its level in A B, the atmospheric pressure will exceed that of the
steam, and the degree or quantity of the excess may be ascertained
exactly in the same way.
If the tube be glass, the difference of levels of the mercury would be
visible: but it is most commonly made of iron; and in order to
ascertain the level, a thin wooden rod with a float is inserted in the
open end of B C; so that the portion of the stick within the tube
indicates the distance of the level of the mercury from its mouth. A
bulb or cistern of mercury might be substituted for the leg A B, as in
the common barometer. This instrument is called the STEAM-GAUGE.
If the steam-gauge be used as a measure of the strength of the steam
which presses on the piston, it ought to be on the same side of the
throttle valve (which is regulated by the governor) as the cylinder;
for if it were on the same side of the throttle valve with the boiler,
it would not be affected by the changes which the steam may undergo in
passing through the throttle valve, when partially closed by the
agency of the governor.
(69.) The force with which the piston is pressed depends on two
things: 1º, the actual strength of the steam which presses on it; and
2º, on the actual strength of the vapour which resists it. For
although the vacuum produced by the method of separate condensation
be much more perfect than what had been produced in the atmospheric
engines, yet still some vapour of a small degree of elasticity is
found to be raised from the hot water in the bottom of the condenser
before it can be extracted by the air-pump. One of these pressures is
indicated by the steam-gauge already described; but still, before we
can estimate the force with which the piston descends, it is necessary
to ascertain the force of the vapour which remains uncondensed, and
resists the motion of the piston. Another gauge, called the
barometer-gauge, is provided for this purpose. A glass tube A B (fig.
39.), more than thirty inches long, and open at both ends, is placed
in an upright or vertical position, having the lower end B immersed in
a cistern of mercury C. To the upper end is attached a metal tube,
which communicates with the condenser, in which a constant vacuum, or
rather high degree of rarefaction, is sustained. The same vacuum must,
therefore, exist in the tube A B, above the level of the mercury; and
the atmospheric pressure on the surface of the mercury in the cistern
C will force the mercury up in the tube A B, until the column which is
suspended in it is equal to the difference between the atmospheric
pressure and the pressure of the uncondensed steam. The difference
between the column of mercury sustained in this instrument and in the
common barometer will determine the strength of the uncondensed steam,
allowing a force proportional to one pound per square inch for every
two inches of mercury in the difference of the two columns. In a
well-constructed engine, which is in good order, there is very little
difference between the altitude in the barometer-gauge and the common
barometer.
To compute the force with which the piston descends, thus becomes a
very simple arithmetical process. First ascertain the difference of
the levels of the mercury in the steam-gauge. This gives the excess of
the steam pressure above the atmospheric pressure. Then find the
height of the mercury in the barometer-gauge. This gives the excess
of the atmospheric pressure above the uncondensed steam. Hence, if
these two heights be added together, we shall obtain the excess of the
impelling force of the steam from the boiler on the one side of the
piston, above the resistance of the uncondensed steam on the other
side. This will give the effective impelling force. Now, if one pound
be allowed for every two inches of mercury in the two columns just
mentioned, we shall have the number of pounds of impelling pressure on
every square inch of the piston. Then if the number of square inches
in the section of the piston be found, and multiplied by the number of
pounds on each square inch, the whole effective force with which it
moves will be obtained.
In the computation of the power of the engine, however, all this
force, thus computed, is not to be allowed as the effective working
power. For it requires some force, and by no means an inconsiderable
portion, to move the engine itself, even when unloaded; all this,
therefore, which is spent in overcoming friction, &c. is to be left
out of account, and only the balance set down as the effective working
power.
From what we have stated, it appears that in order to estimate the
effective force with which the piston is urged, it is necessary to
refer to both the barometer and the steam-gauge. This double
computation may be obviated by making one gauge serve both purposes.
If the end C of the steam-gauge (fig. 38.) instead of communicating
with the atmosphere, were continued to the condenser, we should have
the pressure of the steam acting upon the mercury in the tube B A, and
the pressure of the uncondensed vapour which resists the piston acting
on the mercury in the tube B C. Hence the difference of the levels of
the mercury in the tubes will at once indicate the difference between
the force of the steam and that of the uncondensed vapour, which is
the effective force with which the piston is urged.
(70.) To secure the boiler from accidents arising from the steam
becoming too strong, a safety valve is used, similar to those
described in Papin's steam engine, loaded with a weight equal to the
strength which the steam is intended to have above the atmospheric
pressure; for it is found expedient, even in condensing engines, to
use the steam of a pressure somewhat above that of the atmosphere.
Besides this valve, another of the very opposite kind is sometimes
used. Upon stopping the engine, and extinguishing the fire, it is
found that the steam condensed within the boiler produces a vacuum; so
that the atmosphere, pressing on the external surface of the boiler,
has a tendency to crush it. To prevent this, a safety valve is
provided, which opens inwards, and which being forced open by the
atmospheric pressure when a vacuum is produced within, the air rushes
in, and a balance is obtained between the pressures within and
without.
(71.) We have already explained the manner in which the governor
regulates the supply of steam from the boiler to the cylinder,
proportioning the quantity to the work to be done, and thereby
sustaining a uniform motion. Since, then, the _consumption_ of steam
in the engine is subject to variation, owing to the various quantities
of work it may have to perform, it is evident that the PRODUCTION of
steam in the boiler should be subject to a proportional variation.
For, otherwise, one of two effects would ensue: the boiler would
either fail to supply the engine with steam, or steam would accumulate
in the boiler, from being produced in too great abundance, and would
escape at the safety valve, and thus be wasted. In order to vary the
production of steam in proportion to the demands of the engine, it is
necessary to increase or mitigate the furnace, as production is to be
augmented or diminished. To effect this by any attention on the part
of the engine-man would be impossible; but a most ingenious method has
been contrived, of making the boiler regulate itself in these
respects. Let T (fig. 40.) be a tube inserted in the top of the
boiler, and descending nearly to the bottom. The pressure of the
steam on the surface of the water in the boiler forces water up in the
tube T, until the difference of the levels is equal to the difference
between the pressure of the steam in the boiler and that of the
atmosphere. A weight F, half immersed in the water in the tube, is
suspended by a chain which passes over the wheels P P´, and is
balanced by a metal plate D, in the same manner as the float in fig.
32. is balanced by the weight A. The plate D passes through the mouth
of the flue E, as it issues finally from the boiler; so that when the
plate D falls, it stops the flue and thereby suspends the draught of
air through the furnace, mitigates the fire, and diminishes the
production of steam. If, on the contrary, the plate D be drawn up, the
draught is increased, the fire rendered more effective, and the
production of steam in the boiler stimulated. Now suppose that the
boiler is producing steam faster than the engine consumes it, either
because the load on the engine has been diminished, and therefore its
consumption of steam proportionally diminished, or because the fire
has become too intense. The consequence is, that the steam beginning
to accumulate, will press upon the surface of the water in the boiler
with increased force, and the water will rise in the tube T. The
weight F will therefore be lifted, and the plate D will descend, stop
the draught, mitigate the fire, and check the production of steam; and
it will continue so to do, until the production of steam becomes
exactly equal to the demands of the engine.
If, on the other hand, the production of steam be not equal to the
wants of the machine, either because of the increased load, or the
insufficiency of the fire, the steam in the boiler losing its
elasticity, the surface of the water rises, not sustaining a pressure
sufficient to keep it at its wonted level. Therefore, the surface in
the tube T falls, and the weight F falls, and the plate D rises. The
draught is thus increased, by opening the flue, and the fire rendered
more intense; and thus the production of steam is stimulated, until
it is sufficiently rapid for the purposes of the engine. This
apparatus is called the _self-acting damper_.
(72.) It has been proposed to connect this damper with the
safety-valve invented by the Chevalier Edelcrantz. A small brass
cylinder is fixed to the boiler, and is fitted with a piston which
moves in it, without much friction, and nearly steam-tight. The
cylinder is closed at top, having a hole through which the piston-rod
plays; so that the piston is thus prevented from being blown out of
the cylinder by the steam. The side of the cylinder is pierced with
small holes opening into the air, and placed at short distances above
each other. Let the piston be loaded with a weight proportional to the
pressure of the steam intended to be produced. When the steam has
acquired a sufficient elasticity, the piston will be lifted, and steam
will escape through the first hole. If the production of steam be not
too rapid, and that its pressure be not increasing, the piston will
remain suspended in this manner: but if it increase, the piston will
be raised above the second hole, and it will continue to rise until
the escape of the steam through the holes is sufficient to render the
weight of the piston a counterpoise for the steam. This safety valve
is particularly well adapted to cases where steam of an exactly
uniform pressure is required; for the pressure must necessarily be
always equal to the weight on the piston. Thus, suppose the section of
the piston be equal to a square inch; if it be loaded with 10 lbs.,
including its own weight, the steam which will sustain it in any
position in the cylinder, whether near the bottom or top, must always
be exactly equal in pressure to 10 lbs. per inch. In this respect it
resembles the quality already explained in the governor, and renders
the pressure of the steam uniform, exactly in the same manner as the
governor renders the velocity of the engine uniform.
(73.) The economy of fuel depends, in a great degree, on the
construction of the furnace, independently of the effects of the
arrangements we have already described.
The grate or fire-place of an ordinary furnace is placed under the
boiler; and the atmospheric air passing through the ignited fuel,
supplies sufficient oxygen to support a large volume of flame, which
is carried by the draught into a flue, which circulates twice or
oftener round the boiler, and in immediate contact with it, and
finally issues into the chimney. Through this flue the flame
circulates, so as to act on every part of the boiler near which the
flue passes; and it is frequently not until it passes into the
chimney, and sometimes not until it leaves the chimney, that it ceases
to exist in the state of flame.
The dense black smoke which is observed to issue from the chimneys of
furnaces is formed of a quantity of unconsumed fuel, and may be
therefore considered as so much fuel wasted. Besides this, in large
manufacturing towns, where a great number of furnaces are employed, it
is found that the quantity of smoke which thus becomes diffused
through the atmosphere, renders it pernicious to the health, and
destructive to the comforts of the inhabitants.
These circumstances have directed the attention of engineers to the
discovery of means whereby this smoke or wasted fuel may be consumed
for the use of the engine itself, or for whatever use the furnace may
be applied to. The most usual method of accomplishing this is by so
arranging matters that fuel in a state of high combustion, and,
therefore, producing no smoke, shall be always kept on that part of
the grate which is nearest to the mouth of the flue (and which we
shall call the back); by this means the smoke which arises from the
imperfectly ignited fuel which is nearer to the front of the grate
must pass over the surface of the red fuel, before it enters the flue,
and is thereby ignited, and passes in a state of flame into the flue.
