CHAPTER I.

A CATECHISM OF THE STEAM ENGINE

IN ITS VARIOUS APPLICATIONS TO MINES,

MILLS, STEAM NAVIGATION, RAILWAYS,

AND AGRICULTURE.

WITH

PRACTICAL INSTRUCTIONS FOR THE MANUFACTURE

AND MANAGEMENT OF ENGINES OF EVERY CLASS.

BY

JOHN BOURNE, C.E.

_NEW AND REVISED EDITION._

[Transcriber's Note: Inconsistencies in chapter headings and numbering

of 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 I

was unwilling to allow a new edition to go forth with all the original

faults of the work upon its head, and I have been too much engaged in the

practical construction of steam ships and steam engines to find time for

the thorough revision which I knew the work required. At length, however, I

have sufficiently disengaged myself from these onerous pursuits to

accomplish this necessary revision; and I now offer the work to the public,

with the confidence that it will be found better deserving of the favorable

acceptation and high praise it has already received. There are very few

errors, either of fact or of inference, in the early editions, which I have

had 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. I

have 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 such

extensive additions and improvements as to make the book almost a new one.

One purpose of my emendations has been to render my remarks intelligible to

a tyro, as well as instructive to an advanced student. With this view, I

have devoted the first chapter to a popular description of the Steam

Engine--which all may understand who can understand anything--and in the

subsequent gradations of progress I have been careful to set no object

before the reader for the first time, of which the nature and functions are

not simultaneously explained. The design I have proposed to myself, in the

composition of this work, is to take a young lad who knows nothing of steam

engines, and to lead him by easy advances up to the highest point of

information I have myself attained; and it has been a pleasing duty to me

to smooth for others the path which I myself found so rugged, and to

impart, for the general good of mankind, the secrets which others have

guarded with so much jealousy. I believe I am the first author who has

communicated that practical information respecting the steam engine, which

persons proposing to follow the business of an engineer desire to possess.

My business has, therefore, been the rough business of a pioneer; and while

hewing a road through the trackless forest, along which all might hereafter

travel with ease, I had no time to attend to those minute graces of

composition and petty perfection of arrangement and collocation, which are

the attribute of the academic grove, or the literary parterre. I am,

nevertheless, not insensible to the advantages of method and clear

arrangement in any work professing to instruct mankind in the principles

and practice of any art; and many of the changes introduced into the

present edition of this work are designed to render it less exceptionable

in this respect. The woodcuts now introduced into the work for the first

time will, I believe, much increase its interest and utility; and upon the

whole I am content to dismiss it into circulation, in the belief that those

who peruse it attentively will obtain a more rapid and more practical

acquaintance with the steam engine in its various applications, than they

would be likely otherwise to acquire.

I have only to add that I have prepared a sequel to the present work, in

the shape of a Hand-Book of the Steam Engine, containing the whole of the

rules given in the present work, illustrated by examples worked out at

length, and also containing such useful tables and other data, as the

engineer 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 is

in 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 become

familiar in their several localities; and I shall be happy, in my turn, to

answer any inquiries on engineering subjects which fall within the compass

of my information. If young engineers meet with any difficulty in their

studies, I shall be happy to resolve it if I can; and they may communicate

with me upon any such point without hesitation, in whatever quarter of the

world 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, having

been all sold off in about ten months, I now issue another edition, the

demand for the work being still unabated. It affords, certainly, some

presumption that a work in some measure supplies an ascertained want, when,

though addressing only a limited circle--discoursing only of technical

questions, and without any accident to stimulate it into notoriety,--it

attains so large a circulation as the present work has reached. Besides

being 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 to

those perusing it. I can with truth say, that the hope of rendering some

service to mankind, in my day and generation, has been my chief inducement

in writing it, and if this end is fulfilled, I have nothing further to

desire.

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, in

my Preface of the last year, I referred. I hope to have sufficient leisure

shortly, to give that and some other of my literary designs the necessary

attention. Whatever may have been the other impediments to a more prolific

authorship, certainly one of them has not been the coldness of the

approbation with which my efforts have been received, since my past

performances seem to me to have met with an appreciation far exceeding

their deserts.

JOHN BOURNE.

_February 2d, 1857_.

PUBLISHERS' NOTICE.

In offering to the American public a reprint of a work on the Steam Engine

so deservedly successful, and so long considered standard, the publishers

have not thought it necessary that it should be an exact copy of the

English edition; there were some details in which they thought it could be

improved, and better adapted to the use of American engineers. On this

account, the size of the page has been increased to a full 12mo, to admit

of larger illustrations, which in the English edition are often on too

small a scale; and some of the illustrations themselves have been supplied

by others equally applicable, more recent, and to us more familiar

examples. The first part of Chapter XI, devoted in the English edition to

English portable and fixed agricultural engines, in this edition gives

place entirely to illustrations from American practice, of steam engines as

applied to different purposes, and of appliances and machines necessary to

them. But with the exception of some of the illustrations and the

description of them, and the correction of a few typographical errors, this

edition 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 OF

ENGINES 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 neither

water 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 piston

to the end of the stroke, escapes into the atmosphere, and the impelling

force is therefore that due to the difference between the pressure of the

steam and the pressure of the atmosphere. In the condensing engine the

steam, after having pressed the piston to the end of the stroke, passes

into the condenser, in which a vacuum is maintained, and the impelling

force is that due to the difference between the pressure of the steam above

the 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 urging

the piston, consisting of the pressure shown by the safety-valve on the

boiler, and the pressure of the atmosphere besides.

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

engines?

_A._--Into single acting, rotative, and rotatory engines. Single acting

engines are engines without a crank, such as are used for pumping water.

Rotative engines are engines provided with a crank, by means of which a

rotative motion is produced; and in this important class stand marine and

mill engines, and all engines, indeed, in which the rectilinear motion of

the piston is changed into a circular motion. In rotatory engines the steam

acts at once in the production of circular motion, either upon a revolving

piston 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 the

single acting or pumping engine, and the double acting or rotative engine

can be said to be in actual use. For some purposes, such, for example, as

forcing air into furnaces for smelting iron, double acting engines are

employed, which are nevertheless unfurnished with a crank; but engines of

this kind are not sufficiently numerous to justify their classification as

a distinct species, and, in general, those engines may be considered to be

single acting, by which no rotatory motion is imparted.

4. _Q._--Is not the circular motion derived from a cylinder engine very

irregular, in consequence of the unequal leverage of the crank at the

different parts of its revolution?

_A._--No; rotative engines are generally provided with a fly-wheel to

correct such irregularities by its momentum; but where two engines with

their respective cranks set at right angles are employed, the irregularity

of one engine corrects that of the other with sufficient exactitude for

many purposes. In the case of marine and locomotive engines, a fly-wheel is

not employed; but for cotton spinning, and other purposes requiring great

regularity of motion, its use with common engines is indispensable, though

it is not impossible to supersede the necessity by new contrivances.

5. _Q._--You implied that there is some other difference between single

acting and double acting engines, than that which lies in the use or

exclusion of the crank?

_A._--Yes; single acting engines act only in one way by the force of the

steam, and are returned by a counter-weight; whereas double acting engines

are urged by the steam in both directions. Engines, as I have already said,

are sometimes made double acting, though unprovided with a crank; and there

would be no difficulty in so arranging the valves of all ordinary pumping

engines, as to admit of this action; for the pumps might be contrived to

raise water both by the upward and downward stroke, as indeed in some mines

is already done. But engines without a crank are almost always made single

acting, perhaps from the effect of custom, as much as from any other

reason, and are usually spoken of as such, though it is necessary to know

that 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 can

the pressure of a vacuum be said to be nothing, when a vacuum occasions a

pressure of 15 lbs. on the square inch?

_A._--Because it is not the vacuum which exerts this pressure, but the

atmosphere, which, like a head of water, presses on everything immerged

beneath it. A head of water, however, would not press down a piston, if the

water were admitted on both of its sides; for an equilibrium would then be

established, just as in the case of a balance which retains its equilibrium

when an equal weight is added to each scale; but take the weight out of one

scale, or empty the water from one side of the piston, and motion or

pressure is produced; and in like manner pressure is produced on a piston

by admitting steam or air upon the one side, and withdrawing the steam or

air from the other side. It is not, therefore, to a vacuum, but rather to

the existence of an unbalanced plenum, that the pressure made manifest by

exhaustion is due, and it is obvious therefore that a vacuum of itself

would 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, is

permitted to pass into a vessel called the condenser, where a shower of

cold 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. The

water, which would else be accumulated in the condenser, is continually

being pumped out by a pump worked by the engine. This pump is called the

air pump, because it also discharges any air which may have entered with

the water.

8. _Q._--If a vacuum be an empty space, and there be water in the

condenser, how can there be a vacuum there?

_A._--There is a vacuum above the water, the water being only like so much

iron 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 of

condensation is heated by the steam, and emits a vapor of a tension

represented by about three inches of mercury; that is, when the common

barometer stands at 30 inches, a barometer with the space above the mercury

communicating with the condenser, will stand at about 27 inches.

10. _Q._--Is this imperfection of the vacuum wholly attributable to the

vapor in the condenser?

_A._--No; it is partly attributable to the presence of a small quantity of

air which enters with the water, and which would accumulate until it

destroyed the vacuum altogether but for the action of the air pump, which

expels it with the water, as already explained. All common water contains a

certain quantity of air in solution, and this air recovers its elasticity

when the pressure of the atmosphere is taken off, just as the gas in soda

water flies up so soon as the cork of the bottle is withdrawn.

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

engines?

_A._--Yes; and it is called the vacuum gauge, because it shows the degree

of perfection the vacuum has attained. Another gauge, called the steam

gauge, is applied to the boiler, which indicates the pressure of the steam

by the height to which the steam forces mercury up a tube. Gauges are also

applied to the boiler to indicate the height of the water within it so that

it may not be burned out by the water becoming accidentally too low. In

some cases a succession of cocks placed a short distance above one another

are employed for this purpose, and in other cases a glass tube is placed

perpendicularly in the front of the boiler and communicating at each end

with its interior. The water rises in this tube to the same height as in

the boiler itself, and thus shows the actual water level. In most of the

modern boilers both of these contrivances are adopted.

