LOCOMOTIVE ENGINES ON RAILWAYS.

[Pg321]
TOCINX

We are not, however, to understand that every engine, in which steam is used of a pressure exceeding that of the atmosphere, is what is meant by an high-pressure engine; for in the ordinary engines in common use, constructed on Watt's principle, the safety-valve is loaded with from 3 to 5 lbs. on the square inch; and in Woolf's engines, the steam is produced under a pressure of 40 lbs. on the square inch. These would therefore be more properly called condensing engines than low-pressure engines; a term quite inapplicable to those of Woolf. In fact, by high-pressure engines is meant engines in which no vacuum is produced, and, therefore, in which the piston works against a pressure equal to that of the atmosphere.

In these engines the whole of the condensing apparatus, viz. the cold-water cistern, condenser, air-pump, cold-water pump, &c., are dispensed with, and nothing is retained except the boiler, cylinder, piston, and valves. Consequently, such an engine is small, light, and cheap. It is portable also, and may be moved, if necessary, along with its load, and is therefore well adapted to locomotive purposes.

A (fig. 82.) is the boiler, with the furnace beneath it; C C are two cylinders with solid pistons P P', connected with the working-beams B B', to which are attached the pump-rods R R', of two forcing pumps F F', which communicate with a great force-pipe S; G is a four-way cock (66.) already described. In the position in which it stands in the figure, the steam issues from below the piston P into the atmosphere, and the piston is descending by its own weight; steam from the boiler is at the same time pressing up the piston P', with a force equal to the difference between the pressure of the steam and that of the atmosphere. Thus the piston R of the forcing-pump is being drawn up, and the piston P' is forcing the piston R' down, and thereby driving water into the force-pipe [Pg324] S. On the arrival of the piston P at the bottom of the cylinder C, and P' at the top of the cylinder C', the position of the cock is changed as represented in fig. 83. The steam, which has just pressed up the piston P', is allowed to escape into the atmosphere, while the steam, passing from the boiler below the piston P, presses it up, and thus P ascends by the steam pressure, and P' descends by its own weight. By these means the piston R is forced down, driving before it the water in the pump-cylinder into the force-pipe S, and the piston R' is drawn up to allow the other pump-cylinder to be re-filled; and so the process is continued.

A valve is placed in the bottom of the force-pipes, to prevent the water which has been driven into it from returning. This valve opens upwards; and, consequently, the weight of the water pressing upon it only keeps it more effectually closed. On each descent of the piston, the pressure transmitted to the valve acting upwards being greater than the weight of the water resting upon it, forces it open, and an increased quantity of water is introduced.

The boiler A B is a cylinder with flat circular ends. The fire-place is constructed in the following manner:—A tube enters the cylindrical boiler at one end; and, proceeding onwards near the other extremity, is turned and recurved, so as to be carried back parallel to the direction in which it entered. It is thus conducted out of the boiler, at another part of the same end at which it entered. One of the ends of this tube communicates with the chimney E, which is carried upwards as represented in the figure. The other mouth is furnished [Pg325] with a door; and in it is placed the grate, which is formed of horizontal bars, dividing the tube into two parts; the upper part forming the fire-place, and the lower the ash-pit. The fuel is maintained in a state of combustion, on the bars, in that part of the tube represented at C D; and the flame is carried by the draught of the chimney round the curved flue, and issues at E into the chimney. The flame is thus conducted through the water, so as to expose the latter to as much heat as possible.

A section of the cylinder is represented at F, immersed in the boiler, except a few inches of the upper end, where the four-way cock G is placed for regulating the admission of the steam. A tube is represented at H, which leads from this four-way cock into the chimney; so that the waste steam, after working the piston, is carried off through this tube, and passes into the chimney. The upper end of the piston-rod is furnished with a cross-bar, which is placed in a direction at right angles to the length of the boiler, and also to the [Pg326] piston-rod. This bar is guided in its motion by sliding on two iron perpendicular rods fixed to the sides of the boiler, and parallel to each other. To the ends of this cross-bar are joined two connecting rods, the lower ends of which work two cranks fixed on an axis extending across and beneath the boiler, and immediately under the centre of the cylinder. This axis is sustained in bearings formed in the legs which support the boiler, and upon its extremity is fixed the fly-wheel as represented at B. A large-toothed wheel is placed on this axis; which, being turned with the cranked axle, communicates motion to other wheels; and through them, to any machinery which the engine may be applied to move.

As the four-way cock is represented in the figure, the steam passes from the boiler through the curved passage G above the piston, while the steam below the piston is carried off through a tube which does not appear in the figure, by which it is conducted to the tube H, and thence to the chimney. The steam, therefore, which passes above the piston presses it downwards; while the pressure upwards does not exceed that of the atmosphere. The piston will therefore descend with a force depending on the excess of the pressure of the steam produced in the boiler above the atmospheric pressure. When the piston has arrived at the bottom of the cylinder, the cock is made to assume the position represented in fig. 85. This effect is produced by the motion of the piston-rod. The steam now passes from above the piston, through the tube H, into the chimney, while the steam from the boiler is conducted through another tube below the piston. The pressure above the piston, in this case, does not exceed that of the atmosphere; while the pressure below it will be that of the steam in the boiler. The piston will therefore ascend with the difference of these pressures. On the arrival of the piston at the top of the cylinder, the four-way cock is again turned to the position represented in fig. 85., and the piston again descends; and in the same manner the process is continued. A safety-valve is placed on the boiler at V, loaded with a weight W, proportionate [Pg327] to the strength of the steam with which it is proposed to work.

In the engines now described, this valve was frequently loaded at the rate of from 60 to 80 lbs. on the square inch. As the boilers of high-pressure engines were considered more liable to accidents from bursting than those in which steam of a lower pressure was used, greater precautions were taken against such effects. A second safety-valve was provided, which was not left in the power of the engine-man. By this means he had a power to diminish the pressure of the steam, but could not increase it beyond the limit determined by the valve which was removed from his interference. The greatest cause of danger, however, arose from the water in the boiler being consumed by evaporation faster than it was supplied; and therefore falling below the level of the tube containing the furnace. To guard against accidents arising from this circumstance, a hole was bored in the boiler, at a certain depth, below which the water should not be allowed to fall; and in this hole a plug of metal was soldered with lead, or with some other metal, which would fuse at that temperature which would expose the boiler to danger. Thus, in the event of the water being exhausted, so that its level would fall below the plug, the heat of the furnace would immediately melt the solder, and the plug would fall out, affording a vent for the steam, without allowing the boiler to burst. The mercurial steam-gauge, already described, was also used as an additional security. When the force of the steam exceeded the length of the column of mercury which the tube would contain, the mercury would be blown out, and the tube would give vent to the steam. The water by which the boiler was replenished was forced into it by a pump worked by the engine. In order to economise the heat, this water was contained in a tube T, which surrounded the pipe H. As the waste steam, after working the piston, passed off through H, it imparted a portion of its heat to the water contained in the tube T, which was thus warmed to a certain temperature before it was forced into the boiler by the pump. Thus a part of the heat, which was originally [Pg328] carried from the boiler in the form of steam, was returned again to the boiler with the water with which it was fed.

It is evident that engines constructed in this manner may be applied to all the purposes to which the condensing engines are applicable.

The astonishment with which such performances must be viewed, might be qualified, if the art of transport by steam on railways had been matured, and had attained that full state of perfection which such an art is always capable of receiving from long experience, aided by great scientific knowledge, and the unbounded application of capital. But such is not the present case. The art of constructing locomotive engines, so far from having attained a state of maturity, has not even emerged from its infancy. So complete was the ignorance of its powers which prevailed, even among engineers, previous to the opening of the Liverpool railway, that the transport of heavy goods was regarded as the chief object of the undertaking, and its principal source of revenue. The incredible speed of transport, effected even in the very first experiments in 1830, burst upon the public, and on the scientific world, with all the effect of a new and unlooked-for phenomenon. On the unfortunate occasion which deprived this country of Mr. Huskisson, the wounded body of that statesman was transported a distance of about fifteen miles in twenty-five minutes, being at the rate of thirty-six miles an hour. The revenue of the road arising from passengers since its opening, [Pg330] has, contrary to all that was foreseen, been nearly double that which has been derived from merchandise. So great was the want of experience in the construction of engines, that the company was at first ignorant whether they should adopt large steam engines fixed at different stations on the line, to pull the carriages from station to station, or travelling engines to drag the loads the entire distance. Having decided on the latter, they have, even to the present moment, laboured under the disadvantage of the want of that knowledge which experience alone can give. The engines have been constantly varied in their weight and proportions, in their magnitude and form, as the experience of each successive month has indicated. As defects became manifest they were remedied; improvements suggested were adopted; and each year produced engines of such increased power and efficiency, that their predecessors were abandoned, not because they were worn out, but because they had been outstripped in the rapid march of improvement. Add to this, that only one species of travelling engine has been effectively tried; the capabilities of others remain still to be developed; and even that form of engine which has received the advantage of a course of experiments on so grand a scale to carry it towards perfection, is far short of this point, and still has defects, many of which, it is obvious, time and experience will remove.

