Tread-Mill or Tread-Wheel
Defined in 1 dictionary — Black's (1910)
Black's Law Dictionary
Henry Campbell Black, M.A. · 1910
Is 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 winch they tread, they ascend the next step, and thus drive the wheel. Enc. Brit.