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surface, as it then became obvious to my father that the speed of the engine could not be increased without increasing the evaporative power of the boiler. Increase of surface was in some cases obtained by inserting two tubes, each containing a separate fire, into the boiler; in other cases the same result was obtained by returning the same tube through the boiler; but it was not until he was engaged in making some experiments, during the progress of the Liverpool and Manchester Railway, in conjunction with Mr. Henry Booth, the well-known secretary of the company, that any decided movement in this direction was effected, and that the present multitubular boiler assumed a practicable shape. It was in conjunction with Mr. Booth that my father constructed the “Rocket' engine. “At this stage of the locomotive engine, we have in the multitubular boiler the only important principle of construction introduced, in addition to those which my father had brought to bear at a very early age (between 1815 and 1821) on the Killingworth Colliery Railway. In the ‘Rocket' engine, the power of generating steam was prodigiously increased by the adoption of the multitubular system. Its efficiency was further augmented by narrowing the orifice by which the waste steam escaped into the chimney; for by this means the velocity of the air in the chimney—or, in other words, the draught of the fire—was increased to an extent that far surpassed the expectations even of those who had been the authors of the combination. “From the date of running the “Rocket' on the Liverpool and Manchester Railway, the locomotive engine has received many minor improvements in detail, and especially in accuracy of workmanship; but in no essential particular does the existing locomotive differ from that which obtained the prize at the celebrated competition at Rainhill. “In this instance, as in every other important step in U

science or art, various claimants have arisen for the merit of having suggested the multitubular boiler as a means of obtaining the necessary heating surface. Whatever may be the value of their respective claims, the public, useful, and extensive application of the invention must certainly date from the experiments made at Rainhill. M. Seguin, for whom engines had been made by my father some few years previously, states that he patented a similar multitubular boiler in France, several years before. A still prior claim is made by Mr. Stevens, of New York, who was all but a rival to Mr. Fulton in the introduction of steam-boats on the American rivers. It is stated that as early as 1807 he used the multitubular boiler. These claimants may all be entitled to great and independent merit; but certain it is that the perfect establishment of the success of the multitubular boiler is more immediately due to the suggestion of Mr. Henry Booth, and to my father's practical knowledge in carrying it out.” Mr. Booth's account of the same important invention, with which he has favoured the writer, is as follows: — “I was in almost constant communication with Mr. Stephenson, and I am not aware that he had any intention of competing for the prize till I communicated to him my scheme of a multitubular boiler. This new plan of boiler comprisc d the introduction of numerous small tubes, two or three inches in diameter, and less than an eighth of an inch thick, through which to carry the fire, instead of a single tube or flue, eighteen inches diameter, and about half an inch thick; by which plan we not only obtain a very much larger heating surface, but the heating surface is much more effective, as there intervenes between the fire and the water only a thin sheet of copper or brass, not an eighth of an inch thick, instead of a plate of iron of four times the substance, as well as an inferior conductor of heat.

chap. xxii.] THE TUBES IN THE “ROCKET.” 291

“When the conditions of trial were published, I communicated my multitubular plan to Mr. Stephenson, and proposed to him, that we should jointly construct an engine and compete for the prize. Mr. Stephenson approved the plan ; and agreed to my proposal. He settled the mode in which the fire-box and tubes were to be mutually arranged and connected; and the engine was constructed at the works of Messrs. Robert Stephenson and Co., Newcastle-on-Tyne.”

The fitting of the copper tubes in the boiler of the “Rocket” so as to prevent leakage was a work of some difficulty. They were manufactured by a Newcastle coppersmith, and soldered to brass screws, which were screwed into the boiler ends, standing out in great knobs. When the tubes were thus fitted, and the boiler was filled with water, hydraulic pressure was applied; but the water squirted out at every joint, and the factory floor was soon flooded. Robert went home in despair; and in the first moment of grief, he wrote to his father that the whole thing was a failure. By return of post came a letter from his father, telling him that despair was not to be thought of that he must “try again; ” and he suggested a mode of overcoming the difficulty, which, singularly enough, his son had already anticipated and proceeded to adopt. It was, to bore clean holes in the boiler ends, fit in the smooth copper tubes as tightly as possible, solder up, and then raise the steam. This plan succeeded perfectly, the expansion of the copper tubes completely filling up all interstices, and producing a perfectly water-tight boiler, capable of withstanding extreme internal pressure.

The mode of employing the steam blast for the purpose of increasing the draught in the chimney, was also the subject of numerous experiments. When the engine was first tried, it was thought that the blast in the chimney was not sufficiently strong for the purpose of keeping up the intensity of the fire in the furnace, so as to produce high pressure steam with the required velocity. The expedient was therefore adopted of hammering the copper tubes at the point at which they entered the chimney, whereby the blast was considerably sharpened; and on a further trial it was found that the draught was increased to such an extent as to enable abundance of steam to be raised. The rationale of the blast may be simply explained by referring to the effect of contracting the pipe of a water-hose, by which the force of the jet of water is proportionately increased. Widen the nozzle of the pipe, and the jet is in like manner diminished. So is it with the steam blast in the chimney of the locomotive. Doubts were, however, expressed whether the greater draught secured by the contraction of the blast pipe was not counterbalanced in some degree by the negative pressure upon the piston. A series of experiments was made with pipes of different diameters; and their efficiency was tested by the amount of vacuum that was produced in the smokebox. The degree of rarefaction was determined by a glass tube fixed to the bottom of the smoke-box, and descending into a bucket of water, the tube being open at both ends. As the rarefaction took place, the water would of course rise in the tube; and the height to which it rose above the surface of the water in the bucket was made the measure of the amount of rarefaction. These experiments proved that a considerable increase of draught was obtained by the contraction of the orifice; accordingly, the two blast-pipes opening from the cylinders into either side of the “Rocket” chimney, and turned up within it", were contracted slightly cuAp. xxii.] construction of THE “ROCKET.” 293

* The alteration afterwards made in the blast of the “Rocket,” after the competition at Rainhill, by which the two separate exit pipes were thrown into one, as in the original Killingworth engines, was adopted rather with the view of lessening the space occupied by them in the chimney than because of any increased effect thereby secured, though it is probable that the jet of steam is rather more efficient when thrown upwards in the exact centre of the chimney than when slightly on one side.

below the area of the steam-ports; and before the engine left the factory, the water rose in the glass tube three inches above the water in the bucket. The other arrangements of the “Rocket” were briefly these: —The boiler was cylindrical with flat ends, six feet in length, and three feet four inches in diameter. The upper half of the boiler was used as a reservoir for the steam, the lower half being filled with water. Through the lower part, twenty-five copper tubes of three inches diameter extended, which were open to the fire-box at one end, and to the chimney at the other. The fire-box, or furnace, two feet wide and three feet high, was attached immediately behind the boiler, and was also surrounded with water. The cylinders of the engine were placed on each side of the boiler, in an oblique position, one end being nearly level with the top of the boiler at its after end, and the other pointing towards the centre of the foremost or driving pair of wheels, with which the connexion was directly made from the piston-rod to a pin on the outside of the wheel. The engine, together with its load of water, weighed only four tons and a quarter; and it was supported on four wheels, not coupled. The tender was four-wheeled, and similar in shape to a waggon, —the foremost part holding the fuel, and the hind part a water-cask. When the “Rocket” was completed, it was placed upon the Killingworth Railway for the purpose of experiment. The new boiler arrangement was found perfectly successful. The steam was raised rapidly and continuously, and in a o

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