengulp of water. So soon, however, as the pressure falls in the condenser andsome more air leaks into the pump, the former imperfect action recurs andis again redressed in the same violent manner. 

465. _Q. _--Is this irregular action of the pump the cause of the imperfectvacuum?

_A. _--It is one cause. Sometimes one end of the pump will alone draw andthe other end will be inoperative, although it is equally open to thecondenser, and this will chiefly take place at the stuffing box end, wherea leakage of air is more likely to occur. I find, however, that even whenboth ends of the pump are acting equally and there is no leakage of air atall, the vacuum maintained by a double acting horizontal pump with indiarubber valves, is not so good as that maintained by a single acting pump ofthe kind usual in old engines. 

466. _Q. _--Will you specify more precisely what were the results youobtained?

_A. _--When the vacuum pan was exhausted by the pumps without any boilingbeing carried on in the pan, but only a little cold water being let intoit, and also into the pumps to enable them to act in their best manner, itwas found that whereas with the old pump a vacuum of 114 on the sugarboiler's gauge could be readily obtained, equal to about 29-1/2 inches ofmercury, the lowest that could possibly be got with the new horizontal pumpwas 122 degrees of the sugar boiler's gauge, or 29 inches of mercury, andto get that the engine must not go faster than 10 or 12 strokes per minute. The proper speed of the engine was 75 strokes per minute, but if allowed togo at that speed the vacuum fell to 130 of the sugar maker's gauge, or28-1/2 inches of mercury. When the steam was let into the worms of the panso as to boil the water in it, the vacuum was 134 at 75 revolutions of theengine, and went down to 132 at 40 revolutions, but rose again to 135, equal to about 28-1/4 inches of mercury, at 20 revolutions. 

467. _Q. _--To what do you attribute the circumstance of a better vacuumbeing got at low speeds than at high speeds?

_A. _--It is difficult to assign the precise reason, but it appears to be aconsequence of the largeness of the vacant space between the valve plates. When the piston of the air pump is drawn back, the air contained in thislarge collection of water will cause it to boil up like soda water; andwhen the piston of the pump is forced forward, this air, instead of beingexpelled, will be again driven into the water. There will consequently be aquantity of air in the pump which cannot be got rid of at all, and whichwill impair the vacuum as a matter of course. 

468. _Q. _--What expedient did you adopt to improve the vacuum in the engineto which you have referred?

_A. _--I put blocks of wood on the air pump piston, which at the end of itsstroke projected between the valve plates and forced the water out. I alsointroduced a cock of water at each end of the pump between the valveplates, to insure the presence of water at each end of the pump to forcethe air out. With these ameliorations the pump worked steadily, and thevacuum obtained became as good as in the old pump. I had previouslyintroduced an injection cock into each end of the air pump in steamvessels, from which I had obtained advantageous results; and in allhorizontal air pumps I would recommend the piston and valve plates to be soconstructed that the whole of the water will be expressed by the piston. Iwould also recommend an injection cock to be introduced at each end of thepump. 

PUMPS, COCKS, AND PIPES. 

469. _Q. _--Will you explain the arrangement of the feed pump?

_A. _--In steam vessels, the feed pump plunger is generally of brass, andthe barrel of the pump is sometimes of brass, but generally of cast iron. There should be a considerable clearance between the bottom of the plungerand the bottom of the barrel, as otherwise the bottom of the barrel may beknocked out, should coal dust or any other foreign substance gainadmission, as it probably would do if the injection water were drawn at anytime from the bilge of the vessel, as is usually done if the vessel springsa leak. The valves of the feed pump in marine engines are generally of thespindle kind, and are most conveniently arranged in a chest, which may beattached in any accessible position to the side of the hot well. There aretwo nozzles upon this chest, of which the lower one leads to the pump, andthe upper one to the boiler. The pipe leading to the pump is a suction pipewhen the plunger ascends, and a forcing pipe when the plunger descends. Theplunger in ascending draws the water out of the hot well through the lowestof the valves, and in descending forces it through the centre valve intothe space above it, which communicates with the feed pipe. Should the feedcock be shut so as to prevent any feed water from passing through it, thewater will raise the topmost valve, which is loaded to a pressureconsiderably above the pressure of the steam, and escape into the hot well. This arrangement is neater and less expensive than that of having aseparate loaded valve on the feed pipe with an overflow through the ship'sside, as is the more usual practice. 

470. _Q. _--Will you describe what precautions are to be observed in theconstruction of the cocks used in engines?

_A. _--All the cocks about an engine should be provided with bottoms andstuffing boxes, and reliance should never be placed upon a single boltpassing through a bottom washer for keeping the plug in its place, in thecase of any cock communicating with the boiler; for a great strain isthrown upon that bolt if the pressure of the steam be high, and if the plugbe made with much taper; and should the bolt break, or the threads strip, the plug will fly out, and persons standing near may be scalded to death. In large cocks, it appears the preferable plan to cast the bottoms in; andthe metal of which all the cocks about a marine engine are made, should beof the same quality as that used in the composition of the brasses, andshould be without lead, or other deteriorating material. In some cases thebottoms of cocks are burnt in with hard solder, but this method cannot bedepended upon, as the solder is softened and wasted away by the hot saltwater, and in time the bottom leaks, or is forced out. The stuffing box ofcocks should be made of adequate depth, and the gland should be secured bymeans of four strong copper bolts. The taper of blow-off cocks is animportant element in their construction; as, if the taper be too great, theplugs will have a continual tendency to rise, which, if the packing beslack, will enable grit to get between the faces, while, if the taper betoo little, the plug will be liable to jam, and a few times grinding willsink it so far through the shell that the waterways will no longercorrespond. One eighth of an inch deviation from the perpendicular forevery inch in height, is a common angle for the side of the cock, whichcorresponds with one quarter of an inch difference of diameter in an inchof height; but perhaps a somewhat greater taper than this, or one third ofan inch difference in diameter for every inch of height, is a preferableproportion. The bottom of the plug must be always kept a small distanceabove the bottom of the shell, and an adequate surface must be left aboveand below the waterway to prevent leakage. Cocks formed according to thesedirections will be found to operate satisfactorily in practice, while theywill occasion perpetual trouble if there be any malformation. 

471. _Q. _--What is the best arrangement and configuration of the blow-offcocks?

_A. _--The blow-off cocks of a boiler are generally placed some distancefrom the boiler; but it appears preferable that they should be placed quiteclose to it, as there are no means of shutting off the water from the pipebetween the blow-off cock and the boiler, should fracture or leakage therearise. Every boiler must be furnished with a blow-off cock of its own, independently of the main blow-off cocks on the ship's sides, so that theboilers may be blown off separately, and may be shut off from one another. The preferable arrangement appears to be, to cast upon each blow-off cock abend for attaching the cock to the bottom of the boiler, and the plugshould stand about an inch in advance of the front of the boiler, so thatit may be removed, or re-ground, with facility. The general arrangement ofthe blow-off pipes is to run a main blow-off pipe beneath the floor plates, across the ship, at the end of the engines, and into this pipe to lead aseparate pipe, furnished with a cock, from each boiler. The main blow-offpipe, where it penetrates the ship's side, is furnished with a cock: and inmodern steam vessels Kingston's valves are also used, which consist of aspindle or plate valve, fitted to the exterior of the ship, so that if theinternal pipe or cock breaks, the external valve will still be operative. Some expedient of this kind is almost necessary, as the blow-off cocksrequire occasional regrinding, and the sea cocks cannot be re-groundwithout putting the vessel into dock, except by the use of Kingston'svalves, or some equivalent expedient. 

472. Q. --What is the proper construction and situation of the injectioncocks, and waste water valves?

A. --The sea injection cocks are usually made in the same fashion as the seablow-off cocks, and of about the same size, or rather larger. The injectionwater is generally admitted to the condenser by means of a slide valve, buta cock appears to be preferable, as it is more easily opened, and has notany disposition to shut of its own accord. In paddle vessels the seainjection pipes should be put through the ship's sides in advance of thepaddles, so that the water drawn in may not be injuriously charged withair. The waste water pipe passing from the hot well through the vessel'sside is provided with a stop valve, called the discharge valve, which isusually made of the spindle kind, so as to open when the water coming fromthe air pump presses against it. In some cases this valve is a sluicevalve, but the hot well is then almost sure to be split, if the engine beset on without the valve having been opened. The opening of the waste waterpipe should always be above the load water line, as it will otherwise bedifficult to prevent leakage through the engine into the ship when thevessel is lying in harbor. 

473. Q. --What is the best arrangement of gauge cocks and glass gauges?

A. --Gauge cocks are generally very inartificially made, and occasionneedless annoyance. They are rarely made with bottoms, or with stuffingboxes, and are consequently, for the most part, adorned with stalactites ofsalt after a short period of service. The water discharged from them, too, from the want of a proper conduit, disfigures the front of the boiler, andadds to the corrosion in the ash pits. It would be preferable to combinethe gauge cocks appertaining to each boiler into a single upright tube, connected suitably with the boiler, and the water flowing from them couldbe directed downward into a funnel tube communicating with the bilge. Thecocks of the glass tubes, as well as of the gauge cocks, should befurnished with stuffing boxes and with bottoms, unless the water entersthrough the bottom of the plug, which in gauge cocks is sometimes the case. The glass gauge tubes should always be fitted with a cock at each neckcommunicating with the boiler, so that the water and steam may be shut offif the tube breaks; and the cocks should be so made as to admit of thetubes being blown through with steam to clear them, as in muddy water theywill become so soiled that the water cannot be seen. The gauge cocksfrequently have pipes running up within the boiler, to the end that a highwater level may be made consistent with an easily accessible position ofthe gauge cocks themselves. With the glass tubes, however, this species ofarrangement is not possible, and the glass tubes must always be placed inthe position of the water level. 

474. Q. --What is the proper material of the pipes in steam vessels?

A. --Most of the pipes of marine engines should be made of copper. The steampipes may be of cast iron, if made very strong, but the waste water pipesshould be of copper. Cast iron blow-off pipes have in some cases beenemployed, but they are liable to fracture, and are dangerous. The blow-offand feed pipes should be of copper, but the waste steam pipe may be ofgalvanized iron. Every pipe passing through the ship's side, and every pipefixed at both ends, and liable to be heated and cooled, should be furnishedwith a faucet or expansive joint; and in the case of the cast iron pipes, the part of the pipe fitting into the faucet should be turned. In thedistribution of the faucets of the pipes exposed to pressure, care must betaken that they be so placed that the parts of the pipe cannot be forcedasunder, or turned round by the strain, as serious accidents have occurredfrom the neglect of this precaution. 

475. _Q. _--What is the best mode of making pipes tight where they penetratethe ship's side?

_A. _--In wooden vessels the pipes where they pierce the ship's side, shouldbe made tight, as follows:--the hole being cut, a short piece of lead pipe, with a broad flange at one end, should be fitted into it, the place havingbeen previously smeared with white lead, and the pipe should then be beatenon the inside, until it comes into close contact all around with the wood. A loose flange should next be slipped over the projecting end of the leadpipe, to which it should be soldered, and the flanges should both be nailedto the timber with scupper nails, white lead having been previously spreadunderneath. This method of procedure, it is clear, prevents the possibilityof leakage down through the timbers; and all, therefore, that has to beguarded against after this precaution, is to prevent leakage into the ship. To accomplish this object, let the pipe which it is desired to attach beput through the leaden hause, and let the space between the pipe and thelead be packed with gasket and white lead, to which a little olive oil hasbeen added. The pipe must have a flange upon it to close the hole in theship's side; the packing must then be driven in from the outside, and bekept in by means of a gland secured with bolts passing through the ship'sside. If the pipe is below the water line the gland must be of brass, butfor the waste water pipe a cast iron gland will answer. This method ofsecuring pipes penetrating the side, however, though the best for woodenvessels, will, it is clear, fail to apply to iron ones. In the case of ironvessels, it appears to be the best practice to attach a short iron nozzle, projecting inward from the skin, for the attachment of every pipe below thewater line, as the copper or brass would waste the iron of the skin if theattachment were made in the usual way. 

DETAILS OF THE SCREW AND SCREW SHAFT. 

476. _Q. _--What is the best method of fixing the screw upon the shaft?

_A. _--The best way is to cut two large grooves in the shaft coming up to asquare end, and two corresponding grooves or key seats in the screw bossopposite the arms. Fit into the grooves on the shaft keys with heads, thelength of which is equal to half the depth of the boss, and with the endsof the keys bearing against the ends of the grooves in the shaft. Then shipon the propeller, and drive other keys of an equal length from the otherside of the boss, so that the points of the keys will nearly meet in themiddle; next burr up the edge of the grooves upon the heads of the keys, toprevent them from working back; and finally tap a bolt into the side of theboss to penetrate the shaft. Propellers so fitted will never get slack. 

477. _Q. _--What is the best way of fitting in the screw pipe at the stern?

_A. _--It should have projecting rings, which should be turned; and castiron pieces with holes in them, bored out to the sizes of these rings, should be secured to the stern frames, and the pipe be then shipped throughall. Before this is done, however, the stern post must be bored out by atemplate to fit the pipe, and the pipe is to be secured at the end to thestern post either by a great external nut of cast iron, or by bolts passingthrough the stern post and through lugs on the pipe. The pipe should bebored throughout its entire length, and the shaft should be turned so as toafford a very long bearing which will prevent rapid wear. 

478. _Q. _--How is the hole formed in the deadwood of the ship in which thescrew works?

_A. _--A great frame of malleable iron, the size of the hole, is first setup, and the plating of the ship is brought to the edge of this hole, and isriveted through the frame. It is important to secure this frame very firmlyto the rest of the ship, with which view it is advisable to form a greatpalm, like the palm of a vice, on its inner superior corner, which, projecting into the ship, may be secured by breast-hook plates to thesides, whereby the strain which the screw causes will be distributed overthe stern, instead of being concentrated on the rivets of the frame. 

479. _Q. _--Are there several lengths of screw shaft?

_A. _--There are. 

480. _Q. _--How then are these secured to one another?

_A. _--The best mode of securing the several lengths of shaft together is byforging the shafts with flanges at the ends, which are connected togetherby bolts, say six strong bolts in each, accurately fitted to the holes. 

[Illustration: Fig 44. End of the Screw Shaft of Correo, showing the modeof receiving the Thrust. A, discs; B, tightening wedge. ]

481. _Q. _--How is the thrust of the shaft usually received?

_A. _--In some cases it is received on a number of metal discs set in a boxcontaining oil; and should one of these discs stick fast from friction, theothers will be free to revolve. This arrangement, which is represented infig. 44, is used pretty extensively and answers the purpose perfectly. Itis of course necessary that the box in which the discs A are set, shall bestrong enough to withstand the thrust which the screw occasions. Anotherarrangement still more generally used, is that represented in figs. 55 and56, p. 331. It is a good practice to make the thrust plummer block with avery long sole in the direction of the shaft, so as to obviate any risk ofcanting or springing forward when the strain is applied, as such acircumstance, if occurring even to a slight extent, would be very likely tocause the bearing to heat. 

482. _Q. _--Are there not arrangements existing in some vessels for enablingthe screw to be lifted out of the water while the vessel is at sea?

_A. _--There are; but such arrangements are not usual in merchant vessels. In one form of apparatus the screw is set on a short shaft in the middle ofa sliding frame, which can be raised or lowered in grooves like a windowand the screw shaft within the ship can be protruded or withdrawn byappropriate mechanism, so as to engage or leave free this short shaft asmay be required. When the screw has to be lifted, the screw shaft is drawninto the vessel, leaving the short shaft free to be raised up by thesliding frame, and the frame is raised by long screws turned round by awinch purchase on deck. A chain or rope, however, is better for the purposeof raising this frame, than long screws; but the frame should in such casebe provided with pall catches like those of a windlass, which, if the ropeshould break, will prevent the screw from falling. 

DETAILS OF THE PADDLES AND PADDLE SHAFT. 

483. _Q. _--What are the most important details of the construction ofpaddle wheels?

_A. _--The structure of the feathering wheel will be hereafter described inconnection with an account of the oscillating engine; and it will beexpedient now to restrict any account of the details to the common radialpaddle, as applied to ocean steamers. The best plan of making the paddlecentres is with square eyes, and each centre should be secured in its placeby means of eight thick keys. The shaft should be burred up against thehead of these keys with a chisel, so as to prevent the keys from comingback of their own accord. If the keys are wanted to be driven back, thisburr must be cut off, and if made thick, and of the right taper, they maythen be started without difficulty. The shaft must of course be forged withsquare projections on it, so as to be suitable for the application ofcentres with square eyes. Messrs. Maudslay & Co. Bore out their paddlecentres, and turn a seat for them on the shaft, afterward fixing them onthe shaft with a single key. This plan is objectionable for the tworeasons, that it is insecure when new, and when old is irremovable. Thegeneral practice among the London engineers is to fix the paddle arms atthe centre to a plate by means of bolts, a projection being placed upon theplates on each side of the arm, to prevent lateral motion; but this methodis inferior in durability to that adopted in the Clyde, in which each armis fitted into a socket by means of a cutter--a small hole being leftopposite to the end of each arm, whereby the arm may be forced back by adrift. 

484. _Q. _--How are the arms attached to the outside rings?

_A. _--Some engineers join the paddle arms to the outer ring by means ofbolts; but unless very carefully fitted, those bolts after a time becomeslack sideways, and a constant working of the parts of the wheel goes on inconsequence. Sometimes the part of the other ring opposite the arm isformed into a mortise, and the arms are wedged tight in these holes bywedges driven in on each side; but the plan is an expensive one, and notsatisfactory, as the wedges work loose even though riveted over at thepoint. The best mode of making a secure attachment of the arms to the ring, consists in making the arms with long T heads, and riveting the cross pieceto the outer ring with a number of rivets, not of the largest size, whichwould weaken the outer ring too much. The best way of securing the innerrings to the arms is by means of lugs welded on the arms, and to which therings are riveted. 

485. _Q. _--What are the scantlings of the paddle floats?

_A. _--The paddle floats are usually made either of elm or pine; if of theformer, the common thickness for large sea-going vessels is about 2-1/2inches; if of the latter, 3 inches. The floats should have plates on bothsides, else the paddle arms will be very liable to cut into the wood, andthe iron of the arms will be very rapidly wasted. When the floats have beenfresh put on they must be screwed up several times before they come to abearing. If this be not done, the bolts will be sure to get slack at sea, and all the floats on the weather side may be washed off. The bolts forholding on the paddle floats are made extra strong, on account of thecorrosion to which they are subject; and the nuts should be made large, andshould be square, so that they may be effectually tightened up, even thoughtheir corners be worn away by corrosion. It is a good plan to give thethread of the paddle bolts a nick with a chisel, after the nut has beenscrewed up, which will prevent the nut from turning back. Paddle floats, when consisting of more than one board, should be bolted together edgeways, by means of bolts running through their whole breadth. The floats shouldnot be notched to allow of their projection beyond the outer ring, as, ifthe sides of the notch be in contact with the outer ring, the ring is sooneaten away in that part, and the projecting part of the float, beingunsupported, is liable to be broken off. 

486. _Q. _--Do not the wheels jolt sideways when the vessel rolls?

_A. _--It is usual to put a steel plate at each end of the paddle shaftstightened with a key, to prevent end play when the vessel rolls, but thearrangement is precarious and insufficient. Messrs. Maudslay make theirpaddle shaft bearings with very large fillets in the corner, with the viewof diminishing the evil; but it would be preferable to make the bearings ofthe crank shafts spheroidal; and, indeed, it would probably be animprovement if most of the bearings about the engine were to be made in thesame fashion. The loose end of the crank pin should be made not spheroidal, but consisting of a portion of a sphere; and a brass bush might then befitted into the crank eye, that would completely encase the ball of thepin, and yet permit the outer end of the paddle shaft to fall withoutstraining the pin, the bush being at the same time susceptible of a slightend motion. The paddle shaft, where it passes through the vessel's side, isusually surrounded by a lead stuffing box, which will yield if the end ofthe shaft falls; this stuffing box prevents leakage into the ship from thepaddle wheels: but it is expedient, as a further precaution, to have asmall tank on the ship's side immediately beneath the stuffing box, with apipe leading down to the bilge to catch and conduct away any water that mayenter around the shaft. 

487. _Q. _--How is the outer bearing of the paddle wheels supplied withtallow?

_A. _--The bearing at the outer end of the paddle shaft is sometimessupplied with tallow, forced into a hole in the plummer block cover, as inthe case of water wheels; but for vessels intended to perform long voyages, it is preferable to have a pipe leading down to the oil cup above thejournal from the top of the paddle box, through which pipe oil may at anytime be supplied. 

488. _Q. _--Will you explain the method of putting engines into a steamvessel?

_A. _--As an illustration of this operation it may be advisable to take thecase of a side lever engine, and the method of proceeding is as follows:--First measure across from the inside of paddle bearers to the centre of theship, to make sure that the central line, running in a fore and aftdirection on the deck or beams, usually drawn by the carpenter, is reallyin the centre. Stretch a line across between the paddle bearers in thedirection of the shaft; to this line, in the centre of the ship where thefore and aft mark has been made, apply a square with arms six or eight feetlong, and bring a line stretched perpendicularly from the deck to thekeelson, accurately to the edge of the square: the lower point of the linewhere it touches the keelson will be immediately beneath the marks madeupon the deck. If this point does not come in the centre of the keelson, itwill be better to shift it a little, so as to bring it to the centre, altering the mark upon the deck correspondingly, provided either paddleshaft will admit of this being done--one of the paddle brackets beingpacked behind with wood, to give it an additional projection from the sideof the paddle bearer. Continue the line fore and aft upon the keelson asnearly as can be judged in the centre of the ship; stretch another linefore and aft through the mark upon the deck, and look it out of windingwith the line upon the keelson. Fix upon any two points equally distantfrom the centre, in the line stretched transversely in the direction of theshaft; and from those points, as centres, and with any convenient radius, sweep across the fore and aft line to see that the two are at right angles;and, if not, shift the transverse line a little to make them so. From thetransverse line next let fall a line upon each outside keelson, bringingthe edge of the square to the line, the other edge resting on the keelson. A point will thus be got on each outside keelson, perpendicularly beneaththe transverse line running in the direction of the shaft, and a line drawnbetween those two points will be directly below the shaft. To this line theline of the shaft marked on the sole plate has to be brought, care beingtaken, at the same time, that the right distance is preserved between thefore and aft line upon the sole plate, and the fore and aft line upon thecentral keelson. 

489. _Q. _--Of course the keelsons have first to be properly prepared?

_A. _--In a wooden vessel, before any part of the machinery is put in, thekeelsons should be dubbed fair and straight, and be looked out of windingby means of two straight edges. The art of placing engines in a ship ismore a piece of plain common sense than any other feat in engineering, andevery man of intelligence may easily settle a method of procedure forhimself. Plumb lines and spirit levels, it is obvious, cannot be employedon board a vessel, and the problem consists in so placing the sole plates, without these aids, that the paddle shaft will not stand awry across thevessel, nor be carried forward beyond its place by the framing shoulderingup more than was expected. As a plumb line cannot be used, recourse must behad to a square; and it will signify nothing at what angle with the deckthe keelsons run, so long as the line of the shaft across the keelsons issquare down from the shaft centre. The sole plates being fixed, there is nodifficulty in setting the other parts of the engine in their proper placesupon them. The paddle wheels must be hung from the top of the paddle box toenable the shaft to be rove through them, and the cross stays between theengines should be fixed in when the vessel is afloat. To try whether theshafts are in a line, turn the paddle wheels, and try if the distancebetween the cranks is the same at the upper and under, and the twohorizontal centres; if not, move the end of the paddle shaft up or down, backward or forward, until the distance between the cranks at all the fourcentres is the same. 

490. _Q. _--In what manner are the engines of a steam vessel secured to thehull?

_A. _--The engines of a steamer are secured to the hull by means of boltscalled holding down bolts, and in wooden vessels a good deal of trouble iscaused by these bolts, which are generally made of iron. Sometimes they gothrough the bottom of the ship, and at other times they merely go throughthe keelson, --a recess being made in the floor or timbers to admit of theintroduction of a nut. The iron, however, wears rapidly away in both cases, even though the bolts are tinned; and it has been found the preferablemethod to make such of the bolts as pass through the bottom, or enter thebilge, of Muntz's metal, or of copper. In a side lever engine, four Muntz'smetal bolts may be put through the bottom at the crank end of the framingof each engine, four more at the main centre, and four more at thecylinder, making twelve through bolts to each engine; and it is moreconvenient to make these bolts with a nut at each end, as in that case thebolts may be dropped down from the inside, and the necessity is obviated ofputting the vessel on very high blocks in the dock, in order to give roomto put the bolts up from the bottom. The remainder of the holding downbolts may be of iron, and may, by means of a square neck, be screwed intothe timber of the keelsons as wood screws--the upper part being furnishedwith a nut which may be screwed down upon the sole plate, so soon as thewood screw portion is in its place. If the cylinder be a fixed one itshould be bolted down to the sole plate by as many bolts as are employed toattach the cylinder cover, and they should be of copper or brass, in anysituation that is not easily accessible. 

491. _Q. _--If the engines become loose, how do you refix them?

_A. _--It is difficult to fix engines effectually which have once begun towork in the ship, for in time the surface of the keelsons on which theengines bear becomes worn uneven, and the engines necessarily rock upon it. As a general rule, the bolts attaching the engines to the keelsons are toofew and of too large a diameter: it would be preferable to have smallerbolts, and a greater number of them. In addition to the bolts going throughthe keelsons or the vessel's bottom, there should be a large number of woodscrews securing the sole plate to the keelson, and a large number of boltssecuring the various parts of the engine to the sole plate. In ironvessels, holding down bolts passing through the bottom are not expedient;and there the engine has merely to be secured to the iron plate of thekeelsons, which are made hollow to admit of a more effectual attachment. 

492. _Q. _--What are the proper proportions of bolts?

_A. _--In well formed bolts, the spiral groove penetrates about one twelfthof the diameter of the cylinder round which it winds, so that the diameterof the solid cylinder which remains is five sixths of the diameter over thethread. If the strain to which iron may be safely subjected in machinery isone fifteenth of its utmost strength, or 4, 000 lbs. On the square inch, then 2, 180 lbs. May be sustained by a screw an inch in diameter, at theoutside of the threads. The strength of the holding down bolts may easilybe computed, when the elevating force of the piston or main centre isknown; but it is expedient very much to exceed this strength in practice, on account of the elasticity of the keelsons, the liability to corrosion, and other causes. 

THE LOCOMOTIVE ENGINE. 

493. _Q. _--What is the amount of tractive force requisite to draw carriageson railways?

_A. _--Upon well formed railways with carriages of good construction, theaverage tractive force required for low speeds is about 7-1/2 lbs. Per ton, or 1/300th of the load, though in some experimental cases, where particularcare was taken to obtain a favorable result, the tractive force has beenreduced as low as 1/500th of the load. At low speeds the whole of thetractive force is expended in overcoming the friction, which is made uppartly of the friction of attrition in the axles, and partly of the rollingfriction, or the obstruction to the rolling of the wheels upon the rail. The rolling friction is very small when the surfaces are smooth, and in thecase of railway carriages does not exceed 1/1000th. Of the load; whereasthe draught on common roads of good construction, which is chiefly made upof the rolling friction, is as much as 1/36th of the load. 

494. _Q. _--In reference to friction you have already stated that thefriction of iron sliding upon brass, which has been oiled and then wipeddry, so that no film of oil is interposed, is about 1/11th of the pressure, but that in machines in actual operation, where there is a film of oilbetween the rubbing surfaces, the friction is only about one third of thisamount, or 1/33d of the weight. How then can the tractive resistance oflocomotives at low speeds, which you say is entirely made up of friction, be so little as 1/500th. Of the weight?

_A. _--I did not state that the resistance to traction was 1/500th of theweight upon an average--to which condition the answer given to a previousquestion must be understood to apply--but I stated that the averagetraction was about 1/300th of the load, which nearly agrees with my formerstatement. If the total friction be 1/300th of the load, and the rollingfriction be 1/1000th of the load, then the friction of attrition must be1/429th of the load; and if the diameter of the wheels be 36 in. , and thediameter of the axles be 3 in. , which are common proportions, the frictionof attrition must be increased in the proportion of 36 to 3, or 12 times, to represent the friction of the rubbing surface when moving with thevelocity of the carriage, 12/429ths are about 1/35th of the load, whichdoes not differ much from the proportion of 1/33d as previously determined. 

495. _Q. _--What is the amount of adhesion of the wheels upon the rails?

_A. _--The adhesion of the wheels upon the rails is about 1/5th of theweight when the rails are clean, or either perfectly wet or perfectly dry;but when the rails are half wet or greasy, the adhesion is not more than1/10th or 1/12th of the weight or pressure upon the wheels. The weight of alocomotive of modern construction varies from 20 to 25 tons. 

496. _Q. _--And what is its cost and average performance?

_A. _--The cost of a common narrow gauge locomotive, of average power, varies from £1, 900 to £2, 200; it will run on an average 130 miles per day, at a cost for repairs of 2-1/2d. Per mile; and the cost of locomotivepower, including repairs, wages, oil, and coke, does not much exceed 6d. Per mile run, on economically managed railways. This does not include asinking fund for the renewal of the engines when worn out, which may betaken as equivalent to 10 per cent. On their original cost. 

497. _Q. _--Does the expense of traction increase much with an increasedspeed?

_A. _--Yes; it increases very rapidly, partly from the undulation of theearth when a heavy train passes over it at a high velocity, but chieflyfrom the resistance of the atmosphere and blast pipe, which constitute thegreatest of the impediments to motion at high speeds. At a speed of 30miles an hour, the atmospheric resistance has been found in some cases toamount to about 12 lbs. A ton; and in side winds the resistance evenexceeds this amount, partly in consequence of the additional frictioncaused from the flanges of the wheels being forced against the rails, andpartly because the wind catches to a certain extent the front of everycarriage, whereby the efficient breadth of each carriage, in giving motionto the air in the direction of the train, is very much increased. At aspeed of 30 miles an hour, an engine evaporating 200 cubic feet of water inthe hour, and therefore exerting about 200 horses power, will draw a loadof 110 tons. Taking the friction of the train at 7-1/2 lbs. Per ton, or 825lbs. Operating at the circumference of the driving wheel--which, with 5 ft. 6 in. Wheels, and 18 in. Stroke, is equivalent to 4, 757 lbs. Upon thepiston--and taking the resistance of the blast pipe at 6 lbs. Per squareinch of the pistons, and the friction of the engine unloaded at 1 lb. Persquare inch, which, with pistons 12 in. In diameter, amount together to1, 582 lbs. , and reckoning the increased friction of the engine due to theload at 1/7th of the load, as in some cases it has been foundexperimentally to be, though a much less proportion than this wouldprobably be a nearer average, we have 7018. 4 lbs. For the total load uponthe pistons. At 30 miles an hour the speed of the pistons will be 457. 8feet per minute, and 7018. 4 lbs. Multiplied by 457. 8 ft. Per minute, areequal to 3213023. 5 lbs. Raised one foot high in the minute, which, dividedby 33, 000, gives 97. 3 horses power as the power which would draw 110 tonsupon a railway at a speed of 30 miles an hour, if there were no atmosphericresistance. The atmospheric resistance is at the rate of 12 lbs. A ton, with a load of 110 tons, equal to 1, 320 lbs. , moving at a speed of 30 milesan hour, which, when reduced, becomes 105. 8 horses power, and this, addedto 97. 3, makes 203. 1, instead of 200 horses power, as ascertained by areference to the evaporative power of the boiler. This amount ofatmospheric resistance, however, exceeds the average, and in some of theexperiments for ascertaining the atmospheric resistance, a part of theresistance due to the curves and irregularities of the line has beencounted as part of the atmospheric resistance. 

498. _Q. _--Is the resistance per ton of the engine the same as theresistance per ton of the train?

_A. _--No; it is more, since the engine has not merely the resistance of theatmosphere and of the wheels to encounter, but the resistance of themachinery besides. According to Mr. Gooch's experiments upon a trainweighing 100 tons, the resistance of the engine and tender at 13. 1 milesper hour was found by the indicator to be 12. 38 lbs. ; the resistance perton of the train, as ascertained by the dynamometer, was at the same speed7. 58 lbs. , and the average resistance of locomotive and train was 9. 04 lbs. At 20. 2 miles per hour these resistances respectively became 19. 0, 8. 19, and 12. 2 lbs. At 441 miles per hour the resistances became 34. 0, 21. 10, and25. 5 lbs. , and at 57. 4 miles an hour they became 35. 5, 17. 81, and 23. 8 lbs. 

499. _Q. _--Is it not maintained that the resistance of the atmosphere tothe progress of railway trains increases as the square of the velocity?

_A. _--The atmospheric resistance, no doubt, increases as the square of thevelocity, and the power, therefore, necessary to overcome it will increaseas the cube of the velocity, since in doubling the speed four times, thepower must be expended in overcoming the atmospheric resistance in half thetime. At low speeds, the resistance does not increase very rapidly; but athigh speeds, as the rapid increase in the atmospheric resistance causes themain resistance to be that arising from the atmosphere, the totalresistance will vary nearly as the square of the velocity. Thus theresistance of a train, including locomotive and tender, will, at 15 milesan hour, be about 9. 3 lbs. Per ton; at 30 miles an hour it will be 13. 2lbs. Per ton; and at 60 miles an hour, 29 lbs. Per ton. If we suppose thesame law of progression to continue up to 120 miles an hour, the resistanceat that speed will be 92. 2 lbs. Per ton, and at 240 miles an hour theresistance will be 344. 8 lbs. Per ton. Thus, in doubling the speed from 60to 120 miles per hour, the resistance does not fall much short of beingincreased fourfold, and the same remark applies to the increase of thespeed from 120 to 240 miles an hour. These deductions and other deductionsfrom Mr. Gooch's experiments on the resistance of railway trains, are fullydiscussed by Mr. Clark, in his Treatise on railway machinery, who gives thefollowing rule for ascertaining the resistance of a train, supposing theline to be in good order, and free from curves:--To find the totalresistance of the engine, tender, and train in pounds per ton, at any givenspeed. Square the speed in miles per hour; divide it by 171, and add 8 tothe quotient. The result is the total resistance at the rails in lbs. Perton. 

500. _Q. _--How comes it, that the resistance of fluids increases as thesquare of the velocity, instead of the velocity simply?

_A. _--Because the height necessary to generate the velocity with which themoving object strikes the fluid, or the fluid strikes the object, increasesas the _square_ of the velocity, and the resistance or the weight of acolumn of any fluid varies as the height. A falling body, as has beenalready explained, to have acquired twice the velocity, must have fallenthrough four times the height; the velocity generated by a column of anyfluid is equal to that acquired by a body falling through the height of thecolumn; and it is therefore clear, that the pressure due to any givenvelocity must be as the square of that velocity, the pressure being inevery case as twice the altitude of the column. The work done, however, bya stream of air or other fluid in a given time, will vary as the cube ofthe velocity; for if the velocity of a stream of air be doubled, there willnot only be four times the pressure exerted per square foot, but twice thequantity of air will be employed; and in windmills, accordingly, it isfound, that the work done varies nearly as the cube of the velocity of thewind. If, however, the work done by _a given quantity_ of air moving atdifferent speeds be considered, it will vary as the squares of the speeds. 

501. _Q. _--But in a case where there is no work done, and the resistancevaries as the square of the speed, should not the power requisite toovercome that resistance vary as the square of the speed?

_A. _--It should if you consider the resistance over a given distance, andnot the resistance during a given time. Supposing the resistance of arailway train to increase as the square of the speed, it would take fourtimes the power, so far as atmospheric resistance is concerned, toaccomplish a mile at the rate of 60 miles an hour, that it would take toaccomplish a mile at 30 miles an hour; but in the former case there wouldbe twice the number of miles accomplished in the same time, so that whenthe velocity of the train was doubled, we should require an engine that wascapable of overcoming four times the resistance at twice the speed, or inother words, that was capable of exerting eight times the power, so far asregards the element of atmospheric resistance. We know by experience, however, that it is easier to attain high speeds on railways than in steamvessels, where the resistance does increase nearly as the square of thespeed. 

502. _Q. _--Will you describe generally the arrangement of a locomotiveengine?

_A. _--The boiler and engine are hung upon a framework set on wheels, and, together with this frame or carriage, constitute what is commonly calledthe locomotive. Behind the locomotive runs another carriage, called thetender, for holding coke and water. A common mode of connecting the engineand tender is by means of a rigid bar, with an eye at each end throughwhich pins are passed. Between the engine and tender, however, buffersshould always be interposed, as their pressure contributes greatly toprevent oscillation and other irregular motions of the engine. 

503. _Q. _--How is the framing of a locomotive usually constructed?

_A. _--All locomotives are now made with the framing which supports themachinery situated within the wheels; but for some years a vehementcontroversy was maintained respecting the relative merits of outside andinside framing, which has terminated, however, in the universal adoption ofthe inside framing. It is difficult, in engines intended for the narrowgauge, to get cylinders within the framing of sufficient diameter to meetthe exigencies of railway locomotion; by casting both cylinders in a piece, however, a considerable amount of room may be made available to increasetheir diameters. It is very desirable that the cylinders of locomotivesshould be as large as possible, so that expansion may be adopted to a largeextent; and with any given speed of piston, the power of an engine eitherto draw heavy loads, or achieve high velocities, will be increased withevery increase of the dimensions of the cylinder. The framing oflocomotives, to which the boiler and machinery are attached, and whichrests upon the springs situated above the axles, is formed generally ofmalleable iron, but in some engines the side frames consist of oak withiron plates riveted on each side. The guard plates are in these casesgenerally of equal length, the frames being curved upward to pass over thedriving axle. Hard cast iron blocks are riveted between the guard plates toserve as guides for the axle bushes. The side frames are connected acrossthe ends, and cross stays are introduced beneath the boiler to stiffen theframe sideways, and prevent the ends of the connecting or eccentric rodsfrom falling down if they should be broken. 

504. _Q. _--What is the nature and arrangement of the springs oflocomotives?

