The Sea and Its Work 



of currents will also be established, as in Fig. 47. 

 From the bottom of the piece of ice a clear stream 

 of heavy cold liquid flows down the middle of the 

 glass, like a stream of clear oil, while the neigh- 

 bouring water, which is comparatively warm, 

 flows upwards in currents nearer to the sides 

 of the vessel. 



Unequal cooling or heating of the great natural 

 masses of water will be competent to produce a 

 circulation similar to that just described. Dur- 

 ing the famous voyage of the Challenger the 

 temperature of the sea at different depths was 

 very carefully examined by means of instruments 

 specially constructed to avoid sources of error. 

 These observations show that, as a rule, the tem- 

 perature diminishes as you descend, just as was 

 shown to be the case in the North Atlantic 

 Between Sandy Hook and Bermuda the bottom- 

 water of that part of the ocean has a temperature 

 only a little above 35 F., while, in other places, 

 it is still lower, and may even descend below the 

 freezing-point of fresh water. It appears that 

 the presence of such cold water in the deeper 

 parts of the ocean, even in tropical regions, can 

 hardly be explained otherwise than by assuming 

 a grand movement of water from the polar 

 towards the equatorial regions. Dr Carpenter 

 has brought forward much evidence to prove the 

 existence of such a general oceanic circulation, 

 and he refers the movement mainly to differences 

 of density due to differences of temperature. 

 The cold polar waters sink by their density and 

 165 



