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^^^^^^^^^H%T C. JiABOtl^B, LLC.
Hopkins Transportation Library
STANFORD UNIVEHSITV
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LONDON :
BSADBiniY AKV KYAKB, PBIMTKES, WUITBF&IAB8.
662012
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CONTENTS.
— f—
CHAP. PAOB
L — ^How Steam produces Mechanical Action . . . . S
n. — ^What Steam is, and what are its Properties . . 5
m. — ^How Water is ccmyerted into Steam, and how Steam is
reconverted into Water 7
lY. — ^How much Mechanical Effect is produced by the conversion
of Water into Steam 12
Y. — How much Mechanical Effect is produced by the conversion
of Steam into Water 14
VL — How much Heat is necessary to convert Water into Steam . 15
YiL — How Steam produces Mechanical Force by its Expansion . 19
vin. — ^How a Vacuum is produced without cooling the Vessel
containing the Steam 24
JJL — ^How the Mechanical Action of Steam may be augmented
by Heat imparted to it directly 27
X. — ^How a Piston is made to move alternately from end to end
of a Cylinder with a definite Mechanical Force .28
XL — How the Alternate Motion of the Piston-rod is conveyed to
the Working Beam 35
xn. — How the Alternate Motion of the Working Beam produces
a Motion of continued Rotation 39
xnL — ^How the Steam Engine is rendered a Self-acting Machine . 46
xrv. — How the Mechanical Effect exerted by the Piston is ascer-
tained 48
XV. — ^How the Heat is produced by which Steam ia TOAAsi . - ^^^^
r
CONTENTS.
— How the Draft through tho Futquco of a Steam Bngine ie
maintained i
— Bqw the Mechamcal Virtue of Fuel ie eatdnmted and
eipreaaed <
— How the Power of an Engine is estimatsd and eipressad,
aa diatinguiflhed from its Duty i
—What Dimensions of the Boiler and Furnace are neceesar;
for on Engine of given Power '
— What DimanBiona of tlie Cylinder and other Macbinery ara
requirito for a given Power of Engine ... . '
— How the Intcmul Condiliou of the Boiler and Engine is
rendered eiternally mamffet '
— How the Wanta of the Boiler and Engine are supplied, and
how their Operation is regulated 1
— How the Steam F.ufflDo is adapted to the Working of
— How tho Atmospheric FreSBure comhined with the Pro-
perties of Steam is rendered effioieut in an Engine . I
— How the Steam Engine is constructed in casea where a
Condensing Apparatus is inadmissible ... . !
^How tho Moohacical Pressure of the Staam on tho Piston
is limited, and how the Speed of the Piston is affected
^Ulustrations :
Mercurial Steam Gauge for Low-pressure Boilers . 1!
„ „ „ for High-preasure Boilers . . 1!
Barometer Gauge— Siphon do. 1!
Glass Water Gauge , . i;
Spring Safety Valve for High-pressure Boilers . . i
Wart's Indicator , . 1!
Boilers and their Appendages 11
Waggon Bailer and its Appendages . . II
The Slide ValvBB 1!
General Arrangement of tbe Double-acting Steam
Engine— /ruWMpfeM II
PEEFACE.
In this little book an attempt has been made to
supply, in generally intelligible language, an explanation
of the facts and mechanical principles on which the
structure and operation of Steam Engines depend.
Within the proposed limits of bulk and cost it would
be impossible to give much practical detail. The
object is, therefore, to supply those who desire to learn
how it is that the Steam Engine has accomplished the
miracles of power for which it has been so celebrated
with the means of doing so, without the technicalities
of art and science, and in a form and manner which
will not require a greater amoimt of time and labour
than they can readily bestow upon it.
It is hoped that the simplicity of style and language
and the comprehensive plan which have been adopted
will attain this end, and tiiat «ii£ici^\> ^ "^i^cLQk V^:^^
U PREFACE.
learned to read may in these pages learn how it is
that steam power plays the important part ascribed to
it in the arts and manufactures.
In the Text, the explanations are given with but
little reference to diagrams ; but a selection of illus-
trations is added in Chapter xxvn., by reference to
which the Text will be still more clearly elucidated.
EtTDIMENTAEY TREATISE
THE STEAM ENGINE.
[AP, I, — now 8TEAH PBODHOBa UEOHAHlCilL ACTION.
1. The inBtrument by which ateain accomplishes this ia
almost invaj-iahly a piatoa, moveable in a cylinder.
A cylinder is a tube or pipe, but much larger in its dia-
meter, in proportion to its length, than tubes or pipes usually
are. Thus a common proportion for a cylinder is 3 feet in
diameter, inside measure, and 4 feet op 41 feet in length ; but
this proportion is very variable according to circumstances.
2. The piston is a solid plug, fitting the interior of the
cylinder with auiEcient precision to prevent steam from
passing from the one side to the other, but with sufficient
freedom of motion to enable it to move along the cylinder
without any considerable loss of force to keep it in motion.
3. The ends of the cylinder are imderstood to be closely
stopped by lids. One of these lids is cast with the cylinder,
and forma, in fact, part of it ; the other is attached to it by
screws and nuta, and fitted so exactly that ateam cannot
escape at the joints.
4. Small apertures are provided at each end of the cylin-
der, furnished with stoppers or valves, by which steaJU
be admitted or allowed to escape at pleaaiu^.
5. Now it will he easily understood, tliofc ^S ^ ^a^i
Bteam be admitted at one end o5 ttie c"j\ai&BS SS."*""^"^
I
ot ^m
J
4 KUDI3IENTABT TREATISE
the piston to the other end : if a blast of steam be admifcted
at the other end, that which had previously been admitted
being allowed to escape, the piston will be blown back again.
If we have the means, then, of taking in a blast of steam
alternately at the one end and at the other end of the
cylinder, the piston will be blown constantly backwards and
forwards from end to end.
The force with which this will be effected will depend on
the force of the steam.
6. This alternate motion of the piston from end to end of
the cylinder, made with a certain degree of force, could
accomplish nothing useful if it were confined within the
cylinder ; it must be communicated to something outside
which is required to be set in motion.
7. This is accomplished by an appendage to one side of
the piston, called the pistanrrod. This is a round rod,
firmly fixed into the centre of the piston, and passing
through a hole made in the centre of the cover or lid of the
cylinder, which I have already described, to be attached by
screws and nuts. It must move in this hole as the piston
does in the cylinder, so tightly as not to let any steam
escape, and yet so freely as not to require any considerable
power to urge it.
8. It will be easily understood, that to attain this object
very great precision of form is necessary in the internal
surface of the cylinder and in the piston-rod. The cylinder
is made of cast iron, but the inner surface of it, after being
cast, is reduced to a precise cylindrical form by a boring
machine. This machine scrapes off aU roughness, dnd
reduces every part of the inner surface to an exact circular
form, of precisely the same diameter throughout the entire
length of the cylinder.
9. The piston, which is fiat on either side and circular at
its edge, to correspond with the cylinder, is made to fit the
cylinder in steam-tight contact, and at the same time to
move freely in it by a variety of contrivances which will be
ON THE STEAM EITGINE. 5
noticed hereafter. Por the present it will be sufficient to
assume that mechanical art, in its present state, enables ns
to construct pistons and cylinders with so great a degree of
precision that no steam whatever shall pass between them,
and yet that the motion shall be almost perfectly free.
10. The piston-rod, also of iron, is turned in a lathe so as
to be truly round, and uniformly of the same diameter
throughout its length. The hole through which it plays
in the top of the cylinder is surrounded by a packing of
hemp, soaked in oil and tallow, which is pressed against the
sides of the pi^n^rod ; and in this way, whilst the motion
is £ree, no steam escapes.
11. The piston-rod thus partakes of the alternate motion
which the piston itself receives, and conveys this motion to
any object outside with which it may be connected.
12. Thus the primary motion produced by steam power is
an alternate motion backwards and forwards iu a straight
line ; but by an infinite variety of well-known mechanical
contrivances, this alternate motion may be made to produce
any other kind of motion that may be desired : thus we may
make it keep a wheel in constant rotation, or move a weight
continually in the same straight line and in the same
direction.
13. These points will be hereafter explained : for the pre-
sent we establish the fact that steam can by the means
indicated produce an alternate force backwards and forwards
along a cylinder with a degree of energy proportionate to
the force of the steam, and with a degree of speed -ffopor-
tionate to the rate at which the steam can be supplied.
CHAP. n. — WHAT STEAM IS, AND WHAT ARE ITS PROPERTIES.
1. I have spoken of the piston in the cylinder being driven
from one end to the other by a hlctst of steam. This will at
once suggest the resemblance of steam to an*. Steaxa.^<j3f%.-
sesses, in fact, a set of propertvea t^t^c^^^^ ^^ ^"assjtfe -aa* "sss^
6 BUDIMENTAET TREATISE
if air were heated to the same temperature as steam, it
would, to all intents and purposes, possess the same
mechanical properties; and if it were as manageable in
other respects as steam is, we should have no occasion to
resort to steam engines, but should have nothing but air
engines. Air could blow the piston from end to end of the
cylinder as well and in exactly the same manner as steam
does. It will therefore greatly facilitate the comprehension
of the qualities of steam to attend, in the first instance,
to the corresponding qualities of air.
2. Air is an elastic fluid, — so is steam.
The meaning of an elastic fluid is one which may be
squeezed or compressed into a less bulk ; or, on the other
hand, which will expand itself into a greater bulk spon-
taneously if room be given to it.
3. All fluids, however, do not enjoy this property : water
does not partake of it at all ; it cannot be squeezed by any
practical force into less dimensions than it naturally occu-
pies, and whatever room you may give to it, it will not
expand into greater volume. If air be enclosed in any
vessel, it will spontaneously press on every part of the inner
surface of such vessel with a certain force, tending, as it
were, to burst the vessel. This is what is called its elcu*
ticitt/. If it be squeezed into a vessel of half the size, it
will press on the inner surface of this vessel with just double
the force ; and if, on the other hand, it be allowed a vessel
of twice the size, it will spontaneously expand and fiU every
part of such vessel, but will press on it with a diminished
force, amounting to one-half its original pressure.
4. In short, you may by compression reduce its bidk in
any required proportion, and its bursting or elastic force
will be augmented in exactly the same proportion ; and you
may, on the other hand, permit it to expand to any aug-
mented volume, and its pressure will be diminished in
precisely the proportion in which its voliime will be
increased.
ON THE STEAM ENGINE. 7
5. All these are equally qualities of steam.
Air is an invisible fluid, — so is steam. It is a great mis-
take to imagine that the cloudy vapour that is seen issuing
like white smoke from steam vessels or boilers is steam :
the moment it becomes thus white and cloudy it ceases to
be steam.
These misty particles are particles of water, and not steam.
If a glass vessel were filled with pure steam, it would be as
invisible as when filled with air.
6. Steam is air made firom water.
Air may exist in different states of density, — ^so may
steam. In either case the pressure or elasticity (other cir-
cumstances being the same) is in proportion to the density.
7. But as air is everywhere accessible and disposable, it
may be asked why we may not use it for those mechanical
purposes for which steam has proved so omnipotent, espe-
cially seeing that the production of one is attended with
great cost and trouble, while the other exists in unbounded
quantity, and can be had everywhere and for nothing. To
answer this we must consider those qualities in which steam
differs from air.
CHAP. n. — ^HOW WATER IS CONVERTED INTO STEAM, AND HOW
STEAK IS RECONVERTED INTO WATER.
1. If any source of heat be applied to water, the first and
obvious effect will be to render the water hotter.
2. But to this there will speedily be a limit. It will be
found that when the water has attained a certain heat, no
fiirther application of heat wiU augment its temperature,
but it will then begin to diminish in quantity, and, as it
were, to disappear ; and if the application of heat be con-
tinued, the water wlQ at length altogether vanish. It has
in this case been gradually converted into steam, which
has ascended into the surrounding atmosphere and mMa^^^^'^j.
^th it.
8 BUDIMENTABT TREATISE
3. But tills escape of the steam may be prevented. Let
a second vessel be provided and put in connection with that
in which the water is heated, and let the communication with
the external air be cut off.
4. The steam produced from the water may be collected
in this vessel, and when so collected, and submitted to
examiaation, it will be found, as I have stated, to possess all
the mechanical properties of air.
It thus appears that the liquid water is converted into the
elasti fluid steam by imparting to it a certain quantity of
heat.
5. One of the most remarkable changes which the water
undergoes when it passes into the form of steam is ita
change of bulk, which is quite enormous.
6. It is found that a quart of water evaporated under
ordinary circumstances will produce about 1700 quarts of
steam, but this proportion varies with circumstances, as we
shall now see.
7. Let us suppose that a piston is inserted in a tube, and
that under the piston a small quantity of water is placed.
!For simplicity, let us suppose that quantity of water to be
a cubic inch. Let the piston be arranged to press upon the
water with a force of 15 lb., the magnitude of the sur&ee
of the piston in contact with the water being a square inch ;
and let us in this case put out of consideration any effect of
the pressure of the external atmosphere, this pressure being
represented by the 151b. imputed to the piston. Let a
lamp be supposed to be applied under the tube, so as to heat
the water within. The effect of the lamp for some time will
be merely that of elevating the temperature of the water,
but when the temperature shall have attained to 212® of
[Fahrenheit's thermometer, then the piston will be observed
to begin to ascend in the cylinder, leaving an apparently-
unoccupied space between it and the water. The quantity
of water will at the same time apparently diminish. The
lamp continuing to act, the piston will continue slowly to
OK THE STEAM ENGINE. 9
ascend, and the water slowly to diminish, until at length all
the water shall have disappeared.
8. The piston will then be found to have ascended to such
a height that the space below it in the cylinder will be 1700
times greater than that which the water originally occupied.
This space, which, if seen as it might be through glass,
would appear empty, would in fact be filled with the steam
produced from the water, which, like air, would be
invisible.
9. In this case we have supposed the steam to be pro-
duced under a pressure of 15 lb. on the square inch. Let
us now, however, suppose things restored to their original
state, and the piston to be loaded with 30 lb., or with
15 lb. in addition to the atmospheric pressure, which makes
a total of 30 lb. If the same process as before be repeated,
it will now be found that before the piston begins to ascend,
the temperature of the water will rise, not to 212°, as
before, but to 252° ; the piston will then begin, as before, to
ascend, and will continue to ascend until all the water shall
have disappeared. It will not, however, rise now so as to
leave 1700 times the original bulk of the water below it, but
only the half of that amount, leaving a space for the steam,
thus produced, about 850 times greater than the bidk of the
water.
In short, the piston may be loaded with any pressure
greater or less than that which we have supposed. K loaded
with a less pressure, the water will eicpand into steam of
greater volume ; and if loaded with a greater pressure, it
will expand into steam of less volume. The temperature
also at which the water will begin to be converted into
steam will vary, being higher for greater pressure and lower
for less pressure.
10. When the pressure is doubled, the steam produced
will not be of precisely double the density, but will not vary
much from that proportion. The reason of the variatioiL —
small as it is — is, that when ttie i^Te^^vrc^ \a ^wS!^<5;^^*^^
10 BUDIMEITTABT TBEATISE
temperature of the steam is augmented, and an increase of
volume due to such increase of temperature causes the
density of the steam which results to be a little less than
double the original density. This variation, however, is so
small that we may disregard it in practice, and assume as a
simple and intelligible rule, that the density of steam is in
the direct proportion of its pressure.
11. As it is of great advantage to retain in the memory
the extent to which the volume of water is expanded when
it is converted into steam, the following accidental pro-
portion will be found useful : a cubic foot contains 1728
cubic inches. Now we shall be sufficiently near the truth,
for all practical purposes, if we state that a cubic inch of
water evaporated under a pressure of 15 lb. per square
inch will produce a cubic foot of steam. This statement
is at once so simple and so striking, that it cannot be
forgotten.
12. Ejiowing the volume of steam produced by a given
quantity of water under this pressure, the volumes which
will be produced under other pressures, greater or less, may
be inferred with sufficient practical accuracy by the pro-
portion already given. Under double the pressure, the
volmne woidd be one-half; and imder half the pressure, the
volume would be double. Thus, if water be boiled under a
pressure of 30 lb. per square inch, a cubic inch of water
will produce half a cubic foot of steam ; if it be boiled under
45 lb. per square inch, it will produce one-third of a cubic
foot of steam ; and in like manner, if it be boiled under
7ilb. per square inch, it will produce two cubic feet of
steam ; and under 5 lb. per square inch, three cubic feet of
steam, and so on.
13. This proportion would be strictly accurate but for the
&ct that the temperatures at which the water boils in these
cases are different ; but the difference due to this need not
be now attended to.
14. It may also be observed, that in general, when the
ON THE STEAM EKGIITE. 11
water boiled is eicposed to the atmosphere, the atmosphere
itself produces an average pressure of 16 lb. per square
inch, which is understood to be included in the above
pressures.
15. Having thus described the manner in which water is
converted into steam, let us now see how steam is converted
into water.
The steam which is produced from the water in the manner
we have described has the same temperature as the water
from whence it proceeds. This temperature is indispensable
to it. The moment you deprive it of any heat, that moment
a portion of it returns to the state of water, and by the
continued abstraction of heat from it, it will aU return to the
liquid state.
16. Let us suppose, in the tube which we have already
used for our illustration, that after the piston has ascended,
and the water has been all converted into steam, the tube
be surrounded by any cold medium, such as a cold atmo-
sphere, the lamp being in the meanwhile withdrawn ; imme-
diately a dew will be formed on the inner surface of the
tube, and the piston will begin to descend. The dew thus
formed is the water reproduced from the steam, which has
been restored to its liquid state, in small particles ; these
are swept down before the piston, and at length, when the
piston shall have arrived at its original position, all the water
win have re-appeared at the bottom of the tube.
The steam will, in fact, have been reconverted into
water.
17. Thus, as heat is the agent by which water is con-
verted into steam, the abstraction of heat is the means by
which steam is reconverted into water.
This is one of the most important qualities in which
steam differs from air. No known degree of cold is capable
of converting air into a liquid, although analogy justifies the
inference that some degree of cold, though waftfc^'Kss^^^^
by any means yet known, would efeet ^i\i\^» '^^^^-ic^ ^s^^ 'ess^^a
12 BIJDIHENTABY TREATISE
airs, in fact, on wliich art lias produced this effect, but it has
never been accomplished on the atmosphere.
18. It is precisely this quality, giving us the power of
reconverting steam into water at pleasure, which enables us
to use steam so extensively for mechanical purposes, and
deprives air of the same mechanical utility.
CHAP. rv. — HOW MUCH MECHANICAL EFFECT 18 PRODUCED BY
THE CONVERSION OF WATER INTO STEAM.
1. The most common and general method of estimating
the mechanical effect of any agent is by stating what
weight it would raise a certain height, or to what height
it would elevate a given weight. Thus, if we are told that
such or such a mechanical agent is capable of raising 10 tons
a foot high, we have a distinct notion of its efficiency as a
moving power. In this view of mechanical effect, it will be
seen that we omit the consideration of time altogether ;
whether it be produced in a minute or in an hour, the
mechanical effect accomplished is the same. "We shall con-
sider it in reference to time hereafter.
Now the questions I propose to examine are these ; —
2. "What amount of mechanical effect is produced when
a given quantity of water, as a cubic inch, is converted into
steam P
3. To what extent, if at all, is such mechanical effect in-
fluenced by the pressure under which the water is evaporated
or boiled ?
4. Let it be remembered that in all cases the water is
supposed to be boiled in a close vessel, furnished with a valve
loaded with a given pressure, so that the steam produced
from the water shall* have a pressure equivalent to that of
the valve ; in fact, according to our supposition, it must open
the valve to escape, and consequently its force must be * in
equiUbrio* with it. But for our present purpose we shall
recur to a mode of illustration which wiU be more easily
OK THE STEAM EKGIKE. 13
apprehended. Let us, as before, imagine a cubic inch of
water placed in the bottom of a tube of indefinite length ; a
piston being placed in such tube, resting on the water, and so
fittiQg the tube as not to permit the steam to escape. Let us
suppose this piston, in the first instance, to press on the water
with a force of 15 lb., the surface of the piston in contact with
the water having the magnitude of one square inch.
5. According to what has been already explained, it will
be understood that when heat is applied to the water to
convert it into steam, the piston will be forced upwards, to
give room to the steam thus formed. Now it has been
shown that the room which the steam will thus require will
be 1700 times more than its original volume in the liquid
state. If then the section of the tube be a square inch, the
piston win be raised 1700 inches high, in order to make
room for the steam which will be produced. Thus a weight
of 151b. win be raised 1700 inches, or about 142 feet.
The mechanical effect evolved in the evaporation of a cubic
inch of water under these circimistances is therefore equiva-
lent to 15 lb. raised 142 feet high. But 15 lb. raised 142
feet high is equivalent to 142 times 15 lb. raised 'one foot
high, or to 2130 lb. raised a foot high. Now this weight is
very nearly a ton, and as we are not here concerned with
minute fractional accuracy, the following remarkable fact
wiU follow, and may easily be retained in the memory,
6. A cubic inch of water converted into steam will produce
a mechanical force mfftdent to raise a ton weight afoot high,
7. But it may be objected here, that we have supposed
the water evaporated under a particular pressure, and there-
fore at a particular temperature : may it not happen
therefore, that if evaporated xinder'a different pressure and
at a different temperature, a different mechanical effect wiU
ensue ?
To ascertain this, let us suppose the piston to be loaded
with 30 lb. instead of 151b. We have already aeeiL t\is&»
in such case it would be xaiaed to oiA^ V-ei^i ^2^^ V^AsgQis*^*^^^^
14 BUDIMEKTABT TEEITISB
the steam produced would have double the density. Now
30 lb. raised 71 feet is exactly equal to 16 lb. raised 142
feet, and the same consequences would follow at any other
supposable pressure.
8. The above maxim then is general, and it may be
assimied that in the evaporation of water the mechanical
effect evolved is independent of the pressure under which the
evaporation takes place, and is always at the rate of a ton
raised one foot for a cubic iuch evaporated.
9. It may be well here to observe that this is the entire
mechanical force evolved, and that it must not be supposed
that this effect is practically produced by every cubic inch
of water evaporated in the boiler of a steam-engiue ; a con-
siderable proportion of this force being absorbed by friction
and other causes of the waste of power before the tisefkil
effect can be produced.
CHAP. V. — HOW MUCH MECHANICAL EFFECT IS PRODUCED BY
THE CONVERSION OF STEAK INTO WATER.
1. We have seen that a cubic inch of water makes a cubic
foot of steam at the common pressure. If then a close
vessel be filled with steam at this pressure, and be so exposed
to cold that the steam it contains shall be converted into
water, it will only occupy a cubic inch for every cubic foot of
steam which the vessel previously contained. In fact, the
vessel which was previously filled with steam will now have
only a small quantity of water in it, the remainder of the
space being a vacuum.
2. It is this property by which steam becomes iustru-
mental in doing, by the mere agency of temperature, what is
done by the expenditure of so much labour in air pumps and
common water pumps.
3. By whatever agency a vacuum can be produced, by the
same agency a given mechanical effect will foUow ; for if a
piston be placed in the tube in which the vacuum be created
ON THE STEAK EKQIl^E. 15
•
beneath it, the pressure of the atmosphere will drive the
piston down with a force of 151b. for every square inch in
the section of the piston. In air pumps and common
water pumps, where the vacuum is created by pumping
out the air, the amount of mechanical force expended
in producing the vacuum is equivalent to the amount
of medianical force which the vacuum itself produces when
made ; but when a vacuum is made by converting steam
into water, no mechanical force is expended in producing
the effect ; and consequently steam thus produces a mecha-
nical agent in its reconversion into water, as well as in its
production from water.
4. A cubic foot of steam having a pressure of 15 lb. will
therefore, by being converted into water, produce a mecha-
nical force equivalent to that which a cubic inch of water
produces when converted into a cubic foot of steam.
CHAP. VI. — HOW MUCH HEAT IS NECESSARY TO CONVERT
WATER INTO STEAM.
1. Recurring again to the same mode of illustration, let
us suppose the tube and piston as before, a cubic inch of
water being below the piston ; and let us imagine a lamp
burning in a perfectly uniform manner under the tube, so
that it shall impart heat to the water at an imiform rate.
Let us suppose, at the commencement of the process, the
water to be at the temperature of melting ice, but without
having any ice in it. Let the time be then observed which
shall elapse from the first moment of the application of the
lamp to the moment at which the water begins to be con-
verted into steam, and let us suppose this interval to be an
hour. The application of the lamp being continued, as
before, let the process of evaporation go on until all the
water shall have been converted into steam. It will then
be found that the time necessary to complete the evapora-
tion will be 5| hours.
10 BUDIMEKTABT TREATISE
2. From this then it follows, since we suppose the action
of the lamp to have been uniform, that to convert a given
quantity of water into steam requires 5^ times as much heat
as would be necessary to raise the same water from the
freezing to the boiling point.
3. This is a fact of such capital practical importance that
it ought to be engraven on the memory.
It follows from it, that if a given weight of fuel is con-
sumed in raising a quantity of water from the freezing to
the boiling point, 6i times such weight of ftiel will be con-
sumed in converting the same water into steam.
4. There is another point of view in which it is both
interesting and important to regard this fact.
If a thermometer be immersed in the steam which shall
have been produced from the water, it will show that the
steam has the same temperature as the water : thus, if the
water were boiled under the usual pressure of 151b. per
square inch, its temperature would be 212°; the same
would be the temperature of the steam into which it would
be converted.
6. But it will be naturally asked in this case, what has
become of the enormous quantity of heat which has been
supplied by the lamp ? If in an hour, while the lamp was
raising the water from 32° to 212°, it imparted to such
water a quantity of heat sufficient to raise it 180° higher in
its temperature, it must have imparted an equal quantity of
heat in each succeeding hour, and in 6^ hours it would of
coiirse have imparted as much heat as would have added
6i times 180°, or 990°, to 212°, the temperature of the
water, supposing the latter not to have been converted into
steam: the water would thus, had it not been converted
into steam, have been raised to the temperature of 1202°, or
about 400° hotter than red-hot iron. But in the present
case, in which the water passes from the liquid to the
aeriform state, no augmentation of temperature has takien
place at all; the steam which has received, and which
Oir THE STEAM EKGIITE. 17
actually contams, aU tliis enormous amount of heat, being
no hotter than the water which contained nothing of it.
Where is the heat then ? And why is it not felt or indi-
cated by the thermometer ?
6. The answer to the first question is easy. It can be
practically proved, as we shall presently show, that the heat
is in the steam. But the second question reaches one of
the final points of science, and cannot be answered. The
heat which is in the steam, and yet neither sensible to the
touch nor indicated by a thermometer, is said to be latent,
7. But we must not be deceived by the use of this word ;
it is merely a name given to the fact that the heat is not
sensible, but it discloses to us no reason for that fact.