_A. _--The springs are of the ordinary carriage kind, with plates connectedat the centre, and allowed to slide on each other at their ends. The upperplate terminates in two eyes, through each of which passes a pin, whichalso passes through the jaws of the bridle, connected by a double threadedscrew to another bridle, which is jointed to the framing; the centre of thespring rests upon the axle box. Sometimes the springs are placed betweenthe guard plates, and below the framing which rests upon their extremities. One species of springs which has gained a considerable introduction, consists of a number of flat steel plates with a piece of metal or othersubstance interposed between them at the centre, leaving the ends standingapart. It would be preferable, perhaps, to make the plates of a commonspring with different curves, so that the leaves, though in contact at thecentre, would not be in contact with the ends with light loads, but wouldbe brought into contact gradually, as the strain conies on: a spring wouldthus be obtained that was suitable for all loads. 

505. _Q. _--What is the difference between inside and outside cylinderengines?

_A. _--Outside cylinders are so designated when placed upon the outside ofthe framing, with their connecting rods operating upon pins in the drivingwheels; while the inside cylinders are situated within the framing, and theconnecting rods attach themselves to cranks in the driving axle. 

506. _Q. _--Whether are inside or outside cylinder engines to be preferred?

_A. _--A diversity of opinion obtains as to the relative merits of outsideand inside cylinders. The chief objection to outside cylinders is, thatthey occasion a sinuous motion in the engine which is apt to send the trainoff the rails; but this action may be made less perceptible or be remediedaltogether, by placing a weight upon one side of the wheels, the momentumof which will just balance the momentum of the piston and its connections. The sinuous or rocking motion of locomotives is traceable to the arrestedmomentum of the piston and its attachments at every stroke of the engine, and the effect of the pressure thus created will be more operative ininducing oscillation the farther it is exerted from the central line of theengine. If both cylinders were set at right angles in the centre of thecarriage, and the pistons were both attached to a central crank, therewould be no oscillation produced; or the same effect would be realized byplacing one cylinder in the centre of the carriage, and two at the sides--the pistons of the side cylinders moving simultaneously: but it isimpossible to couple the piston of an upright cylinder direct to the axleof a locomotive, without causing the springs to work up and down with everystroke of the engine: and the use of three cylinders, though adopted insome of Stephenson's engines, involves too much complication to be abeneficial innovation. 

507. _Q. _--Whether are four-wheeled or six-wheeled engines preferable?

_A. _--Much controversial ingenuity has been expended upon the question ofthe relative merits of the four and six-wheeled engines; one partymaintaining that four-wheeled engines are most unsafe, and the other thatsix-wheeled engines are unmechanical, and are more likely to occasionaccidents. The four-wheeled engines, however, appear to have been chargedwith faults that do not really attach to them when properly constructed;for it by no means follows that if the axle of a four-wheeled enginebreaks, or even altogether comes away, that the engine must fall down orrun off the line; inasmuch as, if the engine be properly coupled with thetender, it has the tender to sustain it. It is obvious enough, that such aconnection may be made between the tender and the engine, that either thefore or hind axle of the engine may be taken away, and yet the engine willnot fall down, but will be kept up by the support which the tender affords;and the arguments hitherto paraded against the four-wheeled engines are, sofar as regards the question of safety, nothing more than arguments againstthe existence of the suggested connection. It is no doubt the fact, thatlocomotive engines are now becoming too heavy to be capable of being borneon four wheels at high speeds without injury to the rails; but theobjection of damage to the rails applies with at least equal force to mostof the six-wheeled engines hitherto constructed, as in those engines theengineer has the power of putting nearly all the weight upon the drivingwheels; and if the rail be wet or greasy, there is a great temptation toincrease the bite of those wheels by screwing them down more firmly uponthe rails. A greater strain is thus thrown upon the rail than can exist inthe case of any equally heavy four-wheeled engine; and the engine is madevery unsafe, as a pitching motion will inevitably be induced at highspeeds, when an engine is thus poised upon the central driving wheels, andthere will also be more of the rocking or sinuous motion. Locomotives, however, intended to achieve high speeds or to draw heavy loads, are nowgenerally made with eight wheels, and in some cases the driving wheels areplaced at the end of the engine instead of in the middle. 

508. _Q. _--As the question of the locomotive boiler has been alreadydisposed of in discussing the question of boilers in general, it now onlyremains to inquire into the subject of the engine, and we may commence withthe cylinders. Will you state the arrangement and construction of thecylinders of a locomotive and their connections?

_A. _--The cylinders are placed in the same horizontal plane as the axle ofthe driving wheels, and the connecting rod which is attached to the pistonrod engages either a crank in the driving axle or a pin in the drivingwheel, according as the cylinders are inside or outside of the framework. The cylinders are generally made an inch longer than the stroke, or thereis half an inch of clearance at each end of the cylinder, to permit thesprings of the vehicle to act without causing the piston to strike the topor bottom of the cylinder. The thickness of metal of the cylinder ends isusually about a third more than the thickness of the cylinder itself, andboth ends are generally made removable. The priming of the boiler, when itoccurs, is very injurious to the cylinders and valves of locomotives, especially if the water be sandy, as the grit carried over by the steamwears the rubbing surfaces rapidly away. The face of the cylinder on whichthe valve works is raised a little above the metal around it, both tofacilitate the operation of forming the face and with the view of enablingany foreign substance deposited on the face to be pushed aside by the valveinto the less elevated part, where it may lie without occasioning anyfurther disturbance. The valve casing is sometimes cast upon the cylinder, and it is generally covered with a door which may be removed to permit theinspection of the faces. In some valve casings the top as well as the backis removable, which admits of the valve and valve bridle being removed withgreater facility. A cock is placed at each end of locomotive cylinders, toallow the water to be discharged which accumulates in the cylinder frompriming or condensation; and the four cocks of the two cylinders areusually connected together, so that by turning a handle the whole areopened at once. In Stephenson's engines, however, with variable expansion, there is but one cock provided for this purpose, which is on the bottom ofthe valve chest. 

509. _Q. _--What kind of piston is used in locomotives?

_A. _--The variety of pistons employed in locomotives is very great, andsometimes even the more complicated kinds are found to work verysatisfactorily; but, in general, those pistons which consist of a singlering and tongue piece, or of two single rings set one above the other, soas to break joint, are preferable to those which consist of many pieces. InStephenson's pistons the screws were at one time liable to work slack, andthe springs to break. 

510. _Q. _--Will you explain the connection of the piston rod with theconnecting rod?

_A. _--The piston rods of all engines are now generally either case hardenedvery deeply, or are made of steel; and in locomotive engines the diameterof the piston rod is about one seventh of the diameter of the cylinder, andit is formed of tilted steel. The cone of the piston rod, by which it isattached to the piston, is turned the reverse way to that which is adoptedin common engines, with the view of making the cutter more accessible fromthe bottom of the cylinder, which is made to come off like a door. The topof the piston rod is secured with a cutter into a socket with jaws, throughthe holes of which a cross head passes, which is embraced between the jawsby the small end of the connecting rod, while the ends of the cross headmove in guides. Between the piston rod clutch and the guide blocks, thefeed pump rod joins the cross head in some engines. 

511. _Q. _--What kind of guides is employed for the end of the piston rod?

_A. _--The guides are formed of steel plates attached to the framing, between which work the guide blocks, fixed on the ends of the cross head, which have flanges bearing against the inner edges of the guides. Steel orbrass guides are better than iron ones: Stephenson and Hawthorn attachtheir guides at one end to a cross stay, at the other to lugs on thecylinder cover; and they are made stronger in the middle than at the ends. Stout guide rods of steel, encircled by stuffing boxes on the ends of thecross head, would probably be found superior to any other arrangement. Thestuffing boxes might contain conical bushes, cut spirally, in addition tothe packing, and a ring, cut spirally, might be sprung upon the rod andfixed in advance of the stuffing box, with lateral play to wipe the rodbefore entering the stuffing box, to prevent it from being scratched by theadhesion of dust. 

512. _Q. _--Is any provision made for keeping the connecting rod always ofthe same length?

_A. _--In every kind of locomotive it is very desirable that the length ofthe connecting rod should remain invariable, in spite of the wear of thebrasses, for there is a danger of the piston striking against the cover ofthe cylinder if it be shortened, as the clearance is left as small aspossible in order to economize steam. In some engines the strap encirclingthe crank pin is fixed immovably to the connecting rod by dovetailed keys, and a bolt passes through the keys, rod, and strap, to prevent thedovetailed keys from working out. The brass is tightened by a gib andcutter, which is kept from working loose by three pinching screws and across pin or cutter through the point. The effect of this arrangement is tolengthen the rod, but at the cross head end of the rod the elongation isneutralized by making the strap loose, so that in tightening the brass therod is shortened by an amount equal to its elongation at the crank pin end. The tightening here is also effected by a gib and cutter, which is keptfrom working loose by two pinching screws pressing on the side of thecutter. Both journals of the connecting rod are furnished with oil cups, having a small tube in the centre with siphon wicks. The connecting rod isa thick flat bar, with its edges rounded. 

513. _Q. _--How is the cranked axle of locomotives constructed?

_A. _--The cranked axle of locomotives is always made of wrought iron, withtwo cranks forged upon it toward the middle of its length, at a distancefrom each other answerable to the distance between the cylinders. Bossesare made on the axle for the wheels to be keyed upon, and bearings for thesupport of the framing. The axle is usually forged in two pieces, which areafterward welded together. Sometimes the pieces for the cranks are put onseparately, but the cranks so made are liable to give way. In engines withoutside cylinders the axles are made straight-the crank pins being insertedin the naves of the wheels. The bearings to which the connecting rods areattached are made with very large fillets in the corners, so as tostrengthen the axle in that part, and to obviate side play in theconnecting rod. In engines which, have been in use for some time, however, there is generally a good deal of end play in the bearings of the axlesthemselves, and this slackness contributes to make the oscillation of theengine more violent; but this evil may be remedied by making the bearingsspheroidal, whereby end play becomes impossible. 

514. _Q. _--How are the bearings of the axles arranged?

_A. _--The axles bear only against the top of the axle boxes, which aregenerally of brass; but a plate extends underneath the bearing, to preventsand from being thrown upon it. The upper part of the box in most engineshas a reservoir of oil, which is supplied to the journal by tubes withsiphon wicks. Stephenson uses cast iron axle boxes with brasses, and greaseinstead of oil; and the grease is fed upon the journal by the heat of thebearing melting it, whereby it is made to flow down through a hole in thebrass. Any engines constructed with outside bearings have inside bearingsalso, which are supported by longitudinal bars, which serve also in somecases to support the piston guides; these bearings are sometimes made so asnot to touch the shafts unless they break. 

515. _Q. _--How are the eccentrics of a locomotive constructed?

_A. _--In locomotives the body of the eccentric is of cast iron, in insidecylinder engines the eccentrics are set on the axle between the cranks, andthey are put on in two pieces held together by bolts; but in straight axleengines the eccentrics are cast in a piece, and are secured on the shaft bymeans of a key. The eccentric, when in two pieces, is retained at itsproper angle on the shaft by a pinching screw, which is provided with a jamnut to prevent it from working loose. A piece is left out of the eccentricin casting it to allow of the screw being inserted, and the void isafterward filled by inserting a dovetailed piece of metal. Stephenson andHawthorn leave holes in their eccentrics on each side of the central arm, and they apply pinching screws in each of these holes. The method of fixingthe eccentric to the shaft by a pinching screw is scarcely sufficientlysubstantial; and cases are perpetually occurring, when this method ofattachment is adopted, of eccentrics shifting from their place. In themodern engines the eccentrics are forged on the axles. 

516. _Q. _--How are the eccentric straps constructed?

_A. _--The eccentric hoops are generally of wrought iron, as brass hoops arefound liable to break. When formed of malleable iron, one half of the strapis forged with the rod, the other half being secured to it by bolts, nuts, and jam nuts. Pieces of brass are, in some cases, pinned within themalleable iron hoop; but it appears to be preferable to put brasses withinthe hoop to encircle the eccentric, as in the case of any other bearing. When the brass straps are used, the lugs have generally nuts on both sides, so that the length of the eccentric rod may be adjusted by their means tothe proper length; but it is better for the lugs of the hoops to abutagainst the necks of the screws, and, if any adjustment be necessary fromthe wear of the straps, washers can be interposed. In some engines theadjustment is effected by screwing the valve rod, and the cross headthrough which it passes has a nut on either side of it, by which itsposition upon the valve rod is determined. 

517. _Q. _--Will you describe the eccentric rod and valve levers?

_A. _--In the engines in use before the introduction of the link motion, theforks of the eccentric rod were of steel, and the length of the eccentricrod was the distance between the centre of the crank axle and the centre ofthe valve shaft; but in modern engines the use of the link motion isuniversal. The valve lever in locomotives is usually longer than theeccentric lever, to increase the travel of the valve, if levers areemployed; but it is better to connect the valve rod to the link of the linkmotion without the intervention of levers. The pins of the eccentric leverin the old engines used to wear quickly; Stephenson used to put a ferule ofbrass on these pins, which being loose, and acting like a roller, facilitated the throwing in and out of gear, and when worn could easily bereplaced, so that there was no material derangement of the motion of thevalve from play in this situation. 

518. _Q. _--What is the arrangement of a starting lever?

_A. _--The starting lever travels between two iron segments, and can befixed in any desired position. This is done by a small catch or bell crank, jointed to the bottom of the handle at the end of the lever, and coming upby the side of the handle, but pressed out from it by a spring. The smallerarm of this bell crank is jointed to a bolt, which shoots into notches, made in one of the segments between which the lever moves. By pressing thebell crank against the handle of the lever the bolt is withdrawn, and thelever may be shifted to any other point, when, the spring being released, the bolt flies into the nearest notch. 

519. _Q. _--In what way does the starting handle act on the machinery of theengine to set it in motion?

_A. _--Its whole action lies in raising or depressing the link of the linkmotion relatively with the valve rod. If the valve rod be attached to themiddle of the link, the valve will derive no motion from, it at all, andthe engine will stop. If the attachment be slipped to one end of the linkthe engine will go ahead, and if slipped to the other end it will goastern. The starting handle merely achieves this change of position. 

520. _Q. _--Will you explain the operation of setting the valve of alocomotive?

_A. _--In setting the valves of locomotives, place the crank in the positionanswerable to the end of the stroke of the piston, and draw a straightline, representing the centre line of the cylinder, through the centres ofthe crank shaft and crank pin. From the centre of the shaft describe acircle with the diameter equal to the throw of the valve; another circle torepresent the crank shaft; and a third circle to represent the path of thecrank pin. From the centre of the crank shaft, draw a line perpendicular tothe centre line of the cylinder and crank shaft, and draw anotherperpendicular at a distance from the first equal to the amount of the lapand the lead of the valve: the points in which this line intersects thecircle of the eccentric are the points in which the centre of the eccentricshould be placed for the forward and reverse motions. When the eccentricrod is attached directly to the valve, the radius of the eccentric, whichprecedes the crank in its revolution, forms with the crank an obtuse angle;but when, by the intervention of levers, the valve has a motion, opposed tothat of the eccentric rod, the angle contained by the crank and the radiusof the eccentric must be acute, and the eccentric must follow the crank: inother words, with a direct attachment to the valve the eccentric is set_more_ than one fourth of a revolution in advance of the crank, and with anindirect attachment the eccentric is set _less_ than one fourth of a circlebehind the crank. If the valve were without lead or lap the eccentric wouldbe exactly one fourth of a circle in advance of the crank or behind thecrank, according to the nature of the valve connection; but as the valvewould thus cover the port by the amount of the lap and lead, the eccentricmust be set forward so as to open the port to the extent of the lap andlead, and this is effected by the plan just described. 

521. _Q. _--In the event of the eccentrics slipping round upon the shaft, which you stated sometimes happens, is it necessary to perform theoperation of setting the valve as you have just described it?

_A. _--If the eccentrics shift upon the shaft, they may be easily refixed bysetting the valve open the amount of the lead, setting the crank at the endof the stroke, and bringing round the eccentric upon the shaft till theeccentric rod gears with the valve. It would often be troublesome inpractice to get access to the valve for the purpose of setting it, and thismay be dispensed with if the amount of lap on the valve and the length ofthe eccentric rod be known. To this end draw upon a board two straightlines at right angles to one another, and from their point of intersectionas a centre describe two circles, one representing the circle of theeccentric, the other the crank shaft; draw a straight line parallel to oneof the diameters, and distant from it the amount of the lap and the lead:the points in which his parallel intersects the circle of the eccentric arethe positions of the forward and backward eccentrics. Through these pointsdraw straight lines from the centre of the circle, and mark theintersection of these lines with the circle of the crank shaft; measurewith a pair of compasses the chord of the arc intercepted between either ofthese points, and the diameter which is at right angles with the crank, andthe diameters being first marked on the shaft itself, then by transferringwith the compasses the distance found in the diagram, and marking thepoint, the eccentric may at any time be adjusted without difficulty. 

[Illustration: Fig. 45. ]

522. _Q. _--Will you describe the structure and arrangement of the feedpumps of locomotive engines?

_A. _--The feed pumps of locomotives are generally made of brass, but theplungers are sometimes made of iron, and are generally attached to thepiston, cross head, though in Stephenson's engines they are worked by rodsattached to eyes on the eccentric hoops. There is a ball valve, fig. 45, between the pump and the tender, and two usually in the pipe leading fromthe pump to the boiler, besides a cock close to the boiler, by which thepump may be shut off from the boiler in case of any accident to the valves. The ball valves are guided by four branches, which rise vertically, andjoin together at the top in a hemispherical form. The shocks of the ballagainst this cap have in some cases broken it after one week's work, fromthe top of the cage having been flat, and the branches not having had theirjunction at the top properly filleted. These valve guards are attached indifferent ways to the pipes; when one occurs at the junction of two piecesof pipe it has a flange, which along with the flanges of the pipes and thatof the valve seat are held together by a union joint. It is sometimesformed with a thread at the under end, and screwed into the pipe. The ballsare cast hollow to lessen the shock, as well as to save the metal. In somecases where the feed pump plunger has been attached to the cross head, thepiston rod has been bent by the strain; and that must in all cases occur, if the communication between the pump and boiler be closed when the engineis started, and there be no escape valve for the water. 

523. _Q. _--Are none but ball valves used in the feed pump?

_A. _--Spindle valves have in some cases been used instead of ball valves, but they are more subject to derangement; but piston valves, so contrivedas to shut a portion of water in the cage when about to close, might beadopted with a great diminution of the shock. Slide valves might beapplied, and would probably be found preferable to any of the expedients atpresent in use. In all spindle valves opened and shut rapidly, it isadvisable to have the lower surface conical, to take off the shock of thewater; and a large lift of the valve should be prevented, else much of thewater during the return stroke of the pump will flow out before the valveshuts. 

524. _Q. _--At what part of the boiler is the feed water admitted?

_A. _--The feed pipe of most locomotive engines enters the boiler near thebottom and about the middle of its length. In Stephenson's engine the wateris let in at the smoke box end of the boiler, a little below the waterlevel; by this means the heat is more fully extracted from the escapingsmoke, but the arrangement is of questionable applicability to engines ofwhich the steam dome and steam pipe are at the smoke box end, as in thatcase the entering cold water would condense the steam. 

525. _Q. _--How are the pipes connecting the tender and locomotiveconstructed, so as to allow of play between the engine and tender withoutleakage?

_A. _--The pipes connecting the tender with the pumps should allow access tothe valves and free motion to the engine and tender. This end is attainedby the use of ball and socket joints; and, to allow some end play, onepiece of the pipe slides into the other like a telescope, and is kept tightby means of a stuffing box. Any pipe joint between the engine and tendermust be made in this fashion. 

526. _Q. _--Have you any suggestion to make respecting the arrangement ofthe feed pump?

_A. _--It would be a material improvement if a feed pump was to be set inthe tender and worked by means of a small engine, such as that now used insteam vessels for feeding the boilers. The present action of the feed pumpsof locomotives is precarious, as, if the valves leak in the slightestdegree, the steam or boiling water from the boiler will prevent the pumpsfrom drawing. It appears expedient, therefore, that at least one pumpshould be far from the boiler and should be set among the feed water, sothat it will only have to force. If a pump was arranged in the mannersuggested, the boiler could still be fed regularly, though the locomotivewas standing still; but it would be prudent to have the existing pumpsstill wrought in the usual way by the engine, in case of derangement of theother, or in case the pump in the tender might freeze. 

527. _Q. _--Will you explain the construction of locomotive wheels?

_A. _--The wheels of a locomotive are always made of malleable iron. Thedriving wheels are made larger to increase the speed; the bearing wheelsalso are easier on the road when large. In the goods engines the drivingwheels are smaller than in the passenger engines, and are generally coupledtogether. Wheels are made with much variety in their constructive details:sometimes they are made with cast iron naves, with the spokes and rim ofwrought iron; but in the best modern wheels the nave is formed of the endsof the spokes welded together at the centre. When cast iron naves areadopted, the spokes are forged out of flat bars with T-formed heads, andare arranged radially in the founder's mould, the cast iron, when fluid, being poured among them. The ends of the T heads are then welded togetherto constitute the periphery of the wheel or inner tire; and littlewedge-form pieces are inserted where there is any deficiency of iron. Insome cases the arms are hollow, though of wrought iron; the tire of wroughtiron, and the nave of cast iron; and the spokes are turned where they arefitted into the nave, and are secured in their sockets by means of cutters. Hawthorn makes his wheels with cast iron naves and wrought iron rims andarms; but instead of welding the arms together, he makes palms on theirouter end, which are attached by rivets to the rim. These rivets, however, unless very carefully formed, are apt to work loose; and it would probablybe found an improvement if the palms were to be slightly indented into therim, in cases in which the palms do not meet each other at the ends. Whenthe rim is turned it is ready for the tire, which is now made of steel. 

528. _Q. _--How do you find the length of bar necessary for forming a tire?

_A. _--To find the proper length of bar requisite for the formation of ahoop of any given diameter, add the thickness of the bar to the requireddiameter, and the corresponding circumference in the table ofcircumferences of circles is the length of the bar. If the iron be bentedgewise the breadth of the bar must be added to the diameter, for it isthe thickness of the bar measured radially that is to be taken intoconsideration. In the tires of railway wheels, which have a flange on oneedge, it is necessary to add not only the thickness of the tire, but alsotwo thirds of the depth of the flange; generally, however, the tire barsare sent from the forge so curved that the plain edge of the tire isconcave, and the flange edge convex, while the side which is afterward tobe bent into contact with the cylindrical surface of the wheel is a plane. In this case the addition of the diameter of two thirds of the depth of theflange is unnecessary, for the curving of the flange edge has the effect ofincreasing the real length of the bar. When the tire is thus curved, it isonly necessary to add the thickness of the hoop to the diameter, and thento find the circumference from a table; or the same result will be obtainedby multiplying the diameter thus increased by the thickness of the hoop by3. 1416. 

529. _Q. _--How are the tires attached to the wheels?

_A. _--The materials for wheel tires are first swaged separately, and thenwelded together under the heavy hammer at the steel works; after which theyare bent to the circle, welded, and turned to certain gauges. The tire isnow heated to redness in a circular furnace; during the time it is gettinghot, the iron wheel, turned to the right diameter, is bolted down upon aface plate or surface; the tire expands with the heat, and when at a cherryred, it is dropped over the wheel, for which it was previously too small, and it is also hastily bolted down to the surface plate; the whole mass isthen quickly immersed by a swing crane in a tank of water five feet deep, and hauled up and down till nearly cold; the tires are not afterwardtempered. The tire is attached to the rim with rivets having countersunkheads, and the wheel is then fixed on its axle. 

530. _Q. _--Is it necessary to have the whole tire of steel?

_A. _--It is not indispensable that the whole tire should be of steel; but adovetail groove, turned out of the tire at the place where it bears most onthe rail, and fitted with a band of steel, will suffice. This band may beput in in pieces, and the expedient appears to be the best way of repairinga worn tire; but particular care must be taken to attach these pieces verysecurely to the tire by rivets, else in the rapid revolution of the wheelthe steel may be thrown out by the centrifugal force. In aid of suchattachment the steel, after being introduced, is well hammered, whichexpands it sideways until it fills the dovetail groove. 

531. _Q. _--Is any arrangement adopted to facilitate the passage of thelocomotive round curves?

_A. _--The tire is turned somewhat conical, to facilitate the passage of theengine round curves--the diameter of the outer wheel being virtuallyincreased by the centrifugal force of the engine, and that of the innerwheel being correspondingly diminished, whereby the curve is passed withoutthe resistance which would otherwise arise from the inequality of thespaces passed over by wheels of the same diameter fixed upon the same axle. The rails, moreover, are not set quite upright, but are slightly inclinedinward, in consequence of which the wheels must be either conical orslightly dished, to bear fairly upon the rails. One benefit of incliningthe rails in this way, and coning the tires, is that the flange of thewheels is less liable to bear against the sides of the rail, and with thesame view the flanges of all the wheels are made with large fillets in thecorners. Wheels have been placed loose upon the axle, but they have lessstability, and are not now much used. Nevertheless this plan appears to bea good one if properly worked out. 

532. _Q. _--Are any precautions taken to prevent engines from being thrownoff the rails by obstructions left upon the line?

_A_. --In most engines a bar is strongly attached to the front of thecarriage on each side, and projects perpendicularly downward to within ashort distance of the rail, to clear away stones or other obstructions thatmight occasion accidents if the engine ran over them. 

CHAPTER IX. 

STEAM NAVIGATION. 

 * * * * *

RESISTANCE OF VESSELS IN WATER. 

533. _Q. _--How do you determine the resistance encountered by a vesselmoving in water?

_A. _--The resistance experienced by vessels moving in water varies as thesquare of the velocity of their motion, or nearly so; and the powernecessary to impart an increased velocity varies nearly as the cube of suchincreased velocity. To double the velocity of a steam vessel, therefore, will require four times the amount of tractive force, and as thatquadrupled force must act through twice the distance in the same time, anengine capable of exerting eight times the original power will berequired. [1]

534. _Q. _--In the case of a board moving in water in the manner of a paddlefloat, or in the case of moving water impinging on a stationary board, whatwill be the pressure produced by the impact?

_A. _--The pressure produced upon a flat board, by striking water at rightangles to the surface of the board, will be equal to the weight of a columnof water having the surface struck as a base, and for its altitude twicethe height due to the velocity with which the board moves through thewater. If the board strike the water obliquely, the resistance will beless, but no very reliable law has yet been discovered to determine itsamount. 

535. _Q. _--Will not the resistance of a vessel in moving through the waterbe much less than that of a flat board of the area of the cross section?

_A. _--It will be very much less, as is manifest from the comparativelysmall area of paddle board, and the small area of the circle described bythe screw, relatively with the area of the immersed midship section of thevessel. The absolute speed of a vessel, with any given amount of power, will depend very much upon her shape. 

536. _Q. _--In what way is it that the shape of a vessel influences herspeed, since the vessels of the same sectional area must manifestly put inmotion a column of water of the same magnitude, and with the same velocity?

_A. _--A vessel will not strike the water with the same velocity when thebow lines are sharp as when they are otherwise; for a very sharp bow hasthe effect of enabling the vessel to move through a great distance, whilethe particles of water are moved aside but a small distance, or in otherwords, it causes the velocity with which the water is moved to be verysmall relatively with the velocity of the vessel; and as the resistanceincreases as the square of the velocity with which the water is moved, itis conceivable enough in what way a sharp bow may diminish the resistance. 

537. _Q. _--Is the whole power expended in the propulsion of a vesselconsumed in moving aside the water to enable the vessel to pass?

_A. _--By no means; only a portion, and in well-formed vessels only a smallportion, of the power is thus consumed. In the majority of cases, thegreater part of the power is expended in overcoming the friction of thewater upon the bottom of the vessel; and the problem chiefly claimingconsideration is, in what way we may diminish the friction. 

538. _Q. _--Does the resistance produced by this friction increase with thevelocity?

_A. _--It increases nearly as the square of the velocity. At two nauticalmiles per hour, the thrust necessary to overcome the friction varies as the1. 823 power of the velocity; and at eight nautical miles per hour, thethrust necessary to overcome the friction varies as the 1. 713 power of thevelocity. It is hardly proper, perhaps, to call this resistance by the nameof friction; it is partly, perhaps mainly, due to the viscidity or adhesionof the water. 

539. _Q. _--Perhaps at high velocities this resistance may become less?

_A_. --That appears very probable. It may happen that at high velocities theadhesion is overcome, so that the water is dragged off the vessel, and thefriction thereafter follows the law which obtains in the case of solidbodies. But any such conclusion is mere speculation, since no experimentsillustrative of this question have yet been made. 

540. _Q. _--Will a vessel experience more resistance in moving in salt waterthan in moving in fresh?

_A. _--If the immersion be the same in both cases a vessel will experiencemore resistance in moving in salt water than in moving in fresh, on accountof the greater density of salt water; but as the notation is proportionablygreater in the salt water the resistance will be the same with the sameweight carried. 

541. _Q. _--Discarding for the present the subject of friction, and lookingmerely to the question of bow and stern resistance, in what manner shouldthe hull of a vessel be formed so as to make these resistances a minimum?

_A. _--The hull should be so formed that the water, instead of being awaydriven forcibly from the bow, is opened gradually, so that every particleof water may be moved aside slowly at first, and then faster, like the ballof a pendulum, until it reaches the position of the midship frame, at whichpoint it will have come to a state of rest, and then again, like areturning pendulum, vibrate back in the same way, until it comes to rest atthe stern. It is not difficult to describe mechanically the line which thewater should pursue. If an endless web of paper be put into uniform motion, and a pendulum carrying a pencil or brush be hung in front of it, then suchpendulum will trace on the paper the proper water line of the ship, or theline which the water should pursue in order that no power may be lostexcept that which is lost in friction. It is found, however, in practice, that vessels formed with water lines on this principle are not muchsuperior to ordinary vessels in the facility with which they pass throughthe water: and this points to the conclusion that in ordinary vessels ofgood form, the amount of power consumed in overcoming the resistance due tothe wave at the bow and the partial vacuity at the stern is not so great ashas heretofore been supposed, and that, in fact, the main resistance isthat due to the friction. 

[1] This statement supposes that there is no difference of level betweenthe water at the bow and the water at the stern. In the experiments on thesteamer Pelican, the resistance was found to vary, as the 2. 28th power ofthe velocity, but the deviation from the recognized law was imputed to adifference in the level of the water at the bow and stern. 

EXPERIMENTS ON THE RESISTANCE OF VESSELS. 

542. _Q. _--Have experiments been made to determine the resistance whichsteam vessels experience in moving through the waters?

_A. _--Experiments have been made both to determine the relative resistanceof different classes of vessels, and also the absolute resistance in poundsor tons. The first experiments made upon this subject were conducted byMessrs. Boulton and Watt, and they have been numerous, long continued, andcarefully performed. These experiments were made upon paddle vessels. 

543. _Q. _--Will you recount the chief results of these experiments?

_A. _--The purpose of the experiments was to establish a coefficient ofperformance, which with any given class of vessel would enable the speed, which would be obtained with any given power, to be readily predicted. Thiscoefficient was obtained by multiplying the cube of the velocity of thevessels experimented upon, in miles per hour, by the sectional area of theimmersed midship section in square feet, and dividing by the numbers ofnominal horses power, and this coefficient will be large in the proportionof the goodness of the shape of the vessel. 

544. _Q. _--How many experiments were made altogether?

_A. _--There were five different sets of experiments on five differentclasses of vessels. The first set of experiments was made in 1828, upon thevessels Caledonia, Diana, Eclipse, Kingshead, Moordyke, and Eagle-vesselsof a similar form and all with square bilges and flat floors; and theresult was to establish the number 925 as the coefficient of performance ofsuch vessels. The second set of experiments was made upon the superiorvessels Venus, Swiftsure, Dasher, Arrow, Spitfire, Fury, Albion, Queen, Dart, Hawk, Margaret, and Hero-all vessels having flat floors and roundbilges, where the coefficient became 1160. The third set of experiments wasmade upon the vessels Lightning, Meteor, James Watt, Cinderella, NavyMeteor, Crocodile, Watersprite, Thetis, Dolphin, Wizard, Escape, andDragon-all vessels with rising floors and round bilges, and the coefficientof performance was found to be 1430. The fourth set of experiments was madein 1834, upon the vessels Magnet, Dart, Eclipse, Flamer, Firefly, Ferret, and Monarch, when the coefficient of performance was found to be 1580. Thefifth set of experiments was made upon the Red Rover, City of Canterbury, Herne, Queen, and Prince of Wales, and in the case of those vessels thecoefficient rose to 2550. The velocity of any of these vessels, with anypower or sectional area, may be ascertained by multiplying the coefficientof its class by the nominal horse power, dividing by the sectional area insquare feet, and extracting the cube root of the quotient, which will bethe velocity in miles per hour; or the number of nominal horse powerrequisite for the accomplishment of any required speed may be ascertainedby multiplying the cube of the required velocity in miles per hour, by thesectional area in square feet, and dividing by the coefficient: thequotient is the number of nominal horse power requisite to realize thespeed. 

545. _Q. _--Seeing, however, that the nominal power does not represent aninvariable amount of dynamical efficiency, would it not be better to makethe comparison with reference to the actual power?

_A. _--In the whole of the experiments recited, except in the case of one ortwo of the last, the pressure of steam in the boiler varied between 2-3/4lbs. And 4 lbs. Per square inch, and the effective pressure on the pistonvaried between 11 lbs. And 13 lbs. Per square inch, so that the averageratio of the nominal to the actual power may be easily computed; but itwill be preferable to state the nominal power of some of the vessels, andtheir actual power as ascertained by experiment. 

546. _Q. _--Then state this. 

_A. _--Of the Eclipse, the nominal power was 76, and the actual power 144. 4horses; of the Arrow, the nominal power was 60, and the actual 119. 5;Spitfire, nominal 40, actual 64; Fury, nominal 40, actual 65. 6; Albion, nominal 80, actual 135. 4; Dart, nominal 100, actual 152. 4; Hawk, nominal40, actual 73; Hero, nominal 100, actual 171. 4; Meteor, nominal 100, actual160; James Watt, nominal 120, actual 204; Watersprite, nominal 76, actual157. 6; Dolphin, nominal 140, actual 238; Dragon, nominal 80, actual 131;Magnet, nominal 140, actual 238; Dart, nominal 120, actual 237; Flamer, nominal 120, actual 234; Firefly, nominal 52, actual 86. 6; Ferret, nominal52, actual 88; Monarch, nominal 200, actual 378. In the case of swiftvessels of modern construction, such as the Red Rover, Herne, Queen, andPrince of Wales, the coefficient appears to be about 2550; but in thesevessels there is a still greater excess of the actual over the nominalpower than in the case of the vessels previously enumerated, and theincrease in the coefficient is consequent upon the increased pressure ofthe steam in the boiler, as well as the superior form of the ship. Thenominal power of the Red Rover, Herne, and City of Canterbury is, in eachcase, 120 horses, but the actual power of the Red Rover is 294, of theHerne 354, and of the City of Canterbury 306, and in some vessels theexcess is still greater; so that with such variations it becomes necessaryto adopt a coefficient derived from the introduction of the actual insteadof the nominal power. 

547. _Q. _--What will be the average difference between the nominal andactual powers in the several classes of vessels you have mentioned and therespective coefficients when corrected for the actual power?

_A. _--In the first class of vessels experimented upon, the actual powerwas about 1. 6 times greaterthan the nominal power; in the second class, 1. 67 times greater; in thethird class, 1. 7 timesgreater; and in the fourth, 1. 96 times greater; while in such vessels asthe Red Rover and City ofCanterbury, it is 2. 65 times greater; so that if we adopt the actualinstead of the nominal power infixing the coefficients, we shall have 554 as the first coefficient, 694as the second, 832 for thethird, and 806 for the fourth, instead of 925, 1160, 1430, and 1580 aspreviously specified; whilefor such vessels as the Red Rover, Herne, Queen, and Prince of Wales, weshall have 962 instead of2550. These smaller coefficients, then, express the relative merits ofthe different vessels withoutreference to any difference of efficacy in the engines, and it appearspreferable, with such avariable excess of the actual over the nominal power, to employ theminstead of those first referredto. From the circumstance of the third of the new coefficients beinggreater than the fourth, itappears that the superior result in the fourth set of experiments arosealtogether from a greaterexcess of the actual over the nominal power. 

548. _Q. _--These experiments, you have already stated, were all made onpaddle vessels. Have similar coefficients of performance been obtained inthe case of screw vessels?

_A. _--The coefficients of a greater number of screw vessels have beenobtained and recorded, but it would occupy too much time to enumerate themhere. The coefficient of performance of the Fairy is 464. 8; of the Rattler676. 8; and of the Frankfort 792. 3. This coefficient, however, refers tonautical and not to statute miles. If reduced to statute miles for thepurpose of comparison with the previous experiments, the coefficients willrespectively become 703, 1033, and 1212; which indicate that theperformance of screw vessels is equal to the performance of paddle vessels, but some of the superiority of the result may be imputed to the superiorsize of the screw vessels. 

INFLUENCE OF THE SIZE OF VESSELS UPON THEIR SPEED. 

549. _Q. _--Will large vessels attain a greater speed than small, supposingeach to be furnished with the same proportionate power?

_A. _--It is well known that large vessels furnished with the sameproportionate power, will attain a greater speed than small vessels, asappears from the rule usual in yacht races of allowing a certain part ofthe distance to be run to vessels which are of inferior size. The velocityattained by a large vessel will be greater than the velocity attained by asmall vessel of the same mould and the same proportionate power, in theproportion of the square roots of the linear dimensions of the vessels. Avessel therefore with four times the sectional area and four times thepower of a smaller symmetrical vessel, and consequently of twice thelength, will have its speed increased in the proportion of the square rootof 1 to the square root of 2, or 1. 4 times. 

550. _Q. _--Will you further illustrate this doctrine by an example?

