Valve is a device for controlling the flow of a liquid or gas. When used in pumps, etc., it often consists of a conical hole, provided with an accurately-fitted stopper held in place by a spring or by gravity. Liquid trying to pass in one direction can raise the cone and open the valve, but pressure in the other direction keeps it more tightly closed. In another pattern a hole in the plate is provided on one side with a flap of leather or other flexible material, or in cases where great pressures is to be resisted a number of small holes are used instead of one large one. A stop-valve, used for controlling the supply of steam to an engine, etc., is a kind of tap, a common pattern having a cone forced upon a slating by a screw when the valve is to be closed. The throttle-valve of an engine is a somewhat similar arrangement, which is opened or closed by the governor to adjust the speed. Such a valve is usually balanced, i.e. so arranged that the pressure of steam does not tend either to open or close it.