Horse-power. The word power is of frequent use as a scientific expression. Thus we speak of steam-power, water-power, and so on. When used in this sense it is quite obvious that reference is merely made to the nature of the store of energy in the particular motor under consideration. Again, when we speak of the total energy of a given system as the total power that it has of doing work, it would seem to be almost an insult to the mental power of our auditor to tell him in addition that by the latter phrase we mean simply total work measured in terms of any convenient unit. But it is often necessary to know the quantity of work which can be produced in a given time by a given motor. It is in this connection that the term one horse-power is used to denote the rate at which on the average a horse can do work per unit time, and this rate is adopted in Britain as the unit rate of working. Estimates of its numerical value necessarily differ very much; and so, in order to get a definite unit available for scientific purposes, the convention is made that the original estimate of Boulton and Watt shall be regarded as correct. The value which they gave was 33,000 foot-pounds per minute.
An ordinary rule for calculating the horse-power of a steam-engine is to divide by 33,000 the continued product of the area of the piston in inches, the pressure in pounds weight per square inch, the length of the stroke in feet, and the number of strokes per minute. Thus, by the horse-power of an engine we merely mean the numerical rate at which it can do work, expressed in terms of the above conventional unit, and this number obviously coincides with the number of horses to which the engine is equivalent as regards work in the same time. Of course the available horse-power is less than the actual horse-power as got by the above rule, because of the work which has to be done against friction in the engine itself. See STEAM-ENGINE. The term man-power is similarly used, being usually taken as one-eighth of a horse.