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 

 15'35lbs. 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' 3 5 multiplied by 

 2827, which amounts to 43394 Ibs., but as an allowance of 2lbs. 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 Ibs. ; 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 Ibs. by 252, the velocity in feet per minute, and 

 then dividing the product by 33,000 for the horses' power. 



