(201.)

(201.)The series of experiments which have established these general conclusions have not yet been sufficiently extended and varied to supply a correct practical estimate of the limit which it would be most advantageous to impose upon the[Pg412]gradients of railways; but it is certain that railways may be laid down, without practical disadvantage, with gradients considerably steeper than those to which it has been hitherto the practice to recommend as a limit.

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 were121⁄2inches 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 a121⁄2inch 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 of1⁄95,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 about1⁄300,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.

(202.)In laying out a line of railway the disposition of the gradients should be such as to preserve among them as uniform a character as is practicable, for the weight and power of the engine must necessarily be regulated by the general steepness of the gradients. Thus if upon a railway which is generally level, like that between Liverpool and Manchester, one or two inclined planes of a very steep character occur, as happens upon that line, then the engine which is constructed to work upon the general gradients of the road is unfit to draw the same load up those inclinations which form an exception to the general character of the gradients. In such cases some extraordinary means must generally be provided for surmounting those exceptionable inclinations. Several expedients have been proposed for this purpose, among which the following may be mentioned:—

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.

FOOTNOTES:[30]Some of the preceding observations on inland transport, as well as other parts of the present chapter, appeared in articles written by me in theEdinburgh Reviewfor October, 1832, and October, 1834.[31]Wood on Railroads, 2d edit.[32]The cost of coke has risen considerably since the date of this report.[33]I am indebted to the enlarged edition of Tredgold on the Steam Engine, published by Mr. Weale, for the drawings of this engine. The details of the machine are very fully given in that work, the description of them being supplied by Mr. Stephenson himself.[34]See Table of Pressures, Temperatures, and Volumes, in appendix.

[30]Some of the preceding observations on inland transport, as well as other parts of the present chapter, appeared in articles written by me in theEdinburgh Reviewfor October, 1832, and October, 1834.

[31]Wood on Railroads, 2d edit.

[32]The cost of coke has risen considerably since the date of this report.

[33]I am indebted to the enlarged edition of Tredgold on the Steam Engine, published by Mr. Weale, for the drawings of this engine. The details of the machine are very fully given in that work, the description of them being supplied by Mr. Stephenson himself.

[34]See Table of Pressures, Temperatures, and Volumes, in appendix.

LOCOMOTIVE ENGINES ON TURNPIKE ROADS.

[Pg419]TOCINX

RAILWAYS AND STONE ROADS COMPARED.—MR. GURNEY'S STEAM ENGINE.—CONVENIENCE AND SAFETY OF STEAM CARRIAGES.—HANCOCK'S STEAM ENGINE.—OGLE'S STEAM ENGINE.—TREVETHICK'S INVENTION.—DR. CHURCH'S STEAM ENGINE.

(203.)We have hitherto confined our observations on steam-power, as a means of transport by land, to its application on railways. But modern speculation has not stopped there; various attempts have been made, and attended with more or less success, to work steam-carriages on common roads. The mere practicability of this project had long been regarded as very questionable; but enough has been done to show that the only doubt which can attend it, is as to whether it can be profitably resorted to, as a means of transport, and this question[Pg420]has been materially affected by the recent extension of railways. In comparing the effect of a stone road with an iron railway, there are two circumstances which give great superiority and advantage to the latter: first, the resistance opposed by a railway to the moving power, no matter what that moving power may be, is considerably less in proportion to the load than on a stone road. The average resistance on a good level stone road, to the motion of carriages drawn at the speed usually attained by the application of horse-power, may be taken at about a thirty-sixth part of the load, while the resistance to a load drawn upon a railwayat the same speedprobably does not amount to a tenth part of this resistance. Thus the moving power, whatever it may be, would produce on a railway ten times the useful effect which it would produce on a stone road; secondly, the resistance which is opposed to the moving power on a level railway is much more uniform than on a stone road, and, consequently, the moving power is less subjected to jerks and inequalities. This renders the application of inanimate power more easy on the railway. Those inequalities of surface which increase the amount of resistance on stone roads as compared with railways also produce a jolting motion in the carriage, to counteract which, the use of springs become necessary. These springs render the motion of that part of the carriage which rests upon them different from that part of the carriage which supports them; and in the application of steam-machinery it becomes necessary so to connect the moving power with the wheels that the machinery may have one motion, and the wheels which are put in mechanical connexion with that machinery, and driven by it, shall have another motion. This, it is true, is the case with locomotive engines on railways; but owing to the greater smoothness and equality of the railway surface the difference between the motion of the carriage body suspended on springs and that of the wheels is much less than it would be on a stone road.