A passage called the _feeding-mouth_ leads to the front of the grate,
and both this passage and the grate are generally inclined at a small
angle to the horizon, in order to facilitate the advance of the fuel
according as its combustion proceeds.
When fresh fuel for feeding the boiler is first introduced, it is
merely laid in the feeding-mouth. Here it is exposed to the action of
a part of the heat of the burning fuel on the grate, and undergoes, in
some degree, the process of coking. The door of the feeding-mouth is
furnished with small apertures for the admission of a stream of air,
which carries the smoke evolved by the coking of the fresh fuel over
the burning fuel on the grate, by which this smoke is ignited, and
becomes flame, and in this state enters the flue, and circulates round
the boiler. When the furnace is to be fed, the door of the
feeding-mouth is opened, and the fuel which had been laid in it, and
partially coked, is forced upon the front part of the grate. At first,
its combustion being imperfect, but proceeding rapidly, a dense black
smoke arises from it. The current of air from the open door through
the feeding-mouth carries this over the vividly burning fuel in the
back part of the grate, by which the smoke being ignited, passes in a
state of flame into the flue. When the furnace again requires feeding,
every part of this fuel will be in a state of active combustion, and
it is forced to the back part of the grate next the flue, preparatory
to the introduction of more fuel from the feeding-mouth.
The apertures in the door of the feeding-mouth are furnished with
covers, so that the quantity of air admitted through them can be
regulated by the workmen. The efficiency of these furnaces in a great
degree depends on the judicious admission of the air through the
feeding-mouth: for if less than the quantity necessary to support the
combustion of the fuel be admitted, a part of the smoke will remain
unconsumed; and if more than the proper quantity be admitted, it will
defeat the effects of the fuel by cooling the boiler. If the process
which we have just described be considered, it will not be difficult
to perceive the total impossibility in such a furnace of exactly
regulating the draught of air, so that too much shall not pass at one
time, and too little at another. When the door is open to introduce
fresh fuel into the feeding-mouth, and advance that which occupied it
upon the grate, the workman ceases to have any control whatever over
the draught of air; and even at other times when the door is closed,
his discretion and attention cannot be depended on. The consequence
is, that with these defects the proprietors of steam engines found,
that in the place of economising the fuel, the use of these furnaces
entailed on them such an increased expense that they were generally
obliged to lay them aside.
(74.) Mr. Brunton of Birmingham having turned his attention to the
subject, has produced a furnace which seems to be free from the
objections against those we have just mentioned. The advantages of his
contrivance, as stated by himself, are as follow:--
"First, I put the coal upon the grate by small quantities, and at very
short intervals, say every two or three seconds. 2dly, I so dispose of
the coals upon the grate, that the smoke evolved must pass over that
part of the grate upon which the coal is in full combustion, and is
thereby consumed. 3dly, As the introduction of coal is uniform in
short spaces of time, the introduction of air is also uniform, and
requires no attention from the fireman.
[Illustration: Pl. VIII.]
"As it respects economy: 1st, The coal is put upon the fire by an
apparatus driven by the engine, and so contrived that the quantity of
coal is proportioned to the quantity of work which the engine is
performing, and the quantity of air admitted to consume the smoke is
regulated in the same manner. 2dly, The fire-door is never opened,
excepting to clean the fire; the boiler of course is not exposed to
that continual irregularity of temperature which is unavoidable in the
common furnace, and which is found exceedingly injurious to boilers.
3dly, The only attention required is to fill the coal receiver, every
two or three hours, and clean the fire when necessary. 4thly, The coal
is more completely consumed than by the common furnace, as all the
effect of what is termed stirring up the fire (by which no
inconsiderable quantity of coal is passed into the ash-pit) is
attained without moving the coal upon the grate."
The fire-place is a circular grate placed on a vertical shaft in a
horizontal position. It is capable of revolving, and is made to do so
by the vertical shaft, which is turned by wheelwork, which is worked
by the engine itself; or this shaft or spindle may be turned by a
water-wheel, on which a stream of water is allowed to flow from a
reservoir into which it is pumped by the power of the engine; and by
regulating the quantity of water in the stream, the grate may be made
to revolve with a greater or less speed. In that part of the boiler
which is over the grate, there is an aperture, in which is placed a
hopper, through which fuel is let down upon the grate at the rate of
any quantity per minute that may be required. The apparatus which
admits the coals through this hopper is worked by the engine also, and
by the same means as the grate is turned so that the grate revolves
with a speed proportional to the rapidity with which the fuel is
admitted through the hopper, and by this ingenious arrangement the
fuel falls equally thick upon the grate.
The supply of water which turns the wheel which works the grate and
the machinery in the hopper, is regulated by a cock connected with the
self-regulating damper; so that when the steam is being produced too
fast, the supply of water will be diminished, and by that means the
supply of fuel to the grate will be diminished, and the grate will
revolve less rapidly: and when the steam is being produced too slowly
for the demands of the engine, the contrary effects take place. In
this way the fuel which is introduced into the furnace is exactly
proportioned to the work which the engine has to perform. The hopper
may be made large enough to hold coals for a day's work, so that the
furnace requires no other attendance than to deposite coals in the
hopper each morning.
The coals are let down from the hopper on the grate at that part
which is most remote from the flue; and as they descend in very small
quantities at a time, they are almost immediately ignited. But until
their ignition is complete, a smoke will arise, which, passing to the
flue over the vividly burning fuel, will be ignited. Air is admitted
through proper apertures, and its quantity regulated by the damper in
the same manner as the supply of fuel.
The superiority of this beautiful invention over the common
smoke-consuming furnaces is very striking. Its principle of
self-adjustment as to the supply of coals and atmospheric air, and the
proportioning of these to the quantity of work to be performed by the
engine, not only independent of human labour, but with a greater
degree of accuracy than any human skill or attention could possibly
effect, produces saving of expense, both in fuel and labour.
(75.) Mr. Oldham, engineer to the Bank of Ireland, has proposed
another modification of the self-regulating furnace, which seems to
possess several advantages, and evinces considerable ingenuity.
He uses a slightly inclined grate, at the back or lower end of which
is the flue, and at the front or higher end, the hopper for admitting
the coals. In the bottom or narrow end of the hopper is a moveable
shelf, worked by the engine. Upon drawing back this shelf, a small
quantity of fuel is allowed to descend upon a fixed shelf under it;
and upon the return of the moveable shelf, this fuel is protruded
forward upon the grate. Every alternate bar of the grate is fixed, but
the intermediate ones are connected with levers, by which they are
moved alternately up and down.[22] The effect is, that the coals upon
the bars are continually stirred, and gradually advanced by their own
weight from the front of the grate, where they fall from the hopper,
to the back, where they are deposited in the ash-pit. By the shape and
construction of the bars, the air is conducted upwards between them,
and rushes through the burning fuel, so as to act in the manner of a
blowpipe, and the entire surface of the fire presents a sheet of
flame.
[Footnote 22: Mr. Brunton used moveable bars in a furnace constructed
by him before he adopted the horizontal revolving grate. That plan,
however, does not appear to have been as successful as the latter, as
he has abandoned it. Mr. Oldham states that his furnace has been in
use for several years without any appearance of derangement in the
mechanism, and with a considerable saving of fuel.]
We cannot fail to be struck with the beauty of all these contrivances,
by which the engine is made to regulate itself, and supply its own
wants. It is in fact, all but alive. It was observed by Belidor, long
before the steam engine reached the perfection which it has now
acquired, that it resembled an animal, and that no mere work of man
ever approached so near to actual _life_. Heat is the principle of its
existence. The boiler acts the part of the heart, from which its
vivifying fluid rushes copiously through all the tubes, where having
discharged the various functions of life, and deposited its heat in
the proper places, it returns again to the source it sprung from, to
be duly prepared for another circulation. The healthfulness of its
action is indicated by the regularity of its pulsations; it procures
its own food by its own labour; it selects those parts which are fit
for its support, both as to quantity and quality; and has its natural
evacuations, by which all the useless and innutritious parts are
discharged. It frequently cures its own diseases, and corrects the
irregularity of its own actions, exerting something like moral
faculties. Without designing to carry on the analogy, Mr. Farey, in
speaking of the variations incident to the work performed by different
steam engines, states some further particulars in which it maybe
curiously extended. "We must observe," says he, "that the variation in
the performance of different steam engines, which are constructed on
the same principle, working under the same advantages, is the same as
would be found in the produce of the labour of so many different
horses or other animals when compared with their consumption of food;
for the effects of different steam engines will vary as much from
small differences in the proportion of their parts, as the strength
of animals from the vigour of their constitutions; and again, there
will be as great differences in the performance of the same engine
when in good and bad order, from all the parts being tight and well
oiled, so as to move with little friction, as there is in the labour
of an animal from his being in good or bad health, or excessively
fatigued: but in all these cases there will be a maximum which cannot
be exceeded, and an average which we ought to expect to obtain."
CHAPTER IX.
DOUBLE-CYLINDER ENGINES.
Hornblower's Engine. -- Woolf's Engine. -- Cartwright's Engine.
(76.) The expansive property of steam, of which Watt availed himself
in his single engine by cutting off the supply of steam before the
descent of the piston was completed, was applied in a peculiar manner
by an engineer named Hornblower, about the year 1781, and at a later
period by Woolf. Hornblower was the first who conceived the idea of
working an engine with two cylinders of different sizes, by allowing
the steam to flow freely from the boiler until it fills the smaller
cylinder, and then permitting it to expand into the greater one,
employing it thus to press down two pistons in the manner which we
shall presently describe. The condensing apparatus of Hornblower, as
well as the other appendages of the engine, do not differ materially
from those of Watt; so that it will be sufficient for our present
purpose to explain the manner in which the steam is made to act in
moving the piston.
Let C, fig. 41., be the centre of the great working beam, carrying
two arch heads, on which the chains of the piston rods play. The
distances of these arch heads from the centre C must be in the same
proportion as the length of the cylinders, in order that the same play
of the beam may correspond to the plays of both pistons. Let F be the
steam-pipe from the boiler, and G a valve to admit the steam above the
lesser piston, H is a tube by which a communication may be opened by
the valve I between the top and bottom of the lesser cylinder B. K is
a tube communicating, by the valve L, between the bottom of the lesser
cylinder B and the top of the greater cylinder A. M is a tube
communicating, by the valve N, between the top and bottom of the
greater cylinder A; and P a tube leading to the condenser by the
exhausting valve O.
