James Watt calculated horsepower by measuring the output of an actual horse in order to quantify how many horses his steam engines could replace.


A team of six horses mowing hay in Lancaster County, Pennsylvania

The development of the steam engine provided a reason to compare the output of horses with that of the engines that could replace them. In 1702, Thomas Savery wrote in The Miner’s Friend:

So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…

The idea was later used by James Watt to help market his improved steam engine. He had pre… Continue Reading

Source: https://en.wikipedia.org/wiki/Horsepower#History