But besides the greater smoothness of railways compared with stone roads, the latter have another disadvantage, the effects of which have probably been exaggerated by those who are opposed to this application of steam-power. One of the[Pg421]laws of adhesion long since developed by experiment, and established as a principle of practical science, is that the adhesion is greater between surfaces of the same than between surfaces of a different kind. Thus between two metals of the same kind, the adhesion corresponding to any given pressure is greater than between two metals of different kinds; between two metals of any sort the adhesion is greater than between metal and stone, or between metal and wood. Hence, the wheels of steam-carriages running on a railroad have a greater adhesion with the road, and therefore offer a greater resistance to slip round without the advance of the carriage, than wheels would offer on a turnpike road; for on a railroad the iron tire of the wheel rests in contact with the iron rail, while on a common road the iron tire rests in contact with the surface of stone, or whatever material the road may be composed of. Besides this, the dust and loose matter which necessarily collect on a common road, when pressed between the wheels and the solid base of the road, act somewhat in the manner of rollers, and give the wheels a greater facility to slip than if the road were swept clean, and the wheels rested in immediate contact with its hard surface. The truth of this observation is illustrated on the railroads themselves, where the adhesion is found to be diminished whenever the rails are covered with any extraneous matter, such as dust or moist clay. Although the adhesion of the wheels of a carriage with a common road, however, be less than those of the wheels of a steam-carriage with a railroad, yet still the actual adhesion on turnpike roads is greater in amount than has been generally supposed, and is quite sufficient to propel carriages drawing after them loads of large amount.

The relative facility with which carriages are propelled on railroads and turnpike-roads equally affects any moving power, whether that of horses or steam engines; and whether loads be propelled by the one power or the other, the railroad, as compared with the turnpike-road, will always possess the same proportionate advantage; and a given amount of power, whether of the one kind or the other, will always perform a quantity of work less in the same proportion on a[Pg422]turnpike-road than on a rail-road. But, on the other hand, the expense of original construction, and of maintaining the repairs of a rail-road, is to be placed against the certain facility which it offers to draught.

In the attempts which have been made to adapt locomotive engines to turnpike-roads, the projectors have aimed at the accomplishment of two objects: first, the construction of lighter and smaller engines; and, secondly, increased power. These ends, it is plain, can only be attained, with our present knowledge, by the production of steam of very high temperature and pressure, so that the smallest volume of steam shall produce the greatest possible mechanical effect. The methods of propelling the carriage have been in general similar to that used in the railroad engines, viz. either by cranks placed on the axles, the wheels being fixed upon the same axles, or by connecting the piston rods with the spokes of the wheels. In some carriages, the boiler and moving power, and the body of the carriage which bears the passengers, are placed on the same wheels. In others, the engine is placed on a separate carriage, and draws after it the carriage which transports the passengers, as is always the case on railways.

The chief difference between the steam engines used on railways, and those adapted to propel carriages on turnpike roads, is in the structure of the boiler. In the latter it is essential that, while the power remains undiminished, the boiler should be lighter and smaller. The accomplishment of this has been attempted by various contrivances for so distributing the water as to expose a considerable quantity of surface in contact with it to the action of the fire: spreading it in thin layers on flat plates; inserting it between plates of iron placed at a small distance asunder, the fire being admitted between the intermediate plates; dividing it into small tubes, round which the fire has play; introducing it between the surfaces of cylinders placed one within another, the fire being admitted between the alternate cylinders,—have all been resorted to by different projectors.

(204.)First and most prominent in the history of the application of steam to the propelling of carriages on turnpike roads stands the name of Mr. Goldsworthy Gurney, a medical[Pg423]gentleman, and scientific chemist, of Cornwall. In 1822, Mr. Gurney succeeded Dr. Thompson as lecturer on chemistry at the Surrey Institution; and, in consequence of the results of some experiments on heat, his attention was directed to the project of working steam-carriages on common roads; and he subsequently devoted his exertions in perfecting a steam-engine capable of attaining the end he had in view.

