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A CATECHISM OF THE STEAM ENGINE
IN ITS VARIOUS APPLICATIONS TO MINES,
MILLS, STEAM NAVIGATION, RAILWAYS,
PRACTICAL INSTRUCTIONS FOR THE MANUFACTURE
AND MANAGEMENT OF ENGINES OF EVERY CLASS.
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.]
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.
9 BILLITER STREET,
_March 1st, 1856_.
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
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
_February 2d, 1857_.
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.
Classification of Engines.
Nature and uses of a Vacuum.
Velocity of falling Bodies and Momentum of moving Bodies.
Centres of Gravity, Gyration, and Oscillation.
The Pendulum and Governor.
The Mechanical Powers.
Strength of materials and Strains subsisting in Machines.
CHAP. I.--GENERAL DESCRIPTION OF THE STEAM ENGINE.
The Marine Engine.
The Locomotive Engine.
CHAP. II.--HEAT, COMBUSTION, AND STEAM.
CHAP. III.--EXPANSION OF STEAM AND ACTION OF THE VALVES.
CHAP. IV.--MODES OF ESTIMATING THE POWER AND PERFORMANCE OF
ENGINES AND BOILERS.
Duty of Engines and Boilers.
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.
Steam Room and Priming.
Strength of Boilers.
CHAP. VI.--PROPORTIONS OF ENGINES.
Air Pump, Condenser, and Hot and Cold Water Pumps.
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.
CHAP. VIII.--CONSTRUCTIVE DETAILS OF 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.
CHAP. XI.--ON VARIOUS FORMS AND APPLICATIONS OF THE STEAM ENGINE.
Portable Steam Engines.
Steam Fire Engines.
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
_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
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
_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
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
VELOCITY OF FALLING BODIES AND MOMENTUM OF MOVING BODIES.
15. _Q._--How do you determine the velocity of falling bodies of different
_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
_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
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
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
_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.
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
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
_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
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
_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
_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
_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
_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
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?
41. _Q._--Then will it not be driven round as any other mechanism would be
at a speed proportional to that of the engine?
42. _Q._--Then how can the length of the arms affect the time of
[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
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
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
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
_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
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
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
_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
_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
_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
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
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
_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
_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
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
_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
_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
_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
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
80. _Q._--In a riveted tube, will the riveting be much, damaged by heavy
_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.
GENERAL DESCRIPTION OF THE STEAM ENGINE.
* * * * *
83. _Q._--What are the chief varieties of the steam engine in actual
_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,
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
[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 deg., 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
_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
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
_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
[Illustration: Fig. 7.]
[Illustration: Fig. 8.]
92. _Q._--Is this arrangement different from that obtaining in tubular
_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.]
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
94. _Q._--This reciprocating motion is all that is required in a pumping
_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
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
_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
_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