The Hydrosphere 



ocean depths, and where the basins are enclosed, or 

 partially so, by elevated ridges of the sea floor, some 

 very interesting facts have been observed. For example, 

 in the two great basins of the North Atlantic the 

 temperature at 2000 fathoms is 35, and there is no 

 further decrease at greater depths, the whole of the 

 water below the 2Ooo-fathom line being at the same 

 temperature. In contrast to this is the depression off 

 the coast of Brazil, in which the temperature decreases 

 steadily to the bottom, where it reaches 33 Fahr. 



The difference is due to the fact that the basins of 

 the North Atlantic are separated from the cold Antarctic 

 waters by a ridge over which the water is nowhere more 

 than 2000 fathoms deep, and which prevents the influx 

 of waters at lower temperatures than that appropriate 

 to that depth, while the Brazilian basin is freely open 

 to the south. 



This phenomenon is still more marked in the case of 

 enclosed seas like the Red Sea, where the temperature 

 falls rapidly down to a depth of about 200 fathoms, 

 where it is 70, and does not fall below this even where 

 depths of 1 200 fathoms are met with. 



The high temperature of the great mass of the 

 waters of the Red Sea is undoubtedly due to the 

 shallowness of the Straits of Bab-el-Mandeb, which 

 prevents the influx of the cold polar waters. The 

 diagram Fig. 2 will make this principle clear when we 

 remember that the lower the temperature of the water, 

 the heavier it becomes. 



We have already had occasion to mention the surface 

 currents of the ocean, and as these are the only ones 

 which are directly apparent, and which influence climate 



47 



