in mechanics, a kind of wheel placed round an axis or cylindrical beam, on the top of which are two layers or fixed flaves, for the more easy turning the axis, in order to raise a weight required. The tympanum is much the same with the peritrochium, but that the cylinder of the axis of the peritrochium is much shorter, and less than the cylinder of the tympanum.

in anatomy. See Anatomy, p. 296.