water bottom surface depth fathoms temperature islands southern
NORTH SEA. The North Sea or German Ocean lies between Great Britain and the continent of Europe. It communicates with the North Atlantic by the Straits of Dover in the south, and by the Pentland Firth and the various openings between the Orkney and Shetland Islands in the north. Between the Shetlands and Norway it passes by a wide opening into the sub-polar basin, now generally known as the NORWEGIAN SEA (q.v.), of which indeed it may be regarded as a southern extension. It has communication with the Baltic by the Skagerrack and the Cattegat. The shores of the North Sea have from the earliest times been inhabited by brave and hardy races of men famous for their maritime exploits ; and at the present day it is surrounded by many of the most prosperous and enterprising commercial nations, and is, in consequence, one of the most important highways of the world. Its fisheries give employment to thousands of persons, and are the most valuable that exist. Lighthouses are situated on nearly every available point where they are required, and there are numerous light-ships along the coasts.
The North Sea lies between the parallels of 51° and 61° N. lat. and 2° 30' W. and 8° E. long., its greatest length being about 600 miles, its breadth (from St Abb's Head to the shores of Denmark) 360 miles, and its area about 140,000 square miles. It may be said to be without islands if we except the Orkneys, the Shetlands, and those islands which are situated along the coasts. Its coasts present considerable variety in appearance and geological formation. Scandinavia, composed of ancient rocks, is elevated, deeply indented by fjords, and skirted with numerous islands. The coasts of Britain are bold and rocky in the north, while towards the south they present a succession of low rocky or chalky cliffs and sandy beaches. Denmark, Germany, Holland, and Belgium have low shores, composed of recent formations, and deeply indented. Many parts of Holland and Belgium are indeed below the level of the sea, and are protected from inundation by artificial dykes and extensive natural sand dunes. The sea has repeatedly broken through these artificial and natural barriers and submerged large tracts of country. The Dollart Zee and the Zuyder Zee were formed in this way in 1277 and 1282. Some of the most important rivers of Europe enter the North Sea, as the Elbe, Weser, Ems, Rhine, Scheldt, Thames, Humber, Tyne, Tweed, Forth, and Tay. If we include the Baltic, which enters into it, we may regard the North Sea as receiving the drainage of about one-fourth of the European continent.
Its greatest depth is in a deep gully following closely the trend of the southern portion of the Scandinavian peninsula, where soundings of 160, 200, 300, and 400 fathoms are common. It has been suggested that this deep gully, which extends into the Skagerrack, may have been the bed of a river at a time when the continent of Europe stood at a higher level, and that the Elbe and some of the other rivers now entering the Baltic and North Sea may have united and flowed into the Norwegian basin of the Atlantic through this depression. With the exception of this gully the depth of the North Sea is less than 100 fathoms. It is to be noticed, however, that the bottom of the whole basin is very irregular. The southern half is the shallower, and, generally speaking, the depths in this portion are greater on the eastern and western sides than in the centre, where the Dogger Bank is situated. On this bank the depth is from 8 to 16 fathoms, whereas in the " Silver Pit " immediately to the south there is a depth of 45 fathoms. Similar irregularities are met with in various other parts of this seabed, and are called by the fishermen " pits " and " banks," with various distinguishing names. In the northern half the depth gradually increases towards the north, until a depth of 2000 fathoms is reached in the Norwegian Sea. It is probable that these irregularities met with in the bottom of the North Sea are chiefly due to the moraines and detrital matter left by the great glacier which filled it during the Glacial period. There are besides a great number of shoals and sandbanks lying along the coasts of Holland, Belgium, France, and Britain, which assume the form of ridges running in a direction nearly parallel to the shores, and consist of sand and detrital matter brought down by rivers and arranged apparently by tide streams. The deposits vary considerably in their composition. In the shallow parts a sand predominates, composed of fragments of quartz, felspars, micas, hornblende, augite, magnetite, and calcareous fragments consisting of triturated pieces of mollusc shells, Echinoderms, Polyzoa, Alcyonarian spicules, calcareous Alga, and many Foraminifera. In the deeper water we generally find a mud or clay composed of the above-mentioned mineral and organic fragments, with the addition of fine argillaceous matter, very minute mineral particles, and Diatom frustules. In some places we have stones and gravel, and indeed stones may be met with in all the varieties of deposit.
