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Title: A Catechism of the Steam Engine

Author: John Bourne

Release Date: February 9, 2004 [EBook #10998]

Language: English

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A CATECHISM OF THE STEAM ENGINE

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

WITH

PRACTICAL INSTRUCTIONS FOR THE MANUFACTURE
AND MANAGEMENT OF ENGINES OF EVERY CLASS.


BY

JOHN BOURNE, C.E.


_NEW AND REVISED EDITION._


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


PREFACE

TO THE FOURTH EDITION.

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

For the sake of greater distinctness the work is now divided into chapters.
Some of these chapters are altogether new, and the rest have received such
extensive additions and improvements as to make the book almost a new one.
One purpose of my emendations has been to render my remarks intelligible to
a tyro, as well as instructive to an advanced student. With this view, I
have devoted the first chapter to a popular description of the Steam
Engine--which all may understand who can understand anything--and in the
subsequent gradations of progress I have been careful to set no object
before the reader for the first time, of which the nature and functions are
not simultaneously explained. The design I have proposed to myself, in the
composition of this work, is to take a young lad who knows nothing of steam
engines, and to lead him by easy advances up to the highest point of
information I have myself attained; and it has been a pleasing duty to me
to smooth for others the path which I myself found so rugged, and to
impart, for the general good of mankind, the secrets which others have
guarded with so much jealousy. I believe I am the first author who has
communicated that practical information respecting the steam engine, which
persons proposing to follow the business of an engineer desire to possess.
My business has, therefore, been the rough business of a pioneer; and while
hewing a road through the trackless forest, along which all might hereafter
travel with ease, I had no time to attend to those minute graces of
composition and petty perfection of arrangement and collocation, which are
the attribute of the academic grove, or the literary parterre. I am,
nevertheless, not insensible to the advantages of method and clear
arrangement in any work professing to instruct mankind in the principles
and practice of any art; and many of the changes introduced into the
present edition of this work are designed to render it less exceptionable
in this respect. The woodcuts now introduced into the work for the first
time will, I believe, much increase its interest and utility; and upon the
whole I am content to dismiss it into circulation, in the belief that those
who peruse it attentively will obtain a more rapid and more practical
acquaintance with the steam engine in its various applications, than they
would be likely otherwise to acquire.

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

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


JOHN BOURNE.
9 BILLITER STREET,
LONDON,

_March 1st, 1856_.




PREFACE

TO THE FIFTH EDITION.

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

I regret that circumstances have prevented me from yet issuing the
"Hand-Book" which I have had for some time in preparation, and to which, in
my Preface of the last year, I referred. I hope to have sufficient leisure
shortly, to give that and some other of my literary designs the necessary
attention. Whatever may have been the other impediments to a more prolific
authorship, certainly one of them has not been the coldness of the
approbation with which my efforts have been received, since my past
performances seem to me to have met with an appreciation far exceeding
their deserts.

JOHN BOURNE.

_February 2d, 1857_.




PUBLISHERS' NOTICE.


In offering to the American public a reprint of a work on the Steam Engine
so deservedly successful, and so long considered standard, the publishers
have not thought it necessary that it should be an exact copy of the
English edition; there were some details in which they thought it could be
improved, and better adapted to the use of American engineers. On this
account, the size of the page has been increased to a full 12mo, to admit
of larger illustrations, which in the English edition are often on too
small a scale; and some of the illustrations themselves have been supplied
by others equally applicable, more recent, and to us more familiar
examples. The first part of Chapter XI, devoted in the English edition to
English portable and fixed agricultural engines, in this edition gives
place entirely to illustrations from American practice, of steam engines as
applied to different purposes, and of appliances and machines necessary to
them. But with the exception of some of the illustrations and the
description of them, and the correction of a few typographical errors, this
edition is a faithful transcript of the latest English edition.




CONTENTS.


Classification of Engines.

Nature and uses of a Vacuum.

Velocity of falling Bodies and Momentum of moving Bodies.

Central Forces.

Centres of Gravity, Gyration, and Oscillation.

The Pendulum and Governor.

The Mechanical Powers.

Friction.

Strength of materials and Strains subsisting in Machines.


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

The Boiler.

The Engine.

The Marine Engine.

Screw Engines.

The Locomotive Engine.


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

Heat.

Combustion.

Steam.


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


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

Horses Power.

Duty of Engines and Boilers.

The Indicator.

Dynamometer, Gauges, and Cataract.


CHAP. V.--PROPORTIONS OF BOILERS.

Heating and Fire Grate Surface.

Calorimeter and Vent.

Evaporative Power of Boilers.

Modern Marine and Locomotive Boilers.

The Blast in Locomotives.

Boiler Chimneys.

Steam Room and Priming.

Strength of Boilers.

Boiler Explosions.


CHAP. VI.--PROPORTIONS OF ENGINES.

Steam Passages.

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

Fly Wheel.

Strengths of Land Engines.

Strengths of Marine and Locomotive Engines.


CHAP. VII.--CONSTRUCTIVE DETAILS OF BOILERS.

Land and Marine Boilers.

Incrustation and Corrosion of Boilers.

Locomotive Boilers.


CHAP. VIII.--CONSTRUCTIVE DETAILS OF ENGINES.

Pumping Engines.

Various forms of Marine Engines.

Cylinders, Pistons, and Valves.

Air Pump and Condenser.

Pumps, Cocks, and Pipes.

Details of the Screw and Screw Shaft.

Details of the Paddles and Paddle Shaft.

The Locomotive Engine.


CHAP. IX.--STEAM NAVIGATION.

Resistance of Vessels in Water.

Experiments on the Resistance of Vessels.

Influence of the size of Vessels upon their Speed.

Structure and Operation of Paddle Wheels.

Configuration and Action of the Screw.

Comparative Advantages of Paddle and Screw Vessels.

Comparative Advantages of different kinds of Screws.

Proportions of Screws.

Screw Vessels with full and auxiliary Power.

Screw and Paddles combined.


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

Oscillating Paddle Engines.

Direct acting Screw Engine.

Locomotive Engine.


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

Governor.

Donkey Pumps.

Portable Steam Engines.

Stationary Engines.

Steam Fire Engines.

Steam Excavator.


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

Construction of Engines.

Erection of Engines.

Management of Marine Boilers.

Management of Marine Engines.

Management of Locomotives.




MECHANICAL PRINCIPLES OF THE STEAM ENGINE.



CLASSIFICATION OF ENGINES.


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

_A._--A vacuum means an empty space; a space in which there is neither
water nor air, nor anything else that we know of.

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

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

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

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

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

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

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

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



NATURE AND USES OF A VACUUM.

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

_A._--Because it is not the vacuum which exerts this pressure, but the
atmosphere, which, like a head of water, presses on everything immerged
beneath it. A head of water, however, would not press down a piston, if the
water were admitted on both of its sides; for an equilibrium would then be
established, just as in the case of a balance which retains its equilibrium
when an equal weight is added to each scale; but take the weight out of one
scale, or empty the water from one side of the piston, and motion or
pressure is produced; and in like manner pressure is produced on a piston
by admitting steam or air upon the one side, and withdrawing the steam or
air from the other side. It is not, therefore, to a vacuum, but rather to
the existence of an unbalanced plenum, that the pressure made manifest by
exhaustion is due, and it is obvious therefore that a vacuum of itself
would not work an engine.

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

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

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

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

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

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

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

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

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

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

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

_A._--Yes, if once it be started; but it will be a difficult thing to start
an engine, if the pressure of the steam be not greater than that of the
atmosphere. Before an engine can be started, it has to be blown through
with steam to displace the air within it, and this cannot be effectually
done if the pressure of the steam be very low. After the engine is started,
however, the pressure in the boiler may be lowered, if the engine be
lightly loaded, until there is a partial vacuum in the boiler. Such a
practice, however, is not to be commended, as the gauge cocks become
useless when there is a partial vacuum in the boiler; inasmuch as, when
they are opened, the water will not rush out, but air will rush in. It is
impossible, also, under such circumstances, to blow out any of the sediment
collected within the boiler, which, in the case of the boilers of steam
vessels, requires to be done every two hours or oftener. This is
accomplished by opening a large cock which permits some of the supersalted
water to be forced overboard by the pressure of the steam. In some cases,
in which the boiler applied to an engine is of inadequate size, the
pressure within the boiler will fall spontaneously to a point considerably
beneath the pressure of the atmosphere; but it is preferable, in such
cases, partially to close the throttle valve in the steam pipe, whereby the
issue of steam to the engine is diminished; and the pressure in the boiler
is thus maintained, while the cylinder receives its former supply.

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

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

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

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



VELOCITY OF FALLING BODIES AND MOMENTUM OF MOVING BODIES.

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

_A._--All bodies fall with the same velocity, when there is no resistance
from the atmosphere, as is shown by the experiment of letting fall, from
the top of a tall exhausted receiver, a feather and a guinea, which reach
the bottom at the same time. The velocity of falling bodies is one that is
accelerated uniformly, according to a known law. When the height from which
a body falls is given, the velocity acquired at the end of the descent can
be easily computed. It has been found by experiment that the square root of
the height in feet multiplied by 8.021 will give the velocity.

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

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

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

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

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

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

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

_A._--It is.

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

_A._--Undoubtedly.

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

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

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

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

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

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

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

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



CENTRAL FORCES.

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

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

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

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

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

_A._--Yes. If the velocity in feet per second be divided by 4.01, the
square of the quotient will be four times the height in feet from which a
body must have fallen to have acquired that velocity. Divide this quadruple
height by the diameter of the circle, and the quotient is the centrifugal
force in terms of the weight of the body, so that, multiplying the quotient
by the actual weight of the body, we have the centrifugal force in pounds
or tons. Another rule is to multiply the square of the number of
revolutions per minute by the diameter of the circle in feet, and to divide
the product by 5,870. The quotient is the centrifugal force in terms of the
weight of the body.

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

_A._--Multiply the centrifugal force in terms of the weight of the body by
the diameter of the circle in feet, and multiply the square root of the
product by 4.01; the result will be the velocity of the body in feet per
second.

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

_A._--If we take the tensile strength of cast iron at 15,000 lbs. per
square inch, a fly-wheel rim of one square inch of sectional area would
sustain 30,000 lbs. If we suppose one half of the rim to be so fixed to the
shaft as to be incapable of detachment, then the centrifugal force of the
other half of the rim at the moment of rupture must be equal to 30,000 lbs.
Now 30,000 lbs. divided by 49.48 (the weight of the half rim) is equal to
606.3, which is the centrifugal force in terms of the weight. Then by the
rule given in the last answer 606.3 x 10 = 6063, the square root of which
is 78 nearly, and 78 x 4.01 = 312.78, the velocity of the rim in feet per
second at the moment of rupture.

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

_A._--If we take 2,000 lbs. as the utmost strain per square inch to which
cast iron can be permanently subjected with safety; then, by a similar
process to that just explained, we have 4,000 lbs./49.48 = 80.8 which
multiplied by 10 = 808, the square root of which is 28.4, and 28.4 x 4.01 =
113.884, the velocity of the rim in feet per second, which may be
considered as the highest consistent with safety. Indeed, this limit should
not be approached in practice on account of the risks of fracture from
weakness or imperfections in the metal.

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

_A._--The weight of a malleable iron rim of one square inch sectional area
and 7 feet diameter is 21.991 feet x 3.4 lbs. = 74.76, one half of which is
37.4 lbs. Then by the same process as before, 8,000/37.4 = 213.9, the
centrifugal force in terms of the weight: 213.9 x 7, the diameter of the
wheel = 1497.3, the square root of which, 38.3 x 4.01 = 155.187 feet per
second, the highest velocity of the rims of railway carriage wheels that is
consistent with safety. 155.187 feet per second is equivalent to 105.8
miles an hour. As 4,000 lbs. per square inch of sectional area is the
utmost strain to which iron should be exposed in machinery, railway wheels
can scarcely be considered safe at speed even considerably under 100 miles
an hour, unless so constructed that the centrifugal force of the rim will
be counteracted, to a material extent, by the centripetal action of the
arms. Hooped wheels are very unsafe, unless the hoops are, by some process
or other, firmly attached to the arms. It is of no use to increase the
dimensions of the rim of a wheel with the view of giving increased strength
to counteract the centrifugal force, as every increase in the weight of the
rim will increase the centrifugal force in the same proportion.



CENTRES OF GRAVITY, GYRATION, AND OSCILLATION.

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

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

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

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

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

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



THE PENDULUM AND GOVERNOR.

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

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

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

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

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

_A._--Yes; the length of the second's pendulum being known, the length of
another pendulum, required to perform any given number of vibrations in the
minute, may be obtained by the following rule: multiply the square root of
the given length by 60, and divide the product by the given number of
vibrations per minute; the square of the quotient is the length of pendulum
required. Thus if the length of a pendulum were required that would make 70
vibrations per minute in the latitude of London, then SQRT(39.1393) x 60/70
= (5.363)^2 = 28.75 in. which is the length required.

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

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

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

_A._--And so there is; the length of the second's pendulum at the level of
the sea in London, is 39.1393 inches, and it is from the length of the
second's pendulum that the space through which a body falls in a second has
been determined. As the time in which a pendulum vibrates is to the time in
which a heavy body falls through half the length of the pendulum, as the
circumference of a circle is to its diameter, and as the height through
which a body falls is as the square of the time of falling, it is clear
that the height through which a body will fall, during the vibration of a
pendulum, is to half the length of the pendulum as the square of the
circumference of a circle is to the square of its diameter; namely, as
9.8696 is to 1, or it is to the whole length of the pendulum as the half of
this, namely, 4.9348 is to 1; and 4.9348 times 39.1393 in. is 16-1/12 ft.
very nearly, which is the space through which a body falls by gravity in a
second.

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

_A._--Yes; the motion of the conical pendulum may be supposed to be
compounded of the motions of two common pendulums, vibrating at right
angles to one another, and one revolution of a conical pendulum will be
performed in the same time as two vibrations of a common pendulum, of which
the length is equal to the vertical height of the point of suspension above
the plane of revolution of the balls.

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

_A._--Yes.

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

_A._--It will.

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

[Illustration: Fig. 1.]

_A._--By flying out until they assume a vertical height answering to the
velocity with which they rotate round the central axis. As the speed is
increased the balls expand, and the height of the cone described by the
arms is diminished, until its vertical height is such that a pendulum of
that length would perform two vibrations for every revolution of the
governor. By the outward motion of the arms, they partially shut off the
steam from the engine. If, therefore, a certain expansion of the balls be
desired, and a certain length be fixed upon for the arms, so that the
vertical height of the cone is fixed, then the speed of the governor must
be such, that it will make half the number of revolutions in a given time
that a pendulum equal in length to the height of the cone would make of
vibrations. The rule is, multiply the square root of the height of the cone
in inches by 0.31986, and the product will be the right time of revolution
in seconds. If the number of revolutions and the length of the arms be
fixed, and it is wanted to know what is the diameter of the circle
described by the balls, you must divide the constant number 187.58 by the
number of revolutions per minute, and the square of the quotient will be
the vertical height in inches of the centre of suspension above the plane
of the balls' revolution. Deduct the square of the vertical height in
inches from the square of the length of the arm in inches, and twice the
square root of the remainder is the diameter of the circle in which the
centres of the balls revolve.

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

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

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

_A._--Not in the least; each ball may be supposed to be made up of a number
of small balls or particles, and each particle of matter will act for
itself. Heavy balls attached to a governor are only requisite to overcome
the friction of the throttle valve which shuts off the steam, and of the
connections leading thereto. Though the weight of a ball increases its
centripetal force, it increases its centrifugal force in the same
proportion.



THE MECHANICAL POWERS.

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

_A._--The mechanical powers are certain contrivances, such as the wedge,
the screw, the inclined plane, and other elementary machines, which convert
a small force acting through a great space into a great force acting
through a small space. In the school treatises on mechanics, a certain
number of these devices are set forth as the mechanical powers, and each
separate device is treated as if it involved a separate principle; but not
a tithe of the contrivances which accomplish the stipulated end are
represented in these learned works, and there is no very obvious necessity
for considering the principle of each contrivance separately when the
principles of all are one and the same. Every pressure acting with a
certain velocity, or through a certain space, is convertible into a greater
pressure acting with a less velocity, or through a smaller space; but the
quantity of mechanical force remains unchanged by its transformation, and
all that the implements called mechanical powers accomplish is to effect
this transformation.

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

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

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

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

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

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

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

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

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

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

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

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



FRICTION.

52. _Q._--What is friction?

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

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

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

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

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

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

_A._--It varies with the nature of the rubbing bodies. The friction of iron
sliding upon iron, has generally been taken at about one tenth of the
pressure, when the surfaces are oiled and then wiped again, so that no film
of oil is interposed. The friction of iron rubbing upon brass has generally
been taken at about one eleventh of the pressure under the same
circumstances; but in machines in actual operation, where a film of some
lubricating material is interposed between the rubbing surfaces, it is not
more than one third of this amount or 1/33d of the weight. While this,
however, is the average result, the friction is a good deal less in some
cases. Mr. Southern, in some experiments upon the friction of the axle of a
grindstone--an account of which may be found in the 65th volume of the
Philosophical Transactions--found the friction to amount to less than
1/40th of the weight; and Mr. Wood, in some experiments upon the friction
of locomotive axles, found that by ample lubrication the friction may be
made as little as 1/60th of the weight. In some experiments upon the
friction of shafts by Mr. G. Rennie, he found that with a pressure of from
1 to 5 cwt. the friction did not exceed 1/39th of the pressure when tallow
was the unguent employed; with soft soap it became 1/34th. The fact appears
to be that the amount of the resistance denominated friction depends, in a
great measure, upon the nature of the unguent employed, and in certain
cases the viscidity of the unguent may occasion a greater retardation than
the resistance caused by the attrition. In watchwork therefore, and other
fine mechanism, it is necessary both to keep the bearing surfaces small,
and to employ a thin and limpid oil for the purpose of lubrication, for the
resistance caused by the viscidity of the unguent increases with the amount
of surface, and the amount of surface is relatively greater in the smaller
class of works.

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

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

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

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

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

_A._--Yes, water will answer very well if the surface be very large
relatively with the pressure; and in screw vessels where the propeller
shaft passes through a long pipe at the stern, the stuffing box is
purposely made a little leaky. The small leakage of water into the vessel
which is thus occasioned, keeps the screw shaft in this situation always
wet, and this is all the lubrication which this bearing requires or
obtains.

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

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

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

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

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

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

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

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

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

Sq. in.                 Lb.                            Lb.
8                       56                             21.5
"                      112                             44.
"                      168                             65.5
"                      224                             88.5
"                      336                            140.5
"                      448                            170.75

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

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

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



STRENGTH OF MATERIALS AND STRAINS SUBSISTING IN MACHINES.

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

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

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

_A._--The tensile and crushing strengths of any given material are by no
means the same. The tensile strength, or strength when extended, of good
bar iron is about 60,000 lbs., or nearly 27 tons per square inch of
section; and the tensile strength of cast iron is about 15,000 lbs., or say
6 3/4 to 7 tons per square inch of section. These are the weights which are
required to break them. The crushing strain of cast iron, however, is about
100,000 lbs., or 44 1/2 tons; whereas the crushing strength of malleable
iron is not more than 27,000 lbs., or 12 tons, per square inch of section,
and indeed it is generally less than this. The ultimate tensile strength,
therefore, of malleable iron is four times greater than that of cast iron,
but the crushing strength of cast iron is between three and four times
greater than that of wrought iron. It may be stated, in round numbers, that
the tensile strength of malleable iron is twice greater than its crushing
strength; or, in other words, that it will take twice the strain to break a
bar of malleable iron by drawing it asunder endways, than will cripple it
by forcing it together endways like a pillar; whereas a bar of cast iron
will be drawn asunder with one sixth of the force that will be required to
break or cripple it when forced together endways like a pillar.

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

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

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

_A._--It is about twice greater. A good steel punch will punch through a
plate of wrought iron of a thickness equal to the diameter of the punch. A
punch therefore of an inch diameter will pierce a plate an inch thick. Now
it is well known, that the strain required to punch a piece of metal out of
a plate, is just the same as that required to tear asunder a bar of iron of
the same area of cross section as the area of the surface cut. The area of
the surface cut in this case will be the circumference of the punch, 3.1416
inches, multiplied by the thickness of the plate, 1 inch, which makes the
area of the cut surface 3.1416 square inches. The area of the point of the
punch subjected to the pressure is .7854 square inches, so that the area
cut to the area crushed is as four to one. In other words, it will require
four times the strain to crush steel that is required to tear asunder
malleable iron, or it will take about twice the strain to crush steel that
it will require to break it by extension.

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

_A._--A bar of wrought iron to which a tensile or compressing strain is
applied, is elongated or contracted like a very stiff spiral spring, nearly
in the proportion of the amount of strain applied up to the limit at which
the strength begins to give way, and within this limit it will recover its
original dimensions when the strain is removed. If, however, the strain be
carried beyond this limit, the bar will not recover its original
dimensions, but will be permanently pulled out or pushed in, just as would
happen to a spring to which an undue strain had been applied. This limit is
what is called the limit of elasticity; and whenever it is exceeded, the
bar, though it may not break immediately, will undergo a progressive
deterioration, and will break in the course of time. The limit of
elasticity of malleable iron when extended, or, in other words, the tensile
strain to which a bar of malleable iron an inch square may be subjected
without permanently deranging its structure, is usually taken at 17,800
lbs., or from that to 10 tons, depending on the quality of the iron. It has
also been found that malleable iron is extended about one ten-thousandth
part of its length for every ton of direct strain applied to it.

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

_A._--It is commonly taken at 15,300 lbs. per square inch of section; but
this is certainly much too high, as it exceeds the tensile strength of
irons of medium quality. A bar of cast iron if compressed by weights will
be contracted in length twice as much as a bar of malleable iron under
similar circumstances; but malleable iron, when subjected to a greater
strain than 12 tons per square inch of section, gradually crumples up by
the mere continuance of the weight. A cast-iron bar one inch square and ten
feet long, is shortened about one tenth of an inch by a compressing force
of 10,000 lbs., whereas a malleable iron bar of the same dimensions would
require to shorten it equally a compressing force of 20,000 lbs. As the
load, however, approaches 12 tons, the compressions become nearly equal,
and above that point the rate of the compression of the malleable iron
rapidly increases. A bar of cast iron, when at its breaking point by the
application of a tensile strain, is stretched about one six-hundredth part
of its length; and an equal strain employed to compress it, would shorten
it about one eight-hundredth part of its length.

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

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

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

_A._--To take the case of a beam subjected to a transverse strain, such as
the great beam of an engine, it is clear, if we suppose the beam broken
through the middle, that the amount of strain at the upper and lower edges
of the beam, where the whole strain may be supposed to be collected, will,
with any given pressure on the piston, depend upon the proportion of the
length to the depth of the beam. One edge of the beam breaks by extension,
and the other edge by compression; and the upper and lower edges may be
regarded as pillars, one of which is extended by the strain, and the other
is compressed. If, to make an extreme supposition, the depth of the beam is
taken as equal to its length, then the pillars answering to the edges of
the beam will be compressed, and extended by what is virtually a bellcrank
lever with equal arms; the horizontal distance from the main centre to the
end of the beam being one of the arms, and the vertical height from the
main centre to the top edge of the beam being the other arm. The distance,
therefore, passed through by the fractured edge of the beam during a stroke
of the engine, will be equal to the length of the stroke; and the strain it
will have to sustain will consequently be equal to the pressure on the
piston. If its motion were only half that of the piston, as would be the
case if its depth were made one half less, the strain the beam would have
to bear would be twice as great; and it may be set down as an axiom, that
the strain upon any part of a steam engine or other machine is inversely
equal to the strain produced by the prime mover, multiplied by the
comparative velocity with which the part in question moves. If any part of
an engine moves with a less velocity than the piston, it will have a
greater strain on it, if resisted, than is thrown upon the piston. If it
moves with a greater velocity than the piston, it will have a less strain
upon it, and the difference of strain will in every case be in the inverse
proportion of the difference of the velocity.

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

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

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

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

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

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

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

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

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

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

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

_A._--Iron beams bent alternately in opposite directions, or alternately
deflected and released, will be broken in the course of time with a much
less strain than is necessary to produce immediate fracture. It has been
found, experimentally, that a cast-iron bar, deflected by a revolving cam
to only half the extent due to its breaking weight, will in no case
withstand 900 successive deflections; but, if bent by the cam to only one
third of its ultimate deflection, it will withstand 100,000 deflections
without visible injury. Looking, however, to the jolts and vibrations to
which engines are subject, and the sudden strains sometimes thrown upon
them, either from water getting into the cylinder or otherwise, it does not
appear that a strength answering to six times the breaking weight will give
sufficient margin for safety in the case of cast-iron beams.

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

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

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

_A._--The power of a rod or pillar to resist compression, varies nearly as
the fourth power of the diameter divided by the square of the length. In
the case of hollow cylindrical columns of cast iron, it has been found,
experimentally, that the 3.55th power of the internal diameter, subtracted
from the 3.55th power of the external diameter, and divided by the 1.7th
power of the length, will represent the strength very nearly. In the case
of hollow cylindrical columns of malleable iron, experiment shows that the
3.59th power of the internal diameter, subtracted from the 3.59th power of
the external diameter, and divided by the square of the length, gives a
proper expression for the strength; but this rule only holds where the
strain does not exceed 8 or 9 tons on the square inch of section. Beyond 12
or 13 tons per square inch of section, the metal cannot be depended upon to
withstand the strain, though hollow pillars will sometimes bear 15 or 16
tons per square inch of section.

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

_A._--It manifestly does; for a tube of very thin metal, such as gold leaf
or tin foil, would not stand on end at all, being crushed down by its own
weight. It is found, experimentally, that in malleable iron tubes of the
respective thicknesses of .525, .272, and .124 inches, the resistances per
square inch of section are 19.17, 14.47, and 7.47 tons respectively. The
power of plates to resist compression varies nearly as the cube, or more
nearly as the 2.878th power of their thickness; but this law only holds so
long as the pressure applied does not exceed from 9 to 12 tons per square
inch of section. When the pressure is greater than this the metal is
crushed, and a new law supervenes, according to which it is necessary to
employ plates of twice or three times the thickness, to obtain twice the
resisting power.

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

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

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

_A._--By no means. Some cast iron is very hard and brittle; and although it
will in this state resist compression very strongly, it, will be easily
broken by a blow. Iron which has been remelted many times generally falls
into this category, as it will also do if run into very small castings. It
has been found, by experiment, that iron of which the crushing weight per
square inch is about 42 tons, will, if remelted twelve times, bear a
crushing weight of 70 tons, and if remelted eighteen times it will bear a
crushing weight of 83 tons; but taking its power to resist impact in its
first state at 706, this power will be raised at the twelfth remelting to
1153, and will be sunk at the eighteenth remelting to 149.

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

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




CHAPTER I.


GENERAL DESCRIPTION OF THE STEAM ENGINE.

       *       *       *       *       *

THE BOILER.

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

_A._--There is first the single-acting engine, which is used for pumping
water; the rotative land engine, which is employed to drive mills and
manufactories; the rotative marine engine, which is used to propel steam
vessels; and the locomotive engine, which is employed on railways. The last
is always a high-pressure engine; the others are, for the most part,
condensing engines.

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

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

[Illustration: Fig. 3]

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

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

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

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

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

_A._--On the top of the boiler, near the front, is a short cylinder, with a
lid secured by bolts. This is the manhole door, the purpose of which is to
enable a man to get into the inside of the boiler when necessary for
inspection and repair. On the top of this door is a small valve opening
downward, called the atmospheric valve. The intention of this valve is to
prevent a vacuum from being formed accidentally in the boiler, which might
collapse it; for if the pressure in the boiler subsides to a point
materially below the pressure of the atmosphere, the valve will open and
allow air to get in. A bent pipe, which rises up from the top of the
boiler, immediately behind the position of the manhole, is the steam pipe
for conducting the steam to the engine; and a bent pipe which ascends from
the top of the boiler, at the back end, is the waste-steam pipe for
conducting away the steam, which escapes through the safety valve. This
valve is set in a chest, standing on the top of the boiler, at the foot of
the waste-steam pipe, and it is loaded with iron or leaden weights to a
point answerable to the intended pressure of the steam.

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

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

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

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

[Illustration: Fig. 4.]

[Illustration: Fig. 5.]


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

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

[Illustration: Fig. 6]

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

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

[Illustration: Fig. 7.]

[Illustration: Fig. 8.]

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

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

[Illustration: Fig. 9.]

[Illustration: Fig. 10.]

[Illustration: Fig. 11.]

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

The following are some forms of American boilers:

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

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

[Illustration: Fig. 12.]

[Illustration: Fig. 13.]

[Illustration: Fig. 14.]

[Illustration: Fig. 15.]

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

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

[Illustration: Fig. 16.]

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

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

[Illustration: Fig. 19.]




THE ENGINE.

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

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

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

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

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

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

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

[Illustration: Fig. 20.]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

_A._--The parallel motion is an arrangement of jointed rods, so connected
together that the divergence from the vertical line at any point in the arc
described by the beam is corrected by an equal and opposite divergence due
to the arc performed by the jointed rods during the stroke; and as these
opposite deviations mutually correct one another, the result is that the
piston rod moves in a vertical direction.

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

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

[Illustration: Fig. 21.]

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

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

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

_A._--That is the object of it, and it accomplishes its object in a very
perfect manner, as it gradually arrests the velocity of the piston towards
the end of the stroke, and thus obviates what would otherwise be an
injurious shock upon the machine. When the crank approaches the lowest part
of its throw, and at the same time the piston is approaching the top of the
cylinder, the motion of the crank becomes nearly horizontal, or, in other
words, the piston is only advanced through a very short distance, for any
given distance measured on the circle described by the crank pin. Since,
then, the velocity of rotation of the crank is nearly uniform, it will
follow that the piston will move very slowly as it approaches the end of
the stroke; and the piston is brought to a state of rest by this gradually
retarded motion, both at the top and the bottom of the stroke.

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

_A._--The momentum of the machinery moved by the piston, but more
especially of the fly wheel, which by its operation redresses the unequal
pressures communicated by the crank, and compels the crank shaft to revolve
at a nearly uniform velocity. Everyone knows that a heavy wheel if put into
rapid rotation cannot be immediately stopped. At the beginning and end of
the stroke when the crank is vertical, no force of torsion can be exerted
on the crank shaft by the crank, but this force is at its maximum when the
crank is horizontal. From the vertical point, where this force is nothing,
to the horizontal point, where it is at its maximum, the force of torsion
exerted on the crank shaft is constantly varying; and the fly wheel by its
momentum redresses these irregularities, and carries the crank through that
"dead point," as it is termed, where the piston cannot impart any rotative
force.

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

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


THE MARINE ENGINE.

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

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

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

[Illustration: Fig. 22.]

[Illustration: Fig. 23.]

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

[Illustration: Fig. 24.]

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

_A._--In all paddle vessels, whatever be their subordinate characteristics,
a great shaft of wrought iron, s, turned round by the engine, has to be
carried from side to side of the vessel, on which shaft are fixed the
paddle wheels. The paddle wheels may either be formed with fixed float
boards for engaging the water, like the boards of a common undershot water
wheel, or they may be formed with _feathering_ float boards as they are
termed, which is float boards movable on a centre, and so governed by
appropriate mechanism that they enter and leave the water in a nearly
vertical position. The common fixed or radial floats, however, are the kind
most widely employed, and they are attached to the arms of two or more
rings of malleable iron which are fixed by appropriate centres on the
paddle shaft. It is usual in steam vessels to employ two engines, the
cranks of which are set at right angles with one another. When the paddle
wheels are turned by the engines, the float boards engaging the water cause
a forward thrust to be imparted to the shaft, which propels forward the
vessel on the same principle that a boat is propelled by the action of
oars.

[Illustration: Fig. 25.]

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

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

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

[Illustration: Fig. 26.]