8. It is assumed that the heat has been employed in con-
verting the water from the liquid to the aeriform state,
and being employed in maintamiug the water in such a
state, is not sensible to the thermometer. This, however,
is after all but another mode of stating the fact, and is no
explanation of it.
9. I observed, that the 990° of heat is in the steam,
though not sensible to the thermometer. We might
perhaps be justified in considering this as proved, inasmuch
as the lamp must be supposed to impart heat uniformly
during its action, but we can give a very decisive practical
demonstration of it.
10. Let a cubic foot of steam of the temperature of 212°,
which has been produced from a cubic inch of water, be
supposed to be contaiued in a close vessel. Let 5i cubic
inches of water, at the temperature of 32°, be injected iuto
this vessel. This cold water, mixing with the steam, will
reduce the steam to water, or, to use a technical term, will
condense it, and we shall find in the vessel 6^ cubic inches
of water ; namely, the 5^ cubic inches which were iujeeted,
and the cubic inch which was contaiued in the vessel in the
form of steam, occupying a cubic foot, but which has no^
become water, and occupies oiily ^ cvjW^c Sax-Oo.. ^^^^'^'^ ^*^
18 BUDIHENTAET TREATISE
cubic inches of water will have the temperature of 212**;
that is to say, the same temperature as that of the steam
which was condensed.
Now it is evident that in returning to the state of water,
the steam has given out as much heat as has been sufficient
to raise the 5^ cubic inches of water which were injected
into the vessel from 32° to 212° ; and yet the cubic inch of
water into which such steam has been converted has itself
the temperature of 212°, being the same as that which it
had when in the form of steam. It is clear, then, that the
990° of heat which were in the steam are now in the 5\
cubic inches of water which were injected, and have raised
this, as must have necessarily have been the case, from 82°
to 212°.
11. It is therefore demonstrated that steam has in it as
much heat insensible to the thermometer and to the touch
as would be sufficient to raise 5i times its own weight of
water from the freezing to the boiling point.
12. This result has an important relation to the economy
of steam power. The heat supplied by any fuel of uniform
quality, and used in an uniform manner, will be proportionate
to the quantity of such fuel consumed. It follows, therefore,
that it requires 6^ times as much fuel to convert water into
steam, supposing the process to commence with the water at
32°, as would be sufficient to boil the same quantity of water.
If the process be supposed to commence at the more ordi-
nary temperature of 60°, then a still greater proportion of
fuel will be necessary for evaporation.
13. I have supposed throughout this exposition that the
water has been evaporated imder the common pressure of
15 lb. per square inch, and at the temperature of 212° ; but
it may be asked, what would be the result if the process
were conducted xuider a different pressure and at a different
temperature? Might it not happen that the evaporation
would be effected with a greater economy of heat, which
would be an important fact in the application of steam power ?
ON THE STEAM ENGINE. 19
14. Such, however, is not the case. It is found that no
matter what the pressure may be under which the process is
conducted, the same lamp, or other uniform source of heat,
acting on the same water, wUl take exactly the same time to
convert it into steam. It is true that the quantity of what
is called latent heat will be different, and will be diminished
as the pressure is increased. Thus each degree which is
added to the temperature at which the water boils by
increase of pressure, will be subtracted from the latent heat
of the steam. The manner in which this remarkable fact is
usually expressed is, that the sum of the latent and sensible y
heats of steam is always the same, namely about 1200°.
15. Thus if water be evaporated under such a pressure
that its boiling point shall be 400°, then the latent heat of
the steam produced from it will be 800° ; if it be evaporated
at 300°, the latent heat will be 900°, and so on.
16. This is curious ; but the important fact is, that the
consumption of fuel in the conversion of water into steam is -L^
the same, whatever be the pressure of steam produced.
CHAP. VII. — HOW STEAM PRODUCES MECHANICAL FORCE BY
ITS EXPANSION.
1. We have seen how a piston is urged from one end to
another of a cylinder with a definite force by allowing steam
to flow in upon it, and that increased efficacy is given to this
by creating a vacuum on the side towards which the piston
moves. The steam in this case is supposed to flow from the
boiler, and to press the piston forward with a certain uniform
force. The piston advances because a fresh portion of steam
which enters the cylinder requires more room, to give it
which the motion of the piston is necessary^
When as much steam has entered in this manner as is
sufficient to fill the cylinder, then the piston will be driven
to the extreme end of it. Now itfis well to observe iVsai^W
the production of this effect no (\vxaX\fe5 ^^o^^^ X*'^ ^y^'esss.^
20 BUDIMENTAEY TBEATISE
or which distinguishes steam from any other fluid, is
concerned.
K a liquid (water for example) were made to flow into
the cylinder with the same pressure and in the same quan-
tity, it would produce precisely the same effect ; in fact, the
steam acts thus not because it is an elastic fluid, but because
it is a fluid, and is urged from the boiler with a certain force.
2. I now come to notice, however, a mode of action in
which steam performs what an inelastic fluid could not per-
form ; one, in short, in which it produces a mechanical effect
in virtue of that property which steam enjoys in common
with air and other gaseous fluids, and in which inelastic
fluids, such as water, do not participate.
3. Let us suppose that the steam flowing into the cylinder
acts upon the piston with a certain definite force, as one ton,
and continues so to act as long as it enters the cylinder.
4. Now let us imagine that when the piston has been thus
pushed to the middle of the cylinder, the aperture at which
the steam enters is suddenly closed, so as to prevent any
fresh supply. The piston will then be no longer pushed
forward by any increased quantity of steam coming fix)m the
boiler. It will, nevertheless, be pressed by the elastic force
of the steam, just as it would be by the elastic force
of air under the same circumstances ; it will still be pressed
on by a force of one ton, supposing that no adequate resist-
ance obstructs its motion. It will not, therefore, come to
rest, but will continue to advance. As it advances, the
steam, expanding iuto a larger space, will acquire a propor-
tionally diminishing elastic force, and will press on the
piston with a force less than a ton, in exactly the same
proportion as the space occupied by the steam is greater
than half the cylinder. Ultimately, when the piston arrives
at the end of the cylinder, the steam, which origiually filled
half the cylinder, will fill the whole cylinder; and the
pressure upon the piston, which was origiually a ton, will
then be half a ton.
I
OK THE STEAM ENGINE. 21
6. It appears evident, then, that while the piston is thus
moved through the latter half of the cylinder, it is urged by
a continually decreasing force, which begins with a ton, and
which ends with half a ton.
6. If we could calculate the average amount of this moving
force, we could at once declare the mechanical effect which
is produced through the latter half of the cylinder in virtue
of the expansive power of the steam.
7. At first view it might appear that the average pressure
must be a mean between the original pressure of a ton and
the final pressure of half a ton, and that such mean would
therefore be three-quarters of a ton. But such a conclusion
would be erroneous.
8. The method of calculating the exact average of a force
decreasing in the manner we have described, requires prin-
ciples of the higher mathematics which could not be
introduced properly here. By the application of these
principles it appears that the exact average of the varjdng
pressures, in the case we hate described, would be 1545 lb.
9. The mechanical effect, therefore, obtained in this way
from the expansive action of the steam would be equal to
1545 lb. driven through a space equal to half the length of
the cylinder. It appears, then, that nearly 75 per cent, has
been added to the original mechanical efficacy of the steam
by this expedient.
10. It may be asked whether there be any limit to the
application of this principle. It is known that other fluids,
having the same natural properties as steam, are capable of
expansion indefinitely, and it might at first be imagined that
there is no limit to the augmentation of the mechanical force
which might thus be obtained from steam; but practical
considerations show that there are not only limits, but com-
paratively narrow ones, to its application.
11. It will be observed that the piston, which is urged by
the force of expansive steam, is acted upon by a continually
diminishing power of impulsion. Wlafiii ^JJwa ^^^^vjoctfe <il*^sss5k
22 Rl'DIMENTART TBEATrSV
Htoam booonies bv expansion Ipss than the load which radi
piston clrivort tlirou^h the intervention of machineiT, in-
cluding the nnturnl rcsititancc of the machinery itself^ tiien
it is clear that the moving; power iK-ill cease to be efficacionii
and tliat the pinton nnist conic to rest.
12. Tlie inertia of the Inachincry may continue the
motion somewhat longer than the moment at which an
equilihrium takes place between the resistance of the load
and the pressure on the piston, but this effect must sooa
expire.
13. The expedient by which the cxpansiye principle wsj
be moat conveniently extended is to use, in the commeno&-
ment, steam of high pressure and great density ; such flteam
may allow of considerable expansion before it loses so mndi
of its force as to be reduced to an equilibrium with the
resistance to the piston.
14. In all cases the expansive principle evidently, bh
volves a continual variation in the impelling power of ihd
piston.
Now it seldom happens that there is any similar variation
in the resistance which the piston is required to overcome ;
and in that case an irregularity of action would ensue. In
the commencement, the energy of the impelling force being
greater than the resistance, an accelerated motion would
be produced, and towards the end, the impelling force
becoming less than the resistance, a retarded motion would
be the effect. A great variety of contrivances have been
suggested by mechanical inventors to equalise this varying
action, —
15. The most common and the most beautiM of which is
thefl^'Wheeh This is a heavy wheel of metal, well centered,
and turning upon its axle with but little Motion, so that the
force necessary to keep it in uniform motion is inconsider-
able. The varying action of the piston is transmitted to
this wheel. When the impulsive force is greater than the
resistance of the load, the surplus is imparted to the wheel.
ON THE STEAH EKGINE. 23
to which it gives a slight increase of speed. Owing to the
great mass of matter in the wheel, an increase of speed
which is scarcely sensible absorbs an immense amount of
moving force. When the impulsion of the piston by the
expansion of the steam becomes less than the resistance,
then the ' momentum of the wheel acts upon the load, and
that portion of surplus force which was previously imparted
to it is given back, and the wheel assists, as it were, the
piston in moving the load when the latter becomes enfeebled
by the extreme expansion of the steam.
16. The fly-wheel is thus, as it were, a magazine of force
which gives and takes according to the exigencies of the
machinery. When the moving force is in excess, the fly-
wheel absorbs the surplus ; when the moving force is
deficient, the fly-wheel gives back what it absorbed.
17. Cases occur, however, in the arts in which the
resistance to be overcome by the piston is of a gradually
decreasing nature. In such cases, the expansive action of
the steam, being also gradually decreasing, may be kept in
equilibrio with the work without the intervention of the
equalising action of the fly. Thus if the piston work a
pump by which a column of water is raised, which column
flows off at the top, the length of the column, and therefore
its weight, is greatest when the buckets of the pump begin
to ascend, and least when they arrive at the summit of their
play. The weight in the buckets is in this case of con-
tinually decreasing amount, like the decreasing force of
expanding steam.
18. But in most cases some equalising contrivance is
necessary where the expansive principle is extensively used,
and where any thing approaching to imiform action is
necessary.
19. The expansive action of steam is applied in steam
engines in various ways, but by far the most usual is that
which we have described in the above illustration, by cutting
off the supply of steam at some point befox^ ^;?Ckfc ^ysvsss^^^^-^
21 BUDIMENT.VIIT TBEATISE
of the stroke. In some cases it is cut off at half-stroke, in
some at one-third, and in some at much smaller fractions of
the entire stroke.
CHAP. VIII. — HOW A VACUUM IS PRODUCED WITHOUT COOLDTtt
THE VESSEL CONTAINING THE STEAM.
1. "With whatever force the piston be impelled, the effects
of that force will be evidently augmented by an ability to
produce a vacuum, or even a partial vacuum, in that part of
the cylinder towards which the piston moves.
2. It has been already shown that this may be accom-
plished, if the cylinder be previously filled with steam, by
exposing the steam which has filled it to the contact of cold.
As heat produces steam, cold kills it. Now if a cubic foot
of steam be reconverted into water by cold, it will be
reduced to a cubic inch of that liquid, and we shall have the
entire cubic foot minus one inch, a vacuum ; and, therefore,
for every cubic foot of steam in the cylinder, we shall have a
cubic foot of vacuum minus one cubic inch.
3. But here we encounter a practical difficulty which long
remained without solution. If we produce the vacuum by
cooling the cylinder, and thus condensing the steam it con-
tains, we shall be obliged, on the next stroke of the piston, -
when the cylinder must be refilled with steam, to raise its
temperature again to that of the steam it is intended to
contain; for otherwise the cylinder itself would condense
the steam intended to fill it. Now the heat necessary thus
to warm the cylinder at every stroke of the piston would
entail upon us an enormous waste of fuel ; yet to this waste
was every steam engine exposed from the date of the inven-
tion of that form of the engine called the atmospheric engine,
in the first years of the last century, until the year 1763,
when Watt solved the problem to condense the steam toithotU
cooling the cylinder.
4. Like almost all discoveries of the first order in the
OK THE STEAM ENGINE. 25
»
arts, this seems astonisliinglj obvious now that we know it :
and one only wonders how it could remain for more than
half a century undiscovered, human invention moreover
being stimulated by the prospect of a reward which in the
case of "Watt proved to be a princely fortune.
5. The first expedient suggested in the progress of dis-
covery for the production of a vacuum in the cylinder, by
the condensation of the steam within it, was to cool the
cylinder itself by the application of cold water on its external
surface.
This process was slow, and consequently retarded inju-
riously the rate of action of the machine. Accident suggested
a much more prompt and effectual method.
It happened that a leak took place in the bottom of a
cylinder, at a point where a supply of cold water was placed;
the water, pressed by the atmosphere through the hole,
spirted up in a jet within the cylinder, and in an instant,
by its contact with the steam, condensed it, and produced a
sudden vacuum. The unusally rapid descent of the piston
attracted the attention of the Engineer, the cause was
investigated, and the method of cooling the cylinder on its
exterior surface was thenceforward abandoned. A cock or
valve was placed at the bottom of the cylinder, by which
cold water was injected when it was required to condense
the steam, and another was provided by which the water
and condensed steam were allowed to escape. In this
manner the engine continued to be worked imtil the appli-
cation of the invention which, with so many others, has
conferred immortality on the name of "Watt.
6. Although the condensation by jet has the advantage,
as we have stated, of being prompt, yet the cylinder was
still cooled, and the waste of fiiel attendant upon reheating
it still took place. It is true that a jet of water would in
the first instance condense the steam within the cylinder
without materially lowering the temperature of the cylinder
itself; but the effect would be tlcvsA. WieV^tiJc ol^^ q;f;\sJ^^^.»
26 BUDIMENTABY TREATISE
acting on the water contained within it, would immediatelj
reconvert a portion of such water into steam, and destroj
the vacuum before it could take effect upon the piston. It
was therefore necessary to throw in by the jet as much cold
water as was sufficient not merely to condense the steam,
but also to cool the cylinder down to the temperature of, at
most, 100° ; and even at this temperature a portion of the
vapour was still uncondensed, which impeded injuriously the
action of the machine.
7. The invention of Watt not only had the effect of pro-
ducing an almost perfect vacuum, but it did so without in
the slightest degree lowering the temperature of the cylin-
der. The idea occurred to Watt of placing near the cylinder
another vessel, submerged in cold water, and having a jet of
cold water constantly playing within it. Whenever it was
desired to condense the steam in the cylinder, he opened a
communication by a cock or a valve between this vessel and
the cylinder, and immediately the steam, by its elastic force,
rushed into this vessel and was instantly condensed, leaving
in the cylinder an almost perfect vacuum, and at the same
time exposing the cylinder to no cold which could in the
slightest degree lower its temperature.
8. The vessel here described, immersed in a cistern of
cold water, and having a jet playing in it, was called a
condenser. By the continuance of the process just described,
such vessel would, after a time, not only be filled with
water supplied from the jet and the condensed steam pro-
ceeding from the cylinder, but it would also contain more or
less air which would enter in a fixed form in the water, and
which would be liberated by the warmth of the steam con-
densed by the water. This air would vitiate to some extent
the vacuum in the condenser, into which it would pass in
virtue of its elasticity. These impediments were surmounted
by the adjunction of a pump to the condenser, by which the
water supplied by the jet and the condensed steam, as well
as the air just adverted to, were constantly pumped out.
OK THE STEAM ENGINE. 27
9. This is called tlie air j^vmp.
10. The water surrounding the condenser, unless it were
changed, would in time become warm, and fail to effect the
condensation. This is remedied by the application of a
pump and waste pipe to the cold cistern in which the con-
denser is submerged. The pump continually supplied cold
water, which, by its comparative weight, had a tendency to
sink to the bottom ; and the waste pipe, placed near the
surface, let escape the warm water, which, by its comparative
lightness, ascended: thus, with these arrangements, the
method of separate condensation became complete.
11. The effect of this invention, with a few others which
wiU be described hereafter, was to save about 75 per cent,
of the fuel consumed by the steam engines as previously
worked. Watt and his partner Boulton were content to
receive, as their reward for this gift to the arts, one-third of
the saving which they effected ; and this one-third proved
to be sufficient to enable each of these illustrious men to
leave to their descendants magnificent fortunes.
CHAP. IX. — HOW THE MECHANICAL ACTION OF STEAM MAY BE
AUGMENTED BY HEAT IMPARTEJ) TO IT DIRECTLY.
1. In all the ordinary applications of steam, the heat
imparted is applied to water from which the steam used for
mechanical purposes is raised. Heat, however may be
imparted directly to the steam itself after it has been
separated from the water, and when so applied it will
augment in a certain proportion the mechanical efficacy of
the steam.
It has been thought by some projectors that heat applied
in this way might be rendered more efficacious than when
applied in the evaporation of steam from water. It may,
therefore, be worth while to explain here to what extent
the mechanical power of steam can be augmented in this
way.
28 BUDIMEKTABY TBEATISE
2. It is a remarkable fact, that the effect of heat applied
to air and all species of gases in augmenting their volume is
precisely the same. It is found that if air or any species of
gas be confined within a certain volume, and that heat be
applied to it until its temperature be raised one degree, its
elastic force wiU be augmented by one 480th part of its
whole amount. Thus if a certain surfiwje of the vessel which
contains it suffer a pressure from its elastic force of 480 Ib^
the same surface will suffer a pressure of 481 lb. from the
temperature of the air or gas being raised one degree.
3. Now it is stiU more remarkable, that the very same
law applies to every species of vapour, that of water included.
If then a cylinder containing steam excluded from contact
with water be exposed to any source of heat, it will receive
the above augmentation of pressure for every degree by
which its temperature is elevated. This increase amounts^
in round numbers, to one-fifth per cent, of the whole
mechanical effect.
4. It is scarcely necessary to say, without going into
details for which our limits would not afford us space, that
the same quantity of fuel which would produce this increase
of mechanical effect, applied directly to a vessel containing
steam, would produce a greater mechanical effect, applied to
a boiler to produce steam from water.
It is therefore not necessary to dwell further on this
principle, as invention has not yet profitably employed it in
the case of steam.
CHAP. X. — ^HOW A PISTON 18 MADE TO MOVE ALTERNATELY
FROM END TO END OP A CYLINDER WITH A DEFINITE
MECHANICAL FORCE.
1. It is evident that if steam can be admitted on one side
of the piston, and withdrawn on the other, the piston will
move in obedience to the pressure on the side at which it is
admitted.
OlS THE STEAM ENGINE. 29
2. K, when the piston arrives in this manner at the end
of the cylinder, the steam which has impelled it be withdrawn,
and at the same time steam be admitted on the other side,
the piston will move back again from exactly the same cause.
Thus to Iproduce the alternate motion of the piston it is
only necessary to provide means for the alternate admission
and escape of the steam at each end of the cylinder.
3. This supposes two apertures of some kind at each end,
one for the admission and the other for the escape of the
steam : it supposes also one of these apertures to communi-
cate with the boiler, where the steam is generated, and the
other to communicate with the condenser, where the steam
is destroyed.
4. It supposes, moreover, some means of alternately
stopping and opening each of these apertures.
The means whereby this is effected are very numerous.
5. It may be done by stoppers which fit steam-tight into
holes, from which they are lifted or drawn, and to which
they are returned alternately, just as the stopper of a
decanter would be, only that they are made more conical, in
order that they may be more suddenly opened and closed.
These are usually made of brass or gun-metal, and may be
ground so as to fit with great precision.
These contrivances are caXledpti^et valves. Those which
open a communication with the boiler are called steam valves,
and those which open a communication with the condenser
are called exhausting valves,
6. Now supposing that we are provided with such con-
trivances, and are supplied with the proper mechanism for
opening and closing them, nothing can be more simple than
to work the engine.
7. Although it is not necessary that the cylinder be placed
in a vertical position, and very often it is not so, yet, for the
€onvenience of explanation, we shall here suppose it in that
-position, and we shall distinguish the two steam valves as
the upper and loioer, and the same with the two exis>»»s^sx£s%
30 EUDIMEXTAKT TEEJLTISE
valves. Let us then suppose the piston to begin its motion
at the top of the cylinder, and let the cylinder under it be
imagined to be filled with steam, all the valves being dosed.
Let the upper steam valve and the lower exhausting valve
be simultaneously opened. Steam will flow through the
upper steam valve above the piston, and the steam below the
piston will flow through the lower exhausting valve into the
condenser, where it will bo destroyed. We shall have a
vacuum under the piston, and the pressure of steam above
it. The piston will therefore descend to the bottom of the
cylinder.
8. When it arrives there, let the two valves, which have
just been supposed to be opened, be closed. The top of the
cylinder will now be shut off from the boiler, and the bottom
from the condenser. At the same time let the lower steam
valve and the upper exhausting valve be opened. The steam
which filled the cylinder above the piston wiU immediately
rush to the condenser through the open exhausting valve,
where it wiU be destroyed, and steam from the boiler will
pass below the piston through the lower steam valve. Steam
pressure wlQ therefore act below the piston while there is a
vacuum above it, and the piston will ascend until it reaches
the top of the cylinder. The constant repetition of the same
process of opening and closing the valves in pairs would ob-
viously in this manner contiuue the alternate action of the
piston from end to end of the cylinder.
9. In the earlier steam engines this process of opening
and closing the valves was executed by the hand of an
attendant, and, like all constant mechanical action which
depends on the human will, was done irregularly. It soon
became apparent that the piston itself could be made to
j^jiecute this with the most perfect certainty, regularity, and
priUsion, Tradition says that an uneducated child, named
Htiinphrey Potter, was the inventor of this improvement, by
"which the steam engine first became a self-acting and self-
regulatiQg machine.
OK THE STEAM ENGIl^E. 81
10. From what has been above explained, it will be evi-
dent, that although there are four independent valves, there
is in reality only a single motion, and that all the four may
be easily managed to be connected so that the motion to be
imparted to them may be effected by a single impulse pro-
ceeding from any convenient part of the machinery.
11. When the piston arrives at the top of the cylinder, two
valves, — ^the upper steam valve and lower exhausting valve,
— are required to be opened ; and at the same moment the
two other valves, — the lower steam valve and upper exhaust-
ing valve, — must be closed. Now, as all these movements
are simultaneous, it may be easily^ imagined that the four
valves may be so connected that a single movement im-
parted to them should open one pair and close the other pair.
12. When the piston arrives at the bottom of the cylinder,
a single motion in the contrary direction will evidently effect
the object to be attained, that is to say, to open the lower
steam valve and upper exhausting valve, and close the upper
steam valve and lower exhausting valve.
13. These communications between the ends of the cy-
linder and the boiler on the one hand, and the condenser on
the other, are often governed by means even more simple
than the puppet valves we have just described.
14. The two openings at each end of the cylinder are
sometimes made in flat surfaces, over which two sliding
shutters are moved, these two sliding shutters being con-
nected by a rod or other solid connexion, extending from
end to end of the cylinder. By moving this rod upwards
or downwards, the position of the shutters being properly
adjusted, the openings for the admission or escape of the
steam are covered and uncovered by pairs in the manner
necessary to produce the effect we have described.
15. These contrivances are called slides,
16. If the steam be used expansively, by shutting it off
jbefore the complefion of the stroke, the times of opening
aaoL^ shutting the several apertures will not be the «ax£L^.
82 BUDIMENTABT THEATISE
17. The opening by which the steam is admitted will in
that case be closed at the moment when the piston has
completed a certain part of the stroke and the valve for the
admission of steam at the other end must not be opened till
the end of the stroke.
18. "When a cylinder is so worked, there will then be
three epochs in each stroke at which the valves must be
acted upon, — at the commencement when the steam is first
admitted to impel the piston, at some intermediate point
when its influx is stopped, and at the extremity when it is
let in on the other side. If puppet valves be used, such as
we have first described, each moving independently of the
other, it is easy to conceive how these effects may be pro-
duced : but even with slides they are also managed by so
adjusting the slide to the opening, that by two successive
motions, made at different points of the stroke, the effect is
produced. At the commencement, the slide being advanced
through a certain space, the steam is admitted on the one
side of the piston and withdrawn fi'om the other; at an
intermediate point, the slide being further advanced, the
influx of steam is shut off", but the efflux on the other side
still permitted ; at the termination of the stroke, another
movement of the slide admits the influx on the other side,
and the efflux on the opposite side.
19. There is another class of contrivances for governing
the admission and the emission of steam, which are called
coclcs. These are similar in their mechanical construction to
the common water-cock. A solid metallic cone with the
point cut off*, is capable of revolving in a hollow cone which
it fits steam-tight. This solid cone is pierced with two or
more passages, the openings of which, by tumiug the cock,
may be brought to coiucide with corresponding openings in
the hollow cone in which it revolves. In this way steam
may be admitted to or allowed to escape i&'om the cylinder,
in a manner exactly similar to that by which a liquid is
enabled to flow from a vessel by means of a common cock.
ON THE STEAH ENGO^E. 83
20. The application of tliis expedient evidently supposes
the practicability of bringing the openings for the influx and
efflux of steam communicating with the top and the bottom
of the cylinder to the same point ; but there is no difficulty
in this. It is only necessary to provide tubes or passages,
leading from the point where the cock is placed to the top
and the bottom of the cylinder, through which the steam
may pass to or ho,
21. A practical objection to this expedient is, that at each
stroke as much steam is lost as fills such passages, inasmuch
as such steam has no effect in working the piston. A
source of waste is therefore produced, expressed by the
proportion which the contents of these passages bear to the
magnitude of the cylinder.
22. For this reason, among others, cocks or valves placed
in this manner, at distances more or less considerable from
the ends of the cylinder, are in general used only in small
engines of short stroke. In the larger class of engines, with
very long stroke, valves are placed at each end, close to the
piston, and worked by independent mechanism.
23. The action of the puppet valve, or spindle valve, as it
is sometimes called, has in practice some advantages over that
of slides or cocks ; it is more prompt in opening and closing,
and it is much less likely to leak in consequence of wear ; it
is also obviously subject to less friction.