The mistake which so long prevailed in the application of locomotives on railroads, and which, as we have shown, materially retarded the progress of that invention, was shared by Mr. Gurney. Without reducing the question to the test of experiment, he took for granted, in his first attempts, that the adhesion of the wheels with the road was too slight to propel the carriage. He was assured, he says, by eminent engineers, that this was a point settled by actual experiment. It is strange, however, that a person of his quickness and sagacity did not inquire after the particulars of these "actual experiments." So, however, it was; and, taking for granted the inability of the wheels to propel, he wasted much labour and skill in the contrivance of levers and propellers, which acted on the ground in a manner somewhat resembling the feet of horses, to drive the carriage forward. After various fruitless attempts of this kind, the experience acquired in the trials to which they gave rise at last forced the truth upon his notice, and he found that the adhesion of the wheels was not only sufficient to propel the carriage heavily laden on level roads, but was capable of causing it to ascend all the hills which occur on ordinary turnpike-roads. In this manner it ascended all the hills between London and Barnet, London and Stanmore, Stanmore Hill, Brockley Hill, and mounted Old Highgate Hill, the last at one point rising one foot in nine.

Fig. 114.

Fig. 115.

The boiler of Mr. Gurney's engine is so constructed, that there is no part of it in which metal exposed to the action of the fire is out of contact with water. If it be considered how rapidly the action of an intense furnace destroys metal when water is not present to prevent the heat from accumulating, the advantage of this circumstance will be appreciated. In the boiler of Mr. Gurney, the grate-bars[Pg424]themselves are tubes filled with water, and form, in fact, a part of the boiler itself. This boiler consists of three strong metal cylinders placed in a horizontal position one above the other. A section, made by a perpendicular or vertical plane, is represented infig.114.The ends of the three cylinders just mentioned are represented atD,H, andI. In the side of the lowest cylinderDare inserted a row of tubes, a ground plan of which is represented infig.115.These tubes, proceeding from the side of the lowest cylinderD, are inclined[Pg425]slightly upwards, for a reason which I shall presently explain. From the nature of the section, only one of these tubes is visible infig.114.atC. The other extremities of these tubes atAare connected with the same number of upright tubes, one of which is shown atE. The upper extremitiesGof these upright tubes are connected with another set of tubesK, equal in number, proceeding fromG, inclining slightly upwards, and terminating in the second cylinderH.

Fig. 116.

An end view of the boiler is exhibited infig.116., where the three cylinders are expressed by the same letters. Between the cylindersDandHthere are two tubes of communicationB, and two similar tubes between the cylindersHandI. From the nature of the section these appear only as a single tube infig.114.From the top of the cylinderIproceeds a tubeN, by which steam is conducted to the engine.

It will be perceived that the spaceFis enclosed on every side by a grating of tubes, which have free communication with the cylindersDandH, which cylinders have also a free communication with each other by the tubesB. It follows,[Pg426]therefore, that if water be supplied to the cylinderI, it will descend through the tubes, and first filling the cylinderDand the tubesC, will gradually rise in the tubesBandE, will next fill the tubesKand the cylinderH. The grating of water-pipesC E Kforms the furnace, the pipesCbeing the fire-bars, and the pipesEandKbeing the back and roof of the stove. The fire-door, for the supply of fuel, appears atM, fig. 116. The flue issuing between the tubesFis conducted over the tubesK, and the flame and hot air are carried off through a chimney. That portion of the heat of the burning fuel, which in other furnaces destroys the bars of the grate, is here expended in heating the water contained in the tubesC. The radiant heat of the fire acts upon the tubesK, forming the roof of the furnace, on the tubeEat the back of it, and partially on the cylindersDandH, and the tubesB. The draft of hot air and flame passing into the flue atAacts upon the posterior surfaces of the tubesE, and the upper sides of the tubesK, and finally passes into the chimney.

