RULES FOR CALCULATING THE STEAM ENGINE. 225 



The preceding rules determine the mean pressure, which is equivalent to the entire action 

 of the steam. To get the mean effective pressure it must be diminished by an estimated 

 amount of the resistance and friction of the parts of the engine ; and for condensing engines 

 an addition of about 13 Tbs. per square inch must be added as the pressure acquired by the 

 partial vacuum. 



POWER. 



24. To find the power of an engine. 



-f 

 RULE. Multiply double the length of stroke by the number of strokes per minute, and we 



get the velocity of the piston per minute. 



If the engine works expansively, find the mean effective pressure by the foregoing rules. 



Multiply the square of the cylinder's diameter in inches by the mean effective pressure on 

 the piston in Tbs. per square inch, and by the velocity of the piston ; point off three figures 

 and divide the product by 42, and the quotient will express the number of horses' power. (197.) 



Example. Let the diameter of the cylinder be 36 inches, the length of stroke 4 feet, the 

 number of strokes per minute 24, and the mean effective pressure on the piston 4 Tbs. 

 per square inch. 



8 feet x 24 = 192 feet per minute 

 Diameter 36 inches 

 36 



216 

 108 



1296 

 Mean pressure 4 Tbs. 



5184 

 Velocity 192 feet 



10368 

 46656 



5184 



42) 995)328 (23'7 horse power. 



84 



155 



126 



293 



25. To find the quantity of water required for steam. 



RULE. Find the volume of steam from a cubic foot of water by rule No. 2. 



Multiply the square of the diameter of the cylinder in inches by half the velocity of the 

 piston in inches, or six times the velocity in feet per minute ; cut off two figures from the 

 right hand, divide by 11, and the quotient will express the cubic feet of steam effectively 



