SECT, vi.] CONDENSING ENGINES. 197 



The force of the steam being generally 35 inches of mercury in the boiler, the 

 temperature of the uncondensed steam 120, and its force 3'7 inches ; hence, 

 35 x -632 -- 3 - 7 = 18'42 inches, or 7'1 Ibs. per circular inch, for the mean 

 effective pressure on the piston. l 



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

 square of its diameter in inches, and that product by the velocity in feet per 

 minute, the result will be the effective power in Ibs. raised 1 foot high per 

 minute. 



To find the horse power, divide the result by 33000. 



Example. The diameter of the cylinder of a double engine being 24 inches, 

 the length of the stroke 5 feet, the number of strokes per minute 21|, and the 

 force of the steam in the boiler 35 inches of mercury, or 5 inches above the 

 pressure of the atmosphere ; required its power. 



The velocity is 2 x 5 x 21i = 215 feet per minute, and the mean effective 

 pressure on the piston will be 7*1 Ibs. per circular inch; therefore 7'1 x 24 2 x 215 

 = 879264 Ibs. raised 1 foot high per minute, or 



879264 



330QU = 2o'64 horse power. 



The nominal power of this engine would be only 20 horse power by Boulton and 

 Watt's mode of calculation, but it will be found that the nominal and real power 

 nearly agree when the steam acts expansively (art. 422). 

 The water required for the above engine (art. 415.) will be, 



1-1 X 2 ~ 54 X 215 = ' 50 cubic feet P er minute, or 30 cubic feet per hour ; 



and (art. 190.) 30 x 8'22 = 246'6 Ibs. of caking coal, or ~ = 9'25 Ibs. per hour 



i ) O4 



for each horse power. * 



When an engine is of less than 10 horse power, the consumption of fuel will be 

 greater per horse power about in the ratio given in art. 221. 



418. This engine is applicable to every purpose for which a stationary engine 

 is adapted, and it is only in cases where water is procured with difficulty that it 

 is not applied : it has also lately been brought into use as a moving agent in 

 steam vessels. (See Sect, x.) When the steam acts expansively, the power is 

 obtained with a smaller quantity of fuel, and to save fuel is a great object in 

 every application of steam power. 



1 This is 9-05 Ibs. per square inch. 



2 Mr. Watt states to the effect that 8'7 Ibs. is the quantity equivalent to 1 horse power, but no 

 doubt he means when working expansively. Notes on Robison, vol. ii. p. 145. 



