Steam engine design was greatly improved in the 1770s by Scottish engineer and inventor James Watt (1736–1819). In his early career, Watt was a maker of mathematical instruments, but he soon developed an interest in steam engines. In those days, Thomas Newcomen’s engines were being used for pumping water out of mines. When given one to repair in around 1763, Watt was struck by how inefficient it was: the engine's cylinder was heated and cooled on every cycle. He began work to improve the design. In partnership with the manufacturer Matthew Boulton (1728–1809), Watt's steam engine would make possible the mechanization of factories and mills—triggering the Industrial Revolution.

Watt's steam engine

James Watt’s first steam engine, patented in 1769, included many improvements over Newcomen’s. It had a separate chamber (the condenser) where the steam was condensed (turned back to liquid), allowing the main cylinder to remain hot all the time. Watt also used steam pressure to force the piston down, rather than relying on atmospheric pressure. This increased the power of the engine. An automatic governor controlled the flow of steam to the cylinder, and so regulated its speed.

Rotary motion

In 1775, Watt went into business with Matthew Boulton, who owned an iron foundry near Birmingham. Together, they began to manufacture steam engines following Watt's design. Initial purchases came from mine owners, but Watt's engine, unlike Newcomen's, could easily be adapted for other purposes. in 1781, Watt added sun-and-planet gears, which converted the up-and-down movement of the cylinder into a turning movement, called rotary motion. Wheels and belts could now link the engine to grinding, spinning and weaving machines. Soon, Boulton and Watt engines were installed in factories and mills.