Is a mill worked by the weight of persons treading upon the first movement, which is usually a wide cylindrical wheel, having upon its periphery a series of projecting steps or boards, resembling those of a water-wheel. The weight of the individuals continually climbing these steps, causes it to turn round, and put in motion any other machinery, by means of ordinary gear. Tread-mills are now resorted to pretty generally in this country, as a means of prison discipline; and the result has been, that men cannot be found to work this species of machine out of prison, conceiving the employment to be degrading. The Chinese raise water by a similar contrivance for irrigation.