THE POLAR GLACIERS. 33 



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 correspond- 

 ing 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 

 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 south- 

 ern 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 south- 

 ern hemisphere, would entirely reverse the course of the warm 

 ocean-currents, and carry to the southern continents the heat ab- 

 stracted from the northern. Such a change in the course of 

 ocean-streams has unquestionably followed every change in the 

 glaciation of the hemispheres from astronomical causes. The 

 winds and the water-currents have always helped to increase the 

 difference in temperature which a considerable eccentricity of the 

 earth's orbit must always have produced between the northern 

 and southern halves of our globe. It matters but little which of 

 the two the ocean-currents or the astronomical causes have 

 produced the greater effect, since it is certain that they have ever 

 cooperated in one and the same direction. 



On all the tropical seas, between the terminal lines of the two 

 trade-winds, there is what is called the belt of calms, a tract aver- 

 aging from 300 to 500 miles wide, in which whatever winds there 

 may be are exceedingly light and unreliable. It is here$ as we 

 have seen, that the air and vapor, heated by the vertical rays of 

 the sun, are continually rising and spreading outward in the 

 upper regions. It is a complete dividing line between the cli- 

 mates of the two hemispheres. One may be frigidly cold, while 

 the other is highly heated ; the only difference being that the 

 calm belt would be removed further into the warmer hemisphere. 

 It now ranges from ftve to ten degrees of latitude on this side of 