As the water in the tubesC E Kis heated, it becomes specifically lighter than water of a less temperature, and consequently acquires a tendency to ascend. It passes, therefore, rapidly intoH. Meanwhile the colder portions descend, and the inclined positions of the tubesCandKgive play to this tendency of the heated water, so that a prodigiously rapid circulation is produced, when the fire begins to act upon the tubes. When the water acquires such a temperature that steam is rapidly produced, steam-bubbles are constantly formed in the tubes surrounding the fire; and if these remained stationary in the tubes, the action of the fire would not only decompose the steam, but render the tubes red hot, the water not passing through them to carry off the heat. But the inclined position of the tubes, already noticed, effectually prevents this injurious consequence. A steam-bubble, which is formed either in the tubesCorK, having a tendency to ascend proportional to its lightness as compared with water, necessarily rushes upwards; if inCtowardsA, and if inKtowardsH. But this motion of the steam is also aided by the rapid circulation of the water which is continually maintained[Pg427]in the tubes, otherwise it might be possible, notwithstanding the levity of steam compared with water, that a bubble might remain in a narrow tube without rising. To bring the matter to the test of experiment, I have connected two cylinders, such asDandH, by a system of glass tubes, such as represented atC E K. The rapid and constant circulation of the water was then made evident: bubbles of steam were formed in the tubes, it is true; but they passed with great rapidity into the upper cylinder, and rose to the surface, so that the glass tubes never acquired a higher temperature than that of the water which passed through them.

Every part of the boiler being cylindrical, it has the form which, mechanically considered, is most favourable to strength, and which, within given dimensions, contains the greatest quantity of water. It is also free from the defects arising from unequal expansion, which are found to be most injurious in tubular boilers. The tubesCandKcan freely expand in the direction of their length, without being loosened at their joints, and without straining any part of the apparatus; the tubesE, being short, are subject to a very slight degree of expansion; and it is obvious that the long tubes, with which they are connected, will yield to this without suffering a strain, and without causing any part of the apparatus to be loosened.

When water is converted into steam, any foreign matter which may be combined with it is disengaged, and is deposited on the bottom of the vessel in which the water is evaporated. All boilers, therefore, require occasional cleansing, to prevent the crust thus formed from accumulating; and this operation, for obvious reasons, is attended with peculiar difficulty in tubular boilers. In the case before us, the crust of deposited matter would gather and thicken in the tubesCandK, and if not removed, would at length choke them. But besides this, it would be attended with a still worse effect; for, being a bad conductor, it would intercept the heat in its transit from the fire to the water, and would cause the metal of the tube to become unduly heated. Mr. Gurney of course foresaw this inconvenience, and contrived an ingenious chemical method of removing it, by occasionally injecting[Pg428]through the tubes such an acid as would combine with the deposit, and carry it away. This method was effectual; and although its practical application was found to be attended with difficulty in the hands of common workmen, Mr. Gurney was persuaded to adhere to it by the late Dr. Wollaston, until experience proved the impossibility of getting it effectually performed, under the circumstances in which boilers are commonly used. Mr. Gurney then adopted a method of removing the deposit by mechanical means. Opposite the mouths of the tubes, and on the other side of the cylindersDandH, are placed a number of holes, which, when the boiler is in use, are stopped by pieces of metal screwed into them. When the tubes require to be cleaned, these stoppers are removed, and an iron scraper is introduced through the holes into the tubes, which, being passed backwards and forwards, removes the deposit.

In these engines the draught through the furnace was produced by projecting the waste steam up the chimneys as is practised in railway engines; a method so perfectly effectual, that it is unlikely to be superseded by any other. The objection which has been urged against it in locomotive engines, working on turnpike-roads, is, that the noise which it produces has a tendency to frighten horses.

In the engines on the Liverpool road, the steam is allowed to pass directly from the eduction pipe of the cylinder to the chimney, and it there escapes in puffs corresponding with the alternate motion of the pistons, and produces a noise, which, although attended with no inconvenience on the railroad, would perhaps be objectionable on turnpike-roads. In the engine used in Mr. Gurney's steam-carriage, the steam which passes from the cylinders is conducted to a receptacle, which he calls a blowing box. This box serves the same purpose as the upper chamber of a smith's bellows. It receives the steam from the cylinders in alternate puffs, but lets it escape into the chimney in a continued stream by a number of small jets. Regular draught is by this means produced, and no noise is perceived. Another exit for the steam is also provided, by which the conductor is enabled to increase or diminish, or to suspend altogether, the draught[Pg429]in the chimney, so as to adapt the intensity of the fire to the exigencies of the road. This is a great convenience in practice; because on some roads a draught is scarcely required, while on others a powerful blast is indispensable.

