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 tire of the
wheel and the surface of the road were absolutely smooth and free
from friction, so that the face of the tire 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 tire slipping or
sliding upon the road as the wheel is made to revolve. But if, on
the other hand, the pressure of the face of the tire 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 [Pg336] 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. Trevethick 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. [Pg337]
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._ 86.]
A sketch of these propellers is given in _fig._ 86. 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 [Pg338]
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_ [Pg339] _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.
(186.) 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 kinds 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 waggons.[31]
In 1814, an engine was constructed at Killingworth, by Mr.
Stephenson, having two cylinders with a cylindrical [Pg340]
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._ 87.]
This engine is represented in _fig._ 87., 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 [Pg341]
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 wheel 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.
(187.) The next stimulus which the progress of this [Pg342]
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
waggons 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 waggons or carriages, when drawn by any engine to its
own station, are 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 object to enter. The result of the comparison of the two
systems was, that the capital [Pg343] 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:—
—————————————————————————————————————————————————————————————————
| | | Expense of
| | | taking a
| | Annual | Ton of Goods
| Capital. | Expense. | a Mile.
——————————————————-+——————————————-+——————————————-+—————————————
| £ s. d.| £ s. d.|
Locomotive engines | 58,000 0 0 | 25,517 8 2 | 0·164 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 [Pg344] 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 five hundred pounds 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 fifty pounds 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 Sanspareil, by Hackworth; and
the Novelty, by Messrs. Braithwaite 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 thirty 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 [Pg345] 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 Sanspareil
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.
(188.) 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 draught through the
flues, 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._ 88.]
[Illustration: _Fig._ 89.]
This engine is represented in _fig._ 88. 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
six 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 three inches in
breadth; the magnitude of the box being three feet in length, two
feet in width, and three 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 [Pg346] 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._ 89. 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 [Pg347] 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 draught 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 draught. The
power of this furnace must necessarily depend on the power of
draught 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 draught of the
furnace. In _fig._ 89. 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._ 88.,
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 twenty square
feet, which received heat from the surface of six 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.