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

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

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

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

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

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

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

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

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

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

_A._--The floats are supported by spurs projecting from the rim of the
wheel, and they may be moved upon the points of the spurs, to which they
are attached by pins, by means of short levers proceeding from the backs of
the floats, and connected to rods which proceed towards the centre of the
wheel. The centre, however, to which these rods proceed is not concentric
with the wheel, and the rods, therefore, are moved in and out as the wheel
revolves, and impart a corresponding motion to the floats. In some
feathering wheels the proper motion is given to the rods by means of an
eccentric on the ship's side. The action of paddle wheels, whether radial
or feathering, will be more fully described in the chapter on Steam
Navigation.



SCREW ENGINES.

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

[Illustration: Fig. 27.]

[Illustration: Fig. 28.]

_A._--The engines employed for the propulsion of screw vessels are divided
into two great classes,--geared engines and direct acting engines; and each
of these classes again has many varieties. In screw vessels, the shaft on
which the screw is set requires to revolve at a much greater velocity than
is required in the case of the paddle shaft of a paddle vessel; and in
geared engines this necessary velocity of rotation is obtained by the
intervention of toothed wheels,--the engines themselves moving with the
usual velocity of paddle engines; whereas in direct acting engines the
required velocity of rotation is obtained by accelerating the speed of the
engines, and which are connected immediately to the screw shaft.

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

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

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

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

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

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

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

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

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

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

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

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

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

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



THE LOCOMOTIVE ENGINE.

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

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

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

_A._--They are invariably high pressure engines, and it would be impossible
or at least highly inconvenient, to carry the water necessary for the
purpose of condensation. The steam, therefore, after it has urged the
piston to the end of the stroke, escapes into the atmosphere. In locomotive
engines the waste steam is always discharged into the chimney through a
vertical pipe, and by its rapid passage it greatly increases the intensity
of the draught in the chimney, whereby a smaller fire grate suffices for
the combustion of the fuel, and the evaporative power of the boiler is much
increased.

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

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

[Illustration: Fig. 29.]

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

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

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

[Illustration: Fig. 30.]

[Illustration: Fig. 31.]

_A._--The most recent locomotives resemble in their material features the
locomotive represented in fig. 29. I can, however, give examples of some of
the most powerful engines of recent construction. Fig. 30 represents
Gooch's express engine, adapted for the wide gauge of the Great Western
Railway; and fig. 31 represents Crampton's express engine, adapted for the
ordinary or narrow gauge railways. The cylinders of Gooch's engine are each
18 inches diameter, and 24 inches stroke; the driving wheels are 8 feet in
diameter; the fire grate contains 21 square feet of area; and the heating
surface of the fire box is 153 square feet. There are in all 305 tubes in
the boiler, each of 2 inches diameter, giving a heating surface in the
tubes of 1799 square feet. The total heating surface, therefore, is 1952
square feet. Mr. Gooch states that an engine of this class will evaporate
from 300 to 360 cubic feet of water in the hour, and will convey a load of
236 tons at a speed of 40 miles an hour, or a load of 181 tons at a speed
of 60 miles an hour. The weight of this engine empty is 31 tons; of the
tender 8-1/2 tons; and the total weight of the engine when loaded is 50
tons. In one of Crampton's locomotives, the Liverpool, with one set more of
carrying wheels than the fig., the cylinders are of 24 inches diameter and
18 inches stroke; the driving wheels are 8 feet in diameter; the fire grate
contains 21-1/2 square feet of area; and the heating surface of the fire
box is 154 square feet. There are in all 300 tubes in the boiler of 2-3/16
inches external diameter, giving a surface in the tubes of 2136 square
feet, and a total heating surface of 2290 square feet. The weight of this
engine is stated to be 35 tons when ready to proceed on a journey. Both
engines were displayed at the Great Exhibition in 1851, as examples of the
most powerful locomotive engines then made. The weight of such engines is
very injurious to the railway; bending, crushing, and disturbing the rails,
and trying very severely the whole of the railway works. No doubt the
weight may be distributed upon a greater number of wheels, but if the
weight resting on the driving wheels be much reduced, they will not have
sufficient bite upon the rails to propel the train without slipping. This,
however, is only one of the evils which the demand for high rates of speed
has produced. The width of the railway, or, as it is termed, the _gauge_ of
the rails, being in most of the railways in this kingdom limited to 4 feet
8-1/2 inches, a corresponding limitation is imposed on the diameter of the
boiler; which in its turn restricts the number of the tubes which can be
employed. As, however, the attainment of a high rate of speed requires much
power, and consequently much heating surface in the boiler, and as the
number of tubes cannot be increased without reducing their diameter, it has
become necessary, in the case of powerful engines, to employ tubes of a
small diameter, and of a great length, to obtain the necessary quantity of
heating surface; and such tubes require a very strong draught in the
chimney to make them effective. With a draught of the usual intensity the
whole of the heat will be absorbed in the portion of the tube nearest the
fire box, leaving that portion nearest the smoke box nothing to do but to
transmit the smoke; and with long tubes of small diameter, therefore, a
very strong draught is indispensable. To obtain such a draught in
locomotives, it is necessary to contract the mouth of the blast pipe,
whereby the waste steam will be projected into the chimney with greater
force; but this contraction involves an increase of the pressure on the
eduction side of the piston, and consequently causes a diminution in the
power of the engine. Locomotives with small and long tubes, therefore, will
require more coke to do the same work than locomotives in which larger and
shorter tubes may be employed.




CHAPTER II.

HEAT, COMBUSTION, AND STEAM.



HEAT.

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

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

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

_A._--I mean to express the heat consumed in accomplishing the vaporization
compared with that necessary for producing the temperature. The latent heat
of steam is usually reckoned at about 1000 degrees, by which it is meant
that there is as much heat in any given weight of steam as would raise its
constituent water 1000 degrees if the expansion of the water could be
prevented, or as would raise 1000 times that quantity of water one degree.
The boiling point of water, being 212 degrees, is 180 degrees above the
freezing point of water--the freezing point being 32 degrees; so that it
requires 1180 times as much heat to raise 1 lb. of water into steam, as to
raise 1180 lbs. of water one degree; or it requires about as much heat to
raise a pound of boiling water into steam, as would raise 5-1/2 lbs. of
water from the freezing to the boiling point; 5-1/2 multiplied by 180 being
990 or 1000 nearly.

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

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

Pressure.          Temperature.             Total Heat.      Latent Heat.
  15 lbs.            213.1°                  1178.9°           965.8°
  50                 281.0                   1199.6            918.6
 100                 327.8                   1213.9            886.1

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

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

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

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

_A._--If any body has only half the specific heat of water, then a pound of
that body will, at any given temperature, have only half the heat in it
that is in a pound of water at the same temperature. The specific heat of
air is .2669, that of water being 1; or it is 3.75 times less than that of
water. An amount of heat, therefore, which would raise a pound of water 1
degree would raise a pound of air 3.75 degrees.



COMBUSTION.

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

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

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

_A._--Air consists of oxygen and nitrogen mixed together in the proportion
of 3.29 lbs. of nitrogen to 1 lb. of oxygen. Every pound of coal requires
about 2.66 lbs. of oxygen for its saturation, and therefore for every pound
of coal burned, 8.75 pounds of nitrogen must pass through the fire,
supposing all the oxygen to enter into combination. In practice, however,
this perfection of combination does not exist; from one-third to one-half
of the oxygen will pass through the fire without entering into combination
at all; so that from 16 to 18 lbs. of air are required for every pound of
coal burned. 18 lbs. of air are about 240 cubic feet, which may be taken as
the quantity of air required for the combustion of a pound of coal in
practice.

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

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

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

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

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

_A._--Looking to the quantity of oxygen required to unite chemically with
the various constituents of the coal, we find for example that in 100 lbs.
of anthracite coal, consisting of 91.44 lbs. of carbon, and 3.46 lbs. of
hydrogen, we shall for the 91.44 lbs. of carbon require 243.84 lbs. of
oxygen--since to saturate a pound of carbon by the formation of carbonic
acid, requires 2-2/3 lbs. of oxygen. To saturate a pound of hydrogen in the
formation of water, requires 8 lbs. of oxygen; hence 3.46 Fibs. of hydrogen
will take 27.68 lbs. of oxygen for its saturation. If then we add 243.84
lbs. to 27.68 lbs. we have 271.52 lbs. of oxygen required for the
combustion of 100 lbs. of coal. A given weight of air contains nearly 23.32
per cent of oxygen; hence to obtain 271.52 lbs. of oxygen, we must have
about four times that quantity of atmospheric air, or more accurately, 1164
lbs. of air for the combustion of 100 lbs. of coal. A cubic foot of air at
ordinary temperature weighs about .075 lbs.; so that 100 lbs. of coal
require 15,524 cubic feet of air, or 1 lb. of coal requires about 155 cubic
feet of air, supposing every atom of the oxygen to enter into combination.
If, then, from one-third to one-half of the air passes unconsumed through
the fire, an allowance of 240 cubic feet of air for each pound of coal will
be a small enough allowance to answer the requirements of practice, and in
some cases as much as 300 cubic feet will be required,--the difference
depending mainly on the peculiar configuration of the furnace.

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

_A._--The evaporative efficacy of a pound of carbon has been found
experimentally to be equivalent to that necessary to raise 14,000 lbs. of
water through 1 degree, or 14 lbs. of water through 1000 degrees, supposing
the whole heat generated to be absorbed by the water. Now, if the water be
raised into steam from a temperature of 60°, then 1118.9° of heat will have
to be imparted to it to convert it into steam of 15 lbs. pressure per
square inch. 14,000 / 1118.9 = 12.512 Lbs. will be the number of pounds of
water, therefore, which a pound of carbon can raise into steam of 15 lbs.
pressure from a temperature of 60°. This, however, is a considerably larger
result than can be expected in practice.

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

_A._--The evaporative powers of different coals appear to be nearly
proportional to the quantity of carbon in them; and bituminous coal is,
therefore, less efficacious than coal consisting chiefly of pure carbon. A
pound of the best Welsh or anthracite coal is capable of raising from 9-1/2
to 10 lbs. of water from 212° into steam, whereas a pound of the best
Newcastle is not capable of raising more than about 8-1/2 lbs. of water
from 212° into steam; and inferior coals will not raise more than 6-1/2
lbs. of water into steam. In America it has been found that 1 lb. of the
best coal is equal to 2-1/2 lbs. of pine wood, or, in some cases to 3 lbs.;
and a pound of pine wood will not usually evaporate more than about 2 1/2
lbs. of water, though, by careful management, it may be made to evaporate 4
1/2 lbs. Turf will generate rather more steam than wood. Coke is equal or
somewhat superior to the best coal in evaporative effect.

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

_A._--From 6 to 8 lbs. of water in the generality of land boilers of medium
quality, the difference depending on the kind of boiler, the kind of coal,
and other circumstances. Mr. Watt reckoned his boilers as capable of
evaporating 10.08 cubic feet of water with a bushel or 84 lbs. of Newcastle
coal, which is equivalent to 7 1/2 lbs. of water evaporated by 1 lb. of
coal, and this may be taken as the performance of common land boilers at
the present time. In some of the Cornish boilers, however, a pound of coal
raises 11.8 lbs. of boiling water into steam, or a cwt. of coal evaporates
about 21 cubic feet of water from 212°.

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

_A._--The coals should be broken up into small pieces, and sprinkled thinly
and evenly over the fire a little at a time. The thickness of the stratum
of coal upon the grate should depend upon the intensity of the draught: in
ordinary land or marine boilers it should be thin, whereas in locomotive
boilers it requires to be much thicker. If the stratum of coal be thick
while the draught is sluggish, the carbonic acid resulting from combustion
combines with an additional atom of carbon in passing through the fire, and
is converted into carbonic oxide, which may be defined to be invisible
smoke, as it carries off a portion of the fuel: if, on the contrary, the
stratum of coal be thin while the draught is very rapid, an injurious
refrigeration is occasioned by the excess of air passing through the
furnace. The fire should always be spread of uniform thickness over the
bars of the grate, and should be without any holes or uncovered places,
which greatly diminish the effect of the fuel by the refrigeratory action
of the stream of cold air which enters thereby. A wood fire requires to be
about 6 inches thicker than a coal one, and a turf fire requires to be 3 or
4 inches thicker than a wood one, so that the furnace bars must be placed
lower where wood or turf is burned, to enable the surface of the fire to be
at the same distance from the bottom of the boiler.

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

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

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

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

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

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

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

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

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

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

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

_A._--They are too various for enumeration, but most of them either operate
upon the principle of admitting air into the flues to accomplish the
combustion of the uninflammable parts of the smoke, or seek to attain the
same object by passing the smoke over or through the fire or other
incandescent material. Some of the plans, indeed, profess to burn the
inflammable gases as they are evolved from the coal, without permitting the
admixture of any of the uninflammable products of combustion which enter
into the composition of smoke; but this object has been very imperfectly
fulfilled in any of the contrivances yet brought under the notice of the
public, and in some cases these contrivances have been found to create
weightier evils than they professed to relieve.

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

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

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

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

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

_A._--The whole peculiarity is in the furnace door. The front of the door
consists of metal Venetians, which are opened when the top lever is lifted
up, and shut when that lever descends to its lowest position. When the
furnace door is opened to replenish the fire with coals, the top lever is
raised up, and with it the piston of the small cylinder attached to the
side of the furnace. The Venetians are thereby opened, and a stream of air
enters the furnace, which, being heated in its passage among the numerous
heated plates attached to the back of the furnace door, is in a favorable
condition for effecting the combustion of the inflammable parts of the
smoke. The piston in the small cylinder gradually subsides and closes the
Venetians; and the rate of the subsidence of the piston may obviously be
regulated by a cock, or, as in this case, a small screw valve, so that the
Venetians shall just close when there is no more smoke to be consumed;--the
air or other fluid within the cylinder being forced out by the piston in
its descent.

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

_A._--He tried various methods, but eventually fixed upon the method of
coking the coal on a dead plate at the furnace door, before pushing it into
the fire. That method is perfectly effectual where the combustion is so
slow that the requisite time for coking is allowed, and it is much
preferable to any of the methods of admitting air at the bridge or
elsewhere, to accomplish the combustion of the inflammable parts of the
smoke.

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

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

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

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

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

_A._--The fire grate is made like a round table capable of turning
horizontally upon a centre; a shower of coal is precipitated upon the grate
through a slit in the boiler near the furnace mouth, and the smoke evolved
from the coal dropped at the front part of the fire is consumed by passing
over the incandescent fuel at the back part, from which all the smoke must
have been expelled in the revolution of the grate before it can have
reached that position.

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

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

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

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

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

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

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

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



STEAM.

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

_A._--Yes; very careful experiments. The following rule expresses the
results obtained by Mr. Southern:--To the given temperature in degrees of
Fahrenheit add 51.3 degrees; from the logarithm of the sum, subtract the
logarithm of 135.767, which is 2.1327940; multiply the remainder by 5.13,
and to the natural number answering to the sum, add the constant
fraction .1, which will give the elastic force in inches of mercury. If the
elastic force be known, and it is wanted to determine the corresponding
temperature, the rule must be modified thus:--From the elastic force, in
inches of mercury, subtract the decimal .1, divide the logarithm of the
remainder by 5.13, and to the quotient add the logarithm 2.1327940; find
the natural number answering to the sum, and subtract therefrom the
constant 51.3; the remainder will be the temperature sought. The French
Academy, and the Franklin Institute, have repeated Mr. Southern's
experiments on a larger scale; the results obtained by them are not widely
different, and are perhaps nearer the truth, but Mr. Southern's results are
generally adopted by engineers, as sufficiently accurate for practical
purposes.

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

_A._--Yes, the experiments of M. Regnault upon this subject have been very
elaborate and very carefully conducted, and the results are probably more
accurate than have been heretofore obtained. Nevertheless, it is
questionable how far it is advisable to disturb the rules of Watt and
Southern, with which the practice of engineers is very much identified, for
the sake of emendations which are not of such magnitude as to influence
materially the practical result. M. Regnault has shown that the total
amount of heat, existing in a given weight of steam, increases slightly
with the pressure, so that the sum of the latent and sensible heats do not
form a constant quantity. Thus, in steam of the atmospheric pressure, or
with 14.7 Lbs. upon the square inch, the sensible heat of the steam is 212
degrees, the latent heat 966.6 degrees, and the sum of the latent and
sensible heats 1178.6 degrees; whereas in steam of 90 pounds upon the
square inch the sensible heat is 320.2 degrees, the latent heat 891.4
degrees, and the sum of the latent and sensible heats 1211.0 degrees. There
is, therefore, 33 degrees less of heat in any given weight of water, raised
into steam of the atmospheric pressure, than if raised into steam of 90
Lbs.[1] pressure.

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

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

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

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

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

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

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

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

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

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

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

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

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

_A._--The same as that followed by air, in which the increments in volume
are very nearly in the same proportion as the increments in temperature;
and the increment in volume for each degree of increased temperature is
1/490th part of the volume at 32°. A volume of air which, at the
temperature of 32°, occupies 100 cubic feet, will at 212° fill a space of
136.73 cubic feet. The volume which air or steam--out of contact with
water--of a given temperature acquires by being heated to a higher
temperature, the pressure remaining the same, may be found by the following
rule:--To each of the temperatures before and after expansion, add the
constant number 458: divide the greater sum by the less, and multiply the
quotient by the volume at the lower temperature; the product will give the
expanded volume.

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

_A._--Yes; at the atmospheric pressure, about a cubic inch of water has to
be supplied to the boiler for every cubic foot of steam abstracted; at
other pressures, the relative bulk of water and steam may be determined as
follows:--To the temperature of steam in degrees of Fahrenheit, add the
constant number 458, multiply the sum by 37.3, and divide the product by
the elastic force of the steam in pounds per square inch; the quotient will
give the volume required.

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

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

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

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


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




CHAPTER III.

EXPANSION OF STEAM AND ACTION OF THE VALVES.


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

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

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

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

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

_A._--If the steam be shut off at half stroke, the pressure of the steam,
reckoning the total pressure both below and above the atmosphere, will just
be one-half of what it was at the beginning of the stroke. It is a well
known law of pneumatics, that the pressure of elastic fluids varies
inversely as the spaces into which they are expanded or compressed. For
example, if a cubic foot of air of the atmospheric density be compressed
into the compass of half a cubic foot, its elasticity will be increased
from 15 lbs. on the square inch to 30 lbs. on the square inch; whereas, if
its volume be enlarged to two cubic feet, its elasticity will be reduced to
7-1/2 lbs. on the square inch, being just half its original pressure. The
same law holds in all other proportions, and with all other gases and
vapors, provided their temperature remains unchanged; and if the steam
valve of an engine be closed, when the piston has descended through
one- fourth of the stroke, the steam within the cylinder will, at the end
of the stroke, just exert one-fourth of its initial pressure.

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

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

_A._--Precisely so. Thus in the accompanying figure, (fig. 32), let E be a
cylinder, J the piston, _a_ the steam pipe, _c_ the upper port, _f_ the
lower port, _d_ the steam pipe, prolonged to _e_ the equilibrium valve, _g_
the eduction valve, M the steam jacket, N the cylinder cover, O stuffing
box, _n_ piston rod, P cylinder bottom; let the cylinder be supposed to be
divided in the direction of its length into any number of equal parts, say
twenty, and let the diameter of the cylinder represent the pressure of the
steam, which, for the sake of simplicity, we may take at 10 lbs., so that
we may divide the cylinder, in the direction of its diameter, into ten
equal parts. If now the piston be supposed to descend through five of the
divisions, and the steam valve then be shut, the pressure at each
subsequent position of the piston will be represented by a series, computed
according to the laws of pneumatics, and which, if the initial pressure be
represented by 1, will give a pressure of .5 at the middle of the stroke,
and .25 at the end of it.

If this series be set off on the horizontal lines, it will mark out a
hyperbolic curve--the area of the part exterior to which represents the
total efficacy of the stroke, and the interior area, therefore, represents
the diminution in the power of a stroke, when the steam is cut off at
one- fourth of the descent. If the squares above the point, where the steam
is cut off, be counted, they will be found to amount to 50; and if those
beneath that point be counted or estimated, they will be found to amount to
about 69. These squares are representative of the power exerted; so that
while an amount of power represented by 50 has been obtained by the
expenditure of a quarter of a cylinder full of steam, we get an amount of
power represented by 69, without any expenditure of steam at all, merely by
permitting the steam first used to expand into four times its original
volume.

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

_A._--Yes. The efficacy of a given quantity of steam is more than doubled
by expanding the steam four times, while the efficacy of each stroke is
made nearly one-half less. And, therefore, to carry out the expansive
principle in practice, the cylinder requires to be larger than usual, or
the piston faster than usual, in the proportion in which the expansion is
carried out. Every one who is acquainted with simple arithmetic, can
compute the terminal pressure of steam in a cylinder, when he knows the
initial pressure and the point at which the steam is cut off; and he can
also find, by the same process, any pressure intermediate between the first
and the last. By setting down these pressures in a table, and taking their
mean, he can determine the effect, with tolerable accuracy, of any
particular measure of expansion. It is necessary to remark, that it is the
total pressure of the steam that he must take; not the pressure above the
atmosphere, but the pressure above a perfect vacuum.

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

_A._--Divide the length of stroke through which the steam expands, by the
length of stroke performed with full pressure, which last call 1; the
hyperbolic logarithm of the quotient is the increase of efficiency due to
expansion. According to this rule it will be found, that if a given
quantity of steam, the power of which working at full pressure is
represented by 1, be admitted into a cylinder of such a size that its
ingress is concluded when one-half the stroke has been performed, its
efficacy will be raised by expansion to 1.69; if the admission of the steam
be stopped at one-third of the stroke, the efficacy will be 2.10; at
one- fourth, 2.39; at one-fifth, 2.61; at one-sixth, 2.79; at one-seventh,
2.95; at one-eighth, 3.08. The expansion, however, cannot be carried
beneficially so far as one-eighth, unless the pressure of the steam in the
boiler be very considerable, on account of the inconvenient size of
cylinder or speed of piston which would require to be adopted, the
friction of the engine, and the resistance of vapor in the condenser, which
all become relatively greater with a smaller urging force.

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

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

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

[Illustration: Fig. 33.]

_A._--Yes; and of the slide valve there are many varieties; but the kinds
most in use are the D valve,--so called from its resemblance to a half
cylinder or D in its cross section--and the three ported valve, shown in
fig. 33, which consists of a brass or iron box set over the two ports or
openings into the cylinder, and a central port which conducts away the
steam to the atmosphere or condenser; but the length of the box is so
adjusted that it can only cover one of the cylinder ports and the central
or eduction port at the same time. The effect, therefore, of moving the
valve up and down, as is done by the eccentric, is to establish a
connection alternately between each cylinder port and the central passage
whereby the steam escapes; and while the steam is escaping from beneath the
piston, the position of the valve is such, that a free communication exists
between the space above the piston and the steam in the boiler. The piston
is thus urged alternately up and down--the valve so changing its position
before the piston arrives at the end of the stroke, that the pressure is by
that time thrown on the reverse side of the piston, so as to urge it into
motion in the opposite direction.

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

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

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

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

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

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

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

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

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

_A._--The common stroke of the valve in rotative engines is twice the
breadth or depth of the port, and the length of the valve face will then be
just the breadth of the port when there is lap on neither the steam nor
eduction side. Whatever lap is given, therefore, makes the valve face just
so much longer. In some engines, however, the stroke of the valve is a good
deal more than twice the breadth of the port; and it is to the stroke of
the valve that the amount of lap should properly be referred.

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

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

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

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

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

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

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

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

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

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

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

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

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

_A._--No; the usual way of cutting off the steam is by means of a separate
valve, termed an expansion valve; but such a device appears to be hardly
necessary in ordinary engines. In the Cornish engines, where the steam is
cut off in some cases at one-twelfth of the stroke, a separate valve for
the admission of steam, other than that which permits its escape, is of
course indispensable; but in common rotative engines, which may realize
expansive efficacy by throttling, a separate expansion valve does not
appear to be required.

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

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

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

[Illustration: Fig 34.]

_A._--The structure of expansion apparatus is very various; but all the
kinds operate either on the principle of giving such a motion to the slide
valve as will enable it to cut off the steam, at the desired point, or on
the principle of shutting off the steam by a separate valve in the steam
pipe or valve casing. The first class of apparatus has not been found so
manageable, and is not in extensive use, except in that form known as the
link motion. Of the second class, the most simple probably is the
application of a cam giving motion to the throttle valve, or to a valve of
the same construction, which either accurately fits the steam pipe, or
which comes round to a face, which, however, it is restrained from touching
by a suitable construction of the cam. A kind of expansion valve, often
employed in marine engines of low speed, is the kind used in the Cornish
engines, and known as the equilibrium valve. This valve is represented in
fig. 34. It consists substantially of an annulus or bulging cylinder of
brass, with a steam-tight face both at its upper and lower edges, at which
points it fits accurately upon a stationary seat. This annulus may be
raised or lowered without being resisted by the pressure of the steam, and
in rotative engines it is usually worked by a cam on the shaft. The
expansion cam is put on the shaft in two pieces, which are fastened to each
other by means of four bolts passing through lugs, and is fixed to the
shaft by keys. A roller at one end of a bell-crank lever, which is
connected with the expansion valve, presses against the cam, so that the
motion of the lever will work the valve. The roller is kept against the cam
by a weight on a lever attached to the same shaft, but a spring is
necessary for high speeds. If the cam were concentric with the shaft, the
lever which presses upon it would remain stationary, and also the expansion
valve; but by the projection of the cam, the end of the lever receives a
reciprocating motion, which is communicated to the valve.

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

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

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

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

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

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

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

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

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

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

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

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

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

[Illustration: Fig. 35.]

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

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

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

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

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

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

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


[1] In 1838 I patented an arrangement of expansion valve, consisting of two
movable plates set upon the ordinary slide valve, and which might be drawn
together or asunder by means of a right and left handed screw passing
through both plates. The valve spindle was hollow, and a prolongation of
the screw passed up through it, and was armed on the top with a small
wheel, by means of which the plates might be adjusted while the engine was
at work. In 1839 I fitted an expansion valve in a steam vessel, consisting
of two plates, connected by a rod, and moved by tappets up against the
steam edges of the valve. In another steam vessel I fitted the same species
of valve, but the motion was not derived from tappets, but from a moving
part of the engine, though at the moderate speed at which these engines
worked I found tappets to operate well and make little noise. In 1837 I
employed, as an expansion valve, a rectangular throttle valve, accurately
fitting a bored out seat, in which it might be made to revolve, though it
did not revolve in working. This valve was moved by a pin in a pinion,
making two revolutions for every revolution of the engine, and the
configuration of the seat determined the amount of the expansion. In 1855 I
have again used expansion valves of this construction in engines making one
hundred revolutions per minute, and with perfectly satisfactory results.--
J.B.




CHAPTER IV.

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


HORSES POWER.

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

_A._--An amount of mechanical force that will raise 33,000 lbs. one foot
high in a minute. This standard was adopted by Mr. Watt, as the average
force exerted by the strongest London horses; the object of his
investigation being to enable him to determine the relation between the
power of a certain size of engine and the power of a horse, so that when it
was desired to supersede the use of horses by the erection of an engine, he
might, from the number of horses employed, determine the size of engine
that would be suitable for the work.

210. _Q._--Then when we talk of an engine of 200 horse power, it is meant
that the impelling efficacy is equal to that of 200 horses, each lifting
33,000 lbs. one foot high in a minute?

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

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

_A._--Yes; some raise 52,000 lbs., others 60,000 lbs., and others 66,000
lbs., one foot high in a minute by each nominal horse power. Some engines
indeed work as high as five times above the nominal power, and therefore no
comparison can be made between the performances of different engines,
unless the power actually exerted be first discovered.

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

_A._--By means of an instrument called the indicator, which is a miniature
cylinder and piston attached to the cylinder cover of the main engine, and
which indicates, by the pressure exerted on a spring, the amount of
pressure or vacuum existing within the cylinder. From this pressure,
expressed in pounds per square inch, deduct a pound and a half of pressure
for friction, the loss of power in working the air pump, &c.; multiply the
area of the piston in square inches by this residual pressure, and by the
motion of the piston, in feet per minute, and divide by 33,000; the
quotient is the actual number of horses power of the engine. The same
result is attained by squaring the diameter of the cylinder, multiplying by
the pressure per square inch, as shown by the indicator, less a pound and a
half, and by the motion of the piston, in feet per minute, and dividing by
42,017.

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

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

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

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

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

_A._--The substitution is quite practicable, and will accomplish some
simplification, as the speed of piston proper for the different lengths of
stroke cannot always be remembered. The rule for the nominal power of
condensing engines when thus arranged, will be as follows: multiply the
square of the diameter of the cylinder in inches by the cube root of the
stroke in feet, and divide the product by 47; the quotient is the number of
nominal horses power of the engine, supposing it to be of the ordinary
condensing description. This rule assumes the existence of a uniform
effective pressure upon the piston of 7 lbs. per square inch; Mr. Watt
estimated the effective pressure upon the piston of his 4 horse power
engines at 6-8 lbs. per square inch, and the pressure increased slightly
with the power, and became 6.94 lbs. per square inch in engines of 100
horse power; but it appears to be more convenient to take a uniform
pressure of 7 lbs. for all powers. Small engines, indeed, are somewhat less
effective in proportion than large ones, but the difference can be made up
by slightly increasing the pressure in the boiler; and small boilers will
bear such an increase without inconvenience.

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

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

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

_A._--The nominal horse power of a high pressure engine has never been
defined; but it should obviously hold the same relation to the actual power
as that which obtains in the case of condensing engines, so that an engine
of a given nominal power may be capable of performing the same work,
whether high pressure or condensing. This relation is maintained in the
following rule, which expresses the nominal horse power of high pressure
engines: multiply the square of the diameter of the cylinder in inches by
the cube root of the length of stroke in feet, and divide the product by
15.6. This rule gives the nominal power of a high pressure engine three
times greater than that of a low pressure engine of the same dimensions;
the average effective pressure being taken at 21 lbs. per square inch
instead of 7 lbs., and the speed of the piston in feet per minute being in
both rules 128 times the cube root of the length of stroke.[1]

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

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

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

_A._--The most feasible way of enabling condensing engines to work
satisfactorily at a high speed, appears to lie in the application of
balance weights to the engine, so as to balance the momentum of its moving
parts, and the engine must also be made very strong and rigid. It appears
to be advisable to perform the condensation partly in the air pump, instead
of altogether in the condenser, as a better vacuum and a superior action of
the air pump valves will thus be obtained. Engines constructed upon this
plan may be driven at four times the speed of common engines, whereby an
engine of large power may be purchased for a very moderate price, and be
capable of being put into a very small compass; while the motion, from
being more equable, will be better adapted for most purposes for which a
rotary motion is required. Even for pumping mines and blowing iron
furnaces, engines of this kind appear likely to come into use, for they are
more suitable than other engines for driving the centrifugal pump, which in
many cases appears likely to supersede other kinds of pumps for lifting
water; and they are also conveniently applicable to the driving of fans,
which, when so arranged that the air condensed by one fan is employed to
feed another, and so on through a series of 4 or 5, have succeeded in
forcing air into a furnace with a pressure of 2-1/2 lbs. on the square
inch, and with a far steadier flow than can be obtained by a blast engine
with any conceivable kind of compensating apparatus. They are equally
applicable if blast cylinders be employed.

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

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

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

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

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

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

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

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

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

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

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

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


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

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




DUTY OF ENGINES AND BOILERS.

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

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

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

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

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

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

_A._--Yes, if the power given be the actual, and not the nominal, power.
Divide 166.32 by the number of pounds of coal consumed per actual horse
power per hour; the quotient is the duty in millions of pounds. If you
already have the duty in millions of pounds, and wish to know the
equivalent consumption in pounds per actual horse power per hour, divide
166.32 by the duty in millions of pounds; the quotient is the consumption
per actual horse power per hour. The duty of a locomotive engine is
expressed by the weight of coke it consumes in transporting a ton through
the distance of one mile upon a railway; but this is a very imperfect
method of representing the duty, as the tractive efficacy of a pound of
coke becomes less as the speed of the locomotive becomes greater; and the
law of variation is not accurately known.

