ITS TWO GREAT COURSES. 81 



mere temporary and partial deflections from this 

 grand course. The heated air at the equator rises 

 continually and flows in an upper current towards 

 the pole, getting gradually cooled on its way north. 

 That from the pole flows in an under current 

 towards the equator, getting gradually heated on its 

 way south. We speak only of the Northern 

 Hemisphere, for the sake of simplifying explanation, 

 the action of the great wind-current in the 

 Southern Hemisphere is precisely similar. 



But our broad simple statement about the upper 

 current from the equator, and the under current 

 from the pole, requires a slight modification, which 

 we thought it best not to mingle with the state- 

 ment itself. The heated air from the equator does 

 indeed commence to flow in an upper current, and 

 the cooled air from the pole in an under current ; 

 but, as the upper currents of air are speedily cooled 

 by exposure to space, and the under currents are 

 heated by contact with the earth's surface, they con- 

 stantly change places the lower current becoming 

 the upper, and vice versd. But they do not change 

 direction. The Equatorial Current ascends, rushes 

 north to a point about lat. 30, where, being suffi- 

 ciently cooled, it swoops down, and continues its 

 northward rush along the earth. At another point 

 the Polar Current quits the earth, and soaring up, 

 in consequence of its recently acquired heat, becomes 

 the upper current. This change in the two currents 

 takes place twice in their course. 



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