12. _Q._--Can a condensing engine be worked with a pressure less than that

of the atmosphere?

_A._--Yes, if once it be started; but it will be a difficult thing to start

an engine, if the pressure of the steam be not greater than that of the

atmosphere. Before an engine can be started, it has to be blown through

with steam to displace the air within it, and this cannot be effectually

done 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 be

lightly loaded, until there is a partial vacuum in the boiler. Such a

practice, however, is not to be commended, as the gauge cocks become

useless when there is a partial vacuum in the boiler; inasmuch as, when

they are opened, the water will not rush out, but air will rush in. It is

impossible, also, under such circumstances, to blow out any of the sediment

collected within the boiler, which, in the case of the boilers of steam

vessels, requires to be done every two hours or oftener. This is

accomplished by opening a large cock which permits some of the supersalted

water 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, the

pressure within the boiler will fall spontaneously to a point considerably

beneath the pressure of the atmosphere; but it is preferable, in such

cases, partially to close the throttle valve in the steam pipe, whereby the

issue of steam to the engine is diminished; and the pressure in the boiler

is thus maintained, while the cylinder receives its former supply.

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

rush into it?

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

in.

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

_A._--With the velocity which a body would acquire by falling from the

height of a homogeneous atmosphere, which is an atmosphere of the same

density throughout as at the earth's surface; and although such an

atmosphere does not exist in nature, its existence is supposed, in order to

facilitate the computation. It is well known that the velocity with which

water issues from a cistern is the same that would be acquired by a body

falling from the level of the head to the level of the issuing point; which

indeed is an obvious law, since every particle of water descends and issues

by virtue of its gravity, and is in its descent subject to the ordinary

laws of falling bodies. Air rushing into a vacuum is only another example

of the same general principle: the velocity of each particle will be that

due to the height of the column of air which would produce the pressure

sustained; and the weight of air being known, as well as the pressure it

exerts on the earth's surface, it becomes easy to tell what height a column

of air, an inch square, and of the atmospheric density, would require to

be, to weigh 15 lbs. The height would be 27,818 feet, and the velocity

which the fall of a body from such a height produces would be 1,338 feet

per second.

VELOCITY OF FALLING BODIES AND MOMENTUM OF MOVING BODIES.

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

kinds?

_A._--All bodies fall with the same velocity, when there is no resistance

from the atmosphere, as is shown by the experiment of letting fall, from

the top of a tall exhausted receiver, a feather and a guinea, which reach

the bottom at the same time. The velocity of falling bodies is one that is

accelerated uniformly, according to a known law. When the height from which

a body falls is given, the velocity acquired at the end of the descent can

be easily computed. It has been found by experiment that the square root of

the 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 by

gravity in one second is 16-1/12 feet; in two seconds, 64-4/12 feet; in

three seconds, 144-9/12 feet; in four seconds, 257-4/12 feet, and so on. If

the number of feet fallen through in one second be taken as unity, then the

relation 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 as

the squares of the times of falling.

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

gravity?

_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 uniform

pressure?

_A._--It is.

19. _Q._--Therefore during the first second of falling as much impelling

power will be given by the force of gravity as during every succeeding

second?

_A._--Undoubtedly.

20. _Q._--How comes it, then, that while the body falls 64-4/12 feet in two

seconds, it falls only 16-1/12 feet in one second; or why, since it falls

only 16-1/12 feet in one second, should it fall more than twice 16-1/12

feet in two?

_A._--Because 16-1/12 feet is the average and not the maximum velocity

during the first second. The velocity acquired _at the end_ of the 1st

second is not 16-1/12, but 32-1/6 feet per second, and at the end of the 2d

second a velocity of 32-1/6 feet has to be added; so that the total

velocity at the end of the 2d second becomes 64-2/6 feet; at the end of the

3d, 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., so

that it appears that the velocities acquired by a falling body at different

points, are simply as the times of falling. But if the velocities be as the

times, 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, of

any given weight, is measurable by the height through which it descends, it

follows 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 as

the other, the faster ball has four times the energy or mechanical force

accumulated in it that the slower ball has. If the speed of a fly-wheel be

doubled, it has four times the _vis viva_ it possessed before--_vis viva_

being measurable by a reference to the height through which a body must

have fallen, to acquire the velocity given.

21. _Q._--By what considerations is the _vis viva_ or mechanical energy

proper 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-wheel

rim must have thereto, to keep the irregularities of motion within the

limits which are admissible. It is found in practice, that when the power

resident 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 by

the engine in one half-stroke--the variation, depending on the energy

inherent in the machinery the engine has to drive and the equability of

motion required--the engine will work with sufficient regularity for most

ordinary purposes, but where great equability of motion is required, it

will be advisable to make the power resident in the fly-wheel equal to six

times the power generated by the engine in one half-stroke.

22. _Q._---Can you give a practical rule for determining the proper

quantity of cast iron for the rim of a fly-wheel in ordinary land engines?

_A._--One rule frequently adopted is as follows:--Multiply the mean

diameter of the rim by the number of its revolutions per minute, and square

the product for a divisor; divide the number of actual horse power of the

engine by the number of strokes the piston makes per minute, multiply the

quotient by the constant number 2,760,000, and divide the product by the

divisor found as above; the quotient is the requisite quantity of cast iron

in cubic feet to form the fly-wheel rim.

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

fly-wheel?

_A._--Boulton and Watt's rule for finding the dimensions of the fly-wheel

is as follows:--Multiply 44,000 times the length of the stroke in feet by

the square of the diameter of the cylinder in inches, and divide the

product by the square of the number of revolutions per minute multiplied by

the cube of the diameter of the fly-wheel in feet. The resulting number

will 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 revolving

body tends to fly from the centre; and by centripetal force, I understand

any force which draws it to the centre, or counteracts the centrifugal

tendency. In the conical pendulum, or steam engine governor, which consists

of two metal balls suspended on rods hung from the end of a vertical

revolving shaft, the centrifugal force is manifested by the divergence of

the balls, when the shaft is put into revolution; and the centripetal

force, which in this instance is gravity, predominates so soon as the

velocity is arrested; for the arms then collapse and hang by the side of

the shaft.

25. _Q._--What measures are there of the centrifugal force of bodies

revolving in a circle?

_A._--The centrifugal force of bodies revolving in a circle increases as

the 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 number

of revolutions per minute, but the diameter of one be double that of the

other, the larger will have double the amount of centrifugal force. The

centrifugal force of the _same wheel_, however, increases as the square of

the velocity; so that if the velocity of a fly-wheel be doubled, it will

have four times the amount of centrifugal force.

26. _Q._--Can you give a rule for determining the centrifugal force of a

body of a given weight moving with a given velocity in a circle of a given

diameter?

_A._--Yes. If the velocity in feet per second be divided by 4.01, the

square of the quotient will be four times the height in feet from which a

body must have fallen to have acquired that velocity. Divide this quadruple

height by the diameter of the circle, and the quotient is the centrifugal

force in terms of the weight of the body, so that, multiplying the quotient

by the actual weight of the body, we have the centrifugal force in pounds

or tons. Another rule is to multiply the square of the number of

revolutions per minute by the diameter of the circle in feet, and to divide

the product by 5,870. The quotient is the centrifugal force in terms of the

weight of the body.

27. _Q._--How do you find the velocity of the body when its centrifugal

force 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 by

the diameter of the circle in feet, and multiply the square root of the

product by 4.01; the result will be the velocity of the body in feet per

second.

28. _Q._--Will you illustrate this by finding the velocity at which the

cast iron rim of a fly-wheel 10 feet in diameter would burst asunder by its

centrifugal force?

_A._--If we take the tensile strength of cast iron at 15,000 lbs. per

square inch, a fly-wheel rim of one square inch of sectional area would

sustain 30,000 lbs. If we suppose one half of the rim to be so fixed to the

shaft as to be incapable of detachment, then the centrifugal force of the

other 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 to

606.3, which is the centrifugal force in terms of the weight. Then by the

rule given in the last answer 606.3 x 10 = 6063, the square root of which

is 78 nearly, and 78 x 4.01 = 312.78, the velocity of the rim in feet per

second at the moment of rupture.

29. _Q._--What is the greatest velocity at which it is safe to drive a cast

iron fly-wheel?

_A._--If we take 2,000 lbs. as the utmost strain per square inch to which

cast iron can be permanently subjected with safety; then, by a similar

process to that just explained, we have 4,000 lbs./49.48 = 80.8 which

multiplied 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 be

considered as the highest consistent with safety. Indeed, this limit should

not be approached in practice on account of the risks of fracture from

weakness or imperfections in the metal.

30. _Q._--What is the velocity at which the wheels of railway trains may

run if we take 4,000 lbs. per square inch as the greatest strain to which

malleable iron should be subjected?

_A._--The weight of a malleable iron rim of one square inch sectional area

and 7 feet diameter is 21.991 feet x 3.4 lbs. = 74.76, one half of which is

37.4 lbs. Then by the same process as before, 8,000/37.4 = 213.9, the

centrifugal force in terms of the weight: 213.9 x 7, the diameter of the

wheel = 1497.3, the square root of which, 38.3 x 4.01 = 155.187 feet per

second, the highest velocity of the rims of railway carriage wheels that is

consistent with safety. 155.187 feet per second is equivalent to 105.8

miles an hour. As 4,000 lbs. per square inch of sectional area is the

utmost strain to which iron should be exposed in machinery, railway wheels

can scarcely be considered safe at speed even considerably under 100 miles

an hour, unless so constructed that the centrifugal force of the rim will

be counteracted, to a material extent, by the centripetal action of the

arms. Hooped wheels are very unsafe, unless the hoops are, by some process

or other, firmly attached to the arms. It is of no use to increase the

dimensions of the rim of a wheel with the view of giving increased strength

to counteract the centrifugal force, as every increase in the weight of the

rim 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 be

supposed to be concentrated, and which continually endeavors to gain the

lowest possible position. A body hung in the centre of gravity will remain

at 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 the

whole momentum may be conceived to be concentrated, or in which the whole

effect of the momentum resides. If the ball of a governor were to be moved

in a straight line, the momentum might be said to be concentrated at the

centre of gravity of the ball; but inasmuch as, by its revolution round an

axis, the part of the ball furthest removed from the axis moves more

quickly than the part nearest to it, the momentum cannot be supposed to be

concentrated at the centre of gravity, but at a point further removed from

the 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 swinging

body, such, that if all the matter of the body were to be collected into

that point, the velocity of its vibration would remain unaffected. It is in

fact the mean distance from the centre of suspension of every atom, in a

ratio which happens not to be an arithmetical one. The centre of

oscillation is always in a line passing through the centre of suspension

and the centre of gravity.