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

_A._--The duty of different kinds of engines varies very much, and there
are also great differences in the performance of different engines of the
same class. In ordinary rotative condensing engines of good construction,
10 lbs. of coal per nominal horse power per hour is a common consumption;
but such engines exert nearly twice their nominal power, so that the
consumption per actual horse power per hour may be taken at from 5 to 6
lbs. Engines working very expansively, however, attain an economy much
superior to this. The average duty of the pumping engines in Cornwall is
about 60,000,000 lbs. raised 1 ft. high by a bushel of Welsh coals, which
weighs 94 lbs. This is equivalent to a consumption of 3.1 lbs. of coal per
actual horse power per hour; but some engines reach a duty of above
100,000,000, or 1.74 lbs. of coal per actual horse power per hour.
Locomotives consume from 8 to 10 lbs. of coke in evaporating a cubic foot
of water, and the evaporation of a cubic foot of water per hour may be set
down as representing an actual horse power in locomotives as well as in
condensing engines, if expansion be not employed. When the locomotive is
worked expansively, however, there is of course a less consumption of water
and fuel per horse power, or per ton per mile, than when the full pressure
is used throughout the stroke; and most locomotives now operate with as
much expansion as can be conveniently given by the slide valves.

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

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

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

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

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

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

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

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



THE INDICATOR.

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

[Illustration: Fig. 36]

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

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

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

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

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

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

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

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

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

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

[Illustration: Fig. 37]

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

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

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

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

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

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




DYNAMOMETER, GAUGES, AND CATARACT.


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

_A._--There is the counter for telling the number of strokes the engine
makes, and the dynamometer for ascertaining the tractive power of steam
vessels or locomotives; then there are the gauge cocks, and glass tubes, or
floats, for telling the height of water in the boiler; and in pumping
engines there is the cataract for regulating the speed of the engine.

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

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

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

_A._--The dynamometer employed for ascertaining the traction upon railways
consists of two flat springs joined together at the ends by links, and the
amount of separation of the springs at the centre indicates, by means of a
suitable hand and dial, the force of traction. A cylinder of oil, with a
small hole through its piston, is sometimes added to this instrument to
prevent sudden fluctuations. In screw vessels the forward thrust of the
screw is measured by a dynamometer constructed on the principle of a
weighing machine, in which a small spring pressure at the index will
balance a very great pressure where the thrust is employed; and in each
case the variations of pressure are recorded by a pencil on a sheet of
paper, carried forward by suitable mechanism, whereby the mean thrust is
easily ascertained. The tractive force of paddle wheel steamers is
ascertained by a dynamometer fixed on shore, to which the floating vessel
is attached by a rope. Sometimes the power of an engine is ascertained by a
friction break dynamometer applied to the shaft.

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

_A._--In locomotives and in paddle steamers it will be determined by the
force turning the wheels, and by the smallness of the diameter of the
wheels; for with small wheels the thrust will be greater than with large
wheels. In screw vessels the thrust will be determined by the force turning
round the screw, and by the smallness of the screw's pitch; for with any
given force of torsion a fine pitch of screw will give a greater thrust
than a coarse pitch of screw, just as is the case when a screw works in a
solid nut.

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

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

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

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

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

_A._--A float for telling the height of water in the boiler is employed
only in the case of land boilers, and its action is like that of a buoy
floating on the surface, which, by means of a light rod passing vertically
through the boiler, shows at what height the water stands. The float is
usually formed of stone or iron, and is so counterbalanced as to make its
operation the same as if it were a buoy of timber; and it is generally put
in connection with the feed valve, so that in proportion as the float
rises, the supply of feed water is diminished. The feed water in land
boilers is admitted from a small open cistern, situated at the top of an
upright or stand pipe set upon the boiler, and in which there is a column
of water sufficiently high to balance the pressure of the steam.

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

[Illustration: Fig. 38. ]

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

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

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

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

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




CHAPTER V.


PROPORTION OF BOILERS.


HEATING AND FIRE GRATE SURFACE.

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

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

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

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

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

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

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

_A._--About 9 square feet of flue and furnace surface per horse power is
the usual proportion in wagon boilers, reckoning the total surface as
effective surface, if the boilers be of a considerable size; but in the
case of small boilers the proportion is larger. The total heating surface
of a two horse power wagon boiler is, according to Boulton and Watt's
proportions, 30 square feet, or 15 ft. per horse power; whereas, in the
case of a 45 horse power boiler the total heating surface is 438 square
feet, or 9.6 ft. per horse power. In marine boilers nearly the same
proportions obtain. The original boilers of the Great Western steamer, by
Messrs. Maudslay, were proportioned with about 10 square feet of flue and
furnace surface per horse power, reckoning the total amount as effective;
but in the boilers of the Retribution, by the same makers, but of larger
size, a somewhat smaller proportion of heating surface was adopted. Boulton
and Watt have found that in their marine flue boilers, 9 square feet of
flue and furnace surface are requisite to boil off a cubic foot of water
per hour, which is the proportion of heating surface that is allowed in
their land boilers per horse power; but inasmuch as in most modern engines,
and especially in marine engines, the nominal considerably exceeds the
actual power, they allow 11 or 12 square feet of heating surface per
nominal horse power in their marine boilers, and they reckon as effective
heating surface the tops of the flues, and the whole of the sides of the
flues, but hot the bottoms. For their land engines they still retain Mr.
Watt's standard of power, which makes the actual and the nominal power
identical; and an actual horse power is the equivalent of a cubic foot of
water raised into steam every hour.

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

_A._--Boulton and Watt allow 0.64 of a square foot area of grate bars per
nominal horse power in their marine boilers, and a good effect arises from
this proportion; but sometimes so large an area of fire grate cannot be
conveniently got, and the proportion of half a square foot per horse power,
which is the proportion adopted in the original boiler of the Great
Western, seems to answer very well in engines working with a moderate
pressure, and with some expansion; and this proportion is now very widely
adopted. With this allowance, there will be 22 to 24 square feet of heating
surface per square foot of fire grate; and if the consumption of fuel be
taken at 6 lbs. per nominal horse power per hour, there will be about 12
lbs. of coal consumed per hour on each square foot of grate. The furnaces
should not be more than 6 ft. long, as, if much longer than this, it will
be impossible to work them properly for any considerable length of time, as
they will become choked with clinker at the back ends.

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

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



CALORIMETER AND VENT.

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

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

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

_A._--It is Boulton and Watt's method; but some very satisfactory boilers
have been made by allowing a proportion of 0.6 of a square foot of fire
grate per nominal horse power, and making the sectional area of the flue at
the largest part 1/7th of the area of fire grate, and at the smallest part,
where it enters the chimney, 1/11th of the area of the fire grate. These
proportions are retained whether the boiler is flue or tubular, and from 14
to 16 square feet of tube surface is allowed per nominal horse power.

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

_A._--No. In wagon and tubular boilers very different proportions prevail,
yet the proportions of every kind of boiler are determinable on the same
general principle. In wagon boilers the proportion of the perimeter of the
flue which is effective as heating surface, is to the total perimeter as 1
to 3, or, in some cases as 1 to 2.5; and with any given area of flue,
therefore, the length of the flue must be from 3 to 2.5 times greater than
would be necessary if the total surface were effective, else the requisite
quantity of heating surface will not be obtained. If, then, the vent be the
calorimeter, divided by the length, and the length be made 3 or 2.5 times
greater, the vent must become 3 or 2.5 times less; and in wagon boilers
accordingly, the vent varies from 8 to 11 instead of from 21 to 25, as in
the case of marine flue boilers. In tubular marine boilers the calorimeter
is usually made only about half the amount allowed by Boulton and Watt for
marine flue boilers, or, in other words, the collective sectional area of
the tubes, for the transmission of the smoke, is from 8 to 9 square inches
per nominal horse power. It is better, however, to make the sectional area
larger than this, and to work the boiler with the damper sufficiently
closed to prevent the smoke and flame from rushing exclusively through a
few of the tubes.

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

_A._--In Boulton and Watt's 45 horse wagon boiler the area of flue is 18
square inches per horse power, but the area per horse power increases very
rapidly as the size of the boiler becomes less, and amounts to about 80
square inches per horse power in a boiler of 2 horse power. Some such
increase is obviously inevitable, if a similar form of flue be retained in
the larger and smaller powers, and at the same time the elongation of the
flue in the same proportion as the increase of any other dimension is
prevented; but in the smaller class of wagon boilers the consideration of
facility of cleaning the flues is also operative in inducing a large
proportion of sectional area. Boulton and Watt's 2 horse power wagon boiler
has 30 square feet of surface, and the flue is 18 inches high above the
level of the boiler bottom, by 9 inches wide; while their 12 horse wagon
boiler has 118 square feet of heating surface, and the dimensions of the
flue similarly measured are 36 inches by 13 inches. The width of the
smaller flue, if similarly proportioned to the larger one, would be 6-1/2
inches, instead of 9 inches, and, by assuming this dimension, we should
have the same proportion of sectional area per square foot of heating
surface in both boilers. The length of flue in the 2 horse boiler is 19.5
ft., and in the 12 horse boiler 39 ft., so that the length and height of
the flue are increased in the same proportion.

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

_A._--The Nile steamer, with engines of 110 horse power by Boulton and
Watt, is supplied with steam by two boilers, which are, therefore, of 55
horses power each. The height of the flue winding within the boiler is 60
inches, and its mean width 16-1/2 inches, making a sectional area or
calorimeter of 990 square inches, or 18 square inches per horse power of
the boiler. The length of the flue is 39 ft., making the vent 25, which is
the vent proper for large boilers. In the Dee and Solway steamers, by Scott
and Sinclair, the calorimeter is only 9.72 square inches per horse power;
in the Eagle, by Caird, 11.9; in the Thames and Medway, by Maudslay, 11.34,
and in a great number of other cases it does not rise above 12 square
inches per horse power; but the engines of most of these vessels are
intended to operate to a certain extent expansively, and the boilers are
less powerful in evaporating efficacy on that account.

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

_A._--That is the main difference. The proportion which one part of the
boiler bears to another part is very similar in the cases cited, but the
proportion of boiler relatively to the size of the engine varies very
materially. Thus the calorimeter _of each boiler_ of the Dee and Solway is
1296 square inches; of the Eagle, 1548 square inches; and of the Thames and
Medway, 1134 square inches; and the length of flue is 57, 60, and 52 ft. in
the boilers respectively, which makes the respective vents 22-1/2, 25, and
21. Taking then the boiler of the Eagle for comparison with the boiler of
the Nile, as it has the same vent, it will be seen that the proportions of
the two are almost identical, for 990 is to 1548 as 39 is to 60, nearly;
but Messrs. Boulton and Watt would not have set a boiler like that of the
Eagle to do so much work.

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

_A._--The evaporating power varies as the square root of the area of the
flue, if the length of the flue remain the same; but it varies as the area
simply, if the length of the flue be increased in the same proportion as
its other dimensions. The evaporating power of a boiler is referable to the
amount of its heating surface, and the amount of heating surface in any
flue or tube is proportional to the product of the length of the tube and
the square root of its sectional area, multiplied by a certain quantity
that is constant for each particular form. But in similar tubes the length
is proportional to the square root of the sectional area; therefore, in
similar tubes, the amount of heating surface is proportional to the
sectional area. On this area also depends the quantity of hot air passing
through the flue, supposing the intensity of the draught to remain
unaffected, and the quantity of hot air or smoke passing through the flue
should vary in the same ratio as the quantity of surface.

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

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

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

_A._--It is clear, that with any given area of flue, to increase the
perimeter by adopting a different shape is tantamount to a diminution of
the length of the flue; and, if the perimeter be diminished, the length of
the flue must at the same time be increased, else it will be impossible to
obtain the necessary amount of heating surface. In Boulton and Watt's wagon
boilers, the sectional area of the flue in square inches per square foot of
heating surface is 5.4 in the two horse boiler; in the three horse it is
4.74; in the four horse, 4.35; six horse, 3.75; eight horse, 4.33; ten
horse, 3.96; twelve horse, 3.63; eighteen horse, 3.17; thirty horse, 2.52;
and in the forty-five horse boiler, 2.05 square inches. Taking the amount
of heating surface in the 45 horse boiler at 9 square feet per horse power,
we obtain 18 square inches of sectional area of flue per horse power, which
is also Boulton and Watt's proportion of sectional area for marine boilers
with internal flues.

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

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

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

_A._--The proper relative proportions of the parts of tubular boilers may
easily be ascertained by a reference to the settled proportions of flue
boilers; for the same general principles are operative in both cases. In
the Nile steamer each boiler of 55 horse power has about 497 square feet of
flue surface or 9 square feet per horse power, reckoning the total surface
as effective. The area of the flue, which is rectangular is 990 square
inches, therefore the area is equal to that of a tube 35-1/2 inches in
diameter; and such a tube, to have a heating surface of 497 square feet,
must be 53.4 feet or 640.8 inches in length. The length, therefore, of the
tube, will be about 18 times its diameter, and with the same velocity of
draught these proportions must obtain, whatever the absolute dimensions of
the tube may be. With a calorimeter, therefore, of 18 square inches per
horse power, the length of a tube 3 inches diameter must not exceed 4 feet
6 inches, since the heat will be sufficiently extracted from the smoke in
this length, if the smoke only travels at the velocity due to a calorimeter
of 18 square inches per horse power.

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

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



EVAPORATIVE POWER OF BOILERS.


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

_A._--The proportions of tubular boilers, as of all boilers, should
obviously have reference to the evaporation required, whereas the demand
upon the boiler for steam is very often reckoned contingent upon the
nominal horse power of the engine; and as the nominal power of an engine is
a conventional quantity by no means in uniform proportion to the actual
quantity of steam consumed, perplexing complications as to the proper
proportions of boilers have in consequence sprung up, to which most of the
failures in that department of engineering may be imputed. It is highly
expedient, therefore, in planning boilers for any particular engine, to
consider exclusively the actual power required to be produced, and to
apportion the capabilities of the boiler accordingly.

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

_A._--I would first, as I have already stated, consider the actual power
required to be produced, and then fix the amount of expansion to be
adopted. If the engine had to work up to three times its nominal power, as
is now common in marine engines, I should either increase correspondingly
the quantity of evaporating surface in the boiler, or adopt such an amount
of expansion as would increase threefold the efficacy of the steam, or
combine in a modified manner both of these arrangements. Reckoning the
evaporation of a cubic foot of water in the hour as equivalent to an actual
horse power, and allowing a square yard or 9 square feet as the proper
proportion of flue surface to evaporate a cubic foot of water in the hour,
it is clear that I must either give 27 square feet of heating surface in
the boiler to have a trebled power without expansion, or I must cut off the
steam at one seventh of the stroke to obtain a three-fold power without
increasing the quantity of heating surface. By cutting off the steam,
however, at one third of the stroke, a heating surface of 13-1/2 square
feet will give a threefold power, and it will usually be the most judicious
course to carry the expansion as far as possible, and then to add the
proportion of heating surface necessary to make good the deficiency still
found to exist.

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

_A._--An actual horse power as fixed by Watt is 33,000 lbs. raised one foot
high in the minute; and in Watt's 40 horse power engine, with a 31-1/2 inch
cylinder, 7 feet stroke, and making 17-1/2 strokes a minute, the effective
pressure is 6.92 lbs. on the square inch clear of all deductions. Now, as a
horse power is 33,000 lbs. raised one foot high, and as there are 6.92 lbs,
on the square inch, it is clear that 33,000 divided by 6.92, on 4768 square
inches with 6.92 lbs. on each if lifted 1 foot or 12 inches high, will also
be equal to a horse power. But 4768 square inches multiplied by 12 inches
in height is 57224.4 cubic inches, or 33.1 cubic feet, and this is the
quantity of steam which must be expended per minute to produce an actual
horse power.

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

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

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

_A._--In the Albion Mill engines when working without expansion, it was
found that 1 lb. of water in the shape of steam raised 28,489 lbs. 1 foot
high. A cubic foot of water, therefore, or 62-1/2 lbs., if consumed in the
hour, would raise 1780562.5 lbs. one foot high in the hour, or would raise
29,676 lbs. one foot high in a minute; and if to this we add one tenth for
waste at the ends of the cylinder, a waste which hardly exists in modern
engines, we have 32,643 lbs. raised one foot high in the minute, or a horse
power very nearly. In some cases the approximation appears still nearer.
Thus, in a 40 horse engine working without expansion, Watt found that .674
feet of water were evaporated from the boiler per minute, which is just a
cubic foot per horse power per hour; but it is not certain in this case
that the nominal and actual power were precisely identical. It will be
quite safe, however, to reckon an actual horse power as producible by the
evaporation of a cubic foot of water in the hour in the case of engines
working without expansion; and for boiling off this quantity in flue or
wagon boilers, about 8 lbs. of coal will be required and 9 square feet of
flue surface.




MODERN MARINE AND LOCOMOTIVE BOILERS.

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

_A._--In modern marine boilers the area of fire grate is less than in Mr.
Watt's original boilers, where it was one square foot to nine square feet
of heating surface. The heat in the furnace is consequently more intense,
and a somewhat less amount of surface suffices to evaporate a cubic foot of
water. In Boulton and Watt's modern flue boilers they allow for the
evaporation of a cubic foot of water 8 square feet of heating surface, 70
square inches of fire grate, 13 square inches sectional area of flues, 6
square inches sectional area of chimney, 14 square inches area over furnace
bridges, ratio of area of flue to area of fire grate 1 to 5.4. To evaporate
a cubic foot of water per hour in tubular boilers, the proportions are--
heating surface 9 square feet, fire grate 70 square inches, sectional area
of tubes 10 square inches, sectional area of back uptake 12 square inches,
sectional area of front uptake 10 square inches, sectional area of chimney
7 square inches, ratio of diameter of tube to length of tube 1/28th to
1/30th, cubical content of boiler exclusive of steam chest 6.5 cubic feet,
cubical content of steam chest 1.5 cubic feet.

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

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

                                              Name of Engine

                         Great Britain.   Pallas.      Snake.      Sphinx.
Diameter of cylinder           18 in.      15 in.     14-1/4 in.    18 in.
Length of stroke               24 in.      20 in.     21 in.        24 in.
Diameter of driving wheel       8 ft.       6 ft.     6-1/2 ft.      5 ft.
Inside diameter of fire box    53 in.      55 in.     41-1/3 in.    44 in.
Inside width of fire box       63 in.      42 in.     43-1/4 in. 39-1/2in.
Height of fire box above bars  63 in.      52 in.     48-1/3 in. 55-1/2in.
Number of fire bars            29          ...  32               16
Thickness of fire bars         3/4 in.     1-3/4 in.  5/8 in.        1 in.
Number of Tubes                305         134        181          142
Outside diameter of tubes      2 in.       2 in.      1-7/8 in.     2-1/8
                                                                      in.
Length of tubes                11 ft 3 in  10 ft 6 in 10 ft 3-1/2 in. 14
                                                              ft 3-1/4 in.
Space between tubes            1/2 in.     3/4 in.    1/2 in.
Inside diameter of ferules     1-9/16 in.  1-1/2 in.  1-5/16 in. 1-5/8 in.
Diameter of chimney            17 in.      15 in.     13 in.    15-1/2 in.
Diameter of blast orifice      5-1/2 in.    4-5/8 in. 4-1/2 in.  4-3/4 in.
Area of grate                  21 sq. ft.  16.04 sqft 12.4 sq. ft.  10.56
                                                                   sq. ft
Area of air space of grate     11.4 sqft   4.08 sqft  5.54 sq. ft.    5
                                                                   sq. ft.
Area of tubes                  5.46 sqft   2.40 sqft  2.8 sq. ft.  2.92
                                                                   sq. ft.
Area though ferules            4 sq. ft.   1.64 sqft  2 sq. ft.    2.04
                                                                   sq. ft.
Area of chimney                1.77 sqft   1.23 sqft  .921 sq. ft. 1.31
                                                                   sq. ft.
Area of blast orifice          23.76 sqin  16.8 sqin  14.18 sq. in. 17.7
                                                                   sq. in.
Heating surface of tubes       1627 sqft   668.7 sqft 823 sq. ft.  864
                                                                   sq. ft.



THE BLAST IN LOCOMOTIVES.

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

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

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

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

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

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

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

_A._--It has not been found to be so. A chimney of three or four times its
own diameter in height appears to answer fully as well as a longer one; and
it was found that when in an engine with 17 inch cylinders a chimney of
15-1/4 inches was substituted for a chimney of 17-1/2 inches, a superior
performance was the result. The chimney of a locomotive should have half
the area of the tubes at the ferules, which is the most contracted part,
and the blast orifice should have 1/10th of the area of the chimney. The
sectional area of the tubes through the ferules should be as large as
possible. Tubes without ferules it is found pass one fourth more air, and
tubes with ferules only at the smoke box end pass one tenth more air than
when there are ferules at both ends.

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

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

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

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

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

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

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

_A._--The intensity of the draught may easily be diminished by partially
closing the damper in the chimney, and it may be increased by contracting
the orifice of the blast. A variable blast pipe, the orifice of which may
be enlarged or contracted at pleasure, has been much used. There are
various devices for this purpose, but the best appears to be that adopted
in Stephenson's engine, where a conical nozzle is moved up or down within
the blast pipe, which is made somewhat larger in diameter than the base of
the cone, but with a ring projecting internally, against which the base of
the cone abuts when the nozzle is pushed up. When the nozzle stands at the
top of the pipe the whole of the steam has to pass through it, and the
intensity of the blast is increased by the increased velocity thus given to
the steam; whereas when the nozzle is moved downward the steam escapes
through the annular opening left between the nozzle and the pipe, as well
as through the nozzle itself, and the intensity of the blast is diminished
by the enlargement of the opening for the escape of the steam thus made
available.

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

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

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

_A._--In most locomotives the velocity of the draught is such that it would
require very long tubes to extract the heat from the products of
combustion, if the heat were transmitted through the metal of the tubes
with only the same facility as through the iron of ordinary flue boilers.
The Nile steamer, with engines of 110 nominal horses power each, and with
two boilers having two independent flues in each, of such dimensions as to
make each flue equivalent to 55 nominal horses power, works at 62 per cent.
above the nominal power, so that the actual evaporative efficacy of each
flue would be equivalent to 89 actual horses power, supposing the engines
to operate without expansion; but as the mean pressure in the cylinder is
somewhat less than the initial pressure, the evaporative efficacy of each
flue may be reckoned equivalent to 80 actual horses power. With this
evaporative power there is a calorimeter of 990 square inches, or 12.3
square inches per actual horse power; whereas in Stephenson's locomotive
with 150 tubes, if the evaporative power be taken at 200 cubic feet of
water in the hour, which is a large supposition, the engine will be equal
to 200 actual horses power. If the internal diameter of the tubes be taken
at thirteen eighths of an inch, the calorimeter per actual horse power will
only be 1.1136 square inches, or in other words the calorimeter in the
locomotive boiler will be 11.11 times less than in the flue boiler for the
same power, so that the draught in the locomotive must be 11.11 times
stronger, and the ratio of the length of the tube to its diameter 11.11
times greater than in the flue boiler, supposing the heat to be transmitted
with only the same facility. The flue of the Nile would require to be 35-
1/2 inches in diameter if made of the cylindrical form, and 47-3/4 feet
long; the tubes of a locomotive if 1-3/8ths inch diameter would only
require to be 22.19 inches long with the same velocity of draught; but as
the draught is 11.11 times faster than in a flue boiler, the tubes ought to
be 246.558 inches, or about 20-1/2 feet long according to this proportion.
In practice, however, they are one third less than this, which reduces the
heating surface from 9 to 6 square feet per actual horse power, and this
length even is found to be inconvenient. It is greatly preferable therefore
to increase the calorimeter, and diminish the intensity of the draught.



BOILER CHIMNEYS.

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

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

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

_A._--Boulton and Watt's rule for the dimensions of the chimney of a land
engine is as follows:--multiply the number of pounds of coal consumed under
the boiler per hour by 12, and divide the product by the square root of the
height of the chimney in feet; the quotient is the area of the chimney in
square inches in the smallest part. A factory chimney suitable for a 20
horse boiler is commonly made about 20 in. square inside, and 80 ft. high;
and these dimensions are those which answer to a consumption of 15 lbs. of
coal per horse power per hour, which is a very common consumption in
factory engines. If 15 lbs. of coal be consumed per horse power per hour,
the total consumption per hour in a 20 horse boiler will be 300 lbs., and
300 multiplied by 12 = 3600, and divided by 9 (the square root of the
height) = 400, which is the area of the chimney in square inches. It will
not answer well to increase the height of a chimney of this area to more
than 40 or 50 yards, without also increasing the area, nor will it be of
utility to increase the area much without also increasing the height. The
quantity of coal consumed per hour in pounds, multiplied by 5, and divided
by the square root of the height of the chimney, is the proper collective
area of the openings between the bars of the grate for the admission of air
to the fire.

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

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



STEAM ROOM AND PRIMING.

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

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

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

_A._--Boulton and Watt in their early steam vessels were in the habit of
allowing for the capacity of the steam, space in marine boilers 16 times
the content of the cylinder; but as there were two cylinders, this was
equivalent to 8 times the content of both cylinders, which is the
proportion commonly followed in land engines, and which agrees very nearly
with the proportion of between 5 and 6 cubic feet of steam room per horse
power already referred to. Taking for example an engine with 23 inches
diameter of cylinder and 4 feet stroke, which will be 18.4 horse power--the
area of the cylinder will be 415.476 square inches, which, multiplied by
48, the number of inches in the stroke, will give 19942.848 for the
capacity of the cylinder in cubic inches; 8 times this is 159542.784 cubic
inches, or 92.3 cubic feet; 92.3 divided by 18.4 is rather more than 5
cubic feet per horse power.

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

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

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

_A._--No; in locomotive boilers the proportion of steam room per cubic foot
of water evaporated is considerably less even than this. It does not
usually exceed 1/5 of a cubic foot per cubic foot of water evaporated; and
with clean water, with a steam dome a few feet high set on the barrel of
the boiler, or with a perforated pipe stretching from end to end of the
barrel, and with the steam room divided about equally between the barrel
and the fire box, very little priming is found to occur even with this
small proportion of total steam room. About 3/4 the depth of the barrel is
usually filled with water, and 1/4 with steam.

294. _Q._--What is priming?

_A._--Priming is a violent agitation of the water within the boiler, in
consequence of which a large quantity of water passes off with the steam in
the shape of froth or spray. Such a result is injurious, both as regards
the efficacy of the engine, and the safety of the engine and boiler; for
the large volume of hot water carried by the steam into the condenser
impairs the vacuum, and throws a great load upon the air pump, which
diminishes the speed and available power of the engine; and the existence
of water within the cylinder, unless there be safety valves upon the
cylinder to permit its escape, will very probably cause some part of the
machinery to break, by suddenly arresting the motion of the piston when it
meets the surface of the water,--the slide valve being closed to the
condenser before the termination of the stroke, in all engines with lap
upon the valves, so that the water within the cylinder is prevented from
escaping in that direction. At the same time the boiler is emptied of its
water too rapidly for the feed pump to be able to maintain the supply, and
the flues are in danger of being burnt from a deficiency of water above
them.

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

_A._--The causes of priming are an insufficient amount of steam room, an
inadequate area of water level, an insufficient width between the flues or
tubes for the ascent of the steam and the descent of water to supply the
vacuity the steam occasions, and the use of dirty water in the boiler. New
boilers prime more than old boilers, and steamers entering rivers from the
sea are more addicted to priming than if sea or river water had alone been
used in the boilers--probably from the boiling point of salt water being
higher than that of fresh, whereby the salt water acts like so much molten
metal in raising the fresh water into steam. Opening the safety valve
suddenly may make a boiler prime, and if the safety valve be situated near
the mouth of the steam pipe, the spray or foam thus created may be mingled
with the steam passing into the engine, and materially diminish its
effective power; but if the safety valve be situated at a distance from the
mouth of the steam pipe, the quantity of foam or spray passing into the
engine may be diminished by opening the safety valve; and in locomotives,
therefore, it is found beneficial to have a safety valve on the barrel of
the boiler at a point remote from the steam chest, by partially opening
which, any priming in that part of the boiler adjacent to the steam chest
is checked, and a purer steam than before pusses to the engine.

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

_A._--When a boiler primes, the engineer generally closes the throttle
valve partially, turns off the injection water, and opens the furnace
doors, whereby the generation of steam is checked, and a less violent
ebullition in the boiler suffices. Where the priming arises from an
insufficient amount of steam room, it may be mitigated by putting a higher
pressure upon the boiler and working more expansively, or by the
interposition of a perforated plate between the boiler and the steam chest,
which breaks the ascending water and liberates the steam. In some cases,
however, it may be necessary to set a second steam chest on the top of the
existing one, and it will be preferable to establish a communication with
this new chamber by means of a number of small holes, bored through the
iron plate of the boiler, rather than by a single large orifice. Where
priming arises from the existence of dirty water in the boiler, the evil
may be remedied by the use of collecting vessels, or by blowing off largely
from the surface; and where it arises from an insufficient area of water
level, or an insufficient width between the flues for the free ascent of
the steam and the descent of the superincumbent water, the evil may be
abated by the addition of circulating pipes in some part of the boiler,
which will allow the water to descend freely to the place from whence the
steam rises, the width of the water spaces being virtually increased by
restricting their function to the transmission of a current of steam and
water to the surface. It is desirable to arrange the heating surface in
such a way that the feed water entering the boiler at its lowest point is
heated gradually as it ascends, until toward the superior part of the flues
it is raised gradually into steam; but in all cases there will be currents
in the boiler for which it is proper to provide. The steam pipe proceeding
to the engine should obviously be attached to the highest point of the
steam chest, in boilers of every construction.

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

_A._--According to experiments on the resistance of railway trains at
various rates of speed, made by Mr. Gooch, of the Great Western Railway, it
appears that a train weighing, with locomotive, tender, and carriages,
about 100 tons, experiences, at a speed of 50 miles an hour, a resistance
of about 3,000 lbs., or about 30 lbs. per ton; which resistance includes
the resistance of the engine as well as that of the train. This, therefore,
is the force which must be imparted at the circumference of the driving
wheels, except that small part intercepted by the engine itself, and the
force exerted by the pistons must be greater than that at the circumference
of the driving wheel, in the proportion of their slower motion, or in the
proportion of the circumference of the driving wheel to the length of a
double stroke of the engine. If the diameter of the driving wheel be 5-1/2
feet, its circumference will be 17.278 feet, and if the length of the
stroke be 18 inches, the length of a double stroke will be 3 feet. The
pressure on the pistons must therefore be greater than the traction at the
circumference of the driving wheel, in the proportion of 17.278 to 3, or,
in other words, the mean pressure on the pistons must be 17,278 lbs.; and
the area of cylinders, and pressure of steam, must be such as to produce
conjointly this total pressure. It thus becomes easy to tell the volume and
pressure of steam required, which steam in its turn represents its
equivalent of water which is to be evaporated from the boiler, and the
boiler must be so proportioned, by the rules already given, as to evaporate
this water freely. In the case of a steam vessel, the mode of procedure is
the same, and when the resistance and speed are known, it is easy to tell
the equivalent value of steam.



STRENGTH OF BOILERS.

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

_A._--The iron of boilers, like the iron of machines or structures, is
capable of withstanding a tensile strain of from 50,000 to 60,000 lbs. upon
every square inch of section; but it will only bear a third of this strain
without permanent derangement of structure, and it does not appear
expedient in any boiler to let the strain exceed 4,000 lbs. upon the square
inch of sectional area of metal, especially if it is liable to be weakened
by corrosion.

