Valve, a kind of movable lid, or cover, adapted to the orifice of some tube or passage, and so formed as to open communication in one direction and to close it in the other. Valves are used to regulate the admission or escape of a fluid, such as water, gas or steam. Some valves are self-acting, that is, they are so contrived as to open in the required direction by the pressure of the fluid upon their surface, and immediately to shut and prevent the return of the fluid, when the direction of its pressure changes. Others are operated by some independent external force. Examples of the former kind are presented in the valves of pumps, and in the safety valves of steam boilers. Examples of the latter are the slide valves appended to the cylinder of a steam engine, for the purpose of regulating the admission and escape of the steam. The construction of valves admits of an almost endless variety. See PUMP; STEAM ENGINE.