[ 3S^ I 



■contrary proceed from the fuperfaturation of air with vapours 

 previoufly diffolved, arifing from the fupervening decreafed tem- 

 perature. The inferior ftrata of the atmofphere are fcarce ever 

 fuperfaturated by vapours arifing from water or moifture warmer 

 than the air into which they afcend ; for before the point of fatu- 

 ration can be attained, their affinity to the portion of air to which 

 they are united is weakened, and thence exceeded by the unin- 

 cumbered affinity of the fuperior ftrata, and this happens fac- 

 ceilively on to the higher regions ; but with diminifhed aflivily, by 

 reafon of the diminifhed denfity of the higher ftrata, until their 

 ulterior progrefs is checked by faturation ; but as they are ftill 

 continually recruited from below, their quantity is at laft fo far 

 increafed that they coalefce into clouds. Here the procefs re- 

 commences, for from the furface of thefe clouds a frefh evaporation 

 often takes place, which, after fome progrefs, is again checked 

 in its turn, and clouds are formed at a fuperior height; thefe 

 again give room to a further evaporation, and a new ftage of clouds 

 is formed, until the procefs is at laft arretted by the intenfe cold 

 of the fuperior regions. But the mere cold of congelation is not 

 fufficient to arreft it, for Bouguer informs us that clouds are 

 formed 2500 feet above the lower line of congelation, and that 

 ice itfelf evaporates, though cooled, feveral degrees below the 

 freezing point, is well known. The diflance of the particles, 

 both of air and vapour, from each other, when fo far rarefied as 



they 