24. As I have already stated, these valves are conical, and
rest in a conical seat, being ground so truly as to be steam-
tight. The angle of the cone is usually 45°. If it be less
conical, the valve is apt to get tightened in its seat ; if more
so, it is apt to leak.
25. When slides are used, some expedient is adopted to
enable thein to move against the surface with which they
are in contact so as to be steam-tight. This is either effected
by a packing of hemp soaked in tallow, or by the operation
of some metallic surface urged by springs, technically called
metallic packing.
81 BUDIMENTABT TBEATISE
26. The efficiency of the operation of the piston greatly
depends on its being steam-tight in the cylinder. The least
leakage from the one side to the other would cause the
steam to escape to the vacuum side. It is true that, arriying
there, it would immediately rush to the condenser so that
it might not sensibly impede the action of the piston, but it
would still be a source of waste of power.
27. Pistons are rendered steam-tight either by vegetable
or metallic packing.
28. A common hemp-packed piston consists of two
circular metallic plates, placed one above the other, and
connected together by screws : in the space between these
two plates, round the edge, is left a cavity which is filled with
unspun hemp or soft rope, called gasket^ which, being wound
round the piston, is compressed into an uniform and compact
mass by screwing the top and bottom of the piston together.
29. This packing is pressed afterwards so as to be forced
against the surface of the cylinder ; it is lubricated with
melted tallow, let down on the piston from afrmnel inserted
in the top of the cylinder, and governed by a stop-cock, so
as to prevent the escape of the steam.
80. In the most improved modem engines, however,
metallic packing is generally used. Between the two plates
forming the top and bottom of the piston are placed a
number of metallic rings, one above the other, so as to fill
the space between the two plates, and having their diameters
a little less than that of the cylinder : these rings ure usually
cut into three or four segments, the points at which each
ring is cut not corresponding with those at which the rings
above and below are cut. "Within these segments are placed
springs, which, acting from the centre of the piston, urge
the segments against the surface of the cylinder. The
construction of these and the form of the cylittder itself
have been brought to such a degree of precision, that these
pistons act with complete efficacy; and use, instead of
injuring, improves them.
ON THE STEAM ENGINE. 85
31. In all the preceding explanations it has been supposed
that the steam is admitted at either end of the cylinder at
the moment that the piston has arrived there, and is about
to commence its action in the opposite direction. In
practice, however, it is convenient to admit the steam a
little before the moment when the piston reaches the extre-
mity of the cylinder : this is attended with the advantage of
its assisting to break the shock which would attend the
sudden change in the direction of the motion of the mass of
matter composing the piston and rod, and the other parts
of the machinery which partake of their alternate motion.
The steam admitted just before the motion of the piston is
reversed acts as a sort of cushion to receive the piston.
32. These and other matters of practical detail in the
operation of the engine render the time of opening the valves
a very important matter, and machinery is accordingly pro-
vided for regulating the moment of their opening with the
greatest certainty and precision.
CHAP. XI. — HOW THE ALTERNATE MOTION OF THE riSTON-ROD IS
CONVEYED TO THE WORKING BEAM.
1. "With few exceptions, the power exercised by the piston
in a steam engine is in the first instance imparted to a beam
called the worTdng beam, which is supported on a fixed axis,
and which vibrates alternately upwards and downwards.
K'ow it may at first view appear that, we might at once
impart the motion of the piston to the beam by attaching its
extremity to that of the beam by a common joint and pin, but
the slightest reflection will show that such an arrangement
would be incompatible with what has been already stated.
2. It wiU be remembered that the piston-rod is a thick
rod of iron, accurately formed and polished, that it is firmly
attached to the centre of the piston, and that the construc-
tion and operation of the cylinder and piston require that
the rod should accurately move in a strai^lit Ikaa ^s<$^^i2c^
36 BUDIMENTABT TBEATISE
and downwards. Now the end of the beam, which vibrates
alternately on a horizontal axis, will move alternately
upwards and downwards, but not in a straight line. It will
move alternately in the arc of a circle, the centre of which
will be that of the axis on which the beam vibrates. If then
we attempt to connect immediately the end of the piston
with the end of the beam, the consequence will be that the
end of the piston, following the motion of the end of the
beam, will be moved alternately upwards and downwards in
a circular arc, and consequently would be strained or bent
and its action in the cylinder disturbed.
3. There are several ways of surmounting this difiB.ciilty,
all of which consist in interposing between the end of the
piston-rod and the end of the beam some piece of mechanism
which will allow the rectilinear motion of one and the alter-
nate circular motion of the other.
4. The most simple expedient of this kind consists of a
rod of metal, working at one end by a pivot on the beam,
and at the other by a pivot on the end of the piston-rod. In
this case, however, there would still be a liability to straining
the piston-rod from its rectilinear motion, were it not regu-
lated by some species of guide. A common method of
effecting this is to attach to the top of the piston-rod a cross-
piece, so as to make with it a form like the letter T. The
ends of this cross-piece are made to move on fixed upright
rods, so that these last may resist any tendency to strain the
piston. The joint or joints connecting the piston with the end
of the beam may be attached to the ends of the cross-piece.
5. It is not indispensably necessary that abeam should be
employed at all, and in some engines of small magnitude
and compact form it is omitted. A rod is brought from the
cross head of the piston directly to the object which the
engine is intended to drive.
6. In many cases, and especially in the large class of steam
engines used in England in manufactories, the piston-rod is
connected with the beam by a contrivance called a paralle
OH" THE STEi.M ENGINE.
87
motion. This is a combination of rods, so arranged and
joined together, that while one of their pivots is moved
alternately in a circular arc, like the end of the beam, some
point upon them will be moved alternately upwards and
downwards in a straight line.
7. A great variety of combinations and proportions are
capable of effecting this with sufficient precision for all
mechanical purposes, but that which is best known as the
parallel motion, and which is due to the invention of the
celebrated Watt, is in principle as follows :
8. Let two equal rods c b and o d be attached by pivots
to two fixed points at c and o, on which they shall be at
liberty to play alternately upwards and downwards in the
circular arcs b' b" and d' d" : but let their play be limited
to small arcs. Now let a third rod b d be connected by
pivots with the ends of the two former.
Let a point m be marked at the middle of the rod b d.
Now if c B be made to vibrate on its centre, ci ^'etrMsiu^Ns^
88 BUDniEKTABT TBEATISX
the arc b' b'', which will cause at the same time o d to
vibrate alternately iu the arc b' d", it will be found that the
point M will ascend and descend in a line k' m", which will
not deviate sensibly from a straight line, in a veitieal
direction ; iu fact, if a pencil were attached to the point ic,
and a surface held behind it, such pencil, by the motion of
the rods, would trace a vertical line upon the surfisu^.
Now if we imagine c b to represent the beam of the engine,
and o D and b d rods connected with it in the manner
already described, o being attached to a fixed pivot, then the
point M, being attached to the top of the piston-rod, will
move with it freely upwards and downwards in a true vertical
lino, and will, through the combination of rods just described,
iin))art motion to the end of the beam b.
9. To demonstrate strictly this would require the applica-
tion of mathematical principles not compatible vrith our
present object ; nor indeed is it strictly true, in a geomet-
rical sense, that the motion of the point m takes place in a
straight line : its deviation, however, from a vertical line,
within the limits of the play given to the beam and piston, is
so extremely small as to have no practical effect whatever.
The general effect of the combination here described may
be underHtood thus. When the point b is moved upwards
to b', the upper extremity of the rod b n is drawn a little to
the right, and at the same time the lower extremity n, being
inov(5d to 1)', is drawn a little to the left. "When the ex-
tremity B descends to b", the extremity d descends to n",
and the ends are again drawn, the one a little to the right,
and the other a little to the left. It will be easily under-
stood that in this case, while the ends of the rod b d are
thus alternately made to move right and left, there vdll be
on intermediate point of it which will neither deviate on the
one side nor on the other. The upper half of the rod, in
fact, is continually inclined towards the right, and the lower
half towards the left, the middle point being affected by
neither motion, and therefore being moved vertically up-
ON THE STEAM ENGimS* 39
wards and downwards in a direct straight line. This is the
principle of the parallel motion.
10. In its practical application it appears somewhat more
complicated, for in order to acconmiodate the arrangements
of the beam and piston-rod, a great number of rods and
joints are necessary to be used ; but these are mere matters
of mechanical convenience, and have no effect upon the
principle of the arrangement.
It is therefore now apparent that the alternate motion of
the piston-rod upwards and downwards in a straight line
imparts a corresponding alternate motion to the end of the
working-beam in a circular arch.
11. Although we have, as usual, here described the
arrangements as if the cylinder were vertical and the beam
placed over the piston-rod, this position is neither necessary
nor is it invariably adopted. Sometimes the beam is placed
below the cylinder, and the rods of the parallel motion or
connexions, with the cross head and guides, are made of
sufficient length to extend down to the beam. Sometimes
the cylinder is horizontal and the beam vertical, and cases
even occur in which it is found convenient to place the
cylinder in an inclined position ; but all these are matters
of arrangement to be determined by the circumstances in
which the engine is applied, and have nothing whatever
to do with its mechanical principle.
CHAP. XII. — HOW THE ALTERNATE MOTION OF THE WORKING
BEAM PRODUCES A MOTION OF CONTINUED ROTATION.
1. Of all sorts of motion, that which is most firequently
required in the arts, is one of continued rotation. Mills in
factories of every kind are impelled by machinery which
receives its motion from a wheel kept in constant rotation.
Ships impelled by steam engines over the deep are driven
by paddle-wheels or screws, to which constant rotation must
be imparted. Carriages on railways are propelled by com?-
40
BUDIMENTABT TREATISE
pelling one or more of their wheels to revolve contiiiuallj by
the application of adequate power to it. This is so evident,
that one of the first and most important problems the steam
engineer has to solve, is how to "make the alternate motion
of the piston-rod produce the continued rotation of a wheeL
2. The contriv-
ance by which this
is effected almost
universally is call-
ed a cormecting^od
and crank.
The crank is
an arm sometimes
attached to the
centre of the wheel
to which revolution
is desired to be
imparted, and the
wheel is made to
revolve by it by
the same mode of
action as that by
which a winch
turns a windlass.
Thus, if K be the
centre to which
motion is to be
imparted, k i is an
arm or lever fixed
upon such centre.
A pin, called the
N) \ crank-pin, is at-
'•x/.fjl/:" tached to this at i,
■ ' ' which forms the
joint by which the connecting-rod is united with thp crank.
I H is a strong iron rod, extending from the crank-pin to the
• • •
. • ^" J:
q c — -3 .••
Oir THE STEAM ENGINE. 4L
end of tlie working beam, with which it is connected by a
similar pin. The weight of this connecting-rod is so adjusted
that it exactly balances the weight of the piston and its rod
attached to the other end of the beam. In the figure the
crank is represented by dotted lines in the different positions
which it assumes as it revolves. As the end of the beam is
moved upwards and downwards, the crank will be turned
round the centre k, and a motion of continued rotation will
be produced, which will be communicated to any wheel
fastened upon the axle k.
3. To make the action of the piston upon the crank
perfectly clear, let it be supposed that the piston is in its
descei^mg stroke. The force of the steam upon it is im-
parteoBy the rod and the intermediate mechanism to the
end of the beam which is drawn down. At the same time
the other end of the beam, with the connecting-rod, is drawn
up. The crank is thus made to ascend from its lowest to
its highest position, to which it arrives when the piston has
reached the bottom of the cylinder. When the piston
ascends, the force of the steam is in like manner transmitted
to the beam by the piston-rod, which is made to ascend, and
the opposite extremity, with the connecting-rod, descends,
by which the crank is driven down to its lowest position on
the side opposite to that on which it ascended, and thus a
motion of continued rotation is produced.
4. But in this action there are particulars necessary to be
noticed. There are two positions which the crank assumes,
in each revolution, at which the force of the piston can have
no effect in continuing its motion : these positions are those
which the crank assumes when the piston is at the top and
at the bottom of the cylinder, the points at which it changes
the direction of its motion. When the piston is at the
bottom of the cylinder, the crank-pin is immediately above
the axis to which the crank is attached : in this position the
force of the piston would have no other effect 'than to press
the crank perpendicularly upon the axle, and evidently ^^>aL^
42 BUDIMENTABT TREATISE
haTO no effect whatever in making it revolve. If we were to
Huppose, then, the entire machinery at rest in this position,
the steam acting on it could not put it into motion.
5. Again, if we suppose the piston to be at the top of the
cylinder, the crank-pin will then be at its lowest point, and
will be directly under the axle : the effect of the steam acting
above the piston would then be to press the crank-pin up-
wards against the axle, but it could have no influence in
turning it. If, therefore, the machinery were at rest in this
position, it could not be put in motion by the steam.
In any intermediate position, however, the connecting-rod
woidd act on the crank with a leverage more or less effective,
and would move it. ^
6. The two points which we have here described, Vwhich
the crank-pin assumes its highest and lowest position, are
usually called the dead points.
Now it may be asked why the engine does not cease to
move every time the crank-pin arrives at these dead points,
seeing that there the moving power, however energetic, can
liavo no effect on it.
7. The answer is, that the machinery is extricated from
this mechanical dilemma in virtue of the common property
of matter called inertia, by reason of which, when it has
acquired any definite motion in any certain direction, it
will not suddenly stop, even though it be impelled by no
external force, but will continue to move until the
momentum it had acquired be exhausted by friction and
other resistance.
8. Since, then, the motive power continues to exercise
more or less force up to the dead points, the machinery,
arriving at them, has some definite motion, and the momen-
tum consequent upon that motion carries the crank out of
the critical position we have referred to.
9. But, independently of the dead points, there are other
circumstances attending the action of the connecting-rod on
the crank which are necessary to be explained. By the
ON THE STEAM ENGINE. 43
intervention of the beam, the force of the piston is trans-
mitted to the crank-pin in the direction of the connecting,
rod. Now by observing the diagram above given, showing
the successive positions of the connecting-rod and crank,
it will be seen that twice in each revolution the connecting-
rod is at right-angles with the crank, but that in other
positions it is more or less oblique to it ; the extremes of the
obliquity terminating alternately in the dead points, in one
of which the connecting-rod and crank are brought into a
continued straight line, and in the other the crank is as it
were doubled on the connecting-rod.
10. "Without resorting to the language of technical
geometry, it will be apparent that the action of the connect-
ing-rod on the crank is most energetic when they are at
right angles ; and that according as they become more and
more oblique, and approach the dead points, the action
becomes less and less effective. It diminishes rapidly in
approaching these points, and is altogether extinguished on
arriving at them. It appears then that the action of the
connecting-rod on the crank is subject to a regular variation
in each semi-revolution : a maximum when they are at right
angles, it diminishes, and at length vanishes when it arrives
at the highest point ; then, in descending, it re-appears,
augments, and is a maximum at the point where they are at
right angles ; then it again diminishes gradually, and ulti-
mately vanishes at the lowest point, having passed which, it
again re-appears, augments, and is a maximum when it
assumes its rectangular attitude.
11. Now although the inertia of that portion of the
machinery which is once put in revolution be sufl5.cient to
prevent the motion from ceasing, and the engine coming to
a dead lock when the crank-pin comes to the dead points,
yet it is not generally sufficient to prevent a very great
inequality of motion from arising from the cause which wo
have here explained. An expedient accordingly has been
resorted to, which perfectly counteracts this incon?remgxj^s^.»
iA BUDIMENTiBT TBEATISE
and equalises the motion. This expedient is the flj-wheel,
which we have abready described.
12. The fly-wheel is placed on the same axle k as the
crank, and it is made to revolTe simidtaneouslj with the
crank. This wheel is so nicely balanced on its centre, and
moves with so little friction, that it absorbs a very incon-
siderable portion of the moving power. It is usuallj made
of very large diameter, and its ring or circumference is com-
posed of a very ponderous mass of metal. All this metal is
put in motion by the moving power, and, from its great
mass, has a considerable momentum even when the velocity
is moderate. When the crank is at the dead points, this
mass, by its momentum, continues the revolution, and
carries the crank into a new attitude, where the moving
power exercises an influence on it. "When the crank and
connecting-rod are in such position in which their action
is most energetic, the fly-wheel absorbs a part of the
'moving power. As the crank approaches the position in
which the action of the moving power upon it becomes
enfeebled, the fly-wheel gives back to the machinery such
surplus power as it received when the action of the crank
was most energetic.
13. Between the fly-wheel and the engine there is, there-
fore, a continual reciprocity of action and interchange of
power, which in practice completely equalises the velocity ;
and there is in fact no perceptible difference between the
speed of the movement at the dead points, where the moving
power loses its influence, and at the middle of the stroke,
where its action is most effective.
14. To minds not very familiar with mechanical con-
siderations, it may seem extraordinary that the intense
action of the moving power upon the fly-wheel at the middle
of the stroke should not at these points produce a perceptible
acceleration in its motion, and a corresponding irregularity,
therefore, in the motion of the machinery which it drives ;
but it must be considered that the excessive mechanical
ON THE STEAM ENGrfiTB. 45
force exerted at the middle of the stroke is imparted to a
great mass of metal coUected in the rim of a very large
wheel. "Now the velocity which a given force produces is
diminished in the direct proportion of the mass of matter to
which it is imparted : thus a force which would give a certain
speed to a ton of metal would give only a tenth part of such
speed to 10 tons. The weight collected in the rim of the
fly-wheel is so great that the excess of power of the engine
at the middle of the stroke, when imparted to it, produces
an inconsiderable increase of speed. But this increase of
speed, inconsiderable as it is, is produced on the circum-
ference of a very large circle, and the mass of matter thus
moved must be carried through a very considerable space in
making even a single revolution. Thus, what between the
great mass of metal coUected in the rim of the fly-wheel
and the great diameter of the fly-wheel itself, the unequal
action of the crank is rendered absolutely imperceptible.
15. In elementary works on the steam engine, sometimes
proceeding from persons who, however respectable, their
practical attainments, are deflcient in mathematical know-
ledge, the crank is often represented as an imperfect con-
trivance, and an extensive source of waste of power, owing
to unequal action.
Nothing can be more fallacious than the reasoning of such
writers. It can be demonstrated by the most strict geome-
trical reasoning, and the result is verified by experience,
that in the action of the crank and fly-wheel there is no
other loss of power than such as is incidental to the common
and well understood causes of friction and atmospheric
resistance.
16. Owing to such fallacious notions, much valuable
inventive power has been wasted in attempts after the
contrivance of what are called rotatory steam endues*
A rotatory steam engine is one by means of which a
movement of continued rotation may be immediately given
to a piston, or in other words, by which the power o£ iVw^
46 BUDI2i£NTAJ2T TEEATISB
steam can be immediatelj applied to a reyolying wheel
without the interposition of a piston, cylinder, beam, and
crank. If such an application could be contriyed without
the various countervailing losses of power which have hitherto
invariably attended such projects, it would certainly have
some advantages ; but it is not easy to see how such an
object can be attained, and at all events, notwithstanding
the expenditure of a vast amount of ingenuity and capital, it
has never yet been effected.
17. Cases occur in the arts, in which a fly-wheel cannot
conveniently be attached to the steam engine, and yet where
uniformity of action is necessary. In such cases the object
is usually attained by using two cylinders, which drive two
cranks constructed on the same axle, but having sueh
positions that when either is at its dead point, the other is
at its point of maximum efficiency. Thus, while the efficiency
of one crank increases, the other diminishes, and vice versd^
and the sum of their actions at all times is nearly the same.
CHAP. XIII. — HOW THE STEAM ENGINE IS RBNDBRBD
A SELF-ACTING MACHINE.
1. "We have already stated that this is accomplished hj
making the engine open and close, at the proper times, the
valves by which steam is admitted to and discharged j&x)m
the cylinder. In the earlier engines this was accomplished
by a har or rod attached to the end of the working beam,
and carried down parallel to the cylinder. On this bar were
attached pins, so placed that as it ascended and descended
they struck the handles or levers of the respective valves,
and opened or closed them, as the case might be. This
method is still used in some of the larger class of engines
applied to the pumping of water. Where slides or cocks
are used (as indeed is almost invariably the case), they are
generally moved by an apparatus attached to the crank shaft,
called an eccentric,
2. This consists of a circular plate of metal b d, which is
OK THE STEAM ENGINE.
47
fixed upon a point a at some distance from the geometrical
centre. Eound this eccentric point it is made to revolve,
and in revolving it is evident that its geometrical centre,
revolving round its centre of motion, will be thrown alter-
nately to the right and to the left of such centre.
3. Now let us suppose this circular plate to be surrounded
by a ring, within which it is capable of turning, but so that
the ring shall not turn with it.
Then such ring will be thrown alternately to the left and
to the right of the centre on which the eccentric plate is
made to turn, and the length of its play, right and left, will
be equal to twice the distance of the geometrical centre of
such circular plate from the centre on which it turns. In
the figure annexed, a is the centre on which the circular
plate revolves ; c is its geometrical centre ; e i is the ring
which embraces it, and within which it can turn. To this
ring is attached a grated bar l m h. As the centre o is
thrown alternately right and left of a by the revolution of
the plate, the point M receives a horizontal motion, right
and left, to a like extent. This motion is transmitted by
means of levers to the slides or cocks of the engine by obvious
and well-known mechanical contrivances.
48 BUDIHEITTABY TREATISE
CHAP. XIV. — HOW THE MECHANICAL EFFECT EZEBTED BT
THE PISTON IS ASCERTAINED,
1. Whatever be the circumstances under vrhich the engine
is worked, it will never happen that throughout the entire
length of the stroke the pressure of steam on the piston will
be exactly the same. Still less will it happen that the
vacuum towards which the piston moves will be uniformlj
perfect.
The moment the exhausting valve is opened, the steam
begins to rush from the cylinder to the condenser, but its
condensation is not instantaneous. The first portion which
mingles with the jet produces warm water, from whence
steam is reproduced, and it is not until so much cold water
has been mixed with the steam as will reduce its temperature
considerably below 100°, that the vacuum in the cylinder
will become practically perfect.
2. The more speedily this effect is produced, the more
efficient will be the operation of the machine, but it never is
produced until the piston has already made some portion of
the stroke. The piston therefore begins to move against a
vapour which offers some resistance more or less considerable,
and the impelling power of the steam at the other side is to
such extent neutralised. This resistance gradually dimin-
ishes, and when the piston has made a certain portion of the
stroke, it will have been reduced to its minimum amount.
It is evident then that this resistance must be ascertained
and calculated before we can determine the mechanical effi-
ciency of the piston.
3. But this is not all ; the steam which impels the piston
never acts throughout the stroke with uniform effect. "When
it acts expansively, being cut off at some determinate point
of the stroke, we have already seen that it acts with an
uniformly diminished pressure ; but even where the expan-^
sive principle is not used, the steam is still cut off a little
before the completion of the stroke.
ON THE STEAM ENGINE. 49
4. There is still another point to be attended to. "We are
able by easy means to ascertain the pressure of steam in the
boiler, but it would be a great mistake to assume that this
must be the pressure of the steam in the cylinder. In
passing from the boiler to the cylinder, the steam has to force
its way through various passages, some of which are very con-
tracted, and in so doing it suffers an effect which engineers
express technically by the term mre-dravm. In fact, the
steam loses somwhat of its density before it reaches the
cylinder. If then we would know the real mechanical pressure
on the piston, we must measure directly the pressure of the
steam in the cylinder, and not derive our knowledge from
its pressure in the boiler.
5. If we can at each successive point of the stroke ascer«
tain the exact pressure of the steam which impels the piston,
and also the pressure of the uncondensed vapour which
resists it, we have only to subtract the one from the other to
obtain the efficient pressure on the piston at the moment ;
and if we can do this successively throughout the entire
stroke, we shall obtain the total mechanical efficiency of
the engine.
6. A beautiful little instrument was, among the numerous
results of his fertile genius, invented by Watt for this pur-
pose, called an indicator, (See Chapter xxvn., title * Wattes
Indicator J It consists of a brass cylinder, something less
than 2 inches in its internal diameter, and from 8 to 12
inches in length. It is bored with extreme accuracy, and
a solid piston moves steam-tight in it with very little
friction.
7. This cylinder is open at the top, and the piston-rod is
kept precisely in its axis by passing through a ring placed
near the top. A spiral spring surrounds the rod of the
cylinder, and is attached at one end to the ring throuigh
which the rod plays, and at the other end to the piston.
When no force acts on the piston, and this spring is there-
fore neither extended nor compressed, the ^iato\^%^'ss^S^^
50 BUDIMEKTABT TBEATI8E
the centre of the length of the cylinder ; when any force
presses the piston upwards, the spring is compressed, and
the piston rises ; and when any force presses the piston
downwards, the spring is extended, and the piston descends.
From the known mechanical qualities 'of a spring of this
species, it follows that the space through which the piston
rises or falls always indicates the force by which it is urged*
At the top of the piston-rod, and at a right angle with it^
is attached a pencil, which plays upon a card properly placed^
and traces upon it a line according to the ascent or descent
of the piston.
AVhile the piston of the engine descends, the card is moved
horizontally against the pencil through a certain space ; and
while it ascends, it is moved back again through the same
space: by this combination of movements a geometrical
figure is traced upon the card, the breadth of which, measured
vertically, represents for each point of the stroke the effective
pressure, and the entire area of such figure represents the
total effect.
When the steam acts against the piston of the indicator^
the space through which that piston ascends represents the
excess of the pressure of the steam above that of the atrno*
sphere ; and when it descends by reason of the vacuum, the
space through which it descends represents the excess of the
pressure of the atmosphere above the pressure of the uncon-
densed vapour : consequently the sum of these two spaces
will represent the excess of the pressure of the steam which
impels the piston of the engine above the pressure of the
imcondensed vapour which resists it ; and this being taken
for each successive point of the stroke, it follows that
the entire area of the figure will represent the effective
action of the piston of the engine. This will be more clearly
understood by referring to the figures, with their explana*
tions, in Chap. xxvn.
8. The chief value, however, of this contrivance consisted
more in its indication of the action of the condenser than as
Oy: THE STJEAM EJ^GUTE. 51
affording a direct measure of the effective action of the
machine. It showed at once, and in a manner quite
unequivocal, whether the condenser was doing its duty, and
whether the condensation was sufficiently prompt. The
moment the exhausting valve is opened, the piston of the
indicator ought suddenly to drop ; and although it will sink
lower while the stroke proceeds, the chief motion should be
instantaneous. "When the condensation is not prompt, then
the piston falls more slowly, and shows either that there is
not enough water injected, or that some other impediment
interferes with the due performance of the condenser.