_A. _--The screw steamer Fairy, if enlarged to three times the size whileretaining the same form, would have twenty-seven times the capacity, ninetimes the sectional area, and nine times the power. The length of such avessel would be 434 feet; her breadth 63 feet 4-1/2 inches; her draught ofwater 16-1/2 feet; her area of immersed section 729 square feet; and hernominal power 1080 horses. Now as the lengths of the Fairy and of the newvessel are in the proportion of 1 to 3, the speeds will be in theproportion of the square root of 1 to the square root of 3; or, in otherwords, the speed of the large vessel will be 1. 73 times greater than thespeed of the small vessel. If therefore the speed of the Fairy be 13 knots, the speed of the new vessel will be 22. 49 knots, although the proportion ofpower to sectional area, which is supposed to be the measure of theresistance, is in both cases precisely the same. If the speed of the Fairyherself had to be increased to 22. 29 knots, the power would have to beincreased in the proportion of the cube of 13 to the cube of 22. 49, or 5. 2times, which makes the power necessary to propel the Fairy at that speedequal to 624 nominal horses power. 

STRUCTURE AND OPERATION OF PADDLE WHEELS. 

551. _Q. _--Will you describe the configuration and mode of action of thepaddle wheels in general use?

_A. _--There are two kinds of paddle wheels in extensive use, the one beingthe ordinary radial wheel, in which the floats are fixed on arms radiatingfrom the centre; and the other the feathering wheel, in which each float ishung upon a centre, and is so governed by suitable mechanism as to bealways kept in nearly the vertical position. In the radial wheel there issome loss of power from oblique action, whereas in the feathering wheelthere is little or no loss from this cause; but in every kind of paddlethere is a loss of power from the recession of the water from the floatboards, or the _slip_ as it is commonly called; and this loss is thenecessary condition of the resistance for the propulsion of the vesselbeing created in a fluid. The slip is expressed by the difference betweenthe speed of the wheel and the speed of the vessel, and the larger thisdifference is the greater the loss of power from slip must be--theconsumption of steam in the engine being proportionate to the velocity ofthe wheel, and the useful effect being proportionate to the speed of theship. 

552. _Q. _--The resistance necessary for propulsion will not be situated atthe circumference of the wheel?

_A. _--In the feathering wheel, where every part of any one immerged floatmoves forward with the same horizontal velocity, the pressure or resistancemay be supposed to be concentrated in the centre of the float; whereas, inthe common radial wheel this cannot be the case, for as the outer edge ofthe float moves more rapidly than the edge nearest the centre of the wheel, the outer part of the float is the most effectual in propulsion. The pointat which the outer and inner portions of the float just balance one anotherin propelling effect, is called the _centre of pressure_; and if all theresistances were concentrated in this point, they would have the sameeffect as before in resisting the rotation of the wheel. The resistanceupon any one moving float board totally immersed in the water will, whenthe vessel is at rest, obviously vary as the square of its distance fromthe centre of motion--the resistance of a fluid varying with the square ofthe velocity; but, except when the wheel is sunk to the axle or altogetherimmersed in the water, it is impossible, under ordinary circumstances, forone float to be totally immersed without others being immersed partially, whereby the arc described by the extremity of the paddle arm will becomegreater than the arc described by the inner edge of the float; andconsequently the resistance upon any part of the float will increase in ahigher ratio than the square of its distance from the centre of motion--theposition of the centre of pressure being at the same time correspondinglyaffected. In the feathering wheel the position of the centre of pressure ofthe entering and emerging floats is continually changing from the loweredge of the float--where it is when the float is entering or leaving thewater--to the centre of the float, which is its position when the float iswholly immerged; but in the radial wheel the centre of pressure can neverrise so high as the centre of the float. 

553. _Q. _--All this relates to the action of the paddle when the vessel isat rest: will you explain its action when the vessel is in motion?

_A. _--When the wheel of a coach rolls along the ground, any point of itsperiphery describes in the air a curve which is termed a cycloid; any pointwithin the periphery traces a prolate or protracted cycloid, and any pointexterior to the periphery traces a curtate or contracted cycloid--theprolate cycloid partaking more of the nature of a straight line, and thecurtate cycloid more of the nature of a circle. The action of a paddlewheel in the water resembles in this respect that of the wheel of acarriage running along the ground: that point in the radius of the paddleof which the rotative speed is just equal to the velocity of the vesselwill describe a cycloid; points nearer the centre, prolate cycloids, andpoints further from the centre, curtate cycloids. The circle described bythe point whose velocity equals the velocity of the ship, is called the_rolling circle_, and the resistance due to the difference of velocity ofthe rolling circle and centre of pressure is that which operates in thepropulsion of the vessel. The resistance upon any part of the float, therefore, will vary as the square of its distance from the rolling circle, supposing the float to be totally immerged; but, taking into account thegreater length of time during which the extremity of the paddle acts, whereby the resistance will be made greater, we shall not err far inestimating the resistance upon any point at the third power of its distancefrom the rolling circle in the case of light immersions, and the 2. 5 powerin the case of deep immersions. 

554. _Q. _--How is the position of the centre of pressure to be determined?

_A. _--With the foregoing assumption, which accords sufficiently withexperiment to justify its acceptation, the position of the centre ofpressure may be found by the following rule:--from the radius of the wheelsubstract the radius of the rolling circle; to the remainder add the depthof the paddle board, and divide the fourth power of the sum by four timesthe depth; from the cube root of the quotient subtract the differencebetween the radii of the wheel and rolling circle, and the remainder willbe the distance of the centre of pressure from the upper edge of thepaddle. 

555. _Q. _--How do you find the diameter of the rolling circle?

_A. _--The diameter of the rolling circle is very easily found, for we haveonly to divide 5, 280 times the number of miles per hour, by 60 times thenumber of strokes per minute, to get an expression for the circumference ofthe rolling circle, or the following rule may be adopted:--divide 88 timesthe speed of the vessel in statute miles per hour, by 3. 1416 times thenumber of strokes per minute; the quotient will be the diameter in feet ofthe rolling circle. The diameter of the circle in which the centre ofpressure moves or the effective diameter of the wheel being known, and alsothe diameter of the rolling circle, we at once find the excess of thevelocity of the wheel over the vessel. 

556. _Q. _--Will you illustrate these rules by an example?

_A. _--A steam vessel of moderately good shape, and with engines of 200horses power, realises, with 22 strokes per minute, a speed of 10. 62 milesper hour. To find the diameter of the rolling circle, we have 88 times10. 62, equal to 934. 66, and 22 times 3. 1416, equal to 69. 1152; then 934. 66divided by 69. 1152 is equal to 13. 52 feet, which is the diameter of therolling circle. The diameter of the wheel is 19 ft. 4 in. , so that thediameter of the rolling circle is about 2/3ds of the diameter of the wheel, and this is a frequent proportion. The depth of the paddle board is 2 feet, and the difference between the diameters of the wheel and rolling circlewill be 5. 8133, which will make the difference of their radii 2. 9067; andadding to this the depth of the paddle board, we have 4. 9067, the fourthpower of which is 579. 64, which, divided by four times the depth of thepaddle board, gives us 72. 455, the cube root of which is 4. 1689, which, diminished by the difference of the radii of the wheel and rolling circle, leaves 1. 2622 feet for the distance of the centre of pressure from theupper edge of the paddle board in the case of light immersions. The radiusof the wheel being 9. 6667, the distance from the centre of the wheel to theupper edge of the float is 7. 6667, and adding to this 1. 2622, we get 8. 9299feet as the radius, or 17. 8598 feet as the diameter of the circle in whichthe centre of pressure revolves. With 22 strokes per minute, the velocityof the centre of pressure will be 20. 573 feet per second, and with 10. 62miles per hour for the speed of the vessel, the velocity of the rollingcircle will be 15. 576 feet per second. The effective velocity will be thedifference between these quantities, or 4. 997 feet per second. Now theheight from which a body must fall by gravity, to acquire a velocity of4. 997 feet per second, is about . 62 feet; and twice this height, or 1. 24feet, multiplied by 62-1/2, which is the number of Lbs. Weight in a cubicfoot of water, gives 77-1/2 Lbs. As the pressure on each square foot of thevertical paddle boards. As each board is of 20 square feet of area, andthere is a vertical board on each side of the ship, the total pressure onthe vertical paddle boards will be 2900 Lbs. 

557. _Q. _--What pressure is this equivalent to on each square inch of thepistons?

_A. _--A vessel of 200 horses power will have two cylinders, each 50 inchesdiameter, and 5 feet stroke, or thereabout. The area of a piston of 50inches diameter is 1963. 5 square inches, so that the area of the twopistons is 3927 square inches, and the piston will move through 10 feetevery revolution; and with 22 strokes per minute this will be 220 feet perminute, or 3. 66 feet per second. Now, if the effective velocity of thecentre of pressure and the velocity of the pistons had been the same, thena pressure of 2900 Lbs. Upon the vertical paddles would have been balancedby an equal pressure on the pistons, which would have been in this caseabout . 75 Lbs. Per square inch; but as the effective velocity of the centreof pressure is 4. 997 feet per second, while that of the pistons is only3. 66 feet per second, the pressure must be increased in the proportion of4. 997 to 3. 66 to establish an equilibrium of pressure, or, in other words, it must be 1. 02 Lbs. Per square inch. It follows from this investigation, that, in radial wheels, the greater part of the engine power is distributedamong the oblique floats. 

558. _Q. _--How comes this to be the case?

_A. _--To understand how it happens that more power is expended upon theoblique than upon the vertical floats, it is necessary to remember that theonly resistance upon the vertical paddle is that due to the difference ofvelocity of the wheel and the ship; but if the wheel be supposed to beimmersed to its axle, so that the entering float strikes the waterhorizontally, it is clear that the resistance on such float is that due tothe whole velocity of rotation; and that the resistance to the enteringfloat will be the same whether the vessel is in motion or not. Theresistance opposed to the rotation of any float increases from the positionof the vertical float-where the resistance is that due to the difference ofvelocity of the wheel and vessel--until it reaches the plane of the axis, supposing the wheel to be immersed so far, where the resistance is that dueto the whole velocity of rotation; and although in any oblique float thetotal resistance cannot be considered operative in a horizontal direction, yet the total resistance increases so rapidly on each side of the verticalfloat, that the portion of it which is operative in the horizontaldirection, is in all ordinary cases of immersion very considerable. In thefeathering wheel, where there is little of this oblique action, theresistance will be in the proportion of the square of the horizontalvelocities of the several floats, which may be represented by thehorizontal distances between them; and in the feathering wheel, thevertical float having the greatest horizontal velocity will have thegreatest propelling effect. 

559. _Q. _--Should the floats in feathering wheels enter and leave the watervertically?

_A. _--The floats should be so governed by the central crank or eccentric, that the entering and emerging floats have a direction intermediate betweena radius and a vertical line. 

560. _Q. _--Can you give any practical rules for proportioning paddlewheels?

_A. _--A common rule for the pitch of the floats is to allow one float forevery foot of diameter of the wheel; but in the case of fast vessels apitch of 2-1/2 feet, or even less, appears preferable, as a close pitchoccasions less vibration. If the floats be put too close, however, thewater will not escape freely from between them, and if set too far apartthe stroke of the entering paddle will occasion an inconvenient amount ofvibratory motion, and there will also be some loss of power. To find theproper area of a single float:--divide the number of actual horses power ofboth engines by the diameter of the wheel in feet; the quotient is the areaof one paddle board in square feet proper for sea going vessels, and thearea multiplied by 0. 6 will give the length of the float in feet. In verysharp vessels, which offer less resistance in passing through the water, the area of paddle board is usually one-fourth less than the aboveproportion, and the proper length of the float may in such case be found bymultiplying the area by 0. 7. In sea going vessels about four floats areusually immersed, and in river steamers only one or two floats. There ismore slip in the latter case, but there is also more engine power exertedin the propulsion of the ship, from the greater speed of engine thusrendered possible. 

561. _Q. _--Then is it beneficial to use small floats?

_A. _--Quite the contrary. If to permit a greater speed of the engine thefloats be diminished in area instead of being raised out of the water, noappreciable accession to the speed of the vessel will be obtained; whereasthere will be an increased speed of vessel if the accelerated speed of theengine be caused by diminishing the diameter of the wheels. In vesselsintended to be fast, therefore, it is expedient to make the wheels small, so as to enable the engine to work with a high velocity; and it isexpedient to make such wheels of the feathering kind, to obviate loss ofpower from oblique action. In no wheel must the rolling circle fall belowthe water line, else the entering and emerging floats will carry masses ofwater before them. The slip is usually equal to about one-fourth of thevelocity of the centre of pressure in well proportioned wheels; but it isdesirable to have the slip as small as is possible consistently with theobservance of other necessary conditions. The speed of the engine and alsothe speed of the vessel being fixed, the diameter of the rolling circlebecomes at once ascertainable, and adding to this the slip, we have thediameter of the wheel. 

CONFIGURATION AND ACTION OF THE SCREW. 

562. _Q. _--Will you describe more in detail than you have yet done, theconfiguration and mode of action of the screw propeller?

_A. _--The ordinary form of screw propeller is represented in figs. 46 and47; fig. 46 being a perspective view, and fig. 47 an end view, or view suchas is seen when looking upon the end of the shaft. The screw hererepresented is one with two arms or blades. Some screws have three arms, some four and some six; but the screw with two arms is the most usual, andscrews with more than three arms are not now much employed in this country. The screw on being put into revolution by the engine, preserves a spiralpath in the water, in which it draws itself forward in the same way as ascrew nail does when turned round in a piece of wood, whereas the paddlewheel more resembles the action of a cog wheel working in a rack. 

[Illustration: Fig. 46. Fig. 47. ORDINARY FORM OF SCREW PROPELLER. ]

563. _Q. _--But the screw of a steam vessel has no resemblance to a screwnail?

_A. _--It has in fact a very close resemblance if you suppose only a veryshort piece of the screw nail to be employed, and if you suppose, moreover, the thread of the screw to be cut nearly into the centre to prevent thewood from stripping. The original screw propellers were made with severalconvolutions of screw, but it was found advantageous to shorten them, untilthey are now only made one-sixth of a convolution in length. 

564. _Q. _--And the pitch you have already explained to be the distance inthe line of the shaft from one convolution to the next, supposing the screwto consist of two or more convolutions?

_A. _--Yes, that is what is meant by the pitch. If a thread be wound upon acylinder with an equal distance between the convolutions, it will trace ascrew of a uniform pitch; and if the thread be wound upon the cylinder withan increasing distance between each convolution, it will trace a screw ofan increasing pitch. But two or more threads may be wound upon the cylinderat the same time, instead of a single thread. If two threads be wound uponit they will trace a double-threaded screw; if three threads be wound uponit they will trace a treble-threaded screw; and so of any other number. Nowif the thread be supposed to be raised up into a very deep and thin spiralfeather, and the cylinder be supposed to become very small, like the newelof a spiral stair, then a screw will be obtained of the kind proper forpropelling vessels, except that only a very short piece of such screw mustbe employed. Whatever be the number of threads wound upon a cylinder, ifthe cylinder be cut across all the threads will be cut. A slice cut out ofthe cylinder will therefore contain a piece of each thread. But thethreads, in the case of a screw propeller, answer to the arms, so that inevery screw propeller the number of threads entering into the compositionof the screw will be the same as the number of arms. An ordinary screw withtwo blades is a short piece of a screw of two threads. 

565. _Q. _--In what part of the ship is the screw usually placed?

[Illustration: Fig. 48]

_A. _--In that part of the run of the ship called the dead wood, which is athin and unused part of the vessel just in advance of the rudder. The usualarrangement is shown in fig. 48, which represents the application to avessel of a species of screw which has the arms bent backwards, tocounteract the centrifugal motion given to the water when there is aconsiderable amount of slip. 

566. _Q. _--How is the slip in a screw vessel determined?

_A. _--By comparing the actual speed of the vessel with the speed due to thepitch and number of revolutions of the screw, or, what is the same thing, the speed which the vessel would attain if the screw worked in a solid nut. The difference between the actual speed and this hypothetical speed, is theslip. 

567. _Q. _--In well formed screw propellers what is the amount of slip foundto be?

_A. _--If the screw be properly proportioned to the resistance that thevessel has to overcome, the slip will not be more than 10 per cent. , but insome cases it amounts to 30 per cent. , or even more than this. In othercases, however, the slip is nothing at all, and even less than nothing; or, in other words the vessel passes through the water with a greater velocitythan if the screw were working in a solid nut. 

568. _Q. _--Then it must be by the aid of the wind or some other extraneousforce?

_A. _--No; by the action of the screw alone. 

569. _Q. _--But how is such a result possible?

_A. _--It appears to be mainly owing to the centrifugal action of the screw, which interposes a film or wedge of water between the screw itself and thewater on which the screw reacts. This negative slip, as it is called, chiefly occurs when the pitch of the screw is less than its diameter, andwhen, consequently, the velocity of rotation is greater than if a coarserpitch had been employed. There is, moreover, in all vessels passing throughthe water with any considerable velocity, a current of water following thevessel, in which current, in the case of a screw vessel, the screw willrevolve; and in certain cases the phenomenon of negative slip may beimputable in part to the existence of this current. 

570. _Q. _--Is the screw propeller as effectual an instrument of propulsionas the radial or feathering paddle?

_A. _--In all cases of deep immersion it appears to be quite as effectual asthe radial paddle, indeed, more so; but it is scarcely as effectual as thefeathering paddle, with any amount of immersion, and scarcely as effectualas the common paddle in the case of light immersions. 

COMPARATIVE ADVANTAGES OF PADDLE AND SCREW VESSELS. 

571. _Q. _--Whether do you consider paddle or screw vessels to be on thewhole the most advantageous?

_A. _--That is a large question, and can only receive a qualified answer. Insome cases the use of paddles is indispensable, as, for example, in thecase of river vessels of a limited draught of water, where it would not bepossible to get sufficient depth below the water surface to enable a screwof a proper diameter to be got in. 

572. _Q. _--But how does the matter stand in the case of ocean vessels?

_A. _--In the case of ocean vessels, it is found that paddle vessels fittedwith the ordinary radial wheels, and screw vessels fitted with the ordinaryscrew, are about equally efficient in calms and in fair or beam winds withlight and medium immersions. If the vessels are loaded deeply, however, asvessels starting on a long voyage and carrying much coal must almostnecessarily be, then the screw has an advantage, since the screw acts inits best manner when deeply immersed, and the paddles in their worst. Whena screw and paddle vessel, however, of the same model and power are set toencounter head winds, the paddle vessel it is found has in all cases anadvantage, not in speed, but in economy of fuel. For whereas in a paddlevessel, when her progress is resisted, the speed of the engine diminishesnearly in the proportion of the diminished speed of ship, it happens thatin a screw vessel this is not so, --at least to an equal extent, --but theengines work with nearly the same rate of speed as if no increase ofresistance had been encountered by the ship. It follows from thiscircumstance, that whereas in paddle vessels the consumption of steam, andtherefore of fuel, per hour is materially diminished when head winds occur, in screw vessels a similar diminution in the consumption of steam and fueldoes not take place. 

573. _Q. _--But perhaps under such circumstances the speed of the screwvessel will be the greater of the two?

_A. _--No; the speed of the two vessels will be the same, unless thestrength of the head wind be so great as to bring the vessels nearly to astate of rest, and on that supposition the screw vessel will have theadvantage. Such cases occur very rarely in practice; and in the case of theordinary resistances imposed by head winds, the speed of the screw andpaddle vessel will be the same, but the screw vessel will consume mostcoals. 

574. _Q. _--What is the cause of this peculiarity?

_A. _--The cause is, that when the screw is so proportioned in its length asto be most suitable for propelling vessels in calms, it is too short to besuitable for propelling vessels which encounter a very heavy resistance. Itfollows, therefore, that if it is prevented from pursuing its spiral coursein the water, it will displace the water to a certain extent laterally, inthe manner it does if the engine be set on when the vessel is at anchor;and a part of the engine power is thus wasted in producing a uselessdisturbance of the water, which in paddle vessels is not expended at all. 

575. _Q. _--If a screw and paddle vessel of the same mould and power be tiedstern to stern, will not the screw vessel preponderate and tow the paddlevessel astern against the whole force of her engines?

_A. _--Yes, that will be so. 

576. _Q. _--And seeing that the vessels are of the same mould and power, sothat neither can derive an advantage from a variation in that condition, does not the preponderance of the screw vessel show that the screw must bethe most powerful propeller?

_A. _---No, it does not. 

577. _Q. _--Seeing that the vessels are the same in all respects except asregards the propellers, and that one of them exhibits a superiority, doesnot this circumstance show that one propeller must be more powerful thanthe other?

_A. _--That does not follow necessarily, nor is it the fact in thisparticular case. All steam vessels when set into motion, will forcethemselves forward with an amount of thrust which, setting aside the lossfrom friction and from other causes, will just balance the pressure on thepistons. In a paddle vessel, as has already been explained, it is easy totell the tractive force exerted at the centre of pressure of the paddlewheels, when the pressure urging the pistons, the dimensions of the wheelsand the speed of the vessel are known; and that force, whatever be itsamount, must always continue the same with any constant pressure on thepistons. In a screw vessel the same law applies, so that with any givenpressure on the pistons and discarding the consideration of friction, itwill follow that whatever be the thrust exerted by a paddle or a screwvessel, it must remain uniform whether the vessel is in motion or at rest, and whether moving at a high or a low velocity through the water. Now toachieve an equal speed during calms in two vessels of the same model, theremust be the same amount of propelling thrust in each; and this thrust, whatever be its amount, cannot afterward vary if a uniform pressure ofsteam be maintained. The thrusts, therefore, caused by their respectivepropelling instruments, when a screw and paddle vessel are tied stern tostern, must be the same as at other times; and as at other times thosethrusts are equal, so must they be when the vessels are set in theantagonism supposed. 

578. _Q. _--How comes it then that the screw vessel preponderates?

_A. _--Not by virtue of a larger thrust exerted by the screw in pressingforward the shaft and with it the vessel, but by the gravitation againstthe stern of the wave of water which the screw raises by its rapidrotation. This wave will only be raised very high when the progress of thevessel through the water is nearly arrested, at which time the centrifugalaction of the screw is very great; and the vessel under such circumstancesis forced forward partly by the thrust of the screw, and partly by thehydrostatic pressure of the protuberance of water which the centrifugalaction of the screw raises up at the stern. 

579. _Q. _--Can you state any facts in corroboration of this view?

_A. _--The screw vessel will not preponderate if a screw and paddle vesselbe tied bow to bow and the engines of each be then reversed. In, some screwvessels the amount of thrust actually exerted by the screw under all itsvarying circumstances, has been ascertained by the application of adynamometer to the end of the shaft. By this instrument--which is formed bya combination of levers like a weighing machine for carts--a thrust orpressure of several tons can be measured by the application of a smallweight; and it has been found, by repeated experiment with the dynamometer, that the thrust of the screw in a screw vessel when towing a paddle vesselagainst the whole force of her engines, is just the same as it is when thetwo vessels are maintaining an equal speed in calms. The preponderance ofthe screw vessel must, therefore, be imputable to some other agency than toa superior thrust of the screw, which is found by experiment not to exist. 

580. _Q. _--Has the dynamometer been applied to paddle vessels?

_A. _--It has not been applied to the vessels themselves, as in the case ofscrew vessels, but it has been employed on shore to ascertain the amount oftractive force that a paddle vessel can exert on a rope. 

581. _Q. _--Have any experiments been made to determine the comparativeperformances of screw and paddle vessels at sea?

_A. _--Yes, numerous experiments; of which the best known are probably thosemade on the screw steamer Rattler and the paddle steamer Alecto, eachvessel of the same model, size, and power, --each vessel being of about 800tons burden and 200 horses power. Subsequently another set of experimentswith the same object was made with the Niger screw steamer and the Basiliskpaddle steamer, both vessels being of about 1000 tons burden and 400 horsespower. The general results which were obtained in the course of theseexperiments are those which have been already recited. 

582. _Q. _--Will you recapitulate some of the main incidents of thesetrials?

_A. _--I may first state some of the chief dimensions of the vessels. TheRattler is 176 feet 6 inches long, 32 feet 8-1/2 inches broad, 888 tonsburden, 200 horses power, and has an area of immersed midship section of380 square feet at a draught of water of 11 feet 5-1/2 inches. The Alectois of the same dimensions in every respect, except that she is only of 800tons burden, the difference in this particular being wholly owing to theRattler having been drawn out about 15 feet at the stern, to leave abundantroom for the application of the screw. The Rattler was fitted with adynamometer, which enabled the actual propelling thrust of the screw shaftto be measured; and the amount of this thrust, multiplied by the distancethrough which the vessel passed in a given time, would determine the amountof power actually utilized in propelling the ship. Both vessels were fittedwith indicators applied to the cylinders, so as to determine the amount ofpower exerted by the engines. 

583. _Q. _--How many trials of the vessels were made on this occasion?

_A. _--Twelve trials in all; but I need not refer to those in which similaror identical results were only repeated. The first trial was made understeam only, the weather was calm and the water smooth. At 54 minutes past 4in the morning both vessels left the Nore, and at 30-1/2 minutes past 2 theRattler stopped her engines in Yarmouth Roads, where in 20-1/2 minutesafterward she was joined by the Alecto. The mean speed achieved by theRattler during this trial was 9. 2 knots per hour; the mean speed of theAlecto was 8. 8 knots per hour. The slip of the screw was 10. 2 per cent. Theactual power exerted by the engines, as shown by the indicator, was in thecase of the Rattler 334. 6 horses, and in the case of the Alecto 281. 2horses; being a difference of 53. 4 horses in favor of the Rattler. Theforward thrust upon the screw shaft was 3 tons, 17 cwt. , 3 qrs. , and 14lbs. The horse power of the shaft--or power actually utilized--ascertainedby multiplying the thrust in pounds by the space passed through by thevessel in feet per minute, and dividing by 33, 000, was 247. 8 horses power. This makes the ratio of the shaft to the engine power as 1 to 1. 3, or, inother words, it shows that the amount of engine power utilized inpropulsion was 77 per cent. In a subsequent trial made with the vesselsrunning before the wind, but with no sails set and the masts struck, thespeed realized by the Rattler was 10 knots per hour. The slip of the screwwas 11. 2 per cent. The actual power exerted by the engines of the Rattlerwas 368. 8 horses. The actual power exerted by the engines of the Alecto was291. 7 horses. The thrust of the shaft was equal to a weight of 4 tons, 4cwt. , 1 qr. , 1 lb. The horse power of the shaft was 290. 2 horses, and theratio of the shaft to the engine power was 1 to 1. 2. Here, therefore, theamount of the engine power utilized was 84 per cent. 

584. _Q. _--If in any screw vessel the power of the engine be diminished byshutting off the steam or otherwise, you will then have a larger screwrelatively with the power of the engine than before?

_A. _--Yes. 

585. _Q. _--Was any experiment made to ascertain the effect of thismodification?

_A. _--There was; but the result was not found to be better than before. Theexperiment was made by shutting off the steam from the engines of theRattler until the number of strokes was reduced to 17 in the minute. Theactual power was then 126. 7 horses; thrust upon the shaft 2 tons, 2 cwt. , 3qrs. , 14 lbs; horse power of shaft 88. 4 horses; ratio of shaft to enginepower 1 to 1. 4; slip of the screw 18. 7 per cent. In this experiment thepower utilized was 71 per cent. 

586. _Q. _--Was any experiment made to determine the relative performancesin head winds?

_A. _--The trial in which this relation was best determined lasted for sevenhours, and was made against a strong head wind and heavy head sea. Thespeed of the Rattler by patent log was 4. 2 knots; and at the conclusion ofthe trial the Alecto had the advantage by about half a mile. Owing to anaccidental injury to the indicator, the power exerted by the engines of theRattler in this trial could not be ascertained; but judging from the powerexerted in other experiments with the same number of revolutions, itappears probable that the power actually exerted by the Rattler was about300 horses. The number of strokes per minute made by the engines of theRattler was 22, whereas in the Alecto the number of strokes per minute wasonly 12; so that while the engines of the Alecto were reduced, by theresistance occasioned by a strong head wind, to nearly half their usualspeed, the engines of the Rattler were only lessened about one twelfth oftheir usual speed. The mean thrust upon the screw shaft during thisexperiment, was 4 tons, 7 cwt. , 0 qr. , 16 lbs. The horse power of the shaftwas 125. 9 horses, and the slip of the screw was 56 per cent. Taking thepower actually exerted by the Rattler at 300 horses, the power utilized inthis experiment is only 42 per cent. 

587. _Q. _--What are the dimensions of the screw in the Rattler?

_A. _--Diameter 10 feet, length 1 foot 3 inches, pitch 11 feet. Theforegoing experiments show that with a larger screw a better averageperformance would be obtained. The best result arrived at, was when thevessel was somewhat assisted by the wind, which is equivalent to areduction of the resistance of the hull, or to a smaller hull, which isonly another expression for a larger proportionate screw. 

588. _Q. _--When you speak of a larger screw, what increase of dimension doyou mean to express?

_A. _--An increase of the diameter. The amount of reacting power of thescrew upon the water is hot measured by the number of square feet ofsurface of the arms, but by the area of the disc or circle in which thescrew revolves. The diameter of the screw of the Rattler being 10 feet, thearea of its disc is 78. 5 square feet; and with the amount of thrust alreadymentioned as existing in the first experiment, viz. 8722 lbs. , the reactingpressure on each square foot of the screw's disc will be 108-1/2 lbs. Theimmersed midship section being 380 square feet, this is equivalent to 23lbs. Per square foot of immersed midship section at a speed of 9. 2 knotsper hour. 

589. _Q. _--In smaller vessels of similar form, will the resistance persquare foot of midship section be more than this?

_A. _--It will be considerably more. In the Pelican, a vessel of 109-3/4square feet of midship section, I estimate the resistance per square footof midship section at 30 lbs. , when the speed of the vessel is 9. 7 knotsper hour. In the Minx with an immersed midship section of 82 square feet, the resistance per square foot of immersed midship section was found by thedynamometer to be 41 lbs. At a speed of 8-1/2 knots; and in the Dwarf, avessel with 60 square feet of midship section, I estimate the resistanceper square foot of midship section at 46 lbs. At a speed of 9 knots perhour, which is just double the resistance per square foot of the Rattler. The diameter of the screw of the Minx is 4-1/2 feet, so that the area ofits disc is 15. 9 square feet, and the area of immersed midship section isabout 5 times greater than that of the screw's disc. The diameter of thescrew of the Dwarf is 5 feet 8 inches, so that the area of its disc is25. 22 square feet, and the area of immersed midship section is 2. 4 timesgreater than that of the screw's disc. The pressure per square foot of thescrew's disc is 214 lbs. In the case of the Minx, and 109-1/2 lbs. In thecase of the Dwarf. 

590. _Q. _--From the greater proportionate resistance of small vessels, willnot they require larger proportionate screws than large vessels?

_A. _--They will. 

591. _Q. _--Is there any ready means of predicting what the amount of thrustof a screw will be?

_A. _--When we know the amount of pressure on the pistons, and the velocityof their motion relatively with the velocity of advance made by the screw, supposing it to work in a solid nut, it is easy to tell what the thrust ofthe screw would be if it were cleared of the effects of friction and otherirregular sources of disturbance. The thrust, in fact, would be at oncefound by the principle of virtual velocities; and if we take thistheoretical thrust and diminish it by one fourth to compensate for frictionand lateral slip, we shall have a near approximation to the amount ofthrust that will be actually exerted. [1]

[1] See Treatise on the Screw Propeller, by J. Bourne, C. E. 

COMPARATIVE ADVANTAGES OF DIFFERENT SCREWS. 

592. _Q. _--What species of screw do you consider the best?

_A. _--In cases in which a large diameter of screw can be employed, theordinary screw or helix with two blades seems to be as effective as anyother, and it is the most easily constructed. If, however, the screw isrestricted in diameter, or if the vessel is required to tow, or will haveto encounter habitually strong head winds, it will be preferable to employa screw with an increasing pitch, and also of such other configuration thatit will recover from the water some portion of the power that has beenexpended in slip. 

593. _Q. _--How can this be done?

_A. _--There are screws which are intended to accomplish, this objectalready in actual use. When there is much slip a centrifugal velocity isgiven to the water, and the screw, indeed, if the engine be set on when thevessel is at rest, acts very much as a centrifugal fan would do if placedin the same situation. The water projected outward by the centrifugal forceescapes in the line of least resistance, which is to the surface; and ifthere be a high column of water over the screw, or, in other words, if thescrew is deeply immersed, then the centrifugal action is resisted to agreater extent, and there will be less slip produced. The easiestexpedient, therefore, for obviating loss by slip is to sink the screwdeeply in the water; but as there are obvious limits to the application ofthis remedy, the next best device is to recover and render available forpropulsion some part of the power which has been expended in giving motionto the water. One device for doing this consists in placing the screw wellforward in the dead wood, so that it shall be overhung by the stern of theship. The water forced upward by the centrifugal action of the screw will, by impinging on the overhanging stern, press the vessel forward in thewater, just in the same way as is done by the wind when acting on anoblique sail. I believe, the two revolving vanes without any twist orobliquity on them at all, would propel a vessel if set well forward in thedead wood or beneath the bottom, merely by the ascent of the water up theinclined plane of the vessel's run; and, at all events, a screw so placedwould, in my judgment, aid materially in propelling the vessel when herprogress was resisted by head winds. 

594. _Q. _--But you said there are some kinds of screws which profess toaccomplish this?

[Illustration: Fig. 49. THE EARL OF DUNDONALD'S PROPELLER. ]

_A. _--There are screws which profess to counteract the centrifugal velocitygiven to the water by imparting to it an equal centripetal force, theconsequence of which will be, that the water projected backward by thescrew, instead of taking the form of the frustum of a cone, with its smallend next the screw, will take the form of a cylinder. One of these forms ofscrew is that patented by the Earl of Dundonald in 1843, and which isrepresented in fig. 49. Another is the form of screw already represented infig. 48, and which was patented by Mr. Hodgson in 1844. Mr. Hodgson bendsthe arms of his propellers backward, not into the form of a triangle, butinto the form of a parabola, to the end that the impact of the screw on theparticles of the water may cause them to converge to a focus, as the raysof light would do in a parabolic reflector. But this particularconfiguration is not important, seeing that the same convergence which isgiven to the particles of the water, with a screw of uniform pitch bentback into the form of a parabola, will be given with a screw bent back intothe form of a triangle, if the pitch be suitably varied between the centreand the circumference. 

595. _Q. _--Then the pitch may be varied in two ways?

_A. _--Yes: a screw may have a pitch increasing in the direction of thelength, as would happen in the case of a spiral stair, if every successivestep in the ascent was thicker than the one below it; or it may increasefrom the centre to the circumference, as would happen in the case of aspiral stair, if every step were thinner at the centre of the lower than atits outer wall. When the pitch of a screw increases in the direction of itslength, the leading edge of the screw enters the water without shock orimpact, as the advance of the leading edge per revolution will not begreater than the advance of the vessel. When the pitch of a screw increasesin the direction of its diameter, the central part of the screw willadvance with only the same velocity as the water, so that it cannotcommunicate any centrifugal velocity to the water; and the whole slip, aswell as the whole propelling pressure, will occur at the outer part of thescrew blades. 

596. _Q. _--Is there any advantage derived from these forms of screws?

_A. _--There is a slight advantage, but it is so slight as hardly to balancethe increased trouble of manufacture, and, consequently, they are notgenerally or widely adopted. 

597. _Q. _--What other kinds of screw are there proposing to themselves thesame or similar objects?

_A. _--There is the corrugated screw, the arms of which are corrugated, soas it were to gear with the water during its revolution, and therebyprevent it from acquiring a centrifugal velocity. Then there is Griffith'sscrew, which has a large ball at its centre, which, by the suction itcreates at its hinder part, in passing through the water, produces aconverging force, which partly counteracts the divergent action of thearms. Finally, there is Holm's screw, which has now been applied to a goodnumber of vessels with success. 

598. _Q. _--Will you describe the configuration and action of Holm's screw?

_A. _--First, then, the screw increases in the direction of its length, andthis increase is very rapid at the following edge, so that, in fact, thefollowing edge stands in the plane of the shaft, or in the verticallongitudinal plane of the vessel. Then the ends of the arms are bent overinto a curved flange, the edge of which points astern, and the point wherethis curved flange joins the following edge of the screw is formed, notinto an angle, but into a portion of a sphere, so that this cornerresembles the bowl of a spoon. When the screw is put into revolution, thewater is encountered by the leading edge of the screw without shock, as itsadvance is only equal to the advance of the vessel, and before the screwleaves the water it is projected directly astern. At the same time, thecurved flange at the rim of the screw prevents the dispersion of the waterin a radial direction, and it consequently assumes the form of a column orcylinder of water, projected backward from the ship. 

599. _Q. _--What is the nature of Beattle's screw?

_A. _--Beattie's screw is an arrangement of the screw propeller whereby itis projected beyond the rudder, and the main object of the arrangement isto take away the vibratory motion at the stern, --an intention which itaccomplishes in practice. There is an oval eye in the rudder, to permit thescrew shaft to pass through it. 

600. _Q. _--When the diameter of the cylinder of water projected backward bya screw, and the force urging it into motion are known, may not thevelocity it will acquire be approximately determined?

