SECT, vi.] CONDENSING ENGINES. 193 



Let the force in the boiler be denoted by - 1-000 



The effective pressure on the piston is less than the difference 



between the force of the steam in the boiler, and the 



resistance of the uncondensed steam, 



1. By the force producing the motion of the steam into 



the cylinder, (art. 154.) - -007 



2. By the cooling in the cylinder (art. 160.) and pipes, 



(art. 148.) - -038 



3. By the friction of the piston and loss by escape, 



(art. 474.) - -05 



4. By the force necessary to expel the steam through the 



passages - -007 



5. By the force required to open and close the valves, 



raise injection water, and the friction of the axes - '100 



6. By the steam being cut off before the end of the stroke '100 



7. By the power required to work the air pump, (art. 354.) '100 



402 



598 



The force of the steam in the boiler is commonly 35 inches of mercury ; that of 

 the uncondensed steam (temp. 120) is 3'7 inches ; hence, 35 x -598 = 20'93 

 inches, and 20'93 3*7 = 17-23, or 6*65 Ibs. is the mean effective pressure per 

 circular inch on the piston ; and when the steam in the boiler is of any other 

 force, the mean effective pressure may be determined in the same manner. 



409. RULE. Multiply the mean effective pressure on the piston by the 

 square of its diameter in inches, and by half the velocity in feet per minute, and 

 the product is the effective power in Ibs. raised 1 foot high per minute. 



Divide by 33000, and the result is the number of horse power. 



Example. Let the force of the steam in the boiler be 35 inches of mercury, 

 the diameter of the cylinder 48 inches, and half the velocity 135 feet per minute. 

 Then the mean pressure is 6'65 Ibs., and 6'65 x 48 2 x 135 = 2068416 Ibs. 

 raised 1 foot, or 



2068416 

 33000 = 63 horse power. 



The water required would be 



1-1 x 4* x -7854 x 135 , _ , . ,. 



~1497~~ ~ = cubic feet per minute, or 75 per hour; 



2n 



