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A horsepower in the 18th Century was not a unit of power, but was a power source. A horsepower had a vertical wood shaft pivoted top and bottom to the structural timbers of a building. An arm, also of wood, was attached to the vertical shaft. A horse, hitched to the end of the arm, walked in circles to rotate the shaft. Wood gears at the top of the shaft drove a horizontal shaft to a machine, such as a grist mill or fulling mill. If two horses were used, it was called a two-horsepower.
In the 18th Century, steam power was in its infancy. Power to aid human muscle was either animal power or water power.
In 19th Century, after cast iron became available, portable horsepower were manufactured and used on farms. These had a gear box which was staked down. An arm extended from the top of the gearbox and the output shaft was close to the ground so the horse stepped over it.
About 1770, James Watt chose the horsepower device for power comparison with his newly-invented steam engine. The actual continuous output of the horsepower device depended on many factors, such as the strength and vigor of the horse, how fast and hard the operator drove the horse, and the efficiency of the wood gears. Watt decided upon 33,000 foot-pounds per minute as the standard output of a horsepower, and that has been our definition of the unit of power ever since.
The power produced at the drawbar of a tractor, for example, is the pull in pounds times the speed of the tractor in feet per minute, divided by 33,000. By simple arithmetic, one can convert this formula to rotating power. Horsepower is the torque in pound-feet times the speed in RPM divided by 5252.
Watt's unit of power originated in England and we acquired it along with a lot of other English units. Germany was another early industrial nation and they also adopted horsepower as a unit of power. They call it PS (for Pferdestaerke) and it is used in all of continental Europe. As it is based on metric measurements, it is not quite the same as our horsepower. Because of that problem, the International Standards Organization (ISO) made the decision in the 1960's to use kilowatts as the international standard. So don't be surprised to see a modern engine rated in kilowatts. 1 HP equals 0.746 kilowatts. 1 PS equals 0.736 kilowatts.
It is interesting that scientists named the electrical unit of power after James Watt.
Obviously, a horse can produce a lot of power for a short period, as can a human. Consider a team of horses in a pulling contest, where the team pulls a 5000 pound stone boat at 6 MPH. If we assume a coefficient of friction between steel stone boat and the earth of 0.6, the team is producing 48 HP or 24 HP for each horse.