180 Mr. W. J. M, Rankine on the Mechanical Action of Heat. 



(31.) As an illustration, I shall calculate the maximumnseful 

 effect of one pound, and of one cubic foot of water, in a Cornish 

 double acting engine, in the circumstances taken by M. de Pam- 

 bour as an example for that kind of engine ; that is to say, — 



Clearance one-twentieth of the stroke, or c = -^, 



Ai. Per square ft. 



Resistance not depending on the useful load, /= 72 lbs. 



Pressure of condensation, P3=5761bs. 



Consequently to give the maximum useful effect, 



^2=^3+/ =648 lbs. 



Total pressure of the steam when first admitted, P, = 7300 lbs. 

 Volume of lib. of steam Vi = 8-7835 cubic feet. 

 Therefore PiVi = 63234 lbs. raised one foot. 



P, 7200 , ,, 



:^ = - , and consequently 

 Pg d4o 



/P \t 

 Expansion toproducethemaximumusefuleffects= ( pM = 7*877. 



Space traversed by the piston during the action of one pound of 



steam, =V,(1 -c)s=65-886 cubic feet. 



Gross effect of one pound of steam, in pounds raised one foot high, 



= PiVi(7-6s-"^-^)-P3V,(l-c>. . =112004 



Deduct for resistance of engine when unloaded"! _ a-jaa 



/Vj(i-c> . . . . \ - 



Effect of one pound of steam m overcommg re- 1 107260 



sistance depending on useful load ... J 

 This being multiplied by 62i, gives for the effect 1 



of one cubic foot of water evaporated, in pounds V 6,703,750 



raised one foot J 



It is here necessary to observe, that M. de Pambour distin- 

 guishes the useful resistance into two parts, the resistance of the 

 useful load independently of the engine, and the increase in the 

 resistance of the engine, arising from the former resistance, and 

 found by multiplying it by a constant fraction, which he calls S. 

 In calculating the net useful effect, he takes into account the 

 foi-mer portion of the resistance only ; consequently. 

 Net useful effect as defined by M. de Pambour 



_ Gross effect —/Vj(l—c)s ,_,. 



^ 1 + 6 ^^^> 



The value of S, for double acting steam-engines generally, is 

 considered by M. de Pambour to be | ; consequently, to reduce 



