barrel organ, mechanical musical instrument requiring nothing but the regular rotary motion of a handle to keep it going. It probably originated at the beginning of the 18th cent., and was once used extensively in English churches. A revolving cylinder is fitted with pegs that open valves, permitting air to enter a set of organ pipes. Some larger ones have several sets of pipes and various couplers. They can be operated by clockwork, by weights, and by electric motors. A portable type of barrel organ whose cylinder is turned by a hand crank has been mistakenly called hurdy-gurdy, from which it is fundamentally different.

barrel: see English units of measurement.