Steam engines use the power of steam to produce useful work. They were the principal power source during the British Industrial Revolution in the 18th century. The first successful steam engine was built 1712 by English inventor Thomas Newcomen at Dudley, West Midlands; it was developed further by Scottish mining engineer James Watt from 1769 and by English mining engineer Richard Trevithick, whose high-pressure steam engine 1802 led to the development of the steam locomotive.

Air pressure
In Newcomen's engine, steam was admitted to a cylinder as a piston moved up, and was then condensed by a spray of water, allowing air pressure to force the piston downwards. James Watt improved Newcomen's engine in 1769 by condensing the steam outside the cylinder (thus saving energy formerly used to reheat the cylinder) and by using steam to force the piston upwards. Watt also introduced the double-acting engine, in which steam is alternately sent to each side of the piston.

Engine
The compound engine (1781) uses the exhaust from one cylinder to drive the piston of another. A later development was the steam turbine, still used today to power ships and generators in power stations. In other contexts, the steam engine was superseded by the internal-combustion engine.

Pumping water
The first stationary steam engines were designed for pumping water from mines, to enable mines to be sunk deeper than was possible with hand pumps. The first practical pumping engine was built by Thomas Savery 1698; on the downstroke the surface of the water in the receiver was used as a piston, the water being forced out by the pressure of steam from the boiler when the steam cock was opened; on the return stroke the steam in the receiver was condensed by pouring water over the outside from the water cock; the resultant vacuum was filled by water from the mine ready for the next downstroke. This engine was very inefficient, as the steam had to heat the receiver at each stroke before it could fill it. Further, the height to which it could pump was dependent on the boiler pressure, which in those days was very low.

Cylinder
Its successor, Newcomen's engine 1712, used a piston, piston rod, and beam to actuate a separate pump so that mine water could be forced to heights, depending on the relative sizes of the steam and water cylinders, without requiring a high steam pressure. The cylinder, however, was still used as a condenser (a jet of water being injected at each stroke), which meant a very large steam consumption. The idea of keeping the cylinder hot continuously, by using a separate condenser, came from Watt, who patented a single-acting engine on this system 1769, and brought out his first double-acting engine 1782.

Uses
Watt made a scientific study of the steam engine, and invented the separate condenser, the air pump, expansive working of steam, lagging of the cylinder and steam pipes, the stuffing box, the governor, the indicator, and the parallel motion (of which the modern equivalent is the cross-head and slide bar); the crank was also his invention, but the patent was stolen. The success of his low-pressure condensing engine delayed the coming of the high-pressure noncondensing engine and the compound engine for many years. As his machines were beam engines, the direct-acting engine was a rarity until after 1825. With the invention of the crank 1781, the steam engine came into extensive use for power purposes, and was the only power used until the end of the 19th century.

Power stations
Steam engines worked the first electric power stations, where the need for high speeds was met by vertical engines and the later demand for large powers by enormous vertical triple- and quadruple-expansion engines. When alternating-current power stations became the rule, turbines replaced the steam engine. In more recent times internal combustion engines and electric motors have taken the field, and the steam turbine is supreme as prime mover for large installations, and especially where steam ('back pressure') is needed for other purposes.