The Origin of Trade Winds. 113 



ences the air and ocean Avould be stagnant. There is a constant 

 interchange of atmosphere between the equator and the poles. 

 Cool air from the north blows toward the equator, first in a 

 southwesterl}^, then in a westerly direction, crossing the Atlantic 

 about the tropic of Cancer. Cool air from the south blows in a 

 northwesterly and westerly direction, and crosses the Atlantic 

 near the equator. The difference of solar accession between the 

 equator and the poles gives the northward and southward mo- 

 tion to these currents; the revolution of the earth on its axis 

 gives the westerly motion. 



Tliese air currents are the great trade winds which wafted 

 Columbus across the Atlantic and Magellan across the Pacific. 

 The trade winds of the northern Atlantic are about 20° in width 

 from north to south ; those of the southern Atlantic are not 

 quite so wide. These winds oscillate northward in August and 

 southward in February, following the sun. Between the trade 

 winds of the north and the trade winds of the south there is a 

 zone of calm. 



While the winds blow over the land as well as over the ocean, 

 their movements, interrupted by hills and mountains and af- 

 fected by temperature, lose that broad sweep and uniformity so 

 characteristic of the ocean. 



Return currents of warm air blow across the ocean from the 

 torrid zone toward the northeast in the northern Atlantic, and 

 toward the southeast in the southern Atlantic. The trade winds, 

 or equatorial currents, blow around the world from east to west ; 

 the polar currents blow from west to east. 



The great ocean currents follow the same general courses as 

 the wind system. Their movements are initiated b}^ differences 

 in density, caused chiefly by temperature and by evaporation ; 

 yet the larger part of the motive power is derived from the 

 wind. These movements have been ascertained by years of 

 observation on vessels in every ocean, sea and gulf, by the cumu- 

 lative evidence of drifting objects, some of which have had their 

 influence on the spread of vegetal and animal life and even 

 civilization itself, and by the researches of scientific exploring 

 expeditions to polar regions and remote islands. These oceanic 

 movements are as well understood as those of the great atmos- 

 pheric ocean above us. 



When water has acquired its movement, the configuration of 

 the bottom of the ocean and of the shore line, the rotation of the 



16— Nat. Geog. Mag., vol. V, 1893. 



