An instrument used to regulate the supply of steam to the cylinder of a steam-engine. It consists of two heavy balls at the end of two rods, whose other ends are jointed to a shaft, turned by a strap from the engine. When the engine moves these balls revolve and separate by centrifugal force, becoming wider apart the faster the engine moves. They act upon a rod which operates the throttle-valve of the engine. When the engine is going too fast this valve is partly closed and the supply of steam reduced, when going too slow it is opened wider and more steam let in. By this means the supply of steam and speed of the engine are kept uniform.