70 



THE PHYSICAL GEOGRAPHY OF THE SEA. 



as steady and as constant as the currents of the Mississippi River 



always moving in the same direction (Plate L). As these two 



PLATE L 



DIAGRAM OF THE WINDS. 



currents of air are constantly flowin- from the poles toward the 

 equator, we are safe in assuming that the air which they keep m 

 motion must return by some channel to the place near the poles 

 whence it came in order to supply the trades. If this were not 

 so these winds would soon exhaust the polar regions of atmos- 

 phere, and pile it up about the equator, and then cease to blow 

 for the want of air to make more wind of. 



