roads, the latter have another disadvantage, the effects of which
have probably been exaggerated by those who are opposed to this
application of steam-power. One of the [Pg421] laws of adhesion
long since developed by experiment, and established as a principle
of practical science, is that the adhesion is greater between
surfaces of the same than between surfaces of a different kind. Thus
between two metals of the same kind, the adhesion corresponding to
any given pressure is greater than between two metals of different
kinds; between two metals of any sort the adhesion is greater than
between metal and stone, or between metal and wood. Hence, the
wheels of steam-carriages running on a railroad have a greater
adhesion with the road, and therefore offer a greater resistance to
slip round without the advance of the carriage, than wheels would
offer on a turnpike road; for on a railroad the iron tire of the
wheel rests in contact with the iron rail, while on a common road
the iron tire rests in contact with the surface of stone, or
whatever material the road may be composed of. Besides this, the
dust and loose matter which necessarily collect on a common road,
when pressed between the wheels and the solid base of the road, act
somewhat in the manner of rollers, and give the wheels a greater
facility to slip than if the road were swept clean, and the wheels
rested in immediate contact with its hard surface. The truth of this
observation is illustrated on the railroads themselves, where the
adhesion is found to be diminished whenever the rails are covered
with any extraneous matter, such as dust or moist clay. Although the
adhesion of the wheels of a carriage with a common road, however, be
less than those of the wheels of a steam-carriage with a railroad,
yet still the actual adhesion on turnpike roads is greater in amount
than has been generally supposed, and is quite sufficient to propel
carriages drawing after them loads of large amount.
The relative facility with which carriages are propelled on
railroads and turnpike-roads equally affects any moving power,
whether that of horses or steam engines; and whether loads be
propelled by the one power or the other, the railroad, as compared
with the turnpike-road, will always possess the same proportionate
advantage; and a given amount of power, whether of the one kind or
the other, will always perform a quantity of work less in the same
proportion on a [Pg422] turnpike-road than on a rail-road. But, on
the other hand, the expense of original construction, and of
maintaining the repairs of a rail-road, is to be placed against the
certain facility which it offers to draught.
In the attempts which have been made to adapt locomotive engines to
turnpike-roads, the projectors have aimed at the accomplishment of
two objects: first, the construction of lighter and smaller engines;
and, secondly, increased power. These ends, it is plain, can only be
attained, with our present knowledge, by the production of steam of
very high temperature and pressure, so that the smallest volume of
steam shall produce the greatest possible mechanical effect. The
methods of propelling the carriage have been in general similar to
that used in the railroad engines, viz. either by cranks placed on
the axles, the wheels being fixed upon the same axles, or by
connecting the piston rods with the spokes of the wheels. In some
carriages, the boiler and moving power, and the body of the carriage
which bears the passengers, are placed on the same wheels. In
others, the engine is placed on a separate carriage, and draws after
it the carriage which transports the passengers, as is always the
case on railways.
The chief difference between the steam engines used on railways, and
those adapted to propel carriages on turnpike roads, is in the
structure of the boiler. In the latter it is essential that, while
the power remains undiminished, the boiler should be lighter and
smaller. The accomplishment of this has been attempted by various
contrivances for so distributing the water as to expose a
considerable quantity of surface in contact with it to the action of
the fire: spreading it in thin layers on flat plates; inserting it
between plates of iron placed at a small distance asunder, the fire
being admitted between the intermediate plates; dividing it into
small tubes, round which the fire has play; introducing it between
the surfaces of cylinders placed one within another, the fire being
admitted between the alternate cylinders,—have all been resorted to
by different projectors.
(204.) First and most prominent in the history of the application of
steam to the propelling of carriages on turnpike roads stands the
name of Mr. Goldsworthy Gurney, a medical [Pg423] gentleman, and
scientific chemist, of Cornwall. In 1822, Mr. Gurney succeeded Dr.
Thompson as lecturer on chemistry at the Surrey Institution; and, in
consequence of the results of some experiments on heat, his
attention was directed to the project of working steam-carriages on
common roads; and he subsequently devoted his exertions in
perfecting a steam-engine capable of attaining the end he had in
view.
