1836.] of the Wet -bulb Hygrometer. 407 



own formula was merely an empiric one formed to represent the 

 experimental data of Benares and those of Gay Lussac in the most 

 ready manner, expressing the depressions in terms of the temperature 

 of the air : the former increasing geometrically with arithmetical 



/ 1*275 



increments of the latter, I found d = — — - ; hut this does not cor- 

 respond at all with the higher depressions now ascertained experi- 

 mentally, though it suits those of the former series. We may, there- 

 fore, reject it without further regard : nor need we pause to consider 

 Berzelius' more simple rule, founded, he says, on the experiments of 

 August, Bonenberger and others, viz. that the temperature of the 

 wet-bulb is always an arithmetical mean between that of the air and 

 the dew point, or t" = 2? — t, which, except at certain points of the 

 scale, is utterly erroneous. 



We now come to Professor Apjohn's formula, which will be found 

 not to differ essentially from those of Leslie or Gay Lussac It is 

 /" = f' — m d (at 30 inches pressure) where m is a co-efficient as usual 

 " depending upon the specific heat of air, and the caloric of elasticity 

 of its included vapour," of which the arithmetical value deduced from 



received data is .01149 or the equivalent vulgar fraction -r=- a ^ *"° 



Farh. Now in the case of extreme dryness assumed for our compa- 

 rison,/" = 0; therefore d = 87/'; an expression entirely agreeing in 

 form with Leslie's, but rather smaller in amount, and more nearly, as 

 will be seen, in accordance with the experiments of Tables II and III. 



Dr. Hudson arrives, from different premises, at nearly the same 

 method as Professor Apjohn*. He calculates a column of the " rela- 

 tive quantities of heat (Q) necessary to supply vapour of saturation to 

 dry air at each degree of wet-bulb temperature, V , and then finding 

 from experiment at one point (t, =61°) the actual depression (51.124 

 Apjohn), the depressions at other degrees he assumes to be direct 

 proportionals, or Q (at 61°) : Q' :: 51.124 ; D. 



Now it is evident that in this equation, as in most of the pre- 

 ceding, Q (whence D is directly derived) necessarily depends on 

 the aqueous tension,/', affected by the indispensable co-efficient 

 of the latent heat of water, vapour and air, or as Dr. Hudson 



deduces from Despretz's values, Q = L I ' For 



448 +t 



* Phil. Mag. Oct. 1835, p. 257. 



t If the theory which makes the sum of the latent and thermo metric heat for 

 gaseous hodies a constant quantity be. correct, Dr. Hudson's expression does 