If, then, the locomotive engine, subject thus to all the imperfections inseparable from a novel contrivance—with the restrictions on the free application of skill and capital, arising from the nature of the monopolies granted to railway companies—with the disadvantage of very limited experience, the great parent of practical improvement, having been submitted to experiments hitherto only on a limited scale, and confined almost to one form of machine;—if, under such disadvantages, such effects have been produced as are now daily witnessed by the public, what may not be looked for from this extraordinary power when the enterprise of the country shall be more unfettered—when greater fields of experience are opened—when time, ingenuity, and capital have removed or diminished existing imperfections, and have brought to light new and more powerful principles? This is not mere speculation [Pg331] on abstract possibilities, but refers to what is in actual progress. The points of greatest wealth and population—the centres of largest capital and most active industry throughout the country—will soon be connected by lines of railway; and various experiments are proposed, with more or less prospect of success, for the application of steam engines on stone roads where the intercourse is not sufficient to render railways profitable.

The important commercial and political effects attending such increased facility and speed in the transport of persons and goods, are too obvious to require any very extended notice here. A part of the price (and in many cases a considerable part) of every article of necessity or luxury, consists of the cost of transporting it from the producer to the consumer; and consequently every abatement or saving in this cost must produce a corresponding reduction in the price of every article transported; that is to say, of every thing which is necessary for the subsistence of the poor, or for the enjoyment of the rich—of every comfort, and of every luxury of life. The benefit of this will extend, not to the consumer only, but to the producer: by lowering the expense of transport of the produce, whether of the soil or of the loom, a less quantity of that produce will be spent in bringing the remainder to market, and consequently a greater surplus will reward the labour of the producer. The benefit of this will be felt even more by the agriculturist than by the manufacturer; because the proportional cost of transport of the produce of the soil is greater than that of manufactures. If two hundred quarters of corn be necessary to raise four hundred, and one hundred more be required to bring the four hundred to market, then the net surplus will be one hundred. But if by the use of steam carriages the same quantity can be brought to market with an expenditure of fifty quarters, then the net surplus will be increased from one hundred to one hundred and fifty quarters; and either the profit of the farmer, or the rent of the landlord, must be increased by the same amount.

But the agriculturist would not merely be benefited by an increased return from the soil already under cultivation. Any [Pg332] reduction in the cost of transporting the produce to market would call into cultivation tracts of inferior fertility, the returns from which would not at present repay the cost of cultivation and transport. Thus land would become productive which is now waste, and an effect would be produced equivalent to adding so much fertile soil to the present extent of the country. It is well known, that land of a given degree of fertility will yield increased produce by the increased application of capital and labour. By a reduction in the cost of transport, a saving will be made which may enable the agriculturist to apply to tracts already under cultivation the capital thus saved, and thereby increase their actual production. Not only, therefore, would such an effect be attended with an increased extent of cultivated land, but also with an increased degree of cultivation in that which is already productive.

It has been said, that in Great Britain there are above a million of horses engaged in various ways in the transport of passengers and goods, and that to transport each horse requires as much land as would, upon an average, support eight men. If this quantity of animal power were displaced by steam engines, and the means of transport drawn from the bowels of the earth, instead of being raised upon its surface, then, supposing the above calculation correct, as much land would become available for the support of human beings as would suffice for an additional population of eight millions; or, what amounts to the same, would increase the means of support of the present population by about one third of the present available means. The land which now supports horses for transport would then support men, or produce corn for food.

The objection that a quantity of land exists in the country capable of supporting horses alone, and that such land would be thrown out of cultivation, scarcely deserves notice here. The existence of any considerable quantity of such land is extremely doubtful. What is the soil which will feed a horse and not feed oxen or sheep, or produce food for man? But even if it be admitted that there exists in the country a small portion of such land, that portion cannot exceed, nor indeed equal, what would be sufficient for the number of horses [Pg333] which must after all continue to be employed for the purposes of pleasure, and in a variety of cases where steam must necessarily be inapplicable. It is to be remembered, also, that the displacing of horses in one extensive occupation, by diminishing their price must necessarily increase the demand for them in others.

The reduction in the cost of transport of manufactured articles, by lowering their price in the market, will stimulate their consumption. This observation applies of course not only to home but to foreign markets. In the latter we already in many branches of manufactures command a monopoly. The reduced price which we shall attain by cheapness and facility of transport will still further extend and increase our advantages. The necessary consequence will be, an increased demand for manufacturing population; and this increased population again reacting on the agricultural interests, will form an increased market for that species of produce. So interwoven and complicated are the fibres which form the texture of the highly civilised and artificial community in which we live, that an effect produced on any one point is instantly transmitted to the most remote and apparently unconnected parts of the system.

The two advantages of increased cheapness and speed, besides extending the amount of existing traffic, call into existence new objects of commercial intercourse. For the same reason that the reduced cost of transport, as we have shown, calls new soils into cultivation, it also calls into existence new markets for manufactured and agricultural produce. The great speed of transit which has been proved to be practicable, must open a commerce between distant points in various articles, the nature of which does not permit them to be preserved so as to be fit for use beyond a certain time. Such are, for example, many species of vegetable and animal food, which at present are confined to markets at a very limited distance from the grower or feeder. The truth of this observation is manifested by the effects which have followed the intercourse by steam on the Irish Channel. The western towns of England have become markets for a prodigious quantity of Irish produce, which it had been previously [Pg334] impossible to export. If animal food be transported alive from the grower to the consumer, the distance of the market is limited by the power of the animal to travel, and the cost of its support on the road. It is only particular species of cattle which bear to be carried to market on common roads and by horse carriages. But the peculiar nature of a railway, the magnitude and weight of the loads which may be transported on it, and the prodigious speed which may be attained, render the transport of cattle, of every species, to almost any distance, both easy and cheap. In process of time, when the railway system becomes extended, the metropolis and populous towns will therefore become markets, not as at present to districts within limited distances of them, but to the whole country.

The moral and political consequences of so great a change in the powers of transition of persons and intelligence from place to place are not easily calculated. The concentration of mind and exertion which a great metropolis always exhibits, will be extended in a considerable degree to the whole realm. The same effect will be produced as if all distances were lessened in the proportion in which the speed and cheapness of transit are increased. Towns at present removed some stages from the metropolis, will become its suburbs; others, now at a day's journey, will be removed to its immediate vicinity; business will be carried on with as much ease between them and the metropolis, as it is now between distant points of the metropolis itself. Let those who discard speculations like these as wild and improbable, recur to the state of public opinion, at no very remote period, on the subject of steam navigation. Within the memory of persons who have not yet passed the meridian of life, the possibility of traversing by the steam engine the channels and seas that surround and intersect these islands, was regarded as the dream of enthusiasts. Nautical men and men of science rejected such speculations with equal incredulity, and with little less than scorn for the understanding of those who could for a moment entertain them. Yet we have witnessed steam engines traversing not these channels and seas alone, but sweeping the face of the waters round every coast in Europe. The [Pg335] seas which interpose between our Asiatic dominions and Egypt, and those which separate our own shores from our West Indian possessions, have offered an equally ineffectual barrier to its powers, and the establishment of a regular steam communication between the capitals of the Old and New World has ceased to be a question of practicability, having become merely one of commercial profit. If steam be not used as the only means of connecting the most distant points of our planet, it is not because it is inadequate to the accomplishment of that end, but because the supply of the material, from which at the present moment it derives its powers, is restricted by local and accidental circumstances.[30]

We propose in the present chapter to lay before our readers some account of the means whereby the effects above referred to have been produced; of the manner and degree in which the public have availed themselves of these means; and of the improvements of which they seem to us to be susceptible.

