78 SEA FISHERIES 



descends from the north-east towards the south-west. 

 Having reached the Wyville Thomson ridge, it runs 

 under the warm current, and sends out a thin stream 

 of water which overlaps the latter in the form of a 

 narrow ribbon in the supra-litoral zone of the Icelandic 

 plateau. A vertical section of the ocean in this region 

 would show, between the surface and the summit of 

 the ridge, a warm layer and a cold layer, each of variable 

 depth and flowing in contrary directions. From this 

 arrangement it follows that the warm stream becomes 

 cooled and the cold stream grows warmer. Sometimes 

 the latter predominates, sometimes the former, but as the 

 Wyville Thomson ridge forms a constant barrier there is 

 always an inevitable mixture of the streams. In the 

 winter, moreover, throughout the entire Icelandic plateau 

 we find vertical convection currents, which maintain a 

 temperature of from 44-6 to 46-4, although at the 

 Westmann Islands the atmospheric temperature falls to 

 freezing-point. Thus Iceland, according to the phrase 

 employed by M. Cligny, plays the part of a two- 

 way tap, which emits turn by turn or simultaneously 

 warm and cold water in variable proportions ; and 

 the Gulf Stream in the northern Atlantic " does not 

 assume the definite and typical form of a river, but 

 of an eddy-like circulation subject to oscillations which 

 are more or less extensive according to the time of 

 year." 



The physiology of the North Sea depends upon the 

 following general data. The North Sea receives, by the 

 submarine channel which passes between the Shetlands 

 and the Faroes, the warm waters of the Atlantic, which, 

 as has been proved, are mingled with large quantities of 

 Mediterranean water which has come through the Straits 

 of Gibraltar. The currents which reach it from the 