9. The best and perhaps the only practical method of
ascertaining the real efficient force with which a steam
engine acts, is to attach it to a water-pump, and measure the
quantity of water which it is capable of raising through a
given height : every other test but this is fallacious.
CHAP. XV. — HOW THE HEAT IS PRODUCED BY WHICH
STEAM IS MADE.
1. The cylinder, piston, beam, connecting-rod, crank, and
fly-wheel, are, like all other pieces of mechanism, a mere
contrivance by which mechanical force is transmitted and
modified. There is nothing in them by which mechanical
force can be produced. Once at rest, at rest they would for
ever remain, unless some motive power were applied to them.
2. This moving power, as we have already described, is
derived from the physical phenomena which are exhibited
when water is converted into steam ; but even the water, in
this case, cannot properly be regarded as any more th«n an
instrument by which the mechanical agency of the heat is
developed. Heat then is the prolific parent of the vast
powers of the steam engine, and it is of the utmost practical
importance to comprehend fiilly how this heat can be pro-
duced and applied with the greatest economy and efficiency
3. This will lead us to the consideration of those properties
of combustibles on which the production of b^^b'si;. ^^-^^^e^i^n
52 BUDIMENTAET TEEATI8B
and the construction of the furnaces and boilers by means of
which its application and transmission are effected.
4. The combustibles universally used in the furnaces of
steam engines are either pit-coal or wood. The former is
used almost invariably in Europe, the latter is used in
America, except in particular districts where coal is advan-
tageously attainable.
6. The constituents of coal are chiefly carbon end a gas
called hydrogen, combined occasionally with a small propor-
tion of sulphur and incombustible matter.
6. In the process of combustion, the carbon, the hydrogen,
and the sulphur combine with the oxygen gas, which is a
constituent of the atmosphere, and other products are
formed. In this combination a quantity of heafc is developed.
The incombustible constituents drop from the grate, and are
left in the ash-pit. The goodness of coal depends in some
degree on the small proportion of incombustible matter
which it contains.
7. The proportion of carbon contained in coal varies ; in
good coal it is seldom less than 75 per cent, of the whole,
sometimes considerably more.
8. Hydrogen cannot be said to enter as a constituent of
coal in its pure and simple form. It is always combined
with a portion of carbon, and is the gas called carhuretted
hydrogen^ being that which is commonly used for the
purposes of illumination. This gas may be expelled from
coal by exposing the latter to heat, by which means the gas,
expanding, is forced from the interstices of the coal, and
may, if required, be collected in proper reservoirs. This
process, applied to the coal, is called coking ; and it is in
this manner that the gas is collected in gas-works for the
purpose of illumination.
9. The proportion of carburetted hydrogen, the element
which produces flame, varies in different sorts of coal. The
more bituminous sorts, such as those of Northumberland
and Durham, generally have a considerable proportion ; the
OK THE STEAM EKGIITB. 63
heavy coal called stone-coal, obtained in some of the coal-
fields of "Wales, Pennsylvania, and elsewhere, have very
little. In all cases the proportion of this element by
weight is insignificant.
Carbon bums without flame, the product of the combus-
tion being the gas called carbonic acid, which escapes from
the fuel in a very heated state.
10. These are the general effects of combustion ; but for
the practical purposes of art, something more must be
learned. We must ascertain with some degree of precision
the quantitative proportions in which the various elements
concerned in the phenomena are present.
11. To begin, then, with the chief ingredient of all
combustibles, carbon, —
This substance, when heated to a temperature of 700° or
800°, equal to that of red-hot iron, will enter into chemical
combination with the gas called oxygen ; the result of this
combination will be another gas, called carbonic acid. In
forming this combination a large quantity of heat, previously
latent in the carbon and the oxygen, is rendered sensible,
and is developed in two ways : 1st, in rendering the remain-
der of the carbon incandescent, or white-hot ; and 2ndly, in
raising the temperature of the carbonic acid which has been
produced to a very high point.
12. Erom the luminous or incandescent carbon the heat
escapes by radiation, according to the same principles and
laws that govern the radiation of light. That portion of it
which is carried off by the carbonic acid may be taken from
such gas by placing in contact with it any surface which is a
good conductor of heat, such as metal : the heat of the gas
will be imparted to the metal until the temperatures of the
metal and the gas be equalised.
13. But it is necessary to know the quantity/ of oxygen
gas which is requisite to combine with the carbon.
It is found that a pound of pure carbon will enter into
combination with 12 cubic feet of oxygen at tK<^ opt^sksasssr^
51 BUDnrEyTABY TBEATI8E
temperature aiifl pressure of the air, the result of the com-
bination being 12 cubic feet of carbonic acid, tbis being
supposed to be reduced to the same temperature and pressure.
But ii2 the temperature of the carbonic acid, at the moment
of combination, is very much elevated, it wiU then have an
enlarged volume.
14. Conmion combustion, however, is maintained not by
an atmosphere of pure oxygen, but by that of the common
air.
15. Common air is a mixture of oxygen and azote, in the
proportion by measure of 1 to 4, — five cubic feet of common
atmospheric air containing but one cubic foot of oxygen. To
obtain 12 cubic feet of oxygen, therefore, we must necessarily
have 5 times 12, or 60 cubic feet of common air.
16. Supposing then (which is however in practice not the
case) all tlie oxygen contained in the atmospheric air suppHed
to the fuel in combustion to enter into combination with
such fuel, it would be necessary to supply 60 cubic feet of
atmospheric air for every pound of carbon consumed.
17. The result of this combination would be the pro-
duction of 12 cubic feet of carbonic acid, formed by the
combination of the oxygen of the atmosphere with the
carbon, and 48 cubic feet of azote, which would be mixed
with the carbonic acid so produced. This volume of mixed
gases would escape from the fuel at a very high temperature,
and would in this state pass into the chimney.
18. Hydrogen gas combines with 8 times its own weight
of oxygen, and the result of the combination is water, or,
more properly speaking, steam ; for it is rendered into the
vaporous form by the great heat developed in the combustion.
19. We have stated that a smaU proportion of sulphur
is present in most sorts of coal. In burning, this produces
sulphurous gas. It is inefficient as to its heating power,
and insignificant in its quantity, but most injurious in its
effects on boilers. Coal, therefore, having much of this
element, should be avoided in steam boilers.
OK THE STEAM ENGINE. 55
20. To maintain the fuel in combustion, it is then evident
that it must be continually supplied with atmospheric air.
The rate of this supply mil depend on the rapidity of the
combustion which is required, and the quantity and quality
of the fuel. The fuel is spread on a grate, between the bars
of which the air which sustains the combustion is admitted.
In passing through the fuel, the air enters into combination
with it, and the gases resulting from the combustion, in-
cluding uncombined oxygen and the azote of the atmospheric
air, which last plays no part whatever in the combustion,
issue together into the upper part of the furnace, all having
a very high temperatiu'e : these proceed to the chimney,
which they soon fill vrith a column of heated air, the buoyancy
of which makes it ascend into the atmosphere, and the vacuum
it leaves behind it draws a fresh portion of air through the
grate bars, and so the combustion is continued.
21. The azote which forms so large a constituent of
atmospheric air has qualities in relation to combustion
merely of a negative kind; it does not either check or
stimulate it. Thus, as a supporter of combustion, the
atmosphere may be considered as diluted oxygen, the azote
having the same effect on the particles of the oxygen as
water would have upon a strong spirit mixed with it.
22. In what has been just explained, the calculations are
based upon the supposition that every particle of oxygen
contained in the atmospheric air, urged through the burning
fuel, enters into combination with it. Now this is not
and cannot be the case, even in the most approximative
sense; and therefore, to complete the combustion of the
fuel, a much greater quantity than 60 cubic feet of atmos-
pheric air for a pound of carbon consumed must be drawn
through the fire. The exact quantity which is necessary is
not capable of calculation, for it depends on circumstances
which vary with the form and structure of the grate and the
mode of working the furnace : but it may be safely assumed
that not less than 150 cubic feet of atmospheric air are
56 BUDIHEyTABY TREATISE
necessary in ordinary furnaces for the combustion of each
pound of carbon contained in the fueL
23. It will be understood that when the fuel is laid in a
stratum more or less thick upon the gratd, and when, rapid
currents of air are ascending through its insterstices, a
quantity of the fuel, always existing in a state of powder o^*
small dust, will be carried upwards by the current, unbumed.
24. Besides this, as the heat expels the hydrogen gas
firom the interior of the coal, minute particles of the coal
itself escape with the current, and rise above the fiiel.
Much of this is also unbumed, or, to speak scientifically,
iincombined with oxygen. It is this minute powder or dust,
imcombined with oxygen, that forms what is called smoke.
The gaseous products of combustion, properly so called, have
not the cloudy and opaque appearance which characterises
smoke. The smoke then is imconsumed fuel, and to what-
ever extent it is produced, it escapes into the chimney, and
is a source of waste. It is clear, then, on the grounds of
economy, independently of sanitary considerations rielating
to the neighbourhood of the engine, that the quantity of
fuel, more or less, thus escaping should be arrested, and
burned before it reaches the chinmey.
25. Yarious methods have been adopted in furnaces for
accomplishing this object. Such arrangements are denomi-
nated smoke-consuming furnaces ; but very simple and
obvious arrangements may be adopted in the mode of feeding
common furnaces, which will have the effect of consuming
the smoke.
26. The following arrangement was adopted with complete
success at the establishment of the late Mr. Watt, at Soho,
Birmingham, and it has been found equally efficacious where-
ever the fire-men have been kept under sufficient discipline
to enforce its observance.
27. The grate must be constructed with a slight descent
backwards, to give facility to the removal of the fuel from the
front towards the back of the grate. Let us suppose a layer
OS THE STEAM EKGINE, 67
of coal of the proper depth spread over the entire surfaxje of
the grate, and brought into vivid combustion, so that every
part of it shall be incandescent. There will then be no
smoke.
The gases of combustion, mixed with the azote and uncom-
bined oxygen, of the atmospheric air, will alone issue from
the burning fuel. The doors of the furnace being now
opened, the fire-man, with a proper instrument, pushes back
a portion of the fuel from the front towards the back of the
grate, so as to make a clear space across the front of the
furnace. He then introduces a quantity of fresh fuel, which
he spreads in a layer of a proper thickness over the portion
of the grate which he has thus cleared, and closes the doors.
The heat immediately begins to expel the hydrogen from the
fuel thus introduced, and, technically speaking, cokes the
friel. "With the hydrogen escapes a quantity of dust and
minute portions of coal, forming smoke. This smoke and,
gas are carried by the draft to the back of the grate, where
the entrance of the flues is placed, and in passing through it
is carried over the remainder of the ftiel, which is in vivid
combustion.
28. The gas and smoke are thus burned, and this con-
tinues imtil the portion of ftiel in front of the grate has
been completely coked and reddened. GThe fire-man then
opens the doors, and repeats the process as before, shoving
this portion back, and introducing a fresh feed.
29. After this manner, without any special smoke-con-,
suming apparatus, the ftiel is completely burned, and no
smoke is ever seen issuing from the chimney,
30. To perform this, however, effectually, requires much
attention and activity on the part of the fire-man, frequent
feeding, and a careful distribution of ftiel on the grate.
31. In general it is difficult to enforce from such agents
the necessary attention. GThe ftiel in the grate is allowed to
bum down, and then the doors are opened and a large
quantity thrown in, heaped on every part of the grate from
58 BTTDIME^TABT TBEATI8E
the back to tlio front : wlien this takes place, a pTodigiom
volume of black smoke is suddenly evolved, which is seen
issuing from the chimney, and continues to issue from it
until the mass of fuel has been coked ; it then ceases, and
the combustion is free from smoke until a fresh feed is
introduced.
32. It must be admitted, however, that the process above
described, for the complete combustion of the fuel and
the prevention of smoke, is not without countervailing
disadvantages.
33. Instead of large feeds of fuel at distant intervals of
time, it supposes smaller and more frequent feeds ; instead
of the fiiel being quickly and carelessly thrown in, it is
carefully distributed upon the grate bars.
34. This supposes the frequent opening of the fiimace
doors, and the keeping them open for greater or less
intervals.
35. Cold air thus rushes in over the fuel, where it ought
never to be admitted, and has the tendency of robbing the
boiler of a portion of the heat which it ought to receive.
To remedy this, smoke-consuming furnaces have frequently
attached to them self-acting feeders. The ftiel, being broken
by proper machinery, is sprinkled on the grate by means of a
hopper, and the grate itself, after it has received its charge,
moves from under the hopper by contrivances provided for
that purpose. Eevolving grates have been sometimes
adopted vrith this view. Such contrivances, however, not
only introduce complexity into the machinery, necessitate
expense of construction, are liable to become deranged by
wear, but also require a portion of the moving power to
work them. These disadvantages are to be weighed against
those attending the operation of the simple furnace, properly
tended. I have, however, known these self-acting furnaces,
in places where fuel was expensive, in operation for years
with much advantage.
36. If the heated gases proceeding from the fuel passed
ON THE STEAM ENGINE. 59
directly to the chimney, they would carry with them a much
greater quantity of heat than would be necessary to maintain
the draft, and thus a portion of the heat developed by the
fuel would be lost. To prevent this, the heated air and
flames which escape from the fuel, instead of passing directly
to the chimney, are conducted through passages of greater
or less length in contact with the boiler, and made to impart
a portion of their heat to the water before they enter the
chimney. These passages are called flu^s, and are very
variously constructed, according to the form, magnitude, and
application of the boiler.
37. In some boilers the flues are made to wind round
them, the external part of the flues being made of brick-
work, which, being a bad conductor of heat, takes but little
from the heated air and flame.
38. The shape and proportions of hollers are so adapted as
to accommodate them to such systems of flues. The great
object is to adopt such arrangements as shall secure the
transmission to the water of all the heat developed in the
combustion of the fuel, except such portion of it as may be
necessary to maintain a sufficient draft in the chimney.
39. The boilers most commonly used are either cylindrical
or waggon-shaped. The cylindrical boilers are generally long
in proportion to their diameter, and their ends are often
spherical. This shape is highly conducive to strength, but
in some cases their ends are made flat.
40. The waggon-shaped boilers resemble, as their name
imports, an oblong waggon: the roof is semi-cylindrical;
the sides either flat or slightly concave, the convexity being
inwards ; the bottom is also slightly concave ; thie furnace is
placed at one end of the boiler, having a portion of the con-
cave bottom for its roof. The flame and heated air passing
from the grate are carried backwards through a flue which
extends the entire length of the boiler. Thus the radiant
heat of the lire, issuing directly from the grate, strikes on
the concave bottom of the boiler, which is immediately above
60 BUDIMEyTAET TBEATISE
the grate, and enters the water. The flame and heated air
pass through the flue under the boiler to the remote end,
and act upon the remainder of the bottom : having arriTed
at the remote end, they rise to a point a little above the
bottom, and then are conducted through a flue which winds
completely round the boiler ; and after circulating round it,
the heated air is conducted to the chimney. In this vray it
wiU be seen that the flame and heated air traverse the
length of the boiler three times, once at the bottom, and
once at each side.
41. In cylindrical boilers the furnace is generally placed
within the boiler, in a large tube which extends from end to
end of it. In one end of this tube is placed the grate, and
the remainder of it forms a flue. By this arrangement all
the heat which radiates from the fire, and even from the
ash-pit, acting upon this internal tube, is communicated to
the water. The heated air, traversing the tube to the remote
end, imparts its heat to the water by this means. Hues
circulate round the outside in the same manner as in the
waggon boiler.
42. In some cases more than one internal flue is made in
the boiler, and the heated air passes alternately through the
interior of the boiler, in contrary directions, and is at length
discharged into the chimney.
43. Internal flues have the advantage of imparting all the
heat to the water, saving that portion which in external flues
is imparted to the brick-work.
44. In some forms of boilers, the grate being constructed
at one end, the flame and heated air, instead of passing
through' a single internal flue, traversing the length of the
boUer, are distributed among three or more similar tubular
flues.
45. This subdivision of flues by the multiplication of the
number of tubes, and the diminution of their magnitude, is
carried to an extreme in locomotive boilers, in which from
100 to 200 tubes, not more than 2 inches diameter, traverse
ON THE STEAM ENGINE. 61
the length of tlie boiler, and divide the flame and heated air
into a multiplicity of small threads, so as to enable the water
to deprive them of their heat.
46. "With these a system of returning flues becomes unne-
cessary, the reduction of the temperature being completely
effected in traversing the boiler once.
47. In some arrangements the fl^me and heated air passing
from the furnace enter a number of narrow upright cells,
placed parallel to each other, and traversing the length of
the boiler; arriving at the remote end, another tier of cells, at
a superior elevation, is provided, by which they return. This
is most commonly the expedient adopted in marine boilers.
48. The multiplicity and complexity of flues, whatever be
their form, have the double disadvantage of increasing the
cost of the boiler and diminishing its strength. They are
therefore only resorted to in cases in which circumstances
exclude a great magnitude and weight of boiler, such as in
locomotive and marine engines. In the boilers used in land
engines, the requisite evaporating power can be obtained with
more simple expedients, by merely augmenting the bulk of
the boiler.
49. The two great objects which are to be attained are —
rapidity of evaporation and economy of friel.
50. The evaporating power of the boiler will depend (other
things being the same) upon the extent of surface which it
exposes to the action of the fire, the flume, and the heated
air. This surfia<ce is technically divided into^re surface and
Jlue surface,
51. By fire surface is meant all that surface of the boiler
upon which the radiant heat of the fiimace acts.
52. In the case of a waggon boiler, this is that portion of
the bottom of the boiler which forms the roof of the furnace;
but in well-constructed boilers, the sides and even the bottom
of the furnace form part of the boiler, and contain water
within them. In such cases they are to be reckoned as part
of the fire surfece.
62 BUDIMEyTABY TBEATI8E
53. The flue surface, as the words import, is that portioiL
of the surface of the boiler in contact with which the flame
and heated air, proceeding from the fire, pass before thej
issue into the chimney. This surface is usuallj of consi-
derable length, in order that the flame and heated air may
be detained in contact with the boiler until they have been
reduced to a temperature not greater than is necessary for
the draft.
54. Whatever be the length and arrangement of the flues,
it is indispensably necessary that they should always be below
the level of the water in the boiler, for otherwise the heat
would be imparted to the metal of the boiler without being
transmitted to the water. Steam is a sluggish recipient of
heat, and metal in contact with it might become red-hot
while the steam itself will remain at a comparatiyely' low
temperature.
This would accordingly be the case if the fire or flame acted
upon any part of the metal of the boiler which has not water
withiQ it.
55. In the economy of steam power, an object of capital
importance is to protect the machiuery from every cause by
which heat can be consumed in any other way than in con-
verting water into steam. A great variety of expedients haye
accordingly been adopted for this purpose, differing from each
other in their effects according to the circiunatances in which
the machinery is worked.
56. A boiler beiug a mass of metal of extensive magnitude,
raised to a very elevated temperature, and this naturally
being a good radiator of heat, a considerable quantity of heat
would be lost by the mere radiation from its surface. The
obvious remedy for this is to surround it by some material
which is a bad conductor of heat.
57. One of the most effectual substances for this purpose
is common saw-dust ; this is accordingly applied with great
effect in cases which do not exclude its use.
68. The boiler and its appendages are surrounded by a
OK THE STSAM EKGINIS. 63
thick casing, stuffed with saw-dust, and so completely does
this expedient answer the purpose, that the boiler-room of
a Cornish engine, where this arrangement is applied, is often
the coolest place that can be found.
59. In marine and other engines, a coating of patent felt
is often used with advantage : hemp, and other fibrous and
woollen substances, may be resorted to.
60. Locomotive boilers are cased in wood, which is a
tolerable non-conductor. The cylinders of large stationary
engines are also frequently cased in wood. The steam pipes
and other parts of the machinery containing steam are wrapped
with tow or other similar substances.
61. By these means the loss of heat by radiation may be
reduced almost to nothing.
62. "Where fuel is used which bums with little or no fl^me,
such as stone-coal or coke, the chief effect is produced by the
radiant heat, and a comparatively small effect by the heated
air. In such cases the fire surface should bear a large pro-
portion to the flue surface. In all cases the fire surfoce,
being more active in proportion to its extent than the flue
surface, is more liable to wear by intense heating. It may
be said, that as the surfiice of the metal cannot rise to a
higher temperature than that of the water within, and as the
entire mass of the water within must be maintained at an
uniform temperature, the fire surface cannot rise above the
general temperature of the mass. This would be true if the
boilers and furnaces were worked by a moderate system of
combustion, the fuel being consumed very gradually and
the heat developed slowly, so that a fierce action should not
take place on any part of the boiler. Such is the case, for
example, in the boilers and furnaces of the Cornish engines,
where space is a matter of little importance, and the economy
of fuel pushed to its extreme limit ; but in other cases these
advantages must be sacrificed, and a combustion so intense
maintained in the furnaces that the fire surface becomes
heated to a higher temperature than the water in CQiLta.Qfe
64 BVDIMEKTABY TBEATISE
"with it, and to a much higher temperatope than the flue
surface. Tlie formation of steam in contact with the fire
surface is so rapid that its bubbles do not escape to the
surface quick enough to keep the metal in continual contact
with water.
63. The metal, therefore, is momentarily out of contact
with water, and has a tendency to become overheated.
64. It is true that upon the escape of the steam bubbles
just formed the liquid will again wash the metal and lower
its temperature, but still this effect is such (in the case, for
example, of locomotive engines and sometimes of marine
engines) that the fire surface is exposed to much more rapid
wear by temperature than the fine surfia,ce.
CHAP. XVI. — HOW THE DRAFT THROUGH THE FURNACE OF
A STEAM ENGINE IS MAINTAINED.
1. The most common method of effecting this is by the
ordinary expedient of a chimney.
2. When the products of combustion are allowed to flow
through a chimney of sufficient height, the vertical column
of heated air thus formed has a certain buoyancy or tendency
to ascend into the atmosphere, proportional to the difference
between its weight and the weight of an equal column of
common air. This difference will be so much the greater as
the column has greater magnitude and height, provided only
that every part of it shall be, bulk for bulk, lighter than air.
Hence obviously follows the necessity of a chimney in
creating a draft, whether through the furnace of a steam
engine or in any ordinary maimer.
3. In stationary engines, as used in the arts and manufac-
tures, chimneys of any desired magnitude can generally be
attached to the engine. It is not necessary that the chimney
should be immediately over or contiguous to the fiimace ; it
may be placed at a considerable distance fipom it, provided
only it be connected with it by the proper air passages. This
is often a matter of convenience in factories, and we
ON THE STEAM EKGENTE. 65
accordingly see the chimney frequently erected at a con-
siderable distance from the boilers and ftimaces.
4. But in numerous applications of the steam engine it is
not practicable to use chimneys of such elevation, or so
placed, and in some cases the tube provided for the escape
of the products of combustion must necessarily be so short
as to afford no draft of appropriate amount.
5. Such is the case, for example, in locomotive engines : in
marine engines this is to some extent also true, — the chimney
must be comparatively short.
6. "When sufficient length of chimney is not admissible,
we are compelled either to throw in the gases of combustion
at a very high temperature, so as to make up for want of
height in the column, or to adopt some other expedient for
creating a draft.
7. A wheel is sometimes placed in the flues where they
enter the chimney, by the revolution of which the gases are
driven up the chimney with a force proportional to the
velocity with which the wheel revolves. This expedient is
similar to a sort of bellows commonly used for domestic
purposes, and is called di, farmer, and sometimes a blower, A
portion of the power of the engine is borrowed to keep this
wheel in motion. In this way an upward current is main-
tained in the chimney of any required power, and the
necessary draft sustained through the furnace.
8. Another expedient is used in locomotive engines, and
may always be resorted to where steam of high pressure is
used. This consists of Sijet, or, as it is technically called, a
blast pi^e, which is placed at the base of the chimney, and
presented upwards. A portion of the steam received from
the engine is allowed to escape by puffs, or even in a con-
tinued stream, through this pipe, and, being directed up the
chimney, creates the necessary draft.
CO £UDlM£^^TiLBY TBEATI8I
CHAP. XVII. — ^nOW THE MECHANICAL VIRTUE OP FUEL IS
ESTIMATED AND EXPRESSED,
1. In explaining the mechanical offects of steam, it has
been already shown that whatever be the purpose to which
the force of a steam engine be applied, its effect may always
be represented by a certain weight raised a certain height.
2. Whether an engine be employed to drive a mill-wheel, to
propel a ship, or to draw a carriage, the tension or resistance
to be encountered at the working point may be xuiiversally
represented by an equivalent weight.
3. Thus it is easily understood, if a locomotive engine
draws a train of carriages, that the tension of the chain which
connects the engine with the train will be the same as if the
same chain, in a vertical position, had a certain weight sus-
pended to it ; and the same will be true, whatever be the
nature of the resistance to the moving power, or the manner
in which this moving power may be applied.
4. It has been usual also to express the mechanical efficacy
by the number of pounds raised one foot ; for whatever be the
resistance, and whatever be the space through which the
moving power acts upon it, the effect can always be reduced,
as has been already explained, to an equivalent number of
pounds raised one foot.
5. The mechanical virtue of coals, thus explained and
applied to a steam engine, has been technically called the
duty of the fuel. Thus a bushel of coals consumed in the
furnace of an engine wiy. enable such engine to exert at the
working point a mechanical effect equivalent to a certain num-
ber of pounds raised one foot high : this effect is the duty
of the fuel, or as is sometimes said, the duty of the engine.
6. The duty of the engine is therefore not the entire
mechanical effect developed by the fuel in producing evapo*
ration ; for a portion of the mechanical power of the steam
ovolved in the boiler, and in some cases a very large portion
of it, is expended in moving the machinery of the engine
OK THE STEAM EKGIKE. 67
itself: all such portion is intercepted therefore between the
furnace and the working point. The duty, properly speak-
ing, is the net mechanical force developed by the steam, or
such portion only which is available for the work to which
the engine is applied.
7. The duty of engines varies within very wide limits,
according to the purpose to which they are applied. In this
respect engines may be reduced to three classes : — 1st, Such
as are used in the mining districts of Cornwall, where the
economy of fuel is pushed to its extreme limit ; — 2ndly, The
stationary engines used in the manufactories generally, in
which class may also be included marine engines; — 3rdly,
Locomotive engines on railways.
8. In the Cornish engines, where alone very accurate
observations are made on the mechanical effect produced,
and on the economy of fuel, it has been found, in some cases,
that by the combustion of a bushel of coals an effect has been
produced by the engine equivalent to 125 millions of pounds,
or what is the same, 62,000 tons, raised a foot high. This,
however, is not to be understood as an average result. In
producing it, the utmost care was taken to guard against
every source of waste of power,
9. The more common duty obtained from a well-managed
engine used in the mining districts has been from 80 to 90
millions of pounds, or at the rate of one million of pounds
raised one foot for every pound of coal consumed, — a result
remarkable enough in itself, and easily remembered.
