A measure of force. Watt estimated the “force” of a London dray-horse, working eight hours a day, at 33,000 foot-pounds (q.v.) per minute. In calculating the horse-power of a steam-engine the following is the formula:
times A times L times N33,000 deduct 10 for friction.
P, pressure (in lbs.) per sq. inch on the piston.
A, area (in inches) of the piston.
L, length (in feet) of the stroke.
N, number of strokes per minute.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894