carrying coke for the supply of the furnace, and water for the
boiler. The coke is contained in the space R″, (_fig._ 98. 100.)
surrounded by a tank I″ containing water to feed the boiler. The
feed for the boiler is conducted from the tank through a pipe
descending downwards and in a curved direction, P″ Q″, _fig._ 98.,
and connected with a horizontal pipe K, _fig._ 97. A cock is
provided at P″, by which the supply of water to this pipe may be cut
off at pleasure. Another cock is provided at _t′_, _fig._ 97., where
the curved pipe joins the horizontal pipe by which the quantity of
water supplied to K may be regulated by opening the cock more or
less fully. The handle of this cock rises through the floor of the
engine, so that the engineer may regulate it at discretion. The pipe
K being conducted under the engine, as represented in _fig._ 97.,
terminates in a vertical pipe, of greater diameter, containing two
valves, both of which open upwards, and between these valves to this
vertical pipe is attached a force-pump, by which the water is drawn
from the horizontal pipe K into the vertical pipe K′, and from the
latter is driven into a delivery-pipe by which it is forced into the
boiler. The details of the interior of this feed-pump are
represented on a larger scale in _fig._ 111. The extremity of the
horizontal pipe K′ is represented in section at H, where it is
joined on by a screw to the bottom of the vertical pipe which is
represented in _fig._ 97. at K, and which is here represented in
section. The vertical pipe, represented in _fig._ 97. consists of
several parts screwed together by nuts and bolts passing through
flanges. The lowest piece I is attached by a flange to the piece L:
within these is contained the valve Q resting in a seat made
conical, so that the ball [Pg382] which forms the valve shall rest
in water-tight contact with it. The ball is turned and ground to an
accurate sphere, and whatever position it assumes upon its seat its
contact will be perfect. It is guided in its upward and downward
motion by several vertical bars which confine it, and which are
united at the top, so as to limit the upward motion of the ball. A
screw V′ is inserted in the bottom of the piece I, by removing which
access can be obtained to the valve. The piece L is secured to the
short pipe G by nuts and bolts passed through a flange. The pipe G
is cast upon the end of the feed-pump A. On the foremost end of this
feed-pump is constructed a stuffing-box C of the usual form, having
a gland D forced against packing by nuts and screws E. The plunger B
is turned so as to be truly cylindrical, and moves in water-tight
contact through the gland D. The plunger not being in contact with
the inner surface of the pump-barrel A, the latter need not be
ground. The horizontal rod by which the plunger B is driven is
attached at its foremost extremity to an arm which projects from the
rod of the steam-piston, and consequently this plunger is moved
through a space equal to the stroke of the steam-piston. In this
case that space is eighteen inches. The [Pg383] upper end of the
vertical tube G is attached by screws and a flange to a piece P
containing a valve R similar in all respects to the lower valve Q,
and like it opening upwards. A screw V is introduced at the top by
which access may be obtained to this valve. This screw also presses
on the crown of the guides of the valve, so as to hold it down by
regulated pressure. At the side of this upper piece P is inserted a
horizontal tube M connected with the end of the delivery-pipe N.
This latter is continued to the boiler with which it communicates at
the fire-box. When the plunger B is drawn out of the pump-barrel A,
the spherical valve Q being relieved from its downward pressure is
raised, and water passes from the pipe H through the valve Q into
the vertical pipe G; the lower valve Q then closes and stops the
return of the water. The plunger B returning into the pump-barrel A
then forces the water against the upper valve R and drives it
through the delivery-tube N, from which its return is prevented by
the valve R. When the delivery-tube N is filled with water
throughout its whole length, every stroke of the plunger will
evidently drive into the boiler a volume of water equal to the
magnitude of a part of the plunger eighteen inches in length.