Fogs, mists, and rain occur at all seasons. The winds are variable, the moisture-laden winds from the south-west being the most prevalent, and storms are frequent. The currents depend chiefly upon the direction of the winds, and the navigation is most difficult. The great tidal wave , from the Atlantic on reaching the British Islands breaks into two portions, one passing through the Straits of Dover and the other round the north of Scotland into the North Sea. These two portions meeting produce nodal lines, where they partially neutralize each other, for instance in the Straits of Dover, and less distinctly in lines stretching from the Wash and Moray Firth to the north of Denmark. The North•Sea lies between the January isotherms of 31° and 40°, and the July isotherms of 55° and 65° Fahr., so that the difference between the mean winter and summer temperature is about 24°. The temperature of the surface water ranges in January between 39° and 45° Fahr., and in July between 53° and 63°. Hence the contrast between the temperature of the water and that of the air is greater in winter than in summer, and indeed except during the warmest months the air is colder than the water. In the southern part of the North Sea, south of the Dogger Bank, where the sea is comparatively shallow, there is in summer only a difference of a degree or so between the surface and the bottom water, the bottom water being a little the colder. The difference is greatest in the hollows like the " Silver Pit," where the depth reaches 45 fathoms. The temperature along the British coast appears to be in summer about 3° colder both at the surface and at the bottom than along the coast of Denmark. We have no very reliable information as to the temperature of the water at different depths during the winter months, but we know it takes a very long time before the cooling of the surface water affects the temperature at the depth of a few fathoms, therefore it is most probable that the water at the bottom in the southern part of the sea is much warmer than the surface or intermediate water, and this is likely to be the case especially in the " pits " where the depth is greatest. This is very probably the chief reason why such large catches of soles and other fish are made in these " pits " during very cold winters.
North of the Dogger Bank there is a very considerable difference of temperature between the surface and the bottom water in summer. Off Aberdeen there is a difference of I 11°, the surface temperature in summer being 56° Fahr.
32° Fahr. all the year round. The specific gravity ranges • between 1.0249 and 1-0270, the saltest water being found at the bottom in the Norwegian gully. The lightest water is found in the Skagerrack where the Baltic water enters the North Sea, and in the southern half of the sea where the Continental rivers discharge their waters.
The North Sea has an abundant flora. Algae in great abundance and variety grow on all the shores and in all the shallower waters, while a few species are found at depths of even 50 and 100 fathoms. The surface and subsurface waters swarm with Diatoms, Peridinias, Coccospheres, and other minute Algae. It is a matter of observation that where there is a low specific gravity, indicating a mixture of fresh with salt water, there is usually a great abundance of Diatomaceze in the surface waters. This is the case in bays and estuaries, and in the arctic and antarctic regions, where melting ice lowers the specific gravity. The North Sea has all the characteristic features of a great bay, and has a great abundance of plant life. Its surface or intermediate waters are at times quite discoloured by the enormous abundance of Diatoms or Peridinias which are met with in vast floating banks. The cause of the rapid and great development of these minute organisms at particular times and places appears to depend on physical conditions which are not at present understood. With such a vast food supply it is not surprising that a prolific fauna swarms in the North Sea. Everywhere on the bottom we find Foraminifera, Sponges, Coelenterates, Echinoderms, Worms, Polyzoa, Tunicata, Molluscs, Crustacea, and Fishes. At all depths in the intermediate water we find Protozoa, Medus, Copepods, Amphipods, Schizopods, Sagitta, and various other pelagic animals, together with a great abundance of the larvm of animals living on the bottom. The invertebrates living 'on the bottom and in the water at various depths, in their adult as well as in their larval stages, supply food for those fishes which are so much desired for the table. Most valuable food fish, as the cod, haddock, herring, sprat, holibut, sole, coal-fish, and many others, frequent the North Sea, and are captured in great numbers by the fishermen of all the nations occupying the seaboard. There are also important fisheries for crustaceans such as lobsters, crabs, prawns, and shrimps, and for molluscs such as oysters, mussels, whelks, and periwinkles. Whales and porpoises are numerous, and sea-birds are found in vast numbers on the islands along the shores. The annual value of the North Sea fisheries of various kinds is enormous. In a recent lecture the duke of Edinburgh estimated that the labours of British fishermen supplied annually "fish food amounting to about 615,000 tons weight, which at £12 per ton represents a money value of £7,380,000." By far the larger part of this comes from the North Sea. If we consider that these waters are also fished by Norwegians, Danes, Germans, Dutchmen, Belgians, and Frenchmen, we may form some idea of its fertility. It is very probable that the annual value of all the fisheries exceeds £25,000,000.
No systematic investigation of the North Sea has yet been undertaken, and in consequence our knowledge is in many respects very meagre. This is all the more astonishing when we remember the value of the fisheries and the enterprise of the nations engaging in them. The Admiralty employed a ship for several seasons to examine the currents and tides ; the results are published in the North Sea pilot and admiralty tide tables. The German ship " Pornmerania " was engaged during the year 1872 in examining the North Sea. See Die Expedition. zur physicaliseli-themisehenund biologischen. Untersuchung der Nordsee irn Sommer 1872 (Berlin, 1875). (J. Nu.)