The big idea
Steam from the boiler pushes on one side of the piston while the other side is open to a cold condenser, where the used steam turns back to water and leaves the space nearly empty. Then the valve swaps the sides, so the piston is pushed both ways. The piston rocks the beam, the sun and planet gears turn the flywheel, and the governor keeps the speed steady.
Memory trick: “Keep the cylinder hot, keep the condenser cold.”
A bit of history: Thomas Newcomen's atmospheric engine of 1712, near Dudley in the English Midlands, was the first steam engine that worked well enough to be widely used, pumping water out of mines. In 1763 or 1764 James Watt, an instrument maker at the University of Glasgow, was asked to repair a model Newcomen engine and saw how much steam it wasted. In 1765, on a walk by his own later account, the idea of the separate condenser came to him, and in January 1769 he was granted a patent for “A New Invented Method of Lessening the Consumption of Steam and Fuel in Fire Engines”. In 1775 he went into partnership with Matthew Boulton of Birmingham; an Act of Parliament extended the patent to 1800, and their first engines were at work in 1776. Then came turning motion with William Murdoch's sun and planet gears (patented 1781), the double-acting engine (1782), the parallel motion (1784) and, from about 1788, the centrifugal governor. By 1800 the firm had built about 450 engines, more than half of them for turning machinery. Sources differ by a year or so on some of these dates.
In the psalm that calls all creation to praise the Lord, even fire and vapour have their place: Fire, and hail; snow, and vapour; stormy wind fulfilling his word:
(Psalm 148:8, KJV)