Until within the last few years, locomotive engines were supported
on only four wheels; they are, however, now almost universally
supported on six, the driving wheels being in the middle. To give
greater security to the position of the engine between the rails
it is usual to construct flanges on the tires of all the six
wheels. Mr. Stephenson, however, has been in the practice of
constructing the driving wheels without flanges, and with tires
truly cylindrical, depending on the flanges of the two pairs of
smaller wheels to maintain the engine between the rails. The
wheels of the engine here described are constructed in this
manner. The driving wheels D′ are fixed on the cranked axle C′,
and are five feet in diameter. The other wheels L′ M′, the one
being placed immediately behind the smoke-box, and the other
immediately behind the fire-box, are each three feet six inches in
diameter, and have a flange upon their tires, which running on
the [Pg384] inside of each rail keeps the engine between the
rails. Each pair of these small wheels, like the driving-wheels,
is fixed upon their axle. The axles are 3-5/8 inches diameter, and
project beyond the wheels, the projecting part supporting the
frame of the engine and turning in brasses. Upon these brasses
rest springs, which bear the whole weight of the engine. These
springs having nothing between them and the road but the wheels
and axles intercept and equalise the sudden shocks produced by the
rapid motion upon the road.
When an engine is required for the transport of very heavy loads,
such as those of merchandise, the adhesion of one pair of working
wheels is found to be insufficient, and, in such cases, one of the
two pairs of wheels L′ M′ is made of the same diameter as the
wheels which are placed upon the working axle, and a bar is
attached to points on the outside of the wheels at equal distances
from their centre, connecting them in such a manner that any force
applied to make one pair of wheels revolve must necessarily impart
the same motion to the other pair. By such means the force of the
steam is made to drive both pairs of wheels, and consequently a
proportionally increased adhesion is obtained.
The velocity which an engine is capable of imparting to the load
which it draws depends upon the rate at which the pistons are
capable of being moved in the cylinders. By every motion of each
piston backwards and forwards one revolution of the driving wheels
is produced, and by each revolution of the driving wheels,
supposing them not to slip upon the rails, the load is driven
through a distance upon the road equal to their circumference. As
the two cylinders work together, it follows, that a quantity of
steam sufficient to fill four cylinders supplied by the boiler to
the engine will move the train through a distance equal to the
circumference of the driving wheels; and in accomplishing this,
each piston must move twice from end to end of the cylinder; each
cylinder must be twice filled with steam from the boiler; and that
steam must be twice discharged from the cylinder through the
blast-pipe into the chimney.
[Pg401] If the driving wheels be five feet in diameter their
circumference will be fifteen feet seven inches. To drive a train
with a velocity of thirty miles an hour, it will be necessary that
the engine should be propelled through a space of forty-five feet
per second. To accomplish this with five-feet wheels they must be
therefore made to revolve at the rate of very nearly three
revolutions per second; and as each revolution requires two
motions of the piston in the cylinder, it follows that each piston
must move three times forwards and three times backwards in the
cylinder in a second; that steam must be admitted six times per
second from the steam-chest to each cylinder, and discharged six
times per second from each cylinder into the blast-pipe. The
motion, therefore, of each piston, supposing it to be uniform,
must divide a second into six equal parts, and the puffs of the
blast-pipe in the chimney must divide a second into twelve equal
parts. The motion of the slides and other reciprocating parts of
the machinery must consequently correspond.
This motion of the reciprocating parts of the machinery being
found to be injurious to it, and to produce very rapid wear,
attempts have been made to remedy the defect, and to obtain
greater speed with an equal or diminished rate of motion of the
piston, by the adoption of driving wheels of greater diameter, and
on several of the great lines of railway the magnitude of the
wheels for the passenger-engines have been increased to five feet
and a half and six feet diameter; but such engines have not been
sufficiently long in use to afford grounds for forming a practical
estimate of their effects. Experiments of a much bolder
description have, however, been tried on one of the great lines of
railway by the adoption of driving wheels of much greater
diameter. In some cases their magnitude has been increased even to
ten feet; but from various experiments to which these engines have
been submitted by myself and others, as well as from the
experience which appears to be obtained from the results of their
ordinary work, it does not appear that any advantages have
attended them, and they have been accordingly for the most part
abandoned.
