The piston-rods Y terminate in a fork, by which they are attached to cross heads Z, the ends of which are confined by guide-bars A′, in which they are allowed to play backwards and forwards through a space equal to the stroke of the piston. To these cross heads Z, between the prongs of the fork in which the piston terminates, are attached the foremost ends of the connecting rods B′. These rods are, therefore, driven backwards and forwards by the motion imparted to the cross head Z by the piston-rods Y. The connecting rods B′ are attached at the hinder ends to two cranks formed upon the axles C′ of the driving wheels D′. These two cranks are formed upon the axles precisely at right angles to each other. The left-hand crank is represented in its horizontal position, in fig. 99., and the right-hand crank is seen in its vertical position. A cranked axle is represented on a larger scale in fig. 107., and the two cranks are seen in a position oblique to the plane of the figure. As this axle is the instrument by which the impelling force is conveyed to the load, and as it has to support a great portion of the weight of the engine, it is constructed with great strength and precision. It is made all in one [Pg376] piece, and of the best wrought iron called Back Barrow, or scrap iron. In the engine here described its extreme length is six feet and a half, and its diameter is five inches. At the centre part A it is cylindrical, and is increased to five inches and a quarter at C, where the cranks are formed. The sides D of the cranks are four inches thick, and the crank pins B, which are truly cylindrical, are five inches diameter, and three inches in length, the brasses at the extremities of the connecting rods which play upon them having a corresponding magnitude. The distance from the centre of the crank-pins B to the centre of the axle A must be exactly equal to half the stroke of the piston, and is, therefore, in this case precisely nine inches. Upon the parts F, which are seven inches and a half long, the great driving wheels are firmly fastened, so as to be prevented from turning or shaking upon the axle. The axle projects beyond the wheels at G, where it is reduced to three inches and an eighth diameter. These projecting parts G are five inches long, having collars at the outer ends. Brasses are fixed at the outside frame of the engine which rest upon these projections G of the axle, and upon these brasses the weight of the engine is supported. The entire axle is accurately turned in a lathe, and each of the crank-pins B is likewise turned by suspending the axle on centres corresponding with the centres of the crank-pins, and made on strong cast iron arms, which are firmly fixed on the ends of the axle, and project beyond the cranks so as to balance the axle, and enable it to turn round on the centre of the crank-pin. The axle is by such means made perfectly true, and the cranks are made of exactly the proper length, and precisely at right angles to each other. The corners of the cranks are champered off, as shown in the figure, and the ends of the cylindrical parts well rounded out.

The strength and accuracy of construction indispensable in these cranked axles, in order to make them execute their work, render them very expensive. Those which are here described cost about 50l. each. When properly constructed, however, they are seldom broken, but are sometimes bent when the engine escapes from the rails.

The proper motion to admit and withdraw the steam from [Pg377] either end of the cylinder is imparted to the slide-valves by eccentrics, in a manner and on a principle so similar to that already described in large stationary engines, that it will not be necessary here to enter into any detailed explanation of the apparatus for communicating this motion, which is exhibited in plan and section in figs. 97. 99. The eccentrics are attached to the cranked axles at E′ E″. The eccentric E′ imparts motion by a rod e″ to a lever h″, formed on an axle extending across the frame of the engine. This conveys motion to another lever l″, projecting from the same axle. This lever l″ is jointed to horizontal links m″, which at the foremost ends are attached to the spindle l′, by which the slide is driven. By these means the motion received by the eccentric from the great working axle conveys to the spindle l′ an alternate movement backwards and forwards, and the points at which it is reversed will be regulated by the position given to the eccentric upon the great axle. The eccentric is formed in two separate semicircles, and is keyed on to the great axle, and consequently any position may be given to it which may be required. The position to be given to the eccentrics should be such that they shall be at right angles to their respective cranks, and they should be fixed a quarter of a revolution behind the cranks so as to move the slides to that extent in advance of the pistons, since by the position of the levers h″ and l″, the motion of the eccentric becomes reversed before it reaches the valve spindle.