THE PENDULUM AND GOVERNOR.

34. _Q._--By what circumstance is the velocity of vibration of a pendulous

body determined?

_A._--By the length of the suspending rod only, or, more correctly, by the

distance between the centre of suspension and the centre of oscillation.

The length of the arc described does not signify, as the times of vibration

will be the same, whether the arc be the fourth or the four hundredth of a

circle, or at least they will be nearly so, and would be so exactly, if the

curve 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 sensible

deviation from the cycloidal curve, but in other respects the size of the

arc does not signify.

35. _Q._--If then the length of a pendulum be given, can the number of

vibrations in a given time be determined?

_A._--Yes; the time of vibration bears the same relation to the time in

which a body would fall through a space equal to half the length of the

pendulum, that the circumference of a circle bears to its diameter. The

number of vibrations made in a given time by pendulums of different

lengths, is inversely as the square roots of their lengths.

36. _Q._--Then when the length of the second's pendulum is known the proper

length of a pendulum to make any given number of vibrations in the minute

can readily be computed?

_A._--Yes; the length of the second's pendulum being known, the length of

another pendulum, required to perform any given number of vibrations in the

minute, may be obtained by the following rule: multiply the square root of

the given length by 60, and divide the product by the given number of

vibrations per minute; the square of the quotient is the length of pendulum

required. Thus if the length of a pendulum were required that would make 70

vibrations 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 pendulum

determines the number of vibrations it makes in a given time?

_A._--Because the length of the pendulum determines the steepness of the

circle in which the body moves, and it is obvious, that a body will descend

more rapidly over a steep inclined plane, or a steep arc of a circle, than

over one in which there is but a slight inclination. The impelling force is

gravity, which urges the body with a force proportionate to the distance

descended, and if the velocity due to the descent of a body through a given

height be spread over a great horizontal distance, the speed of the body

must be slow in proportion to the greatness of that distance. It is clear,

therefore, that as the length of the pendulum determines the steepness of

the arc, it must also determine the velocity of vibration.

38. _Q._--If the motions of a pendulum be dependent on the speed with which

a body falls, then a certain ratio must subsist between the distance

through which a body falls in a second, and the length of the second's

pendulum?

_A._--And so there is; the length of the second's pendulum at the level of

the sea in London, is 39.1393 inches, and it is from the length of the

second's pendulum that the space through which a body falls in a second has

been determined. As the time in which a pendulum vibrates is to the time in

which a heavy body falls through half the length of the pendulum, as the

circumference of a circle is to its diameter, and as the height through

which a body falls is as the square of the time of falling, it is clear

that the height through which a body will fall, during the vibration of a

pendulum, is to half the length of the pendulum as the square of the

circumference of a circle is to the square of its diameter; namely, as

9.8696 is to 1, or it is to the whole length of the pendulum as the half of

this, 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 a

second.

39. _Q._--Are the motions of the conical pendulum or governor reducible to

the same laws which apply to the common pendulum?

_A._--Yes; the motion of the conical pendulum may be supposed to be

compounded of the motions of two common pendulums, vibrating at right

angles to one another, and one revolution of a conical pendulum will be

performed in the same time as two vibrations of a common pendulum, of which

the length is equal to the vertical height of the point of suspension above

the plane of revolution of the balls.

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

by the engine?

_A._--Yes.

41. _Q._--Then will it not be driven round as any other mechanism would be

at a speed proportional to that of the engine?

_A._--It will.

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

revolution?

[Illustration: Fig. 1.]

_A._--By flying out until they assume a vertical height answering to the

velocity with which they rotate round the central axis. As the speed is

increased the balls expand, and the height of the cone described by the

arms is diminished, until its vertical height is such that a pendulum of

that length would perform two vibrations for every revolution of the

governor. By the outward motion of the arms, they partially shut off the

steam from the engine. If, therefore, a certain expansion of the balls be

desired, and a certain length be fixed upon for the arms, so that the

vertical height of the cone is fixed, then the speed of the governor must

be such, that it will make half the number of revolutions in a given time

that a pendulum equal in length to the height of the cone would make of

vibrations. The rule is, multiply the square root of the height of the cone

in inches by 0.31986, and the product will be the right time of revolution

in seconds. If the number of revolutions and the length of the arms be

fixed, and it is wanted to know what is the diameter of the circle

described by the balls, you must divide the constant number 187.58 by the

number of revolutions per minute, and the square of the quotient will be

the vertical height in inches of the centre of suspension above the plane

of the balls' revolution. Deduct the square of the vertical height in

inches from the square of the length of the arm in inches, and twice the

square root of the remainder is the diameter of the circle in which the

centres of the balls revolve.

43. _Q._ Cannot the operation of a governor be deduced merely from the

consideration of centrifugal and centripetal forces?

_A._--It can; and by a very simple process. The horizontal distance of the

arm from the spindle divided by the vertical height, will give the amount

of centripetal force, and the velocity of revolution requisite to produce

an equivalent centrifugal force may be found by multiplying the centripetal

force of the ball in terms of its own weight by 70,440, and dividing the

product by the diameter of the circle made by the centre of the ball in

inches; the square root of the quotient is the number of revolutions per

minute. By this rule you fix the length of the arms, and the diameter of

the base of the cone, or, what is the same thing, the angle at which it is

desired the arms shall revolve, and you then make the speed or number of

revolutions such, that the centrifugal force will keep the balls in the

desired 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 number

of small balls or particles, and each particle of matter will act for

itself. Heavy balls attached to a governor are only requisite to overcome

the friction of the throttle valve which shuts off the steam, and of the

connections leading thereto. Though the weight of a ball increases its

centripetal force, it increases its centrifugal force in the same

proportion.

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 convert

a small force acting through a great space into a great force acting

through a small space. In the school treatises on mechanics, a certain

number of these devices are set forth as the mechanical powers, and each

separate device is treated as if it involved a separate principle; but not

a tithe of the contrivances which accomplish the stipulated end are

represented in these learned works, and there is no very obvious necessity

for considering the principle of each contrivance separately when the

principles of all are one and the same. Every pressure acting with a

certain velocity, or through a certain space, is convertible into a greater

pressure acting with a less velocity, or through a smaller space; but the

quantity of mechanical force remains unchanged by its transformation, and

all that the implements called mechanical powers accomplish is to effect

this transformation.

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

_A._--Not any: the power is merely put into another shape, just as the

contents of a hogshead of porter are the same, whether they be let off by

an inch tap or by a hole a foot in diameter. There is a greater gush in the

one case than the other, but it will last a shorter time; when a lever is

used there is a greater force exerted, but it acts through a shorter

distance. It requires just the same expenditure of mechanical power to lift

1 lb. through 100 ft., as to lift 100 lbs. through 1 foot. A cylinder of a

given cubical capacity will exert the same power by each stroke, whether

the cylinder be made tall and narrow, or short and wide; but in the one

case it will raise a small weight through a great height, and in the other

case, 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 by

the use of the crank, because at the top and bottom centres it is capable

of exerting little or no power; but at those times there is little or no

steam 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 perplex

themselves by confounding the vertical with the circumferential velocity.

If the circle of the crank be divided by any number of equidistant

horizontal lines, it will be obvious that there must be the same steam

consumed, and the same power expended, when the crank pin passes from the

level of one line to the level of the other, in whatever part of the circle

it may be, those lines being indicative of equal ascents or descents of the

piston. But it will be seen that the circumferential velocity is greater

with the same expenditure of steam when the crank pin approaches the top

and bottom centres; and this increased velocity exactly compensates for the

diminished leverage, so that there is the same power given out by the crank

in each of the divisions.

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

lever?

_A._--Yes, many plans,--some of them displaying much ingenuity, but all

displaying a complete ignorance of the first principles of mechanics, which

teach that power cannot be gained by any multiplication of levers and

wheels. I have occasionally heard persons say: "You gain a great deal of

power by the use of a capstan; why not apply the same resource in the case

of a steam vessel, and increase the power of your engine by placing a

capstan motion between the engine and paddle wheels?" Others I have heard

say: "By the hydraulic press you can obtain unlimited power; why not then

interpose a hydraulic press between the engines and the paddles?" To these

questions the reply is sufficiently obvious. Whatever you gain in force you

lose in velocity; and it would benefit you little to make the paddles

revolve with ten times the force, if you at the same time caused them to

make only a tenth of the number of revolutions. You cannot, by any

combination of mechanism, get increased force and increased speed at the

same time, or increased force without diminished speed; and it is from the

ignorance of this inexorable condition, that such myriads of schemes for

the realization of perpetual motion, by combinations of levers, weights,

wheels, quicksilver, cranks, and other mere pieces of inert matter, have

been 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 words

pressure acting though space. But if by force you mean mere pressure, then

it furnishes no measure of power. Power is not measurable by force but by

force and velocity combined.

50. _Q._--Is not power lost when two moving bodies strike one other and

come to a state of rest?

_A._--No, not even then. The bodies if elastic will rebound from one

another with their original velocity; if not elastic they will sustain an

alteration of form, and heat or electricity will be generated of equivalent

value to the power which has disappeared.

51. _Q._--Then if mechanical power cannot be lost, and is being daily

called into existence, must not there be a daily increase in the power

existing in the world?

_A._--That appears probable unless it flows back in the shape of heat or

electricity to the celestial spaces. The source of mechanical power is the

sun which exhales vapors that descend in rain, to turn mills, or which

causes winds to blow by the unequal rarefaction of the atmosphere. It is

from 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 of

combustion, and the vegetation thus rendered possible is the source of coal

and other combustible bodies. The combustion of coal under a steam boiler

therefore merely liberates the power which the sun gave out thousands of

years before.

FRICTION.

52. _Q._--What is friction?

