[ 35' ] 



quantity on Mount Blanc, barometer i6, fljould be ,309, for as 

 827,7 (= "28,77^ )isto2 56(= 16^ ) ^'^ '^ '° ^° 0^3091 which 

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



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

 -and partly through that of affinity and of eledricity, fo they fe- 

 parate from it, fometimes from a diminution of that degree of heat 

 which they poffeffed in their nafcent ftate, fometimes from a di- 

 minution of affinity, and fometimes from an alteration in their 

 eledtrical flate. 



In their firft degree of coalefcence wlien feparated from air, they 

 form aggregates of exceeding minute particles, feparated from air 

 by the diminution of affinity, and alfo from each other by elec- 

 trical atraofpheres ; thefe aggregates are of equal, and often lower, 

 fpecific gravity, than the air in which they are formed, and yet 

 are vifible by reafon of their opacity ; when near the earth they 

 are calledyo^^j, mifls or haze^ (which differ only in denfxty) and 

 when at greater heights, clouds. 



Vapours iffuing from water or moifture warmer than the air 

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

 meafure difmiffed ; hence the morning mills obferved in fummer 

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



X X 2 contrary 



