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



Lastly we have a considerable volume of fresh water entering the 

 North Sea from the great rivers of the Continent, and an outward 

 flowing current from the Baltic Sea which consists of water which 

 is always fresher than the water of the sea into which it flows. 



Now the strength of these various currents is not always the 

 same. We have seen that the extent of ocean covered by the 

 Gulf Stream eddy varies from time to time throughout the year, 

 being greatest in November and least in March. This is a 

 periodic variation which depends on the shifting of the equatorial 

 currents as the earth travels round the sun. Because of this 

 variation of the area covered by the Gulf Stream eddy there is 

 in November a greater amount of w^arm w^ater banked up in the 

 temperate Atlantic than in March, and as a consequence the 

 volume of the European Stream in northerly latitudes is greater 

 in the early spring than in the late autumn. This is the periodic 

 variation of the Atlantic flooding of the northern seas. So also 

 we have a periodic variation of the Polar current, less marked, it 

 is true, than that we have been considering, but still considerable 

 enough to complicate the hydrography of the Northern Ocean. 

 Lastly we have variations in the amount of w^ater entering the 

 North Sea from rivers, and variations in the strength of the Baltic 

 current, these two latter obviously depending on the nature of the 

 seasons. Year by year these various currents w^ax and wane, and 

 w^e expect that each of them attains its maximum strength at a 

 definite time of the year. 



But there are variations in the flow of each of these currents 

 which are not easily to be explained. In the case of a planet, the 

 motions of w^hich may be calculated according to our knowledge of 

 various laws, there are found perturbations which are caused by 

 influences which have not been taken into account. Just so in the 

 case of these various currents, the strength and course of which 

 may be expected to recur periodically from year to year, there are 

 perturbations which can only be explained by assuming the exist- 

 ence of some influences of the nature of wdiich w^e know next to 

 nothing. 



When W'e remember that the study of the oceanic circulation 

 of the North-Atlantic Ocean by reliable methods dates back only 

 from the beginning of the International Fishery Investigations in 



