North Sea. The North Sea, or German Ocean, is a southern extension of the Arctic Ocean (q.v.). It communicates freely with that part of the Arctic Ocean lying between Iceland and Norway which has received the name of the Norwegian Sea. Its northern boundary would be represented by a line drawn from the Shetland Islands to the opposite coast of Norway, and its southern boundary is the Strait of Dover; on the west it is bounded by the east coast of Great Britain, and on the east by the coasts of Norway, Denmark, Germany, Holland, and Belgium. With the North Atlantic Ocean it has communication through the Strait of Dover and the English Channel on the south, and on the north by the Pentland Firth and the channel between the Orkney and Shetland Islands; with the Arctic Ocean as already stated; and with the Baltic by the Skagerrack and Cattegat. The North Sea is over 600 miles in length and about 400 miles in greatest width, and its area is estimated at over 160,000 sq. m. By far the greater proportion of this area is less than 100 fathoms in depth, the only part where deeper water is found being off the coast of Norway (the Norwegian Gully or Norwegian Deep, as it has been called), where a depth of 360 fathoms has been recorded; the mean depth of the whole area is estimated at 61 fathoms. The sea is very shallow towards the south and east, and the coasts in this direction are low and flat, being in some places below the level of the sea, whereas to the north and west, where the water is deeper, the sea-coast is high, and the deep Norwegian Gully is faced by the high and bold cliffs of Norway. The sea-bottom is very irregular, a number of banks running across from the Yorkshire coast towards the Skagerrack, the most important of which is the Dogger Bank (q.v.), and there are also depressions like the Silver Pit; off the low-lying coasts of Holland, Belgium, and Britain there are numerous shoals and sandbanks formed of the materials brought down by the rivers. The North Sea is surrounded by continental land and receives the waters of numerous rivers, the principal of which are the Thames, Ouse, Humber, Tyne, Tweed, Forth, and Tay, the Scheldt, Rhine, Weser, and Elbe. The deposits forming on the bottom consequently belong to the class called 'terrigenous,' consisting in the shallower water of sands and gravels and in the deeper water of muds and clays, containing stones and fragments of rocks and minerals derived from the land, along with calcareous fragments of shells and other organisms.
The salinity of the water of the North Sea varies between 1.025 and 1.027, the lightest water occurring in the southern part and in the Skagerrack, where fresh water comes from the Baltic, and the densest water at the bottom in the deep water off Norway. The mean temperature of the air over the North Sea in summer is about 60° F., and in winter about 36°, the range throughout the year being about 34°—from 31° to 65° F. Except in the summer months, the temperature of the surface water is higher than that of the air, the mean temperature of the surface water in summer being about 58°, and in winter about 42° F. The winds are variable over the North Sea, the most prevalent being from the south-west, and the currents are chiefly dependent on the direction of the wind; fogs, mists, and rain occur at all seasons. The great tidal wave of the Atlantic advancing from the west is divided into two portions on striking the British Islands: the one entering the North Sea round the Orkneys and through the Pentland Firth, the other coming up the English Channel. Captain Tizard, R.N., who has made a special study of the tides in the North Sea, says: 'The former undulation seems to run along the east coast of Britain like a wave along a break-water, and makes it successively high-water along the coast from Duncansbay Head to Orfordness; the latter undulation runs along the coasts of France, Belgium, Germany, &c., and finally ends at the Scaw. As the distance from the coast increases, the rise and fall seems to diminish until it is probable that in the centre of the sea there is very little if any. Captain Hewitt found one spot midway between the British and Dutch coasts where there was no rise and fall. The tidal streams do not depend on the times of high and low water at the different ports, but seem to be more dependent on the position where the maximum rise and fall takes place, running towards that spot when the tide is rising there and away from it when falling there; I have not worked it all out yet, but certainly on the east coast from St Abb's Head to the Wash the stream is always running to the southward when the tide is rising in the Wash and to the northward when falling there. From Cromer to the Downs the tide is affected greatly by the undulation from the Channel, and the two undulations seem to pass through each other; but there is a very curious fact in connection with the two tides—viz. all the light-vessels in the North Sea affected by the Channel tide rotate with the hands of a clock, and all affected by the tide round the Orkneys rotate against the hands of a clock.'
The North Sea has been from the earliest times one of the most important highways of the world, and is surrounded by some of the most prosperous commercial nations, famous for their maritime exploits. The fisheries of the North Sea are among the most important in the world, providing employment for thousands of fishermen from the surrounding countries; all the varieties of food-fishes abound, as well as edible molluscs and crustaceans, such as oysters, mussels, lobsters, crabs, and shrimps. The value of these fisheries depends to a great extent upon the abundance of the fauna and flora living on the sea-floor, all the various groups of invertebrates being met with in great profusion in the North Sea, while the surface waters swarm with algae, such as diatoms, &c., which sometimes form extensive floating banks.
See North Sea Pilot and Admiralty publications; also Expedition zur Untersuchung der Nordsee (Berlin, 1875). For North Sea Canals, see BALTIC SEA, CANAL.