10. In the ordinary stationary engines belongiug to the
second class, where the same scrupulous attention to economy
cannot be or is not paid, the duty, according to the commonly
received estimate, is in round numbers about 20 millions of
pounds for a bushel of coal, being four times less than that
of the good Cornish engines, and six times less than the duty
which has in certain cases been obtained.
11. In the locomotive engines worked on railways the
economy of fuel is of course still less ; but in this api^licatLc^^i.
68 BUDUIENTABr TBEi.TI8B
of the engine the economy of fuel becomes a consideration
80 subordinate, that it need not be enlarged on here.
12. The great economy obtained in the engines used in
Cornwall is the result of a variety of contrivances, some of
which, such as the protection of the machinery from radiation,
have been already mentioned. The boilers are constructed of
extraordinary magnitude, in proportion to the power expected
from them ; the furnace is of proportionate size ; the com-
bustion is slow ; the heating surface is very extensive, and
the intensity of heat upon it very slight ; the flues are of
great length, and the heated air is not permitted to escape
until the last available portion of heat has been extracted
from it J the fuel is managed in the furnaces with the most
extreme care, the combustion being perfect. Added to all
this, the steam is used at a pressure of from 35 to 50 lb. per
square inch above the pressure of the atmosphere, and the
expansive principle extensively applied.
13. In giving these last estimates of the duty of fuel in
the engines used in the manufactures generally, it is right to
observe, that ovring partly to the difficulty of ascertaining the
actual mechanical effect produced, and partly to the negligence
of proprietors of engines, the estimates of duty are of the most
loose and inaccurate description. When an engine is applied,
as is generally the case in ComwaU, directly to the elevation
of water or other heavy matter, it is easy to observe the me-
chanical effect it produces ; but when an engine is applied to
give motion to the works of a factory, to drive spinning-frames,
power-looms, or printing-presses, it is not so easy a matter to
reduce the effect it produces to an equivalent weight raised
a given height. In the case of locomotive engines the same
difficulty ought not to exist ; yet it is surprising that until
a very recent period, errors the most monstrous prevailed
respecting the real mechanical effect produced by these
machines. It was, for example, long assiuned as a maxim,
that the resistance offered by a given train of carriages to a
locomotive engine was independent of the speed, or in other
ON THE STEA3C ENGINE. 69
words, the same at all speeds. This error was not brought
to light until the year 1838, when it "was demonstrated, by a
series of experiments conducted by me, that the resistance
was augmented in a very high ratio with the speed.
CHAP. XVIir. — ^HOW THE POWER OF AN ENGINE IS ESTIMATED
AND EXPRESSED, AS DISTINGUISHED FROM ITS DUTY.
1. The duty, as we have seen, is the practical effect pro-
duced by the given weight of coal without reference to time.
Thus, whether a bushel of coal raises 20 millions of pounds
a foot in one hour or in ten hours, the duty of the engine is
exactly the same. But the power of the engine is quite
different.
2. The power of the engine is estimated by the mechanical
effect it is capable of producing in a given time.
"When steam engines were first brought into use, the work
to which they were applied had been previously done by
horses who worked the mills. It was convenient, therefore,
and indeed indispensable, to express the mechanical capabi-
lities of these machines by declaring the number of horses
which one of them would supersede ; and hence the term
now so general, horse-power, came into use. At first this
expression had but a vague signification, and was understood
by the manufacturers and capitalists who intended to employ
the steam engine in the literal sense of the actual number of
horses whose expense would be saved to them by it. But
after the engine had completely superseded horses in the
arts and manufactures, and it became necessary to express
its effects with greater precision, instead of abandoning the
term horse-power, an arbitrary signification was given to it
by Watt, which it has since retained. The word horse-
power, then, as applied to the steam engine, means the
capability of the engine to produce a mechanical effect per
minute equivalent to 33,0001b. raised one foot.
3. Thus an engine of 10 horse-power means one which in
70 BnDIM£I^Ti.BY TBEATI8B
working is capable of producing a mechanical effect per minute
of 330,000 lb. raised one foot, or an effect per bour equivalent
to 20 millions of pounds, verj nearly, raised one foot.
4. When a steam engine is declared to be of such or
such a horse-power, the expression must be understood in a
qualified sense. Thus it is assumed that the furnace is
worked in a certain average manner, and that a proportional
evaporation takes place in the boiler. An engine whose
nominal power is that of 100 horses may, by urging the
furnace in an extraordinary manner, be made to produce an
effect much greater than that of its nominal power ; or, on
the other hand, by keeping the furnace low, it may be, and
frequently is, worked considerably under its nominal power.
CHAP. XIX. — WHAT DIMENSIONS OF THE BOILER AND FUBNACE
ARE NECESSARY FOR AN ENGINE OR GIVEN POWER.
1. The technical rules adopted by engineers for the pro-
portion of engines corresponding to any required power, are
generally understood as applicable only to the second class
of engines enumerated already, namely, those generally
used in the manufiactories and in steam navigation.
2. The Cornish engines, on the one hand, and locomotive
engines on the other, are exceptional extremes, each being
worked in a manner peculiar to itself. In the one, much
larger dimensions are allowed for the production of a
given power, the action of the furnaces being of low
intensity ; while in the other, the dimensions producing a
given power are much smaller, and the consequent action of
the furnaces much more intense.
"What we shall therefore state here will be understood to
have reference to the second class of engines above mentioned.
3. In calculating the mechanical force developed in the
evaporation of water, we have seen that one cubic inch of
water, converted into steam, produces a mechanical force
sufficient to raise a ton weight a foot high. It would there-
fore follow that to raise 20 xniUionB oi ^jjovni^a a. ioo\. \i\^.
ON THE ST3BAM ENGINE. 71
■would require the evaporation of 1000 cubic inches of water.
But this calculation refers to the entire mechanical force
developed in the evaporation. A portion of this force is,
however, expended in moving the engriie itself, and is wasted
in various ways before it reaches the working point ; and it
is customary for engine-makers to allow for this fi:om 35 to
45 per cent, of the entire mechanical force developed in the
evaporation. Now since there are 1728 cubic inches in a
cubic foot, it follows that by such an allowance for waste of
power, the net effect of a cubic foot of water evaporated per
hour would be one nominal horse-power.
4. Such is the general usage of boiler-makers, but it would
be most erroneous to assume that this usage is based upon
even a. loose calculation: there can be no doubt that the
power expended in waste and uncondensed steam, and iii
moving the engine in any tolerably managed machine, must
be considerably less than this. The error, however, lies on
the safe side ; it is better to have superfluous boHer power
than a stint of steam. A boiler having more evaporating
power than is needed, can always be worked as much under
its power as may be desired; but when an engineer is
obliged to push a boiler above its legitimate power, both
waste and danger ensue. It must not therefore be assumed^
as has been done by some writers, that engine-makers adopt
these rules from ignorance. Although they do not in general
seek for an accurate knowledge of the amount of power
expended in moving the engine and in waste steam, they are
nevertheless fully aware that the allowance they make is
greater than its amoxint ; and in the absence of such exact
knowledge, it is clear they are right in adopting an excessive
estimate.
5. Prom what has been stated, therefore, it follows that
for every horse-power which the engine is expected to exert,
a power of evaporating a cubic foot of wat^r per hour ia
provided in the boiler.
6. When the term horse-power ia 05^'^^^, ^t5-t^l<^^.^^
72 BrDnf£3iTJLBY TREATISE
boilers, in reference to their capability of evaporation, it is to
be understood as indicating the evaporation at the rate of a
cubic foot of water per hour : thus, by a boiler of 50 horse-
power, is to be understood a boiler capable of evaporating
50 cubic feet of water per hoiir, the furnaces being worked
in the ordinary way.
7. The magnitude of the grate and the extent of heating
surface necessary to produce a given rate of evaporation, vary
more or less in different engines, and according to the prac-
tice of different engineers ; but still, in common engines
used in the arts and manufactures, there are average
standards which it is useful to know.
8. Thus it is generally agreed, that the dimensions of the
grate necessary for a boiler of a certain power should be
regulated by allowing a square foot of grate surface for
every horse-power in the boiler. Thus it follows, that as
much fuel is consimied per hour upon a square foot of the
surface of the grate, as is necessary and sufficient to evaporate
a cubic foot of water.
9. The dimensions of the surfSsice of the boiler exposed to
the action of heat, whether by radiation or by the contact of
heated air in the flues, is generally estimated at the rate of
15 square feet for a horse-power. Thus a boiler of 50 horse-
power would require a heating surface of 750 square feet,
10. These are not only average standards from which
individual boilers and furnaces of the class we more particu-
larly refer to, vary more or less considerably, but they are
altogether inapplicable to the two extreme classes of boilers,
— the Cornish on the one hand, and the locomotive on the
other.
11. In the Cornish boilers a slow combustion is main-
tained on the grates, and although the fuel is placed upon
them in a thicker layer, the intensity of the heat from a given
surface is considerably less than in the ordinary boilers.
Accordingly, for a given rate of evaporation, at least double
the extent of grate surface is allowed, ^e^ %xi!^,>iJftet^is5ii^^
OK THE STEAK EKGINE. 73
that two square feet are given for every cubic foot of water
per hour to be evaporated.
12. In like manner, as in these boilers the heat acts
with less intensity on a given surface of the boiler, a pro-
portionally greater heating surface is necessary to produce
a given rate of evaporation. In these cases a stiU greater
departure from the common boiler is necessary ; and instead
of 15 square feet being allowed for a cubic foot of water per
hour evaporated, we find 4 and 5 times this surface given.
13. The fiame and heated air are also made to traverse a
much greater length of flues before they enter the chimney.
14. Thus, while 60 feet length of flues are allowed in a
common wagon boiler, 150 or upwards are frequently given
in the Cornish boilers.
15. These circumstances wiU at once indicate the different
mode of operation, and the different quality of these two
classes of boilers.
16. The locomotive boiler is in the other extreme. Instead
of one square foot of grate surface evaporating one cubic foot
of water per hour, it usually evaporates 8 cubic feet. As
the heat developed in a given time may be taken as nearly
proportional to the water evaporated, it follows that the
calorific action of a square foot of the grate of a loco-
motive is 8 times that of a square foot of the grate of a
common stationary engine, and 16 times that of a Cornish
engine.
17. The intensity of the combustion maintained in the
furnaces of locomotive engines may be thus in some measure
conceived.
I have myself witnessed a set of new grate bars partially
fused and rendered useless in a trip of 30 miles. The splen-
dour of the biLming fuel in these furnaces is sometimes so
intense, that it impresses the eye with the same pain as is
sustained in looking at the sun.
18. The Cornish boilers, which differ ^o extec^'vfiL<2v:^Ns^'^^^
mode of operation and effects fromftie\ocoTSiO>Gv??^^,"t^"^'e^^'^^
74 BXTDUSXJXTXRT TBXATI8S
them neYertheless very doselj in their fonoi. Both are
cylindrical, and the flues in both consist of metal tubes,
traversiog the length of the boiler. In the Cornish boilers
the tubes are of iron, and of considerable diameter. In the
locomotive boilers they are of brass, and yery small in
diameter.
19. The diameter of the Cornish boilers is usually about
^th of their length. Where great power is required, it is
found more convenient to use two or more boilers than one
of larger dimensions. A common proportion for these
boilers is from 36 to 40 feet of length, and firom 6 to 7 feet
in diameter. The locomotive boilers are usually from 8 to
10 feet long, and from 3i to 4i in diameter.
20. The common published reports of the consumption of
fuel are usually given by expressing the weight of coal con-
sumed per hour per horse-power ; but unless it be ascer^
tained that the real working power of the engine and the
consumption of fuel are equal to, and do not exceed its
nominal power, such reports lead to erroneous conclusions.
The common allowance of fuel for stationary engines and
marine engines, when working to their fall power, is 10 lb.
per horse-power per hour. The consumption, however, is
undoubtedly less than this when the engines are properly
constructed and carefully worked : 7 and 8 lb. per horse-
power is a very common consumption for well-managed
engines. In the Cornish engines the common consumption
is little more than 5 lb. per horse-power per hour.
CHAP. XX. — WHAT DIMENSIONS OF THE CYLINDER AND OTHER
MACHINERY ARE REQUISITE FOR A GIVEN POWER OF ENGINE.
1. Nothing can be more vague, uncertain, and arbitrary,
than the rules adopted by engineers in reference to this
problem. It may be truly stated that every engine-maker
has his own standards, to which he attaches invariably as
jmueh m&UiWitjr as if this mechanical problem were capable
019^ THE STEAM El^GIIfS. 75
of as certam and demonstrative solution as a problem in
common geometry.
2. It will be obvious, on the slightest consideration, that
the magnitude of the cylinder and piston necessary to pro-
duce a given working power, must depend on the pressure
of the steam after it enters the cylinder, and the velocity
with which the piston is driven, the degree of perfection of
the vacuum on the other side of the piston, and the extent
to which the expansive principle ia introduced. In general,
however, it has been the practice to apply the calculation to
low-pressure engines, that is to say, to those in which the
steam, after it enters the cylinder, has not a pressure
exceeding the atmosphere by more than 4 or 5 lb. per square
inch, and in which the piston ia supposed to move at the
average rate of 200 feet per minute. These conditions
being assumed, and a good vacuum being sustained in the
condenser, 22 square inches of the piston are allowed for
every nominal horse-power of the engine.
3. Where these rules are observed, the nominal power of
an engine may always be obtained by dividing the number of
square inches in the surface of the piston by 22 ; or, which
is the same, by dividing the square of the diameter of the
piston, expressed in inches, by 28.
4. Again, if it be required to find the magnitude of the
piston necessary for an engine of a given power, it is only
necessary to multiply the number expressing the power by
28, and the square root of the product will be the diameter
of the piston.
6. It must be carefully observed, however, that such rules
are only applicable so long as the piston moves with the
above velocity, and is urged by low-pressure steam at the
above rate.
6. Indeed, it may be observed generally that the mode
of expressing the mechanical capabilities of engines by
horse-power, frequently leads to most erroneous confil\i&v<i.\5^^^
and it has lately been accordmg\y Txixvc^ci ^-sasi^^-v^iJi^sss^^^
76 BUDIMENTAET TEEA.TISE
among engineers and scientific men. In locomotive engines
it is not applied at all ; nor, indeed, in the Cornish
engines.
7. The proportion of the diameter to the stroke of the
cylinder, as its length is called, varies very much according
to the purposes to which the engine is applied. In marine
engines, for example, where the cylinder has a vertical
position, and the engine is stinted in height, the stroke very
little exceeds the diameter. In stationary land engines the
proportion of the diameter to the stroke is frequently that of
lto2.
8. The dimensions of the air-pump, condenser, and other
parts of the engine, bear a certain proportion to those of
the cylinder, which are but little departed from by engine-
makers.
9. Thus the air-pump has usually half the stroke and half
the area of the piston, and consequently its capacity is a
quarter of that of the cylinder : nevertheless some engineers
maintain that a larger proportion of air-pump augments the
efficiency of the machine.
CHAP, XXI.— HOW THE INTERNAL CONDITION OF THE BOILER
AND ENOINE IS RENDERED EXTERNALLY MANIFEST.
1. To enable the engine-man to maintain the boiler and
machinery in a state of efficient operation, it is necessary
that he should be at all times informed of their internal
condition. A class of contrivances for indicating this has
therefore exercised the invention of those to whom we
are indebted for the improvement of this department of
mechanical art.
2. One of the most obvious circumstances attending the
internal condition of the boiler, which it is necessary that
the engine-man should at all times know, is the quantity of
water in it. If the level of the water get below the flues, the
boiler incurs the danger of becoming red-hot, and bursting :
if the level of the water be too high, tlae «\ie^m xoom in
ON THE STEAM BNGINB. 77
the boiler becomes insufficient, and the spray of the boiling
water, mingled with the steam, passes through the steam
pipes into the cylinder, producing a waste of heat and other
inconveniences. This effect is called primmg. The level
of the water in the boiler should therefore always be known.
3. The earliest and most simple contrivance for indicating
this is the gauge-cocks. These cocks are two common stop-
cocks, screwed or cemented into the boiler, one above the
point at which the level of the water ought to stand, and the
other below it. When the water is at the proper level, steam
should issue on opening the one, and water on opening the
other. If water issue jfrom the upper cock, the boiler is too
fiill ; and if steam issue from the lower cock, the boiler is too
empty. So long aa steam issues from the upper and water
from the lower, the level of the water is at its right point.
4. In boilers maintained in a very violent ebullition,
where a highly intense .fiirnafte^ i^ used, the agitation near
the surface renders the indicatibli of the gauge-cocks some-
times uncertain, and another contrivance is either substituted
for them, or used in connexion with them.
5. If it were possible to have a glass boiler, the level of the
water would always be visible ; but instead of a boiler all glass,
we may have a strong glass plate inserted into the side or end
of the boiler, at the level at which the water ought to stand,
and through this plate the surface of the water might be seen ;
but the great agitation of the water in ebullition would render
this observation uncertain: the object is therefore accom-
plished by the glass water ga^e, (see Chapter xxyii., title
" Glass Water Oauge,^') which is a strong glass tube placed
in a vertical position outside the boiler, conmiunicating at the
top and bottom by metal tubes with the interior. The water
in the boiler enters the lower end of this tube, and the steam
enters the upper end; and by the common principles of
hydrostatics, the pressure of the steam in the tube and in
the boiler being the same, the water ia t\i<& \,\^a^-^^'^'«s!i^
at the same level as the water m \i\xe\>o^et.
78 B(rDIlLEKTi.BY TBEATI8X
6. To guard against the effects of the accidental fracture
of this tube, stop-cocks are usually placed between the ends
of it and the boiler, by which the communication between it
and the boiler is cut off at pleasure. When the engine-man
desires to ascertain the level of the water in the boiler, he
opens both the stop-cocks, but at other times it is more
prudent to keep them closed.
7. This expedient has the advantage over the gauge-cocks,
inasmuch as it indicates the exact level of the water.
8. Another contrivance used for the same purpose consists
in a floaty formed of a hollow casing of metal ; to this is
attached a rod which passes through the top of the boiler.
As the level of the water rises or falls in the boiler, this
float rises or falls with it, and the rod is pushed upwards or
drawn downwards, as the case may be. An index of any
kind may be attached to this rod, which should play upon a
divided scale, indicating the position of the float and the level
of thp water.
9. Another expedient is sometimes used, which consists of
a tube let in through the top of the boiler, and descending
to a point below which the water ought not to fall : at the
top of this tube is fixed a steam whistle,
10. So long as the level of the water is above the lower
end of the tube, a column of water will be sustained in the
tube by the pressure of the steam within the boiler ; but
when the level subsides below the mouth of the tube, then
steam, rushing through the tube, will issue from the whistle,
and produce an alarm which will give notice of the want of
water in the boiler.
11. This last contrivance can only be used in low-pressure
boilers, where the column of water which wiU balance the
steam is not too high.
12. It is most necessary at all times that the pressure of
the steam within the boiler should be known, and provision
should be made to prevent its exceeding a certain limit.
^^Z^ is accompUahei by the common safety -oolDe.
OK THB STSAH ENGnTE. 79
This valve is an ordinaiy conical valve, placed in the top of
the boiler, and fitting into its seat, so as to be steam-tight.
It is loaded with a weight which determines the maximum
pressure to which the steam is allowed to attain. Thus, if
it be intended, as in low-pressure boilers generally, that the
steam should not exceed 6 lb. per square inch, thrai the
safety valve is loaded with a weight, regulated in such pro-
portion to the magnitude of its surface exposed to the steam,
that whenever the pressure of the steam exceeds this limit,
it forces the valve open, and escapes until the pressure is
reduced to the proper limit.
13. The safety valve, however, affords an indication that
the pressure of the steam does not exceed a certain amount,
rather than an indication oi what that pressure actually is.
14. The steam gauge exhibits the exact amount of this
pressure.
15. The mercurial steam gauge generally used in low-
pressure boilers (see Chap, ixvii., title " Mercurial Steam
Gauge for Low-pressure Boilers ") consists of a siphon tube
with equal legs, half-filled with mercury; one end is cemented
into a pipe which enters that part of the boiler which con-
tains the steam ; the other end is open to the atmosphere.
A stop-cock is usually provided between this gauge and the
boiler, so that it may be put in communication with the
boiler at pleasure. When the stop-cock is open, the steam
acting on the mercury in one leg of the gauge presses it
down, and the mercury in the other leg rises. The difference
between the two columns is the height of mereury which
corresponds to the excess of the pressure of the steam in the
boiler above the pressiu^ of the atmosphere ; or, in other
words, to the effective pressure on the safety vaJve. K half
a pound per inch be allowed for the length of this column,
we shall obtain, in pounds per square inch, the effective
pressure of the steam.
16. If the siphon steam gauge ^et^ Tftsw^^a ^^ ^ass^^ *^i«^
height of the mercurial columii xe^Te^eia^m^^'^ ^^^^ses^sj^k '^
80 BT7DIMEKTABT TBBi.TI8E.
the steam could be obtained bj inspection, a scale being
annexed; but to avoid accidental fractures, this tube ia
usuallj made of iron, and the level of the mercurj is indi-
cated bj a float, haying a rod attached, similar to the gauge-
float already described for indicating the level of the water.
To this rod may be attached any convenient index and scale.
17. Owing to the obstruction which the steam encounters
in passing through the steam pipes and valves, its pressure
undergoes a greater or less diminution on its way to the
cylinder. To ascertain the effective pressure, therefore, in
the cylinder, a steam gauge is sometimes placed upon the
steam pipe, as close as possible to the cylinder.
18. A custom has been adopted too generally of estimating
the pressure of the steam in the cylinder by its pressure in
the boiler, assuming that between the two there is but a
slight difference. Nothing can be more erroneous than this.
Between the pressure of the steam in the boiler and in the
cylinder there may be almost any amount of difference. K
the throttle valve be nearly closed while the pressure of the
steam in the boiler is very high, the pressure of steam which
works the piston may be very low ; and, on the other hand,
if the throttle valve be nearly open, there may not be a
considerable difference between the two.
19. To calculate, therefore, in general the effective power
of the engine, by taking, as is commonly done, the pressure
of the steam in the boiler, and multiplying that by the area
of the piston and its velocity, is a most fallacious method.
The indicator already described may be used to determine
the average pressure of steam on the piston, and thus the
effective action of the piston may be calculated ; or, if the
actual quantity of water transmitted in the state of steam to
the cylinder be known, the mechanical effect of this can be
calculated independently of any consideration of the pressure
of the steam, or even of the magnitude of the piston. It
will, however, be necessary even in this case to determine
the reaiattmce of the uncondenaed steam.
. ON THE STEAM EKGIKE. 81
20. In high-pressure boilers, where steam is worked at 40
and 50 lb. above the atmosphere, or at even higher pressures,
the mercurial steam gauge is inconvenient, owing to the
height of the column of mercury which the pressure would
sustain, and from other causes. This inconvenience is espe-
cially felt in locomotive engines. In stationary engines it is
always possible to provide a permanent mercurial steam
gauge of sufficient height, whatever be the pressure of the
steam ; and indeed it is desirable so to do, for there is no
measure of the force of the steam so certain and exact. In
locomotive engines, however, and in other cases where a tail
column of mercury is inadmiasible, the preesure of the steam
is indicated by a spring steelyard, which is made to act upon
the safety valve. (See Chap, xxvii., title " Spring Safety
Vaher) This instrument is in principle precisely the same
as the common spring steelyards used in domestic economy.
A scale is attached to it, upon which an index plays, by
which the pressure on the valve is expressed in lbs. per
square inch. The instrument is usually screwed down, so
that the valve will only be opened when the steelyard
indicates a certain pressure. .
21. It is customary, more especially in high-pressure
engines, to provide two safety valves, one of which shall be
removed from the interference of the engine-man. GDhis
precaution prevents the danger which would arise from the
engine-man overloadiQg the valve, or from the valve becoming
fixed in its seat from accidental causes, which sometimes
happens.
22. When a boiler ceases to be worked, and the fire has
been extinguished, the steam which filled its interior will be
speedily condensed, and the interior would become a vacuum.
In this case a prodigious amount of amospheric pressure,
acting on the external surface of the boilers inwards, would
have a tendency to crush it. This contingency is sometimes
provided against by a safety valve whick cy^ec&^ss^^st^* "^^
long as the boiler is in operation, tti\a N«XNe> \aV«^^^ A'^'^'^^^^
82 fiVDIM£^TABT TBEATISZ
the pressure of the steam ; when it oeases to be worked, it is
opened by the pressure of the atmosphere.
23. It is most necessary for the efficient operation of the
engine that the state of the vacuum in the condenser should
be at all times known. For this purpose an indicator is
adopted, called the barometer gauge, forming one of the most
important appendages of the condensing steam engine. (See
Chap, ixni., title " Barometer Gauge")
24. This instrument, as its name imports, is a common
barometer, but the top of the tube, instead of being closed, is
made to communicate with the condenser. The atmospheric
pressure, acting as usual in barometers, on the mercury in
the cistern, presses a column of mercury up the tube. If
the vacuum in the condenser were as perfect as that which
is at the top of the barometric tube, then the column of
mercury in this instrument would stand at exactly the same
height as in the common barometer ; but as this is never the
case, there is a difference of height which is due to the pres-
sure of uncondensed steam and air, which, notwithstanding
the action of the air-pump, will always remain in more or less
quantity in the condenser. The difference, therefore, between
the height of the column of mercury in the barometer gauge
communicating with the condenser, and in a true barometer
placed near it, will give, in inches of mercury, the pressure
which re-acts upon the piston against the steam.
25. In well-managed engines the barometer gauge is
seldom more than an inch below the true barometer, which
would give half-a-pound per square inch for the pressure
re-acting on the piston.
26. If the barometer gauge stand too low, it indicates the
presence either of uncondensod vapour or of air in the con-
denser. This may arise either from too Httle or too much
water being thrown in by the condensing jet. If too little
be thrown in, the condensation will be imperfect, and
uncondensed vapour wiU lower the gauge : if too much be
thrown in, an accumulation of air will be produced faster
OK TEE STEAM EITOINE. 83
than the pump can remove it, and the gauge will be similarly-
affected. The adjustment of the jet is a matter, therefore,
that should be carefully attended to. The cock which
governs the jet has a handle to which an index is attached,
plajdng upon a divided scale ; and according to the position
of that index, the cock is more or less opened or closed.
CHAP. XXII. — HOW THJ! WANTS OP THE BOILER AND ENGINE
ARE SUPPLIED, AND HOW THEIR OPERATION IS REGULATED.
