242 PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



coast water at the surface, in the manner which has already been 

 described. 



The general results of Mr. Dickson's own investigations of the 

 conditions and movements of the sea- water to the north of Scotland 

 were as follows. Between the Faroe and Shetland Islands and the sub- 

 marine elevations of which they are the highest points, is a narrow 

 submarine valley extending from the deep basin of the Norwegian Sea. 

 This valley is separated at its south-western end from the deep basin of 

 the Atlantic by the Wyville Thomson ridge, which is only 300 fathoms 

 from the surface of the sea. Over this ridge the water of the Atlantic 

 flows throughout tlie year towards the north-east, its movement being 

 caused by the cyclonic winds in winter and by the greater warmth and 

 consequent higher level of the Atlantic water in summer as compared 

 with the Norwegian Sea. The Atlantic water flowing over the ridge 

 sucks up the cold water from beneath, and mixes with it. Owing to its 

 greater saltness the Atlantic water thus cooled sinks and loses its 

 velocity. The influence of the earth's rotation deflects the current to the 

 right, and thus in summer a mass of Atlantic water tends to collect on 

 the north-western and northern edge of the North Sea bank. At all 

 seasons this water enters with the tides between the Orkneys and Shet- 

 lands, but in winter, westerly winds drive it towards the east, and none 

 passes down the east coast of Britain. But in summer, when the winds 

 are light, and the surface layers of the North Sea are warmer and 

 lighter, the Atlantic water mixes with the cold bottom water of the 

 North Sea, and finds its way along the east coast of Scotland. The 

 causes by which, according to Mr. Dickson's explanation, this is 

 accomplished, I do not profess to fully understand. The explanation 

 given is that the Atlantic water collected at the edge of the North Sea 

 bank mixes with the cold bottom water already there, and increases its 

 salinity but reduces its specific gravity by warming it. At a certain 

 stage of mixture the temperature and salinities of the two waters com- 

 bine to form an axis of maximum specific gravity. This axis, which 

 probably runs N.E. from Shetland at the end of May or in June, turns 

 slowly toward a N. to S. direction and moves eastward. As it retreats, 

 Atlantic water is gradually admitted round the north end of the Shet- 

 lands, passes down the east side of the islands, joins the tidal stream at 

 the south end, and guided by the axis of heavy water, is distributed 

 along the east coast of Scotland probably during July and August. 



In his essay in Natural Science, Dickson states that it is in the 

 oceanic or Atlantic water, thus admitted in summer down the east 

 coast of Scotland, that the summer and autumn herring fishery takes 

 place, and remarks that whether this fresh supply of oceanic water 

 really does contain a markedly greater amount of oxygen than the 