The mistake which so long prevailed in the application of
locomotives on railroads, and which, as we have shown, materially
retarded the progress of that invention, was shared by Mr. Gurney.
Without reducing the question to the test of experiment, he took for
granted, in his first attempts, that the adhesion of the wheels with
the road was too slight to propel the carriage. He was assured, he
says, by eminent engineers, that this was a point settled by actual
experiment. It is strange, however, that a person of his quickness
and sagacity did not inquire after the particulars of these "actual
experiments." So, however, it was; and, taking for granted the
inability of the wheels to propel, he wasted much labour and skill
in the contrivance of levers and propellers, which acted on the
ground in a manner somewhat resembling the feet of horses, to drive
the carriage forward. After various fruitless attempts of this kind,
the experience acquired in the trials to which they gave rise at
last forced the truth upon his notice, and he found that the
adhesion of the wheels was not only sufficient to propel the
carriage heavily laden on level roads, but was capable of causing it
to ascend all the hills which occur on ordinary turnpike-roads. In
this manner it ascended all the hills between London and Barnet,
London and Stanmore, Stanmore Hill, Brockley Hill, and mounted Old
Highgate Hill, the last at one point rising one foot in nine.
[Illustration: _Fig._ 114.]
[Illustration: _Fig._ 115.]
The boiler of Mr. Gurney's engine is so constructed, that there is
no part of it in which metal exposed to the action of the fire is
out of contact with water. If it be considered how rapidly the
action of an intense furnace destroys metal when water is not
present to prevent the heat from accumulating, the advantage of
this circumstance will be appreciated. In the boiler of Mr.
Gurney, the grate-bars [Pg424] themselves are tubes filled with
water, and form, in fact, a part of the boiler itself. This boiler
consists of three strong metal cylinders placed in a horizontal
position one above the other. A section, made by a perpendicular
or vertical plane, is represented in _fig._ 114. The ends of the
three cylinders just mentioned are represented at D, H, and I. In
the side of the lowest cylinder D are inserted a row of tubes, a
ground plan of which is represented in _fig._ 115. These tubes,
proceeding from the side of the lowest cylinder D, are inclined
[Pg425] slightly upwards, for a reason which I shall presently
explain. From the nature of the section, only one of these tubes
is visible in _fig._ 114. at C. The other extremities of these
tubes at A are connected with the same number of upright tubes,
one of which is shown at E. The upper extremities G of these
upright tubes are connected with another set of tubes K, equal in
number, proceeding from G, inclining slightly upwards, and
terminating in the second cylinder H.
[Illustration: _Fig._ 116.]
An end view of the boiler is exhibited in _fig._ 116., where the
three cylinders are expressed by the same letters. Between the
cylinders D and H there are two tubes of communication B, and two
similar tubes between the cylinders H and I. From the nature of
the section these appear only as a single tube in _fig._ 114. From
the top of the cylinder I proceeds a tube N, by which steam is
conducted to the engine.
It will be perceived that the space F is enclosed on every side by
a grating of tubes, which have free communication with the
cylinders D and H, which cylinders have also a free communication
with each other by the tubes B. It follows, [Pg426] therefore,
that if water be supplied to the cylinder I, it will descend
through the tubes, and first filling the cylinder D and the tubes
C, will gradually rise in the tubes B and E, will next fill the
tubes K and the cylinder H. The grating of water-pipes C E K forms
the furnace, the pipes C being the fire-bars, and the pipes E and
K being the back and roof of the stove. The fire-door, for the
supply of fuel, appears at M, fig. 116. The flue issuing between
the tubes F is conducted over the tubes K, and the flame and hot
air are carried off through a chimney. That portion of the heat of
the burning fuel, which in other furnaces destroys the bars of the
grate, is here expended in heating the water contained in the
tubes C. The radiant heat of the fire acts upon the tubes K,
forming the roof of the furnace, on the tube E at the back of it,
and partially on the cylinders D and H, and the tubes B. The draft
of hot air and flame passing into the flue at A acts upon the
posterior surfaces of the tubes E, and the upper sides of the
tubes K, and finally passes into the chimney.
