(129.)

In the steam engine, as well as in every other machine in which fluids act, it is necessary to open or close, occasionally, the tubes or passages through which these fluids move. The instruments by which this is accomplished are called cocks or valves.

Cocks or valves may be classified by the manner in which they are opened: 1st, they may be opened by a motion similar to the lid of a box upon its hinges; 2d, they may be opened by being raised directly upwards, in the same manner as the lid of a pot or kettle; 3d, they may be opened by a sliding motion, like that of the sash of a window or the lid of a box which slides in grooves; 4th, they may be opened by a motion of revolution, in the same manner as the cock of a beer-barrel is opened or closed. The term valve is more properly applied to the first and second of these classes; the third class are usually called slides, and the fourth cocks.