At the commencement of the operation, suppose all the valves opened,
and steam allowed to flow through the entire engine until the air be
completely expelled, and then let all the valves be closed. To start
the engine, let the exhausting valve O and the steam valves G and L be
opened, as in fig. 41. The steam will flow freely from the boiler, and
press upon the lesser piston, and at the same time the steam below the
greater piston will flow into the condenser, leaving a vacuum in the
greater cylinder. The valve L being opened, the steam which is under
the piston in the lesser cylinder will flow through K, and press on
the greater piston, which, having a vacuum beneath it, will
consequently descend. At the commencement of the motion, the lesser
piston is as much resisted by the steam below it as it is urged by the
steam above it; but after a part of the descent has been effected, the
steam below the lesser piston passing into the greater, expands into
an increased space, and therefore loses its elastic force
proportionally. The steam above the lesser piston retaining its full
force by having a free communication with the boiler by the valve G,
the lesser piston will be urged by a force equal to the excess of the
pressure of this steam above the diminished pressure of the expanded
steam below it. As the pistons descend, the steam which is between it
continually increasing in its bulk, and therefore decreasing in its
pressure, from whence it follows, that the force which resists the
lesser piston is continually decreasing, while that which presses it
down remains the same, and therefore the effective force which impels
it must be continually increasing.
On the other hand, the force which urges the greater piston is
continually decreasing, since there is a vacuum below it, and the
steam which presses it is continually expanding into an increased
bulk.
Impelled in this way, let us suppose the pistons to have arrived at
the bottoms of the cylinders, as in fig. 42., and let the valves G, L,
and O be closed, and the valves I and N opened. No steam is allowed to
flow from the boiler, G being closed, nor any allowed to pass into the
condenser, since O is closed, and all communications between the
cylinders is stopped by closing L. By opening the valve I, a free
communication is made between the top and bottom of the lesser piston
through the tube H, so that the steam which presses above the lesser
piston will exert the same pressure below it, and the piston is in a
state of indifference. In the same manner the valve N being open, a
free communication is made between the top and bottom of the greater
piston, and the steam circulates above and below the piston, and
leaves it free to rise. A counterpoise attached to the pump-rods in
this case, draws up the piston, as in Watt's single engine; and when
they arrive at the top, the valves I and N are closed, and G, L, and O
opened, and the next descent of the pistons is produced in the manner
already described, and so the process is continued.
The valves are worked by the engine itself, by means similar to some
of those already described. By computation, we find the power of this
engine to be nearly the same as a similar engine on Watt's expansive
principle. It does not however appear, that any adequate advantage
was gained by this modification of the principle, since no engines of
this construction are now made.
(77.) The use of two cylinders was revived by _Arthur Woolf_, in 1804,
who, in this and the succeeding year, obtained patents for the
application of steam raised under a high pressure to double-cylinder
engines. The specification of his patent states, that he has proved by
experiment that steam raised under a safety-valve loaded with any
given number of pounds upon the square inch, will, if allowed to
expand into as many times its bulk as there are pounds of pressure on
the square inch, have a pressure equal to that of the atmosphere.
Thus, if the safety-valve be loaded with four pounds on the square
inch, the steam, after expanding into four times its bulk, will have
the atmospheric pressure. If it be loaded with 5, 6, or 10 lbs. on the
square inch, it will have the atmospheric pressure when it has
expanded into 5, 6, or 10 times its bulk, and so on. It was, however,
understood in this case, that the vessel into which it was allowed to
expand should have the same temperature as the steam before it
expands.
It is very unaccountable how a person of Mr. Woolf's experience in the
practical application of steam could be led into errors so gross as
those involved in the averments of this patent; and it is still more
unaccountable how the experiments could have been conducted which led
him to conclusions not only incompatible with all the established
properties of elastic fluids--properties which at that time were
perfectly understood--but even involving in themselves palpable
contradiction and absurdity. If it were admitted that every additional
pound avoirdupois which should be placed upon the safety-valve would
enable steam, by its expansion into a proportionally enlarged space,
to attain a pressure equal to the atmosphere, the obvious consequence
would be, that a physical relation would subsist between the
atmospheric pressure and the pound avoirdupois! It is wonderful that
it did not occur to Mr. Woolf, that, granting his principle to be true
at any given place, it would necessarily be false at another place
where the barometer would stand at a different height. Thus if the
principle were true at the foot of a mountain, it would be false at
the top of it; and if it were true in fair weather, it would be false
in foul weather, since these circumstances would be attended by a
change in the atmospheric pressure, without making any change in the
pound avoirdupois.[23]
[Footnote 23: It is strange that this absurdity has been repeatedly
given as unquestionable fact in various encyclopædias on the article
"Steam Engine," as well as in by far the greater number of treatises
expressly on the subject.]
The method by which Mr. Woolf proposed to apply the principle which he
imagined himself to have discovered was by an arrangement of cylinders
similar to those of Hornblower, but having their magnitudes
proportioned to the greater extent of expansion which he proposed to
use. Two cylinders, like those of Hornblower, were placed under the
working beam, having their piston-rods at distances from the axis
proportioned to the lengths of their respective strokes. The relative
magnitudes of the cylinders A and B must be adjusted according to the
extent to which the principle of expansion is intended to be used. The
valves C C´ were placed at each end of the lesser cylinder in tubes
communicating with the boiler, so as to admit steam on each side of
the lesser piston, and cut it off at pleasure. A tube, D´, formed a
communication between the upper end of the lesser and lower end of the
greater cylinder, which communication is opened and closed at pleasure
by the valve E´. In like manner, the tube D forms a communication
between the lower end of the lesser cylinder and the upper end of the
greater, which may be opened and closed by the valve E. The top and
bottom of the greater cylinder communicated with the condenser by
valves F´ F.
Let us suppose that the air is blown from the engine in the usual
way, all the valves closed, and the engine ready to start, the pistons
being at the top of the cylinders. Open the valves C, E, and F. The
steam which occupies the greater cylinder below the piston will now
pass into the condenser through F, leaving a vacuum below the piston.
The steam which is in the lesser cylinder below the piston will pass
through D and open the valve E, and will press down the greater
piston. The steam from the boiler will flow in at C, and press on the
lesser piston. At first the whole motion will proceed from the
pressure upon the greater piston, since the steam, both above and
below the lesser piston, has the same pressure. But, as the pistons
descend, the steam below the less passing into the greater cylinder,
expands into a greater space, and consequently exerts a diminished
pressure, and, therefore, the steam on the other side exerting an
undiminished pressure, acquires an impelling force exactly equal to
the pressure lost in the expansion of the steam between the two
pistons. Thus both pistons will be pressed to the bottoms of their
respective cylinders. It will be observed that in the descent the
greater piston is urged by a continually decreasing force, while the
lesser is urged by continually increasing force.
Upon the arrival of the pistons at the bottoms of the cylinders, let
the valves, C, E, F be closed, and C´, E´, F´ be opened, as in fig.
44. The steam which is above the greater piston now flows through F´
into the condenser, leaving the space above the piston a vacuum. The
steam which is above the lesser piston passes through E´ and D´ below
the greater, while the steam from the boiler is admitted through C´
below the lesser piston. The pressure of the steam entering through E´
below the greater piston, pressing on it against the vacuum above it,
commences the ascent. In the mean time the steam above the lesser
piston passing into the enlarged space of the greater cylinder, loses
gradually its elastic force, so that the steam entering from the
boiler at C´ becomes in part effective, and the ascent is completed
under exactly the same circumstances as the descent, and in this way
the process is continued.
It is evident that the valves may be easily worked by the mechanism of
the engine itself.
In this arrangement the pistons ascend and descend together, and their
rods must consequently be attached to the beam at the same side of the
centre. It is sometimes desirable that they should act on different
sides of the centre of the beam, and consequently that one should
ascend while the other descends. It is easy to arrange the valves so
as to effect this. In fig. 45., the lesser piston is at the bottom of
the cylinder, and the greater at the top. On opening the valves C´,
E´, F´, a vacuum is produced below the greater piston, and steam flows
from the lesser cylinder, through E´, _above_ the greater piston, and
presses it down. At the same time steam being admitted from the boiler
through C´ below the lesser piston, forces it up against the
diminishing force of the steam above it, which expands into the
greater cylinder. Thus as the greater piston descends the lesser
ascends. When each has traversed its cylinder, the valves C´, E´, F´
being closed, and C, E, F opened, the lesser piston will descend, and
the greater ascend, and so on.
(78.) The law according to which the elastic force of steam diminishes
as it expands, of which Mr. Woolf appears to have been entirely
ignorant, is precisely similar to the same property in air and other
elastic fluids. If steam expands into twice or thrice its volume, it
will lose its elastic force in precisely the same proportion as it
enlarges its bulk; and therefore will have only a half or a third of
its former pressure, supposing that as it expands its temperature is
kept up. Although Mr. Woolf's patent contained the erroneous principle
which we have noticed, yet, so far as his invention suggested the idea
of employing steam at a very high-pressure, and allowing it to expand
in a much greater degree than was contemplated either by Watt or
Hornblower, it became the means of effecting a considerable saving in
fuel; for engines used for pumping on a large scale, the steam being
produced under a pressure of forty or fifty pounds or more upon the
square inch, might be worked first through a small space with intense
force, and the communication with the boiler being then cut off, it
might be allowed, with great advantage, to expand through a very large
space. Some double-cylinder engines upon this principle have been
worked in Cornwall, with considerable economy. But the form in which
the expansive principle, combined with high pressure, is now applied
in the engines used for raising water from the mines, is that in which
it was originally proposed by Watt. A single cylinder of considerable
length is employed; the piston is driven through a small proportion of
this length by steam, admitted from the boiler at a very intense
pressure: the steam being then cut off, the piston is urged by the
expansive force of the steam which has been admitted, and is by that
means brought to the bottom of the cylinder.
It is evident, under such circumstances, that the pressure of the
steam admitted from the boiler must be much greater than the
resistance opposed to the piston, and that the motion of the piston
must, in the first instance, be accelerated and not uniform. If the
piston moved from the commencement with a uniform motion, the pressure
of the steam urging it must necessarily be exactly equal to the
resistance opposed to it, and then cutting off the supply of steam
from the boiler, the piston could only continue its motion by inertia,
the steam immediately becoming of less pressure than the resistance;
and after advancing through a very small space, the piston would
recoil upon the steam, and come to a state of rest. The steam,
however, at the moment it is cut off being of much greater pressure
than the amount of resistance upon the piston, will continue to drive
the piston forward, until by its expansion its force is so far
diminished as to become equal to the resistance of the piston. From
that point the impelling power of the steam will cease, and the
piston will move forward by its inertia only. The point at which the
steam is cut off should therefore be so regulated that it shall
acquire a pressure equal to the resistance on the piston by its
expansion, just at such a distance from the end of the stroke as the
piston may be able to move through by its inertia. It is evident the
adjustment of this will require great care and nicety of management.
