<g48 ^^ l*"*^ SLCCTIIC COLUMNS 



whence results, that the part of that aivt which, during the 

 day, rested on the summits of mountains, descending loweT» 

 is followed by the air which was higher before ; and this, 

 as long as the condensation continues in the lower parts, 

 descending from higher regions, and thus passing over the 

 summits in its way downwards, is found, in an increasing 

 degree, drier than that which rested on them in the day. 

 Theoriae of ^^^ y^ Among the atmospheric phenomena, that oi dew^ 

 commonly considered as very simple, has been long, and is 

 still now, an object of controversy among natural philoso- 

 phers, who have not attended to the latest experiments and 

 observations. The first and most plausible explanation 

 was, that the dew descended from the «ir, by the condpnsar 

 tion of the evaporated water spread in it, when heat dimir 

 uishes; but some experimental philosopher, finding that 

 this caus^ was not sufficipnt to explain all the circumstances 

 pf dpw, conceived the idea, which 1 have above mentioned, 

 that it ascends from the ground, bjecause this retains longer 

 the heat of the day, than the air above it ; which circum- 

 stance was considered a* \\\Qre'ds\ng evaporation : both parties 

 alleging in support of their opinion certain facts, which, 

 though not denied, were not decisive. During the naost 

 active time of this controversy, about 60 years ago, I madp 

 with my brother various kinds qf exppri^nent^ find observa^ 

 tions, which, by turns, favoured one or the other of these 

 hypotheses, but neither of thqm decisively ; and the ques- 

 tion would have remained for eyer jn suspense, had not hy- 

 grology and hygrornetry been pursued with the degree of 

 attention and labour, that Mr. de Saussure and myself have 

 bestowed upon them ; frpo) which the phenomenqn of dem 

 has iappeared under a new and quite different aspect, which 

 excludes both the above causes as fundamental in it, and 

 shows why neither pf them cpujd explajp its most essential 

 circumstances. 

 Agrefnicntof Art. VI. With respect to the experimental part, we have 

 nip^t«o7M Vo^'^ ^Sltern^ined, by djrect and unconcerted experiments, 

 de Saussure the effects produced on pur respective hygro^neterSt placed 

 in y mass of air, whjerein, the quantity of evaporated water 

 remaining the same, there was no change but in the degree 

 qf heat. We have made the same kind of experiments on, 



different 



•witi iheau- 



