The measure of power under the hood of a car was established by Scottish engineer James Watt, often credited with inventing the steam engine. To convince potential buyers of the value of his steam engine, he devised a way to rate its power by comparing it to the work done by horses — the usual source of industrial muscle in his day.
Watt watched horses at work, although there’s some disagreement about his actual methods. He may have observed ponies lifting coal in a coal mine or horses working in a mill. He concluded that a horse could lift 550 pounds at the rate of one foot per second. In other words, it could lift 33,000 pounds one foot per minute. And that’s the figure we use today: one unit of horsepower equals 33,000 foot-pounds per minute.
Regardless of his methods, the term “horsepower” stuck. Two hundred years after Watt coined the word, it’s used to sell everything from lawnmowers to garbage disposals.