The pressure of steam in the boiler is limited by two
safety-valves, [Pg402] represented in _fig._ 97. at N and O. The
valve at N is under the control of the engineer, but the valve at
O is inaccessible to him. The structure of the safety-vale
represented at N is exhibited on a larger scale in _fig._ 112.,
which represents its section, and _fig._ 113., which shows a plan
of the valve-seat with the valve removed. The valve A, which is
made of brass, is mitred round the edge at an angle of 45°, and
has a spindle, or stalk B, cast upon it, projecting downwards from
the middle of it. The valve-seat C is also made of brass, and cast
with a flange at the bottom to attach it to the boiler. The mitred
surface of the valve is ground into the valve-seat, so as to rest
in steam-tight contact with it. Across the valve-seat, which is
two and a half inches in diameter, is cast a thin piece D, seen in
plan in _fig._ 113. and in section in _fig._ 112. which extends
from the top to the bottom, and has a longitudinal hole through
it, in which the spindle B of the valve works: by this hole it is
guided when it rises from its seat. A projection E is cast upon
the seat of the valve, in which a standard F is inserted. This
standard is forked at the top, and receives the end of a lever G,
which turns in it upon a centre. A rod H is jointed to this lever
by another pin at three inches from the former, and the lower end
of this rod, ground to a point, presses upon the centre of the
valve A. At the other end of the lever, which is broken off in
_fig._ 112., at a distance of three feet from the centre pin,
inserted in the fork of the pillar F, the rod of a common
spring-balance _w_, _fig._ 101., is attached by a finger-nut _n_.
The bottom of this spring-balance is secured on to the fire-box.
This balance is screwed up by the finger-nut on the valve-lever
until the required pressure on the lever is produced through the
medium of the rod H, this pressure being generally fifty pounds
per square inch above the atmosphere. When the pressure of the
steam in the boiler exceeds this, the valve A is raised from its
seat, and the steam escapes.
[Illustration: _Fig._ 112.]
[Illustration: _Fig._ 113.]
It is evident that the sliding weight by which the pressure
[Pg403] of the safety-valve is sometimes regulated in stationary
engines would not be admissible in a locomotive engine, since the
motion of the engine would constantly jolt it up and down, and
cause the steam to escape. One of the disadvantages attending the
use of the spring-valve is that it cannot be opened to let the
steam escape without increasing its force, so that the steam, when
escaping, must really have a greater pressure than that to which
the valve has been previously adjusted. The longer the lever is,
the greater will be this difference of pressure, inasmuch as a
given elevation of the pin governing the rod H would cause a
proportionally greater motion in that end of the lever attached to
the spring.
The second safety-valve O is enclosed in a case, so that it is
inaccessible, and its purpose is to limit the power of the
engineer to increase the pressure of steam in the boiler. This
valve is similar in construction to the former, but instead of
being held down by a lever, is pressed upon by several small
elliptical springs placed one above another over the valve, and
held down by a screw which turns in a frame Y, fixed into the
valve-seat. By this screw the pressure on the valve can be
adjusted to any required degree; and if the open safety-valve be
screwed down to a greater pressure, the steam will begin to escape
from this second valve.
Also in the case where the boiler produces surplus steam faster
than its escape can be effected at the valve N, the pressure will
sometimes be increased until the valve O is opened, and its escape
will take place from both valves.
The whole weight of the engine bears upon those parts of the six
axles R′, _fig._ 99., which project beyond the wheels. Boxes are
formed in which these parts of the axles turn, and through the
medium of which the weight of the engine rests upon them. Over
these boxes are constructed oil or grease cups, by means of which
the axles are constantly lubricated. It is usual to lubricate the
axles of the engine itself with oil: the axles of the tender, and
other coaches and waggons, are lubricated with a mixture of oil
and tallow. In the middle of the box in which the axle turns, and
between the two oil-cups, is cast a socket, in which the end of
the spindle on [Pg404] which the spring presses rests. The
springs are composed of a number of steel-plates, laid, in the
usual manner, one above the other, increasing in length upwards.
