Watt’s steam engine

Thomas Newcomen had a working atmospheric engine pumping mines by 1712. James Watt's separate condenser, patented in 1769 and made practical with Matthew Boulton in the 1770s, cut the coal waste. 1776 is when their partnership's engines start selling in earnest. Watt did not invent steam. He made it efficient enough to leave the mines.

The contribution is fuel doing muscle's job, continuously. A water wheel needs a river. A steam engine needs coal, iron, and a boiler, and it can sit next to the work. Factories no longer have to be in a valley. Rails become possible once the engine is light enough to move itself.

The engine still depends on iron, fire, and the older idea of a piston. Without metal that holds pressure, steam is a kettle.

Later engines, internal combustion and turbines, replace Watt's design. They do not replace the idea that heat can be turned into rotation on purpose.