PISTON

, a part or member in several machines, particularly pumps, air-pumps, syringes, &c; called also the Embolus, and popularly the Sucker.

The Piston of a pump is a short cylinder of wood or metal, fitted exactly to the cavity of the barrel, or body; and which, being worked up and down alternately, raises the water; and when raised, presses it again, so as to make it force up a valve with which it is furnished, and so escape through the spout of the pump.

There are two sorts of Pistons used in pumps; the one with a valve, called a bucket; and the other without a valve, called a forcer.