Connected with this blowing box is another apparatus of considerable practical importance. The pipe through which the feeding water is conducted from the tank is carried through this blowing box, within which it is coiled in a spiral form, so that an extensive thread of the water is exposed to the heat of the waste steam which has escaped from the cylinders, and which is enclosed in this blowing box. In passing through this pipe the feeding water is raised from the ordinary temperature of about 60° to the temperature of 212°. Fuel is thus economised and weight diminished; but there is another still greater advantage attending this process. The feeding water in the worm just mentioned, while it takes up the heat from the surrounding steam in the blowing box, condenses a part of the waste steam, which is thence conducted to the tank, from which the feeding water is pumped.

When steam is generated so rapidly as is necessarily the case in locomotive boilers, it rises with great violence in numerous bubbles from the bottom of the boiler to the surface of the water, and puts the liquid into a state of foaming turbulence not unlike the sea in a storm. As the steam rushes from the surface into the upper part of the boiler, under these circumstances, it carries with it a spray by which water is scattered in minute subdivision among the steam, and floats there like the spray which rises from the base of a cascade. If the steam be conducted immediately to the cylinder from the boiler in this state, it will carry with it the water which is thus suspended in it, which will pass through the cylinder, and finally be driven into the atmosphere upon the returning stroke of the piston. The hot water thus carried off possesses none of the mechanical properties of steam, and is wholly inefficient as a moving power, and is therefore an extensive source of the waste of heat. In every boiler, some means should be provided for the separation of the water thus suspended in the steam, before the steam is conducted to the cylinder. In ordinary boilers, the large space which[Pg430]remains above the surface of the water serves this purpose. The steam being there subject to no agitation or disturbance, the water mechanically suspended in it descends by its own gravity, and leaves pure steam in the upper part. In the small tubular boilers, this has been a matter, however, of greater difficulty. The contracted space in which the ebullition takes place causes the water to be mixed with the steam in a greater quantity than could happen in common boilers; and the want of the same steam-room renders the separation of the water from the steam a matter of some difficulty. These inconveniences have been attempted to be overcome by various contrivances. I have already described the rapid and regular circulation effected by the arrangement of the tubes. By this a regularity in the currents is established, which has a tendency to diminish the mixture of water with the steam. In addition to this, a method of separation is provided in the vesselI, which is a strong iron cylinder of some magnitude, placed out of the immediate influence of the fire. A partial separation of the steam from the water takes place in the cylinderH; and the steam with the water mechanically suspended in it, technically called moist steam, rises into theseparatorI. Here, being free from all agitation and currents, and being, in fact, quiescent, the particles of water fall to the bottom, while the pure steam remains at the top. This separator, therefore, serves all the purposes of the steam-room above the surface of the water in the large plate boilers. The dry steam is thus collected and ready for the supply of the engine through the tubeN, while the water, which is disengaged from it, is collected at the bottom of the separator, and is conducted through the tubeTto the lowest vesselD, to be again circulated through the boiler.

The pistons of the engine work on the axles of the hind wheels of the carriage which bears the engine, by cranks, as in the locomotives on the Manchester railway, so that the axle is kept in a constant state of rotation while the engine is at work. The wheels placed on this axle are not permanently fixed or keyed upon it, as in the Manchester locomotives; but they are capable of turning upon it in the same manner as ordinary carriage wheels. Immediately within[Pg431]these wheels there are fixed upon the axles two projecting spokes or levers, which revolve with the axle, and which take the position of two opposite spokes of the wheel. These may be occasionally attached to the wheel or detached from it; so that they are capable of compelling the wheels to turn with the axle, or leaving the axle free to turn independently of the wheel, or the wheel independent of the axle, at the pleasure of the conductor. It is by these levers that the engine is made to propel either or both of the wheels. If both pairs of spokes are thrown into connexion with the wheels, the crank shaft or axle will cause both wheels to turn with it, and in that case the operation of the carriage is precisely the same as those of the locomotives already described upon the Liverpool and Manchester line; but this is rarely found to be necessary, since the adhesion of one wheel with the road is generally sufficient to propel the carriage, and consequently only one pair of these fixed levers are used, and the carriage propelled by only one of the two hind wheels. The fore wheels of the carriage turn upon a pivot similar to those of a four-wheeled coach. The position of these wheels is changed at pleasure by a pinion and circular rack, which is moved by the conductor, and in this manner the carriage is guided with precision and facility.

