The unit of nominal power for steam engines, or the usual estimate of dynamical effect per minute of a horse, called by engineers a 'horse power," is thirty-three thousand pounds at a velocity of one foot per minute, or, the effect of a load of two hundred pounds raised by a horse for eight hours a day, at the rate of two and a half miles per hour, or 150 pounds at the rate of 220 feet per minute.

Rule

Multiply the area of the piston in square inches by the average force of the steam in pounds and by the velocity of the piston in feet per minute; divide the product by thirty-three thousand, and 7/10 of the quotient equal the effective power.