The term horsepower was invented by James Watt (1736-1819), famous for his pioneering work on steam engines.
In order to find a measurement that rated the power output of his engines, Watt tested the ability of several horses to lift coal up a shaft using a rope and pulley.
He discovered that, on average, in one minute the horses could do 22,000 foot-pounds of work — that is, for example, raise a 220lb load by 100ft, or a 22lb load by 1,000ft.
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The story goes that in case his horses were not as strong as others, Watt increased that figure by 50%, giving 33,000 ft lb a minute, and called the measurement one horsepower.
In fact, a fit man could probably do one horsepower of work relatively easily over a minute or two, but the measure was calculated to reflect the work horses could do over an extended period of time without needing regular recovery breaks.
Nowadays, engine power is most commonly expressed in terms of “bhp” — brake horsepower. The “brake” came into the phrase because the power of engines came to be measured by seeing how much force was required to stop the engine’s flywheel from turning, ie, to “brake” it.
Not all horsepower is the same. One differing measurement is continental horsepower, abbreviated to PS, which stands for pferdestärke, the German translation of horsepower, and in France the same measurement is sometimes given as CV, for cheval-vapeur.
The measurement was chosen in Europe since it is the closest thing to horsepower that can be expressed in metric units — 75 kilogram metres per second. It is often used interchangeably with horsepower but is in fact a slightly lower unit of power: one PS is about 98.6% of one horsepower.