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TiJOCGH clouds and a difpofition to rain frequently follow 

 the defcent of the 5, yet this defcent is not the immediate con- 

 fequence of either clouds or rain ; on the contrary the ? frequently 

 riles during rain. But the rarefadion of the atmofpherc which 

 produces the defcent of the ^, and which arifes from the remo- 

 val of the fuperior accumulation, is favourable to the produdion 

 of clouds, as a heavy atmofphere, though it fupports vapors 

 once formed, obftrudls evaporation ; when, therefore, its weight 

 is diminifhed, and evaporation encrcafed, it foon becomes fatu- 

 rated in the higher regions, and clouds are formed. But rain 

 feems to arife from a fubtradion of the elcdrical fluid, which, 

 when the air abounds with vapors, is eafily conduced to the 

 earth. In ferene and fettled weather the $ is generally high, 

 becaufe the greateft difturbances of the atmofphere are conneded 

 with its rarefied ftate, which is commonly pretty diflant when 

 the fuperior accumulation is confiderable. 



That the variations of the mercurial heights fhould be greater 

 at the level of the fea than at great elevations above that level 

 is very natural. For fuppofing the ? at the level of the fea to 

 fland at 30 inches, and at a certain elevation above that level at 

 25 inches, then if the weight of the atmofphere be diminiflied 

 ~ part, the ? at the level of the fea fhould fall — part of 

 30 inches = 0,3 of an inch, but that on the elevation fliould fall 

 ■~ of 25 inches =0,25 of an inch; buj: it has been obferved 

 that the variation on high mountains is beyond all proportion 

 fmaller than on the level of the fea, which proceeds from a 

 caufe hitherto unnoticed, vet deferving the greateft attention ; 

 namely, a property which they feem to poffefs of condenfing and 



accumulating 



