For our present purpose we shall assume that the diameter of the pulley (A´) is 4 inches; the length of the lever (C), 3 feet; the speed of the shaft (A) and the pulley, 210 revolutions per minute; and the weight 600 pounds.
Now proceed as follows:
(1) Multiply the diameter of the pulley (A´)[p. 143] (4 inches) by 3.1416, and this will give the circumference 12.5664 inches; or, 1.0472 feet.
(2) Multiply this product (1.0472) by the revolutions per minute. 1.0472 × 210 = 219.912. This equals the speed of the periphery of the pulley.
(3) The next step is to get the length of the lever (C) from the center of the shaft (A) to the point from which the weights are suspended, and divide this by one-half of the diameter of the pulley (A´). 36" ÷ 2" = 18", or 1 1/2 feet. This is the leverage.
(4) Then multiply the weight in pounds by the leverage. 600 × 1 1/2 = 900.
(5) Next multiply this product (900) by the speed, 900 × 219.912 = 197,920.8, which means foot pounds.
(6) As each horse-power has 33,000 foot pounds, the last product should be divided by this figure, and we have 197,920.8 ÷ 33,000 = 5.99 H. P.
The Foot Measure.—How long is a foot, and what is it determined by? It is an arbitrary measure. The human foot is the basis of the measurement. But what is the length of a man's foot? It varied in different countries from 9 to 21 inches.
In England, in early days, it was defined as a measure of length consisting of 12 inches, or 36 barleycorns laid end to end. But barleycorns differ in length as well as the human foot, so the[p. 144] standard adopted is without any real foundation or reason.