296 Progress of Foreign Science. 



Temperature of the Depression of temperature pro- 



dry air at the pres- duced by evaporation below 



sure of '29.9 inches. the temp, of the air. 



14° C. 10.44° C. 



15 10.82 



16 11.20 



17 11.58 



18 11.96 



19 12.34 



20 12.73 



21 13.12 



22 13.51 



23 13.90 



24 14.30 



25 14.70 



The heat given up by the air, during evaporation, depending 

 evidently on its density, it follows, that, all other things being 

 equal, the cold produced ought to increase as the density 

 diminishes. We have hitherto supposed that the air was per- 

 fectly dried ; but, if we take it in the ordinary hygrometric state, 

 the cold produced by evaporation will not be so considerable, 

 and it will be even null, where the air is saturated with humidity. 

 The cold is relative to the quantity of water which the air can 

 suflPer to pass into the state of vapour ; but this quantity is not 

 immediately known, by that already contained in the air, before 

 it arrives at the moist surface. Suppose, in fact, that the tem- 

 perature of the air is 10° C, and that it is half saturated with 

 humidity ; suppose, further, that the cold produced amounts to 

 4°, it is evident that, at this term, the air which was half satu- 

 rated with moisture at 10°, will be more highly so on account 

 of the cooling which it has experienced, and that the quantity 

 of water which can evaporate, is precisely equal to what the air 

 wants at the temperature of 10° — 4°=:6°, in order to be sa- 

 turated. 



" In general, we may succeed in knowing the hygrometric 

 state of the air, according to the cold produced by evaporation ; 

 but as this cold is variable with the pressure of the air, its 

 temperature, its degree of humidity, we would require very ex- 

 tensive tables to determine it with exactness. 1 was willing 

 to undertake this labour, repeating ray experiments on the cold 

 produced by evaporation, and making new ones; but I have 

 been disheartened by its length, as well as the want of sufficient 

 data, and especially by the consideration that the ingenious 

 process of Leroi was susceptible of a more easy application, and 

 that in the actual state of physics, it was much preferable." 

 We heartily concur in this preference of M. Gay-Lussac, which 

 brings a strong additional argument in favour of Mr. Daniell's 

 hygrometer, founded on the principle of Leroi, and against Mr. 