(79.) In 1797 a patent was granted to the _Rev. Mr. Cartwright_, a
gentleman well known for other mechanical inventions, for some
improvements in the steam-engine. His contrivance is at once so
elegant and simple, that, although it has not been carried into
practice, we cannot here pass it over without notice.
The steam-pipe from the boiler is represented cut off at B (fig. 46.);
T is a spindle-valve for admitting steam above the piston, and R is a
spindle-valve in the piston; D is a curved pipe forming a
communication between the cylinder and the condenser, which is of very
peculiar construction. Cartwright proposed effecting a condensation
without a jet, by exposing the steam to contact with a very large
quantity of cold surface. For this purpose, he formed his condenser by
placing two cylinders nearly equal in size one within the other,
allowing the water of the cold cistern in which they were placed to
flow through the inner cylinder, and to surround the outer one. Thus
the thin space between the two cylinders formed the condenser.
[Illustration: Pl. IX.]
[Illustration: Pl. X.]
The air-pump is placed immediately under the cylinder, and the
continuation of the piston-rod works its piston, which is solid and
without a valve: F is the pipe from the condenser to the air-pump,
through which the condensed steam is drawn off through the valve G on
the ascent of the piston, and on the descent, this is forced through a
tube into a hot well, H, for the purpose of feeding the boiler through
the feed-pipe I. In the top of the hot well H is a valve which opens
inwards, and is kept closed by a ball floating on the surface of the
liquid. The pressure of the condensed air above the surface of the
liquid in H forces it through I into the boiler. When the air
accumulates in too great a degree in H, the surface of the liquid is
pressed so low that the ball falls and opens the valve, and allows it
to escape. The air in H is that which is pumped from the condenser
with the liquid, and which was disengaged from it.
Let us suppose the piston at the top of the cylinder; it strikes the
tail of the valve T, and raises it, while the stem of the piston-valve
R strikes the top of the cylinder, and is pressed into its seat. A
free communication is at the same time open between the cylinder,
below the piston and the condenser, through the tube D. The pressure
of the steam thus admitted above the piston, acting against the vacuum
below it, will cause its descent. On arriving at the bottom of the
cylinder, the tail of the piston-valve R will strike the bottom, and
it will be lifted from its seat, so that a communication will be
opened through it with the condenser. At the same moment a projecting
spring, K, attached to the piston-rod, strikes the stem of the
steam-valve T, and presses it into its seat. Thus, while the further
admission of steam is cut off, the steam above the piston flows into
the condenser, and the piston being relieved from all pressure, is
drawn up by the momentum of the fly-wheel, which continues the motion
it received from the descending force. On the arrival of the piston
again at the top of the cylinder, the valve T is opened and R closed,
and the piston descends as before, and so the process is continued.
The mechanism by which motion is communicated from the piston to the
fly-wheel is peculiarly elegant. On the axis of the fly-wheel is a
small wheel with teeth, which work in the teeth of another large wheel
L. This wheel is turned by a crank, which is worked by a cross-piece
attached to the end of the piston-rod. Another equal-toothed wheel, M,
is turned by a crank, which is worked by the other end of the
cross-arm attached to the piston-rod.
One of the peculiarities of this engine is, that the liquid which is
used for the production of steam in the boiler circulates through the
machine without either diminution or admixture with any other fluid,
so that the boiler never wants more feeding than what can be supplied
from the hot-well H. This circumstance forms a most important feature
in the machine, as it allows of ardent spirits being used in the
boiler instead of water, which, since they boil at low heats, promised
a saving of half the fuel. The inventor even proposed, that the engine
should be used as a still, as well as a mechanical power, in which
case the whole of the fuel would be saved.
In this engine, the ordinary method of rendering the piston
steam-tight, by oil or melted wax or tallow poured upon it, could not
be applied, since the steam above the piston must always have a free
passage through the piston-valve R. The ingenious inventor therefore
contrived a method of making the piston steam-tight in the cylinder,
without oil or stuffing, and his method has since been adopted with
success in other engines.
A ring of metal is ground into the cylinder, so as to fit it
perfectly, and is then cut into four equal segments. The inner surface
of this ring being slightly conical, another ring is ground into it,
so as to fit it perfectly, and this is also cut into four segments,
and one is placed within the other, but in such a manner that the
joints or divisions do not coincide. The arrangement of the two rings
is represented in fig. 47. Within the inner ring are placed four
springs, which press the pieces outward against the sides of the
cylinder, and are represented in the diagram. Four pairs of these
rings are placed one over another, so that their joints do not
coincide, and the whole is screwed together by plates placed at top
and bottom. A vertical section of the piston is given in fig. 48.
One of the advantages of this piston is, that the longer it is worked,
the more accurately it fits the cylinder, so that, as the machine
wears it improves.
Metallic pistons have lately come into very general use, and such
contrivances differ very little from the above.
CHAPTER X.
LOCOMOTIVE ENGINES ON RAILWAYS.
High-pressure Engines. -- Leupold's Engine. -- Trevithick and
Vivian. -- Effects of Improvement in Locomotion. -- Historical
Account of the Locomotive Engine. -- Blenkinsop's Patent. --
Chapman's Improvement. -- Walking Engine. -- Stephenson's First
Engines. -- His Improvements. -- Liverpool and Manchester Railway
Company. -- Their Preliminary Proceedings. -- The Great
Competition of 1829. -- The Rocket. -- The Sanspareil. -- The
Novelty. -- Qualities of the Rocket. -- Successive Improvements.
-- Experiments. -- Defects of the Present Engines. -- Inclined
Planes. -- Methods of surmounting them. -- Circumstances of the
Manchester Railway Company. -- Probable Improvements in
Locomotives. -- Their capabilities with respect to speed. --
Probable Effects of the Projected Railroads. -- Steam Power
compared with Horse Power. -- Railroads compared with Canals.
(80.) In the various modifications of the steam engine which we have
hitherto considered, the pressure introduced on one side of the piston
derives its efficacy either wholly or partially from the vacuum
produced by condensation on the other side. This always requires a
condensing apparatus, and a constant and abundant supply of cold
water. An engine of this kind must therefore necessarily have
considerable dimensions and weight, and is inapplicable to uses in
which a small and light machine only is admissible. If the condensing
apparatus be dispensed with, the piston will always be resisted by a
force equal to the atmospheric pressure, and the only part of the
steam pressure which will be available as a moving power, is that
part by which it exceeds the pressure of the atmosphere. Hence, in
engines which do not work by condensation, steam of a much higher
pressure than that of the atmosphere is indispensably necessary, and
such engines are therefore called _high-pressure engines_.
We are not, however, to understand that every engine, in which steam
is used of a pressure exceeding that of the atmosphere, is what is
meant by a _high-pressure engine_; for in the ordinary engines in
common use, constructed on Watt's principle, the safety-valve is
loaded with from 3 to 5 lbs. on the square inch; and in Woolf's
engines, the steam is produced under a pressure of 40 lbs. on the
square inch. These would therefore be more properly called _condensing
engines_ than _high-pressure engines_; a term quite inapplicable to
those of Woolf. In fact, by _high-pressure engines_ is meant engines
in which no vacuum is produced, and, therefore, in which the piston
works against a pressure equal to that of the atmosphere.
In these engines, the whole of the condensing apparatus, _viz._ the
cold-water cistern, condenser, air-pump, cold-water pump, &c. are
dispensed with, and nothing is retained except the boiler, cylinder,
piston, and valves. Consequently, such an engine is small, light, and
cheap. It is portable also, and may be moved, if necessary, along with
its load, and is therefore well adapted to locomotive purposes.
(81.) High-pressure engines were one of the earliest modifications of
the steam engine. The contrivance, which is obscurely described in the
article already quoted (27.), from the Century of Inventions, is a
high-pressure engine; for the power there alluded to is the elastic
force of steam working against the atmospheric pressure. _Newcomen_,
in 1705, applied the working-beam, cylinder, and piston to the
atmospheric engine; and _Leupold_, about 1720, combined the
working-beam and cylinder with the high-pressure principle, and
produced the earliest high-pressure engine worked by a cylinder and
piston. The following is a description of _Leupold's_ engine:--
A (fig. 49.) is the boiler, with the furnace beneath it; C C´ are two
cylinders with two solid pistons, P P´, connected with the
working-beams B B´, to which are attached the pump-rods, R R´, of two
forcing-pumps, F F´, which communicate with a great force-pipe S; G is
a _four-way cock_ (66.) already described. In the position in which it
stands in the figure, the steam is issuing from below the piston P
into the atmosphere, and the piston is descending by its own weight;
steam from the boiler is at the same time pressing up the piston P´,
with a force equal to the difference between the pressure of the steam
and that of the atmosphere. Thus the piston R of the forcing-pump is
being drawn up, and the piston P´ is forcing the piston R´ down, and
thereby driving water into the force-pipe S. On the arrival of the
piston P at the bottom of the cylinder C, and P´ at the top of the
cylinder C´, the position of the cock is changed as represented in
fig. 50. The steam, which has just pressed up the piston P´, is
allowed to escape into the atmosphere, while the steam, passing from
the boiler below the piston P, presses it up, and thus P ascends by
the steam pressure, and P´ descends by its own weight. By these means
the piston R is forced down, driving before it the water in the pump
cylinder into the force-pipe S, and the piston R´ is drawn up to allow
the other pump-cylinder to be re-filled; and so the process is
continued.
A valve is placed in the bottom of the force-pipes, to prevent the
water which has been driven into it from returning. This valve opens
upwards; and, consequently, the weight of the water pressing upon it
only keeps it more effectually closed. On each descent of the piston,
the pressure transmitted to the valve acting upwards being greater
than the weight of the water resting upon it, forces it open, and an
increased quantity of water is introduced.
(82.) From the date of the improvement of Watt until the commencement
of the present century, high-pressure engines were altogether
neglected in these countries. In the year 1802, Messrs. _Trevithick_
and _Vivian_ constructed the first high-pressure engine which was ever
brought into extensive practical use in this kingdom. A section of
this machine, made by a vertical plane, is represented in fig. 51.
The boiler A B is a cylinder with flat circular ends. The fire-place
is constructed in the following manner:--A tube enters the cylindrical
boiler at one end; and, proceeding onwards, near the other extremity,
is turned and recurved, so as to be carried back parallel to the
direction in which it entered. It is thus conducted out of the boiler,
at another part of the same end at which it entered. One of the ends
of this tube communicates with the chimney E, which is carried
upwards, as represented in the figure. The other mouth is furnished
with a door; and in it is placed the grate, which is formed of
horizontal bars, dividing the tube into two parts; the upper part
forming the fire-place, and the lower the ash-pit. The fuel is
maintained in a state of combustion, on the bars, in that part of the
tube represented at C D; and the flame is carried by the draft of the
chimney round the curved flue, and issues at E into the chimney. The
flame is thus conducted through the water, so as to expose the latter
to as much heat as possible.
