90.; the horizontal section being exhibited in _fig._ 91.
[Illustration: _Fig._ 90.]
[Illustration: _Fig._ 91.]
The draught of the furnace is produced in the same manner as in
the Rocket, by ejecting the waste steam coming from [Pg348] 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. Trevethick and Vivian. In the
horizontal section (_fig._ 91.), 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 [Pg349] of the draught 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
two feet; and it was gradually reduced, so that, at the chimney,
its diameter was only fifteen inches. The grate-bars extended five
feet into the tube. The surface of water exposed to the radiant
heat of the fire was sixteen square feet; and that exposed to the
action of the heated air and flame was about seventy-five square
feet. The magnitude of the grate, or sheet of burning fuel which
radiated heat, was ten square feet.
(190.) The Novelty, of Messrs. Braithwaite and Ericson, is
represented in _fig._ 92.; and a section of the generator and
boiler is exhibited in _fig._ 93.; the corresponding parts in the
two figures are marked by the same letters.
[Illustration: _Fig._ 92.]
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, [Pg350] 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._ 93.)
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._ 93.]
In this engine, the magnitude of the surface of burning fuel on
the grate-bars is less than two square feet; the surface exposed
to radiant heat is nine and a half square feet; and the surface of
water exposed to heated air is about thirty-three 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 three to five, it exposes a
greater surface of water to radiant heat, in the proportion of
four to three; and a greater surface of water to heated air, in
the proportion of more than three to two. It was found that
the Rocket, compared with the Sanspareil, consumed fuel, in
the evaporation of a given quantity of water, [Pg351] in the
proportion of eleven to twenty-eight. 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.
(191.) The object to be effected in the boilers of these engines
is, to keep a small quantity of water at an excessive temperature,
by means of a small quantity of fuel kept in the most active state
of combustion. To accomplish this, it is necessary, first, so to
shape the boiler, furnace, and flues, that the water shall be in
contact with as extensive a surface as possible, every part of
which is acted on, either immediately, by the heat radiating from
the fire, or mediately, by the air which has passed through the
fire, and which finally rushes into the chimney: and, secondly,
that such a forcible draught should be maintained in the furnace,
that a quantity of heat shall be extricated from the fuel, by
combustion, sufficient to maintain the water at the necessary
temperature, and to produce the steam with sufficient rapidity. To
accomplish these objects, therefore, the chamber containing the
grate should be completely surrounded by water, and should be
below the level of the water in the boiler. The magnitude of the
surface exposed to radiation should be as great as is consistent
with the whole magnitude of the machine. The comparative advantage
which the Rocket possessed in these respects over the other
engines will be evident on inspection. In the next place, it is
necessary that the heat, which is absorbed by the air passing
through the fuel, and keeping it in a state of combustion, should
be transferred to the water before the air escapes into the
chimney. Air being a bad conductor of heat, to accomplish this it
is necessary that the air in the flues should be exposed to as
great an extent of surface in contact with the water as possible.
No contrivance can be less adapted for the attainment of this end
than one or two large tubes traversing the boiler, as in the
earliest locomotive engines: the body of air which passed through
the centre of these tubes had no contact with their surface, and,
consequently, passed into the chimney at nearly the same
temperature as that which it had when it quitted the fire. The
only portion of air which imparted its heat to the water [Pg352]
was that portion which passed next to the surface of the tube.
Several methods suggest themselves to increase the surface of
water in contact with a given quantity of air passing through it.
This would be accomplished by causing the air to pass between
plates placed near each other, so as to divide the current into
thin strata, having between them strata of water, or it might be
made to pass between tubes differing slightly in diameter, the
water passing through an inner tube, and being also in contact
with the external surface of the outer tube. Such a method would
be similar in principle to the steam-jacket used in Watt's steam
engines, or to the condenser of Cartwright's engine already
described. But, considering the facility of constructing small
tubes, and of placing them in the boiler, that method, perhaps,
is, on the whole, the best in practice; although the shape of a
tube, geometrically considered, is most unfavourable for the
exposure of a fluid contained in it to its surface. The air which
passes from the fire-chamber, being subdivided as it passes
through the boiler by a great number of very small tubes, may be
made to impart all its excess of heat to the water before it
issues into the chimney. This is all which the most refined
contrivance can effect. The Rocket engine was traversed by
twenty-five tubes, each three inches in diameter; and the
principle has since been carried to a much greater extent.
