Horsepower

Horse-power is the engineer's unit of power, or rate of doing work. From certain experiments made some years ago it was settled that the horsepower should be regarded as equivalent to 33,000 foot-pounds of energy performed per minute. Nevertheless, it requires an exceptional horse to supply energy at that rate for any considerable time, and the unit should therefore be regarded as quite arbitrary. Small engines are estimated by man-power, which is from 2,600 to 3,100 foot pounds per minute. Indicated horse-power, or I.H.P. of a steam-engine signifies the power supplied by the steam that is actually passed into the cylinders. This is calculated from indicator observations of the changing pressure of steam throughout the stroke and of the number of strokes per minute. It is greater than the actual horsepower supplied by the engine for external use, there being waste in friction, etc., in the engine itself. The efficient power is called the brake H.P., which is invariably less than the I.H.P. of the same engine.