PNEUMATICS. 305 



Wind. 



415. The principal cause of those currents of air 

 to which we give the name of Winds, is the dis- 

 turbance of the equilibrium of the atmosphere by 

 the unequal distribution of heat. 



In order that an equilibrium may take place in an 

 elastic fluid, circumfused about a solid, to which it 

 gravitates, every level stratum of the fluid, that is, 

 every stratum which, when continued round, cuts 

 the directions of gravity every where at right angles, 

 should be of the same density, and therefore of the 

 same temperature. As this is not the case, the 

 equilibrium of the atmosphere is inconsistent with 

 the actual distribution of heat on the eartrTs sur- 

 face. 



The general tendency, in such circumstances, is for 

 the heavier columns to displace the lighter, and for 

 the air at the surface to move from the Poles to- 

 ward the Equator. The only supply for the air 

 thus constantly abstracted from the higher latitudes, 

 must be produced by a counter-current in the up- 

 per regions of the atmosphere, carrying back the 

 air from the Equator toward the Poles. The quan- 

 tity of air transported by these opposite currents, 

 is so nearly equal, that the average weight of the 

 air, as measured by the barometer, is the same in 

 all places of the earth. 

 VOL. I. U If 



