MR W. J. MACQUORN RANKINE ON THE DENSITY OF STEAM. 151 



ninth degree of Fahrenheit, from 32° to 428°, and have been printed at the end of 

 a work, " On Prime Movers." An account of the original tables was read to the 

 British Association in 1855* 



10. In the unpubHshed paper before mentioned, as having been read to this 

 Society in 1854, the densities of the vapours of ether and bisulphuret of carbon, 

 under the pressure of one atmosphere, as computed by equation (9.), are shown 

 to agree exactly with those calculated from the chemical composition of those 

 vapours. 



11. The method of using equation (9.), to calculate the volume of one pound 

 of steam, is as follows: — 



I. Calculate the total heat of evaporation of steam from 32°, at a given tem- 

 perature T' on Fahrenheit's scale, by Regnault's well-known formula 10917 

 + 0-305 (r- 32°) (10.) 



II. From that total heat subtract the heat required to raise one lb. of water 

 from 32° to T° Fahr , viz., 



ly 3: 



'' cdT 



32° 



c being the specific heat of water ; the remainder will be the latent heat of evapo- 

 ration of one lb. of steam at T° ; that is to say, 



A = 1091-7 + 0-305 (T-32°)_/"^ cdT (11.) 



In computing the value of the integral in this formula, use has been made of 

 an empirical formula founded on M. Regnault's experiments on the specific heat 

 of water, as to which, see the " Transactions" of this Society for 1851, viz. : — 



"T 



/; 



cdT = T-T' + 0-000000103{(T-39°l)3-(T'-39°-l)3}. . . (11-A.) 



III. The absolute temperature is given by the formula, 



T = T + 46r-2Falir., (12.) 



IV. The value of t -^ is deduced from the following formula, first published 

 in the " Edinburgh Philosophical Journal" for July 1849 : — 



com. log. P = A - - - -2 ; . • • • . (13.) 



from which it follows that 



/B 2C\ „ . ^ 



T^ = 2-3026 P ^^ ^^' 

 dr 



* The reason for using 9" Fahr. as the interval of temperature is, that it is equal to 5° Centigrade 

 and to 4° Reaumur ; so that the tables can be applied with ease to any one of those three scales, 

 VOL. XXIII. PART I. 2 T 