The force of traction necessary to propel a carriage upon common roads must vary with the variable quality of the road, and consequently the propelling power, or the pressure upon the pistons of the engine, must be susceptible of a corresponding variation; but a still greater variation becomes necessary from the undulations and hills which are upon all ordinary roads. This necessary change in the intensity of the impelling power is obtained by restraining the steam in the boiler by the throttle-valve, as already described in the locomotive engines on the railroad. This principle, however, is carried much further in the present case. The steam in the boiler maybe at a pressure of from 100 to 200 lbs. on the square inch; while the steam on the working piston may not exceed 30 or 40 lbs. on the inch. Thus an immense increase of power is always at the command of the conductor; so that when a hill is encountered, or a rough piece of road,[Pg432]he is enabled to lay on power sufficient to meet the exigency of the occasion.

The two difficulties which have been always apprehended in the practical working of steam-carriages upon common roads are, first, the command of sufficient power for hills and rough pieces of road; and, secondly, the apprehended insufficiency of the adhesion of the wheels with the road to propel the carriage. The former of these difficulties has been met by allowing steam of very great pressure to be constantly maintained in the boiler with perfect safety. As to the second, all experiments tend to show that there is no ground for the supposition that the adhesion of the wheels is in any case insufficient for the purposes of propulsion. Mr. Gurney states, that he has succeeded in driving carriages thus propelled, up considerable hills on the turnpike roads about London. He made a journey to Barnet with only one wheel attached to the axle, which was found sufficient to propel the carriage up all hills upon that road. The same carriage, with only one propelling wheel, also went to Bath, and surmounted all the hills between Cranford Bridge and Bath, going and returning.

A double stroke of the piston produces one revolution of the propelling wheels, and causes the carriage to move through a space equal to the circumference of those wheels. It will therefore be obvious, that the greater the diameter of the wheels, the better adapted the carriage is for speed; and, on the other hand, wheels of smaller diameter are better adapted for power. In fact, the propelling power of an engine on the wheels will be in the inverse proportion of their diameter. In carriages designed to carry great weights at a moderate speed, smaller wheels will be used; while in those intended for the transport of passengers at considerable velocities, wheels of at least 5 feet diameter are most advantageous.

(205.)Among the numerous popular prejudices to which this new invention has given rise, one of the most mischievous in its effects and most glaring in its falsehood, is the notion that carriages thus propelled are more injurious to roads than carriages drawn by horses. This error has been successfully exposed in the evidence taken before the committee of the[Pg433]House of Commons upon steam carriages. It is there demonstrated, not only that carriages thus propelled do not wear a turnpike road more rapidly than those drawn by horses, but that, on the other hand, the wear by the feet of horses is far more rapid and destructive than any which could be produced by the wheels of carriages. Steam carriages admit of having the tires of the wheels broad, so as to act upon the road more in the manner of rollers, and thereby to give consistency and firmness to the material of which the road is composed. The driving wheels being proved not to slip upon the road, do not produce any effects more injurious than the ordinary rolling wheels; consequently the wear occasioned by a steam carriage upon a road, is not more than that produced by a carriage drawn by horses, of an equivalent weight and the same or equal tires; but the wear produced by the pounding and digging of horses' feet in draught is many times greater than that produced by the wear of any carriage. Those who still have doubts upon this subject, if there be any such persons, will be fully satisfied by referring to the evidence which accompanies the report of the committee of the House of Commons, printed in October, 1831.

The weight of machinery necessary for steam carriages is sometimes urged as an objection to their practical utility. Mr. Gurney states, that, by successive improvements in the details of the machinery, the weight of his carriages, without losing any of the propelling power, may be reduced to 35 cwt., exclusive of the load, and fuel and water: but thinks that it is possible to reduce the weight still further.

