vON THE EARTH'S HISTORY 



directly, but heat the land and the sea, which absorb 

 some of the rays and reflect others and so warm the air 

 in contact with them. But, as will readily be understood, 

 the land and the sea do not absorb and reflect the heat 

 rays in the same way or to the same extent ; nor do the 

 sun's rays fall equally or constantly on all portions of 

 the earth's surface. So that from various causes one 

 part of the earth is always being warmed in a different 

 way from other parts, and the air above the earth is 

 being warmed in an immeasurable number of different 

 ways. Even if the earth's surface were all water or all 

 land, we should expect therefore that there would be 

 movements of the air due to unequal heating. If, how- 

 ever, the earth's surface were quite even and uniform, 

 we should expect that there would be a certain evenness 

 and uniformity about the movements of the air. These 

 movements would be due partly to the regular heating 

 and regular cooling of the surface, and partly due to the 

 fact that the earth is spinning round taking the air with 

 it but not taking it quite evenly. The air does not fit 

 tightly on to the earth. It is rather like a loose, baggy 

 envelope with a tendency to slip as the earth moves 

 round. Furthermore, a point situated on the Equator 

 has much farther to travel in twenty-four hours as the 

 earth spins round than a point situated in the Arctic Circle, 

 where a tape measure placed along one of the parallels of 

 latitude (let us say the eighty-sixth parallel, where Nansen 

 turned back in his search for the Pole) would show the 

 earth's girth there to be, not twenty-four thousand miles, 

 but only so many hundreds. This also would make a 



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