As the water in the tubes C E K is heated, it becomes specifically
lighter than water of a less temperature, and consequently
acquires a tendency to ascend. It passes, therefore, rapidly into
H. Meanwhile the colder portions descend, and the inclined
positions of the tubes C and K give play to this tendency of the
heated water, so that a prodigiously rapid circulation is
produced, when the fire begins to act upon the tubes. When the
water acquires such a temperature that steam is rapidly produced,
steam-bubbles are constantly formed in the tubes surrounding the
fire; and if these remained stationary in the tubes, the action of
the fire would not only decompose the steam, but render the tubes
red hot, the water not passing through them to carry off the heat.
But the inclined position of the tubes, already noticed,
effectually prevents this injurious consequence. A steam-bubble,
which is formed either in the tubes C or K, having a tendency to
ascend proportional to its lightness as compared with water,
necessarily rushes upwards; if in C towards A, and if in K towards
H. But this motion of the steam is also aided by the rapid
circulation of the water which is continually maintained [Pg427]
in the tubes, otherwise it might be possible, notwithstanding the
levity of steam compared with water, that a bubble might remain in
a narrow tube without rising. To bring the matter to the test of
experiment, I have connected two cylinders, such as D and H, by a
system of glass tubes, such as represented at C E K. The rapid and
constant circulation of the water was then made evident: bubbles
of steam were formed in the tubes, it is true; but they passed
with great rapidity into the upper cylinder, and rose to the
surface, so that the glass tubes never acquired a higher
temperature than that of the water which passed through them.
Every part of the boiler being cylindrical, it has the form which,
mechanically considered, is most favourable to strength, and
which, within given dimensions, contains the greatest quantity of
water. It is also free from the defects arising from unequal
expansion, which are found to be most injurious in tubular
boilers. The tubes C and K can freely expand in the direction of
their length, without being loosened at their joints, and without
straining any part of the apparatus; the tubes E, being short, are
subject to a very slight degree of expansion; and it is obvious
that the long tubes, with which they are connected, will yield to
this without suffering a strain, and without causing any part of
the apparatus to be loosened.
When water is converted into steam, any foreign matter which may
be combined with it is disengaged, and is deposited on the bottom
of the vessel in which the water is evaporated. All boilers,
therefore, require occasional cleansing, to prevent the crust thus
formed from accumulating; and this operation, for obvious reasons,
is attended with peculiar difficulty in tubular boilers. In the
case before us, the crust of deposited matter would gather and
thicken in the tubes C and K, and if not removed, would at length
choke them. But besides this, it would be attended with a still
worse effect; for, being a bad conductor, it would intercept the
heat in its transit from the fire to the water, and would cause
the metal of the tube to become unduly heated. Mr. Gurney of
course foresaw this inconvenience, and contrived an ingenious
chemical method of removing it, by occasionally injecting [Pg428]
through the tubes such an acid as would combine with the deposit,
and carry it away. This method was effectual; and although its
practical application was found to be attended with difficulty in
the hands of common workmen, Mr. Gurney was persuaded to adhere to
it by the late Dr. Wollaston, until experience proved the
impossibility of getting it effectually performed, under the
circumstances in which boilers are commonly used. Mr. Gurney then
adopted a method of removing the deposit by mechanical means.
Opposite the mouths of the tubes, and on the other side of the
cylinders D and H, are placed a number of holes, which, when the
boiler is in use, are stopped by pieces of metal screwed into
them. When the tubes require to be cleaned, these stoppers are
removed, and an iron scraper is introduced through the holes into
the tubes, which, being passed backwards and forwards, removes the
deposit.
In these engines the draught through the furnace was produced by
projecting the waste steam up the chimneys as is practised in
railway engines; a method so perfectly effectual, that it is
unlikely to be superseded by any other. The objection which has
been urged against it in locomotive engines, working on
turnpike-roads, is, that the noise which it produces has a
tendency to frighten horses.
