Stirling's hot air engine, c 1816.
© Science Museum / Science & Society Picture Library
Sectioned copy of one of the first hot air engines to work on the Stirling cycle, and one of two similar models believed to have been made by, or for, Stirling according to his patent of 1816. Although the principle of the hot air engine had already been developed by the English engineer George Cayley (1771-1857) in c 1807, it was the Scottish Presbyterian minister Robert Stirling (1790-1878) and his brother James who built the first ever hot air engine. In the Stirling engine, air is heated in a cylinder 3 m (10 ft) high with a bore of 0.6 m (2 ft). As this happens, the air expands and pushes against a piston, causing it to move. The air is then cooled, allowing the cycle to begin again. The original is at the Royal Museum in Edinburgh, Scotland.