In the engine here described, the plates forming the springs of
the driving wheels are thirteen in number, each of which is four
inches in width, and 5/16ths of an inch in thickness. The springs
upon the other wheels are three inches in width. The springs of
the driving wheels are below the axle, while those of the smaller
wheels are above it.
Buffers D″ are placed behind the tender, which act upon a spring C
(_fig._ 100.), to break the collision, when the waggons or
carriages strike upon the tender, and similar buffers are attached
to all passenger-coaches. Some of these buffers are constructed
with a system of springs similar to C, but more elastic, and
combined in greater number under the framing of the carriage, so
that a considerable play is allowed to them. In some cases the
rods of the buffers are made to act upon strong spiral springs
inserted in the sides of the framing of the carriage. This
arrangement gives greater play to the buffers; and as every coach
in a train has several buffers, the combined effect of these is
such, that a considerable shock given to either end of the train
may be rendered harmless by being spent upon the elasticity of
these several systems of springs.
In order to give notice of the approach of a train, a
steam-whistle Z′, _fig._ 97. 101., is placed immediately above the
fire-box at the back of the engine. This is an apparatus composed
of two small hemispheres of brass, separated one from the other by
a small space. Steam is made to pass through a hollow space
constructed in the lower hemisphere, and escapes from a very
narrow circular opening round the edge of that hemisphere, rushing
up with a force proportionate to its pressure. The edge of the
upper hemisphere presented downwards encounters this steam, and an
effect is produced similar to the action of air in organ pipes. A
shrill whistle is produced, which can be heard at a very
considerable distance, and, differing from all ordinary sounds, it
never fails to give timely notice of the approach of a train.
The water tank I″, _fig._ 98. 100., which is constructed on the
tender, is formed of wrought-iron plates 1/8 of an inch thick,
[Pg405] riveted at the corners by angle iron already described.
This tank is 9 feet long, 6-3/4 feet wide, and 2-1/4 feet deep.
The top is covered with a board K″, and a raised platform N″ is
constructed behind, divided into three parts, covered with leads,
which open on hinges. The middle lid covers an opening to the tank
by which water is let in: the lids at either side cover boxes in
which are contained the tools necessary to be carried with the
engine. The curved pipe P″, _fig._ 98., leading from the bottom of
the tank to the pipe Q″, is of copper. The pipe Q″, connecting the
latter with the feed-pipe K′, _fig._ 99., is sometimes formed of
leather or India-rubber cloth, having a spiral spring on the
inside to prevent it from collapsing. It is necessary that this
pipe Q″ should have a power of yielding to a sufficient degree to
accommodate itself to the inequalities of motion between the
engine and tender. A metal pipe is sometimes used, supplied with a
double ball and socket, and a telescopic joint, having sufficient
play to allow for the lateral and longitudinal inequalities of
motion of the engine and tender. The weight of an engine, such as
that here described, supplied with its proper quantity of water
and fuel, is about 12 tons: the tender, when empty, weighs about
3-1/4 tons; and when filled with water and fuel its weight is 7
tons. The tank contains 700 gallons of water, and the tender is
capable of carrying about 800 weight of coke. This supply is
sufficient for a trip of from thirty to forty miles with an
ordinary load.
(198.) It is not usual to express the power of locomotive engines
in the same manner as that of other engines by the term
horse-power. Indeed, until the actual amount of resistance opposed
to these machines, under the various circumstances in which they
are worked, shall be ascertained with some degree of precision, it
is impossible that their power or efficiency can be estimated with
any tolerable degree of approximation. The quantity of water
evaporated, and passed in steam through the cylinders, supplies a
major limit to the power exerted; but even this necessary element
for the calculation of the efficacy of these machines has not been
ascertained by a sufficiently extensive course of observation and
experiment. Mr. Stephenson states, that the engine which [Pg406]
has been here described is capable of evaporating 77 cubic feet of
water per hour, while the early locomotives could only evaporate
16 cubic feet per hour. This evaporation, however, is inferior to
that which I have ascertained myself to be produced by engines in
regular operation on some of the northern railways. In an
experiment made in July, 1839, with the Hecla engine, I found that
the evaporation in a trip of ninety-five miles, from Liverpool to
Birmingham, was at the rate of 93·2 cubic feet per hour, and in
returning the same distance it was at the rate of 85·7 cubic feet
per hour, giving a mean of 89 cubic feet per hour nearly. The
Hecla weighed 12 tons; and its dimensions and proportions
corresponded very nearly with those of the engine above described.
