Working model (scale about 1:8) of a double-acting beam engine with sun and planet motion; steam is distributed by a cock.

Working model (scale about 1:8) of a double-acting beam engine with sun and planet motion, c.1800. This engine incorporates Watt's separate condenser (patented in 1769) and other features including an air pump, parallel motion, and the sun-and-planet gear by which means the engine turned a flywheel. By 1800, when Watt and Boultons partnership ended, 451 engines had been built of which 268 were rotative. By the end of 19th century, the steam engine had become the main source of power for industry and transport.