«5^ 



On the Stale of Fafeur 



Table of the Ratios of tha Quatilit'ies of Vapour at different 

 barometrical H/ightSy the Quantity at the Surface of iht 

 Earth being given. 



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

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

 the quantity on Mount Blanc, barometer 16, (liould be "309 j 

 for as 827*7 (=: 28*77 ) is to 256 (= 16 ), fo is 10 to 0*309, 

 which difl'crs from the truth by only 0"39i of a grain. 



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

 and partly through that of affinity and of ele6tricity, fo they 

 feparate from it, fonietimcs from a diminution of that degree 

 of heat which they poffeffed in their nafcent ftate, fometimes 

 from a diminution of affinity, and fometimes from an altera- 

 tion in their electrical ftate. 



In their firft degree of coalefcence when feparated from 

 air, they form aggregates of exceeding minute particles, fe- 

 parated from air by the diminution of affinity, and alfo from 

 each other by eleiSirical atmofpheres : thefe aggregates are of 

 equal an<l often lower fpecific gravity than the air in which 

 they are formed, and yet are vifible by reafon of their opa- 

 city; when near the earth, they are cd\\e.A fogs, mifs, or 

 haze, (which differ only in denfity,) and when at greater 

 heights, clouds. 



Vapours ifliiing from v/ater or moidure warmer than the 

 air to which thev unite, are foon cooled by it, and tlunce in 

 great mcafure difmifled : hence the morning mifts obferved 

 in funimer and the winter mifts of the colder regions : even- 

 ing mifts, on the contrary, proceed from the fuperfaturatinn 

 pf iiir with vapours previoufly diflblvedj arifing from the fu- 



pervening 



