EXPLANATION OF THE PLATES.
387
MODE OF CALCULATING THE POWER OF AN ENGINE BY THE INDICATOR
CARDS.
The mean pressure on the piston, as proved by the indicator in diagram No. 1, is
15*35 lbs. upon each square inch of its area, and the area of the piston being 2827
inches, it follows that the total pressure on the piston is equal to 15*35 multiplied by
2827, which amounts to 43394 lbs., but as an allowance of 2 lbs. upon each square
inch area is made for the power necessary to overcome the friction of the engine
and to work the pumps, the effective pressure on the piston will be represented by
multiplying 15*35 — 2 = 13*35 by 2827, which gives 37740*4 lbs.; and as this pres
sure is maintained at a velocity of 252 feet per minute, (the piston making 21
double strokes of 6 feet each,) it follows that the power of the engine will be ex
pressed by multiplying 37740*4 lbs. by 252, the velocity in feet per minute, and
then dividing the product by 33,000 for the horses’ power.