176 Mr. W. J. M. Raukiue on the Mechanical Action of Heat. 



while for engines in which the initial pressure lies between four 

 and eight atmospheres, the suitable value is 



5 

 ^=6- 

 The utmost error which can arise from using these exponents 

 is about y^^ of the whole power of the engine, and that only in 

 extreme cases. 

 IMaking, therefore, /ox-- 



we obtain for the value of the expansive action of unity of weight 

 of steam, 



/>.P=P,v,,4,(,-(^)-^) 



(45) 



s being used to denote —-, or the ratio of the volumes occupied 



by steam at the end and at the beginning of the expansion 

 respectively. 



A table to facilitate the computation is given in the sequel. 



The gross mechanical action of unity of weight of steam on 

 one side of the ])iston is found by adding to the above quantity 

 the action of the steam before it begins to expand, or PiV,, and 

 is therefore 



''■^■(t^-t^*'"') ■•■(«) 



the values of the coefficients and exponent being 



(24.) The following deductions have to be made from the gross 

 action, in order to obtain the action effective in overcoming 

 resistance. 



First. For loss of power owing to a portion of the steam being 

 employed in filling steam-passages, and the space called the 

 clearance of the cylinder at one end. Let the bulk of steam so 

 employed be the fraction cSg of the space filled by steam at the 

 €nd of the expansion ; then the loss of power from this cause is 



PlCS2 = C5PiVi. 