_A. _--That will not be very difficult; and I will take for illustration thecase of the Minx, already referred to, which will show how such acomputation is to be conducted. The speed of this vessel, in one of theexperiments made with her, was 8. 445 knots; the number of revolutions ofthe screw per minute, 231. 32; and the pressure on each square foot of areaof the screw's disc, 214 lbs. If a knot be taken to be 6075. 6 feet, thenthe distance advanced by the vessel, when the speed is 8. 445 knots, will be3. 7 feet per revolution, and this advance will be made in about . 26 of asecond of time. Now the distance which a body will fall by gravity, in . 26of a second, is 1. 087 feet; and a weight of 214 lbs. Put into motion bygravity, or by a pressure of 214 lbs. , would, therefore, acquire a velocityof 1. 087 feet during the time one revolution of the screw is beingperformed. The weight to be moved, however, is 3. 7 cubic feet of water, that being the new water seized by the screw each revolution for everysquare foot of surface in the screw's disc; and 3. 7 cubic feet of waterweigh 231. 5 lbs. , so that the urging force of 214 lbs. Is somewhat lessthan the force of gravity, and the velocity of motion communicated to thewater will be somewhat under 1. 087 feet per revolution, or we may say itwill be in round numbers 1 foot per revolution. This, added to the progressof the vessel, will make the distance advanced by the screw through thewater 4. 7 feet per revolution, leaving the difference between this and thepitch, namely 1. 13 feet, to be accounted for on the supposition that thescrew blades had broken laterally through the water to that extent. Itwould be proper to apply some correction to this computation, which wouldrepresent the increased resistance due to the immersion of the screw in thewater; for a column of water cannot be moved in the direction of its axisbeneath the surface, without giving motion to the superincumbent water, andthe inertia of this superincumbent water must, therefore, be taken into theaccount. In the experiment upon the Minx, the depth of this superincumbentcolumn was but small. The total amount of the slip was 36. 53 per cent. ; andthere will not be much error in setting down about one half of this as dueto the recession of the water in the direction of the vessel's track, andthe other half as due to the lateral penetration of the screw blades. 

601. _Q. _--Is it not important to make the stern of screw vessels veryfine, with the view of diminishing the slip, and increasing the speed?

_A. _--It is most important. The Rifleman, a vessel of 486 tons, hadoriginally engines of 200 horses power, which propelled her at a speed of 8knots an hour. The Teazer, a vessel of 296 tons, had originally engines of100 horses power, which propelled her at a speed of 6-1/2 knots an hour. The engines of the Teazer were subsequently transferred to the Rifleman, and new engines of 40 horse power were put into the Teazer. Both vesselswere simultaneously sharpened at the stern, and the result was, that the100 horse engines drove the Rifleman, when sharpened, as fast as she hadpreviously been driven by the 200 horse engines; and the 40 horse enginesdrove the Teazer, when sharpened, a knot an hour faster than she hadpreviously been driven by the 100 horse engines. The immersion of bothvessels was kept unchanged in each case; and the 100 horse engines of theTeazer, when transferred to the Rifleman, drove that vessel, after she hadbeen sharpened, 2 knots an hour faster than they had previously driven avessel not much more than half the size. These are important facts forevery one to be acquainted with who is interested in the success of screwvessels, and who seeks to obtain the maximum of efficiency with the minimumof expense. [1]

[1] See Treatise on the Screw Propeller, by John Bourne, C. E. 

PROPORTIONS OF SCREWS. 

602. _Q. _--In fixing upon the proportions of a screw proper to propel anygiven vessel, how would you proceed?

_A. _--I would first compute the probable resistance of the vessel, and Iwould be able to find the relative resistances of the screw and hull, andin every case it is advisable to make the screw as large in diameter aspossible. The larger the screw is, the greater will be the efficiency ofthe engine in propelling the vessel; the larger will be the ratio of thepitch to the diameter, which produces a maximum effect; and the smallerwill be the length of the screw or the fraction of a convolution to producea maximum effect. 

603. _Q. _--Will you illustrate this doctrine by a practical example?

_A. _--The French screw steamer Pelican was fitted successively with twoscrews of four blades, but the diameter of the first screw was 98. 42inches, and the diameter of the second 54 inches. If the efficiency of thefirst screw by represented by 1, that of the second screw will berepresented by . 823, or, in other words, if the first screw would give aspeed of 10 knots, the second would give little more than 8. The mostadvantageous ratio of pitch to diameter was found to be 2. 2 in the case ofthe large screw, and 1. 384 in the case of the small. The fraction of aconvolution which was found to be most advantageous was . 281 in the case ofthe large screw, and . 450 in the case of the small screw. 

604. _Q_--Were screws of four blades found to be more efficient than screwswith two?

_A_--They were found to have less slip, but not to be moreefficient, the increased slip in those of two blades being balanced by theincreased friction in those of four. Screws of two blades, to secure amaximum efficiency, must have a finer pitch than screws of four. 

605. _Q. _--Are the proportions found to be most suitable in the case of thePelican applicable to the screws of other vessels?

_A. _--Only to those which have the same relative resistance of screw andhull. Taking the relative resistance to be the area of immersed midshipsection, divided by the square of the screw's diameter, it will in the caseof the Rattler be 380/100 or 3. 8. From the experiments made by MM. Bourgoisand Moll on the screw steamer Pelican, they have deduced the proportions ofscrews proper for all other classes of vessels, whether the screws are oftwo, four, or six blades. 

606. _Q. _--Will you specify the nature of their deductions?

_A. _--I will first enumerate those which bear upon screws with two blades. When the relative resistance is 5. 5 the ratio of pitch to diameter shouldbe 1. 006, and the fraction of the pitch or proportion of one entireconvolution should be 0. 454. When the relative resistance is 5, the ratioof pitch to diameter should be 1. 069, and fraction of pitch 0. 428; relativeresistance 4. 5, pitch 1. 135, fraction 0. 402; relative resistance 4, pitch1. 205, fraction 0. 378; relative resistance 3. 5, pitch 1. 279, fraction0. 355; relative resistance 3, pitch 1. 357, fraction 0. 334; relativeresistance 2. 5, pitch 1. 450, fraction 0. 313; relative resistance 2, pitch1. 560, fraction 0. 294; relative resistance 1. 5, pitch 1. 682, fraction0. 275. The relative resistance of 4 is that which is usual in an auxiliaryline of battle ship, 3. 5 in an auxiliary frigate, 3 in a high speed line ofbattle ship, 2. 5 in a high speed frigate, 2 in a high speed corvette, and1. 5 in a high speed despatch boat. 

607. _Q. _--What are the corresponding proportions of screws of four blades?

_A. _--The ratios of the pitches to the diameter being for each of therelative resistances enumerated above, 1. 342, 1. 425, 1. 513, 1. 607, 1. 705, 1. 810, 1. 933, 2. 080, and 2. 243, the respective fractions of pitch orfractions of a whole convolution will be 0. 455, 0. 428, 0. 402, 0. 378, 0. 355, 0. 334, 0. 313, 0. 294, and 0. 275. 

608. _Q. _--And what are the corresponding proportions proper for screws ofsix blades?

_A. _--Beginning with the relative resistance of 5. 5 as before, the properratio of pitch to diameter for that and each of the successive resistancesin the case of screws with six blades, will be 1. 677, 1. 771, 1. 891, 1. 2009, 2. 131, 2. 262, 2. 416, 2. 600, 2. 804; and the respective fractions of pitchwill be 0. 794, 0. 749, 0. 703, 0. 661, 0. 621, 0. 585, 0. 548, 0. 515, and 0. 481. These are the proportions which will give a maximum performance in everycase. [1]

[1] In my Treatise on the Screw Propeller I have gone into these variousquestions more fully than would consort with the limits of thispublication. 

SCREW VESSELS WITH FULL AND AUXILIARY POWER. 

609. _Q. _--Do you consider that the screw propeller is best adapted forvessels of full power, or for vessels with auxiliary power?

_A. _--It is, in my opinion, best adapted for vessels with auxiliary power, and it is a worse propeller than paddle wheels for vessels which havehabitually to encounter strong head winds. Screw vessels are but illcalculated--at least as constructed heretofore--to encounter head winds, and the legitimate sphere of the screw is in propelling vessels withauxiliary power. 

610. _Q. _--Does the screw act well in conjunction with sails?

_A. _--I cannot say it acts better than paddles, except in so far as it isless in the way and is less affected by the listing or heeling over of theship. A small steam power, however, acts very advantageously in aid ofsails, for not only does the operation of the sails in reducing theresistance of the hull virtually increase the screw's diameter, but thescrew, by reducing the resistance which has to be overcome by the sails andby increasing the speed of the vessel, enables the sails to act withgreater efficiency, as the wind will not rebound from them with as great avelocity as it would otherwise do, and a larger proportion of the power ofthe wind will also be used up. In the case of beam winds, moreover, theaction of the screw, by the larger advance it gives to the vessel willenable the sails to intercept a larger column of wind in a given time. Itappears, therefore, that the sails add to the efficiency of the screw, andthat the screw also adds to the efficiency of the sails. 

611. _Q. _--What is the comparative cost of transporting merchandise inpaddle steamers of full power, in screw steamers of auxiliary power, and insailing ships?

_A. _--That will depend very much upon the locality where the comparison ismade. In the case of vessels performing distant ocean voyages, in whichthey may reckon upon the aid of uniform and constant winds, such as thetrade winds or the monsoon, sailing ships of large size will be able tocarry more cheaply than any other species of vessel. But where the windsare irregular and there is not much sea room, or for such circumstances asexist in the Channel or Mediterranean trades, screw vessels with auxiliarypower will constitute the cheapest instrument of conveyance. 

612. _Q. _--Are there any facts recorded illustrative of the accuracy ofthis conclusion?

_A. _--A full paddle vessel of 1000 tons burden and 350 horses power, willcarry about 400 tons of cargo, besides coal for a voyage of 500 miles, andthe expense of such a voyage, including wear and tear, depreciation, &c. , will be about 190_l_. The duration of the voyage will be about 45-1/2hours. A screw vessel of 400 tons burden and 100 horses power, will carrythe same amount of cargo, besides her coals, on the same voyage, and theexpense of the voyage, including wear and tear, depreciation, &c. , will benot much more than 60_l_. An auxiliary screw vessel, therefore, can carrymerchandise at one third of the cost of a full-powered paddle vessel. Bysimilar comparisons made between the expense of conveying merchandise inauxiliary screw steamers and sailing ships on coasting voyages, it appearsthat the cost in screw steamers is about one third less than in the sailingships; the greater expedition of the screw steamers much more thancompensating for the expense which the maintenance of the machineryinvolves. 

SCREW AND PADDLES COMBINED. 

613. _Q. _--Would not a screw combined with paddles act in a similarlyadvantageous way as a screw or paddles when aided by the wind?

_A. _--If in any given paddle vessel a supplementary screw be added toincrease her power and speed, the screw will act in a more beneficialmanner than if it had the whole vessel to propel itself, and for a likereason the paddles will act in a more beneficial manner. There will be lessslip both upon the paddles and upon the screw than if either had beenemployed alone; but the same object would be attained by giving the vessellarger paddles or a larger screw. 

614. _Q. _--Have any vessels been constructed with combined screw andpaddles?

_A. _--Not any that I know of, except the great vessel built under thedirection of Mr. Brunel. The Bee many years since was fitted with bothscrew and paddles, but this was for the purpose of ascertaining therelative efficiency of the two modes of propulsion, and not for the purposeof using both together. 

615. _Q. _--What would be the best means of accelerating the speed of apaddle vessel by the introduction of a supplementary screw?

_A. _--If the vessel requires new boilers, the best course of procedurewould be to work a single engine giving motion to the screw with highpressure steam, and to let the waste steam from the high pressure enginework the paddle engines. In this way the power might be doubled without anyincreased expenditure of fuel per hour, and there would be a diminishedexpenditure per voyage in the proportion of the increased speed. 

616. _Q. _--What would the increased speed be by doubling the power?

_A. _--The increase would be in the proportion of the cube root of 1 to thecube root of 2, or it would be 1. 25 times greater. If, therefore, theexisting speed were 10 miles, it would be increased to 12-1/2 miles bydoubling the power, and the vessel would ply with about a fourth less coalsby increasing the power in the manner suggested. 

617. _Q. _--Is not high pressure steam dangerous in steam vessels?

_A. _--Not necessarily so, and it has now been introduced into a good numberof steam vessels with satisfactory results. In the case of locomotiveengines, where it is used so widely, very few accidents have occurred; andin steam vessels the only additional source of danger is the salting of theboiler. This may be prevented either by the use of fresh water in theboiler, or by practising a larger amount of blowing off, to insure which itshould be impossible to diminish the amount of water sent into the boilerby the feed pump, and the excess should be discharged overboard through avalve near the water level of the boiler, which valve is governed by afloat that will rise or fall with the fluctuating level of the water. Ifthe float be a copper ball, a little water should be introduced into itbefore it is soldered or brazed up, which will insure an equality ofpressure within and without the ball, and a leakage of water into it willthen be less likely to take place. A stone float, however, is cheaper, andif properly balanced will be equally effective. All steam vessels shouldhave a large excess of boiling feed water constantly flowing into theboiler, and a large quantity of water constantly blowing off through thesurface valves, which being governed by floats will open and let thesuperfluous water escape whenever the water level rises too high. In thisway the boiler will be kept from salting, and priming will be much lesslikely to occur. The great problem of steam navigation is the economy offuel, since the quantity of fuel consumed by a vessel will very muchdetermine whether she is profitable or otherwise. Notwithstanding themomentous nature of this condition, however, the consumption of fuel insteam vessels is a point to which very little attention has been paid, andno efficient means have yet been adopted in steam vessels to insure thatmeasure of economy which is known to be attainable, and which has beenattained already in other departments of engineering in which the benefitsof such economy are of less weighty import. It needs nothing more than theestablishment of an efficient system of registration in steam vessels, toinsure a large and rapid economy in the consumption of fuel, as thisquality would then become the test of an engineer's proficiency, and woulddetermine the measure of his fame. In the case of the Cornish engines, asaving of more than half the fuel was speedily effected by the introductionof the simple expedient of registration. In agricultural engines a likeeconomy has speedily followed from a like arrangement; yet in both of thesecases the benefits of a large saving are less eminent than they would be inthe case of steam navigation; and it is to be hoped that this expedient ofimprovement will now be speedily adopted. 

CHAPTER X. 

EXAMPLES OF ENGINES. 

 * * * * *

OSCILLATING PADDLE ENGINES. 

618. _Q. _--Will you describe the structure of an oscillating engine as madeby Messrs. Penn?

_A. _--To do this it will be expedient to take an engine of a given power, and then the sizes may be given as well as an account of the configurationof the parts: we may take for an example a pair of engines of 21-1/2 inchesdiameter of cylinder, and 22 inches stroke, rated by Messrs. Penn at 12horses power each. The cylinders of this oscillating engine are placedbeneath the cranks, and, as in all Messrs. Penn's smaller engines, thepiston rod is connected to the crank pin by means of a brass cap, providedwith a socket, by means of which it is cuttered to the piston rod. There isbut one air pump, which is situated within the condenser between thecylinders, and it is wrought by means of a crank in the intermediateshaft--this crank being cut out of a solid piece of metal as in theformation of the cranked axles of locomotive engines. The steam enters thecylinder through the outer trunnions, or the trunnions adjacent to theship's sides, and enters the condenser through the two midship trunnions--ashort three ported valve being placed on the front of the cylinder toregulate the flow of steam to and from the cylinder in the proper manner. The weight of this valve on one side of the cylinder is balanced by aweight hung upon the other side of the cylinder; but in the most recentengines this weight is discarded, and two valves are used, which balanceone another. The framing consists of an upper and lower frame of cast iron, bound together by eight malleable iron columns: upon the lower frame thepillow blocks rest which carry the cylinder trunnions, and the condenserand the bottom frame are cast in the same piece. The upper frame supportsthe paddle shaft pillow blocks; and pieces are bolted on in continuation ofthe upper frame to carry the paddle wheels, which are overhung from thejournal. 

619. _Q. _--What are the dimensions and arrangement of the framing?

_A. _--The web, or base plate of the lower frame is 3/4 of an Inch thick, and a cooming is earned all round the cylinder, leaving an opening ofsufficient size to permit the necessary oscillation. The cross section ofthe upper frame is that of a hollow beam 6 inches deep, and about 3-1/2inches wide, with holes at the sides to take out the core; and thethickness of the metal is 13/16ths of an inch. Both the upper and the lowerframe is cast in a single piece, with the exception of the continuations ofthe upper frame, which support the paddle wheels. An oval ring 3 incheswide is formed in the upper frame, of sufficient size to permit the workingof the air pump crank; and from this ring feathers run to the ends of thecross portions of the frame which supports the intermediate shaft journals. The columns are 1-1/2 inches in diameter; they are provided with collars atthe lower ends, which rest upon bosses in the lower frame, and with collarsat the upper ends for supporting the upper frame; but the upper collars oftwo of the corner columns are screwed on, so as to enable the columns to bedrawn up when it is required to get the cylinders out. The cross section ofthe bottom frame is also of the form of a hollow beam, 7 inches deep, except in the region of the condenser, where it is, of course, of adifferent form. The depth of the boss for the reception of the columns is alittle more than 7 inches deep on the lower frame, and a little more than 6inches deep on the upper frame; and the holes through them are so coredout, that the columns only bear at the upper and lower edges of the hole, instead of all through it--a formation by which the fitting of the columnsis facilitated. 

620. _Q. _--What are the dimensions of the condenser?

_A. _--The condenser, which is cast upon the lower frame, consists of anoval vessel 22-1/2 inches wide, by 2 feet 4-1/4 inches long, and 1 foot10-1/2 inches deep; it stands 9 inches above the upper face of the bottomframe, the rest projecting beneath it; and it is enlarged at the sides bybeing carried beneath the trunnions. 

621. _Q. _--What are the dimensions of the air pump?

_A. _--The air pump, which is set in the centre of the condenser, is 15-1/4inches in diameter, and has a stroke of 11 inches. The foot valve issituated in the bottom of the air pump, and its seat consists of a disc ofbrass, in which there is a rectangular flap valve, opening upwards, butrounded on one side to the circle of the pump, and so balanced as to enablethe valve to open with facility. The balance weight, which is formed ofbrass cast in the same piece as the valve itself, operates as a stop, bycoming into contact with the disc which constitutes the bottom of the pump;the disc being recessed opposite to the stop to enable the valve to opensufficiently. This disc is bolted to the barrel of the pump by means of aninternal flange, and before it can be removed the pump must be lifted outof its place. The air pump barrel is of brass to which is bolted a castiron mouth piece, with a port for carrying the water to the hot well;within the hot well the delivery valve, which consists of a common flapvalve, is situated. The mouth piece and the air pump barrel are made tightto the condenser, and to one another, by means of metallic joints carefullyscraped to a true surface, so that a little white or red lead interposedmakes an air tight joint. The air pump bucket is of brass, and the valve ofthe bucket is of the common pot lid or spindle kind. The injection waterenters through a single cock in front of the condenser--the jet strikingagainst the barrel of the air pump. The air pump rod is maintained in itsvertical position by means of guides, the lower ends of which are bolted tothe mouth of the pump, and the upper to the oval in the top frame, withinwhich the air pump crank works; and the motion is communicated from thiscrank to the pump rod by means of a short connected rod. The lower frame isnot set immediately below the top frame, but 2-1/2 inches behind it, andthe air pump and condenser are 2-1/2 inches nearer one edge of the lowerframe than the other. 

622. _Q. _--What are the dimensions of the cylinder?

_A. _--The thickness of the metal of the cylinder is 9/16ths of an inch; thedepth of the belt of the cylinder is 9-1/2 inches, and its greatestprojection from the cylinder is 2-1/2 inches. The distance from the loweredge of the belt to the bottom of the cylinder is 11-1/2 inches, and fromthe upper edge of the belt to the top flange of the cylinder is 9 inches. The trunnions are 7-1/4 inches diameter in the bearings, and 3-1/2 inchesin width; and the flanges to which the glands are attached for screwing inthe trunnion packings are 1-1/2 inch thick, and have 7/8ths of an inch ofprojection. The width of the packing space round the trunnions is 5/8ths ofan inch, and the diameter of the pipe passing through the trunnion4-5/8ths, which leaves 11/16ths for the thickness of the metal of thebearing. Above and below each trunnion a feather runs from the edge of thebelt or bracket between 3 and 4 inches along the cylinder, for the sake ofadditional support; and in large engines the feather is continued throughthe interior of the belt, and cruciform feathers are added for the sake ofgreater stiffness. The projection of the outer face of the trunnion flangefrom the side of the cylinder is 6-1/2 inches; the thickness of the flangeround the mouth of the cylinder is 3/4 of an inch, and its projection 1-3/8inch; the height of the cylinder stuffing box above the cylinder cover is4-1/8 inches, and its external diameter 4-3/8 inches--the diameter of thepiston rod being 2-1/8 inches. The thickness of the stuffing box flange is1-1/8 inch. 

623. _Q. _--Will you describe the nature of the communication between thecylinder and condenser?

_A. _--The pipe leading to the condenser from the cylinder is made somewhatbell mouthed where it joins the condenser, and the gland for compressingthe packing is made of a larger internal diameter in every part except atthe point at which the pipe emerges from it, where it accurately fits thepipe so as to enable the gland to squeeze the packing. By this constructionthe gland may be drawn back without being jammed upon the enlarged part ofthe pipe; and the enlargement of the pipe toward the condenser prevents theair pump barrel from offering any impediment to the free egress of thesteam. The gland is made altogether in four pieces: the ring which pressesthe packing is made distinct from the flange to which the bolts areattached which force the gland against the packing, and both ring andflange are made in two pieces, to enable them to be got over the pipe. Thering is half checked in the direction of its depth, and is introducedwithout any other support to keep the halves together, than what isafforded by the interior of the stuffing box; and the flange is halfchecked in the direction of its thickness, so that the bolts which pressdown the ring by passing through this half-checked part, also keep thesegments of the flange together. The bottom of the trunnion packing spaceis contracted to the diameter of the eduction pipe, so as to prevent thepacking from being squeezed into the jacket; but the eduction pipe does notfit quite tight into this contracted part, but, while in close contact onthe lower side, has about 1/32nd of an inch of space between the top of thepipe and the cylinder, so as to permit the trunnions to wear to that extentwithout throwing a strain upon the pipe. The eduction pipe is attached tothe condenser by a flange joint, and the bolt holes are all made somewhatoblong in the perpendicular direction, so as to permit the pipe to beslightly lowered, should such an operation be rendered necessary by thewear of the trunnion bearings; but in practice the wear of the trunnionbearings is found to be so small as to be almost inappreciable. 

624. _Q. _--Will you describe the valve and valve casing?

_A. _--The length of the valve casing is 16-1/2 inches, and its projectionfrom the cylinder is 3-1/2 inches at the top, 4-1/4 inches at the centre, and 2-1/2 inches at the bottom, so that the back of the valve casing is notmade flat, but is formed in a curve. The width of the valve casing is 9inches, but there is a portion of the depth of the belt 1-1/2 inch wider, to permit the steam to enter from the belt into the casing. The valvecasing is attached to the cylinder by a metallic joint; the width of theflange of this joint is 1-1/4 inch, the thickness of the flange on thecasing 1/2 inch, and the thickness of the flange on the cylinder 5/8ths ofan inch. The projection from the cylinder of the passage for carrying thesteam upwards, and downwards, from the valve to the top and bottom of thecylinder, is 2-1/4 inches, and its width externally 8-5/8 inches. The valveis of the ordinary three ported description, and both cylinder and valvefaces are of cast iron. 

625. _Q. _--What description of piston is used?

_A. _--The piston is packed with hemp, but the junk ring is made ofmalleable iron, as cast iron junk rings have been found liable to break:there are four plugs screwed into the cylinder cover, which, when removed, permit a box key to be introduced, to screw down the piston packing. Thescrews in the junk ring are each provided with a small ratchet, cut in awasher fixed upon the head, to prevent the screw from turning back; and thenumber of clicks given by these ratchets, in tightening up the bolts, enables the engineer to know when they have all been tightened equally. Inmore recent engines, and especially in those of large size, Messrs. Pennemploy for the piston packing a single metallic ring with tongue piece andindented plate behind the joint; and this ring is packed behind with hempsqueezed by the junk ring as in ordinary hemp-packed pistons. 

626. _Q. _--Will you describe the construction of the cap for connecting thepiston rod with the crank pin?

_A. _--The cap for attaching the piston rod to the crank pin, is formedaltogether of brass, which brass serves to form the bearing of the crankpin. The external diameter of the socket by which this cap is attached tothe piston rod is 3-5/16 inches. The diameter of the crank pin is 3 inches, and the length of the crank pin bearing 3-7/8 inches. The thickness of thebrass around the crank pin bearing is 1 inch, and the upper portion of thebrass is secured to the lower portion, by means of lugs, which are of sucha depth that the perpendicular section through the centre of the bearinghas a square outline measuring 7 inches in the horizontal direction, 3-7/8inches from the centre of the pin to the level of the top of the lugs, and2-1/2 inches from the centre of the pin to the level of the bottom of thelugs. The width of the lugs is 2 inches, and the bolts passing through themare 1-1/4 inch in diameter. The bolts are tapped into the lower portion ofthe cap, and are fitted very accurately by scraping where they pass throughthe upper portion, so as to act as steady pins in preventing the cover ofthe crank pin bearing from being worked sideways by the alternate thrust oneach side. The distance between the centres of the bolts is 5 inches, andin the centre of the cover, where the lugs, continued in the form of a web, meet one another, an oil cup 1-5/8 inch in diameter, 1-1/8 inch high, andprovided with an internal pipe, is cast upon the cover, to contain oil forthe lubrication of the crank pin bearing. The depth of the cutter forattaching the cap to the piston rod is 1-1/4 inch and its thickness is3/8ths of an inch. 

627. _Q. _--Will you describe the means by which the air pump rod isconnected with the crank which works the air pump?

[Illustration: Fig. 50. AIR PUMP CONNECTING ROD AND CROSS HEAD. Messrs. Penn. ]

_A. _--A similar cap to that of the piston rod attaches the air pump crankto the connecting rod by which the air pump rod is moved, but in thisinstance the diameter of the bearing is 5 inches, and the length of thebearing is about 3 inches. The air pump connecting rod and cross head areshown in perspective in fig. 50. The thickness of the brass encircling thebearing of the shaft is three fourths of an inch upon the edge, and 1-1/8inch in the centre, the back being slightly rounded; the width of the lugsis 1-5/8 inch, and the depth of the lugs is 2 inches upon the upper brass, and 2 inches upon the lower brass, making a total depth of 4 inches. Thediameter of the bolts passing through the lugs is 1 inch, and the bolts aretapped into the lower brass, and accurately fitted into the upper one, soas to act as steady pins, as in the previous instance. The lower eye of theconnecting rod is forked, so as to admit the eye of the air pump rod; andthe pin which connects the two together is prolonged into a cross head, asshown in fig. 50. The ends of this cross head move in guides. The forkedend of the connecting rod is fixed upon the cross head by means of afeather, so that the cross head partakes of the motion of the connectingrod, and a cap, similar to that attached to the piston rod, is attached tothe air pump rod, for connecting it with the cross head. The diameter ofthe air pump rod is 1-1/2 inch, the external diameter of the socketencircling the rod is 2-1/8 inches, and the depth of the socket 4-1/2inches from the centre of the cross head. The depth of the cutter forattaching the socket to the rod is 1 inch, and its thickness 5/16 inch. Thebreadth of the lugs is 1-3/8 inch, the depth 1-1/4 inch, making a totaldepth of 2-1/2 inches; and the diameter of the bolts seven eighths of aninch. The diameter of the cross head at the centre is 2 inches, thethickness of each jaw around the bearing 1 inch, and the breadth of each9/16 inch. 

628. _Q. _--What are the dimensions of the crank shaft and cranks?

_A. _--The diameter of the intermediate shaft journal is 4-3/16 inches, andof the paddle shaft journal 4-3/8 inches; the length of the journal in eachcase is 5 inches. The diameter of the large eye of the crank is 7 inches, and the diameter of the hole through it is 4-3/8 inches; the diameter ofthe small eye of the crank is 5-1/4 inches, the diameter of the holethrough it being 3 inches. The depth of the large eye is 4-1/4 inches, andof the small eye 3-3/4 inches; the breadth of the web is 4 inches at theshaft end, and 3 inches at the pin end, and the thickness of the web is2-5/8 inches. The width of the notch forming the crank in the intermediateshaft for working the air pump is 3-1/2 inches, and the width of each ofthe arms of this crank is 3-15/16 inches. Both the outer and inner cornersof the crank are chamfered away, until the square part of the crank meetsthe round of the shaft. The method of securing the cranks pins into thecrank eyes of the intermediate shaft consists in the application of a nutto the end of each pin, where it passes through the eye, the projecting endof the pin being formed with a thread upon which the nut is screwed. 

629. _Q. _--Will you describe the eccentric and eccentric rod?

[Illustration: Fig. 51. ECCENTRIC AND ROD. Messrs. Penn. ]

_A. _--The eccentric and eccentric rod are shown in fig. 51. The eccentricis put on the crank shaft in two halves, joined in the diameter of largesteccentricity by means of a single bolt passing through lugs on the centraleye, and the back balance is made in a separate piece five eighths of aninch thick, and is attached by means of two bolts, which also help to bindthe halves of the eccentric together. The eccentric strap is half an inchthick, and 1-1/4 inch broad, and the flanges of the eccentric, within whichthe strap works, are each three eighths of an inch thick. The eccentric rodis attached to the eccentric hoop by means of two bolts passing throughlugs upon the rod, and tapped into a square boss upon the hoop; and piecesof iron, of a greater or less thickness, are interposed between thesurfaces in setting the valve, to make the eccentric rod of the rightlength. The eccentric rod is kept in gear by the push of a small horizontalrod, attached to a vertical blade spring, and it is thrown out of gear bymeans of the ordinary disengaging apparatus, which acts in opposition tothe spring, as, in cases where the eccentric rod is not vertical, it actsin opposition to the gravity of the rod. 

630. _Q. _--Will you explain in detail the construction of the valvegearing, or such parts of it as are peculiar to the oscillating engine?

_A. _--The eccentric rod is attached by a pin, 1 inch in diameter, to anopen curved link or sector with a tail projecting upward and passingthrough an eye to guide the link in a vertical motion. The link is formedof iron case-hardened, and is 2-3/4 inches deep at the middle, and 2-3/8inches deep at the ends, and 1 inch broad. The opening in the link, whichextends nearly its entire length, is 1-5/16 inch broad; and into thisopening a brass block 2 inches long is truly fitted, there being a holethrough the block 3/4 inch diameter, for the reception of the pin of thevalve shaft lever. The valve shaft is 1-3/4 inch diameter at the end nextthe link or segment, and diminishes regularly to the other end, but itscross section assumes the form of an octagon in its passage round thecylinder, measuring mid-way 1-1/4 inch deep, by about 3/4 inch thick, andthe greatest depth of the finger for moving the valve is about 1 inch. Thedepth of the lever for moving the valve shaft is 2 inches at the broad, and1-1/4 inch at the narrow end. The internal breadth of the mortice in whichthe valve finger moves is 5/16 inch, and its external depth is 1-3/4 inch, which leaves three eighths of an inch as the thickness of metal round thehole; and the breadth, measuring in the direction of the hole, is 1-1/2inch. The valve rod is three fourths of an inch in diameter, and themortice is connected to the valve rod by a socket 1 inch long, and 1-1/8inch diameter, through which a small cutter passes. A continuation of therod, eleven sixteenths of an inch diameter, passes upward from the mortice, and works through an eye, which serves the purpose of a guide. In additionto the guide afforded to the segment by the ascending tail, it is guided atthe ends upon the columns of the framing by means of thin semicircularbrasses, 4 inches deep, passing round the columns, and attached to thesegment by two 3/8 inch bolts at each end, passing through projectingfeathers upon the brasses and segment, three eighths of an inch inthickness. The curvature of the segment is such as to correspond with thearc swept from the centre of the trunnion to the centre of the valve leverpin when the valve is at half stroke as a radius; and the operation of thesegment is to prevent the valve from being affected by the oscillation ofthe cylinder, but the same action, would be obtained by the employment of asmaller eccentric with more lead. In some engines the segment is not formedin a single piece, but of two curved blades, with blocks interposed at theends, which may be filed down a little, to enable the sides of the slot tobe brought nearer, as the metal wears away. 

631. _Q. _--What kind of plummer blocks are used for the paddle shaftbearings?

_A. _--The paddle shaft plummer blocks are altogether of brass, and areformed in much the same manner as the cap of the piston rod, only that thesole is flat, as in ordinary plummer blocks, and is fitted betweenprojecting lugs of the framing, to prevent side motion. In the bearingsfitted on this plan, however, the upper brass will generally acquire a gooddeal of play after some amount of wear. The bolts are worked slack in theholes, though accurately fitted at first; and it appears expedient, therefore, either to make the bolts very large, and the sockets throughwhich they pass very deep, or to let one brass fit into the other. 

632. _Q. _--How are the trunnion plummer blocks made?

_A. _--The trunnion plummer blocks are formed in the same manner as thecrank shaft plummer blocks; the nuts are kept from turning back by means ofa pinching screw passing through a stationary washer. It is not expedientto cast the trunnion plummer blocks upon the lower frame, as is sometimesdone; for the cylinders, being pressed from the steam trunnions by thesteam, and drawn in the direction of the condenser by the vacuum, have acontinual tendency to approach one another; and as they wear slightlytoward midships, there would be no power of readjustment unless the plummerblocks were movable. The flanges of the trunnions should always fit tightagainst the plummer block sides, but there should be a little play sidewaysat the necks of the trunnions, so that the cylinder may be enabled toexpand when heated, without throwing an undue strain upon the trunnionsupports. 

633. _Q. _--What kind of paddle wheel is supplied with these oscillatingengines?

_A. _--The wheels are of the feathering kind, 9 feet 8 inches in diameter, measuring to the edges of the floats; and there are 10 floats upon eachwheel, measuring 4 feet 6 inches long each, and 18-1/2 inches broad. Thereare two sets of arms to the wheel, which converge to a cast iron centre, formed like a short pipe with large flanges, to which the arms are affixed. The diameter of the shaft, where the centre is put on, is 4-1/2 inches, theexternal diameter of the pipe is 8 inches, and the diameter of the flangesis 20 inches, and their thickness 1-1/4 inch. The flanges are 12 inchesasunder at the outer edge, and they partake of the converging direction ofthe arms. The arms are 2-1/4 inches broad and half an inch thick; the headsare made conical, and each is secured into a recess upon the side of theflange by means of three bolts. The ring which connects together the arms, runs round at a distance of 3 feet 6 inches from the centre, and theprojecting ends of the arms are bent backward the length of the lever whichmoves the floats, and are made very wide and strong at the point where theycross the ring, to which they are attached by four rivets. The featheringaction of the floats is accomplished by means of a pin fixed to theinterior of the paddle box, set 3 inches in advance of the centre of theshaft, and in the same horizontal line. This pin is encircled by a castiron collar, to which rods are attached 1-3/8 inch diameter in the centre, proceeding to the levers, 7 inches long, fixed on the back of the floats inthe line of the outer arms. One of these rods, however, is formed of nearlythe same dimensions as one of the arms of the wheel, and is called thedriving arm, as it causes the cast iron collar to turn round with therevolution of the wheel, and this collar, by means of its attachments tothe floats, accomplishes the feathering action. The eccentricity in thiswheel is not sufficient to keep the floats in the vertical position, but inthe position between the vertical and the radial. The diameter of the pinsupon which the floats turn is 1-3/8 inch, and between the pins and paddlering two stud rods are set between each of the projecting ends of the arms, so as to prevent the two sets of arms from being forced nearer or furtherapart; and thus prevent the ends of the arms from hindering the action ofthe floats, by being accidentally jammed upon the sides of the joints. Stays, crossing one another, proceed from the inner flange of the centre tothe outer ring of the wheel, and from the outer flange of the centre to theinner ring of the wheel, with the view of obtaining greater stiffness. Thefloats are formed of plate iron, and the whole of the joints and joint pinsare steeled, or formed of steel. For sea-going vessels the most approvedpractice is to make the joint pins of brass, and also to bush the eyes ofthe joints with brass; and the surface should be large to diminish wear. 

634. _Q. _--Can you give the dimensions of any other oscillating engines?

_A. _--In Messrs. Penn's 50 horse power oscillating engine, the diameter ofthe cylinder is 3 feet 4 inches, and the length of the stroke 3 feet. Thethickness of the metal of the cylinder is 1 inch, and the thickness of thecylinder bottom is 1-3/4 inch, crossed with feathers, to give it additionalstiffness. The diameter of the trunnion bearings is 1 foot 2 inches, andthe breadth of the trunnion bearings 5-1/2 inches. Messrs. Penn, in theirlarger engines, generally make the area of the steam trunnion less thanthat of the eduction trunnion, in the proportion of 32 to 37; and thediameter of the eduction trunnion is regulated by the internal diameter ofthe eduction pipe, which is about 1/5th of the diameter of the cylinder. But a somewhat larger proportion than this appears to be expedient: Messrs. Rennie make the area of their eduction pipes, in oscillating engines, 1/22dof the area of the cylinder. In the oscillating engines of the Oberon, byMessrs. Rennie, the cylinder is 61 inches diameter, and 1-1/2 inch thickabove and below the belt, but in the wake of the belt it is 1-1/4 inchthick, which is also the thickness of metal of the belt itself. Theinternal depth of the belt is 2 feet 6 inches, and its internal breadth is4 inches. The piston rod is 6-3/4 inches in diameter, and the total depthof the cylinder stuffing box is 2 feet 4 inches, of which 18 inchesconsists of a brass bush--this depth of bearing being employed to preventthe stuffing box or cylinder from wearing oval. 

635. _Q. _--Can you give any other examples?

_A. _--The diameter of cylinder of the oscillating engines of the steamersPottinger, Ripon, and Indus, by Miller & Ravenhill, is 76 inches, and thelength of the stroke 7 feet. The thickness of the metal of the cylinder is1-11/16 inch; diameter of the piston rod 8-3/4 inches; total depth ofcylinder stuffing box 3 feet; depth of bush in stuffing box 4 inches; therest of the depth, with the exception of the space for packing, beingoccupied with a very deep gland, bushed with brass. The internal diameterof the steam pipe is 13 inches; diameter of steam trunnion journal 25inches; diameter of eduction trunnion journal 25 inches; thickness of metalof trunnions 2-1/4 inches; length of trunnion bearings 11 inches;projection of cylinder jacket, 8 inches; depth of packing space intrunnions, 10 inches; width of packing space in trunnions, or space roundthe pipes, 1-1/2 inch; diameter of crank pin 10-1/4 inches; length ofbearing of crank pin 15-1/2, inches. There are six boilers on the tubularplan in each of these vessels; the length of each boiler is 10 feet 6inches, and the breadth 8 feet; and each boiler contains 62 tubes 3 inchesin diameter, and 6 feet 6 inches long, and two furnaces 6 feet 4-1/2 incheslong, and 3 feet 1-1/2 inch broad. 

