152 FOUNDERS OF OCEANOGRAPHY 



Bank, turn steeply downwards to form a cold wall against 

 which the warmer waters of the Gulf Stream run eastwards. 



Layers of water— both surface and deeper — of different 

 temperatures, and having also other distinguishing charac- 

 teristics, can be traced for considerable distances in the 

 ocean, by means of hydrographic observations, and their 

 source determined and ultimate destiny predicted ; and in 

 that way the distribution of various pelagic animals which 

 are affected by the temperature and other characteristics of 

 the water can be explained. Murray and Hjort have in this 

 way shown how the spread of the Pteropod Clione limacina 

 from the sea about Newfoundland towards the west coast of 

 Ireland depends upon the temperature of the water met 

 with. 



If we take the temperatures in another direction through 

 the North Atlantic from the work of the " Michael Sars," 

 we find in a section from the Sargasso Sea to the Norwegian 

 coast at Lofoten that the isotherm of, say, 50° F. can be 

 traced rising from a depth of about 400 fathoms to the 

 surface, showing the gradual cooling of the upper waters in 

 going north. A more complicated case, where waters from 

 three different sources, each having characteristics which are 

 recognizable, occur in the same section, is seen to the west of 

 Norway. Proceeding towards Jan Mayen, after passing 

 through a belt of coastal water, there is an area of warmer 

 and Salter Atlantic water at a temperature of about 7° C. 

 overlying the mass of cold Arctic water which occupies the 

 greater part of the deep channel and has a temperature of 

 3° C. in its upper part, 0° C. in the intermediate depths, and 

 — 1° C. at the bottom. This is an example of cold polar 

 water creeping along the sea bottom towards the equator ; 

 and, as a rule, in the open sea, the bottom isotherms are quite 

 independent of those on the surface. The surface isotherms 

 run generally in an easterly and westerly direction roughly 

 parallel to the equator (though they may be diverted from 

 this course), while the bottom isotherms run more or less 



