Miscellaneous contrivances connected with the. Steam Engine.
Murray's Sliding Valve.—The idea of a sliding valve has been long ago suggested, and seems to have been first applied to the air pump by Lavoisier or some of his associates. Mr. Murray of Leeds, however, has the merit of having applied it with great ingenuity to the steam engine, and may be regarded as its real inventor. This valve is represented in section in Plate DIX, Fig. 21, and the object of it is to cover al ternately the passages a c, c b. The sliding cover is put in motion by the rod o, which passes through a stuffing box. The steam from the boiler is introduced at S, passes through the opening a to the top of the cylinder, when the slide is down as shown in the figure, while the passage c to the condenser is open through the interior portion of the slide. When the slider is up the passage b to the bottom of the cylinder is open, and the passage a from the top, to c, the pas sage to the condenser is open. Owing to the pressure of the steam against the box, the friction of the valve is considerable. This disadvantage, however, has been remedied by Mr. Murdoch, who packs the slide, and this keeps out the steam. Messrs. Taylor and Martineau have substituted for the slide, pistons sliding in a pipe, which is a valuable improvement.
Site's valve is shown in Plate DIX, Fig. 22, and has been extensively used in large en gines. The seat is shown at a a a, a' a' a', and is solid at a a a. 'Apertures are cut in its sides at a' a' a' for the passage of the steam, and at a" a" is the beat, into which it is ground with emery. The valve b b is a plain cylinder very accurately bored, and packed at d d with the usual materials, the packing being kept in its proper place by the ring e e. A second ring ff rests on e e, and ib kept in its place by the screws g g, which help also to keep the packing in a proper state of compression. The valve is lifted by the bar c. It is evident that the steam can exert no pressure to pre vent the valve from being lifted, and when it is closed, the vapour has no power to open it. This valve an swers its purpose so completely, that an infant might lift the valves of a 90 inch engine. It would be de
sirable to dispense with the packing, as it increases the dimensions of the valve. See Dr. Brewster's Journal of Science, No. XI X, p. 42.
Cartwright's Metallic Piston.—Mr. Cartwright took out a patent in 1797 for the method of using metal in place of elastic substances for the packing of pistons. This method is shown in Fig. 23, where a a a are six or more segments of rings made to fit the internal sur face of the cylinder, with a second series b b b crossing the joints of the former. Each series of segments were pressed against each other, and against the cy linder with V springs, and by having two sets with the junction of the rings in the one, set opposite to the solid parts of the rings in the other set, any escape of steam out of the joints was prevented. The upper and under sides of the sets of segments were connected by plates to which the piston rod was united, as shown in the section.
Jessop's expanding Coil very ingenious contrivance was secured by patent in 1823, and is shown in Fig. 24. It consists of an elastic expanding coil of metal AA which winds round the body of the piston in a spiral form, as shown in the section at AA. BB is a bed of hemp packing which prevents the steam from passing at the joints, and presses the springs against the surface of the cylinder. The pis ton is kept tight by the pressure of the steam on the upper and under plates. Mr. Barton's metallic ex panding piston is very ingenious, and has been im proved by Tredgold; but we must refer for an account of it to the work of the latter on the Steam Engine, p. 228. Perkins' metallic piston, from which much has been expected, is considered inferior to those scribed. See the Repertory of Patents, vol. i, p. 224.
On the .flpplication of the Steam Engine.
The Steam Engine is, of course, applicable to every purpose where mechanical power is required, but some of its applications have been of such vast im portance to the progress of the useful arts, that they require to be specially noticed. The most important of these are as follows : 1. Raising of water.