THE POLAR GLACIERS. i 79 



This is the normal order of the wind-currents, and that which 

 would prevail with nearly perfect regularity if the world were a uni- 

 form globe of water or of land, and equally heated on both sides of 

 the equator. But the continents, and particularly mountain eleva- 

 tions, 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 farther from the earth than during the north- 

 ern 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 cur- 

 rent passes over to the south, and the southern trades are much the 

 strongest. In accordance with this theory, it is a matter of observa- 

 tion that the southern trade-winds reach across the equator and into 

 the northern hemisphere 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 the West Indies, assist and give 

 direction to this movement, which finally impels through the Straits 

 of Florida a tide of tropical waters a hundred times greater than the 

 outflow of all the rivers in the world. This great flood of thermal 

 waters spreads out in the Northern Atlantic, imparting to Europe a 

 climate corresponding to countries twenty degrees south of it on 

 this side of the ocean. There is, of course, an under-current from 

 the Arctics to the equator, exactly compensating this enormous 

 northward flow of the surface-waters. The same process and effect 

 are repeated in the Pacific Ocean ; and the great Japan Stream robs 

 the southern hemisphere, for the benefit of our Pacific States, only 

 in a degree less than does the Gulf Stream for the benefit of Europe. 



A change in the relative strength of the trade- winds, such that the 

 northeast trades would blow across the equator into the southern 

 hemisphere, would entirely reverse the course of the warm ocean- 

 currents, and carry to the southern continents the heat abstracted 

 from the northern. Such a change in the course of ocean-streams has 