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

_A._--The question of the strength of boilers was investigated very
elaborately a few years ago by a committee of the Franklin Institute, in
America, and it was found that the tenacity of boiler plate increased with
the temperature up to 550°, at which point the tenacity began to diminish.
At 32°, the cohesive force of a square inch of section was 56,000 lbs.; at
570°, it was 66,500 lbs.; at 720°, 55,000 lbs.; at 1,050°, 32,000 lbs.; at
1,240°, 22,000 lbs.; and at 1,317°, 9,000 lbs. Copper follows a different
law, and appears to be diminished in strength by every addition to the
temperature. At 32° the cohesion of copper was found to be 32,800 lbs. per
square inch of section, which exceeds the cohesive force at any higher
temperature, and the square of the diminution of strength seems to keep
pace with the cube of the increased temperature. Strips of iron cut in the
direction of the fibre were found to be about 6 per cent. stronger than
when cut across the grain. Repeated piling and welding was found to
increase the tenacity of the iron, but the result of welding together
different kinds of iron was not found to be favorable. The accidental
overheating of a boiler was found to reduce the ultimate or maximum
strength of the plates from 65,000 to 45,000 lbs. per square inch of
section, and riveting the plates was found to occasion a diminution in
their strength to the extent of one third. These results, however, are not
precisely the same as those obtained by Mr. Fairbairn.

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

_A._--He found that boiler plate bore a tensile strain of 23 tons per
square inch before rupture, which was reduced to 16 tons per square inch
when joined together by a double row of rivets, and 13 tons, or about
30,000, when joined together by a single row of rivets. A circular boiler,
therefore, with the ends of its plates double riveted, will bear at the
utmost about 36,000 lbs. per square inch of section, or about 12,000 lbs.
per square inch of section without permanent derangement of structure.

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

_A._--In some locomotive boilers, which are worked with a pressure of 80
lbs. upon the square inch, the thickness of the plates is only 5/16ths of
an inch, while the barrel of the boiler is 39 inches in diameter. It will
require a length of 3.2 inches of the boiler when the plates are 5/16ths
thick to make up a sectional area of one square inch, and the separating
force will be 39 times 3.2 multiplied by 80, which makes the separating
force 9,984 lbs., sustained by two square inches of sectional area--one on
each side; or the strain is 4,992 lbs. per square inch of sectional area,
which is quite as great strain as is advisable. The accession of strength
derived from the boiler ends is not here taken into account, but neither is
the weakening effect counted that is caused by the rivet holes. Some
locomotives of 4 feet diameter of barrel and of 3/8ths iron have been
worked to as high a pressure as 200 lbs. on the inch; but such feats of
daring are neither to be imitated nor commended.

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

_A._--The thickness proper for cylindrical boilers of wrought iron, exposed
to an internal pressure, may be found by the following rule:--multiply 2.54
times the internal diameter of the cylinder in inches by the greatest
pressure within the cylinder per circular inch, and divide by 17,800; the
result is the thickness in inches. If we apply this rule to the example of
the locomotive boiler just given, we have 39 x 2.54 x 62.832 (the pressure
per circular inch corresponding to 80 lbs. per square inch) = 6224.1379,
and this, divided by 17,800, gives 0.349 as the thickness in inches,
instead of 0.3125, or 5/16ths, the actual thickness. If we take the
pressure per square inch instead of per circular inch, we obtain the
following rule, which is somewhat simpler:--multiply the internal diameter
of the cylinder in inches by the pressure in pounds per square inch, and
divide the product by 8,900; the result is the thickness in inches. Both
these rules give the strain about one fourth of the elastic force, or 4,450
lbs. per square inch of sectional area of the iron; but 3,000 lbs. is
enough when the flame impinges directly on the iron, as in some of the
ordinary cylindrical boilers, and the rule may be adapted for that strain
by taking 6,000 as a divisor instead of 8,900.

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

_A._--Where the sides of the boiler are flat, instead of being cylindrical,
a sufficient number of stays must be introduced to withstand the pressure;
and it is expedient not to let the strain upon these stays be more than
3,000 lbs. per square inch of section, as the strength of internal stays in
boilers is generally soon diminished by corrosion. Indeed, a strain at all
approaching that upon locomotive boilers would be very unsafe in the case
of marine boilers, on account of the corrosion, both internal and external,
to which marine boilers are subject. The stays should be small and numerous
rather than large and few in number, as, when large stays are employed, it
is difficult to keep them tight at the ends, and oxidation of the shell
follows from leakage at the ends of the stays. All boilers should be
proved, when new, to twice or three times the pressure they are intended to
bear, and they should be proved occasionally by the hand pump when in use,
to detect any weakness which corrosion may have occasioned.

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

_A._--If the pressure of steam be 20 lbs. on the square inch, which is a
very common pressure in tubular boilers, there will be a pressure of 2,880
lbs. on every square foot of flat surface; so that if the strain upon the
stays is not to exceed 3,000 lbs. on the square inch of section, there must
be nearly a square inch of sectional area of stay for every square foot of
flat surface on the top and bottom, sides, and ends of the boiler. This
very much exceeds the proportion usually adopted; and in scarcely any
instance are boilers stayed sufficiently to be safe when the shell is
composed of flat surfaces. The furnaces should be stayed together with
bolts of the best scrap iron, 1-1/4 inch in diameter, tapped through both
plates of the water space with thin nuts in each furnace; and it is
expedient to make the row of stays, running horizontally near the level of
the bars, sufficiently low to come beneath the top of the bars, so as to be
shielded from the action of the fire, with which view they should follow
the inclination of the bars. The row of stays between the level of the bars
and the top of the furnace should be as near the top of the furnace as will
consist with the functions they have to perform, so as to be removed as far
as possible from the action of the heat; and to support the furnace top,
cross bars may either be adopted, to which the top is secured with bolts,
as in the case of locomotives, or stays tapped into the furnace top, with a
thin nut beneath, may be carried to the top of the boiler; but very little
dependence can be put in such stays as stays for keeping down the top of
the boiler; and the top of the boiler must, therefore, be stayed nearly as
much as if the stays connecting it with the furnace crowns did not exist.
The large rivets passing through thimbles, sometimes used as stays for
water spaces or boiler shells, are objectionable; as, from the great amount
of hammering such rivets have to receive to form the heads, the iron
becomes crystalline, so that the heads are liable to come off, and, indeed,
sometimes fly off in the act of being formed. If such a fracture occurs
between the boilers after they are seated in their place, or in any
position not accessible from the outside, it will in general be necessary
to empty the faulty boiler, and repair the defect from the inside.

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

_A._--The stays, where the sides of the boiler are flat, and the pressure
of the steam is from 20 to 30 lbs., should be pitched about a foot or 18
inches asunder; and in the wake of the tubes, where stays cannot be carried
across to connect the boiler sides, angle iron ribs, like the ribs of a
ship, should be riveted to the interior of the boiler, and stays of greater
strength than the rest should pass across, above, and below the tubes, to
which the angle irons would communicate the strain. The whole of the long
stays within a boiler should be firmly riveted to the shell, as if built
with and forming a part of it; as, by the common method of fixing them in
by means of cutters, the decay or accidental detachment of a pin or cutter
may endanger the safety of the boiler. Wherever a large perforation in the
shell of any circular boiler occurs, a sufficient number of stays should be
put across it to maintain the original strength; and where stays are
intercepted by the root of the funnel, short stays in continuation of them
should be placed inside.



BOILER EXPLOSIONS.

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

_A._--The chief cause of boiler explosions is, undoubtedly, too great a
pressure of steam, or an insufficient strength of boiler; but many
explosions have also arisen from the flues having been suffered to become
red hot. If the safety valve of a boiler be accidentally jammed, or if the
plates or stays be much worn by corrosion, while a high pressure of steam
is nevertheless maintained, the boiler necessarily bursts; and if, from an
insufficiency of water in the boiler, or from any other cause, the flues
become highly heated, they may be forced down by the pressure of the steam,
and a partial explosion may be the result. The worst explosion is where the
shell of the boiler bursts; but the collapse of a furnace or flue is also
very disastrous generally to the persons in the engine room; and sometimes
the shell bursts and the flues collapse at the same time; for if the flues
get red hot, and water be thrown upon them either by the feed pump or
otherwise, the generation of steam may be too rapid for the safety valve to
permit its escape with sufficient facility, and the shell of the boiler
may, in consequence, be rent asunder. Sometimes the iron of the flues
becomes highly heated in consequence of the improper configuration of the
parts, which, by retaining the steam in contact with the metal, prevents
the access of the water: the bottoms of large flues, upon which the flame
beats down, are very liable to injury from this cause; and the iron of
flues thus acted upon may be so softened that the flues will collapse
upward with the pressure of the steam. The flues of boilers may also become
red hot in some parts from the attachment of scale, which, from its
imperfect conducting power, will cause the iron to be unduly heated; and if
the scale be accidentally detached, a partial explosion may occur in
consequence.

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

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

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

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

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

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

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

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

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

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




CHAPTER VI.

PROPORTIONS OF ENGINES.

       *       *       *       *       *

STEAM PASSAGES.

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

_A._--About 0.8 of a circular inch per horse power, or a circular inch per
1-1/4 horse power. The following rule, however, will give the dimensions
suitable for all kinds of engines, whether high or low pressure:--multiply
the square of the diameter of the cylinder in inches by the speed of the
piston in feet per minute, and divide the product by 375 times the pressure
on the boiler per square inch; the quotient is the proper area of the
safety valve in square inches. This rule of course supposes that the
evaporating surface has been properly proportioned to the engine power.

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

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

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

_A._--There are always two.

315. _Q._--And are they always pressed down by a spring balance, and never
by weights?

_A._--They are never pressed down by weights; in fact, weights would not
answer on a locomotive at all, as they would jump up and down with the
jerks or jolts of the train, and cause much of the steam to escape. In land
and marine boilers, however, the safety valve is always kept down by
weights; but in steam vessels a good deal of steam is lost in stormy
weather by the opening of the valve, owing to the inertia of the weights
when the ship sinks suddenly in the deep recess between the waves.

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

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

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

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

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

_A._--In slow working engines the common size of the cylinder passages is
one twenty-fifth of the area of the cylinder, or one fifth of the diameter
of the cylinder, which is the same thing. This proportion corresponds very
nearly with one square inch per horse power when the length of the cylinder
is about equal to its diameter; and one square inch of area per horse power
for the cylinder ports and eduction passages answers very well in the case
of engines working at the ordinary speed of 220 feet per minute. The area
of the steam pipe is usually made less than the area of the eduction pipe,
especially when the engine is worked expansively, and with a considerable
pressure of steam. In the case of ordinary condensing engines, however,
working with the usual pressure of from 4 to 8 lbs. above the atmosphere,
the area of the steam pipe is not less than a circular inch per horse
power. In such engines the diameter of the steam pipe may be found by the
following rule: divide the number of nominal horse power by 0.8 and extract
the square root of the quotient, which will be the internal diameter of the
steam pipe.

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

_A._--The size of the steam pipe is so regulated that there will be no
material disparity of pressure between the cylinder and boiler; and in
fixing the size of the eduction passage the same object is kept in view.
When the diameter of the cylinder and the velocity with which the piston
travels are known, it is easy to tell what the velocity of the steam in the
steam pipe will be; for if the area of the cylinder be 25 times greater
than that of the steam pipe, the steam in the steam pipe must travel 25
times faster than the piston, and the difference of pressure requisite to
produce this velocity of the steam can easily be ascertained, by finding
what height a column of steam must be to give that velocity, and what the
weight or pressure is of such a column. In practice, however, this
proportion is always exceeded from the condensation of steam in the pipe.

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

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

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

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

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

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

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

_A._--The early locomotives were made with very little lead, and the
proportions were in fact very much the same as those previously existing in
land engines. About 1832, the benefits of lap upon the valve, which had
been employed by Boulton and Watt more than twenty years before, were
beginning to be pretty generally apprehended; and, in the following year,
this expedient of economy was applied to the steamer Manchester, in the
Clyde, and to some other vessels, with very marked success. Shortly after
this time, lap began to be applied to the valves of locomotives, and it was
found that not only was there a benefit from the operation of expansion,
but that there was a still greater benefit from the superior facility of
escape given to the steam, inasmuch as the application of lap involved the
necessity of turning the eccentric round upon the shaft, which caused the
eduction to take place before the end of the stroke. In 1840, one of the
engines of the Liverpool and Manchester Railway was altered so as to have 1
inch lap on the valve, and 1 inch opening on the eduction side at the end
of the stroke, the valve having a total travel of 4-1/4 inches. The
consumption of fuel per mile fell from 36.3 lbs. to 28.6 lbs, or about 25
per cent., and a softer blast sufficed. By using larger exhaust passages,
larger tubes, and closer fire bars, the consumption was subsequently
brought down to 15 lbs. per mile.



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

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

_A_--Mr. Watt made the air pump of his engine half the diameter of the
cylinder and half the stroke, or one eighth of the capacity, and the
condenser was usually made about the same size as the air pump; but as the
pressure of the steam has been increased in all modern engines, it is
better to make the air pump a little larger than this proportion. 0.6 of
the diameter of the cylinder and half the stroke answers very well, and the
condenser may be made as large as it can be got with convenience, though
the same size as the air pump will suffice.

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

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

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

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

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

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

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

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

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

_A._--Mr. Watt found that the most beneficial temperature of the hot well
of his engines was 100 degrees. If, therefore, the temperature of the steam
be 212°, and the latent heat 1,000°, then 1,212° may be taken to represent
the heat contained in the steam, or 1,112° if we deduct the temperature of
the hot well. If the temperature of the injection water be 50°, then 50
degrees of cold are available for the abstraction of heat; and as the total
quantity of heat to be abstracted is that requisite to raise the quantity
of water in the steam 1,112 degrees, or 1,112 times that quantity one
degree, it would raise one fiftieth of this, or 22.24 times the quantity of
water in the steam, 50 degrees. A cubic inch of water therefore raised into
steam will require 22.24 cubic inches of water at 50 degrees for its
condensation, and will form therewith 23.24 cubic inches of hot water at
100 degrees. Mr. Watt's practice was to allow about a wine pint (28.9 cubic
inches) of injection water, for every cubic inch of water evaporated from
the boiler.

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

_A._--It is.

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

_A._--Yes.

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

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

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

_A._--The area of the injection orifice proper for any engine can easily be
told when the quantity of water requisite to condense the steam is known,
and the pressure is specified under which the water enters the condenser.
The vacuum in the condenser may be taken at 26 inches of mercury, which is
equivalent to a column of water 29.4 ft. high, and the square root of 29.4
multiplied by 8.021 is 43.15, which is the velocity in feet per second that
a heavy body would acquire in falling 29.4 ft., or with which the water
would enter the condenser. Now, if a cubic foot of water evaporated per
hour be equivalent to an actual horse power, and 28.9 cubic inches of water
be requisite for the condensation of a cubic inch of water in the form of
steam, 28.9 cubic feet of condensing water per horse power per hour, or
13.905 cubic inches per second, will be necessary for the engine, and the
size of the injection orifice must be such that this quantity of water
flowing with the velocity of 43.15 ft. per second, or 517.8 inches per
second, will gain admission to the condenser. Dividing, therefore, 13.905,
the number of cubic inches to be injected, by 517.8, the velocity of influx
in inches per second, we get 0.02685 for the area of the orifice in square
inches; but inasmuch as it has been found by experiment that the actual
discharge of water through a hole in a thin plate is only six tenths of the
theoretical discharge on account of the contracted vein, the area of the
orifice must be increased in the proportion of such diminution of effect,
or be made 0.04475, or 1/22d of a square inch per horse power. This, it
will be remarked, is the theoretical area required per actual horse power;
but as the friction and contractions in the pipe further reduce the
discharge, the area is made 1/15th of a square inch per actual horse power,
or rather per cubic foot of water evaporated from the boiler.

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

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

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

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

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

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

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

_A._--Mainly to the inertia of the water, which, if the pump piston be
drawn up very rapidly, cannot follow it with sufficient rapidity; so that
there may be a vacant space between the piston and the water; and at the
return stroke the momentum of the water in the pipe expends itself in
giving a reverse motion to the column of water approaching the pump.
Messrs. Kirchweger and Prusman, of Hanover, have investigated this subject
by applying a revolving cock at the end of a pipe leading from an elevated
cistern containing water, and the water escaped at every revolution of the
cock in the same manner as if a pump were drawing it. With a column of
water of 17 feet, they found that at 80 revolutions of the cock per minute,
the water delivered per minute by the cock was 9.45 gallons; but with 140
revolutions of the cock per minute, the water delivered per minute by the
cock was only 5.42 gallons. They subsequently applied an air vessel to the
pipe beside the cock, when the discharge rose to 12.9 gallons per minute
with 80 revolutions, and 18.28 gallons with 140 revolutions. Air vessels
should therefore be applied to the suction side of fast moving pumps, and
this is now done with good results.

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

_A._--If to condense a cubic inch of water raised into steam 28.9 cubic
inches of condensing water are required, then the cold water pump ought to
be 28.9 times larger than the feed pump, supposing that its losses were
equally great. The feed pump, however, is made sufficiently large to
compensate for leaks in the boiler and loss of steam through the safety
valve, so that it will be sufficient if the cold water pump be 24 times
larger than the feed pump. This ratio is preserved by the following rule:--
multiply the capacity of the cylinder in cubic inches by the total pressure
of the steam per square inch, or the pressure on the safety valve plus 15,
and divide the product by 200. The quotient is the proper capacity of the
cold water pump in cubic inches when the engine is double acting, and the
pump single acting.



FLY WHEEL.

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

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

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

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



STRENGTHS OF LAND ENGINES.

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

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

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

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

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

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

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

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

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

_A._--Some of his connecting rods were of iron and some of wood. To
determine the thickness when of wood, multiply the square of the diameter
of the cylinder in inches by the length of the stroke in feet, and divide
the product by 24. Extract the fourth root of the quotient, which is the
thickness in inches. For iron the rule is the same, only the divisor was
57.6 instead of 24.

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

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

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

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

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

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

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

_A._--If the force acting at the end of an engine beam be taken at 18 lbs.
per circular inch of the piston, then the force acting at the middle will
be 36 lbs. per circular inch of the piston, and the proper strength of the
beam at the centre will be found by the following rule:--divide the weight
in lbs. acting at the centre by 250, and multiply the quotient by the
distance between the extreme centres. To find the depth, the breadth being
given:--divide this product by the breadth in inches, and extract the
square root of the quotient, which is the depth. The depth of a land engine
beam at the ends is usually made one third of the depth at the centre (the
depth at the centre being equal to the diameter of the cylinder in the case
of low pressure engines), while the length is made equal to three times the
length of the stroke, and the mean thickness 1/108th of the length--the
width of the edge bead being about three times the thickness of the web. In
many modern engines the force acting at the end of the beam is more than 18
lbs. per circular inch of the piston, but the above rules are still
applicable by taking an imaginary cylinder with an area larger in the
proportion of the larger pressure.

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

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

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

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

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

_A._--Experiments upon the force requisite to twist off cast iron necks
show that if the cube of the diameter of neck in inches be multiplied by
880, the product will be the force of torsion which will twist them off
when acting at 6 inches radius; on this fact the following rule is founded:
To find the diameter of a cast iron fly wheel shaft:--multiply the square
of the diameter of the cylinder in inches, by the length of the crank in
inches, and extract the cube root of the product, which multiply by 0.3025,
and the result will be the proper diameter of the shaft in inches at the
smallest part, when of cast iron.

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

_A._--Taking the pressure on the piston at 12 lbs. pressure on the square
inch, and supposing this force to be applied at one foot radius, divide the
total pressure of the piston reduced to 1 foot of radius by 31.4, and
extract the cube root of the quotient, which is the diameter of the shaft:
or extract the cube root of 13.7 times the number of cubic feet of steam
required to make one revolution, which is also the diameter of the shaft.

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

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

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

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



STRENGTH OF MARINE AND LOCOMOTIVE ENGINES.

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

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

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

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

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

_A._--The diameter of the connecting rod at the ends, may be found by
multiplying 0.019 times the square root of the pressure on the piston in
lbs. per square inch by the diameter of the cylinder in inches; and the
diameter of the connecting rod in the middle may be found by the following
rule:--to 0.0035 times the length of the connecting rod in inches, add 1,
and multiply the sum by 0.019 times the square root of the pressure on the
piston in lbs. per square inch, multiplied by the diameter of the cylinder
in inches. The strain is equal to 1/6th of the elastic force.

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

_A._--The diameter of the cylinder side rods at the ends may be found by
multiplying 0.0129 times the square root of the pressure on the piston in
lbs. per square inch by the diameter of the cylinder; and the diameter of
the cylinder side rods at the middle is found by the following rule:--to
0.0035 times the length of the rod in inches, add 1, and multiply the sum
by 0.0129 times the square root of the pressure on the piston in lbs. per
square inch, multiplied by the diameter of the cylinder in inches; the
product is the diameter of each side rod at the centre in inches. The
strain upon the side rods is by these rules equal to 1/6th of the elastic
force.

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

_A._--To find the exterior diameter of the large eye of the crank when of
malleable iron:--to 1.561 times the pressure of the steam upon the piston
in lbs. per square inch, multiplied by the square of the length of the
crank in inches, add 0.00494 times the square of the diameter of the
cylinder in inches, multiplied by the square of the number of lbs. pressure
per square inch on the piston; extract the square root of this quantity;
divide the result by 75.59 times the square root of the length of the crank
in inches, and multiply the quotient by the diameter of the cylinder in
inches; square the product and extract the cube root of the square, to
which add the diameter of the hole for the reception of the shaft, and the
result will be the exterior diameter of the large eye of the crank when of
malleable iron. The diameter of the small eye of the crank may be found by
adding to the diameter of the crank pin 0.02521 times the square root of
the pressure on the piston in lbs. per square inch, multiplied by the
diameter of the cylinder in inches.

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

_A._--The thickness of the web of the crank, supposing it to be continued
to the centre of the shaft, would at that point be represented by the
following rule:--to 1.561 times the square of the length of the crank in
inches, add 0.00494 times the square of the diameter of the cylinder in
inches, multiplied by the pressure on the piston in lbs. per square inch;
extract the square root of the sum, which multiply by the diameter of the
cylinder squared in inches, and by the pressure on the piston in lbs. per
square inch; divide the product by 9,000, and extract the cube root of the
quotient, which will be the proper thickness of the web of the crank when
of malleable iron, supposing the web to be continued to the centre of the
shaft. The thickness of the web at the crank pin centre, supposing it to be
continued thither, would be 0.022 times the square root of the pressure on
the piston in lbs. per square inch, multiplied by the diameter of the
cylinder. The breadth of the web of the crank at the shaft centre should be
twice the thickness, and at the pin centre 1-1/2 times the thickness of the
web; the length of the large eye of the crank would be equal to the
diameter of the shaft, and of the small eye 0.0375 times the square root of
the pressure on the piston in lbs. per square inch, multiplied by the
diameter of the cylinder.

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

_A._--The method of computation will be as follows:--to find the dimensions
of a malleable iron paddle shaft, so that the strain shall not exceed
5/6ths of the elastic force, or 5/6ths of the force iron is capable of
withstanding without permanent derangement of structure, which in tensile
strains is taken at 17,800 lbs. per square inch: multiply the pressure in
lbs. per square inch on the piston by the square of the diameter of the
cylinder in inches, and the length of the crank in inches, and extract the
cube root of the product, which, multiplied by 0.08264, will be the
diameter of the paddle shaft journal in inches when of malleable iron,
whatever the pressure of the steam may be. The length of the paddle shaft
journal should be 1-1/4 times the diameter; and the diameter of the part
where the crank is put on is often made equal to the diameter over the
collars of the journal or bearing.

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

_A._--The diameter of the crank pin in inches may be found by multiplying
0.02836 times the square root of the pressure on the piston in lbs. per
square inch, by the diameter of the cylinder in inches. The length of the
pin is usually about 9/8th times its diameter, and the strain if all thrown
upon the end of the pin will be equal to the elastic force; but in ordinary
working, the strain will only be equal to 1/3d of the elastic force.

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

_A._--If the length of the cross head be taken at 1.4 times the diameter of
the cylinder, the dimensions of the cross head will be as follows:--the
exterior diameter of the eye in the cross head for the reception of the
piston rod, will be equal to the diameter of the hole, plus 0.02827 times
the cube root of the pressure on the piston in lbs. per square inch,
multiplied by the diameter of the cylinder in inches; and the depth of the
eye will be 0.0979 times the cube root of the pressure on the piston in
lbs. per square inch, multiplied by the diameter of the cylinder in inches.
The diameter of each cross head journal will be 0.01716 times the square
root of the pressure on the piston in lbs. per square inch, multiplied by
the diameter of the cylinder in inches--the length of the journal being
9/8ths its diameter. The thickness of the web at centre will be 0.0245
times the cube root of the pressure on the piston in lbs. per square inch,
multiplied by the diameter of the cylinder in inches; and the depth of web
at centre will be 0.09178 times the cube root of the pressure on the piston
in lbs. per square inch, multiplied by the diameter of the cylinder in
inches. The thickness of the web at journal will be 0.0122 times the square
root of the pressure on the piston in lbs. per square inch, multiplied by
the diameter of the cylinder in inches; and the depth of the web at journal
will be 0.0203 times the square root of the pressure upon the piston in
lbs. per square inch, multiplied by the diameter of the cylinder in inches.
In these rules for the cross head, the strain upon the web is 1/2.225 times
the elastic force; the strain upon the journal in ordinary working is
1/2.33 times the elastic force; and if the outer ends of the journals are
the only bearing points, the strain is 1/1.165 times the elastic force,
which is very little in excess of the elastic force.

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

_A._--The diameter of the main centre may be found by multiplying 0.0367
times the square root of the pressure upon the piston in lbs. per square
inch, by the diameter of the cylinder in inches, which will give the
diameter of the main centre journal in inches when of malleable iron, and
the length of the main centre journal should be 1-1/2 times its diameter;
the strain upon the main centre journal in ordinary working will be about
1/2 the elastic force.

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

_A._--The depth of gibs and cutters for attaching the piston rod to the
cross head, is 0.0358 times the cube root of the pressure of the steam on
the piston in lbs. per square inch, multiplied by the diameter of the
cylinder; and the thickness of the gibs and cutters is 0.007 times the cube
root of the pressure on the piston in lbs. per square inch, multiplied by
the diameter of its cylinder. The depth of the cutter through the piston is
0.017 times the square root of the pressure on the piston in lbs. per
square inch, multiplied by the diameter of the cylinder in inches; and the
thickness of the cutter through the piston is 0.007 times the square root
of the pressure on the piston in lbs. per square inch, multiplied by the
diameter of the cylinder.

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

_A._--It is obvious, from the varying proportions subsisting in the
different parts of the engine between the strain and the elastic force,
that in engines proportioned by these rules--which represent nevertheless
the average practice of the best constructors--some of the parts must
possess a considerable excess of strength over other parts, and it appears
expedient that this disparity should be diminished, which may best be done
by increasing the strength of the parts which are weakest; inasmuch as the
frequent fracture of some of the parts shows that the dimensions at present
adopted for those parts are scarcely sufficient, unless the iron of which
they are made is of the best quality. At the same time it is quite certain,
that engines proportioned by these rules will work satisfactorily where
good materials are employed; but it is important to know in what parts good
materials and larger dimensions are the most indispensable. In many of the
parts, moreover, it is necessary that the dimensions should be proportioned
to meet the wear and the tendency to heat, instead of being merely
proportioned to obtain the necessary strength; and the crank pin is one of
the parts which requires to be large in diameter, and as long as possible
in the bearing, so as to distribute the pressure, and prevent the
disposition to heat which would otherwise exist. The cross head journals
also should be long and large; for as the tops of the side rods have little
travel, the oil is less drawn into the bearings than if the travel was
greater, and is being constantly pressed out by the punching strain. This
strain should therefore be reduced as far as possible by its distribution
over a large surface. In the rules which are contained in the answers to
the ten preceding questions (358 to 367) the pressure on the piston in lbs.
per square inch is taken as the sum of the pressure of steam in the boiler
and of the vacuum; the latter being assumed to be 15 lbs. per square inch.




CHAPTER VII.


CONSTRUCTIVE DETAILS OF BOILERS.


       *       *       *       *       *


LAND AND MARINE BOILERS.

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

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

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

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

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

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

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

_A._--In building the brickwork for the setting of the boiler, the part
upon which the heat acts with most intensity is to be built with clay
instead of mortar, but mortar is to be used on the outside of the work. Old
bars of flat iron may be laid under the boiler chime to prevent that part
of the boiler from being burned out, and bars of iron should also run
through the brickwork to prevent it from splitting. The top of the boiler
is to be covered with brickwork laid in the best lime, and if the lime be
not of the hydraulic kind, it should be mixed with Dutch terrass, to make
it impenetrable to water. The top of the boiler should be well plastered
with this lime, which will greatly conduce to the tightness of the seams.
Openings into the flues must be left in convenient situations to enable the
flues to be swept out when required, and these openings may be closed with
cast iron doors jointed with clay or mortar, which may be easily removed
when required. Adjacent to the chimney a slit must be left in the top of
the flue with a groove in the brickwork to enable the sliding door or
damper to be fixed in that situation, which by being lowered into the flue
will obstruct the passage of the smoke and moderate the draught, whereby
the chimney will be prevented from drawing the flame into it before the
heat has acted sufficiently upon the boiler.

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

_A._--There is very little difference in the two cases: the whole of the
shells of marine boilers, however, should be double riveted with rivets
11/16ths of an inch in diameter, and 2-3/8th inches from centre to centre,
the weakening effect of double riveting being much less than that of single
riveting. The furnaces above the line of bars should be of the best
Lowmoor, Bowling, or Staffordshire scrap plates, and the portion of each
furnace above the bars should consist only of three plates, one for the top
and one for each side, the lower seam of the side plates being situated
beneath the level of the bars, so as not to be exposed to the heat of the
furnace. The tube plates of tubular boilers should be of the best Lowmoor,
or Bowling iron, seven eighths to one inch thick: the shells should be of
the best Staffordshire, or Thornycroft S crown iron, 7/16ths of an inch
thick.

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

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

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

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

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

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

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

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

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

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

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

_A._--The furnace bridges of marine boilers are walls or partitions built
up at the ends of the furnaces to narrow the opening for the escape of heat
into the flues. They are either made of fire brick or of plate iron
containing water: in the case of water bridges, the top part of the bridge
should be made with a large amount of slant so as to enable the steam to
escape freely, but notwithstanding this precaution the plates of water
bridges are apt to crack at the bend, so that fire brick bridges appear on
the whole to be preferable. In shallow furnaces the bridges often come too
near the furnace top to enable a man to pass over them; and it will save
expense if in such bridges the upper portion is constructed of two or three
fire blocks, which may be lifted off where a person requires to enter the
flues to sweep or repair them, whereby the perpetual demolition and
reconstruction of the upper part of the bridge will be prevented.

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

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

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

_A._--It is not an uncommon practice to place a hanging bridge, consisting
of a plate of iron descending a certain distance into the flue, at that
part of the flue where it enters the chimney, whereby the stratum of hot
air which occupies the highest part of the flue is kept in protracted
contact with the boiler, and the cooler air occupying the lower part of the
flue is that which alone escapes. The practice of introducing a hanging
bridge is a beneficial one in the case of some boilers, but is not
applicable universally, as boilers with a small calorimeter cannot be
further contracted in the flue without a diminution in their evaporating
power. In tubular boilers a hanging bridge is not applicable, but in some
cases a perforated plate is placed against the ends of the tubes, which by
suitable connections is made to operate as a sliding damper which partially
or totally closes up the end of every tube, and at other times a damper
constructed in the manner of a venetian blind is employed in the same
situation. These varieties of damper, however, have only yet been used in
locomotive boilers, though applicable to tubular boilers of every
description.

