displaces the air, alternately exposing it to the hot and cold influences of the cold water and the hot steam, changing its temperature and expansive condition. The fluctuations cause the reciprocation of a piston in a cylinder to whose ends the air-chamber is alternately connected.

In 1829 Arnott patented his air expansion machine where a fire is placed on a grate near the bottom of a close cylinder, and the cylinder is full of fresh air recently admitted. A loose piston is pulled upwards so that all the air in the cylinder above will