Which Scottish engineer's improvements made the steam engine practical?

James Watt, who did not invent the steam engine - Newcomen's had been pumping water out of mines for fifty years - but made it about four times more efficient with one change: a separate condenser.

The Newcomen engine cooled its cylinder with a spray of water on every stroke and then had to reheat it, so most of the fuel went into warming and rewarming a lump of metal. Watt's insight, while repairing a model in 1765, was to condense the steam in a second vessel kept permanently cold and let the working cylinder stay permanently hot. Everything else - the double-acting engine, the governor, the parallel motion linkage - followed from having an engine worth improving.

He was also a formidable businessman, and that is the contested part. The Boulton and Watt patent ran to 1800 and was enforced hard; historians argue about whether it delayed the high-pressure engine, which Watt thought too dangerous and which turned out to be what put engines on rails and ships. He coined horsepower as a sales tool, by measuring what a mill horse could actually do and then rounding it upwards, and the unit of power carries his name.