150 MR W. J. MAGQUORN RANKINE ON THE DENSITY OF STEAM. 



under a constant pressure, and at a constant temperature, it takes the following 

 form : — 



J^ = -^(V-t') (8.) 



where 



J denotes Joule's equivalent, or 772 foot-pounds per British unit of heat 



(a degree of Fahrenheit in a pound of liquid water) ; 

 h. The heat which disappears during the evaporation of 1 lb. of the liquid ; 



that is, its latent heat of evaporation in British units : — 

 ■r, The absolute temperature (= temperature on Fahrenheit's scale + 46l°2 



Fahr.). 

 P, The pressure under which the evaporation takes place. 

 V, The volume of 1 lb. of the vapour. 

 V, The volume of 1 lb. of the liquid. 

 As the latent heat of evaporation of various fluids is much more accurately 

 known by direct experiment than the volume or density of their vapours, the 

 most useful form in which the equation (8.) can be put, is that of a formula for 

 computing the volume of a vapour from its latent heat, viz. : — 



V=v + 'i (9.) 



T 



dr 



8. Results of this formula were calculated by Messrs Joule and Thomson, 

 and by Professor Clausius for steam, and showed, as had been expected, a 

 greater density and less volume than the law of the perfectly gaseous condition 

 would give. Some results of the same kind, and leading to the same conclusion, 

 were also computed by me and published in the " Philosophical Transactions" 

 for 1853-54 But for some years no attempt was made by any one to make a 

 table for practical use of the volumes of steam from equation (9.) ; because the 

 scientific world were in daily expectation of the publication of direct experi- 

 mental researches on that subject by M. Regnault. 



9. At length, in the spring of 1 855, having occasion to deliver, to the class of 

 my predecessor. Professor Gordon, a course of lectures on the mechanical action 

 of heat, and finding it necessary to provide the students with a practical table 

 of densities of steam founded on a more trustworthy basis than the assumption 

 of the laws of the perfectly gaseous condition, I ventured upon the step of pre- 

 paring a table of the densities of steam for every eighteenth degree of Fahren- 

 heit's scale, from 86° to 410° inclusive, with the logarithms of those densities and 

 their differences, arranged so as to enable the densities for intermediate temper- 

 atures to be calculated by interpolation. Those tables were published in a litho- 

 graphed abstractof the course of lectures before mentioned, which is now out of 

 print. The same tables, however, have since been revised, and extended to ever}' 



