Robert Mallett, Esq. of Dublin, in a communication to the Editor of the Mechanic's Magazine, has described an ingenious and original plan of buffing, an extract of which we shall proceed to give. Mr. Mallett observes; - "The employment of elastic fluids to serve as springs, has been frequently proposed, and occasionally brought into practice, but never upon a arge scale, or with very successful results. This has arisen, principally, from the fact, that it is impossible to make any cylinder and piston, or any rod and stuffing box, absolutely and permanently air-tight, especially under the effects of sudden and violent compressions: hence, the air, or other elastic fluid used, has always been found gradually to escape from the vessel provided for its compression, leaving the whole apparatus springless.
"Yet, if this difficulty be once overcome, the advantages accruing from the use of air (as the most available elastic fluid we have) as a spring, are very great.
"It remains aspringat all temperatures, and in all climates; and its properties are such, that the resisting energy of the spring increases with the amount of the force with which it is compressed or urged, and, by a suitable arrangement, may be made to increase according to any assignable law.
"My method of overcoming the one great obstacle to the use of air as a spring consists in confining the air to be compressed by a body of water, or other liquid; and is based on the well known fact, that joints or sliding surfaces can readily be made water-tight, and kept so, which cannot be made or maintained air-tight.
"In the month of February, 1836,1 designed a set of buffing apparatus upon this basis, which was constructed by the firm with which I am connected, and placed on the Dublin and Kingstown railway, attached to one of the open passenger carriages. Fig. 1 shows a plan of the under carriage, fitted with the buffing apparatus; and Fig. 2, a side view of the air cylinder, etc. on a larger scale. The system of thorough buffing, as invented by Mr. Bergin, the intelligent manager of the railway, is in use upon the Dublin and Kingstown line, and hence was adopted in the hydro-pneumatic buffer, as then designed, a a, is a truly bored cylinder of cast-iron, closed at each end by a cover, provided with a large gland or stuffing-box, through which the buffer rod passes, being turned truly like a piston rod, which, in fact, it is. The buffer rod, going from end to end of the carriage, passes right through the cylinder, and carries a solid piston, packed with leather collars and pressed leather caps, which when in a state of repose is situated in the middle of the length of the cylinder. At b b are two air vessels of a cylindrical form, with hemispherical ends cast on to the cylinder, and standing vertical upon its upper side when in situ.
The capacity of each air vessel is equal to that of half the cylinder, minus the bulk of the piston and included portion of buffer rod. The diameter of the cylinder was 6 inches, and that of the buffer rods 2 1/4 inches of solid iron. By means of a suitably situated screw plug, the whole of the cylinder was filled quite full of water, leaving the two air-vessels above it full of air, which by a condensing syringe, was brought to a density of one additional atmosphere, or to about 15 1bs. per square inch, plus pressure.
"In this state of things, it is obvious that; any force acting at one end of the buffer rod would compress the air in one air vessel and ratify that in the other, by carrying the central piston rod towards one end of the cylinder, and thus driving the water at that end up into the air vessel. It is also plain that, as the water will always remain at the lowest part of the vessel, it will be constantly interposed between the air and the only possible places of escape from the cylinder, namely, the end covers and stuffing boxes. The range of the buffer-rod was limited to two feet - a limit, however, which it is scarcely possible it could ever reach by any force, as the air would then be condensed into about one-fiftieth of its original volume; but as a further precaution the buffer heads c c, and the counter buffer-heads d d, were so arranged, that, at the extremity of the range any shock given to the buffer-rod would be visited upon and distributed through every part of the frame of the carriage.
The whole buffing apparatus was secured by bolting to the frame of the under carriage, to which it became a firm and substantial spine or back-bone, as it were, increasing its strength, in place of breaking it.
"The very first experiment made with this buffer, on the Dublin and Kingstown railway, consisted in bringing the carriage upon one of the lines, and causing 10 or 12 of the railway porters to run it, as fast as they could, full tilt against one of the stone walls of the station house, from which it rebounded, uninjured, like a piece of Indian rubber. The piston was not nearly driven home, or to its whole range, but had passed through more than four fifths of it, indicating a blow equal to more than 1500 lbs. The carriage was then connected with a locomotive engine, and drawn along the line, going at various speeds, stopping and reversing as suddenly as possible. The results were in every respect satisfactory."
The before-described buffer continued in use as long as the under carriage lasted. About four years afterwards Mr. Mallett designed two improved forms of air buffers.; one of them being intended for a great terminus buffer, to be placed in a station house or other similar situation, where trains require to be brought up without the possibility of running beyond a given point.
Page 479 is a side elevation of such a buffer. The construction is so simple as scarcely to need description. A large cast-iron cylinder, having a gland at one end, and closed at the other, is firmly secured down in a horizontal position to a mass of masonry, by lateral framing of cast-iron on a bed plate; the gland of this is filled by a turned cylinder of cast-iron, also hollow, bore of 36 inches diameter, open at its inner and closed at its outer end, which is armed with a large padded leather buffer-head. This cylinder is free to slide in the gland horizontally. The outer cylinder has at its lower side a large aperture, communicating with a spherical air vessel 6, by a pipe d dipping into it; this is enclosed in a cavity of the masonry.
"This cylinder is filled with water, simply by pouring in at the top man lid, which is then screwed down, and the buffer is ready for use. The air in the air vessels, which is here of about equal capacity with the cylinder, is of course permanently compressed with a force equal to a column of water of the altitude from the surface of the water in the air vessel to the highest part of the cylinder; and if any impulse be given to the inner cylinder, which may be considered simply as a large plunger or ram, it will be driven forwards into the outer cylinder, and in doing so will drive an equal bulk of water into the air vessel, compressing the air therein, the elastic resistance of which will increase in proportion to the compressing force.
"In this buffer, if the plunger be driven in half way, the air in the vessel will have a density of one atmosphere, and the total resistance afforded at this point would be 15,270lbs. and so on; and if the plunger were driven in so far that ten atmospheres was the density of the air, the buffer-head would equilibrate a blow having a force of about 68 tons. The total range given here to this buffer is 3 1/2 feet; but it is obvious that any length of range desirable may be given to such an instrument, so as to bring up a railway train with any required degree of gentleness. From the properties of elastic bodies, the plunger when struck by an imperfectly elastic system, such as a railway train, and driven in by it, would again rebound with considerable force, and would be liable to be driven wholly out of the gland: this is provided for by a turned rod, marked x, secured in the axis of the plunger, and passing through a small stuffing box in the back end of the outer cylinder, and having a large nut, and a number of leather collars to deaden the blow upon its extremity: this catches the plunger on the return stroke.
Such is the method I propose for bringing up a train at a station."
Mr. Mallett next proceeds to describe another modification of his hydro-pneumatic buffers to railways, for which we must refer the reader to vol. xxxv. page 422 of the Mechanic's Magazine.
Mr. Curtis, who was some time manager of the Locomotive department of the Greenwich railway, with the view to increase the safety of the carriages by reducing the height of the centre of gravity, altered several carriages on that line, by suspending the frame of each carriage below the axle, instead of supporting it above the axle in the usual way.