PNEUMATICS. 295 



399. The sea is preserved of a moderate tempe- 

 rature by the statical principle which makes the 

 heavier columns of a fluid displace the lighter. A 

 more uniform temperature is thus given to the sea, 

 which communicates itself to the air incumbent on 

 it, and to that on every side. 



400. Conversely, the effect of great and unbro 

 ken continents, is favourable to the extremes of 

 heat or of cold. 



The constitution of the surface may' tend to increase 

 and sometimes to diminish this effect. High moun- 

 tains especially, if covered with snow, may enforce 

 the rigour of a cold climate, or temper the heats of 

 a warm one. 



Forests tend to increase the cold, by preventing the 

 sun's rays from striking on the ground. Evapora- 

 tion produces cold ; and marshes and lakes are there- 

 fore favourable to the severity of the weather. The 

 congelation of water produces heat, and moderates 

 the cold ; the melting of ice, on the other hand, in- 

 creases the capacity for heat, and so produces cold. 



401. Height above the level of the sea, causes a 

 diminution of heat at the constant rate of 1 for 

 270 feet nearly, when not far from the surface of 

 the earth. 



It 



