Horse-power
In speaking of the power, or force which an engine exerts, it is necessary to have some measure of force, or standard of inference. That used in this country is a Horse-power, a force equal to that which the average strength of a horse was believed capable of exerting. This has been estimated at 33,000 lb. avoirdupois weight, raised one foot high in a minute. There have been different estimates as to the real power of horses; and it is now considered that taking the most advantageous rate, for using horse-power, the medium power of that animal is equal to 22,000 lb. raised one foot high per minute. However, the other 33,000 lb. is taken as the standard, and is what is meant when a horse-power is spoken of. In comparing the power of a steam-engine with that of horses applied to do the same work, it must be remembered that the engine horse-power is 33,000 lb. raised one foot per minute; the real horse-power only 22,000 lb.; and that the engine will work unceasingly for twenty-four hours, while the horse works at that rate only eight hours. The engine works three times as long as the horse; hence, to do the same work in a day as the engine of one horse-power, 4·5 horses would be required (33,000 × 3 = 99,000; 99,000 ÷ 22,000 = 4·5). The power of a man may be estimated at one-fifth of the real power of a horse, or 44,000 lb. raised one foot per minute.—Hugo Reid on the Steam-Engine.