PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 243 



water it replaces can only be ascertained by actual analysis (which 

 was not carried out), and that the further question, whether the herring 

 comes with this water or to it, must be settled by zoologists. 



In his paper on "The Movements of the Surface Waters of the 

 North Sea," published in the Geographical Journal for March, 1896, 

 Mr. Dickson tells us that in the International Hydrographic Survey, in 

 1893 and 1894, Denmark, Sweden, Germany, Scotland, and Norway 

 took part. The plan was to obtain simultaneous observations in 

 different parts at times arranged beforehand. In May, 1893, Danish 

 and Swedish ships were at work; in August and November, 1893, 

 and February and May, 1894, ships from the Kiel Commission and 

 the Fishery Board for Scotland co-operated ; while a Norwegian vessel 

 also made observations in the latter periods. Mr. Dickson gives a 

 summary of all the observations so far as they refer to the surface 

 waters, combining with them observations made on merchant vessels, 

 obtained from Professor Pettersson and from the Meteorological Office 

 in London. 



In his preliminary remarks Mr. Dickson points out a fact which, 

 obvious enough in itself, is of great importance, namely, that the 

 original sources of the water in the North Sea are the oceanic waters 

 entering from the north and north-west, and, to some extent also, from 

 the Straits of Dover, and land waters entering from the Baltic and 

 the rivers. The other waters, distinguished and appropriately named 

 by Pettersson, are due to mixture of these two. In a large and deep 

 basin the fresher water would overlie the ocean water, and mixture 

 would take place slowly ; but in a shallow basin like the North Sea 

 the influences of wind and tide penetrate to the bottom, and mixture 

 sometimes takes place with great vigour and rapidity, so that North 

 Sea water (34 to 35 per thousand) sometimes occupies almost the 

 entire basin. The varying temperatures of the seasons and the force 

 of winds are very important local influences governing the distribution 

 of the different waters. 



Descriptions are then given of the distribution of the various waters 

 as distinguished by Pettersson in May, August, and November, 1893, 

 and in February and IMay, 1894. The distribution is well shown on 

 the series of coloured maps, one showing the salinities, the other the 

 temperatures of the surface waters. Perhaps the most important point 

 to notice at the different periods is the distribution of oceanic water, 

 which enters from the north southwards, and from the Straits of Dover 

 northwards, but the temperatures are also, in relation to the distribution 

 of fishes and other animals, of great importance. 



At the beginning of May, 1893, oceanic water covers a small area 

 well to the north and north-east, forming a central tongue terminating 



