Piston

Piston (through Fr. from Ital. pistone or pestone, a great pestle, from Late Lat. pistare, to pound, a frequentative form of classical Lat. pinsere), in the steam engine, a disk or partition placed inside the cylinder, from end to end of which it moves alternately under the pressure of the steam. By means of the "piston-rod" attached to it this forward and backward motion is communicated to the machinery which the engine is employed to drive, and is in most cases converted into rotary motion by a "connecting-rod," one end of which is jointed to the "cross-head" carried at the end of the piston-rod, while the other turns the crank on the crank-shaft. The piston in gas, oil and air engines has a similar function, but in a pump, instead of imparting motion, it has motion imparted to it by some prime-mover. In every case the piston must fit the cylinder so accurately that as little as possible of the working fluid, whether it be steam, gas or water, can escape past it, packing of various forms being commonly placed round its periphery in order to secure this fit. In music, the valves which in certain wind instruments, such as the cornet, enable the player to increase the length of the air-column and thus lower the note produced, are known as pistons. (See VALVES.)