Fly-wheel, in Engineering, means a large wheel with heavy rim, worked by an engine for the purpose of storing up energy and so acting- as an accumulator, from which energy may be taken as required. In certain engines that are uniformly driven, and whose energy is uniformly abstracted at the same rate, a fly-wheel is unnecessary. In others the energy is required at a uniform rate, but is supplied irregularly as in the case of a foot-lathe or of a high-pressure engine, where the full pressure of steam in the cylinder only lasts for a small fraction of the stroke. In others the demand for energy is intermittent, so that it could not, except with much waste, be taken direct from the engine; such a case is seen in the rolling-mill (q.v.); while the metal is between the rolls the demand for energy is great; when the metal has passed through, the engine, of course, goes on, but there is little call on it. Finally, in cases like locomotives, the engine works direct on such heavy masses that these act as accumulators without requiring a' rotary motion.