How did the phrase horsepower get started?

The term "horsepower" was popularized by the engineer James Watt in the late 18th century as a marketing tool to compare the output of steam engines with the power of draft horses. Watt calculated that a horse could turn a mill wheel of a certain size at a specific speed, which he quantified as 550 foot-pounds per second. This measurement allowed people to understand the relative power of engines compared to horses, which were commonly used for labor at the time. The term has since become a standard unit of measurement for engine power across various industries.