One Horse Power Per Second.
To lift 550 pounds one foot in one second requires what is known as one horse power. Similarly, a horse power is able to raise twice that weight one foot in twice the time or one-half foot in just that time. Moreover, it can raise half 550 pounds one foot in half a second, or two feet in a second, and so on. Therefore, when we lift one-fourth of that weight, 137½ pounds, four feet in one second, we are exerting a horse power.
Accordingly, when a person who weighs 137½ pounds runs upstairs at the rate of four feet a second, he is exerting the equivalent of a horse power. For a man weighing twice that much, 275 pounds, it would be necessary to climb at the rate of only two feet a second to exert a horse power. It is possible to do much more.
As a matter of fact, a horse often exerts many times a horse power. The average horse can draw a wagon up a hill where a ten-horse power engine with the same load would fail. A horse power does not represent the greatest momentary strength of the average horse, but is a measure of the power which he can exert continuously.