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

_A._--In a flue boiler this cannot be done, but in a tubular boiler it is
an advantage that there should be a division between the tubes pertaining
to each furnace, so that the smoke of each furnace may be kept apart from
the smoke of the furnace adjoining it until the smoke of both enters the
chimney, as by this arrangement a furnace only will be rendered inoperative
in cleaning the fires instead of a boiler, and the tubes belonging to one
furnace may be swept if necessary at sea without interfering injuriously
with the action of the rest. In a steam vessel it is necessary at intervals
to empty out one or more furnaces every watch to get rid of the clinkers
which would otherwise accumulate in them; and it is advisable that the
connection between the furnaces should be such that this operation, when
being performed on one furnace, shall injure the action of the rest as
little as possible.

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

_A._--The sides of the internal furnaces or flues in all boilers should be
so constructed that the steam may readily escape from their surfaces, with
which view it is expedient to make the bottom of the flue somewhat wider
than the top, or slightly conical in the cross section; and the upper
plates should always be overlapped by the plates beneath, so that the steam
cannot be retained in the overlap, but will escape as soon as it is
generated. If the sides of the furnace be made high and perfectly vertical,
they will speedily be buckled and cracked by the heat, as a film of steam
in such a case will remain in contact with the iron which will prevent the
access of the water, and the iron of the boiler will be injured by the high
temperature it must in that case acquire. To moderate the intensity of the
heat acting upon the furnace sides, it is expedient to bring the outside
fire bars into close contact with the sides of the furnace, so as to
prevent the entrance of air through the fire in that situation, by which
the intensity of the heat would be increased. The tube plate nearest the
furnace in tubular boilers should also be so inclined as to facilitate the
escape of the steam; and the short bent plate or flange of the tube plate,
connecting the tube plate with the top of the furnace, should be made with
a gradual bend, as, if the bend be sudden, the iron will be apt to crack or
burn away from the concretion of salt. Where the furnace mouths are
contracted by bending in the sides and top of the furnace, as is the
general practice, the bends should be gradual, as salt is apt to accumulate
in the pockets made by a sudden bend, and the plates will then burn into
holes.

384. _Q._--In what manner is the tubing of boilers performed?

_A._--The tubes of marine boilers are generally iron tubes, three inches in
diameter, and between six and seven feet long; but sometimes brass tubes of
similar dimensions are employed. When brass tubes are employed, the use of
ferules driven into the ends of the tubes is sometimes employed to keep
them tight; but when the tubes are of malleable iron, of the thickness of
Russell's boiler tubes, they may be made tight merely by firmly driving
them into the tube plates, and the same may be done with thick brass tubes.
The holes in the tube plate next the front of the boiler are just sensibly
larger in diameter than the holes in the other tube plate, and the holes
upon the outer surfaces of both tube plates are very slightly countersunk.
The whole of the tubes are driven through both tube plates from the front
of the boiler,--the precaution, however, being taken to drive them in
gently at first with a light hand hammer, until the whole of the tubes have
been inserted to an equal depth, and then they may be driven up by degrees
with a heavy hammer, whereby any distortion of the holes from unequal
driving will be prevented. Finally, the ends of the tubes should be riveted
up so as to fill the countersink; the tubes should be left a little longer
than the distance between the outer surfaces of the tube plates, so that
the countersink at the ends may be filled by staving up the end of the tube
rather than by riveting it over; and the staving will be best accomplished
by means of a mandril with a collar upon it, which is driven into the tube
so that the collar rests upon the end of the tube to be riveted; or a tool
like a blunt chisel with a recess in its point may be used, as is the more
usual practice.

385. _Q._--Should not stays be introduced in substitution of some of the
tubes?

_A._--It appears expedient in all cases that some of the tubes should be
screwed at the ends, so as to serve as stays if the riveting at the tube
ends happens to be burned away, and also to act as abutments to the riveted
tube--or else to introduce very strong rods of about the same diameter as a
tube, in substitution of some of the tubes; and these stays should have
nuts at each end both within and without the tube plates, which nuts should
be screwed up, with white lead interposed, before the tubes are inserted.
If the tubes are long, their expansion when the boiler is being blown off
will be apt to start them at the ends, unless very securely fixed; and it
is difficult to prevent brass tubes of large diameter and proportionate
length from being started at the ends, even when secured by ferules; but
the brass tubes commonly employed are so small as to be susceptible of
sufficient compression endways by the adhesion due to the ferules to
compensate for the expansion, whereby they are prevented from starting at
the ends. In some, of the early marine boilers fitted with brass tubes, a
galvanic action at the ends of the tubes was found to take place, and the
iron of the tube plates was wasted away in consequence, with rapidity; but
further experience proved the injury to be attributable chiefly to
imperfect fitting, whereby a leakage was caused that induced oxidation, and
when, the tubes were well fitted any injurious action at the ends of the
tubes was found to cease.

386. _Q._--What is the best mode of constructing the chimney and the parts
in connection therewith?

_A._--In sea-going steamers the funnel plates are usually about nine feet
long and 3/16ths thick; and where different flues or boilers have their
debouch in the same chimney, it is expedient to run division plates up the
chimney for a considerable distance, to keep the draughts distinct. The
dampers should not be in the chimney but at the end of the boiler flue, so
that they may be available for use if the funnel by accident be carried
away. The waste steam pipe should be of the same height as the funnel, so
as to carry the waste steam clear of it, for if the waste steam strikes the
funnel it will wear the iron into holes; and the waste steam pipes should
be made at the bottom with a faucet joint, to prevent the working of the
funnel, when the vessel rolls, from breaking the pipe at the neck. There
should be two hoops round the funnel, for the attachment of the funnel
shrouds, instead of one, so that the funnel may not be carried overboard if
one hoop breaks, or if the funnel breaks at the upper hoop from the
corrosive action of the waste steam, as sometimes happens. The deck over
the steam chest should be formed of an iron plate supported by angle iron
beams, and there should be a high angle iron cooming round the hole in the
deck through which the chimney ascends, to prevent any water upon the deck
from leaking down upon the boiler. Around the lower part of the funnel
there should be a sheet iron casing to prevent any inconvenient dispersion
of heat in that situation, and another short piece of casing, of a somewhat
larger diameter, and riveted to the chimney, should descend over the first
casing, so as to prevent the rain or spray which may beat against the
chimney from being poured down within the casing upon the top of the
boiler. The pipe for conducting away the waste water from the top of the
safety valve should lead overboard, and not into the bilge of the ship, as
inconvenience arises from the steam occasionally passing through it, if it
has its termination in the engine room.

387. _Q._--Are not the chimneys of some vessels made so that they may be
lowered when required?

_A._--The chimneys of small river vessels which have to pass under bridges
are generally formed with a hinge, so that they may be lowered backward
when passing under a bridge; and the chimneys of some screw vessels are
made so as to shut up like a spyglass when the fires are put out and the
vessel is navigated under sails. In smaller vessels, however, two lengths
of chimney suffice; and in that case there is a standing piece on deck,
which, however, does not project above the bulwarks.

388. _Q._--Will you explain any further details in the construction of
marine boilers which occur to you as important?

_A._--The man-hole and mud-hole doors, unless put on from the outside, like
a cylinder cover, with a great number of bolts, should be put on from the
inside with cross bars on the outside, and the bolts should be strong, and
have coarse threads and square nuts, so that the threads may not be
overrun, nor the nuts become round, by the unskilful manipulations of the
firemen, by whom these doors are removed or replaced. It is very expedient
that sufficient space should be left between the furnace and the tubes in
all tubular boilers to permit a boy to go in to clear away any scale that
may have formed, and to hold on the rivets in the event of repair being
wanted; and it is also expedient that a vertical row of tubes should be
left out opposite to each water space to allow the ascent of the steam and
descent of the water, as it has been found that the removal of the tubes in
that position, even in a boiler with deficient heating surface, has
increased the production of steam, and diminished the consumption of fuel.
The tubes should all be kept in the same vertical line, so as to permit the
introduction of an instrument to scrape them; but they may be zig-zagged in
the horizontal line, whereby a greater strength of metal will be obtained
around the holes in the tube plates, and the tubes should not be placed too
close together, else their heating efficacy will be impaired.



INCRUSTATION AND CORROSION OF BOILERS.


389. _Q._--What is the cause of the formation of scale in marine boilers?

_A._--Scale is formed in all boilers which contain earthy or saline
matters, just in the way in which a scaly deposit, or rock, as it is
sometimes termed, is formed in a tea kettle. In sea water the chief
ingredient is common salt, which exists in solution: the water admitted to
the boiler is taken away in the shape of steam, and the saline matter which
is not vaporizable accumulates in process of time in the boiler, until its
amount is so great that the water is saturated, or unable to hold any more
in solution; the salt is then precipitated and forms a deposit which
hardens by heat. The formation of scale, therefore, is similar to the
process of making salt from sea water by evaporation, the boiler being, in
fact, a large salt pan.

390. _Q._--But is the scale soluble in fresh water like the salt in a salt
pan?

_A._--No, it is not; or if soluble at all, is only so to a very limited
extent. The several ingredients in sea water begin to be precipitated from
solution at different degrees of concentration; and the sulphate and
carbonate of lime, which begin to be precipitated when a certain state of
concentration is reached, enter largely into the composition of scale, and
give it its insoluble character. Pieces of waste or other similar objects
left within a marine boiler appear, when taken out, as if they had been
petrified; and the scale deposited upon the flues of a marine boiler
resembles layers of stone.

391. _Q/_--Is much inconvenience experienced in marine boilers from these
incrustations upon the flues?

_A._--Incrustation in boilers at one time caused much more perplexity than
it does at present, as it was supposed that in some seas it was impossible
to prevent the boilers of a steamer from becoming salted up; but it has now
been satisfactorily ascertained that there is very little difference in the
saltness of different seas, and that however salt the water may be, the
boiler will be preserved from any injurious amount of incrustation by
blowing off, as it is called, very frequently, or by permitting a
considerable portion of the supersalted water to escape at short intervals
into the sea. If blowing off be sufficiently practised, the scale upon the
flues will never be much thicker than a sheet of writing paper, and _no
excuse_ should be accepted from engineers for permitting a boiler to be
damaged by the accumulation of calcareous deposit.

392. _Q._--What is the temperature at which sea water boils in a steam
boiler?

_A._--Sea water contains about 1/33rd its weight of salt, and in the open
air it boils at the temperature of 213.2°; if the proportion of salt be
increased to 2/33rds of the weight of the water, the boiling point will
rise to 214.4°; with 3/33rds of salt the boiling point will be 215.5°;
4/33rds, 216.7°; 5/33rds, 217.9°; 6/33rds, 219°; 7/33rds, 220.2°; 8/33rds,
221.4°; 9/33rds, 222.5°; 10/33rds, 223.7°; 11/33rds, 224.9°; and 12/33rds,
which is the point of saturation, 226°. In a steam boiler the boiling
points of water containing these proportions of salt must be higher, as the
elevation of temperature due to the pressure of the steam has to be added
to that due to the saltness of the water; the temperature of steam at the
atmospheric pressure being 212°, its temperature, at a pressure of 15 lbs.
per square inch above the atmosphere, will be 250°, and adding to this 4.7°
as the increased temperature due to the saltness of the water when it
contains 4/33rds of salt, we have 254.7° as the temperature of the water in
the boiler, when it contains 4/33rds of salt and the pressure of the steam
is 15 lbs. on the square inch.

393. _Q._--What degree of concentration of the salt water may be safely
permitted in a boiler?

_A._--It is found by experience that when the concentration of the salt
water in a boiler is prevented from exceeding that point at which it
contains 2/33rds its weight of salt, no injurious incrustation will take
place, and as sea water contains only 1/33rd of its weight of salt, it is
clear that it must be reduced by evaporation to one half of its bulk before
it can contain 2/33rds of salt; or, in other words, a boiler must blow out
into the sea one half of the water it receives as feed, in order to prevent
the water from rising above 2/33rds of concentration, or 8 ounces of salt
to the gallon.

394. _Q._--How do you determine 8 ounces to the gallon to be equivalent to
twice the density of salt water, or "two salt waters" as it is sometimes
called?

_A._--The density of the water of different seas varies somewhat. A gallon
of fresh water weighs 10 lbs.; a gallon of salt water from the Baltic
weighs 10.15 lbs.; a gallon of salt water from the Irish Channel weighs
10.28 lbs.; and a gallon of salt water from the Mediterranean 10.29 lbs. If
we take an average saltness represented by a weight of 10.25 lbs., then a
gallon of water concentrated to twice this saltness will weigh 10.5 lbs.,
or the salt in it will weigh .5 lbs or 8 oz., which is the proportion of 8
oz. to the gallon. However, the proportion of 2/33rds gives a greater
proportion than 8 oz. to the gallon, for 2/33 = 1/16 nearly, and 1/16 of 10
lbs. = 10 oz. By keeping the density of the water in a marine boiler at the
proportion of 8 or 10 oz. to the gallon, no inconvenient amount of scale
will be deposited on the flues or tubes. The bulk of water, it may be
remarked, is not increased by putting salt in it up to the point of
saturation, but only its density is increased.

395. _Q._--Is there not a great loss of heat by blowing off so large a
proportion of the heated water from the boiler?

_A._--The loss is not very great. Boilers are sometimes worked at a
saltness of 4/33rds, and taking this saltness and supposing the latent heat
of steam to be at 1000° at the temperature of 212°, and reckoning the sum
of the latent and sensible heats as forming a constant quantity, the latent
heat of steam at the temperature of 250° will be 962°, and the total heat
of the steam will be 1212° in the case of fresh water; but as the feed
water is sent into the boiler at the temperature of 100°, the accession of
heat it receives from the fuel will be 1112° in the case of fresh water, or
1112° increased by 3.98° in the case of water containing 4/33ds of salt--
the 3.98° being the 4.7° increase of temperature due to the presence of
4/33rds of salt, multiplied by 0.847 the specific heat of steam. This makes
the total accession of heat received by the steam in the boiler equal to
1115.98°, or say 1116°, which multiplied by 3, as 3 parts of the water are
raised into steam, gives us 3348° for the heat in the steam, while the
accession of heat received in the boiler by the 1 part of residual brine
will be 154.7°, multiplied by 0.85, the specific heat of the brine, or
130.495°; and 3348° divided by 130.495° is about 1/26th. It appears,
therefore, that by blowing off the boiler to such an extent that the
saltness shall not rise above what answers to 4/33rds of salt, about 1/25th
of the heat is blown into the sea; this is but a small proportion, and as
there will be a greater waste of heat, if from the existence of scale upon
the flues the heat can be only imperfectly transmitted to the water, there
cannot be even an economy of fuel in niggard blowing off, while it involves
the introduction of other evils. The proportion of 4/33rds of saltness,
however, or 16 oz. to the gallon, is larger than is advisable, especially
as it is difficult to keep the saltness at a perfectly uniform point, and
the working point should, therefore, be 2/33rds as before prescribed.

396. _Q._--Have no means been devised for turning to account the heat
contained in the brine which is expelled from the boiler?

_A._--To save a part of the heat lost by the operation of blowing off, the
hot brine is sometimes passed through a number of small tubes surrounded by
the feed water; but there is no very great gain from the use of such
apparatus, and the tubes are apt to become choked up, whereby the safety of
the boiler may be endangered by the injurious concentration of its
contents. Pumps, worked by the engine for the extraction of the brine, are
generally used in connection with the small tubes for the extraction of the
heat from the supersalted water; and if the tubes become choked the pumps
will cease to eject the water, while the engineer may consider them to be
all the while in operation.

397._Q._--What is the usual mode of blowing off the supersalted water from
the boiler?

_A._--The general mode of blowing off the boiler is to allow the water to
rise gradually for an hour or two above the lowest Working level, and then
to open the cock communicating with the sea, and keep it open until the
surface of the water within the boiler has fallen several inches; but in
some cases a cock of smaller size is allowed to run water continuously, and
in other cases brine pumps are used as already mentioned. In every case in
which the supersalted water is discharged from the boiler in a continuous
stream, a hydrometer or salt gauge of some convenient construction should
be applied to the boiler, so that the density of the water may at all times
be visible, and immediate notice be given of any interruption of the
operation. Various contrivances have been devised for this purpose, the
most of which operate on the principle of a hydrometer; but perhaps a more
satisfactory principle would be that of a differential steam gauge, which
would indicate the difference of pressure between the steam in the boiler
and the steam of a small quantity of fresh water enclosed in a suitable
vessel, and immerged in the water of the boiler.

398. _Q._--What is the advantage of blowing off from the surface of the
water in the boiler?

_A._--Blowing off from a point near the surface of the water is more
beneficial than blowing off from the bottom of the boiler. Solid particles
of any kind, it is well known, if introduced into boiling water, will lower
the boiling point in a slight degree, and the steam will chiefly be
generated on the surface of the particles, and indeed will have the
appearance of coming out of them; if the particles be small the steam
generated beneath and around them will balloon them to the surface of the
water, where the steam will be liberated and the particles will descend;
and the impalpable particles in a marine boiler, which by their subsidence
upon the flues concrete into scale, are carried in the first instance to
the surface of the water, so that if they be caught there and ejected from
the boiler, the formation of scale will be prevented.

399. _Q._--Are there any plans in operation for taking advantage of this
property of particles rising to the surface?

_A._--Advantage is taken of this property in Lamb's Scale Preventer, which
is substantially a contrivance for blowing off from the surface of the
water that in practice is found to be very effectual; but a float in
connection with a valve at the mouth of the discharging pipe is there
introduced, so as to regulate the quantity of water blown out by the height
of the water level, or by the extent of opening given to the feed cock. The
operation, however, of the contrivance would be much the same if the float
were dispensed with; but the float acts advantageously in hindering the
water from rising too high in the boiler, should too much feed be admitted,
and thereby obviates the risk of the water running over into the cylinder.
In some boilers sheet iron vessels, called sediment collectors, are
employed, which collect into them the impalpable matter, which in Lamb's
apparatus is ejected from the boiler at once. One of these vessels, of
about the size and shape of a loaf of sugar, is put into each boiler with
the apex of the cone turned downwards into a pipe leading overboard, for
conducting the sediment away from the boiler. The base of the cone stands
some distance above the water line, and in its sides conical slits are cut,
so as to establish a free communication between the water within the
conical vessel and the water outside it. The particles of stony matter
which are ballooned to the surface by the steam in every other part of the
boiler, subside within the cone, where, no steam being generated, the water
is consequently tranquil; and the deposit is discharged overboard by means
of a pipe communicating with the sea. By blowing off from the surface of
the water, the requisite cleansing action is obtained with less waste of
heat; and where the water is muddy, the foam upon the surface of the water
is ejected from the boiler--thereby removing one of the chief causes of
priming.

400. _Q._--What is the cause of the rapid corrosion of marine boilers?

_A._--Marine boilers are corroded externally in the region of the steam
chest by the dripping of water from the deck; the bottom of the boiler is
corroded by the action of the bilge water, and the ash pits by the practice
of quenching the ashes with, salt water. These sources of injury, however,
admit of easy remedy; the top of the boiler may be preserved from external
corrosion by covering it with felt upon which is laid sheet lead soldered
at every joint so as to be impenetrable to water; the ash pits may be
shielded by guard plates which are plates fitting into the ash pits and
attached to the boiler by a few bolts, so that when worn they may be
removed and new ones substituted, whereby any wear upon the boiler in that
part will be prevented; and there will be very little wear upon the bottom
of a boiler if it be imbedded in mastic cement laid upon a suitable
platform.

401. _Q._--Are not marine boilers subject to internal corrosion?

_A._--Yes; the greatest part of the corrosion of a boiler takes place in
the inside of the steam chest, and the origin of this corrosion is one of
the obscurest subjects in the whole range of engineering. It cannot be from
the chemical action of the salt water upon the iron, for the flues and
other parts of the boiler beneath the water suffer very little from
corrosion, and in steam vessels provided with Hall's condensers, which
supply the boiler with fresh water, not much increased durability of the
boiler has been experienced. Nevertheless, marine boilers seldom last more
than for 5 or 6 years, whereas land boilers made of the same quality of
iron often last 18 or 20 years, and it does not appear probable that land
boilers would last a very much shorter time if salt water were used in
them. The thin film of scale spread over the parts of a marine boiler
situated beneath the water, effectually protect them from corrosion; and
when the other parts are completely worn out the flues generally remain so
perfect, that the hammer marks upon them are as conspicuous as at their
first formation. The operation of the steam in corroding the interior of
the boiler is most capricious--the parts which are most rapidly worn away
in one boiler being untouched in another; and in some cases one side of a
steam chest will be very much wasted away while the opposite side remains
uninjured. Sometimes the iron exfoliates in the shape of a black oxide
which comes away in flakes like the leaves of a book, while in other cases
the iron appears as if eaten away by a strong acid which had a solvent
action upon it. The application of felt to the outside of a boiler, has in
several cases been found to accelerate sensibly its internal corrosion;
boilers in which there is a large accumulation of scale appear to be more
corroded than where there is no such deposit; and where the funnel passes
through the steam chest the iron of the steam chest is invariably much more
corroded than where the funnel does not pass through it.

402. _Q._--Can you suggest no reason for the rapid internal corrosion of
marine boilers?

_A._--The facts which I have enumerated appear to indicate that the
internal corrosion of marine boilers is attributable chiefly to the
existence of surcharged steam within them, which is steam to which an
additional quantity of heat has been communicated subsequently to its
generation, so that its temperature is greater than is due to its elastic
force; and on this hypothesis the observed facts relative to corrosion
become to some extent explicable. Felt, applied to the outside of a boiler,
may accelerate its internal corrosion by keeping the steam in a surcharged
state, when by the dispersion of a part of the heat it would cease to be in
that state; boilers in which there is a large accumulation of scale must
have worked with the water very salt, which necessarily produces surcharged
steam; for the temperature of steam cannot be less than that of the water
from which it is generated, and inasmuch as the boiling point of water,
under any given pressure, rises with the saltness of the water, the
temperature of the steam must rise with the saltness of the water, the
pressure remaining the same; or, in other words, the steam must have a
higher temperature than is due to its elastic force, or be in the state of
surcharged steam. The circumstance of the chimney flue passing through the
steam will manifestly surcharge the steam with heat, so that all the
circumstances which are found to accelerate corrosion, are it appears such
as would also induce the formation of surcharged steam.

403. _Q._--Is it the natural effect of surcharged steam to waste away iron?

_A._--It is the natural effect of surcharged steam to oxidate the iron with
which it is in contact, as is illustrated by the familiar process for
making hydrogen gas by sending steam through a red hot tube filled with
pieces of iron; and although the action of the surcharged steam in a boiler
is necessarily very much weaker than where the iron is red hot, it
manifestly must have _some_ oxidizing effect, and the amount of corrosion
produced may be very material where the action is perpetual. Boilers with a
large extent of heating surface, or with descending flues circulating
through the cooler water in the bottom of the boiler before ascending the
chimney, will be less corroded internally than boilers in which a large
quantity of the heat passes away in the smoke; and the corrosion of the
boiler will be diminished if the interior of any flue passing through the
steam be coated with fire brick, so as to present the transmission of the
heat in that situation. The best practice, however, appears to consist in
the transmission of the smoke through a suitable passage on the outside of
the boiler, so as to supersede the necessity of carrying any flue through
the steam at all; or a column of water may be carried round the chimney,
into which as much of the feed water may be introduced as the heat of the
chimney is capable of raising to the boiling point, as under this
limitation the presence of feed water around the chimney in the steam chest
will fail to condense the steam.

404. _Q._--In steam vessels there are usually several boilers?

_A._--Yes, in steam vessels of considerable power and size.

405. _Q._--Are these boilers generally so constructed, that any one of them
may be thrown out of use?

_A._--Marine boilers are now generally supplied with stop valves, whereby
one boiler may be thrown out of use without impairing the efficacy of the
remainder. These stop valves are usually spindle valves of large size, and
they are for the most part set in a pipe which runs across the steam
chests, connecting the several boilers together. The spindles of these
valves should project through stuffing boxes in the covers of the valve
chests, and they should be balanced by a weighted lever, and kept in
continual action by the steam. If the valves be lifted up, and be suffered
to remain up, as is the usual practice, they will become fixed by corrosion
in that position, and it will be impossible after some time to shut them on
an emergency. These valves should always be easily accessible from the
engine room; and it ought not to be necessary for the coal boxes to be
empty to gain access to them.

406. _Q._--Should each boiler have at least one safety valve for itself?

_A._--Yes; it would be quite unsafe without this provision, as the stop
valve might possibly jam. Sometimes valves jam from a distortion in the
shape of the boiler when a considerable pressure is put upon it.

407. _Q._--How is the admission of the water into the boiler regulated?

_A._--The admission of feed water into the boiler is regulated by hand by
the engineer by means of cocks, and sometimes by spindle valves raised and
lowered by a screw. Cocks appear to be the preferable expedient, as they
are less liable to accident or derangement than screw valves, and in modern
steam vessels they are generally employed.

408. _Q._--At what point of the boiler is the feed introduced?

_A._--The feed water is usually conducted from the feed cock to a point
near the bottom of the boiler by means of an internal pipe, the object of
this arrangement being to prevent the rising steam from being condensed by
the entering water. By being introduced near the bottom of the boiler, the
water comes into contact in the first place with the bottoms of the
furnaces and flues, and extracts heat from them which could not be
extracted by water of a higher temperature, whereby a saving of fuel is
accomplished. In some cases the feed water is introduced into a casing
around the chimney, from whence it descends into the boiler. This plan
appears to be an expedient one when the boiler is short of heating surface,
and more than a usual quantity of heat ascends the chimney; but in well
proportioned boilers a water casing round the chimney is superfluous. When
a water casing is used the boiler is generally fed by a head of water, the
feed water being forced up into a small tank, from whence it descends into
the boiler by the force of gravity, while the surplus runs to waste, as in
the feeding apparatus of land engines.

409. _Q._--Suppose that the engineer should shut off the feed water from
the boilers while the engine was working, what would be the result?

_A._--The result would be to burst the feed pipes, except for a safety
valve placed on the feed pipe between the engine and the boilers, which
safety valve opens when any undue pressure comes upon the pipes, and allows
the water to escape. There is, however, generally a cock on the suction
side of the feed pump, which regulates the quantity of water drawn into the
pump. But there must be cocks on the boilers also to determine into which
boiler the water shall be chiefly discharged, and these cocks are sometimes
all shut accidentally at the same time.

410. _Q._--Is there no expedient in use in steam vessels for enabling the
position of the water level in the boiler to determine the quantity of feed
water admitted?

_A._--In some steam vessels floats have been introduced to regulate the
feed, but their action cannot be depended on in agitated water, if applied
after the common fashion. Floats would probably answer if placed in a
cylinder which communicates with the water in the boiler by means of small
holes; and a disc of metal might be attached to the end of a rod extending
beneath the water level, so as to resist irregular movements from the
motion of the ship at sea, which would otherwise impair the action of the
apparatus.

411. _Q._--How is the proper level of the water in the boiler of a steam
vessel maintained when, the engine is stopped for some time, and the boiler
is blowing off steam?

_A._--By means of a separate pump worked sometimes by hand, but usually by
a small separate engine called the Donkey engine. This pump, by the aid of
suitable cocks, will pump from the sea into the boiler; from the sea upon
deck either to wash decks or to extinguish fire; and from the bilge
overboard, through a suitable orifice in the side of the ship.



LOCOMOTIVE BOILERS.

412. _Q._--Will you recapitulate the general features of locomotive
boilers?

_A._--Locomotive boilers consist of three portions (see fig. 29): the
barrel E, E, containing the tubes, the fire box B, and the smoke box F; of
which the barrel smoke box, and external fire box are always of iron, but
the internal fire box is generally made of copper, though sometimes also it
is made of iron. The tubes are sometimes of iron, but generally of brass
fixed in by ferules. The whole of the iron plates of a locomotive boiler
Which are subjected to the pressure of steam, should be Lowmoor or Bowling
plates of the best quality; and the copper should be coarse grained, rather
than rich or soft, and be perfectly free from irregularities of structure
and lamination.

413. _Q._--What are the usual dimensions of the barrel?

_A._--The thickness of the plates composing the barrel of the boiler varies
generally from 5/16ths to 3/8ths of an inch, and the plates should run in
the direction of the circumference, so that the fibres of the iron may be
in the direction of the strain. The diameter of the barrel commonly varies
from 3 ft. to 3 ft. 6 inches; the diameter of the rivets should be from
11/16ths to 3/4ths of an inch, and the pitch of the rivets or distance
between their centres should be from 17/8th to 2 inches.

414. _Q._--How are the fire boxes of a locomotive constructed?

_A._--The space between the external and internal fire boxes forms a water
space, which must be stayed every 4-1/2 or 5 inches by means of copper or
iron stay bolts, screwed through the outer fire box into the metal of the
inner fire box, and securely riveted within it: iron stay bolts are as
durable as copper, and their superior tenacity gives them an advantage.
Sometimes tubes are employed as stays. The internal and external fire boxes
are joined together at the bottom by a N-shaped iron, and round the fire
door they are connected by means of a copper ring 1-1/4 in. thick, and 2
in. broad,--the inner fire box being dished sufficiently outward at that
point, and the outer fire box sufficiently inward, to enable a circle of
rivets 3/4 of an inch in diameter passing through the copper ring and the
two thicknesses of iron, to make a water-tight joint. The thickness of the
plates composing the external fire box is in general 3/8ths of an inch if
the fire box is circular, and from 3/8ths to 1/2 inch if the fire box is
square; and the thickness of the internal fire box is in most cases 7/16ths
if copper, and from 3/8ths to 7/16ths of an inch if of iron. Circular
internal fire boxes, if made of iron, should be welded rather than riveted,
as the rivet heads are liable to be burnt away by the action of the fire;
and when the fire boxes are square each side should consist of a single
plate, turned over at the edges with a radius of 3 inches, for the
introduction of the rivets.

415. _Q._--Is there any provision for stiffening the crown of the furnace
in a locomotive?

_A._--The roof of the internal fire box, whether flat as in Stephenson's
engines, or dome shaped as in Bury's, requires to be stiffened with cross
stay bars, but the bars require to be stronger and more numerous when
applied to a flat surface. The ends of these stay bars rest above the
vertical sides of the fire box; and to the stay bars thus extending across
the crown, the crown is attached at intervals by means of stay bolts. There
are projecting bosses upon the stay bars encircling the bolts at every
point where a bolt goes through, but in the other parts they are kept clear
of the fire box crown so as to permit the access of water to the metal;
and, with the view of facilitating the ascent of the steam, the bottom of
each stay bar should be sharpened away in those parts where it does not
touch the boiler.

416. _Q._--Is any inconvenience experienced from the intense heat in a
locomotive furnace?

_A._--The fire bars in locomotives have always been a source of trouble, as
from the intensity of the heat in the furnace they become so hot as to
throw off a scale, and to bend under the weight of the fuel. The best
alleviation of these evils lies in making the bars deep and thin: 4 or 5
inches deep by five eighths of an inch thick on the upper side, and three
eighths of an inch on the under side, are found in practice to be good
dimensions. In some locomotives a frame carrying a number of fire bars is
made so that it may be dropped suddenly by loosening a catch; but it is
found that any such mechanism can rarely be long kept in working order, as
the molten clinker by running down between the frame and the boiler will
generally glue the frame into its place. It is therefore found preferable
to fix the frame, and to lift up the bars by the dart used by the stoker,
when any cause requires the fire to be withdrawn. The furnace bars of
locomotives are always made of malleable iron, and indeed for every species
of boiler malleable iron bars are to be preferred to bars of cast iron, as
they are more durable, and may if thin be set closer together, whereby the
small coal or coke is saved that would otherwise fall into the ash pit. The
ash box of locomotives is made of plate iron, a quarter thick: it should
not be less than 10 in. deep, and its bottom should be about 9 in. above
the level of the rails. The chimney of a locomotive is made of plate iron
one eighth of an inch thick: it is usually of the same diameter as the
cylinder, but is better smaller, and must not stand more than 14 ft. high
above the level of the rails.