The performance of the engine is materially affected by the position of the eccentrics on the working axle. The slide should begin to uncover the steam-port a little before the commencement of the stroke of the piston, in order that the steam impelling the piston should be shut off, and the steam about to impel it in the contrary direction admitted before the termination of the stroke. Through this small space the steam, therefore, must act in opposition to the motion of the piston. This is called the lead of the slide, and the extent generally given to it is about a quarter of an inch. This is accomplished by fixing the eccentrics not precisely at right angles to the respective cranks, but a little in advance of that position. The introduction of the steam to [Pg378] the piston before the termination of the stroke has the effect of bringing it gradually to rest at the end of the stroke, and thereby diminishing the jerk or shock produced by the rapid change of motion. In stationary engines, where the reciprocations of the engine are slow, the necessity for this provision does not arise; but in locomotive engines in which the motion of the piston is changed from four to six times in a second, it becomes necessary. The steam admitted to the piston before the termination of the stroke acts as a spring-cushion to assist in changing its motion, and if it were not applied, the piston could not be kept tight upon the piston-rod. Another advantage which is produced by allowing some lead to the slide is that the waste steam which has just impelled the piston begins to make its escape through the waste-port before the commencement of the next stroke, so that when the impelling steam begins to produce the returning stroke, there is less waste steam on the other side of the piston to resist it.

When the motion of the engine is very rapid, the resistance of the waste steam, as it escapes from the blast-pipe to the piston, has been generally supposed to be very considerable, though we are not aware of any direct experiments by which its amount has been ascertained. In the account of the locomotive engine which has been here described, supplied by Mr. Stephenson for the last edition of Tredgold on the Steam Engine, he states, that the average resisting pressure of the waste steam throughout the stroke is 6 lbs. per square inch, when running at the usual rate of from 25 to 28 miles an hour, and that at greater velocities this negative pressure has been found to increase to more than double that amount. No experiments are, however, cited from which this inference has been drawn.

It has been also thought that the pressure of steam upon the piston in the cylinder, at high velocities, is considerably below the pressure of steam in the boiler; but this has not been, so far as we are informed, ascertained by any satisfactory experimental test. Mr. Stephenson likewise states, that this loss of pressure, causes the negative pressure or resistance of the waste steam to amount to [Pg379] from 30 to 40 per cent. of the positive pressure upon the piston when the engine is running very fast, and that therefore the power of the engine is diminished nearly one half.

But it will be perceived that besides the uncertainty which attends the estimate of the actual amount of pressure on the piston compared with the pressure of steam in the boiler, the inference here drawn does not appear to be compatible with what has been already proved respecting the mechanical effect of steam. No change of pressure which may take place between the boiler and the cylinder can affect the practical efficacy of the steam. As the steam passes through the engine, whatever change of pressure it may be subject to, it still remains common steam; and though its pressure may be diminished, its volume being increased in a nearly equal proportion, its mechanical effect will remain the same. The power of the engine, therefore, estimated as it ought to be, by the whole mechanical effect produced, will not be altered otherwise than by the effect of the increased resistance produced by the blast-pipe. What that resistance is, we repeat, has not, so far as we know, been ascertained by direct experiment, and there are circumstances attending it which render it probable that, even at high velocities, it is less in amount than Mr. Stephenson's estimate.

The position of the eccentrics which is necessary to make the pistons drive the engine forward must be directly the reverse of that which would cause them to drive the engine backwards. To be able, therefore, to reverse the motion of the engine, it would only be necessary to be able to reverse the position of the eccentrics, which may be accomplished by either of two expedients.

1. First, The eccentrics may be capable of revolving on the great working axle, and also of sliding upon it through a small space. Their revolution on the axle may be checked by letting a pin attached to a collar fastened on the axle fall into a hole on the side of the eccentric. Such a pin will drive the eccentric round with the axle, and the position of this pin and the hole will determine the position of the eccentric with reference to the crank. At a short distance [Pg380] on the other side of the eccentric may be a corresponding collar with a pin in the opposite position. By moving the eccentric longitudinally on the axle, the former pin may be withdrawn from the hole, and the latter allowed to fall into the hole on the other side. Proper mechanism may be provided by which the position of the eccentric may thus be reversed in reference to the crank, and by such means the motion of the engine may be reversed.
2. Secondly, Supposing the eccentrics which drive the engine forward to be immovably fixed upon the axle, two other eccentrics may be provided attached to other parts of the same axle, and having a position exactly the reverse with reference to the cranks. Proper mechanism may be provided, by which either or both pairs of eccentrics may be thrown in or out of gear. Such are the means adopted in the engine which has been already described. The eccentrics for driving the engine backwards are placed outside the cranks at F′ F″. A hand lever w″ fig. 101. is provided, by which the engine man may throw either pair of eccentrics into or out of gear, so as to make the engine work either backwards or forwards.

