OK THE ELECTHIC COLUMN. 



247 



submitted in the same manner to the influence of tempera^ 

 ture. 



Art, III, The aqueous vapour^ i. e. the immediate product Aqueous va- 

 of evaporation, is therefore never concealed in the atmos- pe^/eptfJle?^ 

 phere ; and its quantity, in any part of the latter, can always the hygro- 

 be determined by the observation of the hygrometer dm^ the ™st"' 

 thermometer. This^wirf, produced by the evaporation that 

 never ceases on the surface of the vi^ater and of the land, Ascends, 

 being of a specific gravity less than that of air, constantly 

 ascends in the atmosphere, passing through its lower regions, °^f «i™'n^hei 

 where we do not find that it remains; it ought therefore to 

 accumulate in the higher parts. Now, as we ascend on 

 mountains, the hygrometer indicates less and less evaporated 

 water in the' transparent air, I shall soon answer the hy- 

 pothesis, already mentioned as having been imagined for 

 setting aside the conclusion which I have deduced from this 

 phenomenon, namely, a transmutation of thea^u^oti^ vapour \>y cenversion 

 into atmospheric air ; a conclusion however which will be"***''^* 

 found the ultimate result of this series of facts. 



Art. IV. Another phenomenon, which Mr. de Saussure 

 and myself have observed, proves, that dryness is still greater 

 in the region of the atmosphere above the highest moun- 

 tains, where it waa natural to suppose, and I supposed it at 

 first, that the aqueous vapour was accumulating. On plains 

 and small hills, moisture is increasing in the air after sun- 

 set ; and before we possessed our hygrometers, we had reason 

 to suppose, that it was the same upon high mountains, for 

 there also the grass becomes wet. This being the first com- 

 mon symptom of Tnow/i/rf observed after sunset, and even 

 before, was one of the arguments in favour of the idea, that 

 dew proceeds from the ground; but the hygrometer, that Dew 

 neglected inftrument, has shown it to be a phenomenon be- 

 longing to the physiology of plants, and not to meteorology, belongs to the 

 On high mountains, while the grass on the ground becomes ^^^l^ "^* *^ 

 tvet, the hygrometer being suspended at some height above 

 the ground, in some insulated spot where the air is free. Air on nooun. 

 shows an increase of dryness, which continues during the |?'"^. "^''^'^ 

 night. I have determined the cause of this phenomenon by 

 immediate observations ; it proceeds from the condensation 

 •f the columns of air, while the heat diminishes in them j 



whence - 



