512 SCIENTIFIC RECORD FOR 1883. 



Iii the condition of equilibrium this amount of heat will be conveyed 

 (to the wet bulb) from the quantity m of air with specific heat S that 

 flows past the wet-bulb thermometer ; therefore, 



m (t-t>) S=(Pi-Po)^ A 



or — 



Po=Pi-~ ff (t-t') .... (A) 



In this reasoning a small quantity has been neglected, which, how- 

 ever, as can be easily shown, is entirely negligible.* 



* In respect to the individual cases of deduction of this formula, as 

 given by August himself, and by Belli (Gorso di Jisica esperimentale ; 

 Chistoni, Formale psichrometrische, Zeitschrift /. Met., xvi, p. 81), I con- 

 sider it important to demonstrate that with all the accuracy demanded 

 by August and Belli, the above formula, as deduced by Maxwell, is 

 equally accurate. 



August (Pogg. Ann., bd. v, p. 77) considers that aqueous vapor is also 

 contained in the air that is flowing to the thermometer, and he intro- 

 duces air and vapor according to their weights into the formula. If, 

 then, p is the normal weight (at 0°C, and 760 mm ) of the unit volume of 

 air, we have — 



Weight of the dry air = „ t * P 



7G0 ' I -{-at 



Weight of the original aqueous vapor = J!l p g - 



700 1 + at 



Weight of the evaporated aqueous vapor = ^~^- p g 



760 ' l + ati 



Therefore, if the specific heat of aqueous vapor under constant press- 

 ure is fc, the above formula for the condition of equilibrium becomes — 



(P - Pl ) S (* -t') + Po( rJc(t-t') = ( Pl -po) A G 

 or — 



Let us assume ^J- = ^, as is certainly allowable [in the first term], and 

 we have — 



rlO-^-Tr.O*-*-*-* • • - (B) 



[We can estimate the accuracy of this approximation by the follow- 

 ing considerations] : in the most unfavorable cases ^ may equal 0.02; 

 Tcg 



( 1 — -^ ) is a constant, and equals — 0.26, and therefore in the most 

 unfavorable case 



^(\-lc^\ = -0.0052 



or the entire bracket equals 1.0052, and can therefore be put at unity, 

 and the formula remains as given in the above text. 



Belli really deduced his formula uader the same assumptions, it is 

 therefore, a priori certain that it will lead to the same results ; since, 