A section of the cylinder is represented at F, immersed in the boiler,
except a few inches of the upper end, where the four-way cock G is
placed for regulating the admission of the steam. A tube is
represented at H, which leads from this four-way cock into the
chimney; so that the waste steam, after working the piston, is carried
off through this tube, and passes into the chimney. The upper end of
the piston-rod is furnished with a cross-bar, which is placed in a
direction at right angles to the length of the boiler, and also to the
piston-rod. This bar is guided in its motion by sliding on two iron
perpendicular rods fixed to the sides of the boiler, and parallel to
each other. To the ends of this cross-bar are joined two connecting
rods, the lower ends of which work two cranks fixed on an axis
extending across and beneath the boiler, and immediately under the
centre of the cylinder. This axis is sustained in bearings formed in
the legs which support the boiler, and upon its extremity is fixed the
fly-wheel as represented at B. A large-toothed wheel is placed on this
axis; which, being turned with the cranked axle, communicates motion
to other wheels; and, through them, to any machinery which the engine
may be applied to move.
As the four-way cock is represented in the figure, the steam passes
from the boiler through the curved passage G above the piston, while
the steam below the piston is carried off through a tube which does
not appear in the figure, by which it is conducted to the tube H, and
thence to the chimney. The steam, therefore, which passes above the
piston presses it downwards; while the pressure upwards does not
exceed that of the atmosphere. The piston will therefore descend with
a force depending on the excess of the pressure of the steam produced
in the boiler above the atmospheric pressure. When the piston has
arrived at the bottom of the cylinder, the cock is made to assume the
position represented in the figure 52. This effect is produced by the
motion of the piston-rod. The steam now passes from above the piston,
through the tube H, into the chimney, while the steam from the boiler
is conducted through another tube below the piston. The pressure above
the piston, in this case, does not exceed that of the atmosphere;
while the pressure below it will be that of the steam in the boiler.
The piston will therefore ascend with the difference of these
pressures. On the arrival of the piston at the top of the cylinder,
the four-way cock is again turned to the position represented in fig.
51., and the piston again descends; and in the same manner the process
is continued. A safety-valve is placed on the boiler at V, loaded
with a weight W, proportionate to the strength of the steam with which
it is proposed to work.
[Illustration: Pl. XI.]
In the engines now described, this valve was frequently loaded at the
rate of from 60 to 80 lbs. on the square inch. As the boilers of
high-pressure engines were considered more liable to accidents from
bursting than those in which steam of a lower pressure was used,
greater precautions were taken against such effects. A second
safety-valve was provided, which was not left in the power of the
engine-man. By this means he had a power to diminish the pressure of
the steam, but could not increase it beyond the limit determined by
the valve which was removed from his interference. The greatest cause
of danger, however, arose from the water in the boiler being consumed
by evaporation faster than it was supplied; and therefore falling
below the level of the tube containing the furnace. To guard against
accidents arising from this circumstance, a hole was bored in the
boiler, at a certain depth, below which the water should not be
allowed to fall; and in this hole a plug of metal was soldered with
lead, or with some other metal, which would fuse at that temperature
which would expose the boiler to danger. Thus, in the event of the
water being exhausted, so that its level would fall below the plug,
the heat of the furnace would immediately melt the solder, and the
plug would fall out, affording a vent for the steam, without allowing
the boiler to burst. The mercurial steam-gauge, already described, was
also used as an additional security. When the force of the steam
exceeded the length of the column of mercury which the tube would
contain, the mercury would be blown out, and the tube would give vent
to the steam. The water by which the boiler was replenished was forced
into it by a pump worked by the engine. In order to economise the
heat, this water was contained in a tube T, which surrounded the pipe
H. As the waste steam, after working the piston, passed off through H,
it imparted a portion of its heat to the water contained in the tube
T, which was thus warmed to a certain temperature before it was
forced into the boiler by the pump. Thus a part of the heat, which was
originally carried from the boiler in the form of steam, was returned
again to the boiler with the water with which it was fed.
It is evident that engines constructed in this manner may be applied
to all the purposes to which the condensing engines are applicable.
(_e_) To the plates of the English edition has been added one, (plate
A) representing a high-pressure engine as constructed by the West
Point Foundry in the state of New York. The principal parts will be
readily distinguished from their resemblance to the analogous parts of
a condensing engine. The condenser and air-pump of that engine, it
will be observed, are suppressed. At _v x_ and _y z_ are forcing-pumps
by which a supply of water is injected into the boiler at each motion
of the engine. For the four-way cock, used in the English
high-pressure engines, a slide valve at _r s_, is substituted, and is
found to work to much greater advantage. It is set in motion by an
eccentric, in a manner that will be more obvious from an inspection of
the plate than from any description.--A. E.
(_f_) A very safe and convenient boiler for a high-pressure engine has
been invented in the United States by Mr. Babcock. The boiler consists
of small tubes, into which water is flashed by a small forcing-pump at
every stroke of the engine. The tubes are kept so hot in a furnace, as
to generate steam of the required temperature, but not hot enough to
cause any risk of the decomposition of the water. The strength of the
apparatus is such, and the quantity of water exposed to heat at one
time so small, as to leave hardly any risk of danger.--A. E.
(83.) Two years after the date of the patent of this engine, its
inventor constructed a machine of the same kind for the purpose of
moving carriages on railroads; and applied it successfully, in the
year 1804, on the railroad at Merthyr Tydvil, in South Wales. It was
in principle the same as that already described. The cylinder however
was in a horizontal position, the piston-rod working in the direction
of the line of road: the extremity of the piston-rod, by means of a
connecting rod, worked cranks placed on the axletree, on which were
fixed two cogged wheels: these worked in others, by which their motion
was communicated finally to cogged wheels fixed on the axle of the
hind wheels of the carriage, by which this axle was kept in a state of
revolution. The hind wheels being fixed on the axletree, and turning
with it, were caused likewise to revolve; and so long as the weight of
the carriage did not exceed that which the friction of the road was
capable of propelling, the carriage would thus be moved forwards. On
this axle was placed a fly-wheel to continue the rotatory motion at
the termination of each stroke. The fore wheels are described as being
capable of turning like the fore wheels of a carriage, so as to guide
the vehicle. The projectors appear to have contemplated, in the first
instance, the use of this carriage on turnpike roads; but that notion
seems to have been abandoned, and its use was only adopted on the
railroad before mentioned. On the occasion of its first trial, it drew
after it as many carriages as contained 10 tons of iron a distance of
nine miles; which stage it performed without any fresh supply of
water, and travelled at the rate of five miles an hour.
(84.) Capital and skill have of late years been directed with
extraordinary energy to the improvement of inland transport; and this
important instrument of national wealth and civilisation has received
a proportionate impulse. Effects are now witnessed, which, had they
been narrated a few years since, could only have been admitted into
the pages of fiction or volumes of romance. Who could have credited
the possibility of a ponderous engine of iron, loaded with several
hundred passengers, in a train of carriages of corresponding
magnitude, and a large quantity of water and coal, taking flight from
Manchester and arriving at Liverpool, a distance of about thirty
miles, in little more than an hour? And yet this is a matter of daily
and almost hourly occurrence. Neither is the road, on which this
wondrous performance is effected, the most favourable which could be
constructed for such machines. It is subject to undulations and
acclivities, which reduce the rate of speed much more than similar
inequalities affect the velocity on common roads. The rapidity of
transport thus attained is not less wonderful than the weights
transported. Its capabilities in this respect far transcend the
exigencies even of the two greatest commercial marts in Great Britain.
Loads, varying from 50 to 150 tons are transported at the average rate
of 15 miles an hour; but the engines in this case are loaded below
their power; and in one instance we have seen a load--we should rather
say a _cargo_--of wagons, conveying merchandise to the amount of 230
tons gross, transported from Liverpool to Manchester at the average
rate of 12 miles an hour.
The astonishment with which such performances must be viewed, might be
qualified, if the art of transport by steam on railways had been
matured, and had attained that full state of perfection which such an
art is always capable of receiving from long experience, aided by
great scientific knowledge, and the unbounded application of capital.
But such is not the present case. The art of constructing locomotive
engines, so far from having attained a state of maturity, has not even
emerged from its infancy. So complete was the ignorance of its powers
which prevailed, even among engineers, previous to the opening of the
Liverpool railway, that the transport of heavy goods was regarded as
the chief object of the undertaking, and its principal source of
revenue. The incredible speed of transport, effected even in the very
first experiments in 1830, burst upon the public, and on the
scientific world, with all the effect of a new and unlooked-for
phenomenon. On the unfortunate occasion which deprived this country
of Mr. Huskisson, the wounded body of that statesman was transported a
distance of about fifteen miles in twenty-five minutes, being at the
rate of thirty-six miles an hour. The revenue of the road arising from
passengers since its opening, has, contrary to all that was foreseen,
been nearly double that which has been derived from merchandise. So
great was the want of experience in the construction of engines, that
the company was at first ignorant whether they should adopt large
steam-engines fixed at different stations on the line, to pull the
carriages from station to station, or travelling engines to drag the
loads the entire distance. Having decided on the latter, they have,
even to the present moment, laboured under the disadvantage of the
want of that knowledge which experience alone can give. The engines
have been constantly varied in their weight and proportions, in their
magnitude and form, as the experience of each successive month has
indicated. As defects became manifest they were remedied; improvements
suggested were adopted; and each quarter produced engines of such
increased power and efficiency, that their predecessors were
abandoned, not because they were worn out, but because they had been
outstripped in the rapid march of improvement. Add to this, that only
one species of travelling engine has been effectively tried; the
capabilities of others remain still to be developed; and even that
form of engine which has received the advantage of a course of
experiments on so grand a scale to carry it towards perfection, is
far short of this point, and still has defects, many of which,
it is obvious, time and experience will remove. If then travelling
steam engines, with all the imperfections of an incipient
invention--with the want of experience, the great parent of
practical improvements--with the want of the common advantage of the
full application of the skill and capital of the country--subjected to
but one great experiment, and that experiment limited to one form of
engine; if, under such disadvantages, the effects to which we have
referred have been produced, what may we not expect from this
extraordinary power, when the enterprise of the country shall be
unfettered, when greater fields of experience are opened, when time,
ingenuity, and capital have removed the existing imperfections, and
have brought to light new and more powerful principles? This is not
mere speculation on possibilities, but refers to what is in a state of
actual progression. Railways are in progress between the points of
greatest intercourse in the United Kingdom, and travelling steam
engines are in preparation for the common turnpike roads; the
practicability and utility of that application of the steam engine
having not only been established by experiment to the satisfaction of
their projectors, but proved before the legislature in a committee of
inquiry on the subject.
