230 LIGHT SCIENCE FOR LEISURE HOURS. 



do not here pretend to give anything like a complete 

 history of recent investigations into the subject. I 

 select only those facts which bear most significantly on 

 the wider relations the more marked features of 

 oceanic circulation. 



In the first place, a result which had long perplexed 

 physical geographers has been shown to be erroneous. 

 It had been supposed that the temperature of sea-water 

 below a certain depth is in all latitudes constant, and 

 about seven degrees above the temperature at which 

 fresh water freezes. Sir John Herschel, in his ' Phy- 

 sical Geography,' adopted this supposed discovery as 

 well established.. Now, let one consequence of such a 

 relation be carefully noted. The surface water in the 

 tropics is warmer than this supposed constant bottom- 

 temperature ; the surface water in arctic regions is 

 cooler ; at some intermediate latitude the surface water 

 has the same temperature as the water at the bottom. 

 Hence in this intermediate latitude the water is uni- 

 formly warm (according to the supposed relation) from 

 the surface to the bottom. We may therefore regard 

 the water in this latitude as constituting, in effect, a 

 constant barrier between the tropical waters and the 

 arctic waters. "Without regarding it as absolutely im- 

 movable we should yet be compelled to regard it as so 

 far steadfast as to negative the theory of the existence 

 of submarine currents of an importance corresponding 

 to that of the surface currents. Accordingly, the theory 

 put forward by Humboldt and Pouillet to the effect 

 that there is an interchange of waters between polar 