It is obvious that both of these effects may be partially produced; the adhesion of the wheel to the road may be insufficient to prevent slipping altogether, and yet it may be sufficient to prevent the wheel from slipping as fast as it revolves. Under such circumstances the carriage would advance and the wheel would slip. The progressive motion of the carriage during one complete revolution of the wheel would be equal to the difference between the complete circumference of the wheel and the portion through which in one revolution it has slipped.

When the construction of travelling steam engines first engaged the attention of engineers, and for a considerable period afterwards, a notion was impressed upon their minds that the adhesion between the face of the wheel and the surface of the road must necessarily be of very small amount, and that in every practical case the wheels thus driven would either slip altogether, and produce no advance of the carriage, or that a considerable portion of the impelling power would be lost by the partial slipping or sliding of the wheels. It is singular that it should never have occurred to the many ingenious persons who for several years were engaged in such experiments and speculations, to ascertain by experiment the actual amount of adhesion in any particular case between the wheels and the road. Had they done so, we should probably now have found locomotive engines in a more advanced state than that to which they have attained.

To remedy this imaginary difficulty, Messrs. Trevethick and Vivian proposed to make the external rims of the wheels rough and uneven, by surrounding them with projecting heads of nails or bolts, or by cutting transverse grooves on them. They proposed, in cases where considerable elevations were to be ascended, to cause claws or nails to project from the surface during the ascent, so as to take hold of the road. [Pg337]

In seven years after the construction of the first locomotive engine by these engineers, another locomotive engine was constructed by Mr. Blinkensop, of Middleton Colliery, near Leeds. He obtained a patent, in 1811, for the application of a rack-rail. The railroad thus, instead of being composed of smooth bars of iron, presented a line of projecting teeth, like those of a cog-wheel, which stretched along the entire distance to be travelled. The wheels on which the engine rolled were furnished with corresponding teeth, which worked in the teeth of the railroad, and, in this way, produced a progressive motion in the carriage.

The next contrivance for overcoming this fictitious difficulty, was that of Messrs. Chapman, who, in the year 1812, obtained a patent for working a locomotive engine by a chain extending along the middle of the line of railroad, from the one end to the other. This chain was passed once round a grooved wheel under the centre of the carriage; so that, when this grooved wheel was turned by the engine, the chain being incapable of slipping upon it, the carriage was consequently advanced on the road. In order to prevent the strain from acting on the whole length of the chain, its links were made to fall upon upright forks placed at certain intervals, which between those intervals sustained the tension of the chain produced by the engine. Friction-rollers were used to press the chain into the groove of the wheel, so as to prevent it from slipping. This contrivance was soon abandoned, for the very obvious reason that a prodigious loss of force was incurred by the friction of the chain.

The following year, 1813, produced a contrivance of singular ingenuity, for overcoming the supposed difficulty arising from the want of adhesion between the wheels and the road. This was no other than a pair of mechanical legs and feet, which were made to walk and propel in a manner somewhat resembling the feet of an animal.

A sketch of these propellers is given in fig. 86. A is the carriage moving on the railroad, L and L' are the legs, F and F' the feet. The foot F has a joint at O, which corresponds to the ankle; another joint is placed at K, which corresponds to the knee; and a third is placed at L, which corresponds to [Pg338] the hip. Similar joints are placed at the corresponding letters in the other leg. The knee-joint K is attached to the end of the piston of the cylinder. When the piston, which is horizontal, is pressed outwards, the leg L presses the foot F against the ground, and the resistance forces the carriage A onwards. As the carriage proceeds, the angle K at the knee becomes larger, so that the leg and thigh take a straighter position; and this continues until the piston has reached the end of its stroke. At the hip L there is a short lever L M, the extremity of which is connected by a cord or chain with a point S, placed near the shin of the leg. When the piston is pressed into the cylinder, the knee K is drawn towards the engine, and the cord M S is made to lift the foot F from the ground; to which it does not return until the piston has arrived at the extremity of the cylinder. On the piston being again driven out of the cylinder, the foot F, being placed on the road, is pressed backwards by the force of the piston-rod at K; but the friction of the ground preventing its backward motion, the re-action causes the engine to advance: and in the same manner this process is continued.

Attached to the thigh at N, above the knee, by a joint, is a horizontal rod N R, which works a rack R. This rack has beneath it a cog-wheel. This cog-wheel acts in another rack below it. By these means, when the knee K is driven from the engine, the rack R is moved backwards; but the cog-wheel acting on the other rack beneath it, will move the latter in the contrary direction. The rack R being then moved in the [Pg339] same direction with the knee K, it follows that the other rack will always be moved in a contrary direction. The lower rack is connected by another horizontal rod with the thigh of the leg L F', immediately above the knee at N'. When the piston is forced inwards, the knee K' will thus be forced backwards; and when the piston is forced outwards, the knee K' will be drawn forwards. It therefore follows, that the two knees K and K' are pressed alternately backwards and forwards. The foot F', when the knee K' is drawn forward, is lifted by the means already described for the foot F.

It will be apparent, from this description, that the piece of mechanism here exhibited is a contrivance derived from the motion of the legs of an animal, and resembling in all respects the fore legs of a horse. It is however to be regarded rather as a specimen of great ingenuity than as a contrivance of practical utility.

In 1814, an engine was constructed at Killingworth, by Mr. Stephenson, having two cylinders with a cylindrical [Pg340] boiler, and working two pair of wheels, by cranks placed at right angles; so that when the one was in full operation, the other was at its dead points. By these means the propelling power was always in action. The cranks were maintained in this position by an endless chain, which passed round two cogged wheels placed under the engine, and which were fixed on the same axles on which the wheels were placed. The wheels in this case were fixed on the axles, and turned with them.

This engine is represented in fig. 87., the sides being open, to render the interior mechanism visible. A B is the cylindrical boiler; C C are the working cylinders; D E are the cogged wheels fixed on the axle of the wheels of the engine, and surrounded by the endless chain. These wheels being equal in magnitude, perform their revolutions in the same time; so that, when the crank F descends to the lowest point, the crank G rises from the lowest point to the horizontal position D; and, again, when the crank F rises from the lowest point to the horizontal position E, the other crank rises to the highest point; and so on. A very beautiful contrivance was adopted in this engine, by which it was suspended on springs of steam. Small cylinders, represented at H, are screwed by flanges to one side of the boiler, and project within it a few inches; they have free communication at the top with the water or steam of the boiler. Solid pistons are represented at I, which move steam-tight in these [Pg341] cylinders; the cylinders are open at the bottom, and the piston-rods are screwed on the carriage of the engine, over the axle of each pair of wheels, the pistons being presented upwards. As the engine is represented in the figure, it is supported on four pistons, two at each side. The pistons are pressed upon by the water or steam which occupies the upper chamber of the cylinder; and the latter being elastic in a high degree, the engine has all the advantage of spring suspension. The defect of this method of supporting the engine is, that when the steam loses that amount of elasticity necessary for the support of the machine, the pistons are forced into the cylinders, and the bottoms of the cylinders bear upon them. All spring suspension is then lost. This mode of suspension has consequently since been laid aside.

In an engine subsequently constructed by Mr. Stephenson, for the Killingworth railroad, the mode adopted of connecting the wheels by an endless chain and cog-wheels was abandoned; and the same effect was produced by connecting the two cranks by a straight rod. All such contrivances, however, have this great defect, that, if the fore and hind wheels be not constructed with dimensions accurately equal, there must necessarily be a slipping or dragging on the road. The nature of the machinery requires that each wheel should perform its revolution exactly in the same time; and consequently, in doing so, must pass over exactly equal lengths of the road. If, therefore, the circumference of the wheels be not accurately equal, that wheel which has the lesser circumference must be dragged along so much of the road as that by which it falls short of the circumference of the greater wheel; or, on the other hand, the greater wheel must be dragged in the opposite direction, to compensate for the same difference. As no mechanism can accomplish a perfect equality in four, much less in six, wheels, it may be assumed that a great portion of that dragging effect is a necessary consequence of the principle of this machine; and even were the wheels, in the first instance, accurately constructed, it is not possible that their wear could be so exactly uniform as to continue equal.

