From your car, to your lawn mower, to your snow blower, to your chainsaw—the power of almost every engine you deal with is measured in terms of horsepower. None of these things seemingly have anything to do with horses, so why it is so, and from where this unit came.

In the early 1780s, after making a vastly superior steam engine to the then classic Newcomen steam engine, Watt was looking for a way to market his invention, advertising the fact that his engine used about 75 per cent less fuel than a similarly powered Newcomen, among many other improvements.

But according to Science ABC, since a majority of the consumers then used draft horses to carry out mechanical tasks, Watt came up with the ‘horsepower’ unit to compare the power of commonly used draft horses and that of his engine using a single measure.

Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute).The wheel was 12 feet (3.6576 meters) in radius; therefore, the horse travelled 2.4·2π·12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds. So:

Watt defined and calculated the horsepower as 32,572 ft·lbf/min, which was rounded to an even 33,000 ft·lbf/min

In the end, Watt’s engine was revolutionary and played a huge role in the Industrial Revolution. Thanks to this fact, his unit of measure of an engine’s power, horsepower, also became popular. Funny enough, today the SI unit of power, the Watt, which was named in homage to James Watt, has widely come to replace “horsepower” in most applications.

 

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