_A._--Friction is the resistance experienced when one body is rubbed upon

another body, and is supposed to be the result of the natural attraction

which bodies have for one another, and of the interlocking of the

impalpable asperities upon the surfaces of all bodies, however smooth.

There is, no doubt, some electrical action involved in its production, not

yet recognized, nor understood; and it is perhaps traceable to the

disturbance of the electrical equilibrium of the particles of the body

owing to the condensation or change of figure which all bodies must

experience when subjected to a strain. When motion in opposite directions

is given to smooth surfaces, the minute asperities of one surface must

mount upon those of the other, and both will be abraded and worn away, in

which act power must be expended. The friction of smooth rubbing substances

is less when the composition of those substances is different, than when it

is the same, the particles being supposed to interlock less when the

opposite 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 bearing

surfaces of steam engines as large as possible, as there is no increase of

friction by extending the surface, while there is a great increase in the

durability. When the bearings of an engine are made too small, they very

soon 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 the

friction over a given amount of surface be considered; but it increases as

the velocity, if the comparison be made with the time during which the

friction 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 the

friction per stroke is the same, the friction per minute is doubled. The

friction, therefore, of any machine per hour varies as the velocity, though

the friction per revolution remains, at all ordinary velocities, the same.

Of excessive velocities we have not sufficient experience to enable us to

state with confidence whether the same law continues to operate among them.

55. _Q._--Can you give any approximate statement of the force expended in

overcoming friction?

_A._--It varies with the nature of the rubbing bodies. The friction of iron

sliding upon iron, has generally been taken at about one tenth of the

pressure, when the surfaces are oiled and then wiped again, so that no film

of oil is interposed. The friction of iron rubbing upon brass has generally

been taken at about one eleventh of the pressure under the same

circumstances; but in machines in actual operation, where a film of some

lubricating material is interposed between the rubbing surfaces, it is not

more 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 some

cases. Mr. Southern, in some experiments upon the friction of the axle of a

grindstone--an account of which may be found in the 65th volume of the

Philosophical Transactions--found the friction to amount to less than

1/40th of the weight; and Mr. Wood, in some experiments upon the friction

of locomotive axles, found that by ample lubrication the friction may be

made as little as 1/60th of the weight. In some experiments upon the

friction of shafts by Mr. G. Rennie, he found that with a pressure of from

1 to 5 cwt. the friction did not exceed 1/39th of the pressure when tallow

was the unguent employed; with soft soap it became 1/34th. The fact appears

to be that the amount of the resistance denominated friction depends, in a

great measure, upon the nature of the unguent employed, and in certain

cases the viscidity of the unguent may occasion a greater retardation than

the resistance caused by the attrition. In watchwork therefore, and other

fine 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 the

resistance caused by the viscidity of the unguent increases with the amount

of surface, and the amount of surface is relatively greater in the smaller

class of works.

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

bearings?

_A._--The nature of the unguent, proper for different bearings, appears to

depend in a great measure upon the amount of the pressure to which the

bearings are subjected,--the hardest unguents being best where the pressure

is greatest. The function of lubricating substances is to prevent the

rubbing surfaces from coming into contact, whereby abrasion would be

produced, and unguents are effectual in this respect in the proportion of

their viscidity; but if the viscidity of the unguent be greater than what

suffices to keep the surfaces asunder, an additional resistance will be

occasioned; and the nature of the unguent selected should always have

reference, therefore, to the size of the rubbing surfaces, or to the

pressure per square inch upon them. With oil the friction appears to be a

minimum when the pressure on the surface of a bearing is about 90 lbs. per

square inch. The friction from too small a surface increases twice as

rapidly as the friction from too large a surface, added to which, the

bearing, when the surface is too small, wears rapidly away.

57. _Q._--Has not M. Morin, in France, made some very complete experiments

to determine the friction of surfaces of different kinds sliding upon one

another?

_A._--He has; but the result does not differ materially from what is stated

above, though, upon the whole, M. Morin, found the resistance due to

friction to be somewhat greater than it has been found to be by various

other engineers. When the surfaces were merely wiped with a greasy cloth,

but had no film of lubricating material interposed, the friction of brass

upon cast iron he found to be .107, or about 1/10th of the load, which was

also the friction of cast iron upon oak. But when a film of lubricating

material was interposed, he found that the friction was the same whether

the surfaces were wood on metal, wood on wood, metal on wood, or metal on

metal; and the amount of the friction in such case depended chiefly on the

nature of the unguent. With a mixture of hog's lard and olive oil

interposed between the surfaces, the friction was usually from 1/12th to

1/14th 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 large

relatively with the pressure; and in screw vessels where the propeller

shaft passes through a long pipe at the stern, the stuffing box is

purposely made a little leaky. The small leakage of water into the vessel

which is thus occasioned, keeps the screw shaft in this situation always

wet, and this is all the lubrication which this bearing requires or

obtains.

59. _Q._--What is the utmost pressure which may be employed without heating

when oil is the lubricating material?

_A._--That will depend upon the velocity. When the pressure exceeds 800

lbs. per square inch, however, upon the section of the bearing in a

direction parallel with the axis, then the oil will be forced out and the

bearing will necessarily heat.

60. _Q._--But, with, a given velocity, can you tell the limit of pressure

which will be safe in practice; or with a given pressure, can you tell the

limit of velocity?

_A._--Yes; that may be done by the following empirical rule, which has been

derived from observations made upon bearings of different sizes and moving

with different velocities. Divide the number 70,000 by the velocity of the

surface of the bearing in feet per minute. The quotient will be the number

of pounds per square inch of section in the line of the axis that may be

put upon the bearing. Or, if we divide 70,000 by the number of pounds per

square inch of section, then the quotient will be the velocity in feet per

minute 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 the

diameter of the bearing multiplied by its length?

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

bearing.

62. _Q._--What is the amount of friction in the case of surfaces sliding

upon one another in sandy or muddy water--such surfaces, for example, as

are to be found in the sluices of valves for water?

_A._--Various experiments have been made by Mr. Summers of Southampton to

ascertain the friction of brass surfaces sliding upon each other in salt

water, with the view of finding the power required for moving sluice doors

for lock gates and for other similar purposes. The surfaces were planed as

true 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 the

rubbing rubbing Surface. Slide _slowly_ in muddy

Surface. 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 moved

was similar, but three or four times as long.]

These results were the average of eight fair trials; in each case, the

sliding surfaces were totally immersed in muddy salt water, and although

the apparatus used for drawing the slide along was not very delicately

fitted up, the power required may be considered as a sufficient

approximation for practical purposes.

It appears from these experiments, that rough surfaces follow the same law

as regards friction that is followed by smooth, for in each case the

friction 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 steam

engine determined?

_A._--By reference to the amount of the strain or pressure to which they

are subjected, and to the cohesive strength of the iron or other material

of which they are composed. The strains subsisting in engines are usually

characterized as tensile, crushing, twisting, breaking, and shearing

strains; but they may be all resolved into strains of extension and strains

of compression; and by the power of the materials to resist these two

strains, 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 no

means the same. The tensile strength, or strength when extended, of good

bar iron is about 60,000 lbs., or nearly 27 tons per square inch of

section; and the tensile strength of cast iron is about 15,000 lbs., or say

6 3/4 to 7 tons per square inch of section. These are the weights which are

required to break them. The crushing strain of cast iron, however, is about

100,000 lbs., or 44 1/2 tons; whereas the crushing strength of malleable

iron 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 times

greater than that of wrought iron. It may be stated, in round numbers, that

the tensile strength of malleable iron is twice greater than its crushing

strength; or, in other words, that it will take twice the strain to break a

bar of malleable iron by drawing it asunder endways, than will cripple it

by forcing it together endways like a pillar; whereas a bar of cast iron

will be drawn asunder with one sixth of the force that will be required to

break 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 is

about twice as great as that of wrought iron, being about 130,000 lbs. per

square inch of section. The tensile strength of gun metal, such as is used

in engines, is about 36,000 lbs. per square inch of section; of wrought

copper about 33,000 lbs.; and of cast copper about 19,000 lbs. per square

Inch of section.

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

tensile strength?

_A._--It is about twice greater. A good steel punch will punch through a

plate of wrought iron of a thickness equal to the diameter of the punch. A

punch therefore of an inch diameter will pierce a plate an inch thick. Now

it is well known, that the strain required to punch a piece of metal out of

a plate, is just the same as that required to tear asunder a bar of iron of

the same area of cross section as the area of the surface cut. The area of

the surface cut in this case will be the circumference of the punch, 3.1416

inches, multiplied by the thickness of the plate, 1 inch, which makes the

area of the cut surface 3.1416 square inches. The area of the point of the

punch subjected to the pressure is .7854 square inches, so that the area

cut to the area crushed is as four to one. In other words, it will require

four times the strain to crush steel that is required to tear asunder

malleable iron, or it will take about twice the strain to crush steel that

it 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 is

applied, is elongated or contracted like a very stiff spiral spring, nearly

in the proportion of the amount of strain applied up to the limit at which

the strength begins to give way, and within this limit it will recover its

original dimensions when the strain is removed. If, however, the strain be

carried beyond this limit, the bar will not recover its original

dimensions, but will be permanently pulled out or pushed in, just as would

happen to a spring to which an undue strain had been applied. This limit is

what is called the limit of elasticity; and whenever it is exceeded, the

bar, though it may not break immediately, will undergo a progressive

deterioration, and will break in the course of time. The limit of

elasticity of malleable iron when extended, or, in other words, the tensile

strain to which a bar of malleable iron an inch square may be subjected

without permanently deranging its structure, is usually taken at 17,800

lbs., or from that to 10 tons, depending on the quality of the iron. It has

also been found that malleable iron is extended about one ten-thousandth

part 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; but

this is certainly much too high, as it exceeds the tensile strength of

irons of medium quality. A bar of cast iron if compressed by weights will

be contracted in length twice as much as a bar of malleable iron under

similar circumstances; but malleable iron, when subjected to a greater

strain than 12 tons per square inch of section, gradually crumples up by

the mere continuance of the weight. A cast-iron bar one inch square and ten

feet long, is shortened about one tenth of an inch by a compressing force

of 10,000 lbs., whereas a malleable iron bar of the same dimensions would

require to shorten it equally a compressing force of 20,000 lbs. As the

load, however, approaches 12 tons, the compressions become nearly equal,

and above that point the rate of the compression of the malleable iron

rapidly increases. A bar of cast iron, when at its breaking point by the

application of a tensile strain, is stretched about one six-hundredth part

of its length; and an equal strain employed to compress it, would shorten

it about one eight-hundredth part of its length.