The steam engine may be applied, by two distinct methods, to move waggons either on a turnpike road or on a railway. By the one method the steam engine is fixed, and draws the carriage or train of carriages towards it by a chain extending the whole length of road on which the engine works. By this method the line of road over which the transport is conducted is divided into a number of short intervals, at the extremity of each of which an engine is placed. The waggons or carriages, when drawn by any engine to its own station, are detached, and connected with the extremity of the chain worked by the next stationary engine; and thus the journey is performed, from station to station, by separate engines. By the other method the same engine draws the load the whole journey, travelling with it.

The Directors of the Liverpool and Manchester railroad, when that work was advanced towards its completion, employed, in the spring of the year 1829, Messrs. Stephenson and Lock, and Messrs. Walker and Rastrick, experienced engineers, to visit the different railways, where practical information respecting the comparative effects of stationary and locomotive engines was likely to be obtained; and from these gentlemen they received reports on the relative merits, according to their judgment of the two methods. The particulars of their calculations are given at large in the valuable work of Mr. Nicholas Wood on railways; to which we refer the reader, not only on this, but on many other subjects connected with the locomotive steam engine, into which it would be foreign to our object to enter. The result of the comparison of the two systems was, that the capital [Pg343] necessary to be advanced to establish a line of stationary engines was considerably greater than that which was necessary to establish an equivalent power in locomotive engines; that the annual expense by the stationary engines was likewise greater; and that, consequently, the expense of transport by the latter was greater, in a like proportion. The subjoined table exhibits the results numerically:—

On the score of economy, therefore, the system of locomotive engines was entitled to a preference; but there were other considerations which conspired with this to decide the choice of the Directors in its favour. An accident occurring in any part of a road worked by stationary engines must necessarily produce a total suspension of work along the entire line. The most vigilant and active attention on the part of every workman, however employed, in every part of the line, would therefore be necessary; but, independently of this, accidents arising from the fracture or derangement of any of the chains, or from the suspension of the working of any of the fixed engines, would be equally injurious, and would effectually stop the intercourse along the line. On the other hand, in locomotive engines an accident could only affect the particular train of carriages drawn by the engine to which the accident might occur; and even then the difficulty could be remedied by having a supply of spare engines at convenient stations along the line. It is true that the probability of accident is, perhaps, less in the stationary than in the locomotive system; but the injurious consequences, when accident does happen, are prodigiously greater in the former. "The one system," says Mr. Walker, "is like a chain extending from Liverpool to Manchester, the failure [Pg344] of a single link of which would destroy the whole; while the other is like a number of short and unconnected chains," the destruction of any one of which does not interfere with the effect of the others, and the loss of which may be supplied with facility.

The decision of the Directors was, therefore, in favour of locomotive engines; and their next measure was to devise some means by which the inventive genius of the country might be stimulated to supply them with the best possible form of engines for this purpose. With this view, it was proposed and carried into effect to offer a prize for the best locomotive engine which might be produced under certain proposed conditions, and to appoint a time for a public trial of the claims of the candidates. A premium of five hundred pounds was accordingly offered for the best locomotive engine to run on the Liverpool and Manchester railway; under the condition that it should produce no smoke; that the pressure of the steam should be limited to fifty pounds on the inch; and that it should draw at least three times its own weight, at the rate of not less than ten miles an hour; that the engine should be supported on springs, and should not exceed fifteen feet in height. Precautions were also proposed against the consequences of the boiler bursting; and other matters not necessary to mention more particularly here. This proposal was announced in the spring of 1829, and the time of trial was appointed in the following October. The engines which underwent the trial were, the Rocket, constructed by Mr. Stephenson; the Sanspareil, by Hackworth; and the Novelty, by Messrs. Braithwaite and Ericson. Of these, the Rocket obtained the premium. A line of railway was selected for the trial, on a level piece of road about two miles in length, near a place called Rainhill, between Liverpool and Manchester; the distance between the two stations was a mile and a half, and the engine had to travel this distance backwards and forwards ten times, which made altogether a journey of thirty miles. The Rocket performed this journey twice: the first time in 2 hours 14 minutes and 8 seconds; and the second time in 2 hours 6 minutes and 49 seconds. Its speed at different parts of the journey varied: its greatest rate of motion was [Pg345] rather above 29 miles an hour; and its least, about 11¹/₂ miles an hour. The average rate of the one journey was 13⁴/₁₀ miles an hour; and of the other, 14²/₂₀ miles. This was the only engine which performed the complete journey proposed, the others having been stopped from accidents which occurred to them in the experiment. The Sanspareil performed the distance between the stations eight times, travelling 22¹/₂ miles in 1 hour 37 minutes and 16 seconds. The greatest velocity to which this engine attained was something less than 23 miles per hour. The Novelty had only passed twice between the stations when the joints of the boiler gave way, and put an end to the experiment.

This engine is represented in fig. 88. It is supported on four wheels; the principal part of the weight being thrown on one pair, which are worked by the engine. The boiler consists of a cylinder six feet in length, with flat ends; the chimney issues from one end, and to the other end is attached a square box B, the bottom of which is furnished with the grate on which the fuel is placed. This box is composed of two casings of iron, one contained within the other, having between them a space about three inches in breadth; the magnitude of the box being three feet in length, two feet in width, and three feet in depth. The casing which surrounds the box communicates with the lower part of the boiler by a pipe marked C; and the same casing at the top of the box communicates with the upper part of the boiler by another pipe marked D. When water is admitted into the boiler, therefore, it flows freely, through the pipe C, into the casing which [Pg346] surrounds the furnace or fire-box, and fills this casing to the same level as that which it has in the boiler. When the engine is at work, the boiler is kept about half filled with water; and, consequently, the casing surrounding the furnace is completely filled. The steam which is generated in the water contained in the casing finds its exit through the pipe D, and escapes into the upper part of the boiler. A section of the engine, taken at right angles to its length, is represented at fig. 89. Through the lower part of the boiler pass a number of copper tubes of small size, which communicate at one end with the fire-box, and at the other with the chimney, and form a passage for the heated air from the furnace to the chimney. The ignited fuel spread on the grate at the bottom of the fire-box disperses its heat by radiation, and acts in this manner on the whole surface of the casing surrounding the fire-box; and thus raises the temperature of the thin shell of water contained in that casing. The chief [Pg347] part of the water in the casing, being lower in its position than the water in the boiler, acquires a tendency to ascend when heated, and passes into the boiler; so that a constant circulation of the heated water is maintained, and the water in the boiler must necessarily be kept at nearly the same temperature as the water in the casing. The air which passes through the burning fuel, and which fills the fire-box, is carried by the draught through the tubes which extend through the lower part of the boiler; and as these tubes are surrounded on every side with the water contained in the boiler, this air transmits its heat through these tubes to the water. It finally issues into the chimney, and rises by the draught. The power of this furnace must necessarily depend on the power of draught in the chimney; and to increase this, and at the same time to dispose of the waste steam after it has worked the piston, this steam is carried off by a pipe L, which passes from the cylinder to the chimney, and escapes there in a jet which is turned upwards. By the velocity with which it issues from this jet, and by its great comparative levity, it produces a strong current upwards in the chimney, and thus gives force to the draught of the furnace. In fig. 89. the grate-bars are represented at the bottom of the fire-box at F. There are two cylinders, one of which works each wheel; one only appearing in the drawing fig. 88., the other being concealed by the engine. The spokes which these cylinders work are placed at right angles on the wheels; the wheels being fixed on a common axle, with which they turn.

In this engine, the surface of water surrounding the fire-box, exposed to the action of radiant heat, amounted to twenty square feet, which received heat from the surface of six square feet of burning fuel on the bars. The surface exposed to the action of the heated air amounted to 118 square feet. The engine drew after it another carriage, containing fuel and water; the fuel used was coke, for the purpose of avoiding the production of smoke.

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.

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.

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.

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]

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.

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.

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:—

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.

