See Diptera, index to Entomology.

in Mechanics, the name of an appendage to many machines, either as a regulator of their motions or as an accumulator of power. It is usually in the form of a metal disc, or a wheel with a heavy rim, sometimes in that of a cross or bar loaded at the extremities, and is attached to the revolving shaft of the machine in order to distribute the force of the power equally in all parts of its revolution. When used as an accumulator of power for the purpose of producing a very great instantaneous effect, as in the coining press, it is frequently seen in the form of heavy knobs at the opposite extremities of a straight bar. The fly is especially useful in all cases where the power or the resistance acts with an irregular or intermitting force in the different parts of a revolution.