THE OCEAN FEEDING-GROUNDS 79 



north are certainly colder, but they are by no means 

 glacial. For this reason the waters of the Norwegian 

 dike and the Skagerack never become very cold and 

 always exhibit a southern flora and fauna. It must 

 be remembered, however, that in winter the cold, 

 almost brackish waters of the Baltic spread over a 

 great part of the North Sea and depress its superficial 

 temperature. The Channel brings it the warm, salt 

 currents of the Atlantic and the Mediterranean, and 

 the German rivers their brackish waters. 



Ill 



The tidal currents originating in the open sea merely 

 accentuate the factors which have already been explained. 

 There are two tidal waves in the North Sea. One pro- 

 gresses at a speed of eight to nine knots past the north 

 of the Shetlands and the north of Scotland (between the 

 latter and the Orkneys). The second, which carries only 

 a twelfth of the volume of water, arrives by way of the 

 Channel twelve hours later. It takes eight hours to 

 cover the distance from Brest to Dunkirk. The two 

 waves meet before the Thames, resulting in races, eddies, 

 back-washes, and shoals (the Thames Bank, the Belgian 

 shoal, the Goodwin Sands, &c.). The Dogger Bank rises 

 in the midst of the larger eddies, which cover it with 

 material drawn from all parts of the North Sea. The 

 great Atlantic tidal wave, starting from the 33rd meridian, 

 reaches the continental plateau at the end of some three 

 hours. 1 An hour later it is in the Channel and the Irish 



1 The amplitude of the tidal wave is very small in the open 

 sea and is rarely more than 3 feet on distant islands or in 

 inland seas. It reaches its maximum in deep bays, straits, and 

 estuaries. 