It will be recollected that the fire-place is situated at one end of the boiler, immediately below the mouths of the tubes: a powerful draft of air, passing through the fire, carries with it ashes and cinders, which are driven violently through the tubes, and especially the lower ones, situated near the fuel. These tubes are, by this means, subject to rapid wear, the cinders continually acting upon their interior surface. After a short time it becomes necessary to replace single tubes, according as they are found to be worn, by new ones; and it not unfrequently happens, when this is neglected, that tubes burst. After a certain length of time the engines require new tubing. This wear of the tubes might possibly be avoided by constructing the fire-place in a lower position, so as to be more removed from their mouths; or, still more effectually, by interposing a casing of metal, which might be filled with water, between the fire-place and those tubes which are the most exposed to the cinders and ashes. The unequal expansion of the tubes [Pg361] and boilers appears to be an incurable defect, if the present form of the engine be retained. If the fire-place and chimney could be placed at the same end of the boiler, so that the tubes might be recurved, the unequal expansion would then produce no injurious effect; but it would be difficult to clean the tubes, if they were exposed, as they are at present, to the cinders. The next source of expense arises from the wear of the boiler-heads, which are exposed to the action of the fire.

A considerable improvement was subsequently introduced into the method of tubing, by substituting brass for copper tubes. I am not aware that the cause of this improvement has been discovered; but it is certain, whatever be the cause, that brass tubes are subject to considerably slower wear than copper ones.

It appears that, previous to the establishment of the railway, Messrs. Walker and Rastrick, engineers, were employed by the company to visit various places where steam power was applied on railways, for the purpose of forming an estimate of the probable comparative expense of working the railway by locomotive and by fixed power. These engineers recommended the adoption of locomotive power; and their estimate was, that the transport might be effected at the rate of ·278 of a penny, or very little more than a farthing per ton per mile. In the year [Pg362] 1833, five years after this investigation took place, it was found that the actual cost was ·625 of a penny, or something more than a halfpenny, per ton per mile, being considerably above double the estimated rate. Mr. Booth very properly directed his inquiries to ascertain the cause of this discrepancy, by comparing the various circumstances assumed by Messrs. Walker and Rastrick, in making their estimate, with those under which the transport was actually effected. The first point of difference which he observed was the speed of transport: the estimate was founded on an assumed speed of ten miles an hour, and it was stated that a four-fold speed would require an addition of 50 per cent. to the power, without taking into account wear and tear. Now, the actual speed of transport being double the speed assumed in the statement, Mr. Booth holds it to be necessary to add 25 per cent. on that score.

The next point of difference is in the amount of the loads: the estimate is founded upon the assumption, that every engine shall start with its full complement of load, and that with this it shall go the whole distance. "The facts, however, are," says Mr. Booth, "that, instead of a full load of profitable carriage from Manchester, about half the waggons come back empty; and, instead of the tonnage being conveyed the whole way, many thousand tons are conveyed only half the way; also, instead of the daily work being uniform, it is extremely fluctuating." It is further remarked, that in order to accomplish the transport of goods from the branches and from intermediate places, engines are despatched several times a-day, from both ends of the line, to clear the road; the object of this arrangement being rather to lay the foundation of a beneficial intercourse in future, than with a view to any immediate profit. Mr. Booth makes a rough estimate of the disadvantages arising from these circumstances, by stating them at 33 per cent. in addition to the original estimate.

The next point of difference is the fuel. In the original estimate, coal is assumed as the fuel, and it is taken at the price of five shillings and ten-pence per ton: now the act of parliament forbids the use of coal which would produce smoke; the company have, therefore, been obliged to use coke, at [Pg363] seventeen shillings and sixpence a ton.[32] Taking coke, then, to be equivalent to coal, ton for ton, this would add ·162 to the original estimate.

These several discrepancies being allowed for, and a proportional amount being added to the original estimate, the amount would be raised to ·601 of a penny per ton per mile, which is within one fortieth of a penny of the actual cost. This difference is considered to be sufficiently accounted for by the wear and tear produced by the very rapid motion, more especially when it is considered that many of the engines were constructed before the engineer was aware of the great speed that would be required.

"What, then," says Mr. Booth, in the Report already alluded to, "is the result of these opposite and mutually counteracting circumstances? and what is the present position of the company in respect of their moving power? Simply, that they are still in a course of experiment, to ascertain practically the best construction, and the most durable materials, for engines required to transport greater weights, and at greater velocities, than had, till very recently, been considered possible; and which, a few years ago, it had not entered into the imagination of the most daring and sanguine inventor to conceive: and farther, that these experiments have necessarily been made, not with the calm deliberation and quiet pace which a salutary caution recommends,—making good each step in the progress of discovery before advancing another stage,—but amidst the bustle and responsibilities of a large and increasing traffic; the directors being altogether ignorant of the time each engine would last before it would be laid up as inefficient, but compelled to have engines, whether good or bad; being aware of various defects and imperfections, which it was impossible at the time to remedy, yet obliged to keep the machines in motion, under all the disadvantages of heavy repairs, constantly going on during the night, in order that the requisite number of engines might be ready for the morning's work. Neither is this great experiment yet complete; it is still going forward. But the most prominent difficulties have been in a great measure surmounted, [Pg364] and your committee conceive that they are warranted in expecting, that the expenditure in this department will, ere long, be materially reduced,—more especially when they consider the relative performances of the engines at the present time, compared with what it was two years ago."

In the half year ending 31st December, 1831, the six best engines performed as follows:—

In the half year ending 31st December, 1833, the six best engines performed as follows:—

The external appearance of the engine and tender is shown in the engraving at the head of this chapter. In fig. 97. is exhibited a vertical section of the engine made by a plane carried through its length; and in fig. 98. is exhibited a corresponding section of its tender,—the tender being supposed to be joined on to the engine at the part where the connecting points appear to be broken in the drawing. In fig. 99. is exhibited the plan of the working machinery, including the cylinders, pistons, eccentrics, &c. which are under the boiler, by the operation of which the engine is driven. Fig. 100. represents the tender, also taken in plan.

In fig. 101. is represented an elevation of the hinder end of the engine next the fire-box; and in fig. 102. is represented a cross vertical section through the fire-box, and at right angles to the length of the engine, showing the interior of the boiler above and beside the fire-box, the rivets and bolts connecting the internal and external fire-boxes, the regulator, steam funnel, and steam dome.

In fig. 103. is represented an elevation of the front of the engine next the smoke-box, showing the cylinder covers W, buffers T, &c.; and in fig. 104. is represented a section of the interior of the smoke-box, made by a vertical plane at right angles to the engine, showing the tube plate forming the foremost end of the boiler, the branches S of the steam-pipe leading to the cylinders, the blast-pipe p, the cylinders H, and the chimney G.

The same letters of reference are placed at corresponding parts in the different figures.

The boiler, as has been explained in the engines already described, is a cylinder placed upon its side, the section of which is exhibited at A, fig. 97. The fire-box consists of two casings of metal, one within the other. The fire-grate is represented at D. The tubes by which the products of combustion are [Pg366] drawn from the fire-box to the smoke-box F are represented at E. Upon the smoke-box is erected the chimney G. In the engine from which this drawing has been taken, and which was used on the London and Birmingham Railway, the boiler is a cylinder 7¹/₂ feet long, and 3¹/₂ feet in diameter. It is formed of wrought-iron plates ⁵/₁₆ of an inch in thickness, overlapping each other, and bound together by iron rivets ⁷/₈ of an inch in diameter and 1³/₄ inch apart. One of these rivets, as it joins two plates, is represented in fig. 95. The boiler is clothed with a boarding of wood a, an inch in thickness, and bound round by iron hoops screwed together at the bottom. Wood being a slow conductor of heat, this covering has the effect of keeping the boiler warm, and checking the condensation of steam which would otherwise be produced by the rapid motion of the engine through the cold air.

The external fire-box, B B, is a casing nearly square in its plan, being four feet wide outside, and three feet seven and a half inches long, measured in the direction of the boiler. It is constructed of wrought-iron plates, similar to those of the boiler. This box descends about two feet below the boiler, the top being semi-cylindrical, as seen in fig. 102., of a somewhat greater diameter than the boiler, and concentrical with it. The front of the fire-box next the end of the boiler has a circular opening equal in size to the end of the boiler. To the edge of this opening the boiler is fastened by angle irons, and rivets in the manner represented in fig. 96. These rivets are seen in section in fig. 97.

