STATE OF VAPOUR SUBSISTING IN THE ATMOSPHERE. 245 



Thus in the example laft quoted, the quantity of vapour in 

 a cubic foot at Geneva being 10 grains, barometer 28,77, the 

 quantity on Mount B lanc, barometer 16, fliould be ,309, for 

 as 827,7 (:= 28,77 a ) is to 256 (=T6 a ) fo is 10 to 0,309, 

 which differs from the truth by only 0,391 of a grain. 



As vapours unite to air, partly through the agency of heat, Separation of 

 and partly through that of affinity and of elearicity, fo they fe- 2^ a r n ^ f " r 

 parate from it, foinetimes from a diminution of that degree of heat and of 

 heat which they poffeffed in their nafcent ftate, fometimes from eleftricit y« 

 a diminution of affinity, and fometimes from an alteration in 

 their electrical ftate. 



In their firft degree of coalefcence when feparated from air, Firft coalefcence 

 they form aggregrates of exceeding minute particles, feparated ° or ^foudsT * 

 from air by the diminution of affinity, and alfo from each other 

 by eleclrical atmofpheres ; thefe aggregates are of equal, and 

 often lower, fpecific gravity, than the air in which tfjey are 

 formed, and yet are vifible by reafon of their opacity ; when 

 near the earth they are called foggs, mijls or haze, (which differ 

 only in denfity) and when at greater heights, clouds. 



Vapours ifTuing from water or moifture warmer than the air Statement of the 

 to which they unite, are foon cooled by it and thence in great manner in which 

 meafure difmifled ; hence the morning mifts obferved in fum- f orm€ d, 

 mer and the winter mifts of the colder regions ; evening mifts 

 on the contrary proceed from the fuperfatu ration of air with 

 vapours previoufly diflblved, arifing from the fupervening de* 

 creafed temperature. 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 be- 

 fore the point of faturation can be attained, their affinity to the 

 portion of air to which they are united is weakened, and thence 

 exceeded by the unincumbered affinity of the fuperior ftrata, 

 and this happens fucceflively on to the higher regions ; but 

 with diminiftied activity, by reafon of the diminished denfity 

 of the higher ftrata, until their ulterior progrefs is checked by 

 faturation ; but as they are ftill continually recruited from be- 

 low, their quantity is at laft fo far increafed that they coalefce 

 into clouds. Here the procefs recommences, for from the fur- 

 face of thefe clouds a frelh 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 



