Fly, in Mechanics, a wheel with a heavy rim, placed on the shaft of any machinery put in motion by any irregular or intermitting force, for the purpose of rendering the motion equable and regular by means of its momentum. This effect results from a law of nature, that all bodies have a tendency to continue in their state either of motion or of rest, until acted upon by some extraneous force. Thus the rim of a fly-wheel, after a few revolutions, acquires a momentum sufficient to cause it to revolve with a velocity depending upon the resistance of the machinery; and the augmentations and diminutions of the impelling power succeeding each other rapidly, neither cause acts sufficiently long to either augment or diminish the velocity acquired in any considerable degree, so that it remains equable, or nearly so. Thus in the case of a man working at a winch, the power which he exerts in pulling upwards from the lower part considerably exceeds his power in thrusting forwards in the upper quarter; but before the extra force thus exerted has acted sufficiently long to change the velocity of the wheel, the winch arrives at the point where his force is the least, by which time the excessive force previously exerted having taken effect, the equable motion of the fly is maintained; and the resistance of the work being equalized, a man is enabled to raise throughout a whole day a weight of forty pounds with more ease than he could raise thirty pounds without a fly.

In all cases where a rotatory motion is to be obtained from a reciprocating one, by means of a crank, a fly-wheel is necessary to continue the motion at those two points of the revolution in which the crank lies in the direction in which the moving force acts; for in this case the crank affords no leverage to the power either on one or other side of the fulcrum, and consequently no motion could be produced in either direction; but the momentum acquired by the fly urges the crank forward in the direction in which it was previously moving, and continues the rotation until the crank is brought into such a position as to offer sufficient leverage to the power to maintain the impetus of the fly.