The Probert Encyclopaedia of Science & Technology

HORSEPOWER

Horsepower is an imperial unit of power, now replaced by the watt. It was first used by the engineer James Watt, who employed it to compare the power of steam engines with that of horses - one horsepower being the force with which a horse acts when drawing. The mode of ascertaining a horse's power was to find what weight he could raise and to what height in a given time, the horse being supposed to pull horizontally. From a variety of experiments of this sort it was found that a horse, at an average, can raise 160 lbs weight at the velocity of 2.5 miles per hour. The power of a horse exerted in this way was made the standard for estimating the power of a steam-engine. Thus people formerly spoke of an engine of 60 or 80 horsepower, each horsepower being estimated as equivalent to 33,000 lbs. raised one foot high per minute.

Engineers differed widely in their estimate of the work a horse is able to execute. The figure given is the estimate of Boulton and Watt and was based on the work of London dray-horses, but it was considered much too high, 17,400 foot-pounds per minute being generally considered nearer the truth. As it matters little, however, what standard be assumed, provided it be uniformly used, that of Watt was generally adopted. The general rule for estimating the power of a steam-engine in terms of this unit was to multiply together the pressure in pounds on a square inch of the piston, the area of the piston in inches, the length of the stroke in feet, and the number of strokes per minute, the result divided by 33,000 gave the horse-power, deducting one-tenth for friction. As a horse can exert its full force only for about six hours a day, one horse-power of machinery is equal to that of 4.4 horses.

In the UK, one horsepower is now equal to 550 foot-pounds per second or 745.7 watts. In the USA this figure has been rounded to 746 watts, and in the metric system it is 735.5 watts.
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