636. _Q. _--Is it the invariable practice to make the piston rod cap ofbrass in the way you have described?

_A. _--In all oscillating engines of any considerable size, the cover of theconnecting brass, which attaches the crank pin to the connecting rod, isformed of malleable iron; and the socket also, which is cuttered to the endof the piston rod, is of malleable iron, and is formed with a T head, through which bolts pass up through the brass, to keep the cover of thebrass in its place. 

637. _Q. _--Is the piston of an oscillating engine made deeper than incommon engines?

_A. _--It is expedient, in oscillating engines, to form the piston with aprojecting rim round the edge above and below, and a corresponding recessin the cylinder cover and cylinder bottom, whereby the breadth of bearingof the solid part of the metal will be increased, and in many engines thisis now done. 

638. _Q. _--Would any difficulty be experienced in keeping the trunnionstight in a high pressure oscillating engine?

_A. _--It is very doubtful whether the steam trunnions of a high pressureoscillating engine will continue long tight if the packing consists ofhemp; and it appears preferable to introduce a brass ring, to embrace thepipe, cut spirally, with an overlap piece to cover the cut, and packedbehind with hemp. 

639. _Q. _--How is the packing of the trunnions usually effected?

_A. _--The packing of the trunnions, after being plaited as hard aspossible, and cut to the length to form one turn round the pipe, is dippedinto boiling tallow, and is then compressed in a mould, consisting of twoconcentric cylinders, with a gland forced down into the annular space bythree to six screws in the case of large diameters, and one central screwin the case of small diameters. Unless the trunnion packings be wellcompressed, they will be likely to leak air, and it is, therefore, necessary to pay particular attention to this condition. It is also veryimportant that the trunnions be accurately fitted into their brasses byscraping, so that there may not be the smallest amount of play left uponthem; for if any upward motion is permitted, it will be impossible toprevent the trunnion packings from leaking. 

DIRECT ACTING SCREW ENGINE. 

640. _Q. _--Will you describe the configuration and construction of a directacting screw engine?

_A. _--I will take as an example of this species of engine, the engineconstructed by Messrs. John Bourne & Co. , for the screw steamer Alma, avessel of 500 tons burden. This engine is a single steeple engine laid onits side, and in its general features it resembles the engines of theAmphion already described, only that there is one cylinder instead of two. The cylinder is of 42 inches diameter and 42 inches stroke, and the vesselhas been propelled by this single engine at the rate of fourteen miles anhour. 

641. _Q. _--Is not a single engine liable to stick upon the centre so thatit cannot be started or reversed with facility?

_A. _--A single engine is no doubt more liable to stick upon the centre thantwo engines, the cranks of which are set at right angles with one another;but numerous paddle vessels are plying successfully that are propelled by asingle engine, and the screw offers still greater facility than paddles forsuch a mode of construction. In the screw engine referred to, as thecylinder is laid upon its side, there is no unbalanced weight to be liftedup every stroke, and the crank, whereby the screw shaft is turned round, consists of two discs with a heavy side intended to balance the momentum ofthe piston and its connections; but these counter-weights by theirgravitation also prevent the connecting rod and crank from continuing inthe same line when the engine is stopped, and in fact they place the crankin the most advantageous position for starting again when it has to be seton. 

642. _Q. _--Will you explain the general arrangement of the parts of thisengine?

_A. _--The cylinder lies on its side near one side of the vessel, and fromthe end of the cylinder two piston rods extend to a cross head slidingathwartships, in guides, near the other side of the vessel. To this crosshead the connecting rod is attached, and one end of it partakes of themotion of the cross head or piston, while the other end is free to followthe revolution of the crank on the screw shaft. 

643. _Q. _--What is the advantage of two discs entering into the compositionof the crank instead of one?

_A. _--A double crank, such as two discs form with the crank pin, is a muchsteadier combination than would result if only one disc were employed withan over-hung pin. Then the friction on the neck of the shaft is made onehalf less by being divided between the two bearings, and the shortprolongation of the shaft beyond the journal is convenient for theattachment of the eccentrics to work the valves. 

644. _Q. _--Will you enumerate some of the principal dimensions of thisengine?

_A. _--The bottom frame, on which also the condenser is cast, forms the baseof the engine: on one end of it the cylinder is set; on the other end arethe guides for the cross head, and in the middle are the bearings for thecrank shaft. The part where the cylinder stands is two feet high above theengine platform, and the elevation to the centre of the guides or thecentre of the shaft is 10 inches higher than this. The metal both of theside frames and bottom flange is 1-1/4 inch thick. The cylinder has flangescast on its sides, upon which it rests on the bottom frame, and it is sunkbetween the sides of the frame so as to bring the centre of the cylinder inthe same plane as the centre of the screw shaft. The opening left at theguides for the reception of the guide blocks is 6 inches deep, and thebreadth of the bearing surface is 11 inches. The cover of the guides is 8inches deep at the middle, and about half the depth at the ends, and holesare cored through the central web for two oil cups on each guide. The brassfor each of the crank shaft bearings is cut into four pieces so that it maybe tightened in the up and down direction by the bolts, which secure theplummer block cap, and tightened in the athwartship direction, which is thedirection of the strain, by screwing up a wedge-formed plate against theside of the brass, a parallel plate being applied to the other side of thebrass, which may be withdrawn to get out the wedge piece when the shaftrequires to be lifted out of its place. The air pump is bolted to one sideof the bottom frame, and a passage is cast on it conducting from thecondenser to the air pump. In this passage the inlet and outlet valves ateach end of the air pump are situated, and appropriate doors are formedabove them to make them easily accessible. The outlet passage leading fromthe air pump communicates with the waste water pipe, through which thewater expelled by the air pump is discharged overboard. 

645. _Q. _--Is the cylinder of the usual strength and configuration?

_A. _--The cylinder is formed of cast iron in the usual way, and is 1-1/8inch thick in the barrel. The ends are of the same thickness, but are eachstiffened with six strong feathers. The piston is cast open. The bottom ofit is 5/8ths of an inch thick, and it is stiffened by six feathers 3/4 ofan inch thick; but the feather connecting the piston rod eyes is 1-1/4 inchthick, and the metal round the eyes is 2 inches thick. The piston is closedby a disc or cover 5/8ths of an inch thick, secured by 15 bolts, and thiscover answers also the purpose of a junk ring. The piston packing consistsof a single cast iron ring 3-1/2 inches broad, and 1/2 inch thick, packedbehind with hemp. This ring is formed with a tongue piece, with an indentedplate behind the cut; and the cut is oblique to prevent a ridge forming inthe cylinder. The total thickness of the piston is 5-1/2 inches. The pistonrods are formed with conical ends for fitting into the piston, but areconed the reverse way as in locomotives, and are secured in the piston bynuts on the ends of the rods, these nuts being provided with ratchets toprevent them from unscrewing accidentally. 

646. _Q. _--What species of slide valve is employed?

_A. _--The ordinary three ported valve, and it is set on the top of thecylinder. The cylinder ports are 4-1/2 inches broad by 24 inches long; andto relieve the valve from the great friction due to the pressure on solarge a surface, a balance piston is placed over the back of the valve, towhich it is connected by a strong link; and the upward pressure on thispiston being nearly the same as the downward pressure on the valve, itfollows that the friction is extinguished, and the valve can be moved withgreat case with one hand. The balance piston is 21 inches in diameter. Inthe original construction of this balance piston two faults were committed. The passage communicating between the condenser and the top of the balancepiston was too small, and the pins at the ends of the link connecting thevalve and balance piston were formed with an inadequate amount of bearingsurface. It followed from this misproportion that the balance piston, beingadjusted to take off nearly the whole of the pressure, lifted the valve offthe face at the beginning of each stroke. For the escape of the steam intothe eduction passage momentarily impaired the vacuum subsisting there, andowing to the smallness of the passage leading to the space above thebalance piston, the vacuum subsisting in that space could not be impairedwith equal rapidity. The balance piston, therefore, rose by the upwardpressure upon it momentarily predominating over the downward pressure onthe valve; but this fault was corrected by enlarging the communicatingpassage between the top of the balance piston and the eduction pipe. Thesmallness of the pins at the ends of the link connecting the valve andbalance piston, caused the surfaces to cut into one another, and to wearvery rapidly, and the pins and eyes in this situation should be large indiameter, and as long as they can be got, as they are not so easilylubricated as the other bearings about the engine, and are moreover kept ata high temperature by the steam. The balance piston is packed in the sameway as the main piston of the engine. Its cylinder, which is only a fewinches in length, is set on the top of the valve casing, and a trunkprojects upwards from its centre to enable the connecting link to rise upin it to attain the necessary length. 

[Illustration: Fig 52. CONNECTING ROD. Messrs. Bourne & Co. ]

647. _Q. _--What is the diameter of the piston rods and connecting rod?

_A. _--The piston rods, which are two in number, are 3 inches diameter, and12 feet 10 inches long over all. They were, however, found to be rathersmall, and have since been made half an inch thicker. The connecting rodconsists of two rods, which are prolongations of the bolts that connect thesides of the brass bushes which encircle the crank pin and cross head. Theconnecting rod is shown in perspective in fig. 52. The rods composing itare each 2-3/4 inches in diameter. 

648. _Q. _--Will you describe the configuration of the cross head. 

_A. _--The cross head, exhibited in fig. 53, is a round piece of iron like ashort shaft, with two unequal arms keyed upon it, the longer of which _b_works the air pump, and the shorter _c_ works the feed pump. The pistonrods enter these arms at _a A. _ The cross head is 8 inches diameter whereit is embraced by the connecting rod at _e_, and 7 inches diameter wherethe air pump and feed pump arms are fixed on. The ends of the cross head _dd_, for a length of 12 inches, are reduced to 3 inches diameter where theyfit into round holes in the centre of the guide blocks. Those blocks are ofcast iron 6 inches deep, 11 inches wide, and 14 inches long, and they areformed with flanges 1 inch thick on the inner sides of the blocks. Theprojection of the air pump lever from the centre of the cross head is 1foot 9 inches, and it is bent 5-3/4 inches to one side to enable it toengage the air pump rod. The eye of this arm is 6 inches broad and about 2inches thick. At the part where one of the piston rods passes through it, the arm is 8 inches deep and 6 inches wide; but the width thereafternarrows to 3 inches, and finally to 2 inches; and the depth of the web ofthe arm reduces from 8 inches at the piston rod, to 4 inches at the eye, which receives the end of the air pump rod. The feed pump arm is only 3inches thick, and has 9 inches of projection from the centre of the crosshead; but the eye attached to it on the opposite side of the cross head forthe reception of the other piston rod is of the same length as that part ofthe air pump arm which one of the piston rods passes through. The pistonrods have strong nuts on each side of each of these arms to attach them tothe arms, and also to enable the length of the piston rods to be suitablyadjusted, to leave equal clearance between the piston and each end of thecylinder at the termination of the stroke. 

[Illustration: Fig. 53. 

CROSS HEAD AND PUMP ARMS. Messrs. Bourne & Co. ]

649. _Q. _--Will you recapitulate the main particulars of the air pump?

_A. _--The air pump is made of brass 12-1/2 inches diameter and 42 inchesstroke, and the metal of the barrel is 9/16ths of an inch thick. The airpump bucket is a solid piston of brass, 6-1/2 inches deep at the edge, and7 inches deep at the eye; and in the edge three grooves are turned to holdwater which answers the purpose of packing. The inlet and outlet valves ofthe air pump consist of brass plates 1/2 inch with strong feathers acrossthem, and in each plate there are six grated perforations covered by indiarubber discs 7 inches in diameter. These six perforations affordcollectively an area for the passage of the water equal to the area of thepump. The air pump rod is of brass, 2-1/2 inches diameter. 

650. _Q. _--What are the constructive peculiarities of the discs and crankpin?

_A. _--The discs, which are 64 inches diameter, are formed of cast iron, andare 2-1/2 inches thick in the body, and 5 inches broad at the rim. Thecrank shaft is 8-1/2 inches diameter, and the central boss of the discwhich receives the shaft measures 10 inches through the eye, and the metalof the eye is 3 inches thick. In the part of the disc opposite to the crankpin, the web is thickened to 10 inches for nearly the whole semicircle, with the view of making that side of the disc heavier than the other side;and when the engine is stopped, the gravitation of this heavy side raisesthe crank pin to the highest point it can attain, whereby it is placed inmid stroke, and cannot rest with the piston rods and connecting rod in ahorizontal line. The crank pin is 8-1/2 inches diameter, and the length ofthe bearing or rubbing part of it is 16 inches. It is secured at the endsto the discs by flanges 18 inches diameter, and 2 inches thick. Theseflanges are indented into thickened parts of the discs, and are eachattached to its corresponding disc by six bolts 2 inches diameter, countersunk in the back of the disc, and tapped into the malleable ironflange. Besides this attachment, each end of the pin, reduced to 4-1/2inches diameter, passes through a hole in its corresponding disc, and theends of the pin are then riveted over. The crank pin is perforated throughthe centre by a small hole about 3/4 of an inch in diameter, and threeperforations proceed from this central hole to the surface of the pin. Eachcrank shaft bearing is similarly perforated, and pipes are cast in thediscs connecting these perforations together. The result of thisarrangement is, that a large part of the oil or water fed into the bearingsof the shaft is driven by the centrifugal action of the discs to thesurface of the crank pin, and in this way the crank pin may be oiled orcooled with water in a very effectual manner. To intercept the water or oilwhich the discs thus drive out by their centrifugal action, a light paddlebox or splash board of thin sheet brass is made to cover the upper part ofeach of the discs, and an oil cup with depending wick is supported by thetops of these paddle boxes, which wick is touched at each revolution of thecrank by a bridge standing in the middle of an oil cup attached to thecrank pin. The oil is wiped from the wick by the projecting bridge at eachrevolution, and subsides into the cup from whence it proceeds to lubricatethe crank pin bearing. This is the expedient commonly employed to oil thecrank pins of direct acting engines; but in the engine now described, thereare over and above this expedient, the communicating passages from theshaft bearings to the surface of the pin, by which means any amount ofcooling or lubrication can be administered to the crank pin bearing, without the necessity of stopping or slowing the engine. 

[Illustration: Fig. 54. DOUBLE DISC CRANK. Messrs. Bourne & Co. ]

651. _Q. _--What is the diameter of the screw shaft?

_A. _--The screw shaft is 7-1/2 inches diameter, but the bearings on eachside of the disc are 8-1/2 inches diameter, and 16 inches long. Between theside of the disc and the side of the contiguous bearings there is a shortneck extending 4-3/4 inches in the length of the shaft, and hollowed outsomewhat to permit the passage of the piston rod; for one piston rod passesimmediately above the shaft on the one side of the discs, and the otherpiston rod passes immediately below the shaft on the other side of thediscs. A short piece of one piston rod is shown in fig. 54. 

[Illustration: Fig. 55. THRUST BEARING. Messers. Bourne & Co. ]

[Illustration: Fig. 56. COUPLING CRANKS. Messers. Bourne & Co. ]

652. _Q. _--How is the thrust of the screw shaft received?

_A. _--The thrust of the screw shaft is received upon 7 collars, each 1 inchthick, and with 1 inch of projection above the shaft. The plummer block forreceiving the thrust of the shaft is shown in fig. 55, and the coupling toenable the screw propeller to be disconnected from the engine, so that itmay revolve freely when the vessel is under sail, is shown in fig. 56. Whenit is required to disengage the propeller from the engine, the pins passingthrough the opposite eyes shown fig. 56, are withdrawn by means of screwsprovided for that purpose, and the propeller and the engine are thenceforthindependent of one another. 

[Illustration: Fig. 57. LINK MOTION. Messrs. Bourne & Co. ]

653. _Q. _--Will you describe the arrangement of the valve gearing?

_A. _--The end of the screw shaft, after emerging from the bearing besidethe disc, is reduced to a diameter of 4 inches, and is prolonged for 4-1/2inches to give attachment to the cam or curved plate which gives motion tothe expansion valve. This plate is 3-1/2 inches thick, and a stud 3-1/2inches diameter is fixed in the plate at a distance of 5 inches from thecentre of the shaft. To this stud an arm is attached which extends to adistance of 2 inches from the centre of the shaft in the oppositedirection, and the end of this arm carries a pin of 2-1/2 inches diameter. From the pin most remote from the centre of the shaft, a rod 2-1/2 inchesbroad and 1 inch thick extends to the upper end of the link of the linkmotion; and from the pin least remote from the centre of the shaft, asimilar rod extends to the lower end of the link of the link motion. Thislink, which is represented in fig. 57, is 2-1/4 inches broad, 1 inch thick, and is capable of being raised or lowered 25 inches in all. In the openpart of the link is a brass block, which, by raising or lowering the link, takes either the position in which it is represented at the centre of thelink, or a position at either end of it. Through the hole in the brassblock a pin passes to attach the brass to the end of a lever fixed on thevalve shaft; so that whatever motion is imparted to the brass block iscommunicated to the valve through the medium of this lever. If the brassblock be set in the middle of the link, no motion is communicated to it, and the valve being consequently kept stationary and covering both ports, the engine stops. If the link be lowered until the brass block comes to theupper end of the link, the valve receives the motion of the eccentric forgoing ahead, and the engine moves ahead; whereas if the link be raiseduntil the brass block comes to the lower end of the link, the valvereceives the motion of the backing eccentric, and the engine moves astern. Instead of eccentrics, however, pins at the end of the shaft are employedin this engine, the arrangement partaking of the nature of a double crank;but the backing pin has less throw than the going ahead pin, whereby theefficient length of the link for going ahead is increased; and theoperation of backing, which does not require to be performed at the highestrate of speed, is sufficiently accommodated by about half the throw beinggiven to the valve that is given in going ahead. A valve shaft extendsacross the end of the cylinder with two levers standing up, which engagehorizontal side rods extending from a small cross head on the end of thevalve rod. A lever extends downwards from the end of the valve shaft, whichis connected by a pin to the brass block within the link; and the link ismoved up or down by the starting handle, which, by means of a spring boltshooting into a quadrant, holds the starting handle at any position inwhich it may be set. 

654. _Q. _--What is the diameter and pitch of the screw propeller?

_A. _--The diameter is 7 feet and the pitch 14 feet. The propeller is Holm'sconchoidal propeller. Its diameter is smaller than is advisable, beinglimited by the draught of water of the vessel; and the vessel was requiredto have a small draught of water to go over a bar. This engine makes, underfavorable circumstances, 100 strokes per minute. The speed of piston withthis number of strokes is 700 feet per minute, and the engine workssteadily at this speed, the shock and tremor arising from the arrestedmomentum of the moving parts being taken away by the counterbalance appliedat the discs. 

LOCOMOTIVE ENGINE. 

655. _Q. _--Will you describe the principal features of a modern locomotiveengine?

_A. _--I will take for this purpose the locomotive Snake, constructed byJohn V. Gooch for the London and South Western Railway, as an example of amodern locomotive of good construction, adapted for the narrow gauge. Thelength of the wheel base of this engine is 12 feet 8-1/2 inches. There aretwo cylinders, each 14-1/4 inches diameter and 21 inches stroke. The totalweight of the engine is 19 tons; and this weight is so distributed on thewheels as to throw 8 tons on the leading wheels, 6 tons on the drivingwheels, and 5 tons on the hind wheels. The engine is made with outsidecylinders, and the cylinders are raised somewhat out of the horizontal lineto enable them better to clear the leading wheels. 

656. _Q. _--What are the dimensions of the boiler?

_A. _--The interior of the fire box is 3 feet 7-1/4 inches wide by 3 feet5-1/2 inches long, measuring in the direction of the rails. The area of thefire grate is consequently 12. 4 square feet. The bars are somewhat lower onthe side next the fire door than at the side next the tubes, and the meanheight of the crown of the fire box above the bars is 3 feet 10 inches. Thetop edge of the fire door is about 7 inches lower than the crown of thefire box. The fire box is divided transversely by a corrugated feather orbridge of plate iron, containing water, about 3-1/2 inches wide, and ofabout one-third of the height of the fire box in the centre of the feather, and about two-thirds the height of the fire box at the sides where it joinsthe sides of the fire box. The internal shell of the fire box taperssomewhat upwards to facilitate the disengagement of the steam. It is about2 inches narrower and shorter at the top than at the bottom; the waterspace between the external and internal shell of the fire box being 2inches at the bottom and 3 inches at the top. 

657. _Q. _--Of what material is the fire box composed?

_A. _--The external shell of the fire box is formed of iron plates 3/8ths ofan inch thick, and the internal shell is formed of copper plates 1/4 inchthick, but the tube plate is 3/4 inch thick. The fire grate is rectangular, and the internal and external shells are tied together by iron stay bolts3/4 inch diameter, and pitched about 4 inches apart. The roof of the firebox is stiffened by six strong bars extending from side to side of the firebox like beams, and the top of the fire box is secured to these bars, sothat it cannot be forced down without breaking or bending them. 

658. _Q. _--What are the dimensions of the barrel of the boiler?

_A. _--The barrel of the boiler is 3 feet 7-1/2 inches in diameter, and 10feet long. It is formed of iron plates 3/8ths of an inch thick, rivetedtogether. It is furnished with 181 brass tubes 1-7/8 inch diameter and 10feet long, secured at the ends by ferules. The tube plate at the smoke boxend is 5/8ths of an inch thick, and the tube plates above the tubes aretied together by eight iron rods 7/8ths of an inch thick, extending fromend to end of the boiler. The metal of the tubes is somewhat thicker at theend next the fire, being 13 wire gauge at fire box end, and 14 wire gaugeat smoke box end. The rivets of the boiler are 3/4 inch diameter and 1-1/2inch pitch. The plating of the ash pan is 5/16ths of an inch thick, and theplating of the smoke box is 3/16ths of an inch thick. 

659. _Q. _--Will you describe the structure of the framework on which theboiler and its attachments rest, and in which the wheels are set?

_A. _--The framework or framing consists of a rectangular structure of plateiron circumscribing the boiler, with projecting lugs or arms for thereception of the axles of the wheels. In this engine the sides of therectangle are double, or, as far as regards the sides, there are virtuallytwo framings, one for the reception of the driving axles, and the other forthe reception of the axles not connected with the engine. The whole of theparts of the outer and inner framings are connected together by knees atthe corners, and the double sides are elsewhere connected by interveningbrackets and stays, so as to constitute the whole into one rigid structure. The whole of the plating of the inside frame is 3/4 inch thick and 9 inchesdeep. The plating of the outside frame is of the same thickness and depthat the fore part, until it reaches abaft the position of the cylinders andguides, where it reduces to 1/2 inch thick. The axle guard of the leadingwheels is formed of 3/4 plate bolted to the frame with angle iron guides. The axle guards of the trailing wheels are formed of two 1/2 inch plates, with cast iron blocks between them to serve as guides. The ends of therectangular frame are formed of plates 3/4 thick, and at the front endthere is a buffer beam of oak 4-1/2 inches thick and 15 inches deep. Thedraw bolt is 2 inches diameter. There are two strong stays on each side, joining the barrel of the boiler to the inside framing, and one angle ironon each side joining the bottom of the smoke box to the inside framing. 

660. _Q. _--Of what construction are the wheels?

_A. _--The wheels and axles are of wrought iron, and the tires of the wheelsare of steel. The driving wheels are 6 feet 6-1/2 inches in diameter, andthe diameter of crank pin is 3-1/2 inches. The diameter of the smallerwheels is 48-1/2 inches. The axle boxes are of cast iron with bushes ofFenton's metal, and the leading axle has four bearings. The springs areformed of steel plates, 3 feet long, 4 inches broad, and 1\2 inch thick. The axle of the driving wheel has two eccentrics, forged solid upon it, forworking the pumps. 

661. _Q. _--Will you specify the dimensions of the principal parts of theengine?

_A. _--Each of the cylinders which is 14-1/4 inches diameter, has the valvecasing cast upon it. The steam ports are 13 inches long and 1-5/8 inchesbroad, and the exhaust port is 2-1/2 inches broad. The travel of the valveis 4-1/8 inches, the lap 1 inch, and the lead 1/4 inch. The piston is 4inches thick: its body is formed of brass with a cover of cast iron, andbetween the body and the cover two flanges, forged on the piston rod, areintroduced to communicate the push and pull of the piston to the rod. Thepiston rod is of iron, 2-1/2 inches diameter. The guide bars for guidingthe top of the piston rod are of steel, 4 inches broad, fixed to rib ironbearers, with hard wood 1/4 of an inch thick, interposed. The connectingrod is 6 feet long between the centres, and is fitted with bushes of whitemetal. The eccentrics are formed of wrought iron, and have 4-1/8 inches ofthrow. The link of the link motion is formed of wrought iron. It is hung bya link from a pin attached to the framing; and instead of being susceptibleof upward and downward motion, as in the case of the link represented infig. 57 a rod connecting the valve rod with the movable block in the link, is susceptible of this motion, whereby the same result is arrived at as ifthe link were moved and the block was stationary. One or the otherexpedient is preferable, according to the general nature of thearrangements adopted. The slide valve is of brass, and the regulatorconsists of two brass slide valves worked over ports in a chest in thesteam pipe, set in the smoke box. The steam pipe is of brass, No. 14. Wiregauge, perforated within the boiler barrel with holes 1/12th of an inch indiameter along its upper side. The blast pipe, which is of copper, has anorifice of 4-1/4 inches diameter. There is a damper, formed like a Venetianblind, with the plates running athwartships at the end of the tubes. 

[Illustration: Fig. 58. SAFETY VALVE. Gooch. ]

662. _Q. _--Of what construction is the safety valve?

_A. _--There are two safety valves, consisting of pistons 1-3/16 inch indiameter, and which are kept down by spiral springs placed immediately overthem. A section of this valve is given in fig. 58. 

663. _Q. _--What are the dimensions of the feed pumps?

_A. _--The feed pumps are of brass, with plungers 4 inches diameter and3-1/4 inches stroke. The feed pipe is of copper, 2 inches diameter. A gooddeal of trouble has been experienced in locomotives from the defectiveaction of the feed pump, partly caused by the leakage of steam into thepumps, which prevented the water from entering them, and partly from thereturn of a large part of the water through the valves at the return strokeof the pump, in consequence of the valve lifting too high. The pet cock--asmall cock communicating with the interior of the pump--will allow anysteam to escape which gains admission, and the air which enters by the cockcools down the barrel of the pump, so that in a short time it will be in acondition to draw. The most ordinary species of valve in the feed pumps oflocomotives, is the ball valve. 

Notwithstanding the excellent performance of the best examples oflocomotive engines, it is quite certain that there is still much room forimprovement; and indeed various sources of economy are at present visible, which, if properly developed, would materially reduce the expense of thelocomotive power. In all engines the great source of expense is the fuel;and although the consumption of fuel has been greatly reduced within thelast ten or fifteen years, it is capable of being still further reduced bycertain easy expedients of improvement, which therefore it is importantshould be universally applied. One of these expedients consists in heatingthe feed water by the waste steam; and the feed water should in every casebe sent into the boiler _boiling hot_, instead of being quite cold, as isat present generally the case. The ports of the cylinders should be aslarge as possible; the expansion of the steam should be carried to agreater extent; and in the case of engines with outside cylinders, thewaste steam should circulate entirely round the cylinders before escapingby the blast pipe. The escape of heat from the boiler should be morecarefully prevented; and the engine should be balanced by weights on thewheels to obviate a waste of power by yawing on the rails. The mostimportant expedient of all, however, lies in the establishment of a systemof registering the performance of all new engines, in order thatcompetition may stimulate the different constructors to the attainment ofthe utmost possible economy; and under the stimulus of comparison andnotoriety, a large measure of improvement would speedily ensue. Thebenefits consequent on public competition are abundantly illustrated by therapid diminution of the consumption of fuel in the case of agriculturalengines, when this stimulus was presented. 

CHAPTER XI

OF VARIOUS FORMS, APPLICATIONS, AND APPLIANCES OF THE STEAM ENGINE. 

In the English edition of this work, the first part of this chapter isdevoted to examples of Portable and fixed Agricultural engines, ofdifferent makers and styles of workmanship, but not in sufficient detail, nor illustrated on large enough scale to be of practical value as models, forming rather in fact an illustrated catalogue of the manufacturer, than astudy for the mechanic. On this account, they have been entirely omitted, and their place supplied by a few illustrations from American workmanship, not only of Steam Engines, of various forms and applications, but also ofvarious machines, or appliances, connected with the working of engines, asfor the determination, or regulation of pressure, of the boilers; for thesupply or feed of the boilers, the regulation of the speed of the engine, and the like. 

The Gauges used in this country to show the pressures of steam in boilersare of various constructions, but perhaps the most common is the Bourdon, or, as it is known here, the Ashcroft gauge, from the party introducing it, and holding the patent. Fig. 59 represents its interior construction. Itconsists of a thin metallic tube, _a_, bent into nearly a complete circleclosed at one end, the steam being introduced at the other, at _b_. Theeffect of the pressure of the steam on the interior of the tube is toexpand the circle, more or less according to the pressure, the elasticityof the metal returning the circle to its original position, when thepressure is removed. The free or closed end of the tube is connected by alink _c_ with a lever _d_, at the opposite end of which is segmental gear, in gear with a pinion, on which is a hand, which marks the pressure on adial. The dial and hand are not shown on the cut, but are on the exteriorcase removed to show the construction. 

[Illustration: Fig. 59. ]

[Illustration: Fig. 60. ]

Fig. 60 is an elevation of a boiler with Clark's Patent Steam and FireRegulator attached, for the control of the draft of the chimney by thepressure of steam in the boiler. It consists of a chamber, _a_, with aflexible diaphragm or cover on top, in communication with the boiler. Onthis diaphragm rests a plunger or piston, which is held down like a safetyvalve, by a lever and weight, _b_. The end of the lever is connected with abalanced damper, _c_, in the chimney. The weight, _b_, is placed at anyrequired position on the lever, and when the pressure of steam in theboiler, exerted on the diaphragm, becomes sufficient to raise the weight, the lever rises, and the damper begins to close, and to check the draft inthe chimney. When properly adjusted, the machine works on a variation offrom, one to two pounds between the extremes of motion. When the dampersare very large, say 3 feet or over, they should be set on rollers, likecommon grindstone rollers; the regulator should be attached directly to thedamper, the length of the pipe connecting the regulator with the boilerbeing of no account. 

[Illustration: Fig. 61. ]

Porter's Patent Governor, fig. 61, is a modification of the ordinarycentrifugal governor. Very small balls are employed, from 2-1/4 to 2-5/8inches in diameter. These swing from a single joint at the axis of thespindle, which is the most sensitive arrangement, and make from 300 to 350revolutions per minute, at which speed their centrifugal force lifts thecounterpoise. The lower arms are jointed to the upper ones at the centresof the balls, and connect with the slide by joints about two inches apart. The counterpoise may be attached to the slide in any manner; for the sakeof elegance, it is put in the form of a vase rising between the arms, itsstem forming the slide. The vase is hollow and filled with lead, and weighsfrom 60 lbs. To 175 lbs. It moves freely on the spindle, through nearlytwice the vertical distances traversed by the balls, and is capable ofrising from 2-1/2 to 3 inches, before its rim will touch the arms. It isrepresented in the figure as lifted through about one half of its range ofaction. 

The standard is bored out of the solid, forming a long and perfect bearingfor the spindle; the arms and balls are of gun metal, the joint pins ofsteel; every part of the governor is finished bright, except the bracketcarrying the lever, and the square base of the standard, which are painted. The pulley is from 3 to 10 inches in diameter, and makes in the largersizes about 125 revolutions, and in the smaller 230 revolutions per minute;the higher speed of the governor being got up by gearing. 

Mr. Porter warrants the following action in this governor, operating anyregulating valve or cut-off which is in reasonably good order. The engineshould be run with the stop-valve wide open, and, except the usual oiling, will require no attention from the engineer, under any circumstances, afterit is started, until it is to be stopped. No increase in the pressure ofsteam will affect its motion perceptibly. The extreme possible variation inthe speed, between that at which the regulating valve will be held wideopen, and that at which it will be closed, is from 3 to 5 per cent. , beingleast in the largest governors. This is less than 1/6 of the variationrequired by the average of ordinary governors, and is with difficultydetected by the senses. The entire load which the engine is capable ofdriving may be thrown on or off at once, and one watching the revolutionscannot tell when it is done. The governor will be sensibly affected by avariation in the motion of the engine of 1 revolution in 800. Notwithstanding this extreme sensitiveness, or rather by reason of it, itwill not oscillate, but when the load is uniform will stand quite, ornearly, motionless. 

For the supply of the water to the boiler, in many positions, it is veryconvenient to have a pump unconnected with the engine. On this account itis very usual in this country to have what are called donkey pumps orengines independent of the main engines, which can be used to feed theboilers, or for supplying water for many other purposes. 

Fig. 62 is a longitudinal section of the Worthington Steam Pump, the firstof its kind, and for many years in successful operation. 

The general arrangement is that of a Steam Cylinder, the piston rod ofwhich, carried through into the water cylinder and attached directly to thewater plunger, works back and forth without rotary motion, and of coursewithout using either crank or fly wheel. 

[Illustration: Fig. 62. ]

In the figures, _a_ is the Steam Cylinder--_b_, the Steam Chest--_d_, ahandle for regulating the steam valve--_f_, the starting bar _g, g_, tappets attached to the valve rod, which is moved by the contact of the arm_e_, on the piston rod with said tappets--_h_, the double-acting waterplunger working through a packing ring--_o, o_, force valves--_o', o'_, suction valves. The pump piston is represented as moving from right toleft, the arrows indicating the course of the water through the passages. The suction valves _o'_, on the right side, and the force valves _o_, onthe left side, are show open; _x_, is an air chamber made of copper; _s_, the suction pipe terminating in a vacuum chamber; made by prolonging thesuction pipe, and closing it perfectly tight at the top, the connectionbeing made to the pump by a branch as shown; _m, m_, are hand-hole plates, affording easy access to the water valves; _n, n_, small holes through theplunger, which relieve the pressure near the end of the stroke, to givemomentum to throw the valves when working at slow speed. 

[Illustration: Fig. 63. ]

Fig. 63 is a perspective view of H. R. Worthington's Duplex Steam Pump. Theprominent peculiarity of this pump is its valve motion. As seen in the cut, two steam pumps are placed side by side (or end to end, if desired). Eachpump, by a rock shaft connected with its piston rod, gives a constant andeasy motion to the steam valve of the other. Each pump therefore givessteam to and starts its neighbor, and then finishes its own stroke, pausingan instant till its own steam valve, being opened by the other pump, allowsit to make the return stroke. 

This combined action produces a perfectly positive valve motion withoutdead points, great regularity and ease of motion, and entire absence ofnoise or shock of any kind. Both kinds of pumps are made by Mr. Worthington, of various size according to the requirements, the duplexbeing used for boiler feed and for the supply of cities with water. 

Fig. 64 is a side elevation of the Woodward Steam Pump. The pump is directacting. The steam and water piston being on the same rod, but momentum isobtained to throw the valves by means of a fly wheel, placed beyond thepump, and connected with the piston rod by a cross head and a yoke. Themachine is simple in its construction and action, and is extensively used. 

Giffard's Injector, both in Europe and this country, is quite extensivelyused to supply the place of a pump, as independent feed for all classes ofboilers. It is represented in elevation and section, figs. 65 and 66. 

[Illustration: Fig. 64. ]

[Illustration: Fig. 65. ]

[Illustration: Fig. 66. ]

_A_, steam pipe leading from the boiler. _B_, a perforated tube orcylinder, through which the steam passes into the space _b_. _C_ screwedrod for regulating the passage of steam through the annular conical space_c_, and worked by the handle _d/_. _E_, suction pipe, leading from thetank or hot well to small chamber _m_. _F_, annular conical opening ordischarge pipe, the size of which is regulated by the movement of the tubeor cylinder _B_. _G_, hand wheel for actuating the cylinder _B_. _H_, opening, in connection with the atmosphere, intervening between dischargepipe _F_ and the receiving pipe through which the water is forced. _I_, tube through which the water passes to the boiler. _K_, valve forpreventing the return of the water from the boiler when the injector is notworking. _L_, waste or overflow pipe. _M_, nut to tighten the packing rings_g_ and upper packing _i_ in cylinder _B_. _N_, lock nut to hold _M_. 

The pipe _A_ is connected with the steam space of the boiler at its highestpart, to obtain as dry steam as possible. The passage of the steam into _A_is controlled by a cock, as is also the feed pipe to the boiler. Inworking, both are opened, the steam passes through _A_ into the space _b_, and issuing through the nozzle _c_ with the pressure due to its head, and apartial vacuum by its contact with the feed water, it drives this water inconnection with the jet through the pipe _F_ into the pipe _I_ inconnection with the water space of the boiler. 

_Method of Working. _--Turn the wheel so as to permit a small quantity ofwater to flow to the instrument. Open the steam cock connecting theapparatus with the boiler. Turn slightly the handle, which will admit asmall quantity of steam to the apparatus; a partial vacuum is thusproduced, causing the water to enter through the supply pipe. As soon asthis happens, which can be observed at the overflow pipe, the supply ofsteam or water may be increased as required, up to the capacity of theinstrument, regulating either by means of the wheel and handle, so as toprevent any overflow. The quantity of water delivered into the boiler, maybe varied by means of the stop cocks on the steam and water pipes, withoutaltering the handles on the injector; a graduated cock on the water supplypipe is very convenient for this purpose. 

The machines are manufactured by Wm. Sellers & Co. Philadelphia. 

As an example of Portable Steam Engines, of which there are large numbersin this country of different manufacturers, we give the representation(fig. 67) of one made by J. C. Hoadley, of Lawrence, Mass. 