A steam carriage constructed by Mr. Gurney, weighing 35 cwt., working for 8 hours, is found, according to his statement, to do the work of about 30 horses. He calculates that the weight of his propelling carriage, which would be capable of drawing 18 persons, would be equal to the weight of 4 horses; and the carriage in which these persons would be drawn would have the same weight as a common stage coach capable of carrying the same number of persons. Thus the weight of the whole—the propelling carriage and the carriage for passengers taken together—would be the same[Pg434]with the weight of a common stage coach, with 4 horses inclusive.

There are two methods of applying locomotives upon common roads to the transport of passengers or goods; the one is by causing the locomotive to carry, and the other to draw the load; and different projectors have adopted the one and the other method. Each is attended with its advantages and disadvantages. If the same carriage transport the engine and the load, the weight of the whole will be less in proportion to the load carried; also a greater pressure may be produced on the wheels by which the load is propelled. It is also thought that a greater facility in turning and guiding the vehicle, greater safety in descending the hills, and a saving in the original cost, will be obtained. On the other hand, when the passengers are placed in the same carriage with the engine, they are necessarily more exposed to the noise of the machinery and to the heat of the boiler and furnace. The danger of explosion is so slight, that, perhaps, it scarcely deserves to be mentioned; but stillthe apprehensionof danger on the part of the passengers, even though groundless, should not be disregarded. This apprehension will be obviously removed or diminished by transferring the passengers into a carriage separate from the engine; but the greatest advantage of keeping the engine separate from the passengers is the facility which it affords of changing one engine for another in case of accident or derangement on the road, in the same manner as horses are changed at the different stages: or, if such an accident occur in a place where a new engine cannot be procured, the load of passengers may be carried forward by horses, until it is brought to some station where a locomotive may be obtained. There is also an advantage arising from the circumstance, that when the engines are under repair, or in process of cleaning, the carriages for passengers are not necessarily idle. Thus the same number of carriages for passengers will not be required when the engine is used to draw as when it is used to carry.

In case of a very powerful engine being used to carry great loads, it would be quite impracticable to place the engine[Pg435]and loads on four wheels, the pressure being such as no turnpike road could bear. In this case it would be indispensably necessary to place a part of the load at least upon separate carriages to be drawn by the engine.

In the comparison of carriages propelled by steam with carriages drawn by horses, there is no respect in which the advantage of the former is so apparent as the safety afforded to the passenger. Steam power is under the most perfect control, and a carriage thus propelled is capable of being guided with the most admirable precision. It is also capable of being stopped almost suddenly, whatever be its speed: it is capable of being turned within a space considerably less than that which would be necessary for four-horse coaches. In turning sharp corners, there is no danger, with the most ordinary care on the part of the conductor. On the other hand, horse power, as is well known, is under very imperfect control, especially when horses are used adapted to that speed which at present is generally considered necessary for the purposes of travelling. "The danger of being run away with and overturned," says Mr. Farey, in his evidence before the House of Commons, "is greatly diminished in a steam coach. It is very difficult to control four such horses as can draw a heavy stage coach ten miles an hour, in case they are frightened or choose to run away; and, for such quick travelling, they must be kept in that state of courage that they are always inclined to run away, particularly down hill, and at sharp turns in the road. Steam power has very little corresponding danger, being perfectly controllable, and capable of having its power reversed, to retard in going down hill. It must be carelessness that would occasion the overturning of a steam carriage. The chance of breaking down has been hitherto considerable, but it will not be more than in stage coaches when the work is truly proportioned and properly executed. The risk from explosion of the boiler is the only new cause of danger, and that I consider not equivalent to the danger from horses."

That the risk of accident from explosion is extremely slight, may be proved by the fact that the railway between Liverpool and Manchester has now been in operation for about ten[Pg436]years, and that other railways more extensive in length have been worked for a considerable time, and that no instance has ever yet occurred of an accident to passengers from the explosion of a boiler. Generally these machines, when they fail, are attended with no other effect than the extinction of the fire, by the water of the boiler flowing in upon it. I am not aware of more than one instance, in which a serious accident has been produced by explosion; and in that instance, the sufferers were only the engineer and stoker. In the steam-engine of Mr. Gurney, the carriage is drawn after the engine, as represented infig.117.

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