Horsepower is literally the power of a horse or the rate at which it can perform work. With the invention of the steam-engine, it became necessary for Watt to express its power in terms of the horse, and the standard he established has been generally adopted. So the horsepower became equivalent to the raising of thirty-three thousand pounds one foot high in one minute, or to the doing of work at the rate of thirty-three thousand foot pounds per minute. This somewhat exceeds the activity of an average horse except for limited periods. The determination of the horsepower of a steam-engine is a comparatively simple matter. Multiply together the area of the piston in square inches, the length of the stroke in feet, the mean effective pressure of the steam in pounds per square inch, and the number of strokes per minute, and divide the product by thirty-three thousand. It is customary to subtract ten percent for friction.
The unit of power in metric units is the watt, named from the inventor, and is equal to doing work at the rate of one joule, or ten million ergs, per second. The kilowatt is of more practical size, and equals one and one-third horsepower.