[Illustration: Fig. 67. ]

In these machines, the rules and proportions of the locomotive engine areadapted to the requirements of stationary power, for all purposes underforty horse power. The leading ideas are: high velocity, high pressure, good valve motion, large fire-box, numerous and short flues, and steamblast. The characteristic features are: great strength of boiler, fullyadequate to bear with safety 200. Lbs. Pressure per sq. In. , greatcompactness and simplicity, large and adjustable wearing surfaces, and theentire absence of all finish, or polish, for mere show. 

The cylinder is placed over the centre of the boiler, at the fire-box end, so that the strain due to the engine is central to the boiler (which servesas bed plate); the starting valve is under the hand of the engineer when atthe fire door; and both ends of the crank shaft are available for drivingpulleys. 

For the sake of compactness, the cylinders are set low, by means of adepression in the boiler between the stands of the crank shaft, to admit ofthe play of the crank and connecting rod. All the parts are attached to theboiler, which is made of sufficient strength to bear all extra strain dueto the working of the engine. 

They have feed water heater, force pumps, Jackson's governor and valve, belt for governor, belt pulley, turned on the face, steam gauge;everything, in short, necessary to the convenient working of a steamengine. All engines are fired up and tried before they leave the shop, andthey are warranted tight, safe, and complete. 

A strong and convenient running gear, so arranged as to be easily attachedand detached at pleasure, is furnished, if desired; forming, when separate, a useful wagon. 

[Illustration: Fig. 68. ]

Fig. 68 is a compact vertical engine, as built by R. Hoe & Co. , of thiscity. It is intended to drive printing presses, but is adapted to any kindof work, and is especially suited to such places as require economy ofspace. Although the value of expansion has been called in question by someof the engineers of the United States Navy, and under an appropriation fromCongress is now to be made the subject of experiment; yet, in almost allthe manufactories and workshops of the United States, no matter what theform of steam engine, or the purposes to which it is applied, whetherstationary, locomotive, or marine, some form of cut-off, by which expansionof the steam can be availed of, is considered indispensable. Many varietiesare in use, but those engines are most popular in which the cut-off isapplied directly to the valves on the cylinder, opening them quickly andshutting off almost instantly, avoiding all wire drawing of the steam atthe ports, and regulating the speed of the engine promptly. Of this classof engines, those manufactured by the Corliss Steam Engine Company, ofProvidence, R. I. , are perhaps the widest known, not only for theirextensive introduction, but also from having, by a long and successfullitigation, established the claims of the patentee, Mr. George H. Corliss. 

[Illustration: Fig. 70. ]

Fig. 70 is a section of the cylinder and valve chests of a horizontalCorliss engine. _S_ is the steam connection, and _E_ the exhaust; there aretwo distinct sets of valves, the steam _s, s'_, and the exhaust _e, e'_, operated independently of each other. In their construction the valves maybe considered cylindrical plugs, of which portions near the ports are cutaway to admit the steam and reduce the bearing surface; the valves arefitted on the lathe and the seats by boring. The motion given to the valvesis rocking, but it will be observed that the valves are not firmlyconnected to the rocking shaft or cylinder; in the figure the valves areshown shade lined, and the shaft or stem plain; in this way the valves arenot affected by the packing of the valve stem, but always rest upon theface of the ports. In the figure the piston is just about to commence itsoutstroke, the movement of the steam is supposed to be represented by thearrows; the inner steam valve _s_, and the outer exhaust _e'_, are justbeginning to open. It will be observed that the outer steam _s'_ is fullyclosed, whilst the inner exhaust valve _e_ is but barely so, showing thatthere has been a cut-off on the steam valve, but no lead to the exhaust, that it was left fully open till the completion of the stroke. 

[Illustration: Fig. 71. ]

Fig. 71 is a side elevation of the cylinder, with the valve connectionswith the governor. _S_ is the steam pipe; _s, s'_ handles to the steamvalves, and _e, e'_ to the exhaust valves, shown in dotted line in fig. 70. The handles to the exhaust valves are connected directly to a rocking plate_R_, to which motion is given by a connection _x_, with an eccentric on theengine shaft. When once set, therefore the movement of the exhaust valvesis constant, and they will always be opened and closed at the same point ofthe stroke. Connected with the rocking plate _R_, and on opposite sides ofits centre, the same as the exhaust valve connections, there are twolevers, vibrating on a centre _c_, of which one only is shown, as it coversthe other; to the upper ends of these levers pawls are attached, one end ofwhich rests on the stems or rods connected with the handles _s, s'_, of thesteam valves; on these stems there are notches against which the pawlsstrike, and as the levers vibrate inward they push back the stems andthereby open the valves, and this continues for the whole length of theinward motion of the levers, or till the outer extremities of the pawlscome in contact with the end of the short lever _l_, which, pushing downthe outer end of the pawls, relieves the stems at the other ends, and thevalve stem returns to its place through the force of springs attached tothe outer extremities of the valve stems _a_, are cylindrical guides to thevalve stems, at the inner extremities of which are air cushions. The lever_l_ is connected directly with the governor. As the balls rise, theydepress the extremity, which comes in contact with the pawls sooner, andthereby shut the valves earlier; and on the contrary when the balls aredepressed, the valves remain open longer; as the pawls come in contact withthe stems always at one point, the steam valves open constantly, but areclosed at any point by the relief of the pawls, according to the speed ofthe governor. 

Fig. 71 represents, partly in section and partly in plan, the cylinder, steam chests, valves, &c. , of one of the Woodruff & Beach high pressureEngines, Wright's patent. 

Fig. 72 represents, in elevation, the cam shaft, to the upper end of which, not shown in the drawing, is attached the ordinary centrifugal governor. The cylinder, steam chests, valves, &c. , being similar to those of otherengines, need no special notice; but the cam for opening and closing thesteam valves, fig. 72, requires particular attention, as it embodies abeautiful and simple device for cutting off the steam with certainty at anypart of the stroke, the motion being produced automatically by the actionof the governor on this cam, throwing it more or less out of centre withthe spindle of the governor, as the rotation of the balls is less or morerapid, the eccentricity of the cam determining the amount of steam admittedto the working cylinder of the engine. To produce this effect the cam ismade as follows:

_C_ is a hollow cylinder or shell, with a part of one end formed into a camproper. Throughout the whole length of this piece, upon the inside, thereis a spiral groove cut to receive one end of a feather, by which its pitchor eccentricity is regulated. _C'_ is also a hollow cylinder or shell, ofthe same length and diameter as _C_, with a similar spiral groove cut onthe inside, the outside being perfectly smooth and plain, upon which thetoe (_t_) for closing the valves is fastened. The inside piece consists oftwo hubs _D, D'_, eccentric with each other, and made in one piece, _D_being turned to exactly fit the inside of the shell _C_, and _D'_ to fitthe shell _C'_, the hub _D'_ having a socket (_c_) into which the spindle(_s_) of the governor is screwed; the end (_d_) of the hub _D_ forming ajournal or bearing, with a bevel wheel on its extremity to convey motionfrom the crank-shaft gearing to the governor and cut-off. There is a holethroughout the length of the inside hubs _D_ and _D'_, which is continuedthrough the spindle of the governor, and contains the rod (_r_) thatconnects the cam with the governor. This hole is eccentric to the outsidesurface of the hub _D_, as well as to the shell _C_, and concentric withthe hub _D'_ and shell _C'_, and with the governor rod (_r_). 

The shell _C_ and hub _D_, and shell _C'_ and hub _D'_, are connectedtogether by feathers; one piece of each feather is of a spiral form, andthe other a straight or rectangular piece, the two being connected togetherby a stub on the rectangular piece, which fits into a hole or bearing inthe other or spiral piece, so that the latter can turn on the stub andaccommodate itself to the groove in which it has to work. The spiral partof each feather works in the spiral groove on the inside of itscorresponding shell _C_ and _C'_ respectively, and the rectangular pieceswork in a straight groove cut in the hubs _D_ and _D'_, the inner parts ofthe rectangular pieces being fastened to the governor rod (_r_), so thatthe feathers are permanently connected with the governor. 

The shell _C'_ revolves inside of two yokes (_y_) and (_y'_), one attachedto each steam-valve toe, (_a_) and (_a'_) respectively. 

On the inside of each yoke, and opposite to its valve-toe, is a raisedpiece, against which the closing piece (_t_) on the shell (_C'_) acts toclose the valves. 

This shell (_C'_), as before noticed, has a spiral groove on its inside, similar in all respects to that in the cam-shell (_C_); and being actedupon in the same manner and through the same rod by the governor, it isevident that the closing piece (_t_) on its outside will always hold thesame relation to the opening toe on the lower or cam-shell (_C_); andwhatever alteration is made in the one, a corresponding alteration takesplace in the other, thereby insuring the closing of the valves at theproper time at every point of the variation of the cut-off. 

When the several pieces above described are put together, the apparatus foropening and closing the valves and producing the cut-off is complete, asshown in fig. 72, and it operates as follows:

[Illustration: Fig. 71. ]

[Illustration: Fig. 72. ]

Motion is communicated by gearing from the crank-shaft to the bevel wheelon the piece (_d_) on the end of the hub _D_, and is communicated to thespindle of the governor, which is screwed into the socket on _D'_. As theballs rise or fall, through change of centrifugal force due to thevariation in the speed of rotation, they raise or depress the governor-rod, which passes through the spindle and the hubs _D'_ and _D_, and is attachedto the feathers, thereby raising or depressing the feathers, which, actingon their respective spiral grooves, instantly alters the lift of the cam onthe shell (_C_), and brings the closing toe (_t_) on the shell (_C'_) intoproper position for closing, and so regulates the amount of steam admittedto the cylinder. 

[Illustration: Fig. 71. ]

Consequently, any speed may be selected at which the load of the engine isto move, and any variation from that will be instantly felt by thegovernor, and corrected by this simple and beautiful device. There is nojar in the working of the parts; the feathers move noiselessly in theirgrooves; the governor rod moves up and down through the spindle and thehubs _D_ and _D'_, and can be regulated by hand to give any requiredopening of the steam ports to suit the work to be done. Any change in theamount of work will then alter the speed of the engine, and so affect thegovernor and cam, as before said. 

It is unnecessary to insist on the great economy attained by using steamwith a well-regulated cut-off, for practical men know now that theessential points of excellence in the steam engine are a good boiler, whichgenerates the greatest quantity of steam for the least consumption of fuel;and, secondly, a reliable cut-off, which uses the steam to the bestadvantage, by admitting the proper quantity for the work required. 

STEAM FIRE ENGINES. --Portable engines for the extinguishment of fires, arean American invention, and to Messrs. A. B. & E. Latta, of Cincinnati, working on the right principles, is due the credit which they claim intheir circular, as follows:

"We claim to be the _original_ and first _projectors_ of the _firstsuccessful steam fire engine_ in the world's history. There have been manyattempts at making a machine of such construction as would answer toextinguish fires; but none of them proved to be available in a sufficientlyshort space of time to warrant their use as a fire apparatus. We hold thata steam fire engine should be of such nature as to be brought intorequisition in as short a space of time as is necessary to get the machineon the ground, and the hose laid and ready to work: that is, supposing thefire to be within one square of the place where the steamer is located. Theobject in locating a machine at any point is to protect that immediatevicinity; and it is therefore absolutely necessary to have it available inthe shortest space of time, and that with unerring certainty. We think thatreliability is of the greatest importance to the protection of a city fromfire, as everything is dependent on the _working_ of such apparatus intime; and for this reason no expense should be spared on this kind ofmachinery. "

Fig. 73 is a representation of one of the Messrs. Latta's fire engines, ofwhich there are many of different classes, according to the requirements;they say that they can furnish engines as low as $1, 000, and have made somefor $10, 000. 

The first peculiar feature of this engine is the boiler; it differsentirely from all boilers now in use. 

[Illustration: Fig. 73. ]

The fire box or furnace is simply a square box or furnace of any requireddimensions; it is nothing more than a water space surrounding the fire, stay-bolted as all water spaces are. It is made of boiler plate in theusual manner. The water space extends only 2/3 of the height, the balancebeing a single sheet. The bottom of this fire box is crossed by grate barsto support the fuel; in its rear side are fire doors, inserted for firing. The internal arrangements of the boiler are composed of a large number oftubes, lying across in a horizontal position, put together in sections withreturn bends resembling the coils for heating buildings. These coils are ofsmall pipe (say one inch in diameter), and as numerous as may be necessary. They give the required amount of steam. They are secured to wrought-ironplates at each end by rivets. These plates lie close to the box, and aresecured to it, top and bottom. These tubes are wrought iron, firmly screwedinto the bends, so as to prevent any possible breaking. 

The box has a hole through both sheets, in the same manner as a hollowstay-bolt, through which the coil pipe passes, having no connection withthe box. After passing into the box it divides into two pipes, thensubdivides into four, and so on, until its numbers equal the number ofcoils in the box, and to which each limb is attached. The upper ends ofthese coils are the same in number, and are carried through at the top ornearly the top of the box. They then run down outside to the steam chamber, or rather water space, as the box is both steam chamber and water space. These pipes empty their contents into the box, steam and water, as it maycome, all together. It will be observed that these coils of tube aresufficiently separated to allow the fire to pass between them freely, andcover their whole surface. 

The mode of operation of this boiler is this: The fire box is filled 2/3full of water. The coils are dry at starting; the space for fuel beingfilled with good wood, the fire is lighted, and in a few moments theengineer moves his hand pump, which takes its water from the box to whichit is attached, and forces it through the coils. By this means steam isgenerated in from 3 to 5 minutes, so as to start the engine. 

It will be seen that the water performs a complete circuit; it is takenfrom the box and passed through the coils; what is steam remains in thesteam chamber, and what is not (if any) drops back into the box from whereit started. Hence it will be seen that a large surface is exposed to asmall quantity of water, and in a way that it is entirely controllable. Allthe engineer has to do to surcharge his steam, is to reduce the speed ofthe pump (which is independent of the main engine). By raising the heat andquantity of water, any degree of elasticity can be given to the steam, andthat, too, with the least amount of waste heat in giving a natural draft. Hence the great economy of this boiler. 

The next feature of this engine is, it has no wood work about it to perishwith the heat and roughness of the streets. All the wheels are wroughtiron; and, as yet, these are the only ones that have stood a steam fireengine. The frame is wrought iron; truck, on which the front wheel is hung, wrought iron. The axles are cast steel. The engine and pump is adouble-acting piston pump direct, without any rotary motion; with aperfect balance valve, it is balanced at all times, and hence the engineremains quiet without blocking, when at work. The engine is mounted onthree wheels, which enables it to be turned in a very short space. 

Many engines have been constructed by the Messrs. Latta for the firecompanies, of different cities, and have been in successful competitionwith other engines; the farthest throw ever made by one of theirfirst-class engines was 310 feet from a 1-5/8 inch nozzle; steaming time, starting from cold water, 3-1/2 minutes. 

[Illustration: Fig. 74 AMOSKEAG STEAM FIRE ENGINE. ]

Fig. 74 is a representation of one class of steam fire engine, as built bythe Amoskeag Manufacturing Company, at Manchester, N. H. The boiler is anupright tubular boiler, of a peculiar construction, the patent right towhich is vested in the Amoskeag Manufacturing Company. This boiler is verysimple in its combination, and for safety, strength, durability, andcapacity for generating steam is unsurpassed. No fan or artificial bloweris ever used or needed, the natural draft of the boiler being alwayssufficient. Starting with cold water in the boiler, a working head of steamcan be generated in _less than five minutes_ from the time of kindling thefire. The engine "Amoskeag, " owned by the city of Manchester, has playedtwo streams in _three minutes and forty seconds_ after touching the match, at the same time drawing her own water. The boilers are made and proved soas to be safely run at a steam pressure of 140 to 150 lbs. To the squareinch; but the engines are constructed so as to give the best streams at apressure of about 100 lbs. To the square inch, and for service at fires asteam pressure of about 60 lbs. To the square inch is all that is required. 

The various styles of engine are all _vertical_ in their action, and in allthe pumps and steam cylinders are firmly and directly fastened to theboiler, the steam cylinders being attached directly to the steam dome. Thisarrangement obviates the necessity of carrying steam to the cylindersthrough pipes of considerable length, and the machine has very littlevibratory motion when in operation--so little that it is not necessary toblock its wheels to keep it in its place, or to take the weight off thesprings before commencing work. 

The pumps are placed on the engines as near the ground as they can be withsafety, and are arranged so as to attach the suction and leading hose toeither or both sides of the machine, as may be most convenient ordesirable, so that less difficulty will be found in placing an engine forwork, and when required to draw its own water, it has only to draw it theshortest possible distance. 

Each engine has two "feed pumps" for supplying the boiler, and also aconnection between the main forcing pumps and the boiler, so that it can besupplied from that source if desirable. The tank which carries the waterfor supplying the boiler is so placed that the water in it is always abovethe "feed pumps, " an advantage that insures the almost certain working ofthese pumps. These pumps are of brass, the best locomotive pattern, and oneof them running with the engine, when at work, furnishes an ample supply ofwater to the boiler. 

[Illustration: Fig. 75. ]

The engines are exceedingly portable; they can be turned about or placedfor service in as contracted a space as any hand engine, and two goodhorses will draw a first-class engine with the greatest ease, carrying atthe same time water for the boiler, a supply of fuel sufficient to run theengine two hours, the driver, the engineer, and the fireman. 

Fig. 75 is a representation of the class of steam fire engine built bySilsbee, Mynderse & Co. , Seneca Falls, N. Y. Under Holly's patent. 

The boiler is vertical, with vertical water tubes passing directly throughthe fire. These tubes are closed at the bottom and open at the top, wherethey pass through a water-tight plate, and communicate with the water inthe boiler. The arrangement of the tubes causes a constant current, thewater rising on the outside of the tubes as they are heated, and its placebeing supplied by a current flowing downward through the tube to theboiler. The smoke and flame pass among the tubes up through flues. 

Both engine and pump are rotary, and of the same type. They consistessentially of two elliptical rotary pistons, cogged and working into oneanother in an air-tight case. The pistons fit close to the inside of thecase, and gear into each on the line of their conjugate diameters. Theaction is somewhat similar to the old-fashioned rotary pump, consisting oftwo cog wheels in gear with, each other, the spaces at the side of the casebeing filled with water, which at the centre are occupied by the teeth ingear. In Holly's pump, instead of uniform teeth, and depending on the fitof the teeth with the side of the case and with each other for the packing, there are two large teeth in each piston opposite each other, which haveslide pistons, and intermediate with these large teeth are small cogs, which continue the motion of the rotary pistons. The machine works verysmoothly, and performs the work necessary, in ordinary service, under apressure of 50 to 60 lbs. 

There are many other makers of fire engines in this country; but sufficientexamples are given to illustrate the class; so successful have they been, that they are fast superseding hand engines, even in the smaller cities. 

Under a paid department, the following is, in the city of Boston, Mass. , the comparative cost of running the two kinds of engines, viz. :

 STEAM FIRE ENGINE. 1 engineer. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. $720 001 fireman. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 600 001 driver. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 600 001 foreman of hose. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 150 008 hosemen, at $125 each. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 375 00-- --------7 men. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . $2, 445 00Keeping of 2 horses. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 315 00 -------- Total. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. $2, 760 00

 HAND ENGINE. 1 foreman. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . $150 001 assistant foreman. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 125 001 clerk. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 125 001 steward. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 125 003 leading hosemen, at $125 each. .. .. .. .. .. .. .. .. .. .. . 375 0033 men, at $100 each. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 3, 300 00-- ---------40 men. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. $4, 200 00

Here the engineer, fireman, and driver are constantly employed, the hosemenhave other employment in the neighborhood, but all the company sleep in theengine house. 

In the city of Manchester, N. H. , a steam fire engine company is composed offourteen men, all told, one of whom, acting as driver and steward, isconstantly employed, remaining at the engine house with a pair of horsesalways ready to run out with the engine in case of an alarm of fire. Theother members of the company have other employments, and turn out only onan alarm of fire. 

 STEAM FIRE ENGINES. "Amoskcag, " Expenditures. .. .. .. .. .. .. .. .. .. .. $864 32"Fire King, " " . .. .. .. .. .. .. .. .. .. .. 855 78"E. W. Harrington, " " . .. .. .. .. .. .. .. .. .. .. 496 09

The above expense includes pay of members, team expenses, cost of gas, wood, coal, and all necessities incident to service. The "E. W. Harrington"is a second-class engine, stationed in the outskirts of the city, and wasrun cheaper from the fact that no horses were kept for it by the city. 

A first-class hand-engine company is allowed to number, all told, fiftymen, and the members of the company are paid as follows:

 FIRST-CLASS HAND-ENGINE COMPANY. 1 foreman. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . $35 001 assistant foreman. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 28 001 clerk. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 28 001 steward. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 68 0046 men, at $18 each. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 828 00 --------50 men. Total. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . $987 00

By this it will be seen, that in a city like Manchester, with from twentyto twenty-five thousand inhabitants, a first-class steam fire engine can berun at an expense not to exceed that of a first-class hand engine, while inservice it will do at least _four times_ the work. The cost of repairs isfound by experience to be no greater on the steam fire engines than on handengines. 

The Excavator, fig. 76, is the invention of the late Mr. Otis, anapplication of the spoon dredging machine of the docks to railway purposes, with very important modifications. The machine consists of a strong truck, _A_, _A_, mounted on railway wheels, on which is placed the boiler _C_, thecrane _E_, and the requisite gearing. The excavator or shovel, _D_, is abox of wrought iron, with strong points in front to act as picks inloosening the earth, and its bottom hung by a hinge at _d_, so that, bydetaching a catch, it may fly open and discharge the material raised. Tooperate the machine, suppose the shovel _D_ to be in the position shown inthe cut; it is lowered by the chains _o_, _o_, and thrown forward orbackward, if necessary, by the drum _B_, and handle _S_, till the picks inthe front of the shovel are brought in proper contact with the face of thecut; motion forward is now given to the shovel by the drum _B_ and handle_S_, and at the same time it is raised by the chains _o_, _o_. These twomotions can be so adjusted to each other, as to give movement to the shovelto enable it to loosen and scrape up a shovelful of earth. The handle _S_is now left free, and the shovel _D_ is raised vertically by the chains_o_, _o_. The crane is now turned round, till the shovel comes over a railcar on a side track; the bottom of the shovel is opened, and the dirtdeposited in the car. All these motions are performed by the aid of a steamengine, and are controlled by a man who stands on a platform at _f_. 

[Illustration: Fig 76. ]

692. _Q. _--Having now described the most usual and approved forms ofengines applicable to numerous miscellaneous purposes for which a moderateamount of steam power is required, will you briefly recapitulate whatamount of work of different kinds an engine of a given power will perform, so that any one desiring to employ an engine to perform a given amount ofwork, will be able to tell what the power of such engine should be?

_A. _--It will of course be impossible to recapitulate all the purposes towhich engines are applicable, or to specify for every case the amount ofpower necessary for the accomplishment of a given amount of work; but someexamples may be given which will be applicable to the bulk of the casesoccurring in practice. 

693. _Q. _--Beginning, then, with the power necessary for threshing, --a 4horse power engine, with cylinder 6 inches diameter, pressure of steam 45lbs. , per square inch, and making 140 revolutions per minute, will threshout 40 quarters of wheat in 10 hours with a consumption of 3 cwt. Of coals. 

_A. _--Although this may be done, it is probably too much to say that it canbe done on an average, and about three fourths of a quarter of wheat perhorse power would probably be a nearer average. The amount of powerconsumed varies with the yield. 

Messrs. Barrett, Exall, and Andrewes give the following table asillustrative of the work done, and the fuel consumed by their portableengines; but this must be regarded as a maximum performance:--

 Number of | Weight of | Quarters of | Quantity of | Quantity ofHorse Power. | Engine. | Corn thrashed| Coals consumed| Water required | | in 10 Hours. | in 10 Hours. | for 10 Hours | | | | in Gallons. ------------|-----------|--------------|---------------|--------------- |Tons. Cwts. | | Cwts. | 4 | 2 0 | 40 | 3 | 360 5 | 2 5 | 50 | 4 | 380 6 | 2 10 | 60 | 5 | 460 7 | 2 15 | 70 | 6 | 540 8 | 3 0 | 80 | 7 | 620 10 | 3 10 | 100 | 9 | 780-----------------------------------------------------------------------

694. _Q. _--In speaking of horses power, I suppose you mean indicator horsepower?

_A. _--Yes; or rather the dynamometer horse power, which is the same, barring the friction of the engine. At the shows of the Royal AgriculturalSociety, the power actually exerted by the different engines is ascertainedby the application of a friction wheel or dynamometer. 

695. _Q. _--Can you give any other examples of the power necessary forgrinding corn?

_A. _--An engine exerting 23-1/3 horses power by the indicator works twopairs of flour stones of 4 feet 8 inches diameter, two pairs of stonesgrinding oatmeal of 4 feet 8 inches diameter, one dressing machine, onepair of fanners, one dust screen, and one sifting machine. One of the flourstones makes 85, and the other 90 revolutions in the minute. One of theoatmeal stones makes 120, and the other 140 revolutions in the minute. Totake another case:--An engine exerting 26-1/2 indicator horses power workstwo pairs of flour stones, one dressing machine, two pairs of stonesgrinding oatmeal, and one pair of shelling stones. The flour stones, onepair of the oatmeal stones, and shelling stones, are 4 feet 8 inchesdiameter. The diameter of the other pair of oatmeal stones is 3 feet 8inches. The length of the cylinder of the dressing machine is 7 feet 6inches. The flour stones make 87 revolutions in the minute, and the largeroatmeal stone 111 revolutions, but the smaller oatmeal stone and theshelling stone revolve faster than this. At the time the indicator diagramwas taken, each pair of flour stones was grinding at the rate of 5 bushelsan hour; each pair of oatmeal stones about 24 bushels an hour; and theshelling stones were shelling at the rate of about 54 bushels an hour. Thefanners and screen were also in operation. 

696. _Q. _--Have you any other case to enumerate?

_A. _--I may mention one in which the power of the same engine was increasedby giving it a larger supply of steam. The engine when working with 8. 65horses power, gives motion to one pair of oatmeal stones of 4 feet 6 inchesdiameter, and one pair of flour stones 4 feet 8 inches diameter. Theoatmeal stone makes 100 revolutions in the minute, and the flour stone 89. The oatmeal stones grind about 36 bushels in the hour, and the flour stones5 bushels in the hour. The engine when working to 12 horses power drivesone pair of flour stones, 4 feet 8 inches diameter, at 89 revolutions perminute and one pair of stones of the same diameter at 105 revolutions, grinding beans for cattle. The flour mill stones with this proportion ofpower, being more largely fed, ground 6 bushels per hour, and the otherstones also ground 6 bushels per hour. When the power was increased to 18horses, and the engine was burdened in addition with a dressing machinehaving a cylinder of 19 inches diameter, the speed of the flour stone fellto 85, and of the beans stone to 100 revolutions per minute, and the yieldwas also reduced. The dressing machine dressed 24 bushels per hour. 

697. _Q. _--What is the power necessary to work a sugar mill such as is usedto press the juice from canes in the West Indies?

_A. _--Twenty horses power will work a sugar mill having rollers about 5feet long and 28 inches diameter; the rollers making 2-1/3 turns in aminute. If the rollers be 26 inches diameter and 4-1/2 feet long, 18 horsespower will suffice to work them at the same speed, and 16 horses power ifthe length be reduced to 3 feet 8 inches. 12 horses power will be requiredto work a sugar mill with rollers 24 inches diameter and 4 feet 2 incheslong; and 10 horses power will suffice if the rollers be 3 feet 10 incheslong and 23 inches diameter. The speed of the surface of sugar mill rollersshould not be greater than 16 feet per minute, to allow time for the canesto part with their juice. In the old mills the speed was invariably toogreat. The quantity of juice expressed will not be increased by increasingthe speed of the rollers, but more of the juice will pass away in thebegass or woody refuse of the cane. 

698. _Q. _--What is the amount of power necessary to drive cotton mills?

_A. _--An indicator or actual horse power will drive 305 hand mule spindles, with proportion of preparing machinery for the same; or 230 self-actingmule spindles with preparation; or 104 throstle spindles with preparation;or 10-1/2 power looms with common sizing. The throstles referred to are thecommon throstles spinning 34's twist for power loom weaving, and thespindles make 4000 turns per minute. The self-acting mules are Robert's, about one half spinning 36's weft, and spindles revolving 4800 turns perminute; and the other half spinning 36's twist, with the spindles revolving5200 times per minute. Half the hand mules were spinning 36's weft, at 4700revolutions, and the other half 36's twist at 5000 revolutions per minute. The average breadth of the looms was 37 inches, weaving 37 inch cloth, making 123 picks per minute, --all common calicoes about 60 reed, Stockportcount, and 68 picks to the inch. To take another example in the case of amill for twisting cotton yarn into thread:--In this mill there are 27frames with 96 common throstle spindles in each, making in all 2592spindles. The spindles turn 2200 times in a minute; the bobbins are 1-7/8inches diameter, and the part which holds the thread is 2-3/16 inches long. In addition to the twisting frames the steam engine works 4 turning lathes, 3 polishing lathes, 2 American machines for turning small bobbins, twocircular saws, one of 22 and the other of 14 inches diameter, and 24 bobbinheads or machines for filling the bobbins with finished thread. The powerrequired to drive the whole of this machinery is 28-1/2 horses. When allthe machinery except the spindles is thrown off, the power required is 21horses, so that 2592, the total number of spindles, divided by 21, thetotal power, is the number of twisting spindles worked by each actual horsepower. The number is 122. 84. 

699. _Q. _--What work will be done by a given engine in sawing timber, pressing cotton, blowing furnaces, driving piles, and dredging earth out ofrivers?

_A. _--A high pressure cylinder 10 inches diameter, 4 feet stroke, making 35revolutions with steam of 90 to 100 lbs. On the square inch, supplied bythree cylindrical boilers 30 inches diameter and 20 feet long, works twovertical saws of 34 inches stroke, which are capable of cutting 30 feet ofyellow pine, 18 inches deep, in the minute. A high pressure cylinder 14inches diameter and 4 feet stroke, making 60 strokes per minute with steamof 40 lbs. On the square inch, supplied by three cylindrical boilerswithout flues, 30 inches diameter and 26 feet long, with 32 square feet ofgrate surface, works four cotton presses geared 6 to 1, with two screws ineach, of 7-1/2 inches diameter and 1-5/8 pitch, which presses will screw1000 bales of cotton in the twelve hours. Also one high pressure cylinderof 10 inches diameter and 3 feet stroke, making 45 to 60 revolutions perminute, with steam of 45 to 50 lbs. Per square inch, with two hydraulicpresses having 13 inch rams of 41 feet stroke, and force pumps 2 inchesdiameter and 6 inches stroke, presses 30 bales of cotton per hour. Onecondensing engine with cylinder 56 inches diameter, 10 feet stroke, andmaking 15 strokes per minute with steam of 60 lbs. Pressure per squareinch, cut off at 1/4th of the stroke, supplied by six boilers, each 5 feetdiameter, and 24 feet long, with a 22-inch double-return flue in each, and198 square feet of fire grate, works a blast cylinder of 126 inchesdiameter, and 10 feet stroke, at 15 strokes per minute. The pressure of theblast is 4 to 5 lbs. Per square inch; the area of pipes 2300 square inches, and the engine blows four furnaces of 14 feet diameter, each making 100tons of pig iron per week. Two high pressure cylinders, each of 6 inchesdiameter and 18 inches stroke, making 60 to 80 strokes per minute, withsteam of 60 Lbs. Per square inch, lift two rams, each weighing 1000 lbs. , five times in a minute, the leaders for the lift being 24 feet long. Onehigh pressure cylinder of 12 inches diameter and 5 feet stroke, making 20strokes per minute, with steam of 60 to 70 lbs. Pressure per square inch, lifts 6 buckets full of dredging per minute from a depth of 30 feet belowthe water, or lifts 10 buckets full of mud per minute from a depth of 18feet below the water. 

CHAPTER XII. 

MANUFACTURE AND MANAGEMENT OF STEAM ENGINES. 

CONSTRUCTION OF ENGINES. 

700. _Q. _--What are the qualities which should be possessed by the iron ofwhich the cylinder of steam engines are made?

_A. _--The general ambition in making cylinders is to make them sound andhard; but it is expedient also to make them tough, so as to approach asnearly as possible to the state of malleable iron. This may be done bymixing in the furnace as many different kinds of iron as possible; and itmay be set down as a general rule in iron founding, that the greater thenumber of the kinds of metal entering into the composition of any casting, the denser and tougher it will be. The constituent atoms of the differentkinds of iron appear to be of different sizes, and the mixture of differentkinds maintains the toughness, while it adds to the density and cohesivepower. Hot blast iron was at one time generally believed to be weaker thancold blast iron, but it is now questioned whether it is not the stronger ofthe two. The cohesive strength of unmixed iron is not in proportion to itsspecific gravity, and its elasticity and power to resist shocks appear tobecome greater as the specific gravity becomes less. Nos. 3 and 4 are thestrongest irons. In most cases, iron melted in a cupola is not so strong aswhen remelted in an air furnace, and when run into green sand it is notreckoned so strong as when run into dry sand, or loam. The quality of thefuel, and even the state of the weather, exerts an influence on the qualityof the iron: smelting furnaces, on the cold blast principle, have long beenknown to yield better iron in winter than in summer, probably from theexistence of less moisture in the air; and it would probably be found toaccomplish an improvement in the quality of the iron if the blast were madeto pass through a vessel containing muriate of lime, by which the moistureof the air would be extracted. The expense of such a preparation would notbe considerable, as, by subsequent evaporation, the salt might be used overand over again for the same purpose. 

701. _Q. _--Will you explain the process of casting cylinders?

_A. _--The mould into which the metal is poured is built up of bricks andloam, the loam being clay and sand ground together in a mill, with theaddition of a little horse-dung to give it a fibrous structure and preventcracks. The loam board, by which the circle of the cylinder is to be swept, is attached to an upright iron bar, at the distance of the radius of thecylinder, and a cylindrical shell of brick is built up, which is plasteredon the inside with loam, and made quite smooth by traversing theperpendicular loam board round it. A core is then formed in a similarmanner, but so much smaller as to leave a space between the shell and thecore equal to the thickness of the cylinder, and into this space the meltedmetal is poured. Whatever nozzles or projections are required upon thecylinder, must be formed by means of wooden patterns, which are built intothe shell, and subsequently withdrawn; but where a number of cylinders ofthe same kind are required, it is advisable to make these patterns of iron, which will not be liable to warp or twist while the loam is being dried. Before the iron is cast into the mould, the interior of the mould must becovered with finely powdered charcoal--or blackening, as it is technicallytermed; and the secret of making finely skinned castings lies in usingplenty of blackening. In loam and dry sand castings the charcoal should bemixed with thick clay water, and applied until it is an eighth of an inchthick, or more; the surface should be then very carefully smoothed orsleeked, and if the metal has been judiciously mixed, and the mouldthoroughly dried, the casting is sure to be a fine one. Dry sand and loamcastings should be, as much as possible, made in boxes; the moulds maythereby be more rapidly and more effectually dried, and better castingswill be got with a less expense. 

702. _Q. _--Will you explain the next operation which a cylinder undergoes?

_A. _--The next stage is the boring; and in boring cylinders of 74 inchesdiameter, the boring bar must move so as to make one revolution in about4-1/2 minutes, at which speed the cutters will move at the rate of about5 feet per minute. In boring brass, the speed must be slower; the commonrate at which the tool moves in boring brass air pumps is about 3 feet perminute. If this speed be materially exceeded the tool will be spoiled, andthe pump made taper. The speed proper for boring a cylinder will answer forboring the brass air pump of the same engine. A brass air pump of 36-1/2inches diameter requires the bar to make one turn in about three minutes, which is also the speed proper for a cylinder 60 inches in diameter. Tobore a brass air pump 36-1/2 inches in diameter requires a week, an ironone requires 48 hours, and a copper one 24 hours. In turning a malleableiron shaft 12-3/4 inches in diameter, the shaft should make about fiveturns per minute, which is equivalent to a speed in the tool of about 16feet per minute; but this speed may be exceeded if soap and water beplentifully run on the point of the tool. A boring mill, of which the speedmay be varied from one turn in six minutes to twenty-five turns in oneminute, will be suitable for all ordinary wants that can occur in practice. 

703. _Q. _--Are there any precautions necessary to be observed in order thatthe boring may be truly effected?

_A. _--In fixing a cylinder into the boring mill, great care must be takenthat it is not screwed down unequally; and indeed it will be impossible tobore a large cylinder in a horizontal mill without being oval, unless thecylinder be carefully gauged when standing on end, and be set up by screwswhen laid in the mill until it again assumes its original form. A largecylinder will inevitably become oval if laid upon its side; and if whileunder the tension due to its own weight it be bored round, it will becomeoval again when set upon end. If the bottom be cast in, the cylinder willbe probably found to be round at one end and oval at the other, unless avertical boring mill be employed, or the precautions here suggested beadopted. 

704. _Q. _--Does the boring tool make the cylinder sufficiently smooth forthe reception of the piston?

_A. _--Many engine makers give no other finish to their cylinders; butMessrs. Penn grind their cylinders after they are bored, by laying them ontheir side, and rubbing a piece of lead, with a cross iron handle like thatof a rolling stone, and smeared with emery and oil, backward and forward--the cylinder being gradually turned round so as to subject every partsuccessively to the operation. The lead by which this grinding isaccomplished is cast in the Cylinder, whereby it is formed of the rightcurve; but the part of the cylinder in which it is cast should bepreviously heated by a hot iron, else the metal may be cracked by thesudden heat. 

705. _Q. _--How are the parts of a piston fitted together so as to beperfectly steam tight?

_A. _--The old practice was to depend chiefly upon grinding as the means ofmaking the rings tight upon the piston or upon one another; but scraping isnow chiefly relied on. Some makers, however, finish their steam surfaces bygrinding them with powdered Turkey stone and oil. A slight grinding, orpolishing, with powdered Turkey stone and oil, appears to be expedient inordinary cases, and may be conveniently accomplished by setting the pistonon a revolving table, and holding the ring stationary by a cross piece ofwood while the table turns round. Pieces of wood may be interposed betweenthe ring and the body of the piston, to keep the ring nearly in its rightposition; but these pieces of wood should be fitted so loosely as to givesome side play, else the disposition would arise to wear the flange of thepiston into a groove. 