1. If the work executed by a steam engine were subject
to no variation whatever, the rate at which the steam should
be supplied to the cylinder and gen^ated in the boiler
would be uniform also ; and as the production of such steam
necessarily bears an uniform ratio to the development of
heat in the furnace, this last would be also uniform. The
development of heat in the furnace being in direct ratio to
the supply of air, or what is the same, the draft in the
chimney, it would follow that an engine pwfectly uniform in
its action would require an invariable adjustment of the
flues, an invariable rate of evaporation in the boiler, and an
invariable magnitude of communication between the boiler
and cylinder for the supply of steun.
2. But in practice it is found that the work to be executed
by machinery of this kind is subject to more or less variation,
requiring a greater or less intensity from time to time in the
moving power.
3. This necessitates a corresponding variation in the action
of the steam in the cylinder. This variation is produced by
the throttle valve, placed in the pipe by which steam is con-
ducted to the cylinder. (See ^^. art, 17.) This valve is a
circxdar plate, corresponding nearly with the magnitude of
the pipe in which it is placed. It is so constructed as to
turn on an axis which coincides with one of its diameters,
and its movement its governed by a lever or handle on tha
outside of the steam pipe. ^W\ieTv. ^\i\a ^xscvjJ!^^ -sj^a^ Sa»
84 SUDIMEKTABT TBEATISE
turned so as to present its edge to the current of steam,
that current is allowed to pass without obstruction to the
cylinder ; but when it is turned so that its face is. presented
to the steam, the current is altogether stopped. Between
these two extreme positions it may have any intermediate
inclination by which the flow of steapi to the cylinder shall
be regulated in any desired manner.
4. Supposing this valve to be adjusted from time to time,
so as to proportion the quantity of steam admitted to the
cylinder to the quantity of work to be done, the production
of the steam in the boiler will have to be considered. If this
production be uniform, it must be adequate in quantity to the
greatest amount of steam at any time required by the cylinder.
5. When less than this is admitted to the cylinder by the
action of the throttle valve, an accumulation would neces-
sarily take place in the boiler, and the pressure on the safety
valve becoming excessive, the surplus steam would blow off.
This would occasion, of course, a corresponding waste of
fuel. The remedy for this would be a contrivance by which
the rate of evaporation in the boiler can be augmented or
diminished at pleasure, according to the wants of the cylinder.
This will obviously be accomplished by any contrivance which
will stimulate or slacken the fiimace at pleasure. . Now since
the action of the furnace is regulated by the intensity of the
draft, exactly as the action of the piston is regulated by the
intensity of the steam admitted to it, the same kind of regu-
lator may be applied to the one as has been, applied to the
other. A plate called a damper is therefore introduced at
some convenient point in the flue near the chimney. This
plate is generally made like a sliding shutter. When it is
let down it stops the flue altogether, and the fire would be
extinguished ; when it is drawn up to the limit of its play,
the flue is altogether open, and the draft is at its extreme
power ; between these limits the damper may have an inde-
finite variety of positions, leaving more or less of the flue
open, BO as io give to the draft any required intensity.
ON THE STEAM ENGINE. 85
6. It is easy to imagine an attendant working these two
instruments so as to regulate the action of the machinery.
When the resistance on the working point is lightened, the
throttle valve is partially closed, so as to diminish the supply
of steam ; and at the same time the damper is partially closed,
so as to diminish the draft : on the other hand, when the
load on the machinery is increased, the throttle valve is
opened, so as to augment the supply of steam and increase
the action on the piston : and the damper is raised, so as to
increase the intensity of the combustion and augment the
rate of evaporation in the boiler.
7. It would be obviously desirable that these contrivances,
which we have here supposed to be regulated at the discre-
tion of the attendant on the engine, should be regulated by
the wants of the engine itself, so as to be made self-acting, like
the valves which regulate the supply of steam to the cylinder.
8. This is accordingly accomplished by very simple and
effectual means in low-pressure boilers, to which we more
partici^ly advert at present. A tube is inserted, which
descends in the boiler below the level of the water ; the
pressure of the steam supports in this tube a column of
water of a certain height, and as the pressure of the steam
varies, this column varies in height. A float is introduced
in the tube, and supported by this column of water. A
chain attached to this float is conducted over one or more
pulleys, and carried to the damper, which is suspended to
it. Now let us suppose the throttle valve either opened or
closed, as the case may be. K it be opened, the supply of
steam passing from the boiler to the cylinder is augmented ;
the pressure of steam in the boiler is for the moment
diminished by this exhaustion : the column of water in the
tube falls by reason of the diminished pressure; the float
supported by it falls with it, and, drawing down the chain,
draws up the damper; the draft through the furnace is
augmented, the combustion is stimulated, the heat which
acts on the boiler increased, and t\ie e^^^ot^^ovi. ^^^'^^'^'^iws^^
86 BITDnCENTABY TBEATIS£
until tbe production of steam becomes adequate to the
demands of the cylinder.
9. In this way the varying demands of the cylinder on
the boiler are made to vary in a proportional manner the
action of the furnace, on which the generation of steam
depends: when the cylinder consumes much steam, the
damper is kept open ; when little, it is partially closed.
10. The superintendence of the damper by the engine-
man is therefore superseded. The engine itself works it
more regularly and perfectly than could be done by any
manual superintendence.
11. This arrangement is called the self-acting damper,
12. In steam-engines in general, and especially in those
used in the manufactories, the rate at which steam is
supplied to the cylinder ought to be proportionate to the
work which the engine has to perform ; if not, whenever the
resistance on the engine should be diminished, the speed of
the piston would be augmented ; and whenever the resist-
ance should be augmented, the speed of the piston would be
diminished, and a continually varying and irregular motion
would necessarily take place in the engine, and would be
transmitted to the machinery which it works. This is in
general incompatible with the exigencies of the arts and
manufactures, in which there is a certain rate of motion or
speed which ought to be imparted to the machinery, and
which ought neither to be permitted to decline or augment.
13. Now, since occasional variations in the resistance are
inevitable, the only way to maintain an uniform velocity in
the engine and in the machinery it drives, is to provide
means of regulating the supply of steam, so that the rate at
which it shall flow into the cylinder shall be varied in the
exact proportion of the resistance. This might, as I have
already stated, be accomplished by the manual superintend-
ence of the throttle valve, but a much more certain and
efficacious expedient was supplied in the governor, by the
fertile invention of Watt.
OK THE STEAM ENGDng. 87
14. To make the principle of the governor comprehended,
we must refer to a well-known property of the common
pendulum used as the regulator of time-pieces. It is the
property of this instrument, that when it oscillates in obedi-
ence to gravity from side to side in a circular arch, the time
of its vibration will be the same whether the arches in which
it vibrates are long or short, provided only the angle of its
vibration be not considerable : if the arches be short, its
motion will be slow ; if long, its velocity will be proportion-
ally great ; and thus, whether long or short, the time of
accomplishing a complete vibration will be the same. This
well-known property of the pendulum is called isochronism,
15. Now if the pendulous knob, instead of vibrating in a
circular arch, be made to whirl with a circular motion round
an ajLis, the knob, in virtue of the centrifugal force produced
by the rotation, will have a tendency to recede from the axis
round which the motion takes place ; and when it assumes
such a position that the tendency to recede is equal to its
tendency to descend, in virtue of its weight, it will remain at
a fixed distance from the axis round which it revolves, neither
receding from, nor approaching to it.
16. It is a property of this arrangement, quite analogous
to the isochronism of the pendulum, and indeed depending
on the same physical principles, that the time of revolution
necessary to produce this equilibrium, and to keep the knob
at a fixed distance from the axis, without receding from or
approaching to it, is the same whatever be the distance of
the knob from the axis, provided only that the angle of
obliquity of the rod be not considerable ; and even though
such angle have some considerable magnitude, the times of
revolution corresponding to the state of equilibrium will not
be considerably different.
17. This expedient, known by the name of the conical
pendulvm, was applied by Watt, with his usual felicity and
success, to the regulation of the throttle valve. The arrange-
ment, as usually adopted, is re^te^eiA,^^ m ^^ ^^^^"^^cc^%
BimiUBirTABt TBEATISX
figure. Two baUa i are attached to the ends of equal rods
of metal H o. The arrangement is composed of a aeries of
jointed rods h f e, which play upon a vertical spindle c d,
being fiied at h, but capable of eliding upon it at e. "When
the balls are separated so that the rods e o become more
divergent, the arms h f open, and the pivots t, aeparating,
draw down the collar e, which, as I have stated, slides upon
the spindle ; and on the contrary, when the balls approach
each other, the arms h f eiso approach each other, and the
collar E is forced up. Thus, according to the distances of
the balls from the vertical spindle, the collar e ascends or
descends. In the collar e is inserted the forked end E of
the lever NIK. The end n of this lever is connected, as
represented in the figure, with the throttle valve t, and the
proportion and position of the rods are ao adjusted that
when the halls descend towards their. lowest position, the
throttle valve becomes open; and when they separate, it
becomes gradually closed.
AgTooYe^ wheel a b, or oftener a toothed pinion, is fixed
OK THE STEAM ENGINE. 89
upon the axle of the spindle, which receives its motion from
any convenient part of the machineiy.
Now let us suppose that the load on the engine is suddenly
diminished. A momentary augmentation of speed will take
place in the piston, and an increased velocity be imparted to
the wheel a b and the balls oi the governor ; these balls will
consequently fly further from the vertical spindle, the fork k
wiU be drawn down, the throttle valve t partially closed, and
the supply of steam to the cylinder diminished.
If, on the other hand, the load on the engiae be increased,
the speed of the piston will be momentarily slackened, the
velocity of the wheel A b will be diminished, the balls will
descend and approach the vertical spindle, the fork k will be
raised, and the throttle valve t partially opened. In this
manner the governor has the effect of admitting at all times
to the cyHnder just that portion of steam which is necessary
to give to the piston the proper velocity, the quantity being
always proportioned to the load on the engiae.
It is to be understood that this beautiful little instrument
exercises powers circumscribed within narrow limits; but
these limits are sufficiently extended to accommodate them-
selves to the variations incidental to the work which the
engine performs. If the average amount of work varies from
time to time, the governor can be adjusted accordingly.
18. I have already explained in how great a degree the
regular supply of water to the boiler is necessary to the
efficiency of the machine. Since the water in the boiler wiU
be in the direct proportion of the work executed by the
engine and the combustion in the furnace, it seems natural
to seek for some self-regulating mode of feediug the boiler,
analogous to that which we have described as governing the
combustion in the furnace and the supply of steam to the
cylinder. It has been already explained that a float within
the boiler causes a rod bearing an index to ascend and
descend, iudicating always the quantity of water in the boiler.
Now if this rod can be made to a«;\) u^oii^x^'eRsc^Q^s^ ^
90 BXTDIMBKTjLBT tesatise
water communicating with the interior of the boiler, so as to
open the valve and admit water when it descends, and close
the valve so as to stop the supply when it ascends, the desired
object will be attained. Such an arrangement has accord*
ingly been adopted with complete success, and forms what
is called the self-iustmg feeder. To the rod of the float is
attached a cord or chain by which it is connected with the
end of a lever, which opens and closes a valve placed in the
bottom of a small cistern which stands at a sufficient height
above the boiler. A tube is inserted in the bottom of this
cistern under the valve, which tube descends into the boiler,
and in it a column of water is sustained by the pressure of
the steam, as already described.
When the level of the water subsides and the boiler
requires feeding, the float falls, draws down the rod, opens
the valve in the small cistern above, and lets water flow in
through the tube : this continues until the level of the water
is restored to its proper height, when the valve is closed.
19. But to speak more precisely, this valve is not alter-
nately opened and closed. The float and valve will be so
adjusted that the latter is kept just so much open as to
allow a stream of water to descend in the tube which is
exactly equal to the rate of evaporation in the boiler, so that
the level of the water is kept constantly at the same point.
20. This arrangement, however, is only applicable to low-
pressure boilers, for in high-pressure boilers the column of
water which would be sustained in the tube would be too
high.
21. It is customary to supply the feed cistern just men-
tioned with the water pumped from tiie condenser by the air-
pump : this water, having a temperature more elevated than
that of the atmosphere, carries back to the boiler a portion of
heat which would otherwise be wasted.
22. In high-pressure boilers, where this feeding apparatus
would be inapplicable, the necessary quantity of water is
dn'ren into the boiler by forcing pumps, called feed pun^s,
0:Er THE STEAK EKGHnS. 91
which are worked by the engine. The dimensions of these
pumps are regulated according to the average evaporating
power of the boiler, so that the quantity of water which
they throw in shall be exactly equal to the quantity which
passes in the state of steam to the cylinder.
23. As this proportion, however, cannot be always pre-
cisely maintained, it is necessary to provide means for
cutting off the feed pumps, or throwing them into operation
at pleasure.' Arrangements of this kind are accordingly
provided, and placed at the disposal of the engineer.
24. An easy and obvious expedient suggests itself for
cutting off the feed, and supplying it according to the wants
of the boiler, which, however, I do not recollect seeing
adopted in practice.
25. The float which rises' and falls with the level of the
water in the boiler might be made to act by its rod upon the
gearing of the feed pumps, exactly as it acts upon the valve
in the feed cistern in low-pressure boilers ; so that whenever
the level of the water should become too high, the pump
should be thrown out of gear ; and whenever it was too low,
it should be thrown into action.
CHAP. XXIII. — HOW THE STEAM ENGINE IS ADAPTED TO THE
WORKING OF PUBCPS.
1. Hitherto we have considered the piston as driven in
both directions, upwards and downwards, by steam, a vacuum
being produced alternately on the side towards which it
moves.
2. When the engine is applied to work a common pump,
the force being only required to be exerted when the pump
buckets are raised, but not in their descent, an arrangement
would be required in the cylinder by which the piston should
be only driven by steam in its descent, the pump buckets
being then raised at the other end of the beam ; but in its
ascent the piston would be drawn up by the weight of the
descending buckets, without any aiifeoT!£v>^^^^ft«n^'^^^^^^
92 BUDDOKTiLBY TBEATIBE
adapted to work pumps are therefore so arranged that the
yalye shall only admit steam above the piston, a vacuum
being made below it in the descent. Engines constructed in
this manner are called single-acting engines, while those in
which the steam acts both above and below the piston are
called double-acting engines,
8/ The single-acting engine in its principle differs in no
respect from those we have described. A valve is provided
at the top of the cylinder, by which steam is admitted above
the piston when it begins to descend ; another valve is pro-
vided at the bottom, by which the steam' under the piston
passes to the condenser ; and the piston descends exactly in
the same manner as in the double-acting engine. But
when the piston has reached the bottom of the cylinder, a
valve is opened which gives a communication between the
top and the bottom of the cylinder, so that the steam which
has just pressed the piston down now passes equally above
and below it. The piston being then drawn up by the
weight of the descending buckets, the steam which was
above it passes below it, through a tube attached, in which
the valve just mentioned, communicating between the top
and bottom of the cylinder, is placed. When the piston has
reached the top of the cylinder, the steam which previously
fiUed the cylinder above the piston will now fill it below the
piston ; and when the piston is about to descend by the
pressure of fresh steam admitted above it, the steam below
it is discharged to the condenser by another valve, already
mentioned, and so the operation proceeds.
4. These single-acting engines are only applicable to
pumping or to some other operation in which an intermitting
force, acting in one direction only, is required.
5. The double-acting engine may, however, be also applied
to pumping by the use of a double-acting pump, a variety
of forms of which are familiar to engineers.
6. The most remarkable examples of the application of the
steam engine to pumping are presented in the mining districts
ON TKB STEAM ENail^E. 93
of Cornwall, where engines constructed on an enormous
scale are applied to the drainage of the mines. The largest
steam engines in the world are used for this purpose.
Cylinders 8 and 9 feet in diameter are not unprecedented.
The expansive principle may here be applied without limit,
inasmuch as regularity of motion is not necessary. Steam
having a pressure of 50 lb. per square inch above the atmos-
phere is admitted to act on the piston, and cut off afber
performing from ^ to -j^ of the stroke, the remainder of the
stroke being effected by the expansion alone of the steam.
CHAP. XXIV. — HOW THE ATMOSPHERIC PRESSURE COMBINED
WITH THE PROPERTIES OF STEAM IS RENDERED EFFICIENT
IN AN ENGINE.
1. The machine called the atmospheric engine, which was
displaced by the improved steam engine of the celebrated
"Watt, consisted of a cylinder and piston, working beam and
pump-rods, similar in their general arrangement to those of
the single-acting steam engine already described. The dif-
ference consisted in this, that a vacuum being made under
the piston by the condensation of steam, the piston was
urged downwards, not by the pressure of the steam, but by
that of the atmosphere which was admitted above it, the top
of the cylinder being open. In this case the steam was
used not directly as a mechanical agent, but indirectly to
produce a vacuum under the piston, and so give effect to the
atmospheric pressure above it.
2. This system, compared with the single-acting engine,
has many defects, the removal of which was so successfully
accomplished by the invention of "Watt. When the piston
was pressed downwards by the atmosphere, the atmosphere
had a tendency to cool the cylinder ; and when the piston
was made to ascend by admitting steam under it, and thus
giving effect to the weight of the pump-rods at the other end
of the beam, the steam as it entered was more or less con-
densed by the cold cylinder •, and to ^\i"aX»^"^«t ^i^^^vi!^ "^^i^
M BUDIMEKTABT TBSATISB
condensaidon took place, there was a proportional waste of
fuel. When the piston was at the top of the cylinder, and
the cylinder under it filled with steam, a jet was introduced
within it, as we have already described, and the steam was
condensed ; but this method, which produced an unnecessary
waste of fuel, is not essential to the principle of the atmos-
pheric engine.
3. The separate condenser of Watt being attached to it,
the condensation is made under the piston without cooling
the cylinder, in the same manner as in the improved engine
of Watt. There still would remain, however, the evil of
cooling the cylinder by the admission of the atmosphere
above the piston.
4. Nothing, on the other hand, is gained by using the
atmosphere in this way. I^e same steam which is used to
make a vacuum under the piston may be previously used to
press the piston downwards, and we therefore consume as
much mechanical force, in the form of steam, when we use
the atmosphere as when we exclude it.
5. In fftvour of the atmospheric engine, however, as com-
pared with the steam engine, there is a circumstance of
sufficient importance to keep this engine still in use in dis-
tricts where fuel is extremely cheap. In its construction
there is much greater simpUcity and cheapness, and less
liability to get out of order. The arrangements for passing
the top of the piston-rod through the top of the cylinder, so
as to be steam-tight, are unnecessary, as are also those for
parallel motion, and the valves for the admission and
emission of steam at the top of the cylinder. These advan-
tages, however, are but small, and will disappear every day
as- the cost of the construction of engines is diminished.
CHAP. XXV. — HOW THE STEAM-ENGINE IS CONSTRUCTED IN
CASES WHERE A CONDENSING AFPABATUS IS INADMISSIBLE.
1. It will be perceived that the advantages obtained
fy^ the vacuum produced by the condensation of steam are
ON THE STEAM ENGINE. 95
not without draw-backs. The machinery for condensation
is costly, bulky, and heavy, and moreover consumes a
considerable portion of the moving power in working it.
The condense requires a cistern of cold water, in which it is
submerged. This cistern must be kept constantly supplied
with cold water, for which purpose a pump, called the cold
icater ptimp, must be worked by the engiue. The water
and air admitted by the condensing jet must be contuiually
pumped out by the air-pump. In many cases the steam
engine is worked in situations in which a sufficient supply
of cdd water cannot be procured, and where the weight and
bulk of the condenser, air-pump, and cold water piunp,
would be inadmissible. In these cases, the power of the
steam must be worked without the advantage of the vacuum
on the other side of the piston. Engines thus constructed
are called non-condenamg engines, and sometimes, though not
with strict propriety, high-pressure engines. Steam haviug
a greater pressure than that of the atmosphere, being
admitted on one side of the piston, and the other side being
left in open commimication with the atmosphere, the piston
will be lu-ged forwards by a force proportional to the excess
of the steam pressiu*e above the pressure of the atmosphere,
the friction, and other resistances. When the piston is
thus drawn to the other end of the cylinder, the steam being
admitted on the opposite side of the piston, and the contrary
side being open to the atmosphere, the piston will in like
manner be urged back again.
2. Between the mechanism by which the admission and
emission of the steam is effected in this machinery, and that
which we have described in the condensing engine, there is
no real difference. "Whether the steam be allowed to
escape to the condenser, or into the open atmosphere, the
mechanism which governs its admission and escape will be
the same.
3. As the pressure of the steam in such machines must
necessarily exceed that of the atE[io«.T5\i«te, \e^ ^ ^soSj^sssss^
96 EUDIMENTABT TEEATISE
proportion to supply a force necessary for the purpose to
whicli the machine is applied, the pressure is always much
greater than is necessary where condensation is used ; and
hence the application of the term Mgh-pressure engines to
such machines ; but the use of the term is objectionable,
inasmuch as steam of an equally high pressure is often used
in engines in which the steam is condensed and a vacuum
produced. An example of this is presented in the engines
used in Cornwall, where steam haying a pressure of 50 lb.
or upwards on the square inch is used.
4. Properly speaking, therefore, highrpressure engines
consist of two classes ; those in which the steam is not
condensed, and those in which it is condensed.
6. The most proper classification of engines, therefore, is
into condensing and non-condensing engines ; the latter being
always high-pressure engines, and the former sometimes
high-pressure and sometimes low-pressure.
6. By low-pressure engines is to be understood those in
which the safety valve on the boiler is loaded at the rate of
4 to 6 lb. per square inch.
7. High-pressure engines is a term rather indefinite ; but
where the valve is loaded with 20 lb. or upwards per square
inch, the machine is generally so called.
8. In the United States, the use of high-pressure steam
is much more universal than in England, and 20 lb. upon a
square inch of the safety valve would hardly be denominated
high-pressure. This will be understood when it is stated
that from 120 to 150 lb. per square inch is not a very
uncommon pressure to use.
9. In locomotive engines, the condensing apparatus is
excluded for obvious reasons. The pressure in these is
usually from 50 to 60 lb. per square inch. The steam
which escapes from the cylinder, after working the engine,
is ejected up the chimney, where it plays the part of a
blower, and supplies that want of elevation of the chimney
wlu'cli circumstances here exclude.
ON THE STEAM ENGINB. 97
CHAP. XrVl. — HOW THE MECHANICAL PRESSURE OP THE STEAM
ON THE PISTON IS LIMITED,
PISTON IS APPECTED BY THIS.
ON THE PISTON IS LIMITED, AND HOW THE SPEED OP THE
It is commonly but erroneously supposed that the
pressure which the steam exerts on the piston of an engine
can be augmented or diminished at pleasure by augmenting
OP diminishing the pressure of the steam in the boiler.
A moment's attention to some universal principles of
mechanical science will be sufficient to rectify this error.
It is an established principle, that when a body which
offers a definite resistance to motion is impelled by a force
whose pressure is precisely equal to that resistance, the body
so acted upon must be in one of two states, viz., either at
rest, or moving with an uniform velocity.
This principle is convertible. A state of rest or of
uniform motion presumes that the body in such state must
be acted upon by forces in equilihrioy — that is to say, if it be
in motion, the energy of the forces which impel it must be
precisely equivalent to the resistance which it offers to
them.
To illustrate this by a practical example, let us suppose
that a carriage placed on an uniform and level road is
drawn by a horse at a perfectly uniform speed. The
resistance in this case which the carriage offers to the
draft is precisely equivalent to the force impressed by the
horse on the collar.
If an experimental proof of this be required, it may be
easily given. Let a carriage be placed on any level surface,
and drawn by a weight carried over a puUey. When its
motion is uniform, it will be found that the amount of the
weight which gives it such motion is precisely equal to the
resistance of the carriage.
But it will be asked, how can the energy of the impelling
forces be greater or less than the resistance, if the object to
which it is applied be in motion ? If it be ^e«fc^"t '^\ia3£^''^3w^
^
98 EUDIMENtART TREATISE
resistance, it cannot do more than move it ; if it be less than
the resistance, why does not the object stop altogether ?
Admitting that a moving force greater in amount than the
resistance of the body moved can be applied, it may be
further asked, what becomes of the surplus of such moving
force ? It is clear that the resistance cannot absorb more
than its own amount of the moving force : on what, then, is
the surplus expended ?
Let the simple and familiar example of a carriage moved
on a level road be taken. Let us suppose that the force
exercised on the carriage is 150 lbs., while the resistance of
the carriage to the moving power is only 100 lbs. On what
object, then, are the other 50 lbs. expended ?
The answer to this is extremely simple, and easily under-
stood. AYhen the moving force is thus greater in intensity
than the resistance, the motion imparted to the body to
which it is applied is not, as above, an uniform speed, but a
speed constantly accelerated : in every succeeding second of
time, the moving force imparts to the body an increased
velocity, and consequently an increased momentum. It is
by this augmentation of momentiun, then, that the surplus
moving force is absorbed. It is, therefore, a living force.
It is not, properly speaking, extinguished, as is that portion
of the moving force which is in equilibrio with the resistance.
The momentum which it produces in the moving body will
be retained and expended upon something before the moving
body can come to a state of rest.
Accelerated motion is, then, the consequence of the
moving force exceeding in amount the resistance of the
body moved.
Analogy will at once raise the pre.sumption, that a gradu-
ally retarded motion will be the consequence of the moving
forpe being less in intensity than the resistance of the body
moved.
The moving force in this case balances, or as it were
extinguishes, so much of the resistance as is ec^ual to its
ON TUB STEAM ENGINE. 99
intensity ; tbe excess of the resistance, liowever, remains to
be accounted for. WLat is its effect, and. what becomes of
it ? AVe suppose the body to be already in motion ; its
weight or mass has therefore a certain momentum, which,
by the common properties of matter, gives it a tendency to
continue in motion. This tendency is opposed by that
portion of the resistance which is not balanced by the
moving force. This portion of the, resistance, then, gradu-
ally robs the moving body of its momentum, makes it move
more and more slowly, and at length, extinguishing all the
momentum, brings the body to a state of rest.
Thus it will be clearly imderstood that any inequality
between the intensity of tlie pressure, or traction, or impul-
sion, by whichever term the moving force be designated, and
the intensity of the resistance, will be attended with an acce-
lerated or retarded motion in the body moved, according as
the excess lies on the side of the moving power or on the
side of the resistance.
There is nothing new in these principles. Tliey are, in
fact, the established principles of general mechanics, per-
fectly fiuniliar to all who have cultivated the higher depart-
ments of science.
It would, however, certainly appear from the common
language and modes of calculation and reasoning which have
prevailed among engineers and practical men, that they have
either lost sight of these principles, or never known them.
