B.H.P. = (S1 − S2) 3·14 × D x R / 33000
S1 = Reading in lbs. of spring balance No. 1.
S2 = Reading in lbs. of spring balance No. 2.
D = Diameter of fly-wheel and diameter of brake rope in feet.
R = Revolutions of fly-wheel per minute.
As 3·14 × D / 33000 will always remain the same for any given engine and gear, we may call that expression C; then the B.H.P. may be written—
B.H.P. = (S1 − S2) C R.