of a New Hygrometer. 321 



so that, in similar circumstances, a double or triple number of 

 degrees indicate constantly a double or triple quantity of va- 

 pours existing in the atmosphere. 



The hygrometer of Saussure, the most perfect of all, pos- 

 sesses, in an eminent degree, the first quality. It is con- 

 stant in its indications, comparable, and of extreme sensibili- 

 ty ; but its variations are not proportional to those of the ab- 

 solute humidity of the atmosphere. For example, towards 

 extreme dryness, it moves nearly 3° for one unit of difference 

 in the tension of the vapour ; and, on the contrary, towards 

 extreme humidity, it moves only 1° for three units of difference 

 in the same tension. Thus, when the hygrometer marks 75°, 

 for example, at 0° of the centigrade thermometer, this does 

 not denote that the vapours in the air are the same, when at 

 15° of the thermometer it stands at 75°, but only that, in the 

 two cases, the ratio between the humidity of the air, and ex- 

 treme humidity at the same temperature, is the same. 



In order that we may deduce from the indications of the 

 hydrometer exact notions on these two last points, we must 

 take the assistance of tables formed from a numerous series of 

 delicate experiments. Such are the tables contained in the 

 Essai sur V Hygrometrie ; such also is the table constructed 

 from the experiments of M. Gay Lussac, which gives for the 

 temperature of 10° centig., the degree of the hair hygrometer, 

 when we know the tension of the aqueous vapour actually ex- 

 isting in the air, and vice versa. * 



From these considerations, we shall proceed to study the 

 results to which we may be led by employing the process 

 which I have indicated above. 



I plunge the ball of a delicate thermometer into sulphuric 

 ac id ;— I draw it out, giving it a slight shake, so that there 

 may remain around the ball only a thin film of adhering acid. 

 The thermometer rises immediately a certain number of de- 

 grees above that which it indicated before its immersion in 

 the acid ; then it immediately stops and begins to fall. I 

 suppose, however, that we have determined how many de- 

 grees it rises for extreme humidity at the same temperature 



• Biot, Traile dc Physique, torn. i. r>. 538. 

 VOL. III. NO. II. OCTOBER 1825. * 