The internal fire-box C, fig. 97., is similar in shape to the external, only it is flat at the top, and close every where except at the bottom. Between it and the external fire-box an open space of three inches and a half is left all round, and on the side next the boiler this space is increased to four inches. This internal fire-box is made of copper plates, ⁷/₁₆ [Pg367] of an inch in thickness, every where except next the boiler, where the thickness is ⁷/₈.

As the sides and front of the external fire-box, and all the surfaces bounding the internal fire-box, are flat, their form is unfavourable for the resistance of pressure. Adequate means are, therefore, provided for strengthening them. The plates forming the internal fire-box are bent outwards near the bottom, until they are brought into contact with those of the external fire-box, to which they are attached by copper rivets, as represented at f in fig. 97. The plates forming the bounding surfaces of the two fire-boxes are fastened together by stays represented at k in figs. 97. and 102. These stays, which are of copper, have a screw cut upon them through their whole length, and holes are made through the plates of both fire-boxes tapped with corresponding threads. The copper screws are then passed through them, and rivets formed on their heads within and without, as seen in fig. 102. These screw rivets connect all parts of the plating of the two fire-boxes which are opposed to each other: they are placed at about four inches apart over the sides and back of the internal fire-place and that part of the front which is below the boiler.

As the top of the internal fire-box cannot be strengthened by stays of this kind, ribs of wrought-iron, which are seen in their length at l, in fig. 97., and of which an end view is seen in fig. 102., are attached by bolts to it. These ribs are hollowed out, as seen in fig. 97., between bolt and bolt, in order to break their contact with the roof of the fire-box, and allow a more free passage to the heat through it. If they were in continuous contact with the fire-box, the metal composing them would become more highly heated, and would soon wear out, besides intercepting heat from the water. This part of the fire-box is subject to rapid wear, unless care be taken that the level of the water be preserved at its proper height in the boiler. Even when the boiler is properly filled, the depth of water above the roof of the fire-box is not considerable, and on the least neglect the roof may be exposed to the contact of steam, in which case it will soon be destroyed.

To prevent accidents arising from this cause, a leaden plug, [Pg368] represented at m, figs. 97. and 102., is inserted in the roof of the internal fire-box. If the water be allowed to subside, this plug will melt out before the copper is very injuriously heated, and the steam rushing out at the aperture will cause the fire to be extinguished.

Copper fire-boxes are almost universally used; but sometimes, from the consideration of cheapness, the internal fire-box is constructed of iron.

In the plating which forms the back of the external fire-box, an oval aperture is formed, as represented in the back view of the engine, fig. 101., for the fire-door g. The plating of the internal fire-box around this aperture is bent at right angles to meet that of the external fire-box, to which it is fastened by a row of copper rivets. The fire-door is formed of two plates of wrought-iron, riveted together with a space of nine inches and a half between them. The air between these plates being an imperfect conductor of heat, keeps the outer plate of the fire-door at a moderate temperature.

In that part of the surface of the internal fire-box which forms the end of the boiler, holes are made to receive the extremities of the tubes, by which the air proceeding from the fire is drawn to the smoke-box at the remote end of the boiler. These tubes are represented in longitudinal section at E, fig. 97., and their ends are seen in the surface of the internal fire-box in fig. 102., and in the remote end of the boiler where they terminate in the smoke-box in fig. 104. These tubes are formed of the best rolled brass, and their thickness in the engine, to which we now refer, is ¹/₁₃ of an inch. After the brass plating is bent into the form of a tube, and being overlapped, is properly soldered together, and the edges smoothed off, the tubes are made perfectly cylindrical by being drawn through a circular steel die.

The tube-plates (as those parts of the boiler ends in which the tubes are inserted are called) are bored with holes in corresponding positions, truly cylindrical, and corresponding in magnitude to the tubes, so that the tubes, when passed into them, will be just in contact with them. The length of the tubes is so regulated, that when extending from end to end of the boiler, and passing through the holes, they shall [Pg369] project at each end a little beyond the holes. The manner of fastening them so as to be water-tight is as follows:—A steel hoop or ferrule, made slightly conical, a section of which is exhibited at C. fig. 105., the smaller end of which is a little less than the internal diameter of the tube, but which increases towards the outer end, is driven in as represented in the figure. It acts as a wedge, and forces the tube into close contact with the edges of the hole in the tube-plate.

When particular tubes in a boiler are worn out, and require to be replaced, their removal is easily effected. It is only necessary to cut the steel ferrule on the inside, and to bend it off from contact with the tube, by which means it can be loosened and withdrawn, and the tube removed.

In the engine to which this description refers there were one hundred and twenty-four tubes, the external diameter of which was 1⁵/₈ inch. The distance between tube and tube was ³/₄ of an inch. The number of tubes vary in different engines, some having so many as one hundred and fifty, while the number in some is less than ninety. The evaporating power of an engine greatly depends on the proper number and magnitude of its tubes; and the experience which engineers have had on railways have led them gradually to increase the number of tubes, and diminish their magnitude. In the Rocket, already mentioned as having gained the prize on the opening of the Liverpool and Manchester Railway, the number of tubes was twenty-four, and their diameter three inches; but in all the engines subsequently made their number was augmented, and their diameter diminished. The practical inconvenience which limits the size of the tubes is their liability to become choked by cinders and ashes, which get wedged in them when they are too small, and thereby obstruct the draft, and diminish the evaporating power of the boiler. The tubes now in use, of about an inch and a [Pg370] half internal diameter, not only require to be cleared of the ashes and cinders, which get fastened in them after each journey, but it is necessary throughout a journey of any length that the tubes should be picked and cleaned by opening the fire door at convenient intervals.

The substitution of brass for copper tubes, which has been already mentioned as so great an improvement in the construction of locomotive engines, is ascribed to Mr. Dixon, who suggested them in 1833, being then the resident engineer of the Liverpool and Manchester Railway. They are said to last six or eight times as long as copper tubes of the same dimensions.

When tubes fail, they are usually destroyed by the pressure of the water crushing them inwards: the water enters through the rent made in the tube, and flowing upon the fire extinguishes it. When a single tube thus fails upon a journey, the engine, notwithstanding the accident, may generally be made to work to the end of its journey by plugging the ends of the broken tube with hard wood; the water in contact with which will prevent the fire from burning it away.

Tubes of the dimensions here referred to weigh about sixteen pounds, and lose from six to seven pounds before they are worn out. Their cost is about one pound each.