69. _Q._--But to what strain may the iron used in the construction of

engines be safely subjected?

_A._--The most of the working parts of modern engines are made of malleable

iron, and the utmost strain to which wrought iron should be subjected in

machinery is 4000 lbs. per square inch of section. Cast iron should not be

subjected to more than half of this. In locomotive boilers the strain of

4000 lbs. per square inch of section is sometimes exceeded by nearly one

half; but such an excess of strain approaches the limits of danger.

70. _Q._--Will you explain in what way the various strains subsisting in a

steam engine may be resolved into tensile and crushing strains; also in

what way the magnitude of those strains may be determined?

_A._--To take the case of a beam subjected to a transverse strain, such as

the great beam of an engine, it is clear, if we suppose the beam broken

through the middle, that the amount of strain at the upper and lower edges

of 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 the

length 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 be

regarded as pillars, one of which is extended by the strain, and the other

is compressed. If, to make an extreme supposition, the depth of the beam is

taken as equal to its length, then the pillars answering to the edges of

the beam will be compressed, and extended by what is virtually a bellcrank

lever with equal arms; the horizontal distance from the main centre to the

end of the beam being one of the arms, and the vertical height from the

main 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 stroke

of the engine, will be equal to the length of the stroke; and the strain it

will have to sustain will consequently be equal to the pressure on the

piston. If its motion were only half that of the piston, as would be the

case if its depth were made one half less, the strain the beam would have

to bear would be twice as great; and it may be set down as an axiom, that

the strain upon any part of a steam engine or other machine is inversely

equal to the strain produced by the prime mover, multiplied by the

comparative velocity with which the part in question moves. If any part of

an engine moves with a less velocity than the piston, it will have a

greater strain on it, if resisted, than is thrown upon the piston. If it

moves with a greater velocity than the piston, it will have a less strain

upon it, and the difference of strain will in every case be in the inverse

proportion of the difference of the velocity.

71. _Q._--Then, in computing the amount of metal necessary to give due

strength to a beam, the first point is to determine the velocity with which

the 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 the

upper and lower edges or flanges rigidly together, so as to enable the

extending and compressing strains to be counteracted in an effectual manner

by the metal of those flanges. It is only necessary, therefore, to make the

flanges of sufficient strength to resist effectually the crushing and

tensile strains to which they are exposed, and to make the web of the beam

of sufficient strength to prevent a distortion of its shape from taking

place.

72. _Q._--Is the strain greater from being movable or intermittent than if

it was stationary?

_A._--Yes it is nearly twice as great from being movable. Engineers are in

the habit of making girders intended to sustain a stationary load, about

three times stronger than the breaking weight; but if the load be a movable

one, as is the case in the girders of railway bridges, they make the

strength equal to six times the breaking weight.

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

applied?

_A._--If a weight be placed on a long and slender beam propped up in the

middle, and the prop be suddenly withdrawn, so as to allow deflection to

take place, it is clear that the deflection must be greater than if the

load had been gradually applied. The momentum of the weight and also of the

beam itself falling through the space through which it has been deflected,

has necessarily to be counteracted by the elasticity of the beam; and the

beam will, therefore, be momentarily bent to a greater extent than what is

due to the load, and after a few vibrations up and down it will finally

settle at that point of deflection which the load properly occasions. It is

obvious that a beam must be strong enough, not merely to sustain the

pressure due to the load, but also that accession of pressure due to the

counteracted momentum of the weight and of the beam itself. Although in

steam engines the beam is not loaded by a weight, but by the pressure of

the steam, yet the momentum of the beam itself must in every case be

counteracted, and the momentum will be considerable in every case in which

a large and rapid deflection takes place. A rapid deflection increases the

amount of the deflection as well as the amount of the strain, as is seen in

the cylinder cover of a Cornish pumping engine, into which the steam is

suddenly admitted, and in which the momentum of the particles of the metal

put into motion increases the deflection to an extent such as the mere

pressure of the steam could not produce.

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

deflection?

_A._--The momentum of any moving body being proportional to the square of

its velocity, it follows that the strain will be proportional to the square

of the amount of deflection produced in a specified time.

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

the momentum increase deflection?

_A._--No doubt that will be so; but whether in practical cases increase of

mass without reference to strength or load will, upon the whole, increase

or diminish deflection, will depend very much upon the magnitude of the

mass relatively with the magnitude of the deflecting pressure, and the

rapidity with which that pressure is applied and removed. Thus if a force

or weight be very suddenly applied to the middle of a ponderous beam, and

be as suddenly withdrawn, the inertia of the beam will, as in the case of

the collision of bodies, tend to resist the force, and thus obviate

deflection to a considerable extent; but if the pressure be so long

continued as to produce the amount of deflection due to the pressure, the

effect 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 different

directions facilitate its fracture?

_A._--Iron beams bent alternately in opposite directions, or alternately

deflected and released, will be broken in the course of time with a much

less strain than is necessary to produce immediate fracture. It has been

found, experimentally, that a cast-iron bar, deflected by a revolving cam

to only half the extent due to its breaking weight, will in no case

withstand 900 successive deflections; but, if bent by the cam to only one

third of its ultimate deflection, it will withstand 100,000 deflections

without visible injury. Looking, however, to the jolts and vibrations to

which engines are subject, and the sudden strains sometimes thrown upon

them, either from water getting into the cylinder or otherwise, it does not

appear that a strength answering to six times the breaking weight will give

sufficient margin for safety in the case of cast-iron beams.

77. _Q._--Does the same law hold in the case of the deflection of malleable

iron bars?

_A._--In the case of malleable iron bars it has been found that no very

perceptible damage was caused by 10,000 deflections, each deflection being

such as was due to half the load that produced a large permanent

deflection.

78. _Q._--The power of a rod or pillar to resist compression becomes very

little when the diameter is small and the length great?

_A._--The power of a rod or pillar to resist compression, varies nearly as

the fourth power of the diameter divided by the square of the length. In

the case of hollow cylindrical columns of cast iron, it has been found,

experimentally, that the 3.55th power of the internal diameter, subtracted

from the 3.55th power of the external diameter, and divided by the 1.7th

power of the length, will represent the strength very nearly. In the case

of hollow cylindrical columns of malleable iron, experiment shows that the

3.59th power of the internal diameter, subtracted from the 3.59th power of

the external diameter, and divided by the square of the length, gives a

proper expression for the strength; but this rule only holds where the

strain does not exceed 8 or 9 tons on the square inch of section. Beyond 12

or 13 tons per square inch of section, the metal cannot be depended upon to

withstand the strain, though hollow pillars will sometimes bear 15 or 16

tons per square inch of section.

79. _Q._--Does not the thickness of the metal of the pillars or tubes

affect the question?

_A._--It manifestly does; for a tube of very thin metal, such as gold leaf

or tin foil, would not stand on end at all, being crushed down by its own

weight. It is found, experimentally, that in malleable iron tubes of the

respective thicknesses of .525, .272, and .124 inches, the resistances per

square inch of section are 19.17, 14.47, and 7.47 tons respectively. The

power of plates to resist compression varies nearly as the cube, or more

nearly as the 2.878th power of their thickness; but this law only holds so

long as the pressure applied does not exceed from 9 to 12 tons per square

inch of section. When the pressure is greater than this the metal is

crushed, and a new law supervenes, according to which it is necessary to

employ plates of twice or three times the thickness, to obtain twice the

resisting power.

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

strains?

_A._--It will be most affected by percussion. Long-continued impact on the

side of a tube, producing a deflection of only one fifth of that which

would be required to injure it by pressure, is found to be destructive of

the riveting; but in large riveted structures, such as a ship or a railway

bridge, 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 cases

proportional to its power to resist strains?

_A._--By no means. Some cast iron is very hard and brittle; and although it

will in this state resist compression very strongly, it, will be easily

broken by a blow. Iron which has been remelted many times generally falls

into this category, as it will also do if run into very small castings. It

has been found, by experiment, that iron of which the crushing weight per

square inch is about 42 tons, will, if remelted twelve times, bear a

crushing weight of 70 tons, and if remelted eighteen times it will bear a

crushing weight of 83 tons; but taking its power to resist impact in its

first state at 706, this power will be raised at the twelfth remelting to

1153, and will be sunk at the eighteenth remelting to 149.

82. _Q._--From all this it appears that a combination of cast iron and

malleable iron is the best for the beams of engines?

_A._--Yes, and for all beams. Engine beams should be made deeper at the

middle than they are now made; the web should be lightened by holes pierced

in it, and round the edge of the beam there should be a malleable iron hoop

or strap securely attached to the flanges by riveting or otherwise. The

flanges at the edges of engine beams are invariably made too small. It is

in 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 actual

practical use?

_A._--There is first the single-acting engine, which is used for pumping

water; the rotative land engine, which is employed to drive mills and

manufactories; the rotative marine engine, which is used to propel steam

vessels; and the locomotive engine, which is employed on railways. The last

is 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-acting

engine, used for draining mines?

_A._--Permit me then to begin with the boiler, which is common and

necessary to all engines; and I will take the example of a wagon boiler,

such as was employed by Boulton and Watt universally in their early

engines, and which is still in extensive use. This boiler is a long

rectangular 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 beneath

it, and flues constructed of brickwork encircle it, so as to keep the flame

and smoke in contact with the boiler for a sufficient time to absorb the

heat.

[Illustration: Fig. 3]

85. _Q._--This species of boiler has not an internal furnace, but is set in

brickwork, in which the furnace is formed?

_A._--Precisely so. The general arrangement and configuration will be at

once understood by a reference to the annexed figure (fig. 3), which is a

transverse section of a wagon boiler. The line b represents the top of the

grate or fire bars, which slope downward from the front at an angle of

about 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 the

flame passes over a low wall or bridge, and traverses the bottom of the

boiler. The smoke rises up at the back of the boiler, and proceeds through

the flue F along one side to the front, and returns along the other side of

the boiler, and then ascends the chimney. The performance of this course by

the smoke is what is termed a wheel draught, as the smoke wheels once round

the boiler, and then ascends the chimney.

