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.