James Watt actually, the inventor of the steam engine. He came up with horsepower to show his steam engines could do the work of "X" many horses.
What is Horsepower?
How do you define horsepower? Ask a car enthusiast and most of the time you'll get a blank look, a shrug of the shoulders and maybe a guess along the lines of "What a horse can do!".
That answer begs the question: What horse? A thoroughbred race horse that can carry the small weight of a jockey with a lot of speed, or a working horse that can pull heavy loads albeit slowly? Obviously there is a more precise answer. Car manufacturers, despite their reputation for being creative regarding the horsepower ratings of their products for marketing reasons, require a more stable definition.
Horsepower is defined as work done over time. The exact definition of one horsepower is 33,000 lb.ft./minute. Put another way, if you were to lift 33,000 pounds one foot over a period of one minute, you would have been working at the rate of one horsepower. In this case, you'd have expended one horsepower-minute of energy.
Even more interesting is how the definition came to be. It was originated by James Watt, (1736-1819) the inventor of the steam engine and the man whose name has been immortalized by the definition of Watt as a unit of power. The next time you complain about the landlord using only 20 watt light bulbs in the hall, you are honoring the same man.
To help sell his steam engines, Watt needed a way of rating their capabilities. The engines were replacing horses, the usual source of industrial power of the day. The typical horse, attached to a mill that ground corn or cut wood, walked a 24 foot diameter (about 75.4 feet circumference) circle. Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with the figure is not known. Watt observed that a horse typically made 144 trips around the circle in an hour, or about 2.4 per minute. This meant that the horse traveled at a speed of 180.96 feet per minute. Watt rounded off the speed to 181 feet per minute and multiplied that by the 180 pounds of force the horse pulled (181 x 180) and came up with 32,580 ft.-lbs./minute. That was rounded off to 33,000 ft.-lbs./minute, the figure we use today.
Put into perspective, a healthy human can sustain about 0.1 horsepower. Most observers familiar with horses and their capabilities estimate that Watt was a bit optimistic; few horses could maintain that effort for long.