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

boilers?

_A._--No; such boilers sometimes have what is termed a split draught. The

smoke and flame, when they reach the end of the boiler, pass in this case

through an iron flue or tube, reaching from end to end of the boiler; and

on arriving at the front of the boiler, the smoke splits or separates--one

half passing through a flue on the one side of the boiler, and the other

half passing through a flue on the other side of the boiler--both of these

flues 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 a

lid secured by bolts. This is the manhole door, the purpose of which is to

enable a man to get into the inside of the boiler when necessary for

inspection and repair. On the top of this door is a small valve opening

downward, called the atmospheric valve. The intention of this valve is to

prevent a vacuum from being formed accidentally in the boiler, which might

collapse it; for if the pressure in the boiler subsides to a point

materially below the pressure of the atmosphere, the valve will open and

allow air to get in. A bent pipe, which rises up from the top of the

boiler, immediately behind the position of the manhole, is the steam pipe

for conducting the steam to the engine; and a bent pipe which ascends from

the top of the boiler, at the back end, is the waste-steam pipe for

conducting away the steam, which escapes through the safety valve. This

valve is set in a chest, standing on the top of the boiler, at the foot of

the waste-steam pipe, and it is loaded with iron or leaden weights to a

point 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 a

rod, which in its turn is attached to a lever set on the top of a large

upright pipe. The upper part of the pipe is widened out into a small

cistern, through a short pipe in the middle of which a chain passes to the

damper; but any water emptied into this small cistern cannot pass into the

pipe, except through a small valve fixed to the lever to which the rod is

attached. The water for replenishing the boiler is pumped into the small

cistern on the top of the pipe; and it follows from these arrangements that

when the buoy falls, the rod opens the small valve and allows the feed

water 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 the

purpose.

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

_A._--The draught through the furnaces of land boilers is regulated by a

plate of metal or a damper, as it is called, which slides like a sluice up

and down in the flue, and this damper is closed more or less when the

intensity of the fire has to be moderated. In wagon boilers this is

generally accomplished by self-acting mechanism. In the small cistern pipe,

which is called a stand pipe, the water rises up to a height proportional

to the pressure of the steam, and the surface of the water in this pipe

will rise or fall with the fluctuations in the pressure of the steam. In

this pipe a float is placed, which communicates by means of a chain with

the damper. If the pressure of the steam rises, the float will be raised

and the damper closed, whereas, if the pressure in the boiler falls, the

reverse 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 you

have just described?

_A._--No; many land boilers are now made of a cylindrical form, with one or

two internal flues in which the furnace is placed. A boiler of this kind is

represented in Figs. 4 and 5, and which is the species of boiler

principally used in Cornwall. In this boiler a large internal cylinder or

flue runs from end to end. In the fore part of this cylinder the furnace is

placed, and behind the furnace a large tube filled with water extends to

the end of the boiler. This internal tube is connected to the bottom part

of the boiler by a copper pipe standing vertically immediately behind the

furnace bridge, and to the top part of the boiler by a bent copper pipe

which stands in a vertical position near the end of the boiler. The smoke,

after passing through the central flue, circulates round the sides and

beneath 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 steam

vessels?

_A._--These are of two classes, flue boilers and tubular boilers, but the

latter are now most used. In the flue boiler the furnaces are set within

the boiler, and the flues proceeding from them wind backwards and forwards

within 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, or

12 furnaces in all. Fig. 7 is an elevation of 2 boilers, the one to the

right 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 arrows

in plan and longitudinal section, will explain the direction of the smoke

current.

[Illustration: Fig. 7.]

[Illustration: Fig. 8.]

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

boilers?

_A._--In tubular boilers, the smoke after leaving the furnace just passes

once through a number of small tubes and then enters the chimney. These

tubes are sometimes of brass, and they are usually about 3 inches in

diameter, 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 a

vertical longitudinal section, fig. 10 half a front elevation and half a

transverse section, and fig. 11 half a back elevation and half a transverse

section near the end. There is a projecting part on the top of the boiler

called the "steam chest," of which the purpose is to retain for the use of

the cylinder a certain supply of steam in a quiescent state, in order that

it may have time to clear itself of foam or spray. A steam chest is a usual

part 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 smoke

box is usually closed by doors which may be opened when necessary to sweep

the 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 common

form of American marine boiler.

Figs. 14 and 15 are the front and sectional elevation of one of the boilers

of 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. For

land purposes the lowest range of tubes is generally omitted, and the smoke

makes a last return beneath the bottom of the boiler.

Figs. 17 and 18 are the transverse and longitudinal sections of a tubular

boiler, 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 the

cylinder of the engine?

_A._--And presses up and down the piston alternately, being admitted

alternately above and below the piston by suitable valves provided for that

purpose.

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

engine?

_A._--The prevailing form of the pumping engine consists of a great beam

vibrating on a centre like the beam of a pair of scales, and the cylinder

is in connection with one end of the beam and the pump stands at the other

end. The pump end of the beam is usually loaded, so as to cause it to

preponderate when the engine is at rest; and the whole effort of the steam

is employed in overcoming this preponderance until a stroke is performed,

when, the steam being shut off, the heavy end of the beam again falls and

the operation is repeated.

95. _Q._--in the double-acting engine the piston is pushed by the steam

both 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 kind

introduced by Mr. Watt, will be understood by a reference to the annexed

figure (fig. 20), where an engine of this kind is shown in section. A is

the cylinder in which a movable piston, T, is forced alternately up and

down by the alternate admission, to each side, of the steam from the

boiler. The piston, by means of a rod called the piston rod, gives motion

to the beam V W, which by means of a heavy bar, P, called the connecting

rod, moves the crank, Q, and with it the fly wheel, X, from which the

machinery 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 an

elliptical plate of metal swivelling on a spindle passing through its edge

from side to side, and by turning which more or less the opening through

the pipe will be more or less closed. The extent to which this valve is

opened or closed is determined by the governor, D, the balls of which, as

they collapse or expand, move up or down a collar on the governor spindle,

which motion is communicated to the throttle valve by suitable rods and

bell-cranks. The governor, it will be seen, consists substantially of two

heavy balls attached to arms fixed upon an upright shaft, which is kept in

revolution by means of a cord driven by a pulley on the fly wheel shaft.

The velocity with which the balls of the governor revolve being

proportional to that of the fly wheel, it will follow, that if by reason of

too rapid a supply of steam, an undue speed be given to the fly wheel, and

therefore to the balls, a divergence of the balls will take place to an

extent corresponding to the excess of velocity, and this movement being

communicated 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 of

its motion will be reduced. If, on the other hand, the motion of the engine

is slower than is requisite, owing to a deficient supply of steam through

B, then the balls, not being sufficiently affected by centrifugal force,

will fall towards the vertical spindle, and the throttle valve, C, will be

more fully opened, whereby a more ample supply of steam will be admitted to

the cylinder, and the speed of the engine will be increased to the

requisite extent.

97. _Q._--The piston must be made to fit the cylinder accurately so as to

prevent the passage of steam?

_A._--The piston is accurately fitted to the cylinder, and made to move in

it steam tight by a packing of hemp driven tightly into a groove or recess

round the edge of the piston, and which is squeezed down by an iron ring

held by screws. The piston divides the cylinder into two compartments,

between which there is no communication by which steam or any other elastic

fluid can pass. A casing set beside the cylinder contains the valves, by

means of which the steam which impels the piston is admitted and withdrawn,

as the piston commences its motion in each direction. The upper steam box

B, is divided into three compartments by two valves. Above the upper steam

valve V, is a compartment communicating with the steam pipe B. Below the

lower valve E is another compartment communicating with a pipe called the

eduction pipe, which leads downwards from the cylinder to the condenser, in

which 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 of

the cylinder, so as to permit or prevent a supply of steam from the one to

pass to the other. By the valve E a communication may be open or closed

between the top of the cylinder and the condenser, so that the steam in the

top compartment of the cylinder may either be permitted to escape into the

condenser, or may be confined to the cylinder. The continuation of the

steam pipe leads to the lower steam box B', which, like the upper, is

divided into three compartments by two valves V' and E', and the action of

the lower valves is in all respects the same as that of the upper.

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

simultaneously?

_A._--The four valves V, E, V', E' are connected by rods to a single handle

H, which handle is moved alternately up and down by means of pins or

tappets, placed on the rod which works the air pump. When the handle H is

pressed down, the levers in connexion with it open the upper exhausting

valve E, and the lower steam valve V', and close the upper steam valve V

and the lower exhausting valve E'. On the other hand, when the handle H is

pressed up it opens the upper steam valve V and the lower exhausting valve

E', and at the same time closes the upper exhausting valve E, and the lower

steam valve V'.

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

_A._--The condenser K is immerged in a cistern of cold water. At its side

there is a tube I, for the admission of water to condense the steam, and

which is governed by a cock, by opening which to any required extent, a jet

of cold water may be made to play in the condenser. From the bottom of the

condenser a short pipe leads to the air pump J, and in this pipe there is a

flap valve, called the foot valve, opening towards the air pump. The air

pump is a pump set in the same cistern of cold water that holds the

condenser, 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 the

manner of a common sucking pump. The upper part of the air pump

communicates with a small cistern S, called the hot well, through a valve

opening outwards and called the delivery valve. A pump M, called the hot

water pump, lifts hot water out of the hot well to feed the boiler, and

another pump N lifts cold water from a well or other source of supply, to

maintain the supply of water to the cold water cistern, in which the

condenser 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 handle

H will be raised by the lower pin or tappet on the air pump rod, and the

valves V and E' will be opened, and at the same time the other pair of

valves V' and E will be closed. Steam will therefore be admitted above the

piston and the steam or air which had previously filled the cylinder below

the piston will be drawn off to the condenser. It will there encounter the

jet of cold water, which is kept constantly playing there by keeping the

cock I sufficiently open. It will thus be immediately condensed or reduced

to water, and the cylinder below the piston will have a vacuum in it. The

steam therefore admitted from the steam pipe through the open valve V to

the top of the cylinder, not being resisted by pressure below, will press

the 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 pump

rod, and the valves V and E', previously open, will be closed, while the

valves V' and E, previously closed, will be opened. The steam which has

just pressed down the piston, and which now fills the cylinder above the

piston, 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 steam

from the boiler, admitted through the open valve V', will fill the cylinder

below the piston, and press the piston upwards. When the piston has reached

the top of the cylinder, the lower pin on the air pump rod will have struck

the handle upwards, and will thereby have closed the valves V' and E, and

opened the valves V and E'. The piston will then be in the same situation

as in the commencement, and will again descend, and so will continue to be

driven up and down by the steam.