In the engines on the Liverpool road, the steam is allowed to pass
directly from the eduction pipe of the cylinder to the chimney,
and it there escapes in puffs corresponding with the alternate
motion of the pistons, and produces a noise, which, although
attended with no inconvenience on the railroad, would perhaps be
objectionable on turnpike-roads. In the engine used in Mr.
Gurney's steam-carriage, the steam which passes from the cylinders
is conducted to a receptacle, which he calls a blowing box. This
box serves the same purpose as the upper chamber of a smith's
bellows. It receives the steam from the cylinders in alternate
puffs, but lets it escape into the chimney in a continued stream
by a number of small jets. Regular draught is by this means
produced, and no noise is perceived. Another exit for the steam is
also provided, by which the conductor is enabled to increase or
diminish, or to suspend altogether, the draught [Pg429] in the
chimney, so as to adapt the intensity of the fire to the
exigencies of the road. This is a great convenience in practice;
because on some roads a draught is scarcely required, while on
others a powerful blast is indispensable.
Connected with this blowing box is another apparatus of
considerable practical importance. The pipe through which the
feeding water is conducted from the tank is carried through this
blowing box, within which it is coiled in a spiral form, so that
an extensive thread of the water is exposed to the heat of the
waste steam which has escaped from the cylinders, and which is
enclosed in this blowing box. In passing through this pipe the
feeding water is raised from the ordinary temperature of about 60°
to the temperature of 212°. Fuel is thus economised and weight
diminished; but there is another still greater advantage attending
this process. The feeding water in the worm just mentioned, while
it takes up the heat from the surrounding steam in the blowing
box, condenses a part of the waste steam, which is thence
conducted to the tank, from which the feeding water is pumped.
When steam is generated so rapidly as is necessarily the case in
locomotive boilers, it rises with great violence in numerous
bubbles from the bottom of the boiler to the surface of the water,
and puts the liquid into a state of foaming turbulence not unlike
the sea in a storm. As the steam rushes from the surface into the
upper part of the boiler, under these circumstances, it carries
with it a spray by which water is scattered in minute subdivision
among the steam, and floats there like the spray which rises from
the base of a cascade. If the steam be conducted immediately to
the cylinder from the boiler in this state, it will carry with it
the water which is thus suspended in it, which will pass through
the cylinder, and finally be driven into the atmosphere upon the
returning stroke of the piston. The hot water thus carried off
possesses none of the mechanical properties of steam, and is
wholly inefficient as a moving power, and is therefore an
extensive source of the waste of heat. In every boiler, some means
should be provided for the separation of the water thus suspended
in the steam, before the steam is conducted to the cylinder. In
ordinary boilers, the large space which [Pg430] remains above the
surface of the water serves this purpose. The steam being there
subject to no agitation or disturbance, the water mechanically
suspended in it descends by its own gravity, and leaves pure steam
in the upper part. In the small tubular boilers, this has been a
matter, however, of greater difficulty. The contracted space in
which the ebullition takes place causes the water to be mixed with
the steam in a greater quantity than could happen in common
boilers; and the want of the same steam-room renders the
separation of the water from the steam a matter of some
difficulty. These inconveniences have been attempted to be
overcome by various contrivances. I have already described the
rapid and regular circulation effected by the arrangement of the
tubes. By this a regularity in the currents is established, which
has a tendency to diminish the mixture of water with the steam. In
addition to this, a method of separation is provided in the vessel
I, which is a strong iron cylinder of some magnitude, placed out
of the immediate influence of the fire. A partial separation of
the steam from the water takes place in the cylinder H; and the
steam with the water mechanically suspended in it, technically
called moist steam, rises into the _separator_ I. Here, being free
from all agitation and currents, and being, in fact, quiescent,
the particles of water fall to the bottom, while the pure steam
remains at the top. This separator, therefore, serves all the
purposes of the steam-room above the surface of the water in the
large plate boilers. The dry steam is thus collected and ready for
the supply of the engine through the tube N, while the water,
which is disengaged from it, is collected at the bottom of the
separator, and is conducted through the tube T to the lowest
vessel D, to be again circulated through the boiler.
