2o6 BRITISH FISHERIES 



to which it is lowered. Salinity is determined 

 by a chemical method. A solution of nitrate of 

 silver of a known strength is added drop by drop 

 to a measured quantity of water under examination. 

 The silver precipitates the salt as a white curd. 

 From the amount of silver solution required to 

 precipitate all the salt, the amount of the latter 

 present in the sample is very exactly determined. 

 The gaseous contents are estimated by somewhat 

 more complicated chemical methods. 



Now, by making use of these methods it was 

 found by Pettersson and Ekman, and afterwards by 

 the hydrographers of the 1893 survey, that at any 

 one time the water of the North Sea might be 

 derived from one or more of three sources : — (i) 

 North Atlantic water. This enters the North Sea 

 round the north of the Shetland Islands. Its 

 entrance into the latter area is the result of the 

 " Gulf Stream " current. The real Gulf Stream 

 does not, of course, actually reach the British 

 Islands ; but the effect of the prevalence of cyclonic 

 storms reaching our coasts from the south to 

 south-west of the Atlantic is to drive the surface 

 layer of the sea towards and to the north and east 

 of the British Islands. This North Atlantic water 

 is heavy, possessing a normal salinity of over 

 35 parts per thousand. Most of it in the North 

 Sea enters from the north-west, but a small 

 portion enters through the Straits of Dover. (2) 

 Baltic water. This is very light water, with a 