The important commercial and political effects attending such
increased facility and speed in the transport of persons and goods,
are too obvious to require any very extended notice here. A part of
the price (and in many cases a considerable part) of every article of
necessity or luxury, consists of the cost of transporting it from the
producer to the consumer; and consequently every abatement or saving
in this cost must produce a corresponding reduction in the price of
every article transported; that is to say, of everything which is
necessary for the subsistence of the poor, or for the enjoyment of the
rich, of every comfort, and of every luxury of life. The benefit of
this will extend, not to the consumer only, but to the producer: by
lowering the expense of transport of the produce, whether of the soil
or of the loom, a less quantity of that produce will be spent in
bringing the remainder to market, and consequently a greater surplus
will reward the labour of the producer. The benefit of this will be
felt even more by the agriculturist than by the manufacturer; because
the proportional cost of transport of the produce of the soil is
greater than that of manufactures. If 200 quarters of corn be
necessary to raise 400, and 100 more be required to bring the 400 to
market, then the net surplus will be 100. But if by the use of steam
carriages the same quantity can be brought to market with an
expenditure of 50 quarters, then the net surplus will be increased
from 100 to 150 quarters; and either the profit of the farmer, or the
rent of the landlord, must be increased by the same amount.
But the agriculturist would not merely be benefited by an increased
return from the soil already under cultivation. Any reduction in the
cost of transporting the produce to market would call into cultivation
tracts of inferior fertility, the returns from which would not at
present repay the cost of cultivation and transport. Thus land would
become productive which is now waste, and an effect would be produced
equivalent to adding so much fertile soil to the present extent of the
country. It is well known, that land of a given degree of fertility
will yield increased produce by the increased application of capital
and labour. By a reduction in the cost of transport, a saving will be
made which may enable the agriculturist to apply to tracts already
under cultivation the capital thus saved, and thereby increase their
actual production. Not only, therefore, would such an effect be
attended with an increased extent of cultivated land, but also with an
increased degree of cultivation in that which is already productive.
It has been said, that in Great Britain there are above a million of
horses engaged in various ways in the transport of passengers and
goods, and that to support each horse requires as much land as would,
upon an average, support eight men. If this quantity of animal power
were displaced by steam engines, and the means of transport drawn from
the bowels of the earth, instead of being raised upon its surface,
then, supposing the above calculation correct, as much land would
become available for the support of human beings as would suffice for
an additional population of eight millions; or, what amounts to the
same, would increase the means of support of the present population by
about one-third of the present available means. The land which now
supports horses for transport would then support men, or produce corn
for food.
The objection that a quantity of land exists in the country capable of
supporting horses alone, and that such land would be thrown out of
cultivation, scarcely deserves notice here. The existence of any
considerable quantity of such land is extremely doubtful. What is the
soil which will feed a horse and not feed oxen or sheep, or produce
food for man? But even if it be admitted that there exists in the
country a small portion of such land, that portion cannot exceed, nor
indeed equal, what would be sufficient for the number of horses which
must after all continue to be employed for the purposes of pleasure,
and in a variety of cases where steam must necessarily be
inapplicable. It is to be remembered, also, that the displacing of
horses in one extensive occupation, by diminishing their price must
necessarily increase the demand for them in others.
The reduction in the cost of transport of manufactured articles, by
lowering their price in the market, will stimulate their consumption.
This observation applies of course not only to home but to foreign
markets. In the latter we already in many branches of manufacture
command a monopoly. The reduced price which we shall attain by
cheapness and facility of transport will still further extend and
increase our advantages. The necessary consequence will be, an
increased demand for manufacturing population; and this increased
population again reacting on the agricultural interests, will form an
increased market for that species of produce. So interwoven and
complicated are the fibres which form the texture of the highly
civilized and artificial community in which we live, that an effect
produced on any one point is instantly transmitted to the most remote
and apparently unconnected parts of the system.
The two advantages of increased cheapness and speed, besides extending
the amount of existing traffic, call into existence new objects of
commercial intercourse. For the same reason that the reduced cost of
transport, as we have shown, calls new soils into cultivation, it also
calls into existence new markets for manufactured and agricultural
produce. The great speed of transit which has been proved to be
practicable, must open a commerce between distant points in various
articles, the nature of which does not permit them to be preserved so
as to be fit for use beyond a certain time. Such are, for example,
many species of vegetable and animal food, which at present are
confined to markets at a very limited distance from the grower or
feeder. The truth of this observation is manifested by the effects
which have followed the intercourse by steam on the Irish Channel. The
western towns of England have become markets for a prodigious quantity
of Irish produce, which it had been previously impossible to export.
If animal food be transported alive from the grower to the consumer,
the distance of the market is limited by the power of the animal to
travel, and the cost of its support on the road. It is only particular
species of cattle which bear to be carried to market on common roads
and by horse carriages. But the peculiar nature of a railway, the
magnitude and weight of the loads which may be transported on it, and
the prodigious speed which may be attained, render the transport of
cattle, of every species, to almost any distance, both easy and cheap.
In process of time, when the railway system becomes extended, the
metropolis and populous towns will therefore become markets, not as at
present to districts within limited distances of them, but to the
whole country.
The moral and political consequences of so great a change in the
powers of transition of persons and intelligence from place to place
are not easily calculated. The concentration of mind and exertion
which a great metropolis always exhibits, will be extended in a
considerable degree to the whole realm. The same effect will be
produced as if all distances were lessened in the proportion in which
the speed and cheapness of transit are increased. Towns, at present
removed some stages from the metropolis, will become its suburbs;
others, now a day's journey, will be removed to its immediate
vicinity; business will be carried on with as much ease between them
and the metropolis, as it is now between distant points of the
metropolis itself. Let those who discard speculations like these as
wild and improbable, recur to the state of public opinion, at no very
remote period, on the subject of steam navigation. Within the memory
of persons who have not yet passed the meridian of life, the
possibility of traversing by the steam engine the channels and seas
that surround and intersect these islands, was regarded as the dream
of enthusiasts. Nautical men and men of science rejected such
speculations with equal incredulity, and with little less than scorn
for the understanding of those who could for a moment entertain them.
Yet we have witnessed steam engines traversing not these channels and
seas alone, but sweeping the face of the waters round every coast in
Europe. The seas which interpose between our Asiatic dominions and
Egypt, and those which separate our own shores from our West Indian
possessions, have offered an equally ineffectual barrier to its
powers. Nor have the terrors of the Pacific prevented the "Enterprise"
from doubling the Cape, and reaching the shores of India. If steam be
not used as the only means of connecting the most distant points of
our planet, it is not because it is inadequate to the accomplishment
of that end, but because the supply of the material from which at the
present moment it derives its powers, is restricted by local and
accidental circumstances.[24]
[Footnote 24: Some of the preceding observations on inland transport,
as well as other parts of the present chapter, appeared in articles
written by me in the Edinburgh Review for October, 1832, and October,
1834.]
We propose in the present chapter to lay before our readers some
account of the means whereby the effects above referred to have been
produced; of the manner and degree in which the public have availed
themselves of these means; and of the improvements of which they seem
to us to be susceptible.
(85.) It is a singular fact, that in the history of this invention
considerable time and great ingenuity were vainly expended in
attempting to overcome a difficulty, which in the end turned out to be
purely imaginary. To comprehend distinctly the manner in which a wheel
carriage is propelled by steam, suppose that a pin or handle is
attached to the spoke of the wheel at some distance from its centre,
and that a force is applied to this pin in such a manner as to make
the wheel revolve. If the face of the wheel and the surface of the
road were absolutely smooth and free from friction, so that the face
of the wheel would slide without resistance upon the road, then the
effect of the force thus applied would be merely to cause the wheel to
turn round, the carriage being stationary, the surface of the wheel
would slip or slide upon the road as the wheel is made to revolve. But
if, on the other hand, the pressure of the face of the wheel upon the
road is such as to produce between them such a degree of adhesion as
will render it impossible for the wheel to slide or slip upon the road
by the force which is applied to it, the consequence will be, that the
wheel can only turn round in obedience to the force which moves it by
causing the carriage to advance, so that the wheel will roll upon the
road, and the carriage will be moved forward, through a distance equal
to the circumference of the wheel, each time it performs a complete
revolution.
It is obvious that both of these effects may be partially produced;
the adhesion of the wheel to the road may be insufficient to prevent
slipping altogether, and yet it may be sufficient to prevent the wheel
from slipping as fast as it revolves. Under such circumstances the
carriage would advance and the wheel would slip. The progressive
motion of the carriage during one complete revolution of the wheel
would be equal to the difference between the complete circumference
of the wheel and the portion through which in one revolution it has
slipped.
When the construction of travelling steam engines first engaged the
attention of engineers, and for a considerable period afterwards, a
notion was impressed upon their minds that the adhesion between the
face of the wheel and the surface of the road must necessarily be of
very small amount, and that in every practical case the wheels thus
driven would either slip altogether, and produce no advance of the
carriage, or that a considerable portion of the impelling power would
be lost by the partial slipping or sliding of the wheels. It is
singular that it should never have occurred to the many ingenious
persons who for several years were engaged in such experiments and
speculations, to ascertain by experiment the actual amount of adhesion
in any particular case between the wheels and the road. Had they done
so, we should probably now have found locomotive engines in a more
advanced state than that to which they have attained.
To remedy this imaginary difficulty, Messrs. _Trevithick_ and _Vivian_
proposed to make the external rims of the wheels rough and uneven, by
surrounding them with projecting heads of nails or bolts, or by
cutting transverse grooves on them. They proposed, in cases where
considerable elevations were to be ascended, to cause claws or nails
to project from the surface during the ascent, so as to take hold of
the road.
In seven years after the construction of the first locomotive engine
by these engineers, another locomotive engine was constructed by Mr.
_Blinkensop_, of Middleton Colliery, near Leeds. He obtained a patent,
in 1811, for the application of a rack-rail. The railroad thus,
instead of being composed of smooth bars of iron, presented a line of
projecting teeth, like those of a cog-wheel, which stretched along the
entire distance to be travelled. The wheels on which the engine
rolled were furnished with corresponding teeth, which worked in the
teeth of the railroad; and, in this way, produced a progressive motion
in the carriage.
