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 I.). As these two 



PLATE L 



DIAGRAM OF THE WINDS. 



currents of air are constantly flowing 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 we^re 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. 