As all the moving parts of the engine require to be constantly lubricated with oil to diminish the friction, and keep them cool, oil-cups for this purpose are fixed upon them. In some engines these oil-cups are attached separately to all the moving parts: in others they are placed near each other in a row on the boiler, and communicate by small tubes with the several parts required to be lubricated. One of these is requisite for each end of the connecting rods, for each of the guides of the piston-rods, for the piston-rod itself, the spindle of the slide-valve, and other parts. An elevation of one of these oil-cups is shown in fig. 108., a vertical section in fig. 109., and horizontal plan in fig. 110. The cup A is made of brass with a cover B. This cover has a piece projecting from it turning upon a pin in a socket C at the side of the cup A, and square at the end, resting upon a small spring at the bottom of the socket to hold it either open or shut. In the bottom of the [Pg381] cup is inserted an iron tube D extending nearly to the top. This tube projects from the bottom of the cup, where it is tapped for the purpose of fixing the cup on the part of the engine which it is intended to lubricate. The hole into which the cup is screwed communicates with the rubbing surface, and some cotton thread is passed through the tube dipping into the oil in the cup at the one end and touching the moving part at the other. This thread acts as a siphon, and constantly drops oil on the rubbing surface.

The tender is a carriage attached behind the engine and close to it, 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 3⁵⁄₈ 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.

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. fig. 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, 6³⁄₄ feet wide, and 2¹⁄₄ 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 3¹⁄₄ 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.

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 110¹⁄₂ tons gross, engine and tender inclusive, at 30¹⁄₂ miles an hour, evaporated 200 cubic feet of water per hour. The same engine drawing a load of 194¹⁄₂ tons at 18¹⁄₂ miles an hour evaporated 141 cubic feet per hour, and when drawing 45 tons at 38¹⁄₂ 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.

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.

Founded on these principles, a vast number of experiments were made on planes of different inclinations, and with loads of various magnitudes; and it was found, in general, that when a train descended an inclined plane, the rate of acceleration gradually diminished, and at length became uniform; that the uniform speed thus attained depended on the weight, form, and magnitude of the train and the inclination of the plane; that the same train on different inclined planes attained different uniform speeds—on the steeper planes a greater speed being attained. From such experiments it followed, contrary to all that had been previously supposed, that the amount of resistance to railway trains had a dependence on the speed; that this dependence was of great practical importance, the resistance being subject to very considerable variation at different speeds, and that this source of resistance arises from the atmosphere which the train encounters. This was rendered obvious by the different amount of resistance to the motion of a train of coaches and to that of a train of low waggons of equal weight.

The former editions of this work having been published before the discovery which has resulted from these experiments, the average amount of resistance to railway trains, there stated, and the conclusions deduced therefrom, were in conformity with what was then known. It was stated that the resistance to the moving power was practically independent of the speed, and on level rails was at the average rate of about seven pounds and a half per ton. This amount would be equivalent to the gravitation of a load down an inclined plane falling ¹⁄₃₀₀, and consequently in ascending such a plane the moving power would have to encounter twice [Pg411] the resistance opposed to it on a level. As it was generally assumed that a locomotive-engine could not advantageously vary its tractive power beyond this limit, it was therefore inferred that gradients (as inclinations are called) ought not to be constructed of greater steepness than ¹⁄₃₀₀. It was supposed that in descending gradients more steep than this the train would be accelerated and would require the use of the brake to check its motion, while in ascending such planes the engine would be required to exert more than twice the ordinary tractive power required on level rails. As the resistance produced by the air was not taken into consideration, no distinction was made between heavy trains of goods presenting a frontage and magnitude bearing a small proportion to their gross weight and lighter trains of passenger-coaches presenting great frontage and great magnitude in proportion to their weight. The result of the experiments above explained leads to inferences altogether at variance with those which have been given in former editions of the present work, and which were then universally admitted by railway engineers. The tendency of the results of these experiments show that low gradients on railways are not attended with the advantageous effects which have been hitherto ascribed to them; that, on the contrary, the resistance produced by steeper gradients can be compensated by slackening the speed, so that the power shall be relieved from as much atmospheric resistance by the diminution of velocity as is equal to the increased resistance produced by the gravity of the plane which is ascended. And, on the other hand, in descending the plane the speed may be increased until the resistance produced by the atmosphere is increased to the same amount as that by which the train is relieved of resistance by the declivity down which it moves. Thus, on gradients, the inclination of which is confined within practical limits, the resistance to the moving-power may be preserved uniform, or nearly so, by varying the velocity.