The next contrivance for overcoming this fictitious difficulty, was
that of Messrs. _Chapman_, who, in the year 1812, obtained a patent
for working a locomotive engine by a chain extending along the middle
of the line of railroad, from the one end to the other. This chain was
passed once round a grooved wheel under the centre of the carriage; so
that, when this grooved wheel was turned by the engine, the chain
being incapable of slipping upon it, the carriage was consequently
advanced on the road. In order to prevent the strain from acting on
the whole length of the chain, its links were made to fall upon
upright forks placed at certain intervals, which between those
intervals sustained the tension of the chain produced by the engine.
Friction-rollers were used to press the chain into the groove of the
wheel, so as to prevent it from slipping. This contrivance was soon
abandoned, for the very obvious reason that a prodigious loss of force
was incurred by the friction of the chain.
The following year, 1813, produced a contrivance, of singular
ingenuity, for overcoming the supposed difficulty arising from the
want of adhesion between the wheels and the road. This was no other
than a pair of mechanical legs and feet, which were made to walk and
propel in a manner somewhat resembling the feet of an animal.
[Illustration: _Fig. 53._]
A sketch of these propellers is given in fig. 53. A is the carriage
moving on the railroad, L and L´ are the legs, F and F´ the feet. The
foot F has a joint at O, which corresponds to the ankle; another joint
is placed at K, which corresponds to the knee; and a third is placed
at L, which corresponds to the hip. Similar joints are placed at the
corresponding letters in the other leg. The knee-joint K is attached
to the end of the piston of the cylinder. When the piston, which is
horizontal, is pressed outwards, the leg L presses the foot F
against the ground, and the resistance forces the carriage A onwards.
As the carriage proceeds, the angle K at the knee becomes larger, so
that the leg and thigh take a straighter position; and this continues
until the piston has reached the end of its stroke. At the hip L there
is a short lever, L M, the extremity of which is connected by a cord
or chain with a point, S, placed near the shin of the leg. When the
piston is pressed into the cylinder, the knee, K, is drawn towards the
engine, and the cord, M S, is made to lift the foot, F, from the
ground; to which it does not return until the piston has arrived at
the extremity of the cylinder. On the piston being again driven out of
the cylinder, the foot, F, being placed on the road, is pressed
backwards by the force of the piston-rod at K; but the friction of the
ground preventing its backward motion, the re-action causes the engine
to advance: and in the same manner this process is continued.
Attached to the thigh, at N, above the knee, by a joint, is a
horizontal rod, N R, which works a rack, R. This rack has beneath it a
cog-wheel. This cog-wheel acts in another rack below it. By these
means, when the knee K is driven _from_ the engine, the rack R is
moved _backwards_; but the cog-wheel, acting on the other rack beneath
it, will move the latter _in the contrary direction_. The rack R being
then moved _in the same direction with the knee_, K, it follows that
the other rack will always be moved _in a contrary direction_. The
lower rack is connected by another horizontal rod with the thigh of
the leg, L´ F´, immediately above the knee, at N´. When the piston is
forced _inwards_, the knee, K´, will thus be forced _backwards_; and
when the piston is forced _outwards_, the knee, K´, will be drawn
_forwards_. It therefore follows that the two knees, K and K´, are
pressed alternately backwards and forwards. The foot, F´, when the
knee, K´, is drawn forward, is lifted by the means already described
for the foot, F.
It will be apparent, from this description, that the piece of
mechanism here exhibited is a contrivance derived from the motion of
the legs of an animal, and resembling in all respects the fore legs of
a horse. It is however to be regarded rather as a specimen of great
ingenuity than as a contrivance of practical utility.
(86.) It was about this period that the important fact was first
ascertained that the adhesion or friction of the wheels with the rails
on which they moved was amply sufficient to propel the engine, even
when dragging after it a load of great weight; and that in such case,
the progressive motion would be effected without any slipping of the
wheels. The consequence of this fact rendered totally useless all the
contrivances for giving wheels a purchase on the road, such as racks,
chains, feet, &c. The experiment by which this was determined appears
to have been first tried on the Wylam railroad; where it was proved,
that, when the road was level, and the rails clean, the adhesion of
the wheels was sufficient, in all kind of weather, to propel
considerable loads. By manual labour it was first ascertained how much
weight the wheels of a common carriage would overcome without slipping
round on the rail; and having found the proportion which that bore to
the weight, they then ascertained that the weight of the engine would
produce sufficient adhesion to drag after it, on the railroad, the
requisite number of wagons.[25]
[Footnote 25: Wood on Railroads, 2d edit.]
In 1814, an engine was constructed at Killingworth, by Mr.
_Stephenson_, having two cylinders with a cylindrical boiler, and
working two pair of wheels, by cranks placed at right angles; so that
when the one was in full operation, the other was at its dead points.
By these means the propelling power was always in action. The cranks
were maintained in this position by an endless chain, which passed
round two cogged wheels placed under the engine, and which were fixed
on the same axles on which the wheels were placed. The wheels in this
case were fixed on the axles, and turned with them.
[Illustration: _Fig. 54._]
This engine is represented in fig. 54., the sides being open, to
render the interior mechanism visible. A B is the cylindrical boiler;
C C are the working cylinders; D E are the cogged wheels fixed on the
axle of the wheels of the engine, and surrounded by the endless chain.
These wheels being equal in magnitude, perform their revolutions in
the same time; so that, when the crank, F, descends to the lowest
point, the crank, G, rises from the lowest point to the horizontal
position, D; and, again, when the crank, F, rises from the lowest
point to the horizontal position, E, the other crank rises to the
highest point; and so on. A very beautiful contrivance was adopted in
this engine, by which it was suspended on springs of steam. Small
cylinders, represented at H, are screwed by flanges to one side of the
boiler, and project within it a few inches; they have free
communication at the top with the water or steam of the boiler. Solid
pistons are represented at I, which move steam-tight in these
cylinders; the cylinders are open at the bottom, and the piston-rods
are screwed on the carriage of the engine, over the axle of each pair
of wheels, the pistons being presented upwards. As the engine is
represented in the figure, it is supported on four pistons, two at
each side. The pistons are pressed upon by the water or steam which
occupies the upper chamber of the cylinder; and the latter being
elastic in a high degree, the engine has all the advantage of spring
suspension. The defect of this method of supporting the engine is,
that when the steam loses that amount of elasticity necessary for the
support of the machine, the pistons are forced into the cylinders, and
the bottoms of the cylinders bear upon them. All spring suspension is
then lost. This mode of suspension has consequently since been laid
aside.
In an engine subsequently constructed by Mr. _Stephenson_, for the
Killingworth railroad, the mode adopted of connecting the wheels by an
endless chain and cog-wheels was abandoned; and the same effect was
produced by connecting the two cranks by a straight rod. All such
contrivances, however, have this great defect, that, if the fore and
hind wheels be not constructed with dimensions accurately equal, there
must necessarily be a slipping or dragging on the road. The nature of
the machinery requires that each wheel should perform its revolution
exactly in the same time; and consequently, in doing so, must pass
over exactly equal lengths of the road. If, therefore, the
circumference of the wheels be not accurately equal, that wheel which
has the lesser circumference must be dragged along so much of the road
as that by which it falls short of the circumference of the greater
wheel; or, on the other hand, the greater must be dragged in the
opposite direction, to compensate for the same difference. As no
mechanism can accomplish a perfect equality in four, much less in six,
wheels, it may be assumed that a great portion of that dragging
effect is a necessary consequence of the principle of this machine;
and even were the wheels, in the first instance, accurately
constructed, it is not possible that their wear could be so exactly
uniform as to continue equal.
(87.) The next stimulus which the progress of this invention received,
proceeded from the great national work undertaken at Liverpool, by
which that town and the extensive commercial mart of Manchester were
connected by a double line of railway. When this project was
undertaken, it was not decided what moving power it might be most
expedient to adopt as a means of transport on the proposed road: the
choice lay between horse-power, fixed steam engines, and locomotive
engines; but the first, for many obvious reasons, was at once rejected
in favour of one or other of the last two.
The steam engine may be applied, by two distinct methods, to move
wagons either on a turnpike road or on a railway. By the one method
the steam engine is fixed, and draws the carriage or train of
carriages towards it by a chain extending the whole length of road on
which the engine works. By this method the line of road over which the
transport is conducted is divided into a number of short intervals, at
the extremity of each of which an engine is placed. The wagons or
carriages, when drawn by any engine to its own station, or detached,
and connected with the extremity of the chain worked by the next
stationary engine; and thus the journey is performed, from station to
station, by separate engines. By the other method the same engine
draws the load the whole journey, travelling with it.
The Directors of the Liverpool and Manchester railroad, when that work
was advanced towards its completion, employed, in the spring of the
year 1829, Messrs. _Stephenson_ and _Lock_ and Messrs. _Walker_ and
_Rastrick_, experienced engineers, to visit the different railways
where practical information respecting the comparative effects of
stationary and locomotive engines was likely to be obtained; and from
these gentlemen they received reports on the relative merits,
according to their judgment, of the two methods. The particulars of
their calculations are given at large in the valuable work of Mr.
_Nicholas Wood_ on railways; to which we refer the reader, not only on
this, but on many other subjects connected with the locomotive steam
engine into which it would be foreign to our subject to enter. The
result of the comparison of the two systems was, that the capital
necessary to be advanced to establish a line of stationary engines was
considerably greater than that which was necessary to establish an
equivalent power in locomotive engines; that the annual expense by the
stationary engines was likewise greater; and that, consequently, the
expense of transport by the latter was greater, in a like proportion.
The subjoined table exhibits the results numerically:--
+------------------------+------------+-------------+------------------+
| | | Annual | Expense of |
| | Capital. | Expense. | taking a Ton of |
| | | | Goods One Mile. |
+------------------------+------------+-------------+------------------+
| | £ s. d.| £ s. d.| |
| Locomotive Engines | 58,000 0 0 | 25,517 8 2 | 0·164 of a penny.|
| Stationary Engines |121,496 7 0 | 42,031 16 5 | 0·269 |
| +------------+-------------+------------------+
| Locomotive System--less| 63,496 7 0 | 16,514 8 3 | 0·105 |
+------------------------+------------+-------------+------------------+
On the score of economy, therefore, the system of locomotive engines
was entitled to a preference; but there were other considerations
which conspired with this to decide the choice of the Directors in its
favour. An accident occurring in any part of a road worked by
stationary engines must necessarily produce a total suspension of work
along the entire line. The most vigilant and active attention on the
part of every workman, however employed, in every part of the line,
would therefore be necessary; but, independently of this, accidents
arising from the fracture or derangement of any of the chains, or from
the suspension of the working of any of the fixed engines, would be
equally injurious, and would effectually stop the intercourse along
the line. On the other hand, in locomotive engines an accident could
only affect the particular train of carriages drawn by the engine to
which the accident might occur; and even then the difficulty could be
remedied by having a supply of spare engines at convenient stations
along the line. It is true that the _probability_ of accident is,
perhaps, less in the stationary than in the locomotive system; but the
_injurious consequences_, when accident _does_ happen, are
prodigiously greater in the former. "The one system," says Mr.
