An apparatus for converting thermal energy to mechanical motion including a frame mounted onto an axle above a heat source. A flow circuit including at least three elongate chambers connected by fluid conduits is mounted onto the frame, and one-way valves provided in the flow circuit permit one-way fluid flow within the flow circuit. The heat source heats a motive fluid contained within the chambers beyond its boiling point, which increases the vapor pressure within the heated chamber, thereby forcing fluid out of the chamber and into the chamber immediately downstream in the flow circuit. The increased weight of the downstream chamber creates a torque about the axle, rotating the frame in an upstream direction.