417. _Q._--Are locomotive boilers provided with a steam chest?

_A._--The upper portion of the external fire box is usually formed into a
steam chest, which is sometimes dome shaped, sometimes semicircular, and
sometimes of a pyramidical form, and from this steam chest the steam is
conducted away by an internal pipe to the cylinders; but in other cases an
independent steam chest is set upon the barrel of the boiler, consisting of
a plate iron cylinder, 20 inches in diameter, 2 feet high, and three
eighths of an inch thick, with a dome shaped top, and with the seam welded
and the edge turned over to form a flange of attachment to the boiler. The
pyramidical dome, of the form employed in Stephenson's locomotives,
presents a considerable extent of flat surface to the pressure of the
steam, and this flat surface requires to be very strongly stayed with angle
irons and tension rods; whereas the semiglobular dome of the kind employed
in Bury's engines requires no staying whatever. Latterly, however, these
domes over the fire box have been either much reduced in size or abandoned
altogether.

418. _Q._--Is any beneficial use made of the surplus steam of a locomotive?

_A._--To save the steam which is formed when the engine is stationary, a
pipe is usually fitted to the boiler, which on a cock being turned conducts
the steam into the water in the tender, whereby the feed water is heated,
and less fuel is subsequently required. This method of disposing of the
surplus steam may be adopted when the locomotive is descending inclines, or
on any occasion where more steam is produced than the engine can consume.

419._Q._--What means are provided to facilitate the inspection and cleaning
of locomotive boilers?

_A._--The man hole, or entrance into the boiler, consists of a circular or
oval aperture of about 15 in. diameter, placed in Bury's locomotive at the
apex of the dome, and in Stephenson's upon the front of the boiler, a few
inches below the level of the rounded part; and the cover of the man hole
in Bury's engine contains the safety valve seats. In whatever situation
this man hole is placed, the surfaces of the ring encircling the hole, and
of the internal part of the door or cover, should be accurately fitted
together by scraping or grinding, so that they need only the interposition
of a little red lead to make them quite tight when screwed together. Lead
or canvas joints, if of any considerable thickness, will not long withstand
the action of high pressure steam; and the whole of the joints about a
locomotive should be such that they require nothing more than a little
paint or putty, or a ring of wire gauze smeared with white or red lead to
make them perfectly tight. There must be a mud hole opposite the edge of
each water space, if the fire box be square, to enable the boiler to be
easily cleaned out, and these holes are most conveniently closed by screwed
plugs made slightly taper. A cock for emptying the boiler is usually fixed
at the bottom of the fire box, and it should be so placed as to be
accessible when the engine is at work, in order that the engine driver may
blow off some water if necessary; but it must not be in such a position as
to send the water blown off among the machinery, as it might carry sand or
grit into the bearings, to their manifest injury.

420. _Q._--Will you state the dimensions of the tube plate, and the means
of securing the tubes in it?

_A._--The tube plates are generally made from five eighths to three fourths
of an inch thick, but seven eighths of an inch thick appears to be
preferable, as when the plate is thick the holes will not be so liable to
change their figure during the process of feruling the tubes: the distance
between the tubes should never be made less than three fourths of an inch,
and the holes should be slightly tapered so as to enable the tubes to hold
the tube plates together. The tubes are secured in the tube plates by means
of taper ferules driven into the ends of the tubes. The ferules are for the
most part made of steel at the fire box end, and of wrought iron at the
smoke box end, though ferules of malleable cast iron have in some cases
been used with advantage: malleable cast iron ferules are almost as easily
expanded when hammered cold upon a mandrel, as the common wrought iron ones
are at a working heat. Spring steel, rolled with a feather edge, to
facilitate its conversion into ferules, is supplied by some of the
steel-makers of Sheffield, and it appears expedient to make use of steel
thus prepared when steel ferules are employed. In cases where ferules are
not employed, it may be advisable to set out the tube behind the tube plate
by means of an expanding mandrel. There are various forms of this
instrument. One form is that known as Prosser's expanding mandrel, in which
there are six or eight segments, which are forced out by means of a
hexagonal or octagonal wedge, which is forced forward by a screw. When the
wedge is withdrawn, the segments collapse sufficiently to enable them to
enter the tube, and there is an annular protuberance on the exterior circle
of the segments, which protuberance, when the mandrel is put into the tube,
just comes behind the inner edge of the tube plate. When the wedge is
tightened up by the screw, the protuberance on the exterior of the segments
composing the mandrel causes a corresponding bulge to take place in the
tube, at the back of the tube plate, and the tube is thereby brought into
more intimate contact with the tube plate than would otherwise be the case.
There is a steel ring indented into the segments of Prosser's mandrel, to
contract the segments when the central wedge is withdrawn. A more
convenient form of the instrument, however, is obtained by placing the
segments in a circular box, with one end projecting; and supporting each
segment in the box by a tenon, which fits into a mortise in the cylindrical
box. To expand the segments, a round tapered piece of steel, like a drift,
is forced into a central hole, round which the segments are arranged. A
piece of steel tube, also slit up to enable a central drift to expand it,
answers very well; but the thickness of that part of the tube in which
there requires to be spring enough to let the mandrel expand, requires to
be sufficiently reduced to prevent the pieces from cracking when the
central drift is driven in by a hammer. The drift is better when made with
a globular head, so that it may be struck back by the hammer, as well as be
driven in. An expanding mandrel, with a central drift, is more rapid in its
operation than when the expansion is produced by means of a screw.

421. _Q._--Will you explain the means that are adopted to regulate the
admission of steam to the cylinders?

_A._--In locomotives, the admission of the steam from the boiler to the
cylinders is regulated by a valve called the regulator, which is generally
placed immediately above the internal fire box, and is connected with two
copper pipes;--one conducting steam from the highest point of the dome down
to it, and the other conducting the steam that has passed through it along
the boiler to the upper part of the smoke box. Regulators may be divided
into two sorts, viz., those with, sliding valves and steam ports, and those
with conical valves and seats, of which the latter kind are the best. The
former kind have for the most part consisted of a circular valve and face,
with radial apertures, the valve resembling the outstretched wings of a
butterfly, and being made to revolve on its central pivot by connecting
links between its outer edges, or by its central spindle. In some of
Stephenson's engines the regulator consists of a slide valve covering a
port on the top of the valve chests. A rod passes from this valve through
the smoke box below the boiler, and by means of a lever parallel to the
starting lever, is brought up to the engineer's reach. Cocks were at first
used as regulators, but were given up, as they were found liable to stick
fast. A gridiron slide valve has been used by Stephenson, which consists of
a perforated square moving upon a face with an equal number of holes. This
plan of a valve gives, with a small movement, a large area of opening. In
Bury's engines a sort of conical plug is used, which is withdrawn by
turning the handle in front of the fire box: a spiral grove of a very large
pitch is made in the valve spindle, in which fits a pin fixed to the
boiler, and by turning the spindle an end motion is given to it, which
either shuts or opens the steam passage according to the direction in which
it is turned. The best regulator would probably be a valve of the
equilibrium description, such as is used in the Cornish engine: there would
be no friction in such a regulator, and it could be opened or shut with a
small amount of force. Such valves, indeed, are now sometimes employed for
regulators in locomotives.




CHAPTER VIII.


CONSTRUCTIVE DETAILS OF ENGINES.


PUMPING ENGINES.

422. _Q._--Will you explain the course of procedure in the erection of a
pumping engine, such as Boulton and Watt introduced into Cornwall?

_A._--The best instructions on this subject are those of Mr. Watt himself,
which are as follows:--Having fixed on the proper situation of the pump in
the pit, from its centre measure out the distance to the centre of the
cylinder, from which set off all the other dimensions of the house,
including the thickness of the walls, and dig out the whole of the included
ground to the depth of the bottom of the cellar, so that the bottom of the
cylinder may stand on a level with the natural ground of the place, or
lower, if convenient, for the less the height of the house above the
ground, the firmer it will be. The foundations of the walls must be laid at
least two feet lower than the bottom of the cellar, unless the foundation
be firm rock; and care must be taken to leave a small drain into the pit
quite through the lowest part of the foundation of the lever wall, to let
off any water that may be spilt in the engine house, or may naturally come
into the cellar. If the foundation at that depth does not prove good, you
must either go down to a better if in your reach, or make it good by a
platform of wood or piles, or both.

423. _Q._--These directions refer to the foundations?

_A._.--Yes; but I will now proceed to the other parts. Within the house,
low walls must be built to carry the cylinder beams, so as to leave
sufficient room to come at the holding down bolts, and the ends of these
beams must also be lodged in the wall The lever wall must be built in the
firmest manner, and run solid, course by course, with thin lime mortar,
care being taken that the lime has not been long slaked. If the house be
built of stone, let the stones be large and long, and let many headers be
laid through the wall: it should also be a rule, that every stone be laid
on the broadest bed it has, and never set on its edge. A course or two
above the lintel of the door that leads to the condenser, build into the
wall two parallel flat thin bars of iron equally distant from each other,
and from the outside and inside of the wall, and reaching the whole breadth
of the lever wall. About a foot higher in the wall, lay at every four feet
of the breadth of the front, other bars of the same kind at right angles to
the former course, and reaching quite through the thickness of the wall;
and at each front corner lay a long bar in the middle of the side walls,
and reaching quite through the front wall; if these bars are 10 feet or 12
feet long it will be sufficient. When the house is built up nearly to the
bottom of the opening under the great beam another double course of bars is
to be built in, as has been directed. At the level of the upper cylinder
beams, holes must be left in the walls for their ends, with room to move
them laterally, so that the cylinder may be got in; and smaller holes must
be left quite through the walls for the introduction of iron bars, which
being firmly fastened to the cylinder beams at one end, and screwed at the
other or outer end, will serve, by their going through both the front and
back walls, to bind the house more firmly together. The spring beams or
iron bars fastened to them must reach quite through the back wall, and be
keyed or screwed up tight; and they must be firmly fastened to the lever
wall on each side, either by iron bars, firm pieces of wood, or long strong
stones, reaching far back into the wall. They must also be bedded solidly,
and the residue of the opening must be built up in the firmest manner.

424. _Q._--If there be a deficiency of water for the purpose of
condensation, what course should be pursued?

_A._--If there be no water in the neighborhood that can be employed for the
purpose of condensation, it will be necessary to make a pond, dug in the
earth, for the reception of the water delivered by the air pump, to the end
that it may be cooled and used again for the engine. The pond may be three
or four feet deep, and lined with turf, puddled, or otherwise made water
tight. Throwing up the water into the air in the form of a jet to cool it,
has been found detrimental; as the water is then charged with air which
vitiates the vacuum.

425. _Q._--How is the piston of a pumping engine packed?

_A._--To pack the piston, take sixty common-sized white or untarred
rope-yarns, and with them plait a gasket or flat rope as close and firm as
possible, tapering for eighteen inches at each end, and long enough to go
round the piston, and overlapped for that length; coil this rope the thin
way as hard as possible, and beat it with a sledge hammer until its breadth
answers the place; put it in and beat it down with a wooden drift and a
hand mallet, pour some melted tallow all around, then pack in a layer of
white oakum half an inch thick, so that the whole packing may have the
depth of five to six inches, depending on the size of the engine; finally,
screw down the junk ring. The packing should be beat solid, but not too
hard, otherwise it will create so great a friction as to prevent the easy
going of the engine. Abundance of tallow should be allowed, especially at
first; the quantity required will be less as the cylinder grows smooth. In
some of the more modern pumping engines, the piston is provided with
metallic packing, consisting for the most part of a single ring with a
tongue piece to break the joint, and packed behind with hemp. The upper
edge of the metallic ring is sharpened away from the inside so as to permit
more conveniently the application of hemp packing behind it; and the junk
ring is made much the same as if no metallic packing were employed.

426. _Q._--Will you explain the mode of putting the engine into operation?

_A._--To set the engine going, the steam must be raised until the pressure
in the steam pipe is at least equal to three pounds on the square inch; and
when the cylinder jacket is fully warmed, and steam issues freely from the
jacket cock, open all the valves or regulators; the steam will then
forcibly blow out the air or water contained in the eduction pipe, and to
get rid of the air in the cylinder, shut the steam valve after having blown
through the engine for a few minutes. The cold water round the condenser
will condense some of the steam contained in the eduction pipe, and its
place will be supplied by some of the air from the cylinder. The steam
valve must again be opened to blow out that air, and the operation is to be
repeated until the air is all drawn out of the cylinder. When that is the
case shut all the valves, and observe if the vacuum gauge shows a vacuum in
the condenser; when there is a vacuum equivalent to three inches of
mercury, open the injection a very little, and shut it again immediately;
and if this produces any considerable vacuum, open the exhausting valve a
very little way, and the injection at the same time. If the engine does not
now commence its motion, it must be blown through again until it moves. If
the engine be lightly loaded, or if there be no water in the pumps, the
throttle valve must be kept nearly closed, and the top and exhaustion
regulators must be opened only a very little way, else the engine will make
its stroke with violence, and perhaps do mischief. If there is much
unbalanced weight on the pump end, the plug which opens the steam valve
must be so regulated, that the valve will only be opened very slightly; and
if after a few strokes it is found that the engine goes out too slowly, the
valve may be then so adjusted as to open wider. The engine should always be
made to work full stroke, that is, until the catch pins be made to come
within half an inch of the springs at each end, and the piston should stand
high enough in the cylinder when the engine is at rest, to spill over into
the perpendicular steam pipe any water which may be condensed above it; for
if water remain upon the piston, it will increase the consumption of steam.
When the engine is to be stopped, shut the injection valve and secure it,
and adjust the tappets so as to prevent the exhausting valve from opening
and to allow the steam valve to open and remain open, otherwise a partial
vacuum may arise in the cylinder, and it may be filled with water from the
injection or from leaks. A single acting engine, when it is in good order,
ought to be capable of going as slow as one stroke in ten minutes, and as
fast as ten strokes in one minute; and if it does not fulfil these
conditions, there is some fault which should be ascertained and remedied.

427. _Q._--Your explanation has reference to the pumping engine as
introduced into Cornwall by Watt: have any modifications been since made
upon it?

_A._--In the modern Cornish engines the steam is used very expansively, and
a high pressure of steam is employed. In some cases a double cylinder
engine is used, in which the steam, after having given motion to a small
piston on the principle of a high pressure engine, passes into a larger
cylinder, where it operates on the principle of a condensing engine; but
there is no superior effect gained by the use of two cylinders, and there
is greater complexity in the apparatus. Instead of the lever walls, cast
iron columns are now frequently used for supporting the main beam in
pumping engines, and the cylinder end of the main beam is generally made
longer than the pump end in engines made in Cornwall, so as to enable the
cylinder to have a long stroke, and the piston to move quickly, without
communicating such a velocity to the pump buckets as will make them work
with such a shock as to wear themselves out quickly. A high pressure of
steam, too, can be employed where the stroke is long, without involving the
necessity of making the working parts of such large dimensions as would
otherwise be necessary; for the strength of the parts of a single acting
engine will require to be much the same, whatever the length of the stroke
may be.

428. _Q._--What kind of pump is mostly used in draining deep mines?

_A._--The pump now universally preferred is the plunger pump, which admits
of being packed or tightened while the engine is at work; but the lowest
lift of a mine is generally supplied with a pump on the suction principle,
both with the view of enabling the lowest pipe to follow the water with
facility as the shaft is sunk deeper, and to obviate the inconvenience of
the valves of the pump being rendered inaccessible by any flooding in the
mine. The pump valves of deep mines are a perpetual source of expense and
trouble, as from the pressure of water upon them it is difficult to prevent
them from closing with violence; and many expedients have been contrived to
mitigate the evil, of which the valve known as Harvey and West's valve has
perhaps gained the widest acceptation.

429. _Q._--Will you describe Harvey and West's pump valve?

_A._--This valve is a compromise between the equilibrium valve, of the kind
employed for admitting the steam to and from the cylinder in single acting
engines, and the common spindle valve formerly used for that purpose; and
to comprehend its action, it is necessary that the action of the
equilibrium valve, which has been already represented fig. 34, should first
be understood. This valve consists substantially of a cylinder open at both
ends, and capable of sliding upon a stationary piston fixed upon a rod the
length of the cylinder, which proceeds from the centre of the orifice the
valve is intended to close. It is clear, that when the cylinder is pressed
down until its edge rests upon the bottom of the box containing it, the
orifice of the pipe must be closed, as the steam can neither escape past
the edge of the cylinder nor between the cylinder and the piston; and it is
equally clear, that as the pressure upon the cylinder is equal all around
it, and the whole of the downward pressure is maintained by the stationary
piston, the cylinder can be raised or lowered without any further exertion
of force than is necessary to overcome the friction of the piston and of
the rod by which the cylinder is raised. Instead of the rubbing surface of
a piston, however, a conical valve face between the cylinder and piston is
employed, which is tight only when the cylinder is in its lowest position;
and there is a similar face between, the edge of the cylinder and the
bottom of the box in which it is placed. The moving part of the valve, too,
instead of being a perfect cylinder, is bulged outward in the middle, so as
to permit the steam to escape past the stationary piston when the
cylindrical part of the valve is raised. It is clear, that if such a valve
were applied to a pump, no pressure of water within the pump would suffice
to open it, neither would any pressure of water above the valve cause it to
shut with violence; and if an equilibrium valve, therefore, be used as a
pump valve at all, it must be opened and shut by mechanical means. In
Harvey and West's valves, however, the equilibrium principle is only
partially adopted; the lower face is considerably larger in diameter than
the upper face, and the difference constitutes an annulus of pressure,
which will cause the valve to open or shut with the same force as a spindle
valve of the area of the annulus. To deaden the shock still more
effectually, the lower face of the valve is made to strike upon end wood
driven into an annular recess in the pump bucket; and valves thus
constructed work with very little noise or tremor; but it is found in
practice, that the use of Harvey and West's valve, or any contrivance of a
similar kind, adds materially to the load upon the pump, especially in low
lifts where the addition of a load, to the valve makes a material addition
to the total resistance which the engine has to overcome. Instead of end
wood driven into a recess for the valve to strike upon, a mixture of tin
and lead cast in a recess is now frequently used, and is found to be
preferable to the wood.

430. _Q._--Is there any other kind of pump valve which is free from the
shocks incidental to the working of common valves?

_A._--In some cases canvass valves are used for pumps, with the effect of
materially mitigating the shock; but they require frequent renewal, and are
of inferior eligibility in their action to the slide valve, which might in
many cases be applied to pumps without inconvenience.

431. _Q._--Could not a form of pump be devised capable of working without
valves at all?

_A._.--It appears probable, that by working a common reciprocating pump at
a high speed, a continuous flow of water might be maintained through the
pipes in such a way as to render the existence of any valves superfluous
after once the action was begun, the momentum of the moving water acting in
fact as valves. The centrifugal pump, however, threatens to supersede pumps
of every other kind; and if the centrifugal pump be employed there will be
no necessity for pump valves at all. There is less loss of effect by the
centrifugal pump than by the common pump.

432. _Q._--What is the best form of the centrifugal pump?

_A._--There are two forms in which the centrifugal pump may be applied to
mines;--that in which the arms diverge from the bottom, like the letter V;
and that in which revolving arms are set in a tight case near the bottom of
the mine, and are turned by a shaft from the surface. Such pumps both draw
and force; and either by arranging them in a succession of lifts in the
shaft of the mine, or otherwise, the water may be drawn without
inconvenience from any depth. The introduction of the centrifugal pump
would obviously extinguish the single acting engine, as rotative engines
working at a high speed would be the most appropriate form of engine where
the centrifugal pump was employed.

433. _Q._--This would not be a heavy deprivation?

_A._--The single acting engine is a remnant of engineering barbarism which
must now be superseded by more compendious contrivances. The Cornish
engines, though rudely manufactured, are very expensive in production, as a
large engine does but little work; whereas by employing a smaller engine,
moving with a high speed, the dimensions may be so far diminished that the
most refined machinery may be obtained at less than the present cost.

434. _Q._--Are not the Cornish engines more economical in fuel than other
engines?

_A._--It is a mistake to suppose that there is any peculiar virtue in the
existing form of Cornish engine to make it economical in fuel, or that a
less lethargic engine would necessarily be less efficient. The large duty
of the engines in Cornwall is traceable to the large employment of the
principle of expansion, and to a few other causes which may be made of
quite as decisive efficacy in smaller engines working with a quicker speed;
and there is therefore no argument in the performance of the present
engines against the proposed substitution.



VARIOUS FORMS OF MARINE ENGINES.

435. _Q._--What species of paddle engine do you consider to be the best?

_A._--The oscillating engine.

436. _Q._--Will you explain the grounds of that preference?

_A._--The engine occupies little space, consists of few parts, is easily
accessible for repairs, and may be both light and strong at the same time.
In the case of large engines the crank in the intermediate shaft is a
disadvantage, as it is difficult to obtain such a forging quite sound. But
by forging it in three cranked flat bars, which are then laid together and
welded into a square shaft, a sound forging will be more probable, and the
bars should be rounded a little on the sides which are welded to allow the
scoriae to escape during that operation. It is important in so large a
forging not to let the fire be too fierce, else the surface of the iron
will be burnt before the heart is brought to a welding heat. In some cases
in oscillating engines the air pump has been wrought by an eccentric, and
that may at any time be done where doubt of obtaining a sound intermediate
shaft is entertained; but the precaution must be taken to make the
eccentric very wide so as to distribute the pressure over a large surface,
else the eccentric will be apt to heat.

437. _Q._--Have not objections been brought against the oscillating engine?

_A._--In common with every other improvement, the oscillating engine, at
the time of its introduction, encountered much opposition. The cylinder, it
was said, would become oval, the trunnion bearings would be liable to heat
and the trunnion joints to leak, the strain upon the trunnions would be apt
to bend in or bend out the sides of the cylinder; and the circumstance of
the cylinder being fixed across its centre, while the shaft requires to
accommodate itself to the working of the ship, might, it was thought, be
the occasion of such a strain upon the trunnions as would either break them
or bend the piston rod. It is a sufficient reply to these objections to say
that they are all hypothetical, and that none of them in practice have been
found to exist--to such an extent at least as to occasion any
inconvenience; but it is not difficult to show that they are altogether
unsubstantial, even without a recourse to the disproofs afforded by
experience.

438. _Q._--Is there not a liability in the cylinder to become oval from the
strain thrown on it by the piston?

_A._--There is, no doubt, a tendency in oscillating engines for the
cylinder and the stuffing box to become oval, but after a number of years'
wear it is found that the amount of ellipticity is less than that which is
found to exist in the cylinders of side lever engines after a similar
trial. The resistance opposed by friction to the oscillation of the
cylinder is so small, that a man is capable of moving a large cylinder with
one hand; whereas in the side lever engine, if the parallel motion be in
the least untrue, which is, at some time or other, an almost inevitable
condition, the piston is pushed with great force against the side of the
cylinder, whereby a large amount of wear and friction is occasioned. The
trunnion bearings, instead of being liable to heat like other journals, are
kept down to the temperature of the steam by the flow of steam passing
through them; and the trunnion packings are not liable to leak when the
packings, before being introduced, are squeezed in a cylindrical mould.

439. _Q._--Might not the eduction trunnions be immersed in water?

_A._--In some cases a hollow, or lantern brass, about one third or one
fourth the length of the packing space, and supplied with steam or water by
a pipe, is introduced in the middle of the packing, so that if there be any
leakage through the trunnion, it will be a leakage of steam or water, which
will not vitiate the vacuum; but in ordinary cases this device will not be
necessary, and it is not commonly employed. It is clear that there can be
no buckling of the sides of the cylinder by the strain upon the trunnions,
if the cylinder be made strong enough, and in cylinders of the ordinary
thickness such an action has never been experienced; nor is it the fact,
that the intermediate shaft of steam vessels, to which part alone the
motion is communicated by the engine, requires to adapt itself to the
altering forms of the vessel, as the engine and intermediate shaft are
rigidly connected, although the paddle shaft requires to be capable of such
an adaptation. Even if this objection existed, however, it could easily be
met by making the crank pin of the ball and socket fashion, which would
permit the position of the intermediate shaft, relatively with that of the
cylinder, to be slightly changed, without throwing an undue strain upon any
of the working parts.

440. _Q._--Is the trunk engine inferior to the oscillating?

_A._--A very elegant and efficient arrangement of trunk engine suitable for
paddle vessels has latterly been employed by Messrs. Rennie, of which all
the parts resemble those of Penn's oscillating engine except that the
cylinders are stationary instead of being movable; and a round trunk or
pipe set upon the piston, and moving steam tight through the cylinder
cover, enables the connecting rod which is fixed to the piston to vibrate
within it to the requisite extent. But the vice of all trunk engines is
that they are necessarily more wasteful of steam, as the large mass of
metal entering into the composition of the trunk, moving as it does
alternately into the atmosphere and the steam, must cool and condense a
part of the steam. The radiation of heat from the interior of the trunk
will have the same operation, though in vertical trunk engines the loss
from this cause might probably be reduced by filling the trunk with oil, so
far as this could be done without the oil being spilt over the edge.

441. _Q._--What species of screw engine do you consider the best?

_A._--I am inclined to give the preference to a variety of the horizontal
steeple engine, such as was first used in H.M.S. Amphion. In this engine
the cylinders lie on their sides, and they are placed near the side of the
vessel with their mouths pointing to the keel. From each cylinder two long
piston rods proceed across the vessel to a cross head working in guides;
and from this cross head a connecting rod returns back to the centre of the
vessel and gives motion to the crank. The piston rods are so placed in the
piston that one of them passes above the crank shaft, and the other below
the crank shaft. The cross head lies in the same horizontal plane as the
centre of the cylinder, and a lug projects upwards from the cross head to
engage one piston rod, and downwards from the cross head to engage the
other piston rod. The air pump is double acting, and its piston or bucket
has the same stroke as the piston of the engine. The air pump bucket
derives its motion from an arm on the cross head, and a similar arm is
usually employed in engines of this class to work the feed and bilge pumps.

442. _Q._--Is not inconvenience experienced in direct acting screw engines
from the great velocity of their motion?

_A._--Not if they are properly constructed; but they require to be much
stronger, to be fitted with more care, and to have the bearing surfaces
much larger than is necessary in engines moving slowly. The momentum of the
reciprocating parts should also be balanced by a weight applied to the
crank or crank shaft, as is done in locomotives. A very convenient
arrangement for obtaining surface is to form the crank of each engine of
two cast iron discs cast with heavy sides, the excess of weight upon the
heavy sides being nearly equal to that of the piston and its connections.
When the piston is travelling in one direction the weights are travelling
in the opposite; and the momentum of the piston and its attachments, which
is arrested at each reciprocation, is just balanced by the equal and
opposite momentum of the weights. One advantage of the horizontal engine
is, that a single engine may be employed, whereby greater simplicity of the
machinery and greater economy of fuel will be obtained, since there will be
less radiating surface in one cylinder than in two.




CYLINDERS, PISTONS, AND VALVES,


443. _Q._--Is it a beneficial practice to make cylinders with steam
jackets?

_A._--In Cornwall, where great attention is paid to economy of fuel, all
the engines are made with steam jackets, and in some cases a flue winds
spirally round the cylinder, for keeping the steam hot. Mr. Watt, in his
early practice, discarded the steam jacket for a time, but resumed it
again, as he found its discontinuance occasioned a perceptible waste of
fuel; and in modern engines it has been found that where a jacket is used
less coal is consumed than where the use of a jacket is rejected. The cause
of this diminished effect is not of very easy perception, for the jacket
exposes a larger radiating surface for the escape of the heat than the
cylinder; nevertheless, the fact has been established beyond doubt by
repeated trials, that engines provided with a jacket are more economical
than engines without one. The exterior of the cylinder, or jacket, should
be covered with several plies of felt, and then be cased in timber, which
must be very narrow, the boards being first dried in a stove, and then
bound round the cylinder with hoops, like the staves of a cask. In many of
the Cornish engines the steam is let into casings formed in the cylinder
cover and cylinder bottom, for the further economisation of the heat, and
the cylinder stuffing box is made very deep, and a lantern or hollow brass
is introduced into the centre of the packing, into which brass the steam
gains admission by a pipe provided for the purpose; so that in the event of
the packing becoming leaky, it will be steam that will be leaked into the
cylinder instead of air, which, being incondensable, would impair the
efficiency of the engine. A lantern brass, of a similar kind, is sometimes
introduced into the stuffing boxes of oscillating engines, but its use
there is to receive the lateral pressure of the piston rod, and thus take
any strain off the packing.

444. _Q._--Will you explain the proper course to pursue in the production
of cylinders?

_A._--In all engines the valve casing, if made in a separate piece from the
cylinder, should be attached by means of a metallic joint, as such a
barbarism as a rust joint in such situations is no longer permissible. In
the case of large engines with valve casings suitable for long slides, an
expansion joint in the valve casing should invariably be inserted,
otherwise the steam, by gaining admission to the valve casing before it can
enter the cylinder, expands the casing while the cylinder remains unaltered
in its dimensions, and the joints are damaged, and in some cases the
cylinder is cracked by the great strain thus introduced. The chest of the
blow-through valve is very commonly cast upon the valve casing; and in
engines where the cylinders are stationary this is the most convenient
practice. All engines, where the valve is not of such a construction as to
leave the face when a pressure exceeding that of the steam is created in
the cylinder by priming or otherwise, should be provided with an escape
valve to let out the water, and such valve should be so constructed that
the water cannot fly out with violence over the attendants; but it should
be conducted away by a suitable pipe, to a place where its discharge can
occasion no inconvenience. The stuffing boxes of all engines which cannot
be stopped frequently to be repacked, should be made very deep; metallic
packing in the stuffing box has been used in some engines, consisting in
most instances of one or more rings, cut, sprung, and slipped upon the
piston rod before the cross head is put on, and packed with hemp behind.
This species of packing answers very well when the parallel motion is true,
and the piston rod free from scratches, and it accomplishes a material
saving of tallow. In some cases a piece of sheet brass, packed behind with
hemp, has been introduced with good effect, a flange being turned over on
the under edge of the brass to prevent it from slipping up or down with the
motion of the rod. The sheet brass speedily puts an excellent polish upon
the rod, and such a packing is more easily kept, and requires less tallow
than where hemp alone is employed. In side lever marine engines the
attachments of the cylinder to the diagonal stay are generally made of too
small an area, and the flanges are made too thick. A very thick flange cast
on any part of a cylinder endangers the soundness of the cylinder, by
inducing an unequal contraction of the metal; and it is a preferable course
to make the flange for the attachment or the framing thin, and the surface
large--the bolts being turned bolts and nicely fitted. If from malformation
in this part the framing works to an inconvenient extent, the best
expedient appears to be the introduction of a number of steel tapered
bolts, the holes having been previously bored out; and if the flanges be
thick enough, square keys may also be introduced, half into one flange and
half into the other, so as to receive the strain. If the jaw cracks or
breaks away, however, it will be best to apply a malleable iron hoop around
the cylinder to take the strain, and this will in all cases be the
preferable expedient, where from any peculiarities of structure there is a
difficulty in introducing bolts and keys of sufficient strength.

445. _Q._--Which is the most eligible species of piston?

_A._--For large engines, pistons with a metallic packing, consisting of a
single ring, with the ends morticed into one another, and a piece of metal
let in flush over the joint and riveted to one end of the ring, appears to
be the best species of piston; and if the cylinder be oscillating, it will
be expedient to chamfer off the upper edge of the ring on the inner side,
and to pack it at the back with hemp. If the cylinder be a stationary one,
springs may be substituted for the hemp packing, but in any case it will be
expedient to make the vertical joints of the ends of the ring run a little
obliquely, so as to prevent the joint forming a ridge in the cylinder. For
small pistons two rings may be employed, made somewhat eccentric internally
to give a greater thickness of metal in the centre of the ring; these rings
must be set one above the other in the cylinder, and the joints, which are
oblique, must be set at right angles with one another, so as to obviate any
disposition of the rings, in their expansion, to wear the cylinder oval.
The rings must first be turned a little larger than the diameter of the
cylinder, and a piece is then to be cut out, so that when the ends are
brought together the ring will just enter within the cylinder. The ring,
while retained in a state of compression, is then to be put in the lathe
and turned very truly, and finally it is to be hammered on the inside with
the small end of the hammer, to expand the metal, and thus increase the
elasticity.

