44 THE OCEANOGRAPHY OF THE NORTH-WESTERN OCEAN [PART I 



probability. For liquids of different densit}^ and temperature 

 diffuse into each other so slowly that an extensive area of sea may 

 consist of roughly defined streams or strata which only mix slowly 

 at their boundaries and preserve their individuality over a consider- 

 able extent of sea surface. A comparison of such results as they 

 are indicated in the charts published quarterl}^ in the Bulletin des 

 Resultats of the International Council for the Exploration of the Sea 

 will shew that considerable variations in the extent of the sea surface 

 €overed by these different layers occur from time to time. These 

 are indicative of changes in the situation of the water components 

 of the Northern Ocean, that is to say they are evidence of a water 

 circulation in this area. Similar evidence is furnished by a study 

 of the microscopic animals and plants present in the water, since 

 these are characteristic of the place of origin of the stream. 



We have seen that Gulf Stream water, Arctic water from the 

 Polar Sea and fresh water from the great rivers of the land all 

 enter into the composition of the seas of Northern Europe. The 

 first of these components is by far the most important. It is well 

 known that the shores of the British Islands are bathed by water 

 of subtropical origin, usually called " Gulf Stream " water, and the 

 statement that the climate of these islands is much milder than it 

 would otherwise be is a well-worn one. It is quite true that 

 water of subtropical origin, relatively warm and dense, does reach 

 these latitudes from the Atlantic Ocean, but for a long time it has 

 been known that this water is not an offshoot of the Gulf Stream. 



The first complete investigation of the waters of the North 

 Atlantic with reference to the distribution of the waters of the 

 Gulf Stream was carried out in 1899^ when the variations in the 

 temperature, salinity and plankton of the waters of the Gulf 

 Stream region were investigated from the point of view of the 

 distribution of the waters of this famous current. It is shewn that 

 the limits of the Gulf Stream were variable but that at no time 

 in the year did it reach the British Islands. If we consider the 

 isohalines (lines of equal salinity of 36 to 37) we find that these 

 vary with the season. In March they reach no further north than 

 the Azores, and then the sea between these islands and the coast of 



^ Cleve, Ekman et Pettersson, Les variations annuelles de Veau de surface de 

 Vocean Atlantique. 