706. _Q. _--What kind of tool is used for finishing surfaces by scraping?

_A. _--A flat file bent, and sharpened at the end, makes an eligible scraperfor the first stages; or a flat file sharpened at the end and used like achisel for wood. A three-cornered file, sharpened at all the corners, isthe best instrument for finishing the operation. The scraping tool shouldbe of the best steel, and should be carefully sharpened at short intervalson a Turkey stone, so as to maintain a fine edge. 

707. _Q. _--Will you explain the method of fitting together the valve andcylinder faces?

_A. _--Both faces must first be planed, then filed according to theindications of a metallic straight edge, and subsequently of a thickmetallic face plate, and finally scraped very carefully until the faceplate bears equally all over the surface. In planing any surface, thecatches which retain the surface on the planing machine should be relaxedpreviously to the last cut, to obviate distortion from springing. Toascertain, whether the face plate bears equally, smear it over with alittle red ochre and oil, and move the face plate slightly, which will fixthe color upon the prominent points. This operation is to be repeatedfrequently; and as the work advances, the quantity of coloring matter is tobe diminished, until finally it is spread over the face plate in a thinfilm, which only dims the brightness of the plate. The surfaces at thisstage must be rubbed firmly together to make the points of contact visible, and the higher points will become slightly clouded, while the other partsare left more or less in shade. If too small a quantity of coloring matterbe used at first, it will be difficult to form a just conception of thegeneral state of the surface, as the prominent points will alone beindicated, whereas the use of a large quantity of coloring matter in thelatter stages would destroy the delicacy of the test the face plateaffords. The number of bearing points which it is desirable to establish onthe surface of the work, depends on the use to which the surface is to beapplied; but whether it is to be finished with great elaboration, orotherwise, the bearing points should be distributed equally over thesurface. Face plates, or planometers, as they are sometimes termed, aresupplied by most of the makers of engineering tools. Every factory shouldbe abundantly supplied with them, and also with steel straight edges; andthere should be a master face plate, and a master straight edge, for thesole purpose of testing, from time to time, the accuracy of those in use. 

708. _Q. _--Is the operation of surfacing, which you have described, necessary in the case of all slide valves?

_A. _--Yes; and in fitting the faces of a D valve, great care must, inaddition, be taken that the valve is not made conical; for unless the backbe exactly parallel with the face, it will be impossible to keep thepacking from being rapidly cut away. When the valve is laid upon the faceplate, the back must be made quite fair along the whole length, by drawfiling, according to the indications of a straight edge; and the distancefrom the face to the extreme height of the back must be made identical ateach extremity. 

709. _Q. _--When you described the operation of boring the cylinder, youstated that the cylinder, when laid upon its side, became oval; will notthis change of figure distort the cylinder face?

_A. _--It is not only in the boring of the cylinder that it is necessary tobe careful that there is no change of figure, for it will be impossible toface the valves truly in the case of large cylinders, unless the cylinderbe placed on end, or internal props be introduced to prevent the collapsedue to the cylinder's weight. It may be added, that the change of figure isnot instantaneous, but becomes greater after some continuance of the strainthan it was at first, so that in gauging a cylinder to ascertain thedifference of diameter when it is placed on its side, it should have lainsome days upon its side to ensure the accuracy of the operation. 

710. _Q. _--How is any flaw in the valve or cylinder face remedied?

_A. _--Should a hole occur either in the valve, in the cylinder, or anyother part where the surface requires to be smooth, it may be plugged upwith a piece of cast iron, as nearly as possible of the same texture. Boreout the faulty part, and afterward widen the hole with an eccentric drill, so that it will be of the least diameter at the mouth. The hole may go morethan half through the iron: fit then a plug of cast iron roughly by filing, and hammer it into the hole, whereby the plug will become riveted in it, and its surface may then be filed smooth. Square pieces may be let in afterthe same fashion, the hole being made dovetailed, and the pieces thusfitted will never come out. 

711. _Q. _--When cylinders are faced with brass, how is the face attached tothe cylinder?

_A. _--Brass faces are put upon valves or cylinders by means of small brassscrews tapped into the iron, with conical necks for the retention of thebrass: they are screwed by means of a square head, which, when the screw isin its place, is cut off and filed smooth. In some cases the face is madeof extra thickness, and a rim not so thick runs round it, forming a step orrecess for the reception of brass rivets, the heads of which are clear ofthe face. 

712. _Q. _--What is the best material for valve faces?

_A. _--Much trouble is experienced with every modification of valve face;but cast iron working upon cast iron is, perhaps, the best combination yetintroduced. A usual practice is to pin brass faces on the cylinder, allowing the valve to retain its cast iron face. Some makers employ brassvalves, and others pin brass on the valves, leaving the cylinder with acast iron face. If brass valves are used, it is advisable to plane out twogrooves across the face, and to fill them up with hard cast iron to preventrutting. Speculum metal and steel have been tried for the cylinder faces, but only with moderate success. In some cases the brass gets into ruts; butthe most prevalent affection is a degradation of the iron, owing to theaction of the steam, and the face assuming a granular appearance, somethinglike loaf sugar. This action shows itself only at particular spots, andchiefly about the angles of the port or valve face. At first the action isslow; but when once the steam has worked a passage for itself, the cuttingaway becomes very rapid, and, in a short time, it will be impossible toprevent the engine from heating when stopped, owing to the leakage of steamthrough the valve into the condenser. Copper steam pipes seem to have somegalvanic action on valve faces, and malleable iron pipes have sometimesbeen substituted; but they are speedily worn out by oxidation, and thescales of rust which are carried on by the steam scratch the valves andcylinders, so that the use of copper pipes is the least evil. 

713. _Q. _--Will you explain in what manner the joints of an engine aremade?

_A. _--Rust joints are not now much used in engines of any kind, yet it isnecessary that the engineer should be acquainted with the manner of theirformation. One ounce of sal-ammoniac in powder is mingled with 18 ounces ora pound of borings of cast iron, and a sufficiency of water is added to wetthe mixture thoroughly, which should be done some hours before it is wantedfor use. Some persons add about half an ounce of flowers of brimstone tothe above proportions, and a little sludge from the grindstone trough. Thiscement is caulked into the joints with a caulking iron, about threequarters of an inch wide and one quarter of an inch thick, and after thecaulking is finished the bolts of the joints may be tried to see if theycannot be further tightened. The skin of the iron must, in all cases, bebroken where a rust joint is to be made; and, if the place be greasy, thesurface must be well rubbed over with nitric acid, and then washed withwater, till no grease remains. The oil about engines has a tendency todamage rust joints by recovering the oxide. Coppersmiths staunch the edgesof their plates and rivets by means of a cement formed of poundedquicklime, with serum of blood, or white of egg; and in copper boilers sucha substance may be useful in stopping the impalpable leaks which sometimesoccur, though Roman, cement appears to be nearly as effectual. 

714. _Q. _--Will you explain the method of case hardening the parts ofengines?

_A. _--The most common plan for case hardening consists in the insertion ofthe articles to be operated upon among horn or leather cuttings, hone dust, or animal charcoal, in an iron box provided with a tight lid, which is thenput into a furnace for a period answerable to the depth of steel required. In some cases the plan pursued by the gunsmiths may be employed withconvenience. The article is inserted in a sheet iron case amid bone dust, often not burned; the lid of the box is tied on with wire, and the jointluted with clay; the box is heated to redness as quickly as possible andkept half an hour at a uniform heat: its contents are then suddenlyimmersed in cold water. The more unwieldy portions of an engine may be casehardened by prussiate of potash--a salt made from animal substances, composed of two atoms of carbon and one of nitrogen, and which operates onthe same principle as the charcoal. The iron is heated in the fire to adull red heat, and the salt is either sprinkled upon it or rubbed on in alump, or the iron is rubbed in the salt in powder. The iron is thenreturned to the fire for a few minutes, and finally immersed in water. Bysome persons the salt is supposed to act unequally, as if there were greasyspots upon the iron which the salt refused to touch, and the effect underany circumstances is exceedingly superficial; nevertheless, upon all partsnot exposed to wear, a sufficient coating of steel may be obtained by thisprocess. 

715. _Q. _--What kind of iron is most suitable for the working parts of anengine?

_A. _--In the malleable iron work of engines scrap iron has long been used, and considered preferable to other kinds; but if the parts are to be casehardened, as is now the usual practice, the use of scrap iron is to bereprehended, as it is almost sure to make the parts twist in the casehardening process. In case hardening, iron absorbs carbon, which causes itto swell; and as some kinds of iron have a greater capacity for carbon thanother kinds, in case hardening they will swell more, and any such unequalenlargement in the constituent portions of a piece of iron will cause it tochange its figure. In some cases, case hardening has caused such a twistingof the parts of an engine, that they could not afterward be fittedtogether; it is preferable, therefore, to make such parts as are to be casehardened to any considerable depth of Lowmoor, Bowling, or Indian iron, which being homogeneous will absorb carbon equally, and will not twist. 

716. _Q. _--What is the composition of the brass used for engine bearings?

_A. _--The brass bearings of an engine are composed principally of copperand tin. A very good brass for steam engine bearings consists of old copper112 lbs. , tin 12-1/2 lbs. , zinc 2 or 3 oz. ; and if new tile copper be used, there should be 13 lbs. Of tin instead of 12-1/2 lbs. A tough brass forengine work consists of 1-1/2 lb. Tin, 1-1/2 lb. Zinc, and 10 lbs. Copper;a brass for heavy bearings, 2-1/2 oz. Tin, 1/2 oz. Zinc, and 1 lb. Copper. There is a great difference in the length of time brasses wear, as made bydifferent manufacturers; but the difference arises as much from a differentquantity of surface, as from a varying composition of the metal. Brassesshould always be made strong and thick, as when thin they collapse upon thebearing and increase the friction and the wear. 

717. _Q. _--How is Babbitt's metal for lining the bushes of machinerycompounded?

_A. _--Babbitt's patent lining metal for bushes has been largely employed inthe bushes of locomotive axles and other machinery: it is composed of 1 lb. Of copper, 1 lb. Regulus of antimony, and 10 lbs. Of tin, or other similarproportions, the presence of tin being the only material condition. Thecopper is first melted, then the antimony is added, with a small proportionof tin-charcoal being strewed over the surface of the metal in the crucibleto prevent oxidation. The bush or article to be lined, having been castwith a recess for the soft metal, is to be fitted to an iron mould, formedof the shape and size of the bearing or journal, allowing a little in sizefor the shrinkage. Drill a hole for the reception of the soft metal, say1/2 to 3/4 inch diameter, wash the parts not to be tinned with a clay washto prevent the adhesion of the tin, wet the part to be tinned with alcohol, and sprinkle fine sal-ammoniac upon it; heat the article until fumes arisefrom the ammonia, and immerse it in a kettle of Banca tin, care being takento prevent oxidation. When sufficiently tinned, the bush should be soakedin water, to take off any particles of ammonia that may remain upon it, asthe ammonia would cause the metal to blow. Wash with pipe clay, and dry;then heat the bush to the melting point of tin, wipe it clean, and pour inthe metal, giving it sufficient head as it cools; the bush should then bescoured with fine sand, to take off any dirt that may remain upon it, andit is then fit for use. This metal wears for a longer time than ordinarygun metal, and its use is attended with very little friction. If thebearing heats, however, from the stopping of the oil hole or otherwise, themetal will be melted out. A metallic grease, containing particles of tin inthe state of an impalpable powder, would probably be preferable to thelining of metal just described. 

718. _Q. _--Can you state the composition of any other alloys that are usedin engine work?

_A. _--The ordinary range of good yellow brass that files and turns well, isabout 4-1/2 to 9 ounces of zinc to the pound of copper. Flanges to standbrazing may be made of copper 1 lb. , zinc 1/2 oz. , lead 3/8 oz. Brazingsolders when stated in the order of their hardness are:-three parts copperand one part zinc (very hard), eight parts brass and one part zinc (hard), six parts brass, one part tin, and one part zinc (soft); a very commonsolder for iron, copper, and brass, consists of nearly equal parts ofcopper and zinc. Muntz's metal consists of forty parts zinc and sixty ofcopper; any proportions between the extremes of fifty parts of zinc andfifty parts copper, and thirty-seven zinc and sixty-three copper, will rolland work at a red heat, but forty zinc to sixty copper are the proportionspreferred. Bell metal, such as is used for large bells, consists of 4-1/2ounces to 5 ounces of tin to the pound of copper; speculum metal consistsof from 7-1/2 ounces to 8-1/2 ounces of tin to the pound of copper. 

ERECTION OF ENGINES. 

719. _Q. _--Will you explain the operation of erecting a pair of side leverengines in the workshop?

A. --In beginning the erection of side lever marine engines in the workshop, the first step is to level the bed plate lengthways and across, and strikea line up the centre, as near as possible in the middle, which indent witha chisel in various places, so that it may at any time be easily foundagain. Strike another line at right angles with this, either at thecylinder or crank centre, by drawing a perpendicular in the usual manner. Lay the other sole plate alongside at the right distance, and strike a lineat the cylinder or crank centre of it also, shifting either sole plate alittle endways until these two transverse lines come into the same line, which may be ascertained by applying a straight edge across the two soleplates. Strike the rest of the centres across, and drive a pin into eachcorner of each sole plate, which file down level, so as to serve for pointsof reference at any future stage; next, try the cylinder, or plumb it onthe inside roughly, and see how it is for height, in order to ascertainwhether much will be required to be chipped off the bottom, or whether morerequires to be chipped off the one side than the other. Chip the cylinderbottom fair; set it in its place, plumb the cylinder very carefully with astraight edge and silk thread, and scribe it so as to bring the cylindermouth to the right height, then chip the sole plate to suit that height. The cylinder must then be tried on again, and the parts filed wherever theybear hard, until the whole surface is well fitted. Next, chip the place forthe framing; set up the framing, and scribe the horizontal part of the jawwith the scriber used for the bottom of the cylinder, the upright partbeing set to suit the shaft centres, and the angular flange of cylinder, where the stay is attached, having been previously chipped plumb and level. The stake wedges with which the framing is set up preparatorily to theoperation of scribing, must be set so as to support equally thesuperincumbent weight, else the framing will spring from resting unequally, and it will be altogether impossible to fit it well. These directionsobviously refer exclusively to the old description of side lever enginewith cast iron framing; but there is more art in erecting an engine of thatkind with accuracy, than in erecting one of the direct action engines, where it is chiefly turned or bored surfaces that have to be dealt with. 

720. _Q. _--How do you lay out the positions of the centres of a side leverengine?

_A. _--In fixing the positions of the centres in side lever engines, itappears to be the most convenient way to begin with the main centre. Theheight of the centre of the cross head at half stroke above the plane ofthe main centre is fixed by the drawing of the engine, which gives thedistance from the centre of cross head at half stroke to the flange of thecylinder; and from thence it is easy to find the perpendicular distancefrom the cylinder flange to the plane of the main centre, merely by puttinga straight edge along level, from the position of the main centre to thecylinder, and measuring from the cylinder flauge down to it, raising orlowering the straight edge until it rests at the proper measurement. Themain centre is in that plane, and the fore and aft position is to be foundby plumbing up from the centre line on the sole plate. To find the paddleshaft centre, plumb up from the centre line marked on the edge of the soleplate, and on this line lay off from the plane of the main centre thelength of the connecting rod, if that length be already fixed, or otherwisethe height fixed in the drawing of the paddle shaft above the main centre. To fix the centre for the parallel motion shaft, when the parallel bars areconnected with the cross head, lay off from the plane of main centre thelength of the parallel bar from the centre of the cylinder, deduct thelength of the radius crank, and plumb up the central line of motion shaft;lay off on this line, measuring from the plane of main centre, the lengthof the side rod; this gives the centre of parallel motion shaft when theradius bars join the cross head, as is the preferable practice whereparallel motions are used. The length of the connecting rod is the distancefrom the centre of the beam when level, or the plane of the main centre, tothe centre of the paddle shaft. The length of the side rods is the distancefrom the centre line of the beam when level, to the centre of the crosshead when the piston is at half stroke. The length of the radius rods ofthe parallel motion is the distance from the point of attachment on thecross head or side rod, when the piston is at half stroke, to the extremityof the radius crank when the crank is horizontal; or in engines with theparallel motion attached to the cross head, it is the distance from thecentre of the pin of the radius crank when horizontal to the centre of thecylinder. Having fixed the centre of the parallel motion shaft in themanner just described, it only remains to put the parts together when themotion is attached to the cross head; but when the motion is attached tothe side rod, the end of the parallel bar must not move in a perpendicularline, but in an arc, the versed sine of which bears the same ratio to thatof the side lever, that the distance from the top of the side rod to thepoint of attachment bears to the total length of the side rod. 

721. _Q. _--How do you ascertain the accuracy of the parallel motion?

_A. _--The parallel motion when put in its place should be tested by raisingand lowering the piston by means of the crane. First, set the beams level, and shift in or out the motion shaft plummer blocks or bearings, until thepiston rod is upright. Then move the piston to the two extremes of itsmotion. If at both ends the cross head is thrown too much out, the stud inthe beam to which the motion side rod is attached is too far out, and mustbe shifted nearer to the main centre; if at the extremities the cross headis thrown too far in, the stud in the beam is not out far enough. If thecross head be thrown in at the one end, and out equally at the other, thefault is in the motion side rod, which must be lengthened or shortened toremedy the defect. 

722. _Q. _--Will you describe the method pursued in erecting oscillatingengines?

_A. _--The columns here are of wrought iron, and in the case of smallengines there is a template made of wood and sheet iron, in which the holesare set in the proper positions, by which the upper and lower frames areadjusted; but in the case of large engines, the holes are set off by meansof trammels. The holes for the reception of the columns are cast in theframes, and are recessed out internally: the bosses encircling the holesare made quite level across, and made very true with a face plate, and thepillars which have been turned to a gauge are then inserted. The top frameis next put on, and must bear upon the collars of the columns so evenly, that one of the columns will not be bound by it harder than another. Ifthis point be not attained, the surfaces must be further scraped, until aperfect fit is established. The whole of the bearings in the bestoscillating engines are fitted by means of scraping, and on no other modeof fitting can the same reliance be placed for exactitude. 

723. _Q. _--How do you set out the trunnions of oscillating engines, so thatthey shall be at right angles with the interior of the cylinder?

_A. _--Having bored the cylinder, faced the flange, and bored out the holethrough which the boring bar passes, put a piece of wood across the mouthof the cylinder, and jam it in, and put a similar piece in the hole throughthe bottom of the cylinder. Mark the centre of the cylinder upon each ofthese pieces, and put into the bore of each trunnion an iron plate, with asmall indentation in the middle to receive the centre of a lathe, andadjusting screws to bring the centre into any required position. Thecylinder must then be set in a lathe, and hung by the centres of thetrunnions, and a straight edge must be put across the cylinder mouth andlevelled, so as to pass through the line in which the centre of thecylinder lies. Another similar straight edge, and similarly levelled, mustbe similarly placed across the cylinder bottom, so as to pass through thecentral line of the cylinder; and the cylinder is then to be turned roundin the trunnion centres-the straight edges remaining stationary, which willat once show whether the trunnions are in the same horizontal plane as thecentre of the cylinder, and if not, the screws of the plates in thetrunnions must be adjusted until the central point of the cylinder justcomes to the straight edge, whichever end of the cylinder is presented. Toascertain whether the trunnions stand in a transverse plane, parallel tothe cylinder flange, it is only necessary to measure down from the flangeto each trunnion centre; and if both these conditions are satisfied, theposition of the centres may be supposed to be right. The trunnion bearingsare then turned, and are fitted into blocks of wood, in which they runwhile the packing space is being turned out. Where many oscillating enginesare made, a lathe with four centres is used, which makes the use ofstraight edges in setting out the trunnions superfluous. 

724. _Q. _--Will you explain how the slide valve of a marine engine is set?

_A. _--Place the crank in the position corresponding to the end of thestroke, which can easily be done in the shop with a level, or plumb line;but in a steam vessel another method becomes necessary. Draw the transversecentre line, answering to the centre line of the crank shaft, on the soleplate of the engine, or on the cylinder mouth if the engine be of thedirect action kind; describe a circle of the diameter of the crank pin uponthe large eye of the crank, and mark off on either side of the transversecentre line a distance equal to the semi-diameter of the crank pin. Fromthe point thus found, stretch a line to the edge of the circle described onthe large eye of the crank, and bring round the crank shaft till the crankpin touches the stretched line; the crank may thus be set at either end ofits stroke. When the crank is thus placed at the end of the stroke, thevalve must be adjusted so as to have the amount of lead, or opening on thesteam side, which it is intended to give at the beginning of the stroke;the eccentric must then be turned round upon the shaft until the notch inthe eccentric rod comes opposite the pin on the valve lever, and falls intogear: mark upon the shaft the situation of the eccentric, and put on thecatches in the usual way. The same process must be repeated for goingastern, shifting round the eccentric to the opposite side of the shaft, until the rod again falls into gear. In setting valves, regard must ofcourse be had to the kind of engine, the arrangements of the levers, andthe kind of valve employed; and in any general instructions it isimpossible to specify every modification in the procedure thatcircumstances may render advisable. 

725. _Q. _--Is a similar method of setting the valve adopted when the linkmotion is employed. 

_A. _--Each end of the link of the link motion has the kind of motioncommunicated to it that is due to the action of the particular eccentricwith which that end is in connection. In that form of the link motion inwhich the link itself is moved up or down, there is a different amount oflead for each different position of the link, since to raise or lower thelink is tantamount to turning the eccentric round on the shaft. In thatform of the link motion in which the link itself is not raised or lowered, but is susceptible of a motion round a centre in the manner of a doubleended lever, the lead continues uniform. In both forms of the link motion, as the stroke of the valve may be varied to any required extent while thelap is a constant quantity, the proportion of the lap relatively to thestroke of the valve may also be varied to any required extent, and theamount of the lap relatively with the stroke of the valve determines theamount of the expansion. In setting the valve when fitted with the linkmotion, the mode of procedure is much the same as when it is moved by asimple eccentric. The first thing is to determine if the eccentric rods areof the proper length, and this is done by setting the valve at half strokeand turning round the eccentric, marking each extremity of the travel ofthe end of the rod. The valve attachment should be midway between theseextremes; and if it is not so, it must be made so by lengthening orshortening the rod. The forward and backward eccentric rods are to beadjusted in this way, and this being done, the engine is to be put to theend of the stroke, and the eccentric is to be turned round until the amountof lead has been given that is desired. The valve must be tried by turningthe engine round to see that it is right at both centres, for going aheadand also for going astern. In some examples of the link motion, one of theeccentric rods is made a little longer than the other, and the position ofthe point of suspension or point of support powerfully influences theaction of the link in certain cases, especially if the link and this pointare not in the same vertical line. To reconcile all the conditions properto the satisfactory operation of the valve in the construction of the linkmotion, is a problem requiring a good deal of attention and care for itssatisfactory solution; and to make sure that this result is attained, theengine must be turned round a sufficient number of times to enable us toascertain if the valve occupies the desired position, both at the top andbottom centres, whether the engine is going ahead or astern. This shouldalso be tried with the starting handle in the different notches, or, inother words, with the sliding block in the slot or opening of the link indifferent positions. 

MANAGEMENT OF MARINE BOILERS. 

726. _Q. _--You have already stated that the formation of salt or scale inmarine boilers is to be prevented by blowing out into the sea at frequentintervals a portion of the concentrated water. Will you now explain how theproper quantity of water to be blown out is determined?

_A. _--By means of the salinometer, which is an instrument for determiningthe density of the water, constructed on the principle of the hydrometerfor telling the strength of spirits. Some of the water is drawn off fromthe boiler from time to time, and the salinometer is immersed in it afterit has been cooled. By the graduations of the salinometer the saltness ofthis water is at once discovered; and if the saltness exceeds 8 ounces ofsalt in the gallon, more water should be blown out of the boiler to bereplenished with fresher water from the sea, until the prescribed limit offreshness is attained. Should the salinometer be accidentally broken, atemporary one may be constructed of a phial weighted with a few grains ofshot or other convenient weight. The weighted phial is first to be floatedin fresh water, and its line of floatation marked; then to be floated insalt water, and its line of floatation marked; and another mark of an equalheight above the salt water mark will be the blow off point. 

727. _Q. _--HOW often should boilers be blown off in order to keep them freefrom incrustation?

_A. _--Flue boilers generally require to be blown off about twice everywatch, or about twice in the four hours; but tubular boilers may require tobe blown off once every twenty minutes, and such an amount of blowing offshould in every case be adopted, as will effectually prevent any injuriousamount of incrustation. 

728. _Q. _--In the event of scale accumulating on the flues of a boiler, what is the best way of removing it?

_A. _--If the boilers require to be scaled, the best method of performingthe operation appears to be the following:--Lay a train of shavings alongthe flues, open the safety valve to prevent the existence of any pressurewithin the boiler, and light the train of shavings, which, by expandingrapidly the metal of the flues, while the scale, from its imperfectconducting power, can only expand slowly, will crack off the scale; bywashing down the flues with a hose, the scale will be carried to the bottomof the boiler, or issue, with the water, from the mud-hole doors. Thismethod of scaling must be practised only by the engineer himself, and mustnot be intrusted to the firemen who, in their ignorance, might damage theboiler by overheating the plates. It is only where the incrustation uponthe flues is considerable that this method of removing it need bepractised; in partial cases the scale may be chipped off by a hatched facedhammer, and the flues may then be washed down with the hose in the mannerbefore described. 

729. _Q. _--Should the steam be let out of the boiler, after it has blownout the water, when the engine is stopped?

_A. _--No; it is better to retain the steam in the boiler, as the heat andmoisture it occasions soften any scale adhering to the boiler, and cause itto peel off. Care must, however, be taken not to form a vacuum in theboiler; and the gauge cocks, if opened, will prevent this. 

730. _Q. _--Are tubular boilers liable to the formation of scale in certainplaces, though generally free from it?

_A. _--In tubular boilers a good deal of care is required to prevent theends of the tubes next the furnace from becoming coated with scale. Evenwhen the boiler is tolerably clean in other places the scale will collecthere; and in many cases where the amount of blowing off previously found tosuffice for flue boilers has been adopted, an incrustation five eighths ofan inch in thickness has formed in twelve months round the furnace ends ofthe tubes, and the stony husks enveloping them have actually grown togetherin some parts so as totally to exclude the water. 

731. _Q. _--When a tubular boiler gets incrusted in the manner you havedescribed, what is the best course to be adopted for the removal of thescale?

_A. _--When a boiler gets into this state the whole of the tubes must bepulled out, which may be done by a Spanish windlass combined with a pair ofblocks; and three men, when thus provided, will be able to draw out from 50to 70 tubes per day, --those tubes with the thickest and firmestincrustations being, of course, the most difficult to remove. The act ofdrawing out the tubes removes the incrustation; but the tubes shouldafterward be scraped by drawing them backward and forward between the oldfiles, fixed in a vice, in the form of the letter V. The ends of the tubeshould then be heated and dressed with the hammer, and plunged while at ablood heat into a bed of sawdust to make them cool soft, so that they maybe riveted again with facility. A few of the tubes will be so far damagedat the ends by the act of drawing them out, as to be too short forreinsertion: this result might be to a considerable extent obviated bysetting the tube plates at different angles, so that the several horizontalrows of tubes would not be originally of the same length, and the damagedtubes of the long rows would serve to replace the short ones; but thepractice would be attended with other inconveniences. 

732. _Q. _--Is there no other means of keeping boilers free from scale thanby blowing off?

_A. _--Muriatic acid, or muriate of ammonia, commonly called sal-ammoniac, introduced into a boiler, prevents scale to a great extent; but it isliable to corrode the boiler internally, and also to damage the engine, bybeing carried over with the steam; and the use of such intermixtures doesnot appear to be necessary, if blowing off from the surface of the water islargely practised. In old boilers, however, already incrusted with scale, the use of muriate of ammonia may sometimes be advantageous. 

733. _Q. _--Are not the tubes of tubular boilers liable to be choked up bydeposits of soot?

_A. _--The soot which collects in the inside of the tubes of tubular boilersis removed by means of a brush, like a large bottle brush; and thecarbonaceous scale, which remains adhering to the interior of the tubes, isremoved by a circular scraper. Ferules in the tubes interfere with theaction of this scraper, and in the case of iron tubes ferules are nowgenerally discarded; but it will sometimes be necessary to use ferules foriron tubes, where the tubes have been drawn and reinserted, as it may bedifficult to refix the tubes without such an auxiliary. Tubes one tenth ofan inch in thickness are too thin: one eighth of an inch is a betterthickness, and such tubes will better dispense with the use of ferules, andwill not so soon wear into holes. 

734. _Q. _--If the furnace or flue of a boiler be injured, how do youproceed to repair it?

_A. _--If from any imperfection in the roof of a furnace or flue a patchrequires to be put upon it, it will be better to let the patch be appliedupon the upper, rather than upon the lower, surface of the plate; as ifapplied within the furnace a recess will be formed for the lodgment ofdeposit, which will prevent the rapid transmission of the heat in thatpart; and the iron will be very liable to be again burned away. A crack ina plate may be closed by boring holes in the direction of the crack, andinserting rivets with large heads, so as to cover up the imperfection. Ifthe top of the furnace be bent down, from the boiler having beenaccidentally allowed to get short of water, it may be set up again by ascrew jack, --a fire of wood having been previously made beneath the injuredplate; but it will in general be nearly as expeditious a course to removethe plate and introduce a new one, and the result will be moresatisfactory. 

735. _Q. _--In the case of the chimney being carried away by shot orotherwise, what course would you pursue?

_A. _--In some cases of collision, the funnel is carried away and lostoverboard, and such cases are among the most difficult for which a remedycan be sought. If flame come out of the chimney when the funnel is knockedaway, so as to incur the risk of setting the ship on fire, the uptake ofthe boiler must be covered over with an iron plate, or be sufficientlycovered to prevent such injury. A temporary chimney must then be made ofsuch materials as are on board the ship. If there are bricks and clay orlime on board, a square chimney may be built with them, or, if there besheet iron plates on board, a square chimney may be constructed of them. Inthe absence of such materials, the awning stanchions may be set up roundthe chimney, and chain rove in through among them in the manner of wickerwork, so as to make an iron wicker chimney, which may then be plasteredoutside with wet ashes mixed with clay, flour, or any other material thatwill give the ashes cohesion. War steamers should carry short sparefunnels, which may easily be set up should the original funnel be shotaway; and if a jet of steam be let into the chimney, a very short and smallfunnel will suffice for the purpose of draught. 

MANAGEMENT OF MARINE ENGINES. 

736. _Q. _--What are the most important of the points which suggestthemselves to you in connection with the management of marine engines?

_A. _--The attendants upon engines should prepare themselves for anycasualty that may arise, by considering possible cases of derangement, anddeciding In what way they would act should certain accidents occur. Thecourse to be pursued must have reference to particular engines, and nogeneral rules can therefore be given; but every marine engineer should beprepared with the measures to be pursued in the emergencies in which he maybe called upon to act, and where everything may depend upon his energy anddecision. 

737. _Q. _--What is the first point of a marine engineer's duty?

_A. _--The safe custody of the boiler. He must see that the feed ismaintained, being neither too high nor too low, and that blowing out thesupersalted water is practised sufficiently. The saltness of the water atevery half hour should be entered in the log book, together with thepressure of steam, number of revolutions of the engine, and any otherparticulars which have to be recorded. The economical use of the fuel isanother matter which should receive particular attention. If the coal isvery small, it should be wetted before being put on the fire. Next to thesafety of the boiler, the bearings of the engine are the most importantconsideration. These points, indeed, constitute the main parts of the dutyof an engineer, supposing no accident to the machinery to have taken place. 

738. _Q. _--If the eccentric catches or hoops were disabled, how would youwork the valve?

_A. _--If the eccentric catches or hoops break or come off, and the damagecannot readily be repaired, the valve may be worked by attaching the end ofthe starting handle to any convenient part of the other engine, or to somepart in connection with the connecting rod of the same engine. In sidelever engines, with the starting bar hanging from the top of the diagonalstay, as is a very common arrangement, the valve might be wrought byleading a rope from the side lever of the other engine through blocks so asto give a horizontal pull to the hanging starting bar, and the bar could bebrought back by a weight. Another plan would be, to lash a piece of wood tothe cross tail butt of the damaged engine, so as to obtain a sufficientthrow for working the valve, and then to lead a piece of wood or iron, froma suitable point in the piece of wood attached to the cross tail, to thestarting handle, whereby the valve would receive its proper motion. Inoscillating engines it is easy to give the required motion to the valve, byderiving it from the oscillation of the cylinder. 

739. _Q. _--What would you do if a crank pin broke?

_A. _--If the crank pin breaks in a paddle vessel with two engines, theother engine must be made to work one wheel. In a screw vessel the samecourse may be pursued, provided the broken crank is not the one throughwhich the force of the other engine is communicated to the screw. In such acase the vessel will be as much disabled as if she broke the screw shaft orscrew. 

740. _Q. _--Will the unbroken engine, in the case of disarrangement of oneof the two engines of a screw or paddle vessel, be able of itself to turnthe centre?

_A. _--It will sometimes happen, when there is much lead upon the slidevalve, that the single engine, on being started, cannot be got to turn thecentre if there be a strong opposing wind and sea; the piston going up tonear the end of the stroke, and then coming down again without the crankbeing able to turn the centre. In such cases, it will be necessary to turnthe vessel's head sufficiently from the wind to enable some sail to be set;and if once there is weigh got upon the vessel the engine will begin towork properly, and will continue to do so though the vessel be put head towind as before. 

741. _Q. _--What should be done if a crack shows itself in any of the shaftsor cranks?

_A. _--If the shafts or cranks crack, the engine may nevertheless be workedwith moderate pressure to bring the vessel into port; but if the crack bevery bad, it will be expedient to fit strong blocks of wood under the endsof the side levers, or other suitable part, to prevent the cylinder bottomor cover from being knocked out, should the damaged part give way. The sameremark is applicable when flaws are discovered in any of the main parts ofthe engine, whether they be malleable or cast iron; but they must becarefully watched, so that the engines may be stopped if the crack isextending further. Should fracture occur, the first thing obviously to bedone is to throw the engines out of gear; and should there be much weigh onthe vessel, the steam should at once be thrown on the reverse side of thepiston, so as to counteract the pressure of the paddle wheel. 

742. _Q. _--Have you any information to offer relative to the lubrication ofengine bearings?

_A. _--A very useful species of oil cup is now employed in a number of steamvessels, and which, it is said, accomplishes a considerable saving of oil, at the same time that it more effectually lubricates the bearings. Aratchet wheel is fixed upon a little shaft which passes through the side ofthe oil cup, and is put into slow revolution by a pendulum attached to itsoutside and in revolving it lifts up little buckets of oil and empties themdown a funnel upon the centre of the bearing. Instead of buckets a fewshort pieces of wire are sometimes hung on the internal revolving wheel, the drops of oil which adhere on rising from the liquid being deposited. Upon a high part set upon the funnel, and which, in their revolution, thehanging wires touch. By this plan, however, the oil is not well supplied atslow speeds, as the drops fall before the wires are in proper position forfeeding the journal. Another lubricator consists of a cock or plug insertedin the neck of the oil cup, and set in revolution by a pendulum and ratchetwheel, or any other means. There is a small cavity in one side of the plug, which is filled with oil when that side is uppermost, and delivers the oilthrough the bottom pipe when it comes opposite to it. 

743. _Q. _--What are the prevailing causes of the heating of bearings?

_A. _--Bad fitting, deficient surface, and too tight screwing down. Sometimes the oil hole will choke, or the syphon wick for conducting theoil from the oil cup into the central pipe leading to the bearing willbecome clogged with mucilage from the oil. In some cases bearings heat fromthe existence of a cruciform groove on the top brass for the distributionof the oil, the effect of which is to leave the top of the bearings dry. Inthe case of revolving journals the plan for cutting a cruciform channel forthe distribution of the oil does not do much damage; but in other cases, asin beam journals, for instance, it is most injurious, and the brassescannot wear well wherever the plan is pursued. The right way is to make ahorizontal groove along the brass where it meets the upper surface of thebearing, so that the oil may be all deposited on the highest point of thejournal, leaving the force of gravity to send it downward. This channelshould, of course, stop short a small distance from each flange of thebrass, otherwise the oil would run out at the ends. 

744. _Q. _--If a bearing heats, what is to be done?

_A. _--The first thing is to relax the screws, slow or stop the engine, andcool the bearing with water, and if it is very hot, then hot water may befirst employed to cool it, and then cold. Oil with sulphur intermingled isthen to be administered, and as the parts cool down, the screws may beagain cautiously tightened, so as to take any jump off the engine from thebearing being too slack. The bearings of direct acting screw enginesrequire constant watching, as, if there be any disposition to heatmanifested by them, they will probably heat with great rapidity from thehigh velocity at which the engines work. Every bearing of a direct actingscrew engine should have a cock of water laid on to it, which may beimmediately opened wide should heating occur; and it is advisable to workthe engine constantly, partly with water, and partly with oil applied tothe bearings. The water and oil are mixed by the friction into a species ofsoap which both cools and lubricates, and less oil moreover is used than ifwater were not employed. It is proper to turn off the water some timebefore the engine is stopped, so as to prevent the rusting of the bearings. 

MANAGEMENT OF LOCOMOTIVES. 

745. _Q. _--What are the chief duties of the engine driver of a locomotive?

_A. _--His first duties are those which concern the safety of the train; hisnext those which concern the safety and right management of the engine and

boiler. The engine driver's first solicitude should be relative to theobservation and right interpretation of the signals; and it is only afterthese demands upon his attention have been satisfied, that he can look tothe state of his engine. 

746. _Q. _--As regards the engine and boiler, what should his main dutiesbe?

