HORSE-POWER is the name given to the unit in terms of which engineers measure the power of steam-engines, water-wheels, and other prime movers. It is defined to be the rate at which an engine works when it does 33,000 foot-pounds of work per minute, a foot-pound being the amount of work necessary to raise a pound weight a foot high. We must go back to the early history of the steam-engine to discover the reason why this number was adopted. The first steam-engines were employed to drive mills, pumps, and other machinery which had previously been driven by horses; and it seemed natural to express their working-power in terms of the number of horses whose work they were got to accomplish. This led to experiments being made in order to get an estimate of the average working-power of a horse. Several such estimates have been given, all differing considerably from each other; but the one adopted whereby to express horse-power is that obtained by Boulton and Watt from observations on the strong dray horses employed at the London breweries working eight hours a day. They found that a horse was able to go at the rate of 2∣ miles per hour and at the same time raise a weight of 150 lb by means of a rope led over a pulley. This is easily seen to be equivalent to 33,000 S> raised one foot per minute, and hence the number given above. In connexion with this subject it is necessary to distinguish clearly between “horse-power indicated’’and “horsepower nominal” as applied to steam-engines. The horsepower indicated is got from an examination of the indicator diagram (see Diagrams, vol vii. p. 152). The area of the closed curve traced on the diagram, or “card” as it is technically called, gives the work done by the steam on the piston during each complete stroke. This ...