When the steam engine began to do the work of horses in the mines during the early 1800s, the mine owners began to ask how many horses an engine would replace.

James Watt, who invested steam engines, figured out a mathematical way to equate horses to engine power. Thus the term horsepower was invented.

Watt measures the capability of of a big horse to pull a load and found it could pull a weight of 150 pounds, while walking at a speed of 2.5 miles per hour. This works out to 33000 foot-pounds per minute or 550 foot-pounds per second.