_A. _--The engineer of a locomotive should constantly be upon the foot boardof the engine, so that the regulator, the whistle or the reversing handlemay be used instantly, if necessary; he must see that the level of thewater in the boiler is duly maintained, and that the steam is kept at auniform pressure. In feeding the boilers with water, and the furnaces withfuel, a good deal of care and some tact are necessary, as irregularity inthe production of steam will often occasion priming, even though the waterbe maintained at a uniform level; and an excess of water will of itselfoccasion priming, while a deficiency is a source of obvious danger. Theengine is generally furnished with three gauge cocks, and water shouldalways come out of the second gauge cock, and steam out of the top one whenthe engine is running: but when the engine is at rest, the water in theboiler is lower than when in motion, so that when the engine is at rest, the water will be high enough if it just reaches to the middle gauge cock. In all boilers which generate steam rapidly, the volume of the water isincreased by the mingled steam, and in feeding with cold water the level atfirst falls; but it rises on opening the safety valve, which causes thesteam in the water to swell to a larger volume. In locomotive boilers, therise of the water level due to the rapid generation of steam is termed"false water. " To economize fuel, the variable expansion gear, if theengine has one, should be adjusted to the load, and the blast pipe shouldbe worked with the least possible contraction; and at stations the dampershould be closed to prevent the dissipation of heat. 

747. _Q. _--In starting from a station, what precautions should be observedwith respect to the feed?

_A. _--In starting from a station, and also in ascending inclined planes, the feed water is generally shut off; and therefore before stopping orascending inclined planes, the boiler should be well filled up with water. In descending inclined planes an extra supply of water may be introducedinto the boiler, and the fire may be fed, as there, is at such times asuperfluity of steam. In descending inclined planes the regulator must bepartially closed, and it should be entirely closed if the plane be verysteep. The same precaution should be observed in the case of curves, orrough places on the line, and in passing over points or crossings. 

748. _Q. _--In approaching a station, how should the supply of water andfuel be regulated?

_A. _--The boiler should be well filled with water on approaching a station, as there is then steam to spare, and additional water cannot beconveniently supplied when the engine is stationary. The furnace should befed with small quantities of fuel at a time, and the feed should be turnedoff just before a fresh supply of fuel is introduced. The regulator may, atthe same time, be partially closed; and if the blast pipe be a variableone, it will be expedient to open it widely while the fuel is beingintroduced, to check the rush of air in through the furnace door, and thento contract it very much so soon as the furnace door is closed, in order torecover the fire quickly. The proper thickness of coke upon the gratedepends upon the intensity of the draught; but in heavily loaded engines itis usually kept up to the bottom of the fire door. Care, however, must betaken that the coke does not reach up to the bottom row of tubes so as tochoke them up. The fuel is usually disposed on the grate like a vault; andif the fire box be a square one, it is heaped high in the corners, thebetter to maintain the combustion. 

749. _Q. _--How can you tell whether the feed pumps are operating properly?

_A. _--To ascertain whether the pumps are acting well, the pet cock must beturned, and if any of the valves stick they will sometimes be induced toact again by working with the pet cock open, or alternately open and shut. Should the defect arise from a leakage of steam into the pump, whichprevents the pump from drawing, the pet cock remedies the evil bypermitting the steam to escape. 

750. _Q. _--What precautions should be taken against priming in locomotives?

_A. _--Should priming occur from the water in the boiler being dirty, aportion of it may be blown out; and should there be much boiling downthrough the glass gauge tube, the stop cock may be partially closed. Thewater should be wholly blown out of locomotive boilers three times a week, and at those times two mud-hole doors at opposite corners of the boilershould be opened, and the boiler be washed internally by means of a hose. If the boiler be habitually fed with dirty water, the priming will be aconstant source of trouble. 

751. _Q. _--What measures should the locomotive engineer take, to check thevelocity of the train, on approaching a station where he has to stop?

_A. _--On approaching a station the regulator should be gradually closed, and it should be completely shut about half a mile from the station if thetrain be a very heavy one: the train may then be brought to rest by meansof the breaks. Too much reliance, however, must not be put upon the breaks, as they sometimes give way, and in frosty weather are nearly inoperative. In cases of urgency the steam may be thrown upon the reverse side of thepiston, but it is desirable to obviate this necessity as far as possible. At terminal stations the steam should be shut off earlier than at roadsidestations, as a collision will take place at terminal stations if the trainovershoots the place where it ought to stop. There should always be a goodsupply of water when the engine stops, but the fire may be sufferedgradually to burn low toward the conclusion of the journey. 

752. _Q. _--What is the duty of an engine man on arriving at the end of hisjourney?

_A. _--So soon as the engine stops it should be wiped down, and be thencarefully examined: the brasses should be tried, to see whether they areslack or have been heating; and, by the application of a gauge, it shouldbe ascertained occasionally whether the wheels are square on their axles, and whether the axles have end play, which should be prevented. Thestuffing boxes must be tightened, and the valve gear examined, and theeccentrics be occasionally looked at to see that they have not shifted ontheir axles, though this defect will be generally intimated by theirregular beating of the engines. The tubes should also be examined andcleaned out, and the ashes emptied out of the smoke box through the smallash door at the end. If the engine be a six-wheeled one, with the drivingwheels in the middle, it will be liable to pitch, and oscillate if too muchweight be thrown upon the driving wheels; and where such faults are foundto exist, the weight upon the drivings wheels should be diminished. Thepractice of blowing off the boiler by the steam, as is always done inmarine boilers, should not be permitted as a general rule in locomotiveboilers, when the tubes are of brass and the fire box of copper; but whenthe tubes and fire boxes are of iron, there will not be an equal risk ofinjury. Before starting on a journey, the engine man should take a summaryglance beneath the engine--but before doing so he ought to assure himselfthat no other engine is coming up at the time. The regulator, when theengine is standing, should be closed and locked, and the eccentric rod befixed out of gear, and the tender break screwed down; the cocks of the oilvessels should at the same time be shut, but should all be opened a shorttime before the train starts. 

753. _Q. _--What should be done if a tube bursts in the boiler?

_A. _--When a tube bursts, a wooden or iron plug must be driven into eachend of it, and if the water or steam be rushing out so fiercely that theexact position of the imperfection cannot be discovered, it will beadvisable to diminish the pressure by increasing the supply of feed water. Should the leak be so great that the level of the water in the boilercannot be maintained, it will be expedient to drop the bars and quench thefire, so as to preserve the tubes and fire box from injury. 

754. _Q. _--If any of the working parts of a locomotive break or becomederanged, what should be done?

_A. _--Should the piston rod or connecting rod break, or the cutters fallout or be clipped off--as sometimes happens to the piston cutter when theengine is suddenly reversed upon a heavy train--the parts should bedisconnected, if the connection cannot be restored, so as to enable oneengine to work; and of course the valve of the faulty engine must be keptclosed. If one engine has not power enough to enable the train to proceedwith the blast pipe full open, the engine may perhaps be able to take on apart of the carriages, or it may run on by itself to fetch assistance. Thesame course must be pursued if any of the valve gearing becomes deranged, and the defects cannot be rectified upon the spot. 

755. _Q. _--What are the most usual causes of railway collisions?

_A. _--Probably fogs and inexactness in the time kept by the trains. Collisions have sometimes occurred from carriages having been blown from asiding on to the rails by a high wind; and the slippery state of the rails, or the fracture of a break, has sometimes occasioned collisions at terminalstations. Collision has also repeatedly taken place from one engine havingovertaken another, from the failure of a tube in the first engine, or fromsome other slight disarrangement; and collision has also taken place fromthe switches having been accidentally so left as to direct the train into asiding, instead of continuing it on the main line. Every train now carriesfog signals, which are detonating packets, which are fixed upon the railsin advance or in the rear of a train which, whether from getting off therails or otherwise, is stopped upon the line, and which are exploded by thewheels of any approaching train. 

756. _Q. _--What other duties of an engine-driver are there deservingattention?

_A. _--They are too various to be all enumerated here, and they also varysomewhat with the nature of the service. One rule, however, of universalapplication, is for the driver to look after matters himself, and notdelegate to the stoker the duties which the person in charge of the engineshould properly perform. Before leaving a station, the engine-driver shouldassure himself that he has the requisite supply of coke and water. Besidesthe firing tools and rakes for clearing the tubes, he should have with himin the tender a set of signal lamps and, torches, for tunnels and fornight, detonating signals, screw keys, a small tank of oil, a small cask oftallow, and a small box of waste, a coal hammer, a chipping hammer, somewooden and iron plugs for the tubes, and an iron tube holder for insertingthem, one or two buckets, a screw jack, wooden and iron wedges, split wirefor pins, spare cutters, some chisels and files, a pinch bar, oil cans andan oil syringe, a chain, some spare bolts, and some cord, spun yarn, andrope. 

INDEX. 

Accidents in steam vessels, proper preparation for. Admiralty rule for horse power. Adhesion of wheels of locomotives to rails. Air, velocity of, entering a vacuum, required for combustion of coal; law of expansion of, by heat;Air pump, description of, action of; proper dimensions of. Air pump of marine engines, details of. Air pump of oscillating engine. Air pump of direct acting screw engines. Air pumps made both single and double acting, difference of, explained. Air pumps, double acting valves of, bad vacuum in; causes and remedy. Air pump rods, brass or copper, in marine engines. Air pump bucket, valves of. Air pump, connecting rod and cross head of oscillating engine. Air pump rod of oscillating engine. Air pump arm. Air vessels applied to suction side of pumps. "Alma, " engine of, by Messrs. John Bourne & Co. "Amphion, " engines of. Amoskeag steam fire engine. Angle iron in boilers, precautions respecting. Apparatus for raising screw propeller. Atmospheric valve. Atmospheric resistance to railway trains. Auxiliary power, screw vessels with. Axle bearings of locomotives. Axle guards. Axles and wheels of modern locomotives. "Azof, " slide valve of. 

Babbitt's metal, how to compound. Balance piston to take pressure off slide valve. Ball valves. Barrel of boiler of modern locomotives. Beam, working of land engine, main or working strength proper for. Bearings of engines or other machinery, rule for determining proper surface of. Bearings, heating of, how to prevent or remedy, journals should always bottom, as, if they grip on the sides, the pressure is infinite. Beattie's screw. Belidor's valves might be used for foot and delivery valves. Bell-metal, composition of. Blast pipe of locomotives, description of. Blast in locomotives, exhaustion produced by, proper construction of the blast pipe; the blast pipe should be set below the root of the chimney so much that the cone of escaping steam shall just fill the chimney. Blast pipe with variable orifice, at one time much used. Blow-off cock of locomotives. Blow-off cocks of marine boilers, proper construction of. Blow-off cocks, description of. Blowing off supersalted water from marine boilers. Blowing off, estimate of heat lost by, mode of. Blow through valve, description of. Blowing furnaces, power necessary for. Bodies, falling, laws of. Bodmer, expansion valve by. Boilers, general description of: the wagon boiler, the Cornish boiler; the marine flue boiler; the marine tubular boiler; locomotive boiler--_see_ Locomotives. Boilers proportions of: heating surface of, fire grate, surface of; consumption of fuel on each square foot of fire bars in wagon, Cornish, and locomotive boilers; calorimmeter and vent of boilers; comparison of proportions of wagon, flue, and tubular boilers; evaporative power of boilers; power generated by evaporation of a cubic foot of water; proper proportions of modern marine boilers both flue and tubular; modern locomotive boilers; exhaustion produced by blast in locomotives; increased evaporation from increased exhaustion; strength of boilers; experiments on, by Franklin Institute; by Mr. Fairbairn; mode of computing strength of boilers; staying of. Boilers, marine, prevented from salting by blowing off, early locomotive and contemporaneous marine boilers compared; chimneys of land; rules for proportions of chimneys; chimneys of marine boilers. Boilers, constructive details of: riveting and caulking of land boilers, proving of; seams payed with mixture of whiting and linseed oil; setting of wagon boilers; riveting of marine boilers; precautions respecting angle iron; how to punch the rivet holes and shear edges of plates; setting of marine boilers in wooden vessels; mastic cement for setting marine boilers; composition of mastic cement; best length of furnace; configuration of furnace bars; advantages and construction of furnace bridges; various forms of dampers; precautions against injury to boilers from intense heat; tubing of boilers; proper mode of staying tube plates; proper mode of constructing steamboat chimneys; waste steam-pipe and funnel casing; telescope chimneys; formation of scale in marine boilers; injury of such incrustations; amount of salt in sea water; saltness permissible in boilers; amount of heat lost by blowing off; mode of discharging the supersalted water; Lamb's scale preventer; internal corrosion of marine boilers; causes of internal corrosion; surcharged steam produced from salt water; stop valves between boilers; safety or escape valve on feed pipe; locomotive boilers consist of the fire box, barrel for holding tubes, and smoke box; dimensions of the barrel and thickness of plates; mode of staying fire box and furnace crown; fire bars, ash box, and chimney; steam dome used only in old engines; manhole, mudholes, and blow-oft cock; tube plate, and mode of securing tubes; expanding mandrels; various forms of regulator. Boilers of modern locomotives. Boiler, the, proper care of, the first duty of the engineer. Bolts, proper proportions of. Boring of cylinders. Boulton and Watt's rules for fly wheel, proportions of marine flue boilers; rule for proportions of chimneys of land boilers; of marine boilers; experiments on the resistance of vessels in water. Bourdon's steam and vacuum gauges. Bourne, expansion valves by. Bourne, Messrs. J. & Co. , direct acting screw engines by. Brass for bearings, composition of. Brazing solders. Bridges in furnaces, benefits of. Burning of boilers, precautions against. Bursting velocity of fly wheel, and of railway wheels. Bursting of boilers, causes of; precautions against; may be caused by accumulations of salt. Butterfly valves of air pump. 

Cabrey, expansion valve by. Calorimeter of boilers, definition of. Cams, proper forms of. Cast iron, strength of, proportions of cast iron beams; effects of different kinds of strains on beams; strength to resist shocks not proportional to strength to resist strains; to attain maximum strength should be combined with wrought iron. Casting of cylinders. Case-hardening, how to accomplish. Cataract, explanation of nature and uses of. Caulking of land boilers. Cement, mastic, for setting marine boilers. Central forces. Centre of pressure of paddle wheels. Centres of gravity, gyration and oscillation. Centres for fixing arms of paddle wheel. Centres of an engine, how to lay off. Centrifugal force, nature of, rule for determining; bursting velocity of fly wheel; and of railway wheels. Centrifugal pump will supersede common pump. Centripetal force, nature of. Chimney of locomotives. Chimney of steam vessels, what to do if carried away. Chimneys of land boilers, Boulton and Watt's rule for proportions of; of marine boilers. Chimneys, exhaustion produced by, high and wide chimneys in locomotives injurious. Chimneys of steamboats, telescope. Clark's patent steam fire regulator. Coal, constituents of, combustion of air required for; evaporative efficacy of; of wood, turf, and coke. Cocks, proper construction of. Cog wheels for screw engines. Coke, evaporative efficacy of. Cold water pump, description of, rule for size of. Combustion, nature of. Combustion of coal, air required for. Combustion, slow and rapid, comparative merits of, rapid combustion necessary in steam vessels, and enables less heating, surface in the boiler to suffice. Conchoidal propeller. Condensation of steam, water required for. Condenser, description of, action of; proper dimensions of. Condenser of oscillating engine. Condenser of direct acting screw engine. Condensing engine, definition of. Condensing water, how to provide when deficient. Conical pendulum or governor. Connecting rod, description of, strength proper for. Connecting rod of direct acting screw engines, of locomotives. Consumption of fuel on each square foot of fire bars in wagon, Cornish, and locomotive boilers. Copper, strength of. Corliss's steam engine. Corrosion produced by surcharged steam. Corrosion of marine boilers, causes of. Cost of locomotives. Cotton spinning, power necessary for. Counter for counting strokes of an engine. Crank, description of, unequal leverage of, corrected by fly wheel; no power lost by; action of; strength proper for. Crank of direct acting screw engines. Crank pin, strength proper for. Crank pin of direct acting screw engines. Cranked axle of locomotives. Cross head, description of, strength proper for. Cross head of direct acting screw engines. Cross tail, description of. Cylinder, description of, strength proper for. Cylinder of oscillating engine, of direct acting screw engine. Cylinders should have a steam jacket, and be felted and planted, should have escape valves. Cylinders of locomotives should be large, proper arrangement of. Cylinders, how to cast, how to bore; how to grind. Cylinder jacket, advantages of. 

Damper. Dampers, various forms of. Deadwood, hole in, for screw. Delivery valve, description of. Delivery or discharge valves, proper dimensions of. Delivery valves might be made on Belidor's plan. Delivery valves in mouth of air pump, of india rubber. Direct acting screw engines should be balanced. Direct acting screw engine by Messrs. John Bourne &, Co. , cylinder; discs; guides; screw shaft brasses; air pump; slide valve; balance piston; connecting rod; piston rods; cross head; air pump arm; feed pump; crank pin; screw shaft; thrust plummer block; link motion; screw propeller. Discharge valves. Disc valves of india rubber for air pumps. Discs of direct acting screw engine instead of crank. Dodds, expansion valve by. Double acting engines, definition of. Double acting air pumps, valves of; faults of. Draw bolt. Dredging earth out of rivers, power necessary for. Driving wheels of locomotives. Driving piles, power necessary for. Duplex pump, Worthington's. Dundonald, Earl of, screw by. Duty of engines and boilers, how the duty is ascertainable. Dynamometer, description of. Dynamometric power of screw vessels. 

Eccentric, description of, sometimes made loose for backing. Eccentric and eccentric rod of oscillating engine. Eccentric notch should be fitted with a brass bush. Eccentric straps of locomotives, rods of locomotives. Eccentrics of locomotives, how to readjust. Economy of fuel in steam vessels. Edwards, expansion valve by. Elasticity, limits of. Engine, high pressure, definition of, low pressure, definition of. Engines, classification of, rotative, definition of; rotatory, definition of; single acting, definition of; double acting, definition of; mode of erecting in a vessel; how to refix if they have become loose. Engineers of steam vessels should make proper preparation for accidents. Equilibrium slide valve, grid-iron valve. Erecting engines in a vessel. Erection of engines in the workshop. Escape valve on feed pipe. Escape valves for letting water out of cylinders. Evaporative efficacy of coal, of wood, turf, and coke. Evaporative power of boilers, power generated by evaporation of a cubic foot of water; increase of evaporation due to increased exhaustion in locomotives. Excavator, Otis's. Exhaustion produced by chimneys, by the blast in locomotives; increased evaporation from increased exhaustion. Expanding mandrels for tubing boilers. Expansion of air by heat. Expansion of surcharged steam by heat. Expansion of steam, pressure of steam inversely as the space occupied; law of expansion; rule for computing the increase of efficiency produced by working expansively; necessity of efficient provisions against refrigeration in working expansively; advantages of steam jacket; Forms of apparatus for working expansively: lap on the slide valve wire drawing the steam; Cornish expansion valve, in rotative engines worked by a cam; mode of varying the degree of expansion; proper forms of cams; the link motion; expansion valves, by Cabrey, Fenton, Dodds, Farcot, Edwards, Lavagrian, Bodmer, Meyer, Hawthorn, Gonzenbach, and Bourne. Expansion joint in valve casing. Expansion valves, Cornish, the link motion; by Cabrey, Fenton, Dodds, Farcot, Edwards, Lavagrian, Bodmer, Meyer, Hawthorn, Gouzenbach, and Bourne. Explosions of boilers, causes of explosions; precautions against; dangers of accumulations of salt. 

Face plates or planometers. Falling bodies, laws of. Farcot, expansion valve by. Feathering paddle wheels, description of, details of. Feed pump, description of, action of; proper dimensions of; rule for proportioning. Feed pump plunger, and valves. Feed pumps of locomotives, details of. Feed pumps of direct acting screw engines. Fenton, expansion valve by. Fire bars of locomotives. Fire box of locomotives, mode of staying. Fire box of modern locomotives. Fire engines, cost of running. Fire grate surface of boilers. Fire grate in locomotives should be of small area, coke proper to be burned per hour on each square foot of bars. Firing furnaces, proper mode of. Flaws in valves or cylinders, how to remedy. Float for regulating water level in boilers. Floats of paddle. Floats of paddle wheels, increased resistance of, if oblique, floats should be large. Fly wheel corrects unequal leverage of crank, proper energy for; Boulton and Watt's rule for; bursting velocity of; description of; action of, in redressing irregularities of motion. Foot valve, description of, proper dimensions of. Foot valves might be made on Belidor's plan, of india rubber. Frame at stern for holding screw propeller. Framing of locomotives. Framing of oscillating engine. Franklin Institute, experiments on steam by. French Academy, experiments on steam by. Friction, nature of, does not vary as the rubbing surfaces, but as the retaining pressure; does not increase with the velocity per unit of distance, but increases with the velocity per unit of time; measures of friction; effect of unguents; kind of unguent should vary with the pressure; Morin's experiments; rule for determining proper surfaces of bearings; friction of rough surfaces. Friction of the water the main cause of the resistance of vessels of good shape. Fuel burnt on each square foot of fire bars in wagon, Cornish, and locomotive boilers, economy of, in steam vessels. Funnel casing. Funnel, what to do if carried away. Funnels of steam boats. _See_ Chimneys. Furnaces, proper mode of firing, smoke burning: Williams's argand; Prideaux's; Boulton and Watt's dead plate; revolving crate; Juckes's; Maudslay's; Hull's, Coupland's, Godson's, Robinson's, Stevens's, Hazeldine's, &c. . Furnaces of marine boilers, proper length of. Furnace bridges, benefits of. Fusible metal plugs useless as antidotes to explosions. 

Gauges, vacuum, steam; gauge cocks and glass tubes for showing level of water in boiler, description of. Gauge cocks for showing level of water in boiler. Gearing for screw engines. Gibs and cutters, strengths proper for. Giffard's injector. Glass tubes for showing water level in boilers. Glass tube cocks. Gonzeubach, expansion valve by. Gooch's indicator. Gooch's locomotive. Governor or conical pendulum, description of. Governor, Porter's patent. Gravity, centre of. "Great Western, " boilers of, by Messrs. Maudslay. Gridiron valve. Griffith's screw. Grinding corn, power necessary for. Grinding of cylinders. Gudgeons, strength proper for. Guides of locomotives. Guides of direct acting screw engine. Gun metal, strength of. Gyration, centre of. 

Harvey and West's pump valves. Hawthorn, expansion valve by. Heat, latent, definition of. Heat, specific, definition of. Heat, Regnault's experiments on. Heat, loss of, by blowing off marine boilers. Heating surface of boilers. Heating surface per square foot of fire bars in locomotives, a cubic foot of water evaporates by five square feet of heating surface. Heating of bearings, causes of, bearings should always be slack at the sides, else the pressure is infinite. High pressure engine, definition of. High pressure engines, power of. High speed engines, arrangements proper for high speeds. Hoadley's portable engine. Hodgson's screw. Hoe & Co. 's steam engine. Holding down bolts of marine engines, or bolts for securing engines to hull. Holms's screw propeller. Horses power, definition of, nominal horse power; actual power ascertained by the indictator; Admiralty rule for. Hot water or feed pump, description of. Hot well, description of. 

Increasing pitch of screw. Incrustation in boilers. _See_ also Salt. India rubber valves for air pump. Indicator, description of the, by McNaught, structure and mode of using; Gooch's continuous indicator. Injection cock. Injection cocks of marine engines at ship's sides. Injection orifice, proper area of. Injector, Giffard's. Injection valve. Inside cylinder locomotives. Iron, strength of, limits of elasticity of; proper strain to be put upon iron in engines and machines; aggravation of strain by being intermittent; increase of strain due to deflection; strength of pillars and tubes, combination of malleable and cast iron. Iron, cast, strength of, cast iron beams; may be strong to resist strains, but not strong to resist shocks; should be combined with wrought iron to obtain maximum strength. Iron, if to be case hardened, should be homogeneous. 

Jacket of cylinder, advantages of. Joints, rust, how to make. 

Kingston's valves. 

Lamb's scale preventer. Lantern brass in stuffing boxes. Lap and lead of the valve, meaning of. Large vessels have least proportionate resistance. Latent heat, definition of. Latta's steam fire engine. Lavagrian, expansion valve by. Lead and lap of the valve, meaning of. Lead of the valve, benefits of. Lever, futility of plans for deriving power from a lever. Lifting apparatus for screw propeller. Limits of elasticity. Links, main description of. 

Link motion of direct acting screw engine. Link motion, how to set. Locomotive engines, general description of the locomotive; Stephenson's locomotive; Gooch's locomotive for the wide gauge; Crampton's locomotive for the narrow gauge. Locomotives, adhesion of wheels of, cost and performance of; framing of; cylinders of; springs of; outside and inside cylinders; sinuous motion of; rocking motion of; pitching motion of; pistons; piston rods; guides; cranked axle; axle bearings; eccentrics; eccentric rod; starting handle; link motion; valves, how to set; eccentrics, how to readjust; feed pumps; connection of engine and tender, driving wheels; wheel tires. Locomotive engine of modern construction, example of, fire box; barrel of boiler; tubes; tube plate; framing; axle guards; draw bolt; wheels and axles; cylinders; valve; piston; piston rod; guides; connecting rod; eccentrics; link motion; regulator; blast pipe; safety valve; feed pump; tendencies of improvement in locomotives. Locomotives, management of. Locomotive boilers, examples of modern proportions. Locomotive boilers, details of. Low pressure or condensing engine, definition of. Lubrication of rubbing surfaces, the friction depends mainly on the nature of lubricant; oil forced out of bearings, if the pressure exceeds 800 lbs. Per square inch longitudinal section; water a good lubricant if the surfaces are large enough. Lubrication of engine bearings. 

McNaught's indicator. Main beam, strength proper for. Main centre, description of, strength proper for. Main links, description of, strength proper for. Mandrels, expanding, for tubing boilers. Manhole door. Manhole of locomotives. Marine flue boilers, proportions of. _See_ also Boilers. Marine boilers of modern construction, proper proportions of. Marine engines. _See_ Steam Engines, marine. Mastic cement for setting marine boilers. Maudslay, Messrs. , boilers of "Retribution" and "Great Western, " by, Mechanical powers, misconceptions respecting. Mechanical power, definition of, indestructible and eternal; the sun the source of mechanical power. Metallic packing for pistons. Metallic packing for stuffing boxes. Meyer, expansion valve by. Miller, Ravenhill & Co. 's mode of fixing piston rod to piston. Modern locomotives. Momentum, or _vis viva_. Morin, experiments on friction by. Mudholes of locomotives. Muntz's metal, composition of. 

"Niger" and "Basilisk, " trials of. "Nile, " boilers of the, by Boulton and Watt. Notch of eccentric should be fitted with brass bush. 

Oils for lubrication. _See_ Lubrication. Oscillation, centre of. Oscillating paddle engine, description of. Oscillating engine, advantages of, futility of objections to; details of cylinder; framing; condenser; air pump; trunnions; valve and valve casing; piston; piston rod; air pump connecting rod and cross head; air pump rod; eccentric and eccentric rod; valve gear; valve sector; shaft plummer blocks; trunnion plummer blocks; feathering paddle wheels; packing of trunnions. Oscillating engines, how to erect. Otis's excavator. Outside and inside cylinder locomotives. 

Packing for stuffing box of Watt's engine. Packing of piston of pumping engines, how to accomplish. Packing of trunnions. Paddle bolts, proper mode of forming. Paddle centres. Paddle floats. Paddle shaft, description of. Paddle shaft, details of. Paddle shaft plummer blocks of oscillating engines. Paddle wheels, details of, structure and operation of; slip of; centre of pressure of; rolling circle; action of oblique floats; rule for proportioning paddle wheels; benefits of large floats. Paddle wheels, feathering, description of; details of. Paddles and screw combined. Parallel motion, description of, how to lay off centres of. Pendulum, cause of vibrations of; relation of vibrations of pendulum to velocity of falling bodies; conical pendulum or governor. Penn, Messrs. , engines of "Great Britain, " by, direct acting screw engines by; trunk engines by. Performance of locomotives. Pillars, hollow, strength of, law of strength varies with thickness of metal. Pipe for receiving screw shaft. Pipes of marine engines. Piston, description of, how to pack with hemp. Pistons, metallic packing for. Pistons for oscillating engines. Pistons, how to fit and finish. Pistons of locomotives. Piston rod, description of, strength proper for. Piston rods of locomotives. Piston rod of oscillating engine. Piston rods of direct acting screw engine. Pitch of the screw. Pitch, increasing or expanding. Pitching motion in locomotives. Planometers, or face plates. Plummer blocks of shafts and trunnions of oscillating engines. Plummer blocks for receiving thrust of screw propeller. Plunger of feed pump. Portable engine, Hoadley's. Porter's patent governor. Ports of the cylinder, area of. Pot-lid valves of air pump. Powers, mechanical, misconception respecting. Power, horses, definition of, nominal and actual power; power of high pressure engines. Power necessary for thrashing and grinding corn, working sugar mills, spinning cotton, sawing timber, grossing cotton, blowing furnaces, driving piles, and dredging earth out of rivers. Pressing cotton, power necessary for. Priming, nature and causes of. Priming, if excessive, may occasion explosion. Propeller, screw, description of. Proportions of screws with, two, four, and six blades. Proving of boilers. Prussiate of potash for case hardening. Pumping engines, mode of erecting, mode of starting. Pumps, loss of effect in, at high speed and with hot water, causes of this loss; remedy for. Pumps used for mines. Pump, air, description, of, action of. Pumps, air, proper proportions of, single and double acting. Pump, centrifugal, better than common pump. Pump, cold water, description of. Pump, feed, description of, action of; proper dimensions of; rule for proportioning; plunger of; valves of; independent. Pump valves for mines, &c. Punching and shearing boiler plates. 

Railway wheels, bursting velocity of. Railway trains, resistance of. Rarefaction or exhaustion produced by chimneys. "Rattler" and "Alecto, " trials of. Registration, benefits of. Regnault, experiments on heat by. Regulator, a valve for regulating the admission of steam in locomotives, description of; various forms of. Regulator, Clark's, patent steam and fire. Rennie, experiments on friction by. Resistance, experienced by railway trains. Resistance of vessels in water, mainly made up of friction; experiments on. Resistance and speed of vessels influenced by their size. "Retribution, " boilers of, by Messrs. Mandslay. Riveting and caulking of land boilers. Rocking motion of locomotives. Rolling circle of paddle wheels. Rotatory engines, definition of. Rotative engines, definition of. Rust joints, how to make. 

Safety valve, area of, in low pressure engines, in locomotives. Salinometer, or salt gauge, how to use, how to construct. Salt, accumulation of, prevented in marine boilers by blowing off, if allowed to accumulate in boilers may occasion explosion; amount of, in sea water. Salt water produces surcharged steam. Salting of boilers, what to do if this takes place. Sawing timber, power necessary for. Scale in marine boilers. _See_ also Salt. Scale preventer, Lamb's. Scrap iron, unsuitable for case hardening. Scraping tools for metal surfaces. Screw. Screw engine, geared oscillating, description of, direct acting, description of. Screw engine, direct acting, by Messrs. John Bourne & Co. Screw engines, best forms of. Screw frame in deadwood. Screw propeller, description of. Screw propeller, mode of fixing on shaft, modes of receiving thrust; apparatus for lifting; configuration of; action of; pitch of the screw; screws of increasing or expanding pitch; slip of the screw; positive and negative slip; screw and paddles compared; test of the dynamometer; trials of "Rattler" and "Alecto, " and "Niger" and "Basilisk"; indicator and dynamometer power; loss of power in screw vessels in head winds; the screw should be deeply immersed; screws of the Earl of Dundonald, Hodgson, Griffith, Holm, and Beattie; lateral and retrogressive slip; sterns of screw vessels should be sharp; proportions of screws with two, four, and six blades; screw vessels with auxiliary power; screw and paddles combined; economy of fuel in steam vessels; benefits of registration. Screw propeller, Holm's conchoidal. Screw shaft, details of. Screw shaft pipe at stern. Screw shaft brasses of direct acting screw engines. Sea water, amount of salt in. Sea injection cocks. Setting of wagon boilers, of marine boilers. Setting the valves of locomotives. Shaft, paddle, details of. Shaft of screw propeller, details of. Shafts, strength of. Shank's steam gauge. Shocks may not be well resisted by iron that can well resist strains, effect of inertia in resisting shocks. Side levers or beams, description of. Side lever marine engines, description of. Side lever engines, how to erect. Side rods, description of, strength proper for. Silsbee, Mynderse & Co. 's steam fire engine. Single acting engines, definition of. Single acting or pumping engines, mode of erecting; mode of starting. Sinuous motion of locomotives. Slide valve, various forms of; long D and three ported valve, description of; action of the slide valve; lead and lap of the valve; rules for determining the proportions of valves; advantages of lead in swift moving engines. Slide valve, equilibrium. Slide valve with balance piston of direct acting screw engine. Slide valve, how to finish. Slide valves of marine engines, how to set. Slip of paddle wheels. Slip of the screw, positive and negative slip; lateral and retrogressive slip. Smoke, modes of consuming. Smoke burning furnaces, Williams's argand; Prideaux's; Boulton and Watt's dead plate; revolving grate; Juckes's; Maudslay's; Hall's, Coupland's, Godson's, Robinson's, Stevens's, Hazeldine's, &c. "Snake" locomotive. Southern, experiments on friction by; experiments on steam by. Specific heat, definition of. Speed of vessels influenced by their size. Spheroidal condition of water in boilers. Springs of locomotives. Stand pipe for low pressure boilers. Starting handle of locomotives. Staying of boilers. Staying tube plates, mode of. Staying fire boxes of locomotives. Steam, experiments on by Southern, French Academy, Franklin Institute, and M. Regnault. Steam pump, Worthington's; Woodward's. Steam and water, relative bulks of. Steam, expansion of; pressure of; inversely as space occupied; _See also_ Expansion of Steam. Steam engine, applications and appliances of the. Steam engine, general description of Watt's double acting engine; R. Hoe & Co. 's; Corliss's; Woodruff & Beach's. Steam engine, various forms of, for propelling vessels; paddle engines and screw engines; principal varieties of paddle engines; different kinds of paddle wheels; the side lever engine; description of the side lever engine; oscillating paddle engine; description of feathering paddle wheels; direct acting screw engine. Steam dome of locomotives. Steam fire engine, Latta's. Amoskeag. Silsbee, Mynderse & Co. 's. Steam gauge, Bourdon's; Shank's. Steam jacket, benefits of;Steam passages, area of;Steam room in boilers;Steam, surcharged, law of expansion by heat;Steel, strength of;Stephenson, link motion by;Stop valves between boilers;Straight edges;Strains subsisting in machines;Strain proper to be put upon iron in engines;Strains in machines, vary inversely as the velocity of the part to which the strain isapplied. Aggravated by being intermittent. Increase of strain due to deflection. Effects of alternate strains in opposite directions. Strength of materials. Strength of hollow pillars, law of; strength varies with thickness of metal. Strength of cast iron to resist shocks does not vary as the strength to resist strains, increase of strength by combination with cast iron. Strength of boilers, experiments on, by Franklin Institute; by Mr. Fairbairn; mode of computing; mode of staying for strength. Strength of engines: cylinder, trunnions; piston rod; main links; connecting rod; studs of the beam; gudgeons; working beam; cast iron shaft; malleable iron shaft; teeth of wheels; side rods; crank; crank pin; cross head; main centre; gibs and cutter. Studs, strength proper for. Stuffing box, description of. Stuffing boxes with metallic packing, with sheet brass packed behind with hemp; sometimes fitted with a lantern, brass. Sugar mills, power necessary to work. Summers' experiments on the friction of rough surfaces. Surcharged steam, law of expansion of, by heat. Surcharged steam produced by salt water, corrosive action of. Surfaces, how to make true. Sweeping the tubes of boilers clean of soot. 

Teeth of wheels. Telescope chimneys. Tender of a locomotive, description of, attachment of, to engine. Thrashing corn, power necessary for. Throttle valve, description of. Thrust of the screw propeller, modes of receiving. Thrust plummer block. Tires of locomotive wheels. Traction on railways. Trunk engine by Messrs. Rennie, disadvantages of. Trunk engines by Messrs. Penn. Trunnions of oscillating engines, description of, strength proper for; details of. Trunnion packing. Trunnion plummer blocks. Tube plates, mode of staying. Tube plates of modern locomotives. Tubes of modern locomotive boilers. Tubes of boilers, how to sweep clean of soot. Tubing of boilers. Tubing locomotive boilers. 

Valve, atmospheric. Valve casing, description of. Valve casing should have expansion joint. Valve and valve casing of oscillating engine. Valve delivery, description of, action ofValve, equilibrium slide. Valve, foot, description of, action of. Valve gear of Watt's engine, action of. Valve gear of oscillating engine. Valve, gridiron. Valve, slide. _See_ Slide Valve. Valve, slide, how to finish. Valves, ball, Belidor's might be used for foot and delivery valves; butterfly, of air pump; concentric ring, for air pump bucket. Valves, equilibrium. Valves, escape, for cylinders. Valves, expansion. _See_ Expansion Valves. Valves of feed pumps. Valves, india rubber, for air pump. Valves, Kingston's. Valves of locomotives, how to set. Valves, pot-lid, of air pump. Vacuum, meaning of, nature and uses of; how maintained in engines. Vacuum sometimes occurs in boilers, evils of a vacuum in boilers. Vacuum, velocity with which air rushes into a. Vacuum gauge, Bourdon's. Velocity of air entering a vacuum. Velocity of falling bodies. Vent of boilers, definition of. Vessels, resistance of, mainly made up of friction in good forms; experiments on; influence of size. _Vis viva_, or mechanical power. 

Waste steam pipe. Waste water pipe. Water required for condensation, pumps for supplying. Watt's double acting engine, description of. Wedge. Wheels, toothed, for screw engines. Wheels, teeth of. Wheels of locomotives, adhesion of. Wheels, driving, of locomotives. Wheel tires. Wheels and axles of modern locomotives. Wood, experiments on friction by. Wood, evaporative efficacy of. Woodman's steam pump. Woodruff & Beach's steam engine. Working beam of land engine, description of. Worthington's steam pump, duplex pump. 

THE END.