_Walker_, "is like a chain extending from Liverpool to Manchester, the
failure of a single link of which would destroy the whole; while the
other is like a number of short and unconnected chains," the
destruction of any one of which does not interfere with the effect of
the others, and the loss of which may be supplied with facility.
The decision of the Directors was, therefore, in favour of locomotive
engines; and their next measure was to devise some means by which the
inventive genius of the country might be stimulated to supply them
with the best possible form of engines for this purpose. With this
view, it was proposed and carried into effect to offer a prize for the
best locomotive engine which might be produced under certain proposed
conditions, and to appoint a time for a public trial of the claims of
the candidates. A premium of 500_l._ was accordingly offered for the
best locomotive engine to run on the Liverpool and Manchester Railway;
under the condition that it should produce no smoke; that the pressure
of the steam should be limited to 50 lbs. on the inch; and that it
should draw at least three times its own weight, at the rate of not
less than ten miles an hour; that the engine should be supported on
springs, and should not exceed fifteen feet in height. Precautions
were also proposed against the consequences of the boiler bursting;
and other matters, not necessary to mention more particularly here.
This proposal was announced in the spring of 1829, and the time of
trial was appointed in the following October. The engines which
underwent the trial were, the Rocket, constructed by Mr. _Stephenson_;
the Sanspariel, by _Hackworth_; and the Novelty, by Messrs.
_Braithwait_ and _Ericson_. Of these, the Rocket obtained the premium.
A line of railway was selected for the trial, on a level piece of road
about two miles in length, near a place called Rainhill, between
Liverpool and Manchester; the distance between the two stations was a
mile and a half, and the engine had to travel this distance backwards
and forwards ten times, which made altogether a journey of 30 miles.
The Rocket performed this journey twice: the first time in 2 hours 14
minutes and 8 seconds; and the second time, in 2 hours 6 minutes and
49 seconds. Its speed at different parts of the journey varied: its
greatest rate of motion was rather above 29 miles an hour; and its
least, about 11-1/2 miles an hour. The average rate of the one journey
was 13-4/10 miles an hour; and of the other, 14-2/20 miles. This was
the only engine which performed the complete journey proposed, the
others having been stopped from accidents which occurred to them in
the experiment. The Sanspariel performed the distance between the
stations eight times, travelling 22-1/2 miles in 1 hour 37 minutes and
16 seconds. The greatest velocity to which this engine attained was
something less than 23 miles per hour. The Novelty had only passed
twice between the stations when the joints of the boiler gave way, and
put an end to the experiment.
(88.) The great object to be attained in the construction of these
engines was, to combine with sufficient lightness the greatest
possible heating power. The fire necessarily acts on the water in two
ways: first, by its radiant heat; and, second, by the current of
heated air which is carried by the draft through the fire, and finally
passes into the chimney. To accomplish this object, therefore, it is
necessary to expose to both these sources of heat the greatest
possible quantity of surface in contact with the water. These ends
were attained by the following admirable arrangement in the Rocket:--
[Illustration: _Fig. 55._]
[Illustration: _Fig. 56._]
This engine is represented in fig. 55. It is supported on four wheels;
the principal part of the weight being thrown on one pair, which are
worked by the engine. The boiler consists of a cylinder 6 feet in
length, with flat ends; the chimney issues from one end, and to the
other end is attached a square box, B, the bottom of which is
furnished with the grate on which the fuel is placed. This box is
composed of two casings of iron, one contained within the other,
having between them a space about 3 inches in breadth; the magnitude
of the box being 3 feet in length, 2 feet in width, and 3 feet in
depth. The casing which surrounds the box communicates with the lower
part of the boiler by a pipe marked C; and the same casing at the top
of the box communicates with the upper part of the boiler by another
pipe marked D. When water is admitted into the boiler, therefore, it
flows freely through the pipe C, into the casing which surrounds the
furnace or fire-box, and fills this casing to the same level as that
which it has in the boiler. When the engine is at work, the boiler is
kept about half filled with water; and, consequently, the casing
surrounding the furnace is completely filled. The steam which is
generated in the water contained in the casing finds its exit through
the pipe D, and escapes into the upper part of the boiler. A section
of the engine, taken at right angles to its length is represented at
fig. 56. Through the lower part of the boiler pass a number of copper
tubes of small size, which communicate at one end with the fire-box,
and at the other with the chimney, and form a passage for the heated
air from the furnace to the chimney. The ignited fuel spread on the
grate at the bottom of the fire-box disperses its heat by radiation,
and acts in this manner on the whole surface of the casing surrounding
the fire-box; and thus raises the temperature of the thin shell of
water contained in that casing. The chief part of the water in the
casing, being lower in its position than the water in the boiler,
acquires a tendency to ascend when heated, and passes into the boiler;
so that a constant circulation of the heated water is maintained, and
the water in the boiler must necessarily be kept at nearly the same
temperature as the water in the casing. The air which passes through
the burning fuel, and which fills the fire-box, is carried by the
draft through the tubes which extend through the lower part of the
boiler; and as these tubes are surrounded on every side with the water
contained in the boiler, this air transmits its heat through these
tubes to the water. It finally issues into the chimney, and rises by
the draft. The power of this furnace must necessarily depend on the
power of draft in the chimney; and to increase this, and at the same
time to dispose of the waste steam after it has worked the piston,
this steam is carried off by a pipe L, which passes from the cylinder
to the chimney, and escapes there in a jet which is turned upwards. By
the velocity with which it issues from this jet, and by its great
comparative levity, it produces a strong current upwards in the
chimney, and thus gives force to the draft of the furnace. In fig. 56.
the grate-bars are represented at the bottom of the fire-box at F.
There are two cylinders, one of which works each wheel; one only
appearing in the drawing, fig. 55., the other being concealed by the
engine. The spokes which these cylinders work are placed at right
angles on the wheels; the wheels being fixed on a common axle, with
which they turn.
In this engine, the surface of water surrounding the fire-box, exposed
to the action of radiant heat, amounted to 20 square feet, which
received heat from the surface of 6 square feet of burning fuel on the
bars. The surface exposed to the action of the heated air amounted to
118 square feet. The engine drew after it another carriage, containing
fuel and water; the fuel used was coke, for the purpose of avoiding
the production of smoke.
(89.) The Sanspareil of Mr. _Hackworth_ is represented in fig. 57.;
the horizontal section being exhibited in fig. 58.
[Illustration: _Fig. 57._]
[Illustration: _Fig. 58._]
The draft of the furnace is produced in the same manner as in the
Rocket, by ejecting the waste steam coming from the cylinder into the
chimney; the boiler, however, differs considerably from that of the
Rocket. A recurved tube passes through the boiler, somewhat similar to
that already described in the early engine of Messrs. _Trevithick_ and
_Vivian_. In the horizontal section (fig. 58.), D expresses the
opening of the furnace at the end of the boiler, beside the chimney.
The grate-bars appear at A, supporting the burning fuel; and a curved
tube passing through the boiler, and terminating in the chimney, is
expressed at B, the direction of the draft being indicated by the
arrow; C is a section of the chimney. The cylinders are placed, as in
the Rocket, on each side of the boiler; each working a separate
wheel, but acting on spokes placed at right angles to each other. The
tube in which the grate and flue are placed diminishes in diameter as
it approaches the chimney. At the mouth where the grate was placed,
its diameter was 2 feet; and it was gradually reduced, so that, at the
chimney, its diameter was only 15 inches. The grate-bars extended 5
feet into the tube. The surface of water exposed to the radiant heat
of the fire was 16 square feet; and that exposed to the action of the
heated air and flame was about 75 square feet. The magnitude of the
grate or sheet of burning fuel which radiated heat, was 10 square
feet.
(90.) The Novelty, of Messrs. _Braithwait_ and _Ericson_, is
represented in fig. 59.; and a section of the generator and boiler is
exhibited in fig. 60.: the corresponding parts in the two figures are
marked by the same letters.
[Illustration: _Fig. 59._]
A is the generator or receiver, containing the steam which works the
engine; this communicates with a lower generator, B, which extends in
a horizontal direction the entire length of the carriage. Within the
generator, A, is contained the furnace, F, which communicates in a
tube, C; carried up through the generator, and terminated at the top
by sliding shutters, which exclude the air, and which are only opened
to supply fuel to the grate, F. Below the grate the furnace is not
open, as usual, to the atmosphere, but communicates by a tube, E, with
a bellows, D; which is worked by the engine, and which forces a
constant stream of air, by the tube E, through the fuel on F, so as to
keep that fuel in vivid combustion. The heated air, contained in the
furnace, F, is driven on, by the same force, through a small curved
tube marked _e_, which circulates like a worm (as represented in fig.
60.) through the horizontal generator or receiver; and, tapering
gradually, until reduced to very small dimensions, it finally issues
into the chimney, G. The air, in passing along this tube, imparts its
heat to the water by which the tube is surrounded, and is brought to a
considerably reduced temperature when discharged into the chimney. The
cylinder, which is represented at K, works one pair of wheels, by
means of a bell-crank; the other pair, when necessary, being connected
with them.
[Illustration: _Fig. 60._]
In this engine, the magnitude of the surface of burning fuel on the
grate-bars is less than 2 square feet; the surface exposed to radiant
heat is 9-1/2 square feet; and the surface of water exposed to heated
air is about 33 square feet.
The superiority of the Rocket may be attributed chiefly to the greater
quantity of surface of the water which is exposed to the action of the
fire. With a less extent of grate-bars than the Sanspareil, in the
proportion of 3 to 5, it exposes a greater surface of water to radiant
heat, in the proportion of 4 to 3; and a greater surface of water to
heated air, in the proportion of more than 3 to 2. It was found that
the Rocket, compared with the Sanspareil, consumed fuel, in the
evaporation of a given quantity of water, in the proportion of 11 to
28. The suggestion of using the tubes to conduct through the water the
heated air to the chimney is due to Mr. _Booth_, treasurer of the
Liverpool and Manchester Railway Company; and, certainly, nothing has
been more conducive to the efficiency of the engines since used than
this improvement. It is much to be regretted that the ingenious
gentleman who suggested this has reaped none of the advantages to
which a patentee would be legally entitled.[26]
[Footnote 26: Mr. _Booth_ received a part of the premium of 500_l._,
but has not participated in any degree in the profits of the
manufacture of the engines.]