FLY

, in Mechanics, a heavy weight applied to certain machines, to regulate their motions, as in a jack, or to increase their effect, as in the coining engine, &c; by means of which the force of the power is not only preserved, but equally distributed in all the parts of the revolution.

The Fly is either like a cross, with heavy weights at the ends of its arms; or like a heavy wheel at rightangles to the axis of motion. It may be applied to various sorts of engines, whether moved by men, horses, wind, or water; and is of great use in those parts of an engine having a quick circular motion, and where the power or resistance acts unequally in the different parts of a revolution. In this case the Fly becomes a moderator, making the motion of revolution almost everywhere equal.