The pistons of the engine work on the axles of the hind wheels of
the carriage which bears the engine, by cranks, as in the
locomotives on the Manchester railway, so that the axle is kept in
a constant state of rotation while the engine is at work. The
wheels placed on this axle are not permanently fixed or keyed upon
it, as in the Manchester locomotives; but they are capable of
turning upon it in the same manner as ordinary carriage wheels.
Immediately within [Pg431] these wheels there are fixed upon the
axles two projecting spokes or levers, which revolve with the
axle, and which take the position of two opposite spokes of the
wheel. These may be occasionally attached to the wheel or detached
from it; so that they are capable of compelling the wheels to turn
with the axle, or leaving the axle free to turn independently of
the wheel, or the wheel independent of the axle, at the pleasure
of the conductor. It is by these levers that the engine is made to
propel either or both of the wheels. If both pairs of spokes are
thrown into connexion with the wheels, the crank shaft or axle
will cause both wheels to turn with it, and in that case the
operation of the carriage is precisely the same as those of the
locomotives already described upon the Liverpool and Manchester
line; but this is rarely found to be necessary, since the adhesion
of one wheel with the road is generally sufficient to propel the
carriage, and consequently only one pair of these fixed levers are
used, and the carriage propelled by only one of the two hind
wheels. The fore wheels of the carriage turn upon a pivot similar
to those of a four-wheeled coach. The position of these wheels is
changed at pleasure by a pinion and circular rack, which is moved
by the conductor, and in this manner the carriage is guided with
precision and facility.
The force of traction necessary to propel a carriage upon common
roads must vary with the variable quality of the road, and
consequently the propelling power, or the pressure upon the pistons
of the engine, must be susceptible of a corresponding variation; but
a still greater variation becomes necessary from the undulations and
hills which are upon all ordinary roads. This necessary change in
the intensity of the impelling power is obtained by restraining the
steam in the boiler by the throttle-valve, as already described in
the locomotive engines on the railroad. This principle, however, is
carried much further in the present case. The steam in the boiler
maybe at a pressure of from 100 to 200 lbs. on the square inch;
while the steam on the working piston may not exceed 30 or 40 lbs.
on the inch. Thus an immense increase of power is always at the
command of the conductor; so that when a hill is encountered, or a
rough piece of road, [Pg432] he is enabled to lay on power
sufficient to meet the exigency of the occasion.
The two difficulties which have been always apprehended in the
practical working of steam-carriages upon common roads are, first,
the command of sufficient power for hills and rough pieces of
road; and, secondly, the apprehended insufficiency of the adhesion
of the wheels with the road to propel the carriage. The former of
these difficulties has been met by allowing steam of very great
pressure to be constantly maintained in the boiler with perfect
safety. As to the second, all experiments tend to show that there
is no ground for the supposition that the adhesion of the wheels
is in any case insufficient for the purposes of propulsion. Mr.
Gurney states, that he has succeeded in driving carriages thus
propelled, up considerable hills on the turnpike roads about
London. He made a journey to Barnet with only one wheel attached
to the axle, which was found sufficient to propel the carriage up
all hills upon that road. The same carriage, with only one
propelling wheel, also went to Bath, and surmounted all the hills
between Cranford Bridge and Bath, going and returning.
A double stroke of the piston produces one revolution of the
propelling wheels, and causes the carriage to move through a space
equal to the circumference of those wheels. It will therefore be
obvious, that the greater the diameter of the wheels, the better
adapted the carriage is for speed; and, on the other hand, wheels
of smaller diameter are better adapted for power. In fact, the
propelling power of an engine on the wheels will be in the inverse
proportion of their diameter. In carriages designed to carry great
weights at a moderate speed, smaller wheels will be used; while in
those intended for the transport of passengers at considerable
velocities, wheels of at least 5 feet diameter are most
advantageous.