32 THE POLAR GLACIERS. 



the velocity toward the east which it acquired at the equator, and 

 when it strikes the slower-moving latitudes, it will be traveling 

 much faster than the regions it comes down upon. Hence the 

 winds blowing towards the east, that prevail almost constantly in 

 the middle latitudes. 



This is the normal order of the wind-currents, and that w T hich 

 would prevail with nearly perfect regularity if the world were 

 a uniform globe of water or of land, and equally heated on both 

 sides of the equator. But the continents, and particularly moun- 

 tain elevations, produce great disturbances, unequal rainfalls and 

 ever-varying atmospheric pressures. When also from any cause, 

 one of the trade-winds, notably the southern, is increased in its 

 violence so as to push a tornado tongue across the dividing line 

 into the opposite system of winds, there is started one of those 

 cyclones, or great circular storms, which ravage the tropics and 

 whirl through the temperate zones, finally exhausting themselves 

 in the higher latitudes to the eastward. 



The southern hemisphere is at the present time colder than the 

 northern, owing primarily to the fact that the winters there are 

 eight days longer than the northern, and the sun, during those 

 seasons, about 3,000,000 miles further from the earth than during 

 the northern winters. The difference of temperature therefore 

 between the warm air that rises at the equator and the cold air 

 that comes in from the south, is greater than that on the north 

 side. And as it is difference of temperature that produces the 

 whole movement of the air-currents, of course the greater 

 strength of that movement must be on the southern side. Hence 

 the larger share of the equatorial current passes over to the south, 

 and the southern trades are much the strongest. In accordance 

 with this theory it is a matter of observation that the southern 

 trade-winds reach across the equator and into the northern hem- 

 isphere in some places ten to fifteen degrees. 



In obedience to and perfect accord with this great system of 

 winds, the waters of the oceans move. The strong southeast 

 trades blow up from Southern Africa, cross the equator, and 

 drive the waters of the South Atlantic into the Caribbean Sea. 

 The lighter northeast trades, blowing between North Africa and 



