JNTRODUCTTONTi ^gi, 



and fometimes as high as 91. The annual mean 

 temperature cannot, therefore, either much exceed 

 or fall much fliort of 80, as indicated by the wells. 

 Pbilof. Tranf, 



That heat and cold very much depend on the 

 clearnefs or darknefs of the iky, is attefted by facl \ 

 for the winter gold in South America is very fliarp^ 

 becaufe the atmofphere is loaded with clouds and icy 

 particles, which intercept the rays of the fun. At 

 Lima, in latitude 12° fouth, where the iky is never 

 free from vapours, the heat is moderate; but at the 

 diftance of a few miles, the fky is more ferene, and 

 there the heat is greater : And at Carthagena, in 

 latitude ii"" north, the heat on this account is 

 intolerable. 



The different degrees of heat and cold in different 

 places, depend, in a very great meafure, upon the 

 accidents of fituation, with regard to mountains or 

 valleys, rivers, feas, and wood, and the nature of 

 the foil. Mountains, efpecially when they are lofty 

 and covered with fnow, greatly help to chill the air, 

 by the winds which come over them, and v/hich blov/ 

 in eddies through the levels beyond. 



Mountains fometimes turning a concave fide to- 

 wards the fun, have the eiTed of a burning mirror, 

 on the fubjed plain ; and the like efiecl may fome- 

 times be had from the concave or convex parts of 

 clouds, either by refradion or refiedion : And fome 

 even take thefe to be fufficient to kindle the exhala- 

 tions lodged in the air, and produce thunder and 

 lightning. 



