This mechanism is a twin gear driving a common slider crank. The symmetry keeps the center link moving in a straight-line motion. This linkage is shown in Thurston's 'A History of the Growth of the Steam Engine' (1904). Cartwright was a clergyman and distinguished inventor. He sought to improve upon Watt's steam engine by using two concentric cylinders on of which was to be used as a condenser. The engine was described in the Philosophic Magazine, June 1798. His engine was supposed to improve the problems of friction and imperfect vacuum.

A modern version of the Cartwright mechanism is used in Stirling engines.

Francis Moon 2003-05-29