Steam engine, first important heat engine, supplying the power that made the Industrial Revolution possible. It was the principal power source for industry and transport (notably railroad locomotives and steamships) until the 20th-century advent of steam turbines and internal-combustion engines. The steam engine is an external-combustion engine, the steam being raised in a boiler heated by a furnace. The first working example was that of Thomas Newcomen (1712). Steam was admitted to a cylinder as a piston moved up and was condensed by a water spray inside the cylinder, whereupon the air pressure outside forced the piston down again. James Watt radically improved Newcomen's engine (1769) by condensing the steam outside the cylinder (thus no longer having to reheat the cylinder at each stroke) and by using the steam pressure to force the piston up. Watt also invented the double-action principle—both strokes being powered by applying the steam alternately to each end of the piston—and devices for converting the piston's linear motion to rotary motion. The compound engine (1781) made more efficient use of the steam by using the exhaust steam from one cylinder to drive the piston of a second cylinder. Later developments included the use of high-pressure steam.