VALVE

, in Hydraulics, Pneumatics, &c, is a kind of lid or cover to a tube or vessel, contrived to open one way; but which, the more forcibly it is pressed the other way, the closer it shuts the aperture: so that it either admits the entrance of a fluid into the tube, or vessel, and prevents its return; or permits it to escape, and prevents its re-entrance.

Valves are of great use in the air-pump, and other wind machines; in which they are usually made of pieces of bladder. In hydraulic engines, as the emboli of pumps, they are mostly of strong leather, of a round figure, and fitted to shut the apertures of the barrels or pipes. Sometimes they are made of two round pieces of leather enclosed between two others of brass; having divers perforations, which are covered with another piece of brass, moveable upwards and downwards, on a kind of axis, which goes through the middle of them all. Sometimes they are made of brass, covered over with leather, and furnished with a fine spring, which gives way upon a force applied against it; but upon the ceasing of that, returns the Valve over the aperture. See Pump. See also Desaguliers' Exper. Philos. vol. 2, p. 156, and p. 180.