TREAD-MILL, a penal appliance introduced by Sir William Cubitt in 1818 and intended by him as a means of employing criminals usefully. It was a large hollow cylinder of wood on an iron frame, round the circumference of which were a series of steps about 71 in. apart. The criminal, steadying himself by hand-rails on either side, trod on these, his weight causing the mill to revolve and compelling him to take each step in turn. In the brutalizing system formerly in vogue the necessary resistance was obtained by weights, thus condemning the offender to useless toil and defeating the inventor's object. The tread-mill, however, was subsequently utilized for grinding corn, pumping water and other prison purposes. The speed of the wheel was regulated by a brake. Usually it revolved at the rate of 32 ft. per minute. The prisoner worked for 6 hours each day, 3 hours at a time. He was on the wheel for r5 minutes and then rested for 5 minutes. Thus in the course of his day's labour he climbed 8640 ft. Isolation of prisoners at their work was obtained by screens of wood on each side of the mill, converting the working space into a separate compartment. Each prisoner was medically examined before going to the mill.
By the Prison Act 1865 every male prisoner over 16, sentenced to hard labour, had to spend three months at least of his sentence in labour of the first class. This consisted primarily of the tread-mill, or, as an alternative, the crank. The latter consisted of a small wheel, like the paddle-wheel of a steamer, and a handle turned by the prisoner made it revolve in a box partly filled with gravel. The amount of gravel regulated the hard labour; or the necessary resistance was obtained by a brake, by which a pressure, usually of 12 lb, was applied. The prisoner had to make 8000 or io,000 revolutions during his 6 hours' work, according to his strength, the number being registered on a dial. The crank too, however, was subsequently made to serve useful purposes. Both tread-mill and crank have gradually been abolished; in 1895 there were 39 tread-mills and 29 cranks in use in English prisons, and these had dwindled down to 13 and 5 respectively in Igor. They are now disused.
The fundamental idea of Cubitt's invention, i.e. procuring rotary motion for industrial purposes by the weight of men or animals, is very old. "Tread-wheels," of this type, usually consist of hollow cylinders, round the inner surface of which a horse, dog or man walks, foothold being kept by slabs of wood nailed across at short intervals.
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