It all comes from Watt trying to calculate how to sell his machines to the owners of the mines, who depended before on ponies: http://ffden-2.phys.uaf.edu/103_

It all comes from Watt trying to calculate how to sell his machines to the owners of the mines, who depended before on ponies: http://ffden-2.phys.uaf.edu/103_fall2003.web.dir/Chris_Peterson/horsepowerandtorque.htm http://ffden-2.phys.uaf.edu/103_fall2003.web.dir/Chris_Peter... "Watt was working with ponies lifting coal at a coal mine, and he wanted a way to talk about the power available from one of these animals. He found that, on average, a mine pony could do 22,000 foot-pounds of work in a minute. He then increased that number by 50 percent and pegged the measurement of horsepower at 33,000 foot-pounds of work in one minute." "A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute" https://www.popularmechanics.com/science/animals/a26074/horsepower-explainer/ https://www.popularmechanics.com/science/animals/a26074/hors... "his business partner Matthew Boulton standardized the figure at 33,000. The number wasn't quite scientific: Watt looked for horses at peak performance, and for his purposes, it didn't truly matter how accurate his measurement of a horse's power was as long as it was close enough to be believable. Much like his engine, the metric struck."