408 Experimental Researches on the Depressions [J uly, 



ordinary temperatures, Q on an average will be found = 50 f 



51 124 O 



and D is assumed from Apjohn's experiments = — ^ — 1.9 Q; 



25.9 

 so that by this formula (at 30 inches,) D = 98/', nearly; being a 

 little in excess of Leslie's original formula. This is attributable to 

 Apjohn's single experimental depression assumed as the basis of the 

 whole calculus being somewhat too great. 



It cannot be said after the preceding list, that the wet-bulb theory 

 has been neglected. On the contrary, it may be rather feared that 

 the researches of its earliest investigators, particularly those of Leslie 

 and Gay Lussac, have been neglected ; for it is certain that their 

 formula? are nearly as well adapted to the actual phenomena as any 

 that have been since suggested. This cannot be more strikingly 

 exemplified than in the accompanying diagram, (PI. XXI.) which 

 has been filled up from the preceding data. The abscissae represent 

 the temperatures (t), and the ordinates the maximum depressions in 

 dry air (D). The experimental determinations are shewn by dots*, and 

 the principal theoretical curves delineated, are distinguished by the 

 name of their authors. 



The following table also embraces a comparative view for every 

 ten degrees of temperature, the experimental entries being adapted 

 by interpolation from the observations before set forth. 



not seem open to objection. The volumes of air at different temperature 

 being as 448 + t directly : the densities are as 448 + t inversely; and for 

 n. / 660/ 



aDy other pressure _)!_ the density of air at t will be t-T". ™ 



30 ' 448 -j- t X 30 



22/ 



Further, allowing the atomic theory of volumes, the density 



448 4. t 



.625X22/ 

 of vapour at t will be = — • Compounding this expression with that 



4484" t 



of the latent heat of vapour at t which is 1168 — t (being 956 at 212°); we have 



as above the quantity of heat necessary for the vapour of saturation at t = 



1168 — t X 22 /' 



• — . The author has steered clear of what he considers the 



448 4* t 



disputed points, of the capacity of air and vapour for heat : but it may be reason- 

 ably doubted whether the assumption of the equality of / 4- £ be a whit more 

 tenable. 



* In the portion of the curve marked " Prinsep's experiments," both the un- 

 corrected and the corrected observations are entered ; the latter, distinguished 

 by a dotted line passing through them, are alone to be attended to. The cor- 

 rected places of the sulphuric acid experiments have been omitted, because they 

 are necessarily doubtful. The flexure of Gay Lussac's curve seems to be the 

 most suitable to experiment, were its ordinates a little increased. 



