What was a treadmill?

A treadmill was an apparatus of punishment used in prisons. It was a large hollow cylinder, rather like the wheel of a water mill, with twenty-four steps. Each prisoner held on a bar in front of him and kept on treading on the steps, thus making the cylinder revolve. The machines were used for pumping or grinding, or simply as a punishment. The treadmill was introduced in the early 1800s but fell into disuse long ago.