The abstraction of a great quantity of heat from the air before it
reaches the chimney is attended with one consequence, which, at
first view, would present a difficulty apparently insurmountable;
the chimney would, in fact, lose its power of draught. This
difficulty, however, was removed by using the waste steam, which
had passed from the cylinder after working the engine, for the
purpose of producing a draught. This steam was urged through a jet
presented upwards in the chimney, and driven out with such force
in that direction as to create a sufficient draught to work the
furnace.
It will be observed that the principle of draught in the Novelty
is totally distinct from this: in that engine the draught is
produced by a bellows worked by the engine. The question, as far
as relates to these two methods, is, whether more power [Pg353]
is lost in supplying the steam through the jet, as in the Rocket,
or in working the bellows, as in the Novelty. The force requisite
to impel the steam through the jet must be exerted by the
returning stroke of the piston, and, consequently, must rob the
working effect to an equivalent amount. On the other hand, the
power requisite to work the bellows in the Novelty must be
subducted from the available power of the engine. The former
method has been hitherto found to be the more effectual and
economical.
The importance of these details will be understood, when it is
considered that the only limit to the attainment of speed by
locomotive engines is the power to produce, in a given time, a
certain quantity of steam. Each stroke of the piston causes one
revolution of the wheels, and consumes four cylinders full of
steam: consequently, a cylinder of steam corresponds to a certain
number of feet of road travelled over: hence it is that the
production of a rapid and abundant supply of heat, and the
imparting of that heat quickly and effectually to the water, is
the key to the solution of the problem to construct an engine
capable of rapid motion.
The method of subdividing the flue into tubes was carried much
further by Mr. Stephenson after the construction of the Rocket;
and, indeed, the principle was so obvious, it is only surprising
that, in the first instance, tubes of smaller diameter than three
inches were not used. In engines since constructed, the number of
tubes vary from ninety to one hundred and twenty, the diameter
being reduced to two inches or less; and in some instances tubes
have been introduced, even to the number of one hundred and fifty,
of one and a half inch diameter. In the Meteor, twenty square feet
are exposed to radiation, and one hundred and thirty-nine to the
contact of heated air; in the Arrow, twenty square feet to
radiation, and one hundred and forty-five to the contact of heated
air. The superior economy of fuel gained by this means will be
apparent by inspecting the following table, which exhibits the
consumption of fuel which was requisite to convey a ton weight a
mile in each of four engines, expressing also the rate of the
motion:— [Pg354]
————————————————————————————————————————————————————————————
| Average Rate of | Consumption of Coke
Engines. | Speed in Miles per | in Pounds per Ton
| Hour. | per Mile.
————————————————————————————————————————————————————————————
No. 1. Rocket | 14 | 2·41
2. Sanspareil | 15 | 2·47
3. Phœnix | 12 | 1·42
4. Arrow | 12 | 1·25
————————————————————————————————————————————————————————————
(192.) Since the period at which this railway was opened for the
actual purposes of transport, the locomotive engines have been in
a state of progressive improvement. Scarcely a month has passed
without suggesting some change in the details, by which fuel might
be economised, the production of steam rendered more rapid, the
wear of the engine rendered slower, the proportionate strength of
the different parts improved, or some other desirable end
obtained.
Engines constructed in the form of the Rocket, were subject to two
principal defects. The cylinders, being placed outside the engine,
were exposed to the cold of the atmosphere, which produced a waste
of heat more or less considerable by condensation. The points at
which the power of the steam to turn the wheels was applied, being
at the extremities of the axle and on the exterior of the wheel, a
considerable strain was produced, owing to the distance of the
point of application of the power from the centre of resistance.
If it were possible that the impelling power could act in drawing
the train at all times with equal energy on both sides of the
engine, then no injurious strain would be produced; but from the
relative position of the points on the opposite wheels to which it
was necessary to attach the connecting rods, it was inevitable
that, at the moment when one of the pistons exerts its full power
in driving the wheel, the other piston must be altogether
inactive. The impelling power, therefore, at alternate moments of
time, acted on opposite wheels, and on each of them at the
greatest possible distance from the centre of the axle.
[Illustration: _Fig._ 94.]