The tubes act as stays, connecting the ends of the boiler to strengthen them. Besides these, there are rods of wrought iron extended from end to end of the boiler above the roof of the internal fire-place. These rods are represented at o in their length in fig. 97., and an end view of them is seen in fig. 102. The smoke-box F, fig. 97. 104., containing the cylinders, steam-pipe, and blast-pipe, is four feet wide, and two feet long. It is formed of wrought iron plates, half an inch thick on the side next the boiler, and a quarter of an inch elsewhere. The plates are riveted in the same manner as those of the fire-box already described. From the top of the smoke-box, which, like the fire-box, is semi-cylindrical, as seen in elevation in fig. 103., and in section fig. 104., rises the chimney G, fifteen inches diameter, and formed of ¹/₈ inch iron plates, riveted and bound round by hoops. It is flanged to the top of the [Pg371] smoke-box, as represented in fig. 104. Near the bottom of the smoke-box the working cylinders are placed, side by side, in a horizontal position, with the slide valves upwards. In the top of the external fire-box a circular aperture is formed fifteen inches in diameter, and upon this aperture is placed the steam-dome T (figs. 97. 101, 102.) two feet high, and attached around the circular aperture by a flange and screw secured by nuts. This steam dome is made of brass ³/₈ inch thick. In stationary boilers, where magnitude is not limited, it has been already explained, that the space allowed for steam is sufficiently large to secure the complete separation of the vapour from the spray which is mixed with it when it issues immediately from the water. In locomotive boilers sufficient space cannot be allowed for this, and the separation of the water from the steam is effected by the arrangement here represented. A funnel-shaped tube d' (figs. 97. 102.), with its wide end upwards, rises into the steam-dome, and reaches nearly to the top of it. This funnel bends towards the back of the fire-box, and is attached by a flange and screws to the great steam-pipe S, which traverses the whole length of the boiler. The steam rising from the boiler fills the steam-dome T, and descends in the funnel-shaped tube d'. The space it has thus to traverse enables the steam to disengage itself almost completely from the priming. The wider part of the great steam-pipe a is flanged and screwed at the hinder end to a corresponding aperture in the back plate of the fire-box. This opening is covered by a circular plate, secured by screws, having a stuffing-box in its centre, of the same kind as is used for the piston-rods of steam-cylinders. Through this stuffing-box the spindle a of the regulator passes, and to its end is attached a winch h', by which the spindle a is capable of being turned. This winch is limited in its play to a quarter of a revolution. The other end of the spindle a is attached to a plate e' seen edgeways in fig. 97., and the face of which is seen in fig. 102.: this circular plate e is perforated with two apertures somewhat less than quadrants. That part of the plate, therefore, which remains not pierced forms two solid pieces somewhat greater than quadrants. This plate is ground so as to move in steam-tight [Pg372] contact with a fixed plate under it, which terminates at the wide end of the conical mouth of the steam-pipe S. This fixed circular plate is likewise pierced with two nearly quadrantal apertures, corresponding with those in the movable plate e'. When the movable plate e' is turned round by the winch h', the apertures in it may be made to correspond with those of the fixed circular plate on which it moves, in which position the steam-pipe S communicates with the funnel d' by the two quadrantal apertures thus open. If, on the other hand, the winch h' be moved from this position through a quarter revolution, then the quadrantal openings in the movable plate will be brought over the solid parts of the fixed plate on which it moves, and these solid parts being a little more than quadrants, while the openings are a little less, all communication between the steam-pipe S and the funnel d' will be stopped, for in this case the quadrantal openings in the fixed and movable plates respectively will be stopped by the solid parts of these plates. It will be evident that as the winch h' of the regulator is moved from the former position to the latter, in every intermediate position the aperture communicating between the funnel d' and the steam-pipe S will be less in magnitude than the complete quadrant. It will in fact be composed of two openings having the form of sectors of a circle less than a quadrant, and these sectors may be made of any magnitude, however small, until the opening is altogether closed.

By such means the admission of steam from the boiler to the steam-pipe S may be regulated by the winch h'.

The steam being admitted to the steam-pipe passes through it to the front end of the boiler, and the pipe being enclosed within the boiler the temperature of the steam is maintained. The steam-pipe passing through the tube-plate at the front end of the boiler is carried to a small distance from the tube-plate in the same direction, where it is flanged on to a cross horizontal pipe proceeding to the right and to the left as represented in fig. 104. This cross pipe is itself flanged to two curved steam-pipes S (fig. 104.), by which the steam is conducted to the valve-boxes V V. The lower ends of these curved arms are flanged on to the valve-boxes of the two cylinders [Pg373] at the ends nearest to the boiler. The opening of one of these is exhibited in the right hand cylinder in fig. 99. By these pipes the steam is conducted into the valve-boxes or steam-chests, from which it is admitted by slide-valves to the cylinders to work the pistons in the same manner as has been already described in the large stationary engines.

On the upper sides of the cylinders are formed the steam-chests or valve-boxes, which are exhibited at U (figs. 97. 99. 104.). These are made of cast-iron half an inch thick, and are bolted to the upper side of each cylinder. At the front end they are also secured by bolts to the smoke-box, and at the hinder end are attached to the tube-plate. These valve-boxes communicate with the passages m and n fig. 99. leading to the top and bottom of the cylinder: these are called the steam-ports. They also communicate with a passage o leading to the mouth of a curved horizontal pipe p' connecting the front ends of the two cylinders, as seen in figs. 99. 104. These curved pipes unite in a single vertical pipe p, called the blast-pipe, seen in figs. 97. 104.: this vertical pipe becomes gradually small towards the top, and terminates a little above the base of the funnel or chimney G. In the valve-box is placed the slide-valve v to which is attached the spindle l'. This spindle moves through a stuffing-box k', and is worked by gearing, which will be described hereafter. According to the position given to the slide, a communication may be opened between the steam-chest, or the waste-port, and either end of the cylinders. Thus when the slide is in the position represented in fig. 97. the steam-chest communicates with the front end of the cylinder, while the waste-port communicates with the hinder end. If, on the other hand, the spindle l' being pressed forward, move the slide to its extreme opposite position, the steam-port n would communicate with the waste-port o, while the steam-chest would communicate with the steam-port m, steam would, therefore, be admitted to the hinder end of the cylinder, while the foremost end would communicate with the waste-port. It will be perceived that this arrangement is precisely similar to that of the slide-valves already described (133.). The slide-valve is represented on a larger scale in fig. 106., where A is the hinder steam-port, [Pg374] B the foremost steam-port, and C the waste-port. The surfaces D, separating the steam-ports from the waste-ports, are called the bars: they are planed perfectly smooth, so that the surfaces F and G of the slide-valve, also planed perfectly smooth, may move in steam-tight contact with them. These surfaces are kept in contact by the pressure of the steam in the steam-chest, by which the slide-valve is always pressed down. In its middle position, as represented by the dotted lines in the figure, both the steam-ports are stopped by the slide-valve, so that at that moment no steam is admitted to either end of the cylinder. On either side of this intermediate position the slide has an inch and a half play, which is sufficient to open successively the two steam-ports.

The cylinders are inserted at one end in the plate of the smoke-box, and at the other in the tube-plate of the boiler. They are closed at either end by cast iron covers, nearly an inch thick, flanged on by bolts and screws. In the cover of the cylinder attached to the tube-plate is a stuffing-box, in which the piston rod plays. The metallic pistons used in locomotive engines do not differ materially from those already described, and therefore need not be here particularly noticed. From their horizontal position they have a tendency to wear unequally in the cylinders, their weight pressing them on one side only; but from their small magnitude this effect is found to be imperceptible in practice. In the engine here described the stroke of the piston is eighteen inches, and this is the most usual length of stroke in locomotive engines. The piston, in its play, comes at either end within about half an inch of the inner surface of the covers of the cylinders, this space being allowed to prevent collision. In the foremost cover of the cylinder is inserted a cock q' (fig. 97. 99.), by which any water which may collect in the cylinder by condensation or priming may be discharged. A cock r' (fig. 97.), communicating with a small tube proceeding from the branches of the waste pipe p' (fig. 104.), is likewise provided to discharge from that pipe any water which may be [Pg375] collected in it. After the steam has been admitted to work the piston through the slide-valve, and has been discharged through the waste-port by shifting that valve, it passes through the pipe p' into the blast-pipe p, from the mouth of which it issues, with great force, up the funnel G. When the motion of the engine is rapid, the steam from the two cylinders proceeds in an almost uninterrupted current from the blast-pipe, and causes a strong draft up the chimney. The heated air which passes from the mouths of the tubes into the smoke-box is drawn up by this current, and a corresponding draft is produced in the fire-box.

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.

The principle of compensation by varied speed being admitted, it will follow that the time of transit between terminus and terminus of a line of railway laid down with gradients, varying from twenty to thirty feet a mile, will be practically the same as it would be on a line of the same length constructed upon a dead level; and not only will the time of transport be equal, but the quantity of moving power expended will not be materially different. The difference between the circumstances of the transport in the two cases will be merely that, on the undulating line, a varying velocity will be imparted to the train and a varying resistance opposed to the moving power; while on the level line the train would be moved at a uniform speed, and the engine worked against a uniform resistance. These conclusions have been abundantly confirmed by the experiments made in last July with the Hecla engine above referred to. The line of railway between Liverpool and Birmingham on which the experiment was made extended over a distance of ninety-five miles, and the gradients on which the effects were observed varied from a level to thirty feet per mile, a great portion of the line being a dead level. The following table shows the uniform speed with which the train ascended and descended the several gradients, and also the mean of the ascent and descent in each case, as well as the speed upon the level parts of the line:—

[Pg413] From this table it is apparent that the gradients do possess the compensating power with respect to speed already mentioned. The discrepancies existing among the mean values of the speed are only what may be fairly ascribed to casual variations in the moving power. The experiment was made under favourable circumstances: little disturbance was produced from the atmosphere; the day was quite calm. In the same experiment it was found that the water evaporated varied very nearly in proportion to the varying resistance, and the amount of that evaporation may be taken as affording an approximation to the mean amount of resistance. Taking the trip to and from Birmingham over the distance of 190 miles, the mean evaporation per mile was 3·36 cubic feet of water. The volume of steam produced by this quantity of water will be determined approximately by calculating the number of revolutions of the driving wheels necessary to move the engine one mile. The driving wheels being 5 feet in diameter, their circumference was 15·7 feet, and consequently in passing over a mile they would have revolved 336·3 times. Since each revolution consumes four cylinders full of steam, the quantity of steam supplied by the boiler to the cylinders per mile will be found by multiplying the contents of the cylinder by four times 336·3, or 1345·2.

