SECT, vi.] CONDENSING ENGINES. 201 



the capacity of the large cylinder should be 1'82 times the small one ; therefore 

 2'3 x log. 1-82 = '575 : and as each inch of mercury is equivalent to - 385 Ibs. on 

 a circular inch, 120 x -385 = 46'2 Ibs., and 4G'2 x -575 = 26'56 Ibs. on a circular 

 inch, for the mean pressure collected at the small piston. 



428. RULE. The mean pressure being found as above, let it be multiplied by 

 the square of the diameter of the small cylinder in inches, and by the velocity of 

 the small piston in feet per minute, the result will be the power in Ibs. raised 1 

 foot per minute. 



Divide by 33000 for the horse power. 



Example. If the force of the steam be 120 inches of mercury, the diameter of 

 the small cylinder 11 inches, and the velocity of its piston 160 feet per minute, 

 then the mean pressure is 26'56 Ibs. ; and 26'56 x II 2 x 160 = 514200 Ibs. raised 

 1 foot per minute, or 



514200 



33000" = 2 norse P ower - 



429. The quantity of steam required per minute will be equal to the area of 

 the small cylinder in feet multiplied by the velocity ; and the quantity of water 

 will be found by dividing by the volume which the steam from a cubic foot of 

 water occupies, when of the force it is in the boiler, allowing one-tenth for waste. 

 In the above example it is, 



- = -242 cubic feet of water per minute, or 14'52 per hour. 



The fuel will therefore be 14'52 x 8*22 = 119'35 Ibs. of caking coal per hour, or 

 15 =7-7 Ibs. of coal per hour for each horse power. 



Comparing this with art. 422. we find there is no advantage in using two 

 cylinders as regards economy of fuel. 



430. The effects that may be obtained by engines of different species have 

 now been reduced for the first time to definite measures, and their proportions 

 referred to scientific principles. I have in these two sections endeavoured to render 

 assistance to the practical engineer in as condensed and easy a form as possible, and 

 yet with the minute circumstances in detail which are susceptible of variation by 

 improvement in action or construction. He will see that the sum of the particulars 

 must be near the truth, and the circumstances which increase or diminish any one 

 of them must be apparent, or easily known by a reference to the article where it is 

 investigated ; and if he will be careful to distinguish actual practice from preten- 

 sion, he will find that science and practice go hand in hand, the one supporting the 

 conclusions of the other. It is an undeniable proposition, that the ultimate bearing 



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