446. _Q._--The rings should be carefully fitted to one another laterally?

_A._--The rings are to be fitted laterally to the piston, and to one
another, by scraping--a steady pin being fixed upon the flange of the
piston, and fitting into a corresponding hole in the lower ring, to keep
the lower ring from turning round; and a similar pin being fixed into the
top edge of the lower ring to prevent the upper ring from turning round;
but the holes into which these pins fit must be made oblong, to enable the
rings to press outward as the rubbing surfaces wear. In most cases it will
be expedient to press the packing rings out with springs where they are not
packed behind with hemp, and the springs should be made very strong, as the
prevailing fault of springs is their weakness. Sometimes short bent
springs, set round at regular intervals between the packing rings and body
of the piston, are employed, the centre of each spring being secured by a
steady pin or bolt screwed into the side of the piston; but it will not
signify much what kind of springs is used, provided they have sufficient
tension. When pistons are made of a single ring, or of a succession of
single rings, the strength of each ring should be tested previously to its
introduction into the piston, by means of a lever loaded by a heavy weight.

447. _Q._--What kind of piston is employed by Messrs. Penn?

_A._--Messrs. Penn's piston for oscillating engines has a single packing
ring, with a tongue piece, or mortice end, made in the manner already
described. The ring is packed behind with hemp packing, and the piece of
metal which covers the joint is a piece of thick sheet copper or brass, and
is indented into the iron of the ring, so as to offer no obstruction to the
application of the hemp. The ring is fitted to the piston only on the under
edge; the top edge is rounded to a point from the inside, and the junk ring
does not bear upon it, but the junk ring squeezes down the hemp packing
between the packing ring and the body of the piston.

448. _Q._--How should the piston rod be secured to the piston?

_A._--The piston rod, where it fits into the piston, should have a good
deal of taper; for if the taper be too small the rod will be drawn through
the hole, and the piston will be split asunder. Small grooves are sometimes
turned out of the piston rod above and below the cutter hole, and hemp is
introduced in order to make the piston eye tight. Most piston rods are
fixed to the piston by means of a gib and cutter, but in some cases the
upper portion of the rod within the eye is screwed, and it is fixed into
the piston by means of an indented nut. This nut is in some cases
hexagonal, and in other cases the exterior forms a portion of a cone which
completely fills a corresponding recess in the piston; but nuts made in
this way become rusted into their seat after some time, and cannot be
started again without much difficulty. Messrs. Miller, Ravenhill & Co. fix
in their piston rods by means of an indented hexagonal nut, which may be
started by means of an open box key. The thread of the screw is made flat
upon the one side and much slanted on the other, whereby a greater strength
is secured, without creating any disposition to split the nut. In side
lever engines it is a judicious practice to add a nut to the top of the
piston rod, in addition to the cutter for securing the piston rod to the
cross head. In a good example of an engine thus provided, the piston rod is
7 in. in diameter, and the screw 5 in.; the part of the rod which fits into
the cross head eye is 1 ft. 5-1/2 in. long, and tapers from 6-1/2 in. to
6-13/16 in. diameter. This proportion of taper is a good one; if the taper
be less, or if a portion of the piston rod within the cross head eye be
left untapered, as is sometimes the case, it is very difficult to detach
the parts from one another.

449. _Q._--Which is the most beneficial construction of slide valve?

_A._--The best construction of slide valve appears to be that adopted by
Messrs. Penn for their larger engines, and which consists of a three ported
valve, to the back of which a ring is applied of an area equal to that of
exhaustion port, and which, by bearing steam tight against the back of the
casing, so that a vacuum may be maintained within the ring, puts the valve
in equilibrium, so that it may be moved with an inconsiderable exercise of
force. The back of the valve casing is put on like a door, and its internal
surface is made very true by scraping. There is a hole through the valve so
as to conduct away any steam which may enter within the ring by leakage,
and the ring is kept tight against the back of the casing by means of a
ring situated beneath the bearing ring, provided with four lugs, through
which bolts pass tapped into bosses on the back of the valve; and, by
unscrewing these bolts,--which may be done by means of a box key which
passes through holes in the casing closed with screwed plugs,--the lower
ring is raised upwards, carrying the bearing ring before it. The rings must
obviously be fitted over a boss upon the back of the valve; and between the
rings, which are of brass, a gasket ring is interposed to compensate by its
compressibility for any irregularity of pressure, and each of the bolts is
provided with a ratchet collar to prevent it from turning back, so that the
engineer, in tightening these bolts, will have no difficulty in tightening
them equally, if he counts the number of clicks made by the ratchet. Where
this species of valve is used, it is indispensable that large escape valves
be applied to the cylinder, as a valve on this construction is unable to
leave the face. In locomotive engines, the valve universally employed is
the common three ported valve.

450. _Q._--Might not an equilibrium valve be so constructed by the
interposition of springs, as to enable it to leave the cylinder face when
an internal force is applied?

_A._--That can no doubt be done, and in some engines has been done. In the
screw steamer Azof, the valve is of the equilibrium construction, but the
plate which carries the packing on which the top ring rests, is an octagon,
and fits into an octagonal recess on the back of the valve. Below each side
of the octagon there is a bent flat spring, which lifts up the octagonal
plate, and with it the packing ring against the back of the valve casing;
and should water get into the cylinder, it escapes by lifting the valve,
which is rendered possible by the compressibility of the springs. An
equivalent arrangement is shown in figs. 39 and 40, where the ring is
lifted by spiral springs.


[Illustration: Fig. 39. EQUILIBRIUM GRIDIRON SLIDE VALVE. Longitudinal
Section. Scale 3/4 inch = 1 foot.]

451. _Q._--What species of valve is that shown in figs. 39 and 40?

[Illustration: Fig. 40. EQUILIBRIUM GRIDIRON SLIDE VALVE. Back View with
Ring removed. Scale 3/4 inch = 1 foot.]

_A._--It is an equilibrium gridiron valve; so called because it lets the
steam in and out by more than one port.  A A are the ordinary steam
passages to the top and bottom of the cylinder; B B is the ring which rubs
against the back of the valve casing, and D is the eduction passage, S S S
S shows the limits of the steam space, for the steam penetrates to the
central chamber S S by the sides of the valve. When the valve is opened
upon the steam side, the cylinder receives steam through both ports at that
end of the cylinder, and both ports at the other end of the cylinder are at
the same time open to the eduction. The benefit of this species of valve
is, that it gives the same opening of the valve that is given in ordinary
engines, with half the amount of travel; or if three ports were made
instead of two, then it would give the same area of opening that is given
in common engines with one third the amount of travel. For direct acting
screw engines this species of valve is now extensively used.

452. _Q._--Will you describe the configuration and mode of attachment of
the eccentric by which the valve is moved?

_A._--In marine engines, whether paddle or screw, if moving at a slow rate
of speed, the eccentric is generally loose upon the shaft, for the purpose
of backing, and is furnished with a back balance and catches, so that it
may stand either in the position for going ahead, or in that for going
astern. The body of the eccentric is of cast iron, and it is put on the
shaft in two pieces. The halves are put together with rebated joints to
keep them from separating laterally, and they are prevented from sliding
out by round steel pins, each ground into both halves; square keys would
probably be preferable to round pins in this arrangement, as the pins tend
to wedge the jaws of the eccentric asunder. In some cases the halves of the
eccentric are bolted together by means of flanges, which is, perhaps, the
preferable practice. The eccentric hoop in marine and land engines is
generally of brass; it is expedient to cast an oil cup on the eccentric
hoop, and, where practicable, a pan should be placed beneath the eccentric
for the reception of the oil droppings. The notch of the eccentric rod for
the reception of the pin of the valve shaft is usually steeled, to prevent
inconvenient wear; for when the sides of the notch wear, the valve movement
is not only disturbed, but it is very difficult to throw the eccentric rod
out of gear. It is found to be preferable, however, to fit this notch with
a brass bush, for the wear is then less rapid, and it is an easy thing to
replace this bush with another when it becomes worn. The eccentric catches
of the kind usually employed in marine engines, sometimes break off at the
first bolt hole, and it is preferable to have a bolt in advance of the
catch face, or to have a hoop encircling the shaft with the catches welded
on it, the hoop itself being fixed by bolts or a key. This hoop may either
be put on before the cranks in one piece or afterwards in two pieces.

453. _Q._--Are such eccentrics used in direct acting screw engines?

_A._--No; direct acting screw engines are usually fitted with the link
motion and two fixed eccentrics.



AIR PUMP AND CONDENSER.

454. _Q._--What are the details of the air pump?

_A._--The air pump bucket and valves are all of brass in modern marine
engines, and the chamber of the pump is lined with copper, or made wholly
of brass, whereby a single boring suffices. When a copper lining is used,
the pump is first bored out, and a bent sheet of copper is introduced,
which is made accurately to fill the place, by hammering the copper on the
inside. Air pump rods of Muntz's metal or copper are much used. Iron rods
covered with brass are generally wasted away where the bottom cone fits
into the bucket eye, and if the casing be at all porous, the water will
insinuate itself between the casing and the rod and eat away the iron. If
iron rods covered with brass be used, the brass casing should come some
distance into the bucket eye; the cutter should be of brass, and a brass
washer should cover the under side of the eye, so as to defend the end of
the rod from the salt water. Rods of Muntz's metal are probably on the
whole to be preferred. It is a good practice to put a nut on the top of the
rod, to secure it more firmly in the cross head eye, where that plan can be
conveniently adopted. The part of the rod which fits into the cross head
eye should have more taper when made of copper or brass, than when made of
iron; as, if the taper be small, the rod may get staved into the eye,
whereby its detachment will be difficult.

455. _Q._--What species of packing is used in air pumps?

_A._--Metallic packing has in some instances been employed in air pump
buckets, but its success has not been such as to lead to its further
adoption. The packing commonly employed is hemp. A deep solid block of
metal, however, without any packing, is often employed with a satisfactory
result; but this block should have circular grooves cut round its edge to
hold water. Where ordinary packing is employed, the bucket should always be
made with a junk ring, whereby the packing may be easily screwed down at
any time with facility. In slow moving engines the bucket valve is
generally of the spindle or pot-lid kind, but butterfly valves are
sometimes used. The foot and delivery valves are for the most part of the
flap or hanging kind. These valves all make a considerable noise in
working, and are objectionable in many ways. Valves on Belidor's
construction, which is in effect that of a throttle valve hung off the
centre, were some years ago proposed for the delivery and foot valves; and
it appears probable that their operation would be more satisfactory than
that of the valves usually employed.

456. _Q._--Where is the delivery valve usually situated?

_A._--Some delivery valve seats are bolted into the mouth of the air pump,
whereby access to the pump bucket is rendered difficult: but more commonly
the delivery valve is a flap valve exterior to the pump. If delivery valve
seats be put in the mouth of the air pump at all, the best mode of fixing
them appears to be that adopted by Messrs. Maudslay. The top of the pump
barrel is made quite fair across, and upon this flat surface a plate
containing the delivery valve is set, there being a small ledge all round
to keep it steady. Between the bottom of the stuffing box of the pump cover
and the eye of the valve seat a short pipe extends encircling the pump rod,
its lower end checked into the eye of the valve seat, and its upper end
widening out to form the bottom of the stuffing box of the pump cover. Upon
the top of this pipe some screws press, which are accessible from the top
of the stuffing box gland, and the packing also aids in keeping down the
pipe, the function of which is to retain the valve seat in its place. When
the pump bucket has to be examined the valve seat may be slung with the
cover, so as to come up with the same purchase. For the bucket valves of
such pumps Messrs. Maudslay employ two or more concentric ring valves with
a small lift. These valves have given a good deal of trouble in some cases,
in consequence of the frequent fracture of the bolts which guide and
confine the rings; but this is only a fault of detail which is easily
remedied, and the principle appears to be superior to that of any of the
other metallic air pump valves at present in common use.

[Illustration: Fig. 41. TRUNK AIR PUMP. Scale 3/4 inch to 1 foot.]

457. _Q._--Are not air pump valves now very generally made of india rubber?

_A._--They are almost invariably so made if the engines are travelling
fast, as in the case of direct acting screw engines, and they are very
often made of large discs or rings of india rubber, even when the engines
travel slowly. A very usual and eligible arrangement for many purposes is
that shown in fig. 41, where both foot and delivery valves are situated in
the ends of the pump, and they, as well as the valve in the bucket are made
of india rubber rings closing on a grating. The trunk in the air pump
enables guide rods to be dispensed with.

[Illustration: Fig. 42. PENN'S DISK VALVE FOR AIR PUMP. Section.]

[Illustration: Fig. 43. PENN'S DISK VALVE FOR AIR PUMP. Ground Plan.]

[Illustration: Fig. 44. MAUDSLAY'S DISC VALVE FOR AIR PUMP. Section.]

458. _Q._--The air pump, when double acting, has of course inlet and outlet
valves at each end?

_A._--Yes; and the general arrangement of the valves of double acting air
pumps, such as are usual in direct acting screw engines, is that
represented in the figure of Penn's trunk engine already described in
Chapter I. Each inlet and outlet valve consists of a number of india rubber
discs set over a perforated brass plate, and each disc is bound down by a
bolt in the middle, which bolt also secures a brass guard set above the
disc to prevent it from rising too high. The usual configuration of those
valves is that represented in figs. 42, 43, and 44; figs. 42 and 43 being a
section and ground plan of the species of valve used by Messrs. Penn, and
fig. 44 being a section of that used by Messrs. Maudslay. It is important
in these valves to have the india rubber thick,--say about an inch thick
for valves eight inches in diameter. It is also advisable to make the
central bolts with a nut above and a nut below, and to form the bolt with a
counter sunk neck, so that it will not fall down when the top nut is
removed. The lower point of the bolt should be riveted over on the nut to
prevent it from unscrewing, and the top end should have a split pin through
the point for the same purpose. The hole through which the bolt passes
should be tapped, though the bolt is not screwed into it, so that if a bolt
breaks, a temporary stud may be screwed into the hole without the necessity
of taking out the whole plate. The guard should be large, else the disc may
stretch in the central hole until it comes over it; but the guard should
not permit too much lift of the valve, else a good deal of the water and
air will return into the pump at the return stroke before the valve shuts.
Penn's guard is rather small, and Maudslay's permits too much lift.

459. _Q._--What is the proper area through the valve gratings?

_A._--The collective area should be at least equal to the area of the pump
piston, and the lower edges of the perforations should be rounded off to
afford more free ingress or egress to the water.

460. _Q._--Is there much strain thrown on the plates in which the valves
are set?

_A._--A good deal of strain; and in the earlier direct acting screw engines
these plates were nearly in every case made too light. They should be made
thick, have strong feathers upon them, and be very securely bolted down
with split pins at the points of the bolts, to prevent them from
unscrewing. The plate will be very apt to be broken should some of the
bolts become loose. Of course all the bolts and split pins, as well as the
plates and guards, must be of brass.

461. _Q._--How are the plates to be taken out should that become necessary?

_A._--They are usually taken out through a door in the top of the hot well
provided for that purpose, which door should be as large as the plates
themselves; and it is a good precaution to cast upon this door--which will
be of cast iron--six or eight stout projecting feet which will press upon
the top of the outlet or delivery valve plate when the door is screwed
down. The upper or delivery valve plate and the lower or foot valve plate
should have similar feet. A large part of the strain will thus be
transferred from the plates to the door, which can easily be made strong
enough to sustain it. It is advisable that the plates should lie at an
angle so that the shock of the water may not come upon the whole surface at
once.

462. _Q._--Does the double acting air pump usual in direct acting screw
engines, produce as good a vacuum as the single acting air pump usual in
paddle engines?

_A._--It will do so if properly constructed; but I do not know of any case
of a double acting air pump, with india rubber valves, which has been
properly constructed.

463. _Q._--What is the fault of such pumps?

_A._--The pump frequently works by starts, as if at times it did not draw
at all, and then again on a sudden gorged itself with water, so as to throw
a great strain upon the working parts. The vacuum, moreover, is by no means
so good as it should be, and it is a universal vice of direct acting screw
engines that the vacuum is defective. I have been at some pains to
investigate the causes of this imperfection; and in a sugar house engine
fitted with pumps like those of a direct acting screw engine to maintain a
vacuum in the pans, I found that a better vacuum was produced when the
engine was going slowly than when it was going fast; which is quite the
reverse of what was to have been expected, as the hot water which had to be
removed by the condensation of the steam proceeding from the pan, was a
constant quantity. In this engine, too, which was a high pressure one, the
irregularities of the engine consequent upon the fitful catching of the
water by the pump, was more conspicuous, as the working of this vacuum pump
was the only work that the engine had to perform.

464. _Q._--And were you able to discover the cause of these irregularities?

_A._--The main cause of them I found to be the largeness of the space left
between the valve plates in this class of pumps, and out of which there is
nothing to press the air or water which may be lying there. It consequently
happens, that if there be the slightest leakage of air into the pump, this
air is merely compressed, and not expelled, by the advance of the air pump
piston. It expands again to its former bulk on the return of the pump
piston, and prevents the water from entering until there is such an
accumulation of pressure in the condenser as forces the water into the
pump, when the air being expelled by the water, causes a good vacuum to be
momentarily formed in the pump when it gorges itself by taking a sudden
gulp of water. So soon, however, as the pressure falls in the condenser and
some more air leaks into the pump, the former imperfect action recurs and
is again redressed in the same violent manner.

465. _Q._--Is this irregular action of the pump the cause of the imperfect
vacuum?

_A._--It is one cause. Sometimes one end of the pump will alone draw and
the other end will be inoperative, although it is equally open to the
condenser, and this will chiefly take place at the stuffing box end, where
a leakage of air is more likely to occur. I find, however, that even when
both ends of the pump are acting equally and there is no leakage of air at
all, the vacuum maintained by a double acting horizontal pump with india
rubber valves, is not so good as that maintained by a single acting pump of
the kind usual in old engines.

466. _Q._--Will you specify more precisely what were the results you
obtained?

_A._--When the vacuum pan was exhausted by the pumps without any boiling
being carried on in the pan, but only a little cold water being let into
it, and also into the pumps to enable them to act in their best manner, it
was found that whereas with the old pump a vacuum of 114 on the sugar
boiler's gauge could be readily obtained, equal to about 29-1/2 inches of
mercury, the lowest that could possibly be got with the new horizontal pump
was 122 degrees of the sugar boiler's gauge, or 29 inches of mercury, and
to get that the engine must not go faster than 10 or 12 strokes per minute.
The proper speed of the engine was 75 strokes per minute, but if allowed to
go at that speed the vacuum fell to 130 of the sugar maker's gauge, or
28-1/2 inches of mercury. When the steam was let into the worms of the pan
so as to boil the water in it, the vacuum was 134 at 75 revolutions of the
engine, and went down to 132 at 40 revolutions, but rose again to 135,
equal to about 28-1/4 inches of mercury, at 20 revolutions.

467. _Q._--To what do you attribute the circumstance of a better vacuum
being got at low speeds than at high speeds?

_A._--It is difficult to assign the precise reason, but it appears to be a
consequence of the largeness of the vacant space between the valve plates.
When the piston of the air pump is drawn back, the air contained in this
large collection of water will cause it to boil up like soda water; and
when the piston of the pump is forced forward, this air, instead of being
expelled, will be again driven into the water. There will consequently be a
quantity of air in the pump which cannot be got rid of at all, and which
will impair the vacuum as a matter of course.

468. _Q._--What expedient did you adopt to improve the vacuum in the engine
to which you have referred?

_A._--I put blocks of wood on the air pump piston, which at the end of its
stroke projected between the valve plates and forced the water out. I also
introduced a cock of water at each end of the pump between the valve
plates, to insure the presence of water at each end of the pump to force
the air out. With these ameliorations the pump worked steadily, and the
vacuum obtained became as good as in the old pump. I had previously
introduced an injection cock into each end of the air pump in steam
vessels, from which I had obtained advantageous results; and in all
horizontal air pumps I would recommend the piston and valve plates to be so
constructed that the whole of the water will be expressed by the piston. I
would also recommend an injection cock to be introduced at each end of the
pump.




PUMPS, COCKS, AND PIPES.

469. _Q._--Will you explain the arrangement of the feed pump?

_A._--In steam vessels, the feed pump plunger is generally of brass, and
the barrel of the pump is sometimes of brass, but generally of cast iron.
There should be a considerable clearance between the bottom of the plunger
and the bottom of the barrel, as otherwise the bottom of the barrel may be
knocked out, should coal dust or any other foreign substance gain
admission, as it probably would do if the injection water were drawn at any
time from the bilge of the vessel, as is usually done if the vessel springs
a leak. The valves of the feed pump in marine engines are generally of the
spindle kind, and are most conveniently arranged in a chest, which may be
attached in any accessible position to the side of the hot well. There are
two nozzles upon this chest, of which the lower one leads to the pump, and
the upper one to the boiler. The pipe leading to the pump is a suction pipe
when the plunger ascends, and a forcing pipe when the plunger descends. The
plunger in ascending draws the water out of the hot well through the lowest
of the valves, and in descending forces it through the centre valve into
the space above it, which communicates with the feed pipe. Should the feed
cock be shut so as to prevent any feed water from passing through it, the
water will raise the topmost valve, which is loaded to a pressure
considerably above the pressure of the steam, and escape into the hot well.
This arrangement is neater and less expensive than that of having a
separate loaded valve on the feed pipe with an overflow through the ship's
side, as is the more usual practice.

470. _Q._--Will you describe what precautions are to be observed in the
construction of the cocks used in engines?

_A._--All the cocks about an engine should be provided with bottoms and
stuffing boxes, and reliance should never be placed upon a single bolt
passing through a bottom washer for keeping the plug in its place, in the
case of any cock communicating with the boiler; for a great strain is
thrown upon that bolt if the pressure of the steam be high, and if the plug
be made with much taper; and should the bolt break, or the threads strip,
the plug will fly out, and persons standing near may be scalded to death.
In large cocks, it appears the preferable plan to cast the bottoms in; and
the metal of which all the cocks about a marine engine are made, should be
of the same quality as that used in the composition of the brasses, and
should be without lead, or other deteriorating material. In some cases the
bottoms of cocks are burnt in with hard solder, but this method cannot be
depended upon, as the solder is softened and wasted away by the hot salt
water, and in time the bottom leaks, or is forced out. The stuffing box of
cocks should be made of adequate depth, and the gland should be secured by
means of four strong copper bolts. The taper of blow-off cocks is an
important element in their construction; as, if the taper be too great, the
plugs will have a continual tendency to rise, which, if the packing be
slack, will enable grit to get between the faces, while, if the taper be
too little, the plug will be liable to jam, and a few times grinding will
sink it so far through the shell that the waterways will no longer
correspond. One eighth of an inch deviation from the perpendicular for
every inch in height, is a common angle for the side of the cock, which
corresponds with one quarter of an inch difference of diameter in an inch
of height; but perhaps a somewhat greater taper than this, or one third of
an inch difference in diameter for every inch of height, is a preferable
proportion. The bottom of the plug must be always kept a small distance
above the bottom of the shell, and an adequate surface must be left above
and below the waterway to prevent leakage. Cocks formed according to these
directions will be found to operate satisfactorily in practice, while they
will occasion perpetual trouble if there be any malformation.

471. _Q._--What is the best arrangement and configuration of the blow-off
cocks?

_A._--The blow-off cocks of a boiler are generally placed some distance
from the boiler; but it appears preferable that they should be placed quite
close to it, as there are no means of shutting off the water from the pipe
between the blow-off cock and the boiler, should fracture or leakage there
arise. Every boiler must be furnished with a blow-off cock of its own,
independently of the main blow-off cocks on the ship's sides, so that the
boilers may be blown off separately, and may be shut off from one another.
The preferable arrangement appears to be, to cast upon each blow-off cock a
bend for attaching the cock to the bottom of the boiler, and the plug
should stand about an inch in advance of the front of the boiler, so that
it may be removed, or re-ground, with facility. The general arrangement of
the blow-off pipes is to run a main blow-off pipe beneath the floor plates,
across the ship, at the end of the engines, and into this pipe to lead a
separate pipe, furnished with a cock, from each boiler. The main blow-off
pipe, where it penetrates the ship's side, is furnished with a cock: and in
modern steam vessels Kingston's valves are also used, which consist of a
spindle or plate valve, fitted to the exterior of the ship, so that if the
internal pipe or cock breaks, the external valve will still be operative.
Some expedient of this kind is almost necessary, as the blow-off cocks
require occasional regrinding, and the sea cocks cannot be re-ground
without putting the vessel into dock, except by the use of Kingston's
valves, or some equivalent expedient.

472. Q.--What is the proper construction and situation of the injection
cocks, and waste water valves?

A.--The sea injection cocks are usually made in the same fashion as the sea
blow-off cocks, and of about the same size, or rather larger. The injection
water is generally admitted to the condenser by means of a slide valve, but
a cock appears to be preferable, as it is more easily opened, and has not
any disposition to shut of its own accord. In paddle vessels the sea
injection pipes should be put through the ship's sides in advance of the
paddles, so that the water drawn in may not be injuriously charged with
air. The waste water pipe passing from the hot well through the vessel's
side is provided with a stop valve, called the discharge valve, which is
usually made of the spindle kind, so as to open when the water coming from
the air pump presses against it. In some cases this valve is a sluice
valve, but the hot well is then almost sure to be split, if the engine be
set on without the valve having been opened. The opening of the waste water
pipe should always be above the load water line, as it will otherwise be
difficult to prevent leakage through the engine into the ship when the
vessel is lying in harbor.

473. Q.--What is the best arrangement of gauge cocks and glass gauges?

A.--Gauge cocks are generally very inartificially made, and occasion
needless annoyance. They are rarely made with bottoms, or with stuffing
boxes, and are consequently, for the most part, adorned with stalactites of
salt after a short period of service. The water discharged from them, too,
from the want of a proper conduit, disfigures the front of the boiler, and
adds to the corrosion in the ash pits. It would be preferable to combine
the gauge cocks appertaining to each boiler into a single upright tube,
connected suitably with the boiler, and the water flowing from them could
be directed downward into a funnel tube communicating with the bilge. The
cocks of the glass tubes, as well as of the gauge cocks, should be
furnished with stuffing boxes and with bottoms, unless the water enters
through the bottom of the plug, which in gauge cocks is sometimes the case.
The glass gauge tubes should always be fitted with a cock at each neck
communicating with the boiler, so that the water and steam may be shut off
if the tube breaks; and the cocks should be so made as to admit of the
tubes being blown through with steam to clear them, as in muddy water they
will become so soiled that the water cannot be seen. The gauge cocks
frequently have pipes running up within the boiler, to the end that a high
water level may be made consistent with an easily accessible position of
the gauge cocks themselves. With the glass tubes, however, this species of
arrangement is not possible, and the glass tubes must always be placed in
the position of the water level.

474. Q.--What is the proper material of the pipes in steam vessels?

A.--Most of the pipes of marine engines should be made of copper. The steam
pipes may be of cast iron, if made very strong, but the waste water pipes
should be of copper. Cast iron blow-off pipes have in some cases been
employed, but they are liable to fracture, and are dangerous. The blow-off
and feed pipes should be of copper, but the waste steam pipe may be of
galvanized iron. Every pipe passing through the ship's side, and every pipe
fixed at both ends, and liable to be heated and cooled, should be furnished
with a faucet or expansive joint; and in the case of the cast iron pipes,
the part of the pipe fitting into the faucet should be turned. In the
distribution of the faucets of the pipes exposed to pressure, care must be
taken that they be so placed that the parts of the pipe cannot be forced
asunder, or turned round by the strain, as serious accidents have occurred
from the neglect of this precaution.

475. _Q._--What is the best mode of making pipes tight where they penetrate
the ship's side?

_A._--In wooden vessels the pipes where they pierce the ship's side, should
be made tight, as follows:--the hole being cut, a short piece of lead pipe,
with a broad flange at one end, should be fitted into it, the place having
been previously smeared with white lead, and the pipe should then be beaten
on the inside, until it comes into close contact all around with the wood.
A loose flange should next be slipped over the projecting end of the lead
pipe, to which it should be soldered, and the flanges should both be nailed
to the timber with scupper nails, white lead having been previously spread
underneath. This method of procedure, it is clear, prevents the possibility
of leakage down through the timbers; and all, therefore, that has to be
guarded against after this precaution, is to prevent leakage into the ship.
To accomplish this object, let the pipe which it is desired to attach be
put through the leaden hause, and let the space between the pipe and the
lead be packed with gasket and white lead, to which a little olive oil has
been added. The pipe must have a flange upon it to close the hole in the
ship's side; the packing must then be driven in from the outside, and be
kept in by means of a gland secured with bolts passing through the ship's
side. If the pipe is below the water line the gland must be of brass, but
for the waste water pipe a cast iron gland will answer. This method of
securing pipes penetrating the side, however, though the best for wooden
vessels, will, it is clear, fail to apply to iron ones. In the case of iron
vessels, it appears to be the best practice to attach a short iron nozzle,
projecting inward from the skin, for the attachment of every pipe below the
water line, as the copper or brass would waste the iron of the skin if the
attachment were made in the usual way.



DETAILS OF THE SCREW AND SCREW SHAFT.

476. _Q._--What is the best method of fixing the screw upon the shaft?

_A._--The best way is to cut two large grooves in the shaft coming up to a
square end, and two corresponding grooves or key seats in the screw boss
opposite the arms. Fit into the grooves on the shaft keys with heads, the
length of which is equal to half the depth of the boss, and with the ends
of the keys bearing against the ends of the grooves in the shaft. Then ship
on the propeller, and drive other keys of an equal length from the other
side of the boss, so that the points of the keys will nearly meet in the
middle; next burr up the edge of the grooves upon the heads of the keys, to
prevent them from working back; and finally tap a bolt into the side of the
boss to penetrate the shaft. Propellers so fitted will never get slack.

477. _Q._--What is the best way of fitting in the screw pipe at the stern?

_A._--It should have projecting rings, which should be turned; and cast
iron pieces with holes in them, bored out to the sizes of these rings,
should be secured to the stern frames, and the pipe be then shipped through
all. Before this is done, however, the stern post must be bored out by a
template to fit the pipe, and the pipe is to be secured at the end to the
stern post either by a great external nut of cast iron, or by bolts passing
through the stern post and through lugs on the pipe. The pipe should be
bored throughout its entire length, and the shaft should be turned so as to
afford a very long bearing which will prevent rapid wear.

478. _Q._--How is the hole formed in the deadwood of the ship in which the
screw works?

_A._--A great frame of malleable iron, the size of the hole, is first set
up, and the plating of the ship is brought to the edge of this hole, and is
riveted through the frame. It is important to secure this frame very firmly
to the rest of the ship, with which view it is advisable to form a great
palm, like the palm of a vice, on its inner superior corner, which,
projecting into the ship, may be secured by breast-hook plates to the
sides, whereby the strain which the screw causes will be distributed over
the stern, instead of being concentrated on the rivets of the frame.

479. _Q._--Are there several lengths of screw shaft?

_A._--There are.

480. _Q._--How then are these secured to one another?

_A._--The best mode of securing the several lengths of shaft together is by
forging the shafts with flanges at the ends, which are connected together
by bolts, say six strong bolts in each, accurately fitted to the holes.

[Illustration: Fig 44. End of the Screw Shaft of Correo, showing the mode
of receiving the Thrust. A, discs; B, tightening wedge.]

481. _Q._--How is the thrust of the shaft usually received?