Let us apply them to the case of a steam engine.
The piston is in this case the body moved. The boiler is the
source of the moving power. To simplify the case, we shall
imagine the motion of the piston to take place constantly in
one direction, instead of being reciprocated from end to end
of the cylinder.
Now it follows, from what has just been explained, that if*
the motion of the piston in the cylinder be uniform, the pres-
sure of the steam which impels it cannot by any mechanical
possibility be different from the amoxxnfe ol ^^ ^^'sss^jiiiS!^^^
100 EUDIMENTAET TEEATISE
wluch the piston offers : you may load the safety valve as
you please ; you may vary the condition of the boiler in any
imaginable manner, and the pressure of the steam in that
vessel may have any intensity whatever ; but it is demon-
strably certain that the pressure of the steam in the cylinder
cannot be either greater or less than such as would be neces-
sary on the entire surface of the piston to produce an action
equal to its resistance. This is as certain as the conclusion
of any problem in common Geometry.
But then, it may be objected, we can have no power to
vary the pressure of the steam in the boiler, inasmuch as the
resistance of the piston has no connection with the source of
the moving power.
I have explained in a former chapter that the pressure of
steam in the boiler, though it can never be less than the
pressure of steam in the cylinder, may be to any desired
extent greater; — the action of the throttle- valve explains
this: the more the throttle-valve is contracted, and the
smaller the orifice through which the steam has to pass into
the cylinder, the greater will be the ratio of its pressure in
the boiler to its pressure in the cylinder. There is, then, a
minor limit to the pressure of steam in the boiler. It cannot
be less than such a pressure as would produce on the piston
an action equal to its resistance.
What is, on the other hand, the major limit of the pressure
of steam in the boiler ? This limit is obviously determiaed
by the load on the safety valve: when the steam exceeds
this limit, the safety valve will be opened, and the surplus
pressure reduced by escape.
It thus appears that the piston and the safety valve supply
the two limits of the possible pressure of steam in the boiler.
The pressure per square inch of the steam in the boiler
cannot be less than the resistance per square inch of the
piston, nor greater than the pressure per square inch on the
safety valve.
Jn the ordinary action of an engme, t\ie motloii must in
ON THE STEAM EITGINE. 101
the maiii be uniform. Acceleration or retardation are con-
ditions exceptional and occasional. When the piston is first
put in motion from .a state of rest, its motion is accelerated
until it has attweii its normal and regular speed : when
the engine is about- 16 be stopped, its motion is gradually
retarded until the resisiance extinguishes the momentum of
the machinery. ' yyy
When the piston and -other reciprocating parts of the
machinery change the direotiok of their motion at each
extremity of the stroke, they ,will be for a short interval,
before and after the moment the direction changes, retarded
and accelerated ; and this retardation and acceleration would
be very perceptible, were it not for the fly-wheel : but the
momentum of the fly-wheel, as well in' consequence of its
weight as of the velocity of the matter formjng its rim, so
prodigiously exceeds the momentum of the. -reciprocating
parts of the machinery, that the effect of acceieratioji and
retardation in the latter is altogether effaced by the -great
momentum of the revolving mass of the former.
It is for this reason that the fly-wheel justifies us prac-
tically in our reasoning in assiuning the piston as moving
uniformly and constantly in one direction, instead of
reciprocating.
When the steam is used expansively, being cut off at one-
half, or any other fraction of the stroke, the impelling power
necessarily varies in intensity ; and as the resistance does
not vary in intensity, or at least does not vary in the same
manner and proportion, there will consequently not be an
equilibrium between the moving power and the resistance,
and the motion therefore cannot be uniform.
When steam is thus applied, the pressure, when first
admitted on the piston, is greater than the resistance;
and so long as the steam valve is open, the motion of the
piston will be accelerated. When it is closed, and the steam
begins to expand, it gradually diminishes in intensity. The
accelerated motion of the piston ^^,\io^^Net,^<i'Q^cffikS\fcNia^^
I
102 BUDIMENTABT TBEATISE
the pressure of the steam becomes equal to the resistance.
Purther expansion rendering it less powerful than the resist-
ance, the motion of the piston will be retarded to the end of
the stroke. / ; ;
This series of effects is repeated ^t-^acli stroke of the
piston. , \ \' '
Now although in this case the^jjrtion of the piston during
any one stroke is variable, yet ^iJj^ average motion of the
machine will be uniform: "iiljbough throughout a single
stroke the piston be alterfl^fely accelerated and retarded,
yet the number of strbfes 'performed by the machine per
minute will be the* same. The average velocity will be
uniform, although: Jhe Velocity within the limit of a single
stroke be not ^6^ • - '
But even tbj^ variation within the limits of each stroke is
almost effa6eS4)y the action of the fly-wheel, which absorbs
the acceleiwtion and repairs the retardation by giving and
takii% Wiomentum, as already described.
I have spoken of the uniform velocity of the piston, which,
whether it be maintained in the literal sense of the term, or
only on the average, as estimated by the number of strokes
per minute, must in every case be the result of an equili-
brium between the average moving force of the steam and
the resistance of the machinery. But what, it may be asked,
determines the rate of this uniform speed ? What condi-
tions are they which can determine whether the piston shall
move 200 feet or 500 feet per minute ?
This is obviously determined by the rate at which the
boiler is capable of supplying steam of the requisite pres-
sure to the cylinder. Let the resistance on the piston be
estimated ; say that it is 20 lbs. per square inch of its
surface ; then the boiler must be capable of supplying steam
of 20 lbs. pressure per square inch, in such measure as to
enable the piston to move at the required speed.
Let us assume, for example, that the required speed is
200 feet per minute, or 12,000 feet per \io\a, wad tKat the
ON THE STEAM ENGINE. 103
area of the piston is 5 square feet; then, to enable the
piston to advance through 12,000 feet, a column of steam
must follow it, 12,000 feet in length and 5 square feet in its
section, which gives 60,000 cubic feet of steam. But steam
having the pressure of 20 lbs. per square inch bears to the
bulk of water which produces it the proportion of 1281 to 1 ;
therefore, if we divide 60,000 by 1281, we shall find the
number of cubic, feet of water which must be supplied in the
state of steam by the boiler to the .cylinder in an hour.
This division gives 47, very nearly. The boiler, there-
fore, must in this case evaporate 47 cubic feet of water per
hour, or, according to the conventional standard of boiler-
makers, be a boiler of 47 horse-power.
In general this calculation may be made by the aid of the
following Tables.
104
EUDIMENTABT TEEATISE
TABLE I.— AREAS OF PISTONS.
Dia.
Inch.
1
i
Area. Dia. Area. Dia. Area. Dia. Area. Dia. Area
I
?
i
i
i
i
i
i
5
i
i
i
I
*
i
6
i
i
f
i
Indies.
•785
'994
1-227
1-484
1-767
2-073
2-405
2-761
3-141
3-546
3-976
4*430
4-908
5-411
6-939
6-491
7-068
7*669
8-295
8-946
9-621
10-320
11-044
11-793
12-566
13-364
14-186
15-033
15-904
16-800
17-720
18-665
19-635
20-629
21-647
22-690
23-768
24-850
25-967
27-108
28-274
29-464
30-679
31-919
33-183
34-471
Inches.
38-484
39-871
41-282
42-718
44-178
45-663
47-173
48-707
60-266
61-848
53-456
66-088
56-746
58-426
60-132
61-862
63-617
65-396
67-200
69-029
70-882
72-759
74-662
76-688
78-640
80-515
82-516
84-640
86-590
88-664
90-762
92-886
95-033
97-205
99-402
101-62
103-86
106-13
108-43
110-76
118-09
115-46
117-85
120-27
122-71
126-18
127-67
i /3r'122ll i llSO'19
Inches.
132-73
135-29
137-88
140-50
143-13
145-80
148-48
151-20
163-93
166-69
159-48
162-29
165-13
167-98
170-87
173-78
176-71
179-67
182-65
185-66
188-69
191-74
194-82
197-93
201-06
204-21
207-39
210-69
213-82
217-07
220-36
223-65
226-98
230-33
233-70
237-10
240-62
243-97
247-46
260-94
254-46
258-01
261-68
265-18
268-80
272-44
276-11
279-81
Inches.
283-52
287-27
291-03
294-83
298-64
302-48
306-36
310-24
314-16
318-09
322-06
326-05
330-06
334-10
33816
342-25
346-36
350-49
354-65
358-84
363-05
367-28
371-54
376-82
380-13
384-46
388-82
393-20
397-60
402-03
406-49
410-97
415-47
420-00
424-65
429-13
433-73
438-36
443-01
447-69
452-39
457-11
461-86
466-63
471-43
476-26
481-10
Inches.
490-87
496-79
600-74
606-71
610-70
615-72
620-76
626-83
630-93
536-04
541-18
646-36
661-54
556-76
662-00
667-26
672-66
577-87
583-20
588-67
693-95
699-37
604-80
610-26
616-75
621-26
626-79
632-36
637-94
643-64
649-18
654-83
660-62
666-22
671-95
677-71
683-49
689-29
695-12
700-98
706-86
712-76
718-69
724-64
730-61
736-61
742-64
<5 THE BTXAM EKQINi;.
Table I.— CoDtinueil.
Di.
A«.
1,,,
A^a.
>,i.
A.,.
DI
An^
.«..
"a™..
Inc
lllchH.
^
Irche.
li^
Inches.
^
lDCh»
liich.
rnehu.
SI
764'76
3
1075-2
43
1452-2
4i
' 1885-7
65
2375-8
760'86
10B2-4
1460-6
1895-3
2386-6
788-90
1089-r
1469-1
1006-0
3397-4
773-14
1097-1
U77-B
1914-7
2408-S
779-31
1104-4
1488-1
1B24-4
2419-2
786-61
14B4-7
1B34-1
3430-1
791-73
1119-2
1503-3
1943-9
2441-0
7B7-B7
1128-6
1511-9
1
1963-6
2462-0
3
810-54
3
1134-1
1U15
4
1620-5
1529-1
60
i
1963-S
1973-3
5
2463'D
2474-0
816-88
1149-0
1537-8
19S3-1
2485-0
823-21
1156-6
1548-5
1993-8
2496-1
829'67
li64-!
] 655-2
2002-9
2507-1
83S'97
1171-T
1564-0
2012-8
2S18-2
812-3B
1179-3
lS72-a
2022-8
2529-4
848-83
1186-9
1581-8
2032-8
2540-5
3!
855-30
3
1194-6
4
1590-4
5
2042-8
67
2651-7
861 -78
1202-2
1
1599-2
2052-8
2562-9
868-30
1S09-9
18081
2062-9
2574-1
874-84
1217-6
i
1817-0
207-J-O
2566-4
8SI-41
1225-4
1
1025-9
2083-0
2598-7
888-00
1233-1
j
1634-B
2003-2
il
2008-0
894-81
1240-9
1643-8
2103-3
1
2019-3
901-25
1248-7
lG5-i-a
211 3-6
1
2630-7
3J
907-92
914-81
4
1258-5
1284-5
4B
1661-9
1870-B
6
2123-7
2133-9
58
2642-0
2853-4
921-32
1272-3
1880-0
2141-1
2804-9
928-06
1280-3
2154-4
2876-3
934-82
941-80
948-41
966-26
1288-3
1296-2
1304-2
1312-2
1
1698-2
1707-3
1716-5
1725-7
2164-7
2175-0
21S5-4
2195-7
1
2887-8
2099-3
2710-8
2722-4
3
982-11
41
1320-2
4
1734-9
5
2208-1
5B
2733-9
i
968-90
1328-3
1744-1
h
2218-e
2746-5
976-90
1836-4
17S3-4
2227-0
2757-1
982-84
1344-5
17627
2237-5
2788-8
989-80
13E2-6
1773-0
2248-0
2780-6
996-78
1
1360-8
1781-3
2256-5
2792-2
1003-7
1389-0
17S0-7
228B-0
2803-9
1010-8
1
137T-2
1800-1
2279-8
2816-Q
S
j
1017-8
1024-9
42
k
1385-4
1393-7
*
1809-6
18180
2290-2
2300-8
60
1
2827-4
2339-2
]
1032-0
1401-9
1828-4
2311-4
i
2861-0
j
1089-1
UlO-2
1
1837-9
2322-1
1
2862-8
i046-8
H18-6
1847-4
2332-8
1
2874-7
1053-6
1428-9
j
1356 -B
2343-5
i
2886-6
1060-7
1435-3
1866-5
2SS4-1
Ift-Jft-^.
iogp9
■
1443-7
_i_
laltt-lW \\^?ftli-tt
\ \\1>*X^-^N
BUSIMESTARY TREATISE
Tiiblo I. — CoDtinuad.
.«..
A«,..
Dli
,™,
D...
i™.
Dia.
A„.
Di>
a™.
~
7,^
Incb.
Inchon,
r^
iDchci.
iMb
Incbe«.
riirti
Inctie..
Hi'
■:<.>-i-i-i
67
8525-6
73'
4185-3
79
4801-6
65
5674-6
■J 1^34 -4
3538-8
4199-7
4917-2
5691-2
■294a'4
3552-0
4214-1
4932-7
5707-9
295S'fl
3565-2
4228-5
49483
5724-6
2970-5
3678-4
^242-9
4963-9
6741-4
2382-6
3591-7
4257-3
4970-5
6768-2
2B34-T
860B-0
4271-8
4995-1
5775-0
3006-9
3618-3
4289-3
5010-8
6791-B
6
3019-0
3631-6
7
4300-a
8
5026-5
8
5808-8
3051-2
3645-0
4315-3
6042-2
5825-7
3043'4
3658-4
4329-9
6068-0
5842-6
S03:;-7
3071-8
4314-B
5073-7
5859-5
soar-9
86SS-2
4369-1
5089-5
6876-5
3030-2
3608-7
4373-8
510B-4
68B3-5
3092-5
371-2-2
4388-4
6121-2
5B10-6
3104-8
3725-7
4403-1
5137-1
B927-6
e3
3117-2
3129-fl
3142-0
3154-4
3106-9
3179-4
6
373B-2
3752-8
3786-4
3780-0
37B3-6
3807-3
7
4417-8
4432-5
4447-3
4492-1
4476-9
4491-8
8
6153-0
6168-9
6184-8
6200-8
6216-8
6232-8
S
5944-8
5981-7
6996-0
6013-2
8030-4
3191-9
3204-4
3821-0
3834-7
4506-9
4621-G
6248-8
5264-9
6047-6
6064-8
e
3216-9
7
3848-4
7
4536-4
8
6281-0
8
6082-1
3229-5
3862-2
4661-4
6297-1
0099-4
32121
3875-9
4566-8
6313-2
8116-7
J
3254-3
3889-8
4581-8
532B-4
6134-0
3267-4
3903-e
4590-3
6345-6
8151-4
3280-1
3917-4
4611-3
5381-8
9193-8
3282-8
3931-3
4636-4
6378-0
9166-2
3305-5
3945-2
4941-6
5394-3
6203-9
6
3318-3
3331-0
3343-3
'
3969-2
3973-1
3987-1
4966-6
4671-7
4686-9
8
5410-6
G428-9
6443-2
8
6221-1
6238-6
8260-1
3358-7
*00l-l
4702-1
6469-6
6273-6
3369-G
1016-1
4717-3
6476-0
6281-2
1
3362-4
4029-2
4733-5
5*82-4
8308-3
1
3395-3
4043-2
4747-7
5508-8
1
340S-2
I
4067-3
4763-0
5525-3
6344-0
66
i
1
3421-2
3434-1
3447-1
3460-1
72
1
4071-5
4085-8
409B-8
4114-0
'
4778-3
47937
480B-0
4824-4
8
6641-7
5668-2
5674-8
5691-3
9
63B1-7
6379-4
6397-1
8414-3
I
34T3-2
1
4128-2
4889-8
5607-9
6432-6
1
3486-3
1
4142-5
4866-2
5624-5
8460-4
/
A
1.1J2-5 II
i
41S6-7
41 71-0
4870-T
48BS-1
\ 15641-1
i
9463-2
ON TU£ STEAM EKGIXE.
107
Table L — Continued.
Dla.
Inch.
91
J
I
92
JL
Area.
Inches.
6503-8
6621-7
6589-6
6557-6
6575-5
6593-5
6611-5
6629-5
6647-6
66657
6683-8
6701-9
6720-0
6738-2
6756-4
6776-4
Dia.
Area.
Inch.
93
i
I
h
i
i
94
r
-
i
i
i
i
J
Tnclies.
6792-9
6811-1
6829-4
6847-8
6866-1
6884-5
6902-9
6921 -3
6939-7
6968-2
6976-7
6995-2
7013-8
7032-3
7050-9
7069-5
Dia.
Inch.
95
i
9(5
i
i
Area.
1
Dia.
Inch.
Inches.
7088-2
97
7106-9
k
7125-5
i
7144-3
i
7163-0
*
7181-8
1
7200-5
i
7219-4
I
7238-2
98
7257-1
\
7275-9
7294-9
j}
7313-8
i
7332-8
J
7351-7
i
7370-7
i
Area.
Inches.
7389-8
7408-8
7427-9
7447-0
7466-2
7485-3
7504-5
7523-7
7542-9
7562-2
7581-5
7600-8
7620-1
7639-4
7658-8
Dia. Area, i
Inches.
7697-7
7717-1
7736-6
7756-1
7775-6
7795-2
7814-7
7834-3
7854-0
By this Table, when the number of inches in the diameter
of the piston is known, the number of square inches in its
area can be found on inspection.
Question I. — Oivea the diameter of the piston in inches, to find its
area in sqitare feet.
Rule 1. — ^Find in Table I. the number of square inches in the area.
Divide the number thus found by 144. The quotient will be the area
of the piston in square feet.
ExAMFLK — To find the area of a piston in square feet whose diameter
is 86} inches.
By Table I. we find that the area in square inches is 5910-5. Dividing
this by 144 we obtain
144 ^5910-5
41-04
which is the area in square feet.
Question II. — Given the diameter of the piston in inches, and its
speed in feet per minute, to find the number of cubic feet of
steam per hour which passes through the cj/linder.
Bulb 2. — ^By Rule 1, find the area of the piston in square feet. Mul-
tiply this by the speed of the piston in feet per minute, and the product
will be the number of cubic feet of steam which passes through the
cylinder per minute. Multiply this last by 60, and the product is the
number ofcuhio feet per hour.
108
EUDIMENTAET TEEATISE
Example. — ^A 50-inch piston moves at the rate of 180 feet per minute.
What number of cubic feet of steam per hour passes through the
cylinder ?
By Rule 1, we find the area of the piston to be 17*36 square feet.
Multiply this by 180 :
17-36
180
3124-80
60
187488-00
which is the number of cubic feet of steam per hour which passes
through the cylinder.
In the following table is given, in the 1st column, the total
pressure of steam in pounds per square inch ; in the 2nd
column, the corresponding temperature ; in the 3rd column,
the number of cubic inches of steam, which would be produced
by one cubic inch of water ; and in the 4th column, the total
mechanical effect produced by the evaporation of a cubic inch
of water under the pressure expressed in the first column.
TABLE 11.
L
Total
Cubic Inches of
Mechanical Effect of a
Pressure in
Steam produced by
Cubic Inch of Wat^r
Pounds per
CorresT^ViAiTk^
a Cubic Inch of
evaporated in Pounds
Square Inch.
Temperature.
Water.
raised One Foot.
1
102-9
20868
1739
2
126-1
10874
1812
8
141-0
7437
1859
4
152-3
5685
1895
5
161-4
4617
1924
6
169-2
3897
1948
7
176-9
3376
1969
8
182-0
2983
1989
9
187-4
2674
2006
10
192-4
2426
2022
11
197-0
2221
2036
12
201-3
2050
2050
13
205-8
1904
2063
14
209-1
1778
2074
15
212-8
1669
2086
16
216-3
1578
2097
17
219-6
1488
2107
18
222-7
1411
2117
19
225-6
1343
2126
20
228-5
1281
2185
21 I
2S1'2
1225
^ 2144
22
233-8
1174
\ ^l^'iL
\
ON TUB STEAU ENGISE.
Table II.— ContiQued.
ToUl
Cubic InchB.=f
fiuim produced bj
CutilclDchDlWsMr
PoDDdrpe"
iCDbtclDoHof
Bquuc iDCb.
■Wabr.
rmised dm Tool.
23
236-3
1127
2160
U
2387
1084
2168
SS
241-0
1044
2175
SS
243'3
1007
2182
2T
246'5
973
2189
S8
247-6
941
2196
as
21B-6
911
2202
3Q
251-6
833
2209
SI
253-6
857
2216
SS
265-6
833
2221
SS
267'3
8H)
2226 1
SI
259-1
788
2232 1
SS
260-8
787
2238
S3
262-6
748
2243
37
264-3
729
2248
38
26S-9
712
2253
SB
267-5
695
2269
40
26B-1
679
2284
41
270-6
684
2268
4S
272-1
649
2273
43
273-6
635
2278
U
276-0
622
2282
4S
27B-4
610
2287
46
277-8
598
2291
47
279-3
688
2298
48
280-5
675
2300
49
2819
664
2304
51)
61
283-2
284-4
554
544
2308
2312
52
285-7
634
2318
63
286-9
625
2320
64
288-1
618
2324
66
60S
2327
60
290-6
600
2331
67
291 -r
4B2
2335
58
292-9
484
2339
68
294-2
477
2343
296-8
29Q-9
470
463
2847
2351
62
298-1
466
2356
63
299-2
449
2368
64
SOO-3
443
3362
BG
301-3
437
3385
68
302-4
431
2389
67
303-4
425
2372
68
304-4
4ia
I ^W^
«fl
306-4
4U
\ '^l'*. \
110
BUDIMENTAET TBEATISE
Table XL — Continued.
Total
m
Cubic Inches of
Mechanical Effect of a
Pressure in
Steam produced by
Cubic Inch of Water
Pounds per
Corresponding
a Cubic Inch of
evaporated in Pounds
Square Inch.
Temperature.
Water.
raised One Foot.
70
306-4
408
2382
71
307-4
403
2385
72
308-4
398
2388
73
309-3
393
2391
74
310-3
: 388
2394
75
311-2
383
2397
. 76
312-2
379
2400
77
3131
374
2403
78
i 314-0
370
2405
79
314-9
366
2408
80
315-8
362
2411
81
316-7
358
2414
82
317-6
354
2417
83
318-4
350
2419
84
319-3
346
2422
85
320-1
342
2426
86
321-0
339
2427
87
321-8
336
2430
88
322-6
' 332
2432
89
323-5
328
2435
90
324-3
325
2438
91
325-1
322
2440
92
325-9
319
2443
93
326-7
316
2445
94
327-5
313
2448
95
328-2
310
2450
96
329-0
307
2453
97
329-8
304
2455
98
330-5
301
2467
99
331-3
298
2460
100
332-0
295
2462
110
339-2
271
2486
120
346-8
251
2507
130
362-1
233
2527
140
357-9
218
2646
150
363-4
205
2661
160
368-7
193
2677
170
373-6
183
2693
180
378-4
174
2608
190
382-9
166
2622
200
387-3
168
2636
210
391-5
161
2650
220
395-5
145
2663
230
399-4
140
2676
240
403-1
134
2687
/ /
ox THE STEAM ENGINE. Ill
Haying these tables before us, we shall be enabled to solve,
by the common principles of arithmetic, a multitude of prac-
tical problems of considerable utility, the investigation of
which will further illustrate and familiarise the principles
which have been delivered in general terms throughout this
volume.
By the power of a boiler, I would be understood to "mean,
in what follows, the number of cubic feet of water which the
boiler would evaporate per hour in regular operation.
By the speed of the piston, I mean to express the average
number of feet per minute through which the piston is moved.
The engine being understood to be in regular and uniform
operation, the total resistance of the piston will be equal to
the total pressure of the steam upon it ; and the resistance
of the piston per square inch of surface will therefore be
equal to the pressure of the steam in the cylinder per square
inch of surface. These terms, therefore, may be taken as
synonymous. In general, the term pressure of steam is
understood to mean pressure per square inch.
The 3rd column in Table II., which is given as expressing
the number of cubic inches of steam of a given pressiu:^ pro-
duced by the evaporation of a cubic inch of water, will
equally express the number of cubic feet of steam produced
by a cubic foot of water, or, in general, the ratio of the volume
cf steam to the volume of water from which is it produced.
Question III. — Given the power of the boiler , the pressure of the
steam in the cylinder, and the speed of the piston, to find the
diameter.
Rule 3. — In the first column of Table II. find the given pressure ;
the corresponding number in the third column is the ratio of the
volume of such steam to the volume of water which produced it.
Multiply the power of the boiler by such number, and the product will
be the number of cubic feet of steam per hour which passes through
the cylinder, which, divided by 60, gives the number of cubic feet per
minute which passes through the cylinder. Divide this by the speed
of the piston expressed in feet per minute, and the cijiotient will be
the area of the piston expressed in square tee\i. . IJLxiX'Cv^i *0Ks&\r5\>A.,
112 BUDIMEKTAET TREATISE
and the product will be the area of the piston expressed in square
inches. Find this number, or the nearest to it, in the second column
of Table I., and the corresponding number in the first column will be
the diameter of the piston in inches.
Example. — A boiler evaporates 55 cubic feet of water per hour.
The pressure of steam in the cylinder is 20 lbs. per square inch.
What must be the diameter of the cylinder, so as to give the piston a
speed of 200 feet per minute ?
By reference to the first column of Table 11., we find, opposite the
pressure of 201bs. in the first column, 1281 in the third column.
Multiply 1281 by 55 :
1281
55
Divide this by 60 :
Divide this by 200 :
Multiply this by 144 :
70455
60 J 70455
1174-25
200 ;il74'25
5-8712
5-8712
144
845-4528
In the second column of Table I. we find 842-39 opposite 32| in. or
32^ in., and 84883 opposite 32} or 32^
If, then, we take a mean between these, we may assume the diameter
of the cylinder required to be 32]J inches.
Question IV. — Given the diameter of the piston in inches, the
total resistance it opposes to the moving power, and its speed,
to find the power of the boiler.
Rule 4. — ^Find in the first column of Table I. the given diameter.
The corresponding number in the second column will be the area in
square inches. Divide the total resistance of the piston by this number,
and the quotient will be the resistance per square inch, or the pressure
of the steam. Find this pressure in the first column of Table II., and
the corresponding nimiber in the third colunm will be the ratio of the
volume of steam to the volume of water which produces it The volimie
of steam will be found by Rule 2. Let this column be divided by the
number obtained as above from Table II., and the quotient will be the
power of the boiler.
JSxAMPLK — It is required to find how many c>3iV>\c ieeV. ot -skater per
ON THE STEAM ENGINE. " 113
hour the boiler must eyaporate to drive a piston of 34 inches diameter,
at the rate of 200 feet per minute, against a gross resistance of 18,000 lbs.