In a course of experiments which I made upon the engines then in
use on the Grand Junction Railway in the autumn of 1838 I found
that the ordinary evaporating power of these engines varied from
eighty to eighty-five cubic feet per hour.
Engines of much greater dimensions, and consequently of greater
evaporating power, are used on the Great Western Railway. In the
autumn of 1838 experiments were made upon these engines by Mr.
Nicholas Wood and myself, when we found that the most powerful
engine on that line, the North Star, drawing a load of 110-1/2
tons gross, engine and tender inclusive, at 30-1/2 miles an hour,
evaporated 200 cubic feet of water per hour. The same engine
drawing a load of 194-1/2 tons at 18-1/2 miles an hour evaporated
141 cubic feet per hour, and when drawing 45 tons at 38-1/2 miles
an hour evaporated 198 cubic feet of water per hour.
It has been already shown that a cubic foot of water evaporated
per hour produces a gross amount of mechanical force very little
less than two-horse power, and consequently the gross amount of
mechanical power evolved in these cases by the evaporation of the
locomotive boilers will be very nearly twice as many horse-power
as there are cubic feet of water evaporated per hour. Thus the
evaporation of the Hecla, in the experiments made in July, 1839,
gave a gross power of about one hundred and eighty horses, while
the evaporation of the North Star gave a power of about four
hundred horses. In stationary engines about half the gross
[Pg407] power evolved in the evaporation is allowed for waste,
friction, and other sources of resistance not connected with the
load. What quantity should be allowed for this in locomotive
engines is not yet ascertained, and therefore it is impossible to
state what proportion of the whole evaporation is to be taken as
representing the useful horse-power.
(199.) The great uniformity of resistance produced by the traction
of carriages upon a railway is such as to render the application
of steam power to that purpose extremely advantageous. So far as
this resistance depends on mechanical defects, it is probably
rendered as uniform as is practicable, and in proportion to the
quantity of load carried is reduced to as small an amount as it is
likely to attain under any practicable circumstances. Until a
recent period this resistance was ascribed altogether, or nearly
so, to mechanical causes. The inequalities of the road-surface,
the friction of the axles of the wheels in their bearings, and the
various sources of resistance due to the machinery of the engine,
being the principal of these resistances, were for the most part
independent of the speed with which the train was moved; and it
was accordingly assumed in all calculations respecting the power
of locomotive engines that the resistance would be practically the
same whatever might be the speed of the train. It had been well
understood that so far as the atmosphere might offer resistance to
the moving power this would be dependent on the speed, and would
increase in a very high ratio with the speed; but it was
considered that the part of the resistance due to this cause
formed a fraction of the whole amount so insignificant that it
might be fairly disregarded in practice, or considered as a part
of the actual computed resistance taken at an average speed.
It has been, until a late period, accordingly assumed that the
total amount of resistance to railway trains which the locomotive
engines have had to overcome was about the two hundred and
fiftieth part of the gross weight of the load drawn: some
engineers estimated it at a two hundred and twentieth; others at a
two hundred and fiftieth; others at a three hundred and thirtieth
part of the load; and the two hundred and fiftieth part of the
gross load drawn may perhaps be [Pg408] considered as a mean
between these much varying estimates. What the experiments were,
if any, on which these rough estimates were based, has never
appeared. Each engineer formed his own valuation of this effect,
but none produced the experimental grounds of their opinion. It
has been said that the trains run down the engine, or that the
drawing chains connecting the engine slacken in descending an
inclination of sixteen feet in a mile, or 1/330. Numerous
experiments, however, made by myself, as well as the constant
experience now daily obtained on railways, show that this is a
fallacious opinion, except at velocities so low as are never
practised on railways.