An instrument of prison discipline, being a wheel or cylinder with an horizontal axis, having steps attached to it up which the prisoners walk, and thus put the axis in motion. The men hold on by a fixed rail, and, as their weight presses down the step upon which they tread, they ascend the next step, and thus drive the wheel. Enc. Brit.
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Law Dictionary » T » TREAD-MILL, OR TREAD-WHEEL