SECT, vi.] CONDENSING ENGINES. 191 



This is equivalent to 6'25 Ibs. on a circular inch : the excess of force of the steam 

 in the boiler is a full compensation for some other causes of loss of power. 



403. RULE. Multiply 6 - 25 times the square of the diameter of the piston, 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 quotient will be the number of horse power. 



Example. If the diameter of a cylinder be 32 inches, and half the velocity be 

 110 feet per minute; then 6-25 x 32 2 x 110 = 704000 Ibs. raised 1 foot per 

 minute, or 



704000 



330QQ- = 21 J horse power. 



404. As the quantity of water required for the boiler is found by art. 401. the 

 quantity of fuel is easily found from art. 190. In the example of the preceding 



H x (4) 2 x -7854 x 110 ,, c 



article we have 1480 " = water per minute, or 29'4 



feet per hour ; consequently 29'4 x 8'22 = 246 Ibs. of caking coal per hour ; or 



-sj- = 11*7 Ibs. per horse power. 



For the proportions of the boiler, see Sect. in. ; and for the beams and other 

 parts, in reference to strength, see Sect. vin. 



405. For raising water this species of atmospheric engine is admirably 

 adapted ; it can be constructed without difficulty by ordinary workmen ; and for 

 water works, drainage, irrigation, canals, and other cases where water is required in 

 considerable quantities, it is an economical mode of obtaining power. 



STEAM PRESSURE ENGINES. 



406. Boulton and Watt's single engine. The essential parts and operation of a 

 single engine having been described, (art. 386.) we have only to show the con- 

 struction as it regards effect. Fig. 4. Plate v. shows a section of the cylinder C, 

 condenser B, and air pump A, of a single engine, arranged as is most convenient 

 for exhibiting the parts. The steam enters from the boiler to the cylinder by the 

 pipe S, through the valve c ; and presses down the piston P, which is supposed to 

 be taken at the time of its descent : the steam below it goes into the condenser, 

 and is condensed by the jet which plays into it. The air pump bucket p is 

 descending in the air and vapour which the pump had received from the condenser 

 during the previous ascent. When the piston is at the bottom of the cylinder, a 