(193.) The next step in the improvement of the machine was made
with a view to remove these two defects. The cylinders were
transferred from the exterior of the engine to the [Pg355]
interior of the casing called the smoke-box, B, _fig._ 94., which
supports the chimney, and which receives the heated air issuing
from the tubes which traverse the boiler. Thus placed, the
cylinders are always maintained as hot as the air which issues
from the flues, and all condensation of steam by their exposure is
prevented. The piston-rods are likewise brought closer together,
and nearer the centre of the engine: the connecting rods, no
longer attached to the wheels, are made to act upon two cranks
constructed upon the axle of the wheels, and placed at right
angles to each other. From the position of these cranks, one would
always be at its dead point when the other is in full action. The
action of the steam upon them would, therefore, be generally
unequal; but this would not produce the same strain as when the
connecting rods are attached to points upon the exterior of the
wheels, owing to the cranks being constructed on the axle at
points so much nearer its centre. By this means it was found that
the working of the machine was more even, and productive of much
less strain, than in the arrangement adopted in the Rocket, and
the earlier engines. On the other hand, a serious disadvantage was
incurred by a double-cranked axle. The weakness necessarily
arising from such a form of axle could only be removed by great
thickness [Pg356] and weight of metal; and even this precaution,
at first, did not prevent their occasional fracture. The forging
of them was, however, subsequently much improved: the cranks,
instead of being formed by bending the metal when softened by
heat, were made by cutting the square of the crank out of the
solid metal; and now it rarely happens that one of these axles
fails.
The adoption of smaller tubes, and a greater number of them, with
a view more perfectly to extract the heat from the air in passing
to the chimney, rendered a more forcible draft necessary. This was
accomplished by the adoption of a more contracted blast-pipe
leading from the eduction-pipes of the cylinders and presented up
the chimney. A representation of such a blast-pipe, with the two
tubes leading from the cylinders and uniting together near the
point, which is presented up the chimney, is given at _p p_ in
_fig._ 104. The engine thus improved is represented in _fig._ 94.
A represents the cylindrical boiler, the lower half of which is
traversed by tubes, as described in the Rocket. They are usually
from eighty to one hundred in number, and about an inch and a half
in diameter; the boiler is about seven feet in length; the
fire-chamber is attached to one end of it, at F, as in the Rocket,
and similar in construction: the cylinders are inserted in a
chamber at the other end, immediately under the chimney. The
piston-rods are supported in the horizontal position by guides;
and connecting rods extend from them, under the engine, to the two
cranks placed on the axle of the large wheels. The effects of an
inequality in the road are counteracted by springs, on which the
engine rests; the springs being below the axle of the great
wheels, and above that of the less. The steam is supplied to the
cylinders, and withdrawn, by means of the common sliding valves,
which are worked by an eccentric wheel placed on the axle of the
large wheels of the carriage. The motion is communicated from this
eccentric wheel to the valve by sliding rods. The stand is placed
for the attendant at the end of the engine, next the fire-place F;
and two levers L project from the end which communicate with the
valves by means of rods, by which the engine is governed so as to
reverse the motion. [Pg357]
The wheels of these engines have been commonly constructed of wood
with strong iron ties, furnished with flanges adapted to the
rails. But Mr. Stephenson afterward substituted, in some
instances, wheels of iron with hollow spokes. The engine draws
after it a tender carriage containing the fuel and water; and,
when carrying a light load, is capable of performing the whole
journey from Liverpool to Manchester without a fresh supply of
water. When a heavy load of merchandise is drawn, it is usual to
take in water at the middle of the trip.
(194.) In reviewing all that has been stated, it will be perceived
that the efficiency of the locomotive engines used on this railway
is mainly owing to three circumstances: 1st, The unlimited power
of draft in the furnace, by projecting the waste steam into the
chimney; 2d, The almost unlimited abstraction of heat from the air
passing from the furnace, by arrangement of tubes traversing the
boiler; and, 3d, Keeping the cylinders warm, by immersing them in
the chamber under the chimney. There are many minor details which
might be noticed with approbation, but these constitute the main
features of the improvements.
The successive introduction of improvements in the engines, some
of which we have mentioned, was accompanied by corresponding
accessions to their practical power, and to the economy of fuel.
In the spring of the year 1832, I made several experiments on the
Manchester Railway, with a view to determine, in the actual state
of the locomotive engines at that time, their powers with respect
to the amount of load and the economy of fuel, from which I select
the following as examples:—
I.
On Saturday, the 5th of May, the engine called the "Victory"
took 20 waggons of merchandise, weighing gross 92 tons 19 cwt.
