84 THE OCEAN 



portion of the intertropical part of the Pacific, 

 the difference in temperature being small at 

 the surface, but this difference increases 

 downwards to about 100 fathoms, where it 

 may exceed 20° F., then decreases again down 

 to about 200 fathoms. 



Similar areas of low temperature occur in 

 the equatorial part of the Indian Ocean to 

 the south of Ceylon, and in the equatorial 

 part of the Atlantic to the west of Lower 

 Guinea, the Atlantic area lying to the south 

 of the equator, while the Indian and Pacific 

 areas are situated to the north of the equator, 

 that is to say, on that side of the equator 

 where the warm waters of the surface, owing 

 to the diminished density, are not diffused or 

 conveyed so quickly downwards to greater 

 depths. 



The expanse of low temperature at 100 

 fathoms in the Pacific is bounded on each side 

 by those tracts of the ocean where the trade 

 winds are strongest, and these trades impel 

 to the westward warm surface currents, 

 which, by friction, extend to a considerable 

 depth in the ocean. The result is that the 

 currents of the trades proper take a large 

 portion of their supply from the sea directly 

 underlying the calm intervening region 

 between the north and south trades ; hence 

 there necessarily sets in an upwelling. These 



