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. 
cm-rents of air are constantly flowing from the poles toward the 
equator, we are safe in assuming that the air which they keep in 
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. 
