CH. Il] THE OCEANOGRAPHY OF THE NORTH-WESTERN OCEAN 43 



in the region of the Atlantic Equatorial Stream contains about 

 37 grams of dissolved salts in 1000 grams, while in the compara- 

 tively fresh water of the eastern Baltic this weight of water 

 contains only eight grams of total salts. Generally speaking the 

 salinity of the Atlantic south of the Wyville-Thomson Ridge is 

 over 35, and a similar value holds for the surface of a considerable 

 part of the Norwegian Sea. In the North Sea the mean salinity 

 varies from 35"2 to 34'6 and is highest at the entrance to the 

 Straits of Dover in the south and between the Shetlands and 

 Norway in the north ^. In the North Sea in a line drawn from 

 Grimsby to Texel the salinity is lowest, that is 34' 5. In the 

 Irish Sea the values for the salinity vary much as in the North 

 Sea. 



Three components enter into the composition of the water 

 of the North-Atlantic Ocean: (1) "Gulf Stream" water, (2) cold 

 bottom water from the Norwegian Sea, and (3) fresh water from 

 great rivers. Extensive water bodies retain their heat for a long 

 time, and diffusion in liquids being a slow process, the mixture of 

 different layers takes place very slowly. We find then that over 

 wide areas the three components mentioned above may be traced 

 with more or less distinctness. The density of sea water depends 

 on the temperature no less than on the salinity. Thus a water 

 layer at a high temperature and having a high salinity may still 

 be specifically lighter than a water layer at a low temperature but 

 with a low salinity. This is the case with the Atlantic stream 

 which enters the Faeroe Channel. It is really richer in dissolved 

 salts than the water normally belonging to the Norwegian Sea, 

 but since it has a higher temperature than the latter it is lighter 

 on that account, and so it flows on the surface of the cold fresher 

 water until when it has cooled down it sinks beneath the surface 

 and still flows on as an undercurrent. 



If a great number of observations of the temperature and 

 salinity of the sea over a very considerable area be made at about 

 the same time, and if these simultaneous observations be repeated 

 at frequent intervals, say every three months, then a picture of the 

 water circulation in the area may be made with some degree of 



1 A chart shewing the mean salinities is given by Martin Knudsen and Miss K. 

 Smith in Rapports et Proces-Verbaux ; Cons. Perm. Int. Explor. Mer, Vol. vi. 1906. 