_A._--In some cases it is received on a number of metal discs set in a box
containing oil; and should one of these discs stick fast from friction, the
others will be free to revolve. This arrangement, which is represented in
fig. 44, is used pretty extensively and answers the purpose perfectly. It
is of course necessary that the box in which the discs A are set, shall be
strong enough to withstand the thrust which the screw occasions. Another
arrangement still more generally used, is that represented in figs. 55 and
56, p. 331. It is a good practice to make the thrust plummer block with a
very long sole in the direction of the shaft, so as to obviate any risk of
canting or springing forward when the strain is applied, as such a
circumstance, if occurring even to a slight extent, would be very likely to
cause the bearing to heat.

482. _Q._--Are there not arrangements existing in some vessels for enabling
the screw to be lifted out of the water while the vessel is at sea?

_A._--There are; but such arrangements are not usual in merchant vessels.
In one form of apparatus the screw is set on a short shaft in the middle of
a sliding frame, which can be raised or lowered in grooves like a window
and the screw shaft within the ship can be protruded or withdrawn by
appropriate mechanism, so as to engage or leave free this short shaft as
may be required. When the screw has to be lifted, the screw shaft is drawn
into the vessel, leaving the short shaft free to be raised up by the
sliding frame, and the frame is raised by long screws turned round by a
winch purchase on deck. A chain or rope, however, is better for the purpose
of raising this frame, than long screws; but the frame should in such case
be provided with pall catches like those of a windlass, which, if the rope
should break, will prevent the screw from falling.



DETAILS OF THE PADDLES AND PADDLE SHAFT.

483. _Q._--What are the most important details of the construction of
paddle wheels?

_A._--The structure of the feathering wheel will be hereafter described in
connection with an account of the oscillating engine; and it will be
expedient now to restrict any account of the details to the common radial
paddle, as applied to ocean steamers. The best plan of making the paddle
centres is with square eyes, and each centre should be secured in its place
by means of eight thick keys. The shaft should be burred up against the
head of these keys with a chisel, so as to prevent the keys from coming
back of their own accord. If the keys are wanted to be driven back, this
burr must be cut off, and if made thick, and of the right taper, they may
then be started without difficulty. The shaft must of course be forged with
square projections on it, so as to be suitable for the application of
centres with square eyes. Messrs. Maudslay & Co. bore out their paddle
centres, and turn a seat for them on the shaft, afterward fixing them on
the shaft with a single key. This plan is objectionable for the two
reasons, that it is insecure when new, and when old is irremovable. The
general practice among the London engineers is to fix the paddle arms at
the centre to a plate by means of bolts, a projection being placed upon the
plates on each side of the arm, to prevent lateral motion; but this method
is inferior in durability to that adopted in the Clyde, in which each arm
is fitted into a socket by means of a cutter--a small hole being left
opposite to the end of each arm, whereby the arm may be forced back by a
drift.

484. _Q._--How are the arms attached to the outside rings?

_A._--Some engineers join the paddle arms to the outer ring by means of
bolts; but unless very carefully fitted, those bolts after a time become
slack sideways, and a constant working of the parts of the wheel goes on in
consequence. Sometimes the part of the other ring opposite the arm is
formed into a mortise, and the arms are wedged tight in these holes by
wedges driven in on each side; but the plan is an expensive one, and not
satisfactory, as the wedges work loose even though riveted over at the
point. The best mode of making a secure attachment of the arms to the ring,
consists in making the arms with long T heads, and riveting the cross piece
to the outer ring with a number of rivets, not of the largest size, which
would weaken the outer ring too much. The best way of securing the inner
rings to the arms is by means of lugs welded on the arms, and to which the
rings are riveted.

485. _Q._--What are the scantlings of the paddle floats?

_A._--The paddle floats are usually made either of elm or pine; if of the
former, the common thickness for large sea-going vessels is about 2-1/2
inches; if of the latter, 3 inches. The floats should have plates on both
sides, else the paddle arms will be very liable to cut into the wood, and
the iron of the arms will be very rapidly wasted. When the floats have been
fresh put on they must be screwed up several times before they come to a
bearing. If this be not done, the bolts will be sure to get slack at sea,
and all the floats on the weather side may be washed off. The bolts for
holding on the paddle floats are made extra strong, on account of the
corrosion to which they are subject; and the nuts should be made large, and
should be square, so that they may be effectually tightened up, even though
their corners be worn away by corrosion. It is a good plan to give the
thread of the paddle bolts a nick with a chisel, after the nut has been
screwed up, which will prevent the nut from turning back. Paddle floats,
when consisting of more than one board, should be bolted together edgeways,
by means of bolts running through their whole breadth. The floats should
not be notched to allow of their projection beyond the outer ring, as, if
the sides of the notch be in contact with the outer ring, the ring is soon
eaten away in that part, and the projecting part of the float, being
unsupported, is liable to be broken off.

486. _Q._--Do not the wheels jolt sideways when the vessel rolls?

_A._--It is usual to put a steel plate at each end of the paddle shafts
tightened with a key, to prevent end play when the vessel rolls, but the
arrangement is precarious and insufficient. Messrs. Maudslay make their
paddle shaft bearings with very large fillets in the corner, with the view
of diminishing the evil; but it would be preferable to make the bearings of
the crank shafts spheroidal; and, indeed, it would probably be an
improvement if most of the bearings about the engine were to be made in the
same fashion. The loose end of the crank pin should be made not spheroidal,
but consisting of a portion of a sphere; and a brass bush might then be
fitted into the crank eye, that would completely encase the ball of the
pin, and yet permit the outer end of the paddle shaft to fall without
straining the pin, the bush being at the same time susceptible of a slight
end motion. The paddle shaft, where it passes through the vessel's side, is
usually surrounded by a lead stuffing box, which will yield if the end of
the shaft falls; this stuffing box prevents leakage into the ship from the
paddle wheels: but it is expedient, as a further precaution, to have a
small tank on the ship's side immediately beneath the stuffing box, with a
pipe leading down to the bilge to catch and conduct away any water that may
enter around the shaft.

487. _Q._--How is the outer bearing of the paddle wheels supplied with
tallow?

_A._--The bearing at the outer end of the paddle shaft is sometimes
supplied with tallow, forced into a hole in the plummer block cover, as in
the case of water wheels; but for vessels intended to perform long voyages,
it is preferable to have a pipe leading down to the oil cup above the
journal from the top of the paddle box, through which pipe oil may at any
time be supplied.

488. _Q._--Will you explain the method of putting engines into a steam
vessel?

_A._--As an illustration of this operation it may be advisable to take the
case of a side lever engine, and the method of proceeding is as follows:--
First measure across from the inside of paddle bearers to the centre of the
ship, to make sure that the central line, running in a fore and aft
direction on the deck or beams, usually drawn by the carpenter, is really
in the centre. Stretch a line across between the paddle bearers in the
direction of the shaft; to this line, in the centre of the ship where the
fore and aft mark has been made, apply a square with arms six or eight feet
long, and bring a line stretched perpendicularly from the deck to the
keelson, accurately to the edge of the square: the lower point of the line
where it touches the keelson will be immediately beneath the marks made
upon the deck. If this point does not come in the centre of the keelson, it
will be better to shift it a little, so as to bring it to the centre,
altering the mark upon the deck correspondingly, provided either paddle
shaft will admit of this being done--one of the paddle brackets being
packed behind with wood, to give it an additional projection from the side
of the paddle bearer. Continue the line fore and aft upon the keelson as
nearly as can be judged in the centre of the ship; stretch another line
fore and aft through the mark upon the deck, and look it out of winding
with the line upon the keelson. Fix upon any two points equally distant
from the centre, in the line stretched transversely in the direction of the
shaft; and from those points, as centres, and with any convenient radius,
sweep across the fore and aft line to see that the two are at right angles;
and, if not, shift the transverse line a little to make them so. From the
transverse line next let fall a line upon each outside keelson, bringing
the edge of the square to the line, the other edge resting on the keelson.
A point will thus be got on each outside keelson, perpendicularly beneath
the transverse line running in the direction of the shaft, and a line drawn
between those two points will be directly below the shaft. To this line the
line of the shaft marked on the sole plate has to be brought, care being
taken, at the same time, that the right distance is preserved between the
fore and aft line upon the sole plate, and the fore and aft line upon the
central keelson.

489. _Q._--Of course the keelsons have first to be properly prepared?

_A._--In a wooden vessel, before any part of the machinery is put in, the
keelsons should be dubbed fair and straight, and be looked out of winding
by means of two straight edges. The art of placing engines in a ship is
more a piece of plain common sense than any other feat in engineering, and
every man of intelligence may easily settle a method of procedure for
himself. Plumb lines and spirit levels, it is obvious, cannot be employed
on board a vessel, and the problem consists in so placing the sole plates,
without these aids, that the paddle shaft will not stand awry across the
vessel, nor be carried forward beyond its place by the framing shouldering
up more than was expected. As a plumb line cannot be used, recourse must be
had to a square; and it will signify nothing at what angle with the deck
the keelsons run, so long as the line of the shaft across the keelsons is
square down from the shaft centre. The sole plates being fixed, there is no
difficulty in setting the other parts of the engine in their proper places
upon them. The paddle wheels must be hung from the top of the paddle box to
enable the shaft to be rove through them, and the cross stays between the
engines should be fixed in when the vessel is afloat. To try whether the
shafts are in a line, turn the paddle wheels, and try if the distance
between the cranks is the same at the upper and under, and the two
horizontal centres; if not, move the end of the paddle shaft up or down,
backward or forward, until the distance between the cranks at all the four
centres is the same.

490. _Q._--In what manner are the engines of a steam vessel secured to the
hull?

_A._--The engines of a steamer are secured to the hull by means of bolts
called holding down bolts, and in wooden vessels a good deal of trouble is
caused by these bolts, which are generally made of iron. Sometimes they go
through the bottom of the ship, and at other times they merely go through
the keelson,--a recess being made in the floor or timbers to admit of the
introduction of a nut. The iron, however, wears rapidly away in both cases,
even though the bolts are tinned; and it has been found the preferable
method to make such of the bolts as pass through the bottom, or enter the
bilge, of Muntz's metal, or of copper. In a side lever engine, four Muntz's
metal bolts may be put through the bottom at the crank end of the framing
of each engine, four more at the main centre, and four more at the
cylinder, making twelve through bolts to each engine; and it is more
convenient to make these bolts with a nut at each end, as in that case the
bolts may be dropped down from the inside, and the necessity is obviated of
putting the vessel on very high blocks in the dock, in order to give room
to put the bolts up from the bottom. The remainder of the holding down
bolts may be of iron, and may, by means of a square neck, be screwed into
the timber of the keelsons as wood screws--the upper part being furnished
with a nut which may be screwed down upon the sole plate, so soon as the
wood screw portion is in its place. If the cylinder be a fixed one it
should be bolted down to the sole plate by as many bolts as are employed to
attach the cylinder cover, and they should be of copper or brass, in any
situation that is not easily accessible.

491. _Q._--If the engines become loose, how do you refix them?

_A._--It is difficult to fix engines effectually which have once begun to
work in the ship, for in time the surface of the keelsons on which the
engines bear becomes worn uneven, and the engines necessarily rock upon it.
As a general rule, the bolts attaching the engines to the keelsons are too
few and of too large a diameter: it would be preferable to have smaller
bolts, and a greater number of them. In addition to the bolts going through
the keelsons or the vessel's bottom, there should be a large number of wood
screws securing the sole plate to the keelson, and a large number of bolts
securing the various parts of the engine to the sole plate. In iron
vessels, holding down bolts passing through the bottom are not expedient;
and there the engine has merely to be secured to the iron plate of the
keelsons, which are made hollow to admit of a more effectual attachment.

492. _Q._--What are the proper proportions of bolts?

_A._--In well formed bolts, the spiral groove penetrates about one twelfth
of the diameter of the cylinder round which it winds, so that the diameter
of the solid cylinder which remains is five sixths of the diameter over the
thread. If the strain to which iron may be safely subjected in machinery is
one fifteenth of its utmost strength, or 4,000 lbs. on the square inch,
then 2,180 lbs. may be sustained by a screw an inch in diameter, at the
outside of the threads. The strength of the holding down bolts may easily
be computed, when the elevating force of the piston or main centre is
known; but it is expedient very much to exceed this strength in practice,
on account of the elasticity of the keelsons, the liability to corrosion,
and other causes.




THE LOCOMOTIVE ENGINE.

493. _Q._--What is the amount of tractive force requisite to draw carriages
on railways?

_A._--Upon well formed railways with carriages of good construction, the
average tractive force required for low speeds is about 7-1/2 lbs. per ton,
or 1/300th of the load, though in some experimental cases, where particular
care was taken to obtain a favorable result, the tractive force has been
reduced as low as 1/500th of the load. At low speeds the whole of the
tractive force is expended in overcoming the friction, which is made up
partly of the friction of attrition in the axles, and partly of the rolling
friction, or the obstruction to the rolling of the wheels upon the rail.
The rolling friction is very small when the surfaces are smooth, and in the
case of railway carriages does not exceed 1/1000th. of the load; whereas
the draught on common roads of good construction, which is chiefly made up
of the rolling friction, is as much as 1/36th of the load.

494._Q._--In reference to friction you have already stated that the
friction of iron sliding upon brass, which has been oiled and then wiped
dry, so that no film of oil is interposed, is about 1/11th of the pressure,
but that in machines in actual operation, where there is a film of oil
between the rubbing surfaces, the friction is only about one third of this
amount, or 1/33d of the weight.  How then can the tractive resistance of
locomotives at low speeds, which you say is entirely made up of friction,
be so little as 1/500th. of the weight?

_A._--I did not state that the resistance to traction was 1/500th of the
weight upon an average--to which condition the answer given to a previous
question must be understood to apply--but I stated that the average
traction was about 1/300th of the load, which nearly agrees with my former
statement. If the total friction be 1/300th of the load, and the rolling
friction be 1/1000th of the load, then the friction of attrition must be
1/429th of the load; and if the diameter of the wheels be 36 in., and the
diameter of the axles be 3 in., which are common proportions, the friction
of attrition must be increased in the proportion of 36 to 3, or 12 times,
to represent the friction of the rubbing surface when moving with the
velocity of the carriage, 12/429ths are about 1/35th of the load, which
does not differ much from the proportion of 1/33d as previously determined.

495. _Q._--What is the amount of adhesion of the wheels upon the rails?

_A._--The adhesion of the wheels upon the rails is about 1/5th of the
weight when the rails are clean, or either perfectly wet or perfectly dry;
but when the rails are half wet or greasy, the adhesion is not more than
1/10th or 1/12th of the weight or pressure upon the wheels. The weight of a
locomotive of modern construction varies from 20 to 25 tons.

496. _Q._--And what is its cost and average performance?

_A._--The cost of a common narrow gauge locomotive, of average power,
varies from £1,900 to £2,200; it will run on an average 130 miles per day,
at a cost for repairs of 2-1/2d. per mile; and the cost of locomotive
power, including repairs, wages, oil, and coke, does not much exceed 6d.
per mile run, on economically managed railways. This does not include a
sinking fund for the renewal of the engines when worn out, which may be
taken as equivalent to 10 per cent. on their original cost.

497. _Q._--Does the expense of traction increase much with an increased
speed?

_A._--Yes; it increases very rapidly, partly from the undulation of the
earth when a heavy train passes over it at a high velocity, but chiefly
from the resistance of the atmosphere and blast pipe, which constitute the
greatest of the impediments to motion at high speeds. At a speed of 30
miles an hour, the atmospheric resistance has been found in some cases to
amount to about 12 lbs. a ton; and in side winds the resistance even
exceeds this amount, partly in consequence of the additional friction
caused from the flanges of the wheels being forced against the rails, and
partly because the wind catches to a certain extent the front of every
carriage, whereby the efficient breadth of each carriage, in giving motion
to the air in the direction of the train, is very much increased. At a
speed of 30 miles an hour, an engine evaporating 200 cubic feet of water in
the hour, and therefore exerting about 200 horses power, will draw a load
of 110 tons. Taking the friction of the train at 7-1/2 lbs. per ton, or 825
lbs. operating at the circumference of the driving wheel--which, with 5 ft.
6 in. wheels, and 18 in. stroke, is equivalent to 4,757 lbs. upon the
piston--and taking the resistance of the blast pipe at 6 lbs. per square
inch of the pistons, and the friction of the engine unloaded at 1 lb. per
square inch, which, with pistons 12 in. in diameter, amount together to
1,582 lbs., and reckoning the increased friction of the engine due to the
load at 1/7th of the load, as in some cases it has been found
experimentally to be, though a much less proportion than this would
probably be a nearer average, we have 7018.4 lbs. for the total load upon
the pistons. At 30 miles an hour the speed of the pistons will be 457.8
feet per minute, and 7018.4 lbs. multiplied by 457.8 ft. per minute, are
equal to 3213023.5 lbs. raised one foot high in the minute, which, divided
by 33,000, gives 97.3 horses power as the power which would draw 110 tons
upon a railway at a speed of 30 miles an hour, if there were no atmospheric
resistance. The atmospheric resistance is at the rate of 12 lbs. a ton,
with a load of 110 tons, equal to 1,320 lbs., moving at a speed of 30 miles
an hour, which, when reduced, becomes 105.8 horses power, and this, added
to 97.3, makes 203.1, instead of 200 horses power, as ascertained by a
reference to the evaporative power of the boiler. This amount of
atmospheric resistance, however, exceeds the average, and in some of the
experiments for ascertaining the atmospheric resistance, a part of the
resistance due to the curves and irregularities of the line has been
counted as part of the atmospheric resistance.

498. _Q._--Is the resistance per ton of the engine the same as the
resistance per ton of the train?

_A._--No; it is more, since the engine has not merely the resistance of the
atmosphere and of the wheels to encounter, but the resistance of the
machinery besides. According to Mr. Gooch's experiments upon a train
weighing 100 tons, the resistance of the engine and tender at 13.1 miles
per hour was found by the indicator to be 12.38 lbs.; the resistance per
ton of the train, as ascertained by the dynamometer, was at the same speed
7.58 lbs., and the average resistance of locomotive and train was 9.04 lbs.
At 20.2 miles per hour these resistances respectively became 19.0, 8.19,
and 12.2 lbs. At 441 miles per hour the resistances became 34.0, 21.10, and
25.5 lbs., and at 57.4 miles an hour they became 35.5, 17.81, and 23.8 lbs.

499. _Q._--Is it not maintained that the resistance of the atmosphere to
the progress of railway trains increases as the square of the velocity?

_A._--The atmospheric resistance, no doubt, increases as the square of the
velocity, and the power, therefore, necessary to overcome it will increase
as the cube of the velocity, since in doubling the speed four times, the
power must be expended in overcoming the atmospheric resistance in half the
time. At low speeds, the resistance does not increase very rapidly; but at
high speeds, as the rapid increase in the atmospheric resistance causes the
main resistance to be that arising from the atmosphere, the total
resistance will vary nearly as the square of the velocity. Thus the
resistance of a train, including locomotive and tender, will, at 15 miles
an hour, be about 9.3 lbs. per ton; at 30 miles an hour it will be 13.2
lbs. per ton; and at 60 miles an hour, 29 lbs. per ton. If we suppose the
same law of progression to continue up to 120 miles an hour, the resistance
at that speed will be 92.2 lbs. per ton, and at 240 miles an hour the
resistance will be 344.8 lbs. per ton. Thus, in doubling the speed from 60
to 120 miles per hour, the resistance does not fall much short of being
increased fourfold, and the same remark applies to the increase of the
speed from 120 to 240 miles an hour. These deductions and other deductions
from Mr. Gooch's experiments on the resistance of railway trains, are fully
discussed by Mr. Clark, in his Treatise on railway machinery, who gives the
following rule for ascertaining the resistance of a train, supposing the
line to be in good order, and free from curves:--To find the total
resistance of the engine, tender, and train in pounds per ton, at any given
speed. Square the speed in miles per hour; divide it by 171, and add 8 to
the quotient. The result is the total resistance at the rails in lbs. per
ton.

500._Q._--How comes it, that the resistance of fluids increases as the
square of the velocity, instead of the velocity simply?

_A._--Because the height necessary to generate the velocity with which the
moving object strikes the fluid, or the fluid strikes the object, increases
as the _square_ of the velocity, and the resistance or the weight of a
column of any fluid varies as the height. A falling body, as has been
already explained, to have acquired twice the velocity, must have fallen
through four times the height; the velocity generated by a column of any
fluid is equal to that acquired by a body falling through the height of the
column; and it is therefore clear, that the pressure due to any given
velocity must be as the square of that velocity, the pressure being in
every case as twice the altitude of the column. The work done, however, by
a stream of air or other fluid in a given time, will vary as the cube of
the velocity; for if the velocity of a stream of air be doubled, there will
not only be four times the pressure exerted per square foot, but twice the
quantity of air will be employed; and in windmills, accordingly, it is
found, that the work done varies nearly as the cube of the velocity of the
wind. If, however, the work done by _a given quantity_ of air moving at
different speeds be considered, it will vary as the squares of the speeds.

501. _Q._--But in a case where there is no work done, and the resistance
varies as the square of the speed, should not the power requisite to
overcome that resistance vary as the square of the speed?

_A._--It should if you consider the resistance over a given distance, and
not the resistance during a given time. Supposing the resistance of a
railway train to increase as the square of the speed, it would take four
times the power, so far as atmospheric resistance is concerned, to
accomplish a mile at the rate of 60 miles an hour, that it would take to
accomplish a mile at 30 miles an hour; but in the former case there would
be twice the number of miles accomplished in the same time, so that when
the velocity of the train was doubled, we should require an engine that was
capable of overcoming four times the resistance at twice the speed, or in
other words, that was capable of exerting eight times the power, so far as
regards the element of atmospheric resistance. We know by experience,
however, that it is easier to attain high speeds on railways than in steam
vessels, where the resistance does increase nearly as the square of the
speed.

502. _Q._--Will you describe generally the arrangement of a locomotive
engine?

_A._--The boiler and engine are hung upon a framework set on wheels, and,
together with this frame or carriage, constitute what is commonly called
the locomotive. Behind the locomotive runs another carriage, called the
tender, for holding coke and water. A common mode of connecting the engine
and tender is by means of a rigid bar, with an eye at each end through
which pins are passed. Between the engine and tender, however, buffers
should always be interposed, as their pressure contributes greatly to
prevent oscillation and other irregular motions of the engine.

503. _Q._--How is the framing of a locomotive usually constructed?

_A._--All locomotives are now made with the framing which supports the
machinery situated within the wheels; but for some years a vehement
controversy was maintained respecting the relative merits of outside and
inside framing, which has terminated, however, in the universal adoption of
the inside framing. It is difficult, in engines intended for the narrow
gauge, to get cylinders within the framing of sufficient diameter to meet
the exigencies of railway locomotion; by casting both cylinders in a piece,
however, a considerable amount of room may be made available to increase
their diameters. It is very desirable that the cylinders of locomotives
should be as large as possible, so that expansion may be adopted to a large
extent; and with any given speed of piston, the power of an engine either
to draw heavy loads, or achieve high velocities, will be increased with
every increase of the dimensions of the cylinder. The framing of
locomotives, to which the boiler and machinery are attached, and which
rests upon the springs situated above the axles, is formed generally of
malleable iron, but in some engines the side frames consist of oak with
iron plates riveted on each side. The guard plates are in these cases
generally of equal length, the frames being curved upward to pass over the
driving axle. Hard cast iron blocks are riveted between the guard plates to
serve as guides for the axle bushes. The side frames are connected across
the ends, and cross stays are introduced beneath the boiler to stiffen the
frame sideways, and prevent the ends of the connecting or eccentric rods
from falling down if they should be broken.

504. _Q._--What is the nature and arrangement of the springs of
locomotives?

_A._--The springs are of the ordinary carriage kind, with plates connected
at the centre, and allowed to slide on each other at their ends. The upper
plate terminates in two eyes, through each of which passes a pin, which
also passes through the jaws of the bridle, connected by a double threaded
screw to another bridle, which is jointed to the framing; the centre of the
spring rests upon the axle box. Sometimes the springs are placed between
the guard plates, and below the framing which rests upon their extremities.
One species of springs which has gained a considerable introduction,
consists of a number of flat steel plates with a piece of metal or other
substance interposed between them at the centre, leaving the ends standing
apart. It would be preferable, perhaps, to make the plates of a common
spring with different curves, so that the leaves, though in contact at the
centre, would not be in contact with the ends with light loads, but would
be brought into contact gradually, as the strain conies on: a spring would
thus be obtained that was suitable for all loads.

505. _Q._--What is the difference between inside and outside cylinder
engines?

_A._--Outside cylinders are so designated when placed upon the outside of
the framing, with their connecting rods operating upon pins in the driving
wheels; while the inside cylinders are situated within the framing, and the
connecting rods attach themselves to cranks in the driving axle.

506. _Q._--Whether are inside or outside cylinder engines to be preferred?

_A._--A diversity of opinion obtains as to the relative merits of outside
and inside cylinders. The chief objection to outside cylinders is, that
they occasion a sinuous motion in the engine which is apt to send the train
off the rails; but this action may be made less perceptible or be remedied
altogether, by placing a weight upon one side of the wheels, the momentum
of which will just balance the momentum of the piston and its connections.
The sinuous or rocking motion of locomotives is traceable to the arrested
momentum of the piston and its attachments at every stroke of the engine,
and the effect of the pressure thus created will be more operative in
inducing oscillation the farther it is exerted from the central line of the
engine. If both cylinders were set at right angles in the centre of the
carriage, and the pistons were both attached to a central crank, there
would be no oscillation produced; or the same effect would be realized by
placing one cylinder in the centre of the carriage, and two at the sides--
the pistons of the side cylinders moving simultaneously: but it is
impossible to couple the piston of an upright cylinder direct to the axle
of a locomotive, without causing the springs to work up and down with every
stroke of the engine: and the use of three cylinders, though adopted in
some of Stephenson's engines, involves too much complication to be a
beneficial innovation.

507. _Q._--Whether are four-wheeled or six-wheeled engines preferable?

_A._--Much controversial ingenuity has been expended upon the question of
the relative merits of the four and six-wheeled engines; one party
maintaining that four-wheeled engines are most unsafe, and the other that
six-wheeled engines are unmechanical, and are more likely to occasion
accidents. The four-wheeled engines, however, appear to have been charged
with faults that do not really attach to them when properly constructed;
for it by no means follows that if the axle of a four-wheeled engine
breaks, or even altogether comes away, that the engine must fall down or
run off the line; inasmuch as, if the engine be properly coupled with the
tender, it has the tender to sustain it. It is obvious enough, that such a
connection may be made between the tender and the engine, that either the
fore or hind axle of the engine may be taken away, and yet the engine will
not fall down, but will be kept up by the support which the tender affords;
and the arguments hitherto paraded against the four-wheeled engines are, so
far as regards the question of safety, nothing more than arguments against
the existence of the suggested connection. It is no doubt the fact, that
locomotive engines are now becoming too heavy to be capable of being borne
on four wheels at high speeds without injury to the rails; but the
objection of damage to the rails applies with at least equal force to most
of the six-wheeled engines hitherto constructed, as in those engines the
engineer has the power of putting nearly all the weight upon the driving
wheels; and if the rail be wet or greasy, there is a great temptation to
increase the bite of those wheels by screwing them down more firmly upon
the rails. A greater strain is thus thrown upon the rail than can exist in
the case of any equally heavy four-wheeled engine; and the engine is made
very unsafe, as a pitching motion will inevitably be induced at high
speeds, when an engine is thus poised upon the central driving wheels, and
there will also be more of the rocking or sinuous motion. Locomotives,
however, intended to achieve high speeds or to draw heavy loads, are now
generally made with eight wheels, and in some cases the driving wheels are
placed at the end of the engine instead of in the middle.

508. _Q._--As the question of the locomotive boiler has been already
disposed of in discussing the question of boilers in general, it now only
remains to inquire into the subject of the engine, and we may commence with
the cylinders. Will you state the arrangement and construction of the
cylinders of a locomotive and their connections?

_A._--The cylinders are placed in the same horizontal plane as the axle of
the driving wheels, and the connecting rod which is attached to the piston
rod engages either a crank in the driving axle or a pin in the driving
wheel, according as the cylinders are inside or outside of the framework.
The cylinders are generally made an inch longer than the stroke, or there
is half an inch of clearance at each end of the cylinder, to permit the
springs of the vehicle to act without causing the piston to strike the top
or bottom of the cylinder. The thickness of metal of the cylinder ends is
usually about a third more than the thickness of the cylinder itself, and
both ends are generally made removable. The priming of the boiler, when it
occurs, is very injurious to the cylinders and valves of locomotives,
especially if the water be sandy, as the grit carried over by the steam
wears the rubbing surfaces rapidly away. The face of the cylinder on which
the valve works is raised a little above the metal around it, both to
facilitate the operation of forming the face and with the view of enabling
any foreign substance deposited on the face to be pushed aside by the valve
into the less elevated part, where it may lie without occasioning any
further disturbance. The valve casing is sometimes cast upon the cylinder,
and it is generally covered with a door which may be removed to permit the
inspection of the faces. In some valve casings the top as well as the back
is removable, which admits of the valve and valve bridle being removed with
greater facility. A cock is placed at each end of locomotive cylinders, to
allow the water to be discharged which accumulates in the cylinder from
priming or condensation; and the four cocks of the two cylinders are
usually connected together, so that by turning a handle the whole are
opened at once. In Stephenson's engines, however, with variable expansion,
there is but one cock provided for this purpose, which is on the bottom of
the valve chest.

509. _Q._--What kind of piston is used in locomotives?

_A._--The variety of pistons employed in locomotives is very great, and
sometimes even the more complicated kinds are found to work very
satisfactorily; but, in general, those pistons which consist of a single
ring and tongue piece, or of two single rings set one above the other, so
as to break joint, are preferable to those which consist of many pieces. In
Stephenson's pistons the screws were at one time liable to work slack, and
the springs to break.

510. _Q._--Will you explain the connection of the piston rod with the
connecting rod?

_A._--The piston rods of all engines are now generally either case hardened
very deeply, or are made of steel; and in locomotive engines the diameter
of the piston rod is about one seventh of the diameter of the cylinder, and
it is formed of tilted steel. The cone of the piston rod, by which it is
attached to the piston, is turned the reverse way to that which is adopted
in common engines, with the view of making the cutter more accessible from
the bottom of the cylinder, which is made to come off like a door. The top
of the piston rod is secured with a cutter into a socket with jaws, through
the holes of which a cross head passes, which is embraced between the jaws
by the small end of the connecting rod, while the ends of the cross head
move in guides. Between the piston rod clutch and the guide blocks, the
feed pump rod joins the cross head in some engines.

511. _Q._--What kind of guides is employed for the end of the piston rod?

_A._--The guides are formed of steel plates attached to the framing,
between which work the guide blocks, fixed on the ends of the cross head,
which have flanges bearing against the inner edges of the guides. Steel or
brass guides are better than iron ones: Stephenson and Hawthorn attach
their guides at one end to a cross stay, at the other to lugs on the
cylinder cover; and they are made stronger in the middle than at the ends.
Stout guide rods of steel, encircled by stuffing boxes on the ends of the
cross head, would probably be found superior to any other arrangement. The
stuffing boxes might contain conical bushes, cut spirally, in addition to
the packing, and a ring, cut spirally, might be sprung upon the rod and
fixed in advance of the stuffing box, with lateral play to wipe the rod
before entering the stuffing box, to prevent it from being scratched by the
adhesion of dust.

512. _Q._--Is any provision made for keeping the connecting rod always of
the same length?

_A._--In every kind of locomotive it is very desirable that the length of
the connecting rod should remain invariable, in spite of the wear of the
brasses, for there is a danger of the piston striking against the cover of
the cylinder if it be shortened, as the clearance is left as small as
possible in order to economize steam. In some engines the strap encircling
the crank pin is fixed immovably to the connecting rod by dovetailed keys,
and a bolt passes through the keys, rod, and strap, to prevent the
dovetailed keys from working out. The brass is tightened by a gib and
cutter, which is kept from working loose by three pinching screws and a
cross pin or c