Mr. W. J. M. Rankine on the Mechanical Action of Heat. 173 



(the ratio of dilatation of a perfect gas from 0° to 100^ Centi- 

 grade), the result gives for the volume of steam of saturation at 

 100° Centigrade as compared with tliat of water at 



4°-l 1695-72, . . . (37) 



and for its density . 0-00058972. 



The agreement of those results with the known volume and 

 density of steam is sufficiently close to show, that at pressures 

 less than one atmosphere it may be regarded as a gas sensibly 

 pei-fect ; from which it may be concluded, that in the absence of 

 more precise data, the errors arising from treating it as a perfect 

 gas at such higher pressures as occm- in practice, will not be of 

 much importance*. 



Representing, then, by v the volume of unity of weight of 

 water at 4°-l Centigrade, that of unity of weight of steam at 

 any pressure and temperature will be given by the formula 



,- 1696?;ot t .oo\ 



^=-(;r"p' ^ ^ 



CT representing the number of units of weight per unit of area 

 in the pressure of one atmosphere, and (t) the absolute tempe- 

 rature at which the pressure of saturation is one atmosphere ; 

 being for the Centigrade scale 374!°-6, and for Fahrenheit's 

 scale 674°-28. 



Th^ mechanical action of unity of weight of steam at the 

 temperature t and pressure P, during its entrance into a cylinder, 

 before it is permitted to expand, is represented by the product 

 of its pressure and volume, or by 



PV=i^.T (39) 



The coefficient — r-r — represents a certain depth of fall per 

 degree of absolute temperature, and is the same with the coeffi- 

 cient rr^ already referred to. By taking the following values 

 CniM 



of the factors, 



?; = 0-016 cubic foot per pound avoirdupois, 

 cj-=:2117 pounds avoirdupois per square foot, 



we find this coefficient to be 



153-35 feet = 46-74 metres per Centigrade degree,"! 



85-19 feet per degree of Fahrenheit ; J 



this determination may be considered correct to about jg^'u o P^"^* 



* These remarks arc applicable to the calculation of the power of steam- 

 engines uuly. 