101. Q.--But what becomes of the cold water which is let into the condenser

to condense the steam?

A.--It is pumped out by the air pump in the shape of hot water, its

temperature having been raised considerably by the admixture of the steam

in it. When the air pump piston ascends it leaves behind it a vacuum; and

the foot valve being relieved from all pressure, the weight of the water in

the condenser forces it open, and the warm water flows from the condenser

into the lower part of the air pump, from which its return to the condenser

is 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 to

the bottom of the pump barrel, the warm water which was below it will all

have passed above it, and cannot return. When the bucket next ascends, the

water above it, not being able to return through the bucket valve, will be

forced into the hot well through the delivery valve S. The hot water pump

M, pumps a small quantity of this hot water into the boiler, to compensate

for 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 the

cylinder 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, is

furnished with a recess called a stuffing box, into which a stuffing or

packing of plaited hemp is forced, which, pressing on the one side against

the interior of the stuffing box, and on the other side against the piston

rod, which is smooth and polished, prevents any leakage in this situation.

The packing of this stuffing box is forced down by a ring of metal

tightened by screws. This ring, which accurately fits the piston rod, has a

projecting flange, through which bolts pass for tightening the ring down

upon the packing; and a similar expedient is employed in nearly every case

in 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 are

usually made of the same length as the crank, and their purpose is to

enable the end of the great beam to move in the arc of a circle while the

piston rod maintains the vertical position. The point of junction,

therefore, of the links and the piston rod is of the form of a knuckle or

bend at some parts of the stroke.

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

vertical position?

_A._--Some engines have guide rods set on each side of the piston rod, and

eyes on the top of the piston rod engage these guide rods, and maintain the

piston rod in a vertical position in every part of the stroke. More

commonly, however, the desired end is attained by means of a contrivance

called the parallel motion.

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

_A._--The parallel motion is an arrangement of jointed rods, so connected

together that the divergence from the vertical line at any point in the arc

described by the beam is corrected by an equal and opposite divergence due

to the arc performed by the jointed rods during the stroke; and as these

opposite deviations mutually correct one another, the result is that the

piston 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_ has

a link _f g_ hung on each side of the beam, and a short cross bar, called a

cross head, extends from the bottom of one of these links to the bottom of

the other, which cross head is perforated with a hole in the middle for the

reception of the piston rod. There are similar links _b d_ at the point of

the 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 they

always 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 other

rods _c d_, of which the ends at _c_ are attached to stationary pins, while

the 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 arc

described by the point _d_, with _c_ as a centre, is opposite to the arc

described 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 extent

equal 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 a

curve.

[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, passing

from 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 central

axis 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 the

link, therefore, where these opposite motions meet, will follow no arc at

all, 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 a

reciprocating motion?

_A._--That is the object of it, and it accomplishes its object in a very

perfect manner, as it gradually arrests the velocity of the piston towards

the end of the stroke, and thus obviates what would otherwise be an

injurious shock upon the machine. When the crank approaches the lowest part

of its throw, and at the same time the piston is approaching the top of the

cylinder, the motion of the crank becomes nearly horizontal, or, in other

words, the piston is only advanced through a very short distance, for any

given distance measured on the circle described by the crank pin. Since,

then, the velocity of rotation of the crank is nearly uniform, it will

follow that the piston will move very slowly as it approaches the end of

the stroke; and the piston is brought to a state of rest by this gradually

retarded 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 more

especially of the fly wheel, which by its operation redresses the unequal

pressures communicated by the crank, and compels the crank shaft to revolve

at a nearly uniform velocity. Everyone knows that a heavy wheel if put into

rapid rotation cannot be immediately stopped. At the beginning and end of

the stroke when the crank is vertical, no force of torsion can be exerted

on the crank shaft by the crank, but this force is at its maximum when the

crank is horizontal. From the vertical point, where this force is nothing,

to the horizontal point, where it is at its maximum, the force of torsion

exerted on the crank shaft is constantly varying; and the fly wheel by its

momentum redresses these irregularities, and carries the crank through that

"dead point," as it is termed, where the piston cannot impart any rotative

force.

110. _Q._--Are the configuration and structure of the steam engine, as it

left the hand of Watt, materially different from those of modern engines?

_A._--There is not much difference. In modern rotative land engines, the

valves for admitting the steam to the cylinder or condenser, instead of

being clack or pot-lid valves moved by tappets on the air pump rod, are

usually sluice or sliding valves, moved by an eccentric wheel on the crank

shaft. Sometimes the beam is discarded altogether, and malleable iron is

more largely used in the construction of engines instead of the cast iron,

which formerly so largely prevailed. But upon the whole the steam engine of

the present day is substantially the engine of Watt; and he who perfectly

understands the operation of Watt's engine, will have no difficulty in

understanding the operation of any of the numerous varieties of engines

since introduced.

THE MARINE ENGINE.

111. _Q._--Will you describe the principal features of the kind of steam

engine 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 rotation

at each side of the ship: in the other case, the propelling instrument is a

screw, consisting of two or more twisted vanes, revolving beneath the water

at 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 or

employed, such as the trunk (fig. 22), double cylinder (fig. 23), annular,

Gorgon (fig. 24), steeple (fig. 25), and many others. The side lever

engine, however, and the oscillating engine, are the only kinds of paddle

engines which have been received with wide or general favor.

[Illustration: Fig. 24.]

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

engine?

_A._--In all paddle vessels, whatever be their subordinate characteristics,

a great shaft of wrought iron, s, turned round by the engine, has to be

carried from side to side of the vessel, on which shaft are fixed the

paddle wheels. The paddle wheels may either be formed with fixed float

boards for engaging the water, like the boards of a common undershot water

wheel, or they may be formed with _feathering_ float boards as they are

termed, which is float boards movable on a centre, and so governed by

appropriate mechanism that they enter and leave the water in a nearly

vertical position. The common fixed or radial floats, however, are the kind

most widely employed, and they are attached to the arms of two or more

rings of malleable iron which are fixed by appropriate centres on the

paddle shaft. It is usual in steam vessels to employ two engines, the

cranks of which are set at right angles with one another. When the paddle

wheels are turned by the engines, the float boards engaging the water cause

a forward thrust to be imparted to the shaft, which propels forward the

vessel on the same principle that a boat is propelled by the action of

oars.

[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 described

to be such a modification of the land beam engine already described, as

will 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 of

which is set on each side of the engine as low down as possible. The cross

head which engages the piston rod is made somewhat longer than the diameter

of the cylinder, and two great links or rods proceed one from each end of

the cross head to one of the side levers or beams. A similar cross bar at

the other end of the beams serves to connect them together and to the

connecting 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 an

example?

[Illustration: Fig. 26.]

_Q._--Fig. 26 is a side elevation of a side lever engine; x x represent the

beams or keelsons to which the engines are attached, and on which the

boilers rest. The engines are tied down by strong bolts passing through the

bottom of the vessel, but the boiler keeps its position by its weight

alone. The condenser and air pump are worked off the side levers by means

of 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 to

support the side levers or beams. L is the piston rod, which, by means of

the cross head and side rods, is connected to the side levers or beams, one

of which is shown at H H. The line M represents the connecting rod, to

which motion is imparted by the beams, through the medium of the cross tail

extending between the beams, and which by means of the crank turns the

paddle shaft S. The eccentric which works the slide valve is placed upon

the paddle shaft. It consists of a disc of metal encircled by a hoop, to

which a rod is attached, and the disc is perforated with a hole for the

shaft, not in the centre, but near one edge. When, therefore, the shaft

revolves, carrying the eccentric with it, the rod attached to the

encircling hoop receives a reciprocating motion, just as it would do if

attached 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 connection

between the eccentric and the slide valve is broken, by lifting the end of

the 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 valve

called the _blow through valve_, which permits steam from the boiler to

enter the engine both above and below the piston, and also to fill the

condenser and air pump. This steam expels the air from the interior of the

engine, and also any water which may have accumulated there; and when this

has been done, the blow through valve is shut, and a vacuum very soon forms

within the engine, by the condensation of the steam. If now the slide valve

be moved by hand, the steam from the boiler will be admitted on one side of

the 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 the

end of the stroke, the valve has to be moved in the reverse direction, when

the 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 by

allowing the notch on the end of the eccentric rod to engage the pin on the

valve lever, when the valve will be thereafter moved by the engine in the

proper manner. It will, of course, be necessary, when the engine begins to

move, to open the injection cock a little, to enable water to enter for the

condensation of the steam. In the most recent marine engines, a somewhat

different mechanism from this is used for giving motion to the valves, but

that 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 have

been constructed, with high pressure engines. In general, however, marine

engines are low pressure or condensing engines.

118. _Q._--Will you now describe the chief features of the oscillating

paddle marine engine?

_A._--In the oscillating paddle marine engine, the arrangement of the

paddle shaft and paddle wheels is the same as in the case already

described, but the whole of the side levers, side rods, cross head, cross

tail, and connecting rod are discarded. The cylinder is set immediately

under the crank; the top of the piston rod is connected immediately to the

crank pin; and, to enable the piston rod to accommodate itself to the

movement of the crank, the cylinder is so constructed as to be susceptible

of vibrating or oscillating upon two external axes or trunnions. These

trunnions are generally placed about half way up on the sides of the

cylinder; and through one of them steam is received from the boiler, while

through the other the steam escapes to the condenser. The air pump is

usually worked by means of a crank in the shaft, which crank moves the air

pump 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 by

Messrs. Ravenhill & Salked, for the Holyhead Packets. Fig. 27 is a

longitudinal 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 the

wheels. There are two cylinders in this vessel, and one air pump, which

lies in an inclined position, and is worked by a crank in the shaft which

stretches between the cylinders, and which is called the _intermediate

shaft_. A A, is one of the cylinders, B B the piston rod, and C C the

crank. D is the crank in the intermediate shaft, which works the air pump

E. There are double eccentrics fixed on the shaft, whereby the movement of

the slide valves is regulated. The purpose of the double eccentrics is to

enable an improved arrangement of valve gear to be employed, which is

denominated the _link motion_, and which will be described hereafter. I I

are the steam pipes leading to the steam trunnions K K, on which, and on

the eduction trunnions connected with the pipe M, the cylinders oscillate.

