266 RELATIONS OF AIR AND WATER TO TEMPERATURE AND LIFE. 



sion between the equator and the poles gives the northward and south- 

 ward motion to these currents; the revolution of the earth on its axis 

 gives the westerly motion. 



These air currents are the great trade-winds which wafted Columbus 

 across the Atlantic and IMagellan across the I'acitic. The trade- winds 

 of the nortliern Atlantic are about 20'^ in width from north to south ; 

 those of the southern Atlantic are not quite so wide. These winds oscil- 

 late 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 attected by tem- 

 perature, lose that broad sweep and uniformity so characteristic of the 

 ocean. 



Eeturn 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 by differences in density, caused 

 chietiy by temperature and by evaporation ; yet the larger part of the 

 motive power is derived from the wind. These movements have been 

 ascertaiiu'd by years of observation on vessels in every ocean, sea, and 

 gulf, by the cumulative evidence of drifting objects, some of which liave 

 had their influence on the spread of vegetal and animal life and even 

 civilization itself, and by the researches of scientific exploring expedi- 

 tions to polar regions and remote islands. These oceanic movements 

 are as well understood as those of the great atmospheric ocean above us. 



When water has acquired its movement, the configuration of the bot- 

 tom of the ocean and of the shore line, the rotation of the globe on its 

 axis, and the direction and velocity of the wind modify its movement. 



SOUTH AMERICA. 



By this circulation the equatorial waters of the Atlantic blow across 

 that ocean, impinge against thecoastof South America, and are deflected 

 northward and southward. The southeasterly trade-winds blowing 

 over it become surcharged with moisture and pass directly up the val- 

 ley of the Amazon, watering the earth with tmpient rains for 2,000 

 miles to the foothills of the Andes, where some of this moisture is 

 deflected by the mountains southeastward to water southern Brazil; 

 the remainder ascends the slopes of the Andes until it is condensed 

 and falls as rain and snow, and only dry winds blow across the com- 

 paratively narrow plains between the Andes and the Pacific. The vapor 

 from the Atlantic falling in rain over the valley of the Amazon and 

 along the eastern slope of the Andes and the Cordilleras flows back 



