1836.] of the Wet-bulb Hygrometer. 405 



degree marked by his hygrometer, which is equal to the 2880th part 

 for each degree of depression by the common thermometer. 



Now p (Barometric height) may be substituted for the weight of 

 the air, and/' for the saturation weight of vapour at t' : therefore by 



d x 2880 d 



the above data/' will be = <jq- ^ "qq~ or ' ( as d is the object 



sought) d (or D) = 96/', at the pressure 30. 



This simple enunciation, making D in the direct ratio of/', is unduly 

 criticized by M. Anderson in his elaborate treatise on hygrometry 

 in Brewster's Encyclopedia ; but while in reality it will be found closely 

 to agree with the experimental data, and with the subsequent formulae 

 of others, the new expression deduced from " the laborious investiga- 

 tions" of the critic, turns out to be wholly at variance with experiment, 

 except accidentally at the temperature of the single trial he has him- 

 self recorded : his formula (omitting the correction for the barometer) 



is D _ ( 36 J (f — f'J which, when/" = 0, is convertible into 



D=/X36— R 



making the depression depend on the tension at t, instead of at t'. 



M. Gat Lussac's memoir should, I fancy, precede Mr. Anderson's. 

 It was written in 1815, though not published until 1822. The rati- 

 onale of his formula is explained in these words : — 



" Le froid produit (par l'evaporation) est a son maximum lorsque le 

 calorique absorbe par la vapeur est egal a celui que perd l'air pour 

 se mettre en equilibre de temperature et de pression avec elle, plus a 

 celui verse sur la surface evaporante par les corps environnans ; mais 

 la quantite de ce dernier, lorsque le froid produit n'est que de quelques 

 degres, esttres petite en comparaison de l'autre, et peut etre negligee." 

 If, therefore, on one side the latent heat of vapour (I) and its density 

 (8) be combined with its weight (f'J ; these should counterbalance the 

 weight of air (p — /') combined with its capacity (cj and the number 

 of degrees cooled (D or t — V) ; that is,/' s I = (p — /') (t — t') c 

 or, at 30 inches, /' X-625 X 960 = 30—/' X d X.2669 and 



30—/' ' 

 depending as before on/'. With dry air, the divisor in this equation 

 should, I imagine, lose — /' altogether, which would elicit the 

 value oid, = 74.9 /' ; a value lower than Leslie's, but almost exactly 

 agreeing with M. Gay Lussac's own experiments detailed in Table I. 

 Captain Herbert's formula was founded on the proposition that 

 " when the equilibrium or stationary point of the wet-bulb is attained, 