Opposite 34 in the first colunm of Table L we find in the second
column 907*92.
Divide 18,000 by 907*92 :
907*92 ^18000
19-8
Looking in the first column of Table IL, the nearest mmiber to 19 '8
IB 20, opposite to which, in the third column, we find 1281 :
By Bule 1, we find the area of the piston to be in sq^uare feet.
144 ; 907-92
6-305
By Rule 2, multiply this by 200 :
6-305
200
Multiply this by 60 :
Divide this by 1281 :
1261*
1261
60
75660
1281^75660
59-06
The boiler must therefore evaporate 59 cubic feet of water per hour.
Question V. — Given the power of the boiler, the diameter of the
piston and its speed, to find the pressure of steam upon the
piston, or, whaZ is the sam£, its resistance per square in^h.
Rule 6. — By Rules 1 and 2, find the number of cubic feet of steam
per hour which passes through the cylinder. Divide this by the power
of the boiler, and the quotient will be the number of cubic inches of
steam which would be produced by a cubic inch of water. Find this
number, or the nearest to it, in the third column of Table II., aud the
corresponding number in the first column will be the pressure of steam
in the cylinder, or the resistance of the piston per square inch.
Example. — ^What total resistance per square inch will a 35-inch piston,
supplied by a boiler evaporating 55 cubic feet an hour, drive at the
rate of 200 feet per minute ?
In Rules 1 and 2, we find the number of cubic feet which pass through
the cylinder as follows : the diameter of the piston being 35 inches, we
find by Table I. that its area is 962-11 square inches ; and by Rule 1,
that this is equal to 6*68 square feet. Multiplying this by 200, by
Rule 2, it gives the product 1336, which, multiplied by 60, gives 80*160
as the number of cubic feet of steam whicla. -ipaa&e^^iJMaNX.^*^^ ^s^^ocaSksst
per hour. Divide this by 66, and we fiiid t\ie c^o^AfioJ^'^^^'^V V^^^^^aasj
' 114 BUDIMENTAEY TREATISE
in the third column of Table II., we find the number 1488 opposite 17,
and 1411 opposite 18. Taking a mean between which, we may assume
the required pressure to be 17^ lbs. per square inch.
Question VI. — Given the power of the boiler, the pressure of steam
in the cylinder, arid the diameter of the piston, to find its speed.
Rule 6. — In the first coliunn of Table II. find the given resistance or
pressure : the corresponding number in the third column, multiplied
by the power of the boiler, will give the number of cubic feet of steam
per hour which passes through the cylinder. Divide this by the area
of the piston in square feet, found by Rule 1, and the quotient will be
the speed of the piston in feet per hour, which, divided by 60, will be
the speed of the piston.
Example. — With what speed will a 35-inch piston be driven against
a resistance of 20 lbs. per square inch by a boiler which evaporates 56
cubic feet of water per hour ?
Opposite to 20 in the first column of Table II. we find, in the third
column, 1281. Multiply this by 66 :
1281
56
71736
By Rule I, we find that the area of the piston in square feet is 6 '68.
Divide 71736 by 668 :
6-68^)71736
10739
Divide this by 60, and the quotient, 179, very nearly, will be the
speed of the piston.
CHAP. XXVIL — ILLUSTRATIONS.
The following diagrams and descriptions of the principal
parts of steam engines, which have been explained in general
terms in the preceding chapters, will render the principles
which govern the operation and structure of these machines
still more clearly and easily understood.
MERCURIAL STEAM GAUGE FOR LOW-PRESSURE BOILERS.
In the following figure this instrument is represented :
<? is a tube leading from that part of the boiler within which
steam is contained ; d a stop-cock to open or • close the
communication at pleasure ; M 5 w is a siphon tube of iron,
which extends to a lieight sufficiently great iot «>» coYxjcam. o^
^GTcuiyr representing the pressure of steam mt\v€i\>cf^«t.
OS TCE STEAM EXOISE.
At M m ore two Bmall apertureB,
stopped by screws, ivhicli can be
opened or closed at pleasure. The
tube is filled through an opening at b
until the mercury shall flow from the
holes U m. The opening it is thcu
cloBed as well as the apertures k m,
a tsmsil quantity of water having been
preriously let in through the opening
B, on the sur&ce of the mercurj' at m,
A. fioat is placed upon the mercury in
the longer leg of the siphou, from
which a string is carried over the
' pulley^, towhichasmall index (s)ia
attached, which plays upon a divided
scale.
Let us now suppose the stop-cock
d opened, steam will flow from the
boiler and press upon the fluid in o.
The column of mercury in the leg u b
will be pressed down to some point,
such as le, and the column in the
longer leg of the siphou wUl be raised
to a point x, as much above in as a: iu
the short leg is below m.
Ab the mercury in the long leg
rises, it will raise the float, the coun-
terpoise of which (s) will of course
descend, and the scale is so adjusted
that it indicates the height of the
column of mercury from x in the
short leg to a; iu the long leg, which
column balances the pressure of
steam in the boiler, or more correctly
speaking, it balances the excess of
the pressure of the steam in t\\e
116
EUDIMENTAKT TKEATIBE
boiler above the atmoapbere ; in fact, the atmosphere, presa-
iog through the open mouth of the tube upon the mercury
in the longer leg, combines with the column of mercury a: x
in balancing the pressure of steam in the boiler. If, then,
2 inches of mercury be taken to express a pound per square
inch, to which it is very nearly eqaal, such gauge wiD at once
indicate the number of pounds per square inch by which
the pressure of the steam in the boiler exceeds that of the
atmc^phere.
UEROUBIAL STBAH OAUfiE FOR HIQH-PRESSUBE BOUEKS.
In high-pressure boilers, a mercurial gauge of the form
^__^^ shown in the preceding figure would be incon-
f \ venient, owing to the great height of the
column of mercury which would be necessary.
In this case a gauge of another form is made
use of, an example of which is shown in the
annexed figure. Let a b be a cistern of mer-
cury ; let i be a glass tube, open at the lower
end and closed at the upper end, immersed in
the mercury, and containing air in its ordinary
state. When the stop-cock d is open, the
steam from the boUer rushes through the pas-
sage C, and pressing on the mercury in the cis-
tern, will raise a column of mercury in the
tube, by which the air in the tube will be com-
pressed. "When the air is compressed into
half its original bulk, its pressure will be
doubled ; when it ia compressed into one-third,
ita pressure will be increased in a three-fold pro-
portion, and so on. The pressure of the steam,
therefore will be measured by the space into
which it is able to compress the air in the tube.
"When great accuracy is required, a slight cor-
rection will have to be made for the column of
mercury sustained in the tube, t a lb. per square
JJici being added to the preaauie in^ca-tei \)7 'Ctie wno^Te^
OS THE BTXAH «MGINE.
rion of the air for erery inch of mercury sustained
tabe.
BAROUETEK QAUOE.
This gauge is constiructed in various forms.
annexed figure the cistern i. contains mercury ;
tiie barometer tube is immereed in it, and the top
of the tube, formed into a, siphon, communicates
with the condenser ; a stop-cock p being placed
between them, bo as to open or close the com-
mnidcation at pleasure.
EIFHOir BAROMETER OAOOE.
The following figure is another form, in which
the barometer is a siphon, like the steam gauge.
The tnbe and stop-oock P communicate with the
condenser, and the other leg of the siphon is open
to the atmosphere. A hole, stopped by the screw q,
is placed in one of the legs : mercury being poured
in at the other leg, the siphon is filled until the
mercury begins to flow from the hole q. The
fluid then will stand at the same level in both
legs. The hole q being then stopped, and the
slop-cock p opened, the upper part p q of the tube
filled with the uncondensed vapour of the
condenser, which will of course press upon _
the column of mercury in the siphon.
The other leg of the siphon x, being open
to the atmosphere, will he subject to the
atmospheric pressure ; and the column of
mercuiy in the leg p Q, which is above the
level X, will represent the excess of the pres-
sure of the atmosphere above the pressure of
the uncondensed steam, which is the indi-
cation the barometer gauge is required to
give.
This siphon being made of iron, a ftont ia
placed on the mercury at a:, ba.\mg a xoi.
lis BUSIMZNTABY TBEATISE
at the top of which ia an iudes, which plays upon a scale so
graduated aa to eipresa the difference of level of the mercury
in the two legs of the siphon.
GLASS WATER OAUOE.
lu the annexed figure is represented the glass water gauge
described in the test. Its communica-
tions with the boiler are opened and
closed at pleasure by the cocks r. "When
the cocks r are both open, the upper
end of the tube a ia iu free communica^
tiou with the upper part of the boiler
where steam ia contained, and the lower
end of the tube a is in communication
with the lower part of the boiler where
water ia coutmned.
Water enters below and steam sbo^-e,
and as the pressure in the gauge tube is
the same as the pressure in the boiler,
the leyel of the water in the tube will be
the same aa the level of the water in the
boiler. At the bottom of the tube is
placed a stop-cock «, for the occasional discharge of water
from the tube.
THE SPRING SAFETY VALVE FOR niail-PRESSDRB
02f THE STEAM ENGINE. 119
In the preceding figure is represented the safety valve, as
used in high pressure engines. The conical valve is repre-
sented in its seat, its spindle 8 bemg pressed down at a bv
the lever b a c. c is a fixed pivot, on whicli the lever plays.
The pressure on the spindle of the valve at a is produced by
a nut at b, which presses that end of the lever downwards.
This nut works upon a screw, which screw is attached to a
spring balance l, the lower end of which is fimily attached
to a fixed point p. The nut at b, may be turned so as to
submit the valve to any pressure within the limit of the
action of the sprmg balance. As the nut is turned, the
spring becomes more and more compressed. An index and
scale are attached to the balance, the scale being so divided
as to express the number of pounds per square inch by
which the valve is pressed upon its seat. Thus, if the nut b
be turned until the index shows the pressure of 50 lbs., then
the force on the valve will be at the rate of 50 lbs. per square
inch, and the steam will be confined in the boiler until it
has attained such pressure : when the pressure exceeds that
limit, the lever at b will, by the action of the steam on the
valve, press the nut upwards with a force greater than the
energy of the spring, and the spring will consequently be
further compressed, the valve at the same time opening and
allowing the escape of the steam.
There is notlimg in the principle of this valve (essentially
different from the common safety valve, directly loaded with
a weight ; but in boilers where high-pressures are used, the
quantity of weight which it would be necessary to place on
the valve, would be inconvenient. A comparatively small
force, holding b downwards, will produce a multiplied eficct
at A, in the proportion of the length of the lever b c to a c.
Thus, if B c be 20 times a c, a force of 5 lbs. at b Avill produce
100 lbs. at A.
KTJBIMBFTABT TEKATISE
I rendered
watt's ikdicatob.
This little instrument, already deaeribed in the text,
I intelligible by the annexed dia-
gram ; fig, 1 representing a
front view in section, and
fig. 2 & side elevation. The
rod attached to the piston
plays through a collar at a.
At i ia a pencil holder. At
« is a screw hy which the in-
strument is inserted in a bole
proTided for it in the top of
the cylinder. At </ is a stop-
cock, by which a communica-
tion may be open or shut at
pleasure between the indicator
and the cylinder. The piaton-
rod of the indicator ia sur-
rounded by a apiral apring,
the lower eitremity of which
is attached to the piston,
"' ■ and the upper extremity to
a fixed piece a, containing the hole through which the
piston-rod plays. "When the piston rises, the spring ia
compressed ; and when it falla, the apring ia extended.
The spring is in equilibrio when the piston ia at the middle
of the cylinder, and the apace through which it rises
and feJla ia, from the known properties of thia apeciea of
spring, proportional to the force which presses the piston
upwards or downwards. When both extremities of the
cylinder are open to the atmoaphere, the apring is at rest,
and the piston in the middle of the cylinder; but when
steam ia allowed to paas from the cylinder to the indicator,
by opening the atop-cock d auch ateam will preas the piston
upwards, and eompreaa the spring with & force equal to the
exoeBB of the preBBvie of the eteam a^io^e ftiB>^ c& ft*
7
OK THE STEAM ENGINE. 121
atmosphere. "When, on the other hand, a vacuum is pro-
duced in the cylinder by tbe condensation of the steam, the
same vacuum will be produced under the piston in the
indicator, and the piston will be forced downwards by the
excess of the pressure of the atmosphere above that of the
uncondensed vapour in the cylinder.
If an index were placed near the extremity of the piston-
rod tf the pencil, ascending and descending on this index,
would indicate by the space through which it would ascend
the excess of the pressure of the steam over that of the
atmosphere, and by the space through which it would
descend, the excess of the pressure of the atmosphere over
that of the imcondensed vapour. Both spaces added to-
gether, or the entire play of the piston, would therefore
indicate the excess of the pressure of the steam above the
pressure of the uncondensed vapour which resists it, and
would therefore indicate the ejQfective force of the piston,
exclusive of friction.
But as the piston of the indicator would be in rapid and
continued motion, it would not be easy to observe and
record the limits of its play, and still more difl&cult to note
the rapidity of its motion. An ingenious expedient was
therefore contrived to enable the engine itself to record
these effects, which converted the indicator into a self-
registering instrument. A small square frame a b was
constructed, the breadth of which was somewhat greater
than the extreme play of the piston of the indicator. In it
was placed a card, capable of sliding in a horizontal
direction in grooves : a string e was fastened to the side of
the card, and, passing under a pulley, was carried upwards
towards 5, and attached to some part of the machinery which
rises and faUs with the piston of the engine. Another string f
was attached to the other side of the card, and carried over a
pulley and fixed to a small weight w. When the piston rises,
the string e is drawn to the left, the card drawn in the same
direction, and the weight w rises. Wheiitlaa '^l^to^Q.^aH^^^J^ife
weight w, actiag on the string /, dora'^^ >Jafe eajt^X^^^^'cs!^?:^'
122 BTJDIMENTABY TBEATISE
Thus, as the piston rises and falls, the card is drawn
alternately through a certain space left and right.
Let us now suppose steam admitted above the piston of
the engine, pressing the piston down; this steam presses
the piston *of the indicator up, and the pencil t, passing on
the card, would, if the card were at rest, mark upon it a
straight line, the length of which would indicate the
pressure of the steam ; but as the card is drawn from left
to right while the piston fells, the pencil will describe upon
it a curve by the combined effects of the vertical motion of
the pencil and the horizontal motion of the card. The
suddenness of the curvature thus described wiU indicate the
rapidity of the action of the steam on the piston.
"When the piston has reached the bottom of the cylinder,
and the upper exhausting valve is opened, a vacuum is
produced in the cyUnder, which the vacuum extends to the
indicator, the piston of which therefore descends, the pencil
t descending at the same time and at the same rate. While
this takes place, the card is moved from right to left, and a
corresponding curve described upon it by the pencil, the
curvature of which will indicate the suddenness with which
the vacuum is produced, as well as its degree of perfection.
Erom what has been stated, it will appear that in a single
ascent and descent of the piston, or in one stroke, as it is
technically called, a diagram will be formed upon the card,
which wiU exhibit not only the entire mechanical effect of
the steam acting on one side against the imcondensed vapour
on the other, but will show the entire character of its
progressive action at every point of the stroke. Such a
diagram is exhibited in the following figure. Let o x be a
horizontal line. Let o t be the vertical scale which
measures the pressure of the steam according to the move-
ment of the indicator. Let o be the level to which the
pencil would be depressed, if there were a perfect vacuum
in the cjlinder ; then the height of the pencil at any moment
above the level of the horizontal ^e ox^wV^Lmdiftate the
absolute presaure of the steam in the cy\miCT/\si^<&^eiTA«^^
on THE 8TEAU £MOim;.
of any consideration of the preaeure of the atmosphere. Let
A be the porition of the pencO at the moment steam ia
admitted above the piaton. By the action of the steam the
pencil will anddenly start up to b, and after the piston has
i its action, it will rise a little higher, the card
meanwhile being drawn to the left- The line will be traced
on the card by these means, as represented at b m m' and m".
Aa the piston approaches the bottom of the cylinder, if the
steun be cut off before the completion of the stroke, the
pressure will diminish, and from o to e the pencil will tall.
Ldi X be its position at the end of the stroke, the card being
imderstood to be moved from right to left through the space
p Q during the stroke. We may consider this motion of the
eard as representing the motion of the piston, with which it
is simultaneous and proportionate. At the commencement
of the stroke, the height a p of the pencil above o x repre-
sents the pressure of the uncondenaed vapour which was
then above the piston; the height b p represents the
pressure of the steam immediately on it« admiseion ; the
height m p represents its nearly uniform pressure throughout
the former half of the stroke ; and the decreasing height of
the curve from c to e, above the line o x, represents the
decreasing presaure of the steam throughout the remainder
of the stroke, e q represents the pressure of the steam at
the termination of the stroke.
The piston now commences its n/x^A- Toa -q-yS?* '^^"
baaatiog ralve being opened, ani ftie eXi^axb BSwaw*&.'wi^"«
124 BUDIMENTAET TEEATISE
to the condenser, according as it is condensed a vacuum is
formed while the piston is rising, and while the card is
moved back from left to right under the pencil. Starting
from E, the pencil begins to fall, and falls more and more as
the vacuum becomes more perfect. At g the vacuum attains
its most perfect state, and the line from G towards a con-
tinues nearly horizontal, its height above o x representing
the nearly uniform pressure of the uncondensed steam ;
but just before the termination of the stroke the steam is
admitted from the boiler, and the pencil rises to a. The
height of the curve e G a at every point represents the
varying pressure of the uncondensed vapour which resists
the ascent of the piston.
Now although that portion of the curve below the line a e
represents the state of the vacuum above the piston during
its ascent, it may be taken to represent the state of the
vacuum below the piston in its descent, for the same circum-
stances which affect one equally affect the other ; and we may
consider the diagram generally as representing not only the
pressure of the steam which urges the piston downwards, but
also that of the uncondensed vapour which resists its descent.
It appears then that the varying heights of the points of
the upper curve b o e represent the varying pressures on the
piston during its descent ; and the average pressure upon the
piston may be obtained by taking the average of these heights.
In like manner, the heights of the lower curve age may
be taken to represent the varying pressures or resistances of
the uncondensed vapour under the piston during its descent :
and the average of all these heights will give the average of
such resistances. If then we subtract the average of these
resistances, represented by the lower curve, from the average
of the pressures represented by the upper curve, we shall
o]?tain the effective pressure of the steam in urging the piston.
However accurately such an instrument as this may be
constructed, it must be admitted that it cannot be depended
on as affording any exact measure of t\ve po^et oi tVie -^iatoTi.
It^s chief value, as stated in the text, \a t\ie m^cftJcvssvx \^
ON THE BTEAU ENGnTE.
125
afibrds of the degree of perfection of the Tacuum and of the
soddennees of its formation. The curve £ a should fall to
its leant height speedily. It is not untU it attains its least
bedght that the vacuum has attained its greatest perfection.
7<nr the rest, the use of the instrument is Buffiinentlj
explained in the text.
In fig. 1 is represented
» Waggon boiler
•oction, and in fig. 2 the
same iu longitudinal sei"-
tion. The same lel.tera
mdicate the correspond-
ing parts in the two
drawings.
a is the grate sup-
porting the burning fuel;
b and b represent the
flue whiyh aurrounda tlie
boiler ; e e are the gaugc-
coe^ described in the
126
BtTDIUENT^BT TBEATISE
test ; s is the Bteam pipe which leads from the boiler to the
cylinder ; g p ia the safety-valve, the presaure upon which
may be regulated by the sliding weight ff.
The lever y p has a fixed pivot at p.
The spindle of tlie salve ia attached to
it at i ; A is a fork to keep the lever gj'
ill its position. The weight y produi.'es
an efiert at i, which ia multiplied by the
tevor in the proportion ofijp to iji : thus
if g p be 3 times ip, then 2 ib. sua-
pended at y will produt
6 lb. at I
The opening which appears
immediately above the valve
is the end of a discharge pipe
for conducting away the
steam which escapes from
the safety ralre. When the preaaure o£ ftie ateam.^B'iXift^iciiiei
Oy THE STEAM ENGTKE. 127
exceeds the pressure produced by the weight upon the safety
yalye, the latter will be raised, steam will escape around it,
and issue through the waste pipe. Sometimes this steam is
allowed to escape into the atmosphere, and sometimes it is
conducted into the cistern of water by which the boiler is
fed, where it is condensed, and has the effect of raising the
temperature of the water. By this means a portion of heat
which would have been otherwise wasted is carried back to
the boiler. The internal safety valve is represented at a: y z.
This valve presses at n within the boiler, and is drawn up
into its seat by the end of the lever z, y is the pivot which
supports the lever, and a weight suspended from x draws z
upwards. When a vacuum is produced within tlie boiler by
the condensation of the steam, the pressure of the external
atmosphere forces the valve u open, and the air enters and
fills the boiler.
The self-acting feeding apparatus is represented likiWuTc,
Ifc, — ^A tube I is attached to the top of the boiler, and
descends within it to a point below the level at which the
water should stand. The pressure of the steam within the
boiler, acting upon the water, supports a column of water in
this tube I : on the surface of this water at o rests a float,
sustained by a chain g', which passes over two pulleys repre-
sented in figure 1, and which, descending from the second, is
attached to a rod r, which supports the damper. This chain,
as it rises and falls, raises and lowers the damper, and opens
or closes, more or less, the flue across which the damper
passes.
When the pressure of steam in the boiler is unduly aug-
mented, the column of water it supports in I rises ; with it
rises the float o, and consequently the damper r falls, con-
tracts the flue, diminishes the draft, mitigates the intensity
of the fiimace, and renders the evaporation less rapid in the
boiler. When, on the other hand, the evaporation in the
boiler does not proceed fast enough, the pressure of the
steam in it is unduly diminished, and the column of water it
128 EUDIMENTAET TEEATISB
supports in the tube I is lowered : the float o falls, and the
damper r rises ; the opening of the flue is enlarged, the
draft increased, the Aimace stimulated, and the evaporation
augmented.
In this manner the varying demands of the engine on the
boiler are supplied bj the varying power of the furnace, the
wants of the engine producing the requisite effect on the
boiler.
The float m rests on the surface of the water within the
boiler; a wire sustaLoing it passes steam-tight through a
coUar in the top of the boiler, and is attached to the
extremity o of a lever which is balanced by a weight w at the
opposite end ; a rod is attached at u to this lever, which
descends to the bottom of the small hole in the hot water
cistern k, and is attached to a valve at the bottom of this
cistern which opens upwards. When u rises, this valve is
opened ; when it is pressed down, this valve is closed. The
cistern k is supplied by a small pump called the hot water
pwnp, which draws water fipom a reservoir which receives the
discharge of the condenser of the engine, as thrown out by
means of the air pump.
This water is thus pumped by the engine itself into
the cistern k, and a waste pipe is provided for the
discharge of so much of it as is not consumed by the
boiler.
When the water in the boiler begins to be exhausted, the
level falls, and with it the float m; this draws down the
extremity v of the lever, and raises u, by which the valve o is
opened, and the water from the cistern k allowed to descend
by the tube I ; and this continues until the level of the
water in the boiler is raised to the proper point : the float m
is raised with it, and the end v of the lever also raised, and
the valve o closed.
In fact, however, the effect produced is not that of opening
and closing the feeding valve o ; the latter becomes adjusted
in such a manner as to let a contiiiuoua stream from the
OS THE STEAM ENGllTE.
120
rastem i into the tube I, by which the level of the water
in the boiler is maintained at it« proper height.
AH theae arrangementa will be still more clearly undejv
Btood by means of the anneied drawing, which represents
the waggon boiler, with all its appendages, in perapectjve.
The grate and a part of the flues are rendered visible by the
removal of a portion of the masonry in which the boiler is
set. The interior of the boiler is also shown by cutting off
one-balfoftbe semi-cylindrical Toot.
I
BUDIMENTABY I'KEATISB
THE SLIDE TALTEB
In the annexed figures are represented the moat usual
forms of slide valvea.
Fig. 1 represents in section the cylinder, piston, and slide :
8 is the mouth of the steam pipe coming bom the boiler ;
e ia the pipe leading to the condenser ; t is the rod which
is attached to the slide, moving through a stuffing-box m n.
This slide is represented in longitudinal section, separately,
in Sg. d, and in transverse section in fig 4, In the position
of the Blide represented in fig. 1, the ateoin ^saWai^fecna tbe
ON THE STEAM ENGIITE. 131
boiler enters at s, and passes to the bottom of the cylinder
through the opening h, and acts below the piston causing it
to ascend. The steam which was above the piston escapes
through the opening at a, and descending through a longi-
tudinal opening in the slide behind the mouth of the steam-
pipe, finds its way to the pipe e, and through that to the
condenser.
When the piston has reached the top of the cylinder, the
slide will have been moved to the position represented in
fig. 2. The steam now entering at s passes through the
opening a into the cylinder above the piston, while the
steam which was below it escapes through the opening h
and the pipe e to the condenser.
The form of the valve from which it derives its name of
D-valve, is represented in fig. 4. The longitudinal opening
through which the steam descends, then appears in section
of a semicircular form. The packing at the back of the slide
is represented at k ; this is pressed agaiost the surface of
the valve box.
GENERAL ABRAXOEMENT OF THE DOUBLE-ACTING STEAM
ENGINE.
In the figure facing the title of this volume is represented
the * ensemble ' in section of a double-acting steam engine,
on the principle of Watt, as constructed by Mr. Fairbaim,
of Manchester.
s is the steam pipe leading from the boiler; o is the
cylinder ; b t t' t' is the parallel motion ; i" is the end of
the air pump rod attached to the parallel motion ; d is the
upper steam valve ; d' is the lower steam valve ; b b' are the
upper and lower valve boxes ; i is the air pump, the piston
being represented while descending, and the valves being
open ; k' is the feed pump, the plunger of which is driven by
a rod k" attached to the beam at t''. o is the centre of the
beam, f the point at which ttie coxai<bc\im%-^<A Ssa^ ^^"^k^J?^^
/
132 TBEATISE ON THE STEAM ENaOTE.
to it ; a is the crank pin ; J^ is the crank ; I is the jointed
arm, one end of which forms the eccentric, and the other end
works the valves. The fly-wheel is represented as toothed,
and is supposed to drive a pinion, p is a pillar wliich
supports the governor jt, which is connected by a series of
levers p' p" p'" with the throttle valve ; q mm' ia a part of
the machinery which transmits the action of the eccentric to
the valves.
THE END.
BBADBURT AMD RVAMS, PBIKTSBS, WHITEFBIAR8.
1
TJ 4e4.L3201B52
A nidlmantary traatlM m
Stanford UtitM
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