120. _Q._--By what species of mechanism are the positions of the paddle

floats of feathering wheels governed?

_A._--The floats are supported by spurs projecting from the rim of the

wheel, and they may be moved upon the points of the spurs, to which they

are attached by pins, by means of short levers proceeding from the backs of

the floats, and connected to rods which proceed towards the centre of the

wheel. The centre, however, to which these rods proceed is not concentric

with the wheel, and the rods, therefore, are moved in and out as the wheel

revolves, and impart a corresponding motion to the floats. In some

feathering wheels the proper motion is given to the rods by means of an

eccentric on the ship's side. The action of paddle wheels, whether radial

or feathering, will be more fully described in the chapter on Steam

Navigation.

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 divided

into two great classes,--geared engines and direct acting engines; and each

of these classes again has many varieties. In screw vessels, the shaft on

which the screw is set requires to revolve at a much greater velocity than

is required in the case of the paddle shaft of a paddle vessel; and in

geared engines this necessary velocity of rotation is obtained by the

intervention of toothed wheels,--the engines themselves moving with the

usual velocity of paddle engines; whereas in direct acting engines the

required velocity of rotation is obtained by accelerating the speed of the

engines, and which are connected immediately to the screw shaft.

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

engines?

_A._--A good many of the geared engines for screw vessels are made in the

same manner as land engines, with a beam overhead, which by means of a

connecting rod extending downwards, gives motion to the crank shaft, on

which are set the cog wheels which give motion to pinions on the screw

shaft,--the teeth of the wheels being generally of wood and the teeth of

the pinions of iron. There are usually several wheels on the crank shaft

and several pinions on the screw shaft; but the teeth of each do not run in

the same line, but are set a little in advance of one another, so as to

divide the thickness of the tooth into as many parts as there are

independent wheels or pinions. By this arrangement the wheels work more

smoothly 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 the

cylinders of a locomotive engine. In other cases vertical trunk engines are

employed; and in other cases vertical oscillating engines.

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

engine?

_A._--The engines of a geared oscillating engine are similar to the paddle

wheel engines (figs. 27 and 28), but the engines are placed lengthways of

the ship, and instead of a paddle wheel on the main shaft, there is a

geared wheel which connects with a pinion on the screw shaft. The engines

of the Great Britain are made off the same patterns as the paddle engines

constructed by Messrs. John Penn & Son, for H.M.S. Sphinx. The diameter of

each cylinder is 82-1/2 inches, the length of travel or stroke of the

piston is 6 feet, and the nominal power is 500 horses. The Great Britain is

of 3,500 tons burden, and her displacement at 16 feet draught of water is

2,970 tons. The diameter of the screw is 15-1/2 feet, length of screw in

the line of the shaft, 3 feet 2 inches, and the pitch of the screw, 19

feet.

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 screw

of large diameter like a spiral stair, and the pitch of a spiral stair is

the 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 of

a windmill. The multiple of the gearing in the Great Britain is 3 to 1, and

there are 17-1/2 square feet of heating surface in the boiler for each

nominal 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 on

the screw shaft, cause the screw to make three revolutions for every

revolution 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 the

cylinders 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 set

above the screw shaft on appropriate framing; and connecting rods attached

to 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 by

Messrs. Penn.

_A._--It is a horizontal trunk engine. In this engine a round pipe called a

trunk penetrates the piston, to which it is fixed, being in fact cast in

one piece with it; and the trunk also penetrates the top and bottom of the

cylinder, through which it moves, and is made tight therein by means of

stuffing boxes. The connecting rod is attached at one end to a pin fixed in

the middle of the trunk, while the other end engages the crank in the usual

manner. The air pump is set within the condenser, and is wrought by a rod

which is fixed to the piston and derives its motion therefrom. The air pump

is of that species which is called double-acting. The piston or bucket is

formed without valves in it, but an inlet and outlet valve is fixed to each

end of the pump, through the one of which the water is drawn into the pump

barrel, 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 locomotive

engine?

_A._--The locomotive employed to draw carriages upon railways, consists of

a cylindrical boiler filled with brass tubes, through which the hot air

passes on its progress from the furnace to the chimney, and attached to the

boiler are two horizontal cylinders fitted with pistons, valves, connecting

rods, and other necessary apparatus to enable the power exerted by the

pistons to turn round the cranked axle to which the driving wheels are

attached. There are, therefore, two independent engines entering into the

composition of a locomotive, the cranks of which are set at right angles

with one another, so that when one crank is at its dead point, the other

crank is in a position to act with its maximum efficacy. The driving

wheels, which are fixed on the crank shaft and turn round with it, propel

the locomotive forward on the rails by the mere adhesion of friction, and

this is found sufficient not merely to move the locomotive, but to draw a

long 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 impossible

or at least highly inconvenient, to carry the water necessary for the

purpose of condensation. The steam, therefore, after it has urged the

piston to the end of the stroke, escapes into the atmosphere. In locomotive

engines the waste steam is always discharged into the chimney through a

vertical pipe, and by its rapid passage it greatly increases the intensity

of the draught in the chimney, whereby a smaller fire grate suffices for

the combustion of the fuel, and the evaporative power of the boiler is much

increased.

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

form?

_A._--To do this I will take the example of one of Hawthorn's locomotive

engines with six wheels represented in fig. 29; not one of the most modern

construction now in use, nor yet one of the most antiquated. M is the

cylinder, R the connecting rod, C C the eccentrics by which the slide valve

is moved; J J is the steam pipe by which the steam is conducted from the

steam dome of the boiler to the cylinder. Near the smoke stack end of this

pipe is a valve K or regulator moved by a handle _p_ at the front of the

boiler, and of which the purpose is to regulate the admission of the steam

to the cylinder; _f_ is a safety valve kept closed by springs; N is the

eduction pipe, or, as it is commonly termed in locomotives, the _blast

pipe_, by which the steam, escaping from the cylinder after the stroke has

been performed, is projected up the chimney H. The water in the boiler of

course covers the tubes and also the top of the furnace or fire box. It

will be understood that there are two engines in each locomotive, though,

from the figure being given in section, only one engine can be shown. The

cylinders of this engine are each 14 inches diameter; the length of the

stroke of the piston is 21 inches. There are two sets of driving wheels, 5

feet 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 is

to contain coke for feeding the furnace, and water for replenishing the

boiler.

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 the

locomotive represented in fig. 29. I can, however, give examples of some of

the most powerful engines of recent construction. Fig. 30 represents

Gooch's express engine, adapted for the wide gauge of the Great Western

Railway; and fig. 31 represents Crampton's express engine, adapted for the

ordinary or narrow gauge railways. The cylinders of Gooch's engine are each

18 inches diameter, and 24 inches stroke; the driving wheels are 8 feet in

diameter; the fire grate contains 21 square feet of area; and the heating

surface of the fire box is 153 square feet. There are in all 305 tubes in

the boiler, each of 2 inches diameter, giving a heating surface in the

tubes of 1799 square feet. The total heating surface, therefore, is 1952

square feet. Mr. Gooch states that an engine of this class will evaporate

from 300 to 360 cubic feet of water in the hour, and will convey a load of

236 tons at a speed of 40 miles an hour, or a load of 181 tons at a speed

of 60 miles an hour. The weight of this engine empty is 31 tons; of the

tender 8-1/2 tons; and the total weight of the engine when loaded is 50

tons. In one of Crampton's locomotives, the Liverpool, with one set more of

carrying wheels than the fig., the cylinders are of 24 inches diameter and

18 inches stroke; the driving wheels are 8 feet in diameter; the fire grate

contains 21-1/2 square feet of area; and the heating surface of the fire

box is 154 square feet. There are in all 300 tubes in the boiler of 2-3/16

inches external diameter, giving a surface in the tubes of 2136 square

feet, and a total heating surface of 2290 square feet. The weight of this

engine is stated to be 35 tons when ready to proceed on a journey. Both

engines were displayed at the Great Exhibition in 1851, as examples of the

most powerful locomotive engines then made. The weight of such engines is

very injurious to the railway; bending, crushing, and disturbing the rails,

and trying very severely the whole of the railway works. No doubt the

weight may be distributed upon a greater number of wheels, but if the

weight resting on the driving wheels be much reduced, they will not have

sufficient 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 speed

has produced. The width of the railway, or, as it is termed, the _gauge_ of

the rails, being in most of the railways in this kingdom limited to 4 feet

8-1/2 inches, a corresponding limitation is imposed on the diameter of the

boiler; which in its turn restricts the number of the tubes which can be

employed. As, however, the attainment of a high rate of speed requires much

power, and consequently much heating surface in the boiler, and as the

number of tubes cannot be increased without reducing their diameter, it has

become necessary, in the case of powerful engines, to employ tubes of a

small diameter, and of a great length, to obtain the necessary quantity of

heating surface; and such tubes require a very strong draught in the

chimney to make them effective. With a draught of the usual intensity the

whole of the heat will be absorbed in the portion of the tube nearest the

fire box, leaving that portion nearest the smoke box nothing to do but to

transmit the smoke; and with long tubes of small diameter, therefore, a

very strong draught is indispensable. To obtain such a draught in

locomotives, it is necessary to contract the mouth of the blast pipe,

whereby the waste steam will be projected into the chimney with greater

force; but this contraction involves an increase of the pressure on the

eduction side of the piston, and consequently causes a diminution in the

power of the engine. Locomotives with small and long tubes, therefore, will

require more coke to do the same work than locomotives in which larger and

shorter tubes may be employed.