1 qr., together with the tender containing fuel and water, of
the weight of which I have no account, from Liverpool to
Manchester (30 miles), in 1 h. 34 min. 45 sec. The train
stopped to take in water half-way, for 10 minutes, [Pg358]
not included in the above-mentioned time. On the inclined
plane rising 1 in 96, and extending 1-1/2 mile, the engine was
assisted by another engine called the "Samson," and the ascent
was performed in 9 minutes. At starting, the fire-place was
well filled with coke, and the coke supplied to the tender
accurately weighed. On arriving at Manchester, the fire-place
was again filled, and the coke remaining in the tender
weighed. The consumption was found to amount to 929 pounds net
weight, being at the rate of one third of a pound per ton per
mile.
Speed on the level was 18 miles an hour; on a fall of 4 feet
in a mile, 21-1/2 miles an hour; fall of 6 feet in a mile,
25-1/2 miles an hour; on the rise over Chatmoss, 8 feet in a
mile, 17-5/8 miles an hour; on level ground sheltered from the
wind, 20 miles an hour. The wind was moderate, but direct
ahead. The working wheels slipped three times on Chatmoss, and
the train was retarded from 2 to 3 minutes.
The engine, on this occasion, was not examined before or after
the journey, but was presumed to be in good working order.
II.
On Tuesday, the 8th of May, the same engine performed the same
journey, with 20 waggons, weighing gross 90 tons 7 cwt. 2
qrs., exclusive of the unascertained weight of the tender. The
time of the journey was 1 h. 41 min. The consumption of coke
1040 lbs. net weight, estimated as before. Rate of speed:—
Level 17-5/8 miles per hour.
Fall of 4 feet in a mile 22
—— 6 22-1/2
Rise of 8 15
On this occasion there was a high wind ahead on the quarter,
and the connecting rod worked hot, owing to having been keyed
too tight. On arriving at Manchester, I caused the cylinders
to be opened, and found that the pistons were [Pg359] so
loose, that the steam blew through the cylinders with great
violence. By this cause, therefore, the machine was robbed of
a part of its power during the journey; and this circumstance
may explain the slight decrease in speed, and increase in the
consumption of fuel, with a lighter load, in this journey,
compared with that performed on the 5th of May.
The Victory weighs 8 tons 2 cwt., of which 5 tons 4 cwt. rest
on the drawing wheels. The cylinders are 11 inches diameter,
and 16 inches stroke, and the diameter of the drawing wheels
is 5 feet.
III.
On the 29th of May, the engine called the "Samson" (weighing
10 tons 2 cwt., with 14-inch cylinders, and 16-inch stroke;
wheels 4 feet 6 inches diameter, both pairs being worked by
the engine; steam 50 lbs. pressure, 130 tubes) was attached to
50 waggons, laden with merchandise; net weight about 150 tons;
gross weight, including waggons, 223 tons 6 cwt. The tender
weighed 7 tons, making a gross load (including the engine) of
240 tons 8 cwt. The engine with this load travelled from
Liverpool to Manchester (30 miles) in 2 hours and 40 min.,
exclusive of delays upon the road for watering, &c.; being at
the rate of nearly 12 miles an hour. The speed varied
according to the inclinations of the road. Upon a level, it
was 12 miles an hour; upon a descent of 6 feet in a mile, it
was 16 miles an hour; upon a rise of 8 feet in a mile, it was
about 9 miles an hour. The weather was calm, the rails very
wet; but the wheels did not slip, even in the slowest speed,
except at starting, the rails being at that place soiled and
greasy with the slime and dirt to which they are always
exposed at the stations. The coke consumed in this journey,
exclusive of what was raised in getting up the steam, was 1762
lbs., being at the rate of a quarter of a pound per ton per
mile.
(195.) The great original cost, and the heavy expense of keeping
the engines used on the railway in repair, have pressed severely
on the resources of the undertaking. One of the best [Pg360]
constructed of the later engines costs originally 1500_l._ and
sometimes more. The original cost, however, is far from being the
principal source of expense: the wear and tear of these machines,
and the occasional fracture of those parts on which the greatest
strain has been laid, have greatly exceeded what the directors had
anticipated. Although this source of expense must be in part
attributed to the engines not having yet attained that state of
perfection, in the proportion and adjustment of their parts, of
which they are susceptible, and to which experience alone can
lead, yet there are some obvious defects which demand attention.
The heads of the boilers are flat, and formed of iron, similar to
the material of the boilers themselves. The tubes which traverse
the boiler were, until recently, copper, and so inserted into the
flat head or end as to be water-tight. When the boiler was heated,
the tubes were found to expand in a greater degree than the other
parts of the boiler; which frequently caused them either to be
loosened at the extremities, so as to cause leakage, or to bend
from want of room for expansion. The necessity of removing and
refastening the tubes caused, therefore, a constant expense.