The cylinders of the Hecla were 12¹/₂ inches diameter, and 18 inches in length, and consequently their contents were 1·28 cubic feet for each cylinder: this being multiplied by 1345·2 gives 1721·86 or 1722 cubic feet of steam per mile. It appears, therefore, that supposing the priming either nothing or insignificant, which was considered to be the case in these experiments, 3·36 cubic feet of water produced 1722 cubic feet of steam, of the density worked in the cylinders. The ratio, therefore, of the volume of this steam to that of the water producing it, was 1722 to 3·36, or 512·5 to 1. The pressure of steam of this density would be 54·5 pounds per square inch.[34] Such, therefore, was the limit of the average total pressure of the steam in the cylinders. In this experiment the safety-valve of the boiler was screwed down to 60 pounds per square [Pg414] inch above the atmospheric pressure, which was therefore the major limit of the pressure of steam in the boiler; but as the actual pressure in the boiler must have been less than this amount, the difference between the pressure in the cylinder and boiler could not be ascertained. This difference, however, would produce no effect on the moving power of the steam, since the pressure of steam in the cylinders obtained by the above calculation is quite independent of the pressure in the boiler, or of any source of error except what might arise from priming. The pressure of 54·5 pounds per square inch, calculated above, being the total pressure of the steam on the pistons, let 14·5 pounds be deducted from it, to represent the atmospheric pressure against which the piston must act, and the remaining 40 pounds per square inch will represent the whole available force drawing the train and overcoming all the resistances arising from the machinery of the engine, including that of the blast-pipe. The magnitude of a 12¹/₂ inch piston being 122·7 square inches, the total area of the two pistons would be 245·2 square inches, and the pressure upon each of 40 pounds per inch would give a total force of 9816 on the two pistons. Since this force must act through a space of three feet, while the train is impelled through a space of 15·7 feet, it must be reduced in the proportion of 3 to 15·7, to obtain its effect at the point of contact of the wheels upon the rails: this will give 1875 pounds as the total force exerted in the direction of the motion of the train. The gross weight of the train being 80 tons, including the engine and tender, this would give a gross moving force along the road of about 23·4 pounds per ton of the gross load, this force being understood to include all the resistances due to the engine. This resistance corresponds to the gravitation of a plane rising at the rate of ¹/₉₅, and therefore it appears that such would be the inclination of the plane by the gravitation of which the gross resistance would be doubled, instead of such inclination being about ¹/₃₀₀, as has been hitherto supposed.

Since the remarkable and unexpected results of this series of experiments became known various circumstances were brought to light, which were before unnoticed, and which [Pg415] abundantly confirm them. Among these may be mentioned the fact, that in descending the Madeley plane, on the Grand Junction Railway, which falls for above three miles at the rate of twenty-nine feet a mile, the steam can never be entirely cut off. But, on the other hand, to maintain the necessary speed in descending, the power of the engine is always necessary. As this plane greatly exceeds that which would be sufficient to cause the free motion of the train down it, the power of the engine expended in descending it, besides all that part of the gravitating power of the plane which exceeds the resistance due to friction and other mechanical causes must be worked against the atmosphere.

This estimate of the resistance is also in conformity with the results of a variety of experiments made by me with trains of different magnitudes down inclined planes of various inclinations.

1. Upon arriving at the foot of the plane the load is divided, and the engine carries it up in several successive trips, descending the plane unloaded after each trip. The objection to this method is the delay which it occasions—a circumstance which is incompatible with a large transport of passengers. From what has been stated, it would be necessary, when the engine is fully loaded on a level, to divide its load into two or more parts, to be successively [Pg416] carried up when the incline rises 52 feet per mile. This method has been practised in the transport of merchandise occasionally, when heavy loads were carried on the Liverpool and Manchester line, upon the Rainhill incline.

2. A subsidiary or assistant locomotive engine may be kept in constant readiness at the foot of each incline, for the purpose of aiding the different trains, as they arrive, in ascending. The objection to this method is the cost of keeping such an engine with its boiler continually prepared, and its steam up. It is necessary to keep its fire continually lighted, whether employed or not; otherwise, when the train would arrive at the foot of the incline, it should wait until the subsidiary engine was prepared for work. In cases where trains would start and arrive at stated times, this objection, however, would have less force. This method is at present generally adopted on the Liverpool and Manchester line.

3. A fixed steam-engine may be erected on the crest of the incline, so as to communicate by ropes with the train at the foot. Such an engine would be capable of drawing up one or two trains together, with their locomotives, according as they would arrive, and no delay need be occasioned. This method requires that the fixed engine should be kept constantly prepared for work, and the steam continually up in the boiler.

4. In working on the level, the communication between the boiler and the cylinder in the locomotives may be so restrained by partially closing the throttle-valve, as to cause the pressure upon the piston to be less in a considerable degree than the pressure of steam in the boiler. If under such circumstances a sufficient pressure upon the piston can be obtained to draw the load on the level, the throttle-valve may be opened on approaching the inclined plane, so as to throw on the piston a pressure increased in the same proportion as the previous pressure in the boiler was greater than that upon the piston. If the fire be sufficiently active to keep up the supply of steam in this manner during the ascent, and if the rise be not greater in proportion than the power thus obtained, the locomotive will draw the load up the incline without further assistance. It is, however, to be observed, that in this case [Pg417] the load upon the engine must be less than the amount which the adhesion of its working wheels with the railroad is capable of drawing; for this adhesion must be adequate to the traction of the same load up the incline, otherwise, whatever increase of power might be obtained by opening the throttle-valve, the drawing wheels would revolve without causing the load to advance. This method has been generally practised upon the Liverpool and Manchester line in the transport of passengers; and, indeed, it is the only method yet discovered which is consistent with the expedition necessary for that species of traffic.

In the practice of this method considerable aid may be derived also by suspending the supply of feeding water to the boiler during the ascent. It will be recollected that a reservoir of cold water is placed in the tender which follows the engine, and that the water is driven from this reservoir into the boiler by a forcing pump, which is worked by the engine itself. This pump is so constructed that it will supply as much cold water as is equal to the evaporation, so as to maintain constantly the same quantity of water in the boiler. But it is evident, on the other hand, that the supply of this water has a tendency to check the rate of evaporation, since in being raised to the temperature of the water with which it mixes it must absorb a considerable portion of the heat supplied by the fire. With a view to accelerate the production of steam, therefore, in ascending the inclines, the engine man may suspend the action of the forcing pump, and thereby stop the supply of cold water to the boiler; the evaporation will go on with increased rapidity, and the exhaustion of water produced by it will be repaid by the forcing pump on the next level, or still more effectually on the next descending incline. Indeed the feeding pump may be made to act in descending an incline, if necessary, when the action of the engine itself is suspended, and when the train descends by its own gravity, in which case it will perform the part of a brake upon the descending train.

5. The mechanical connexion between the piston of the cylinder and the points of contact of the working wheels with the road may be so altered, upon arriving at the incline, as to [Pg418] give the piston a greater power over the working wheels. This may be done in an infinite variety of ways, but hitherto no method has been suggested sufficiently simple to be applicable in practice; and even were any means suggested which would accomplish this, unless the intensity of the impelling power were at the same time increased, it would necessarily follow that the speed of the motion would be diminished in exactly the same proportion as the power of the piston over the working wheels would be increased. Thus, on the inclined plane, which rises fifty-five feet per mile, upon the Liverpool line, the speed would be diminished to nearly one fourth of its amount upon the level.