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 an intermediate layer of water from a distance which takes part in the double system of vertical circulation as is indicated in fig. 2. The actual direction of this circulation is strongly modified by the influence of the earth’s rotation. The existence of a layer of water of low salinity at a depth of 500 fathoms in the tropical oceans of the southern hemisphere is to be referred to this action of the melting ice of the Antarctic regions. Pettersson’s view that ice-melting dominates the whole circulation of the oceans and regulates in particular the currents of the seas round northern Europe must, however, be looked on as carrying the explanation too far.

Differences of density between the waters of enclosed seas and of the ocean are brought about in some instances by concentration of the water of the sea on account of active evaporation, and in other instances by dilution on account of the great influx of land water. A very powerful vertical circulation is thus set up between enclosed seas and the outer ocean. The very dense water of the Red Sea and the Mediterranean makes the column of water salter and heavier and the level lower than in the ocean beyond the straits. Hence a strong surface current sets inwards through the Straits of Bab-el-Mandeb and Gibraltar, while an undercurrent flows outwards, raising the temperature and salinity of the ocean for a long distance beyond the straits. Through the Bosporus and Dardanelles at the entrance of the Black Sea, and through the sound and belts at the entrance of the Baltic, streams of fresh surface-water flow outwards to the salter Mediterranean and North Sea, while salter water enters in each case as an undercurrent. Wind and tide greatly alter the strength of these currents due to difference of density, and the surface outflow may either be stopped or, in the case of the belts, actually reversed by a strong and steady wind. Both outflowing and inflowing currents are subject to the deflection towards the right imposed by the earth’s rotation.

Modern oceanography has found means to calculate quantitatively the circulatory movements produced by wind and the distribution of temperature and salinity not only at the surface but in deep water. The methods first suggested by H. Mohn and subsequently elaborated by V. Bjerknes have been usefully applied in many cases, but they cannot take the place of direct observations of currents and of the fundamental processes and conditions underlying them. The determination of the exact relationship of cause and effect in the origin of ocean currents is a matter of great practical importance. The researches of Pettersson, Meinardus, H. N. Dickson and others leave no doubt, for example, that the variations in the intensity of the Gulf Stream, whether these be measured by the change in the strength of the current or in the heat stored in the water, produce great variations in the character of the weather of northern Europe. The connexion between variations of current strength and the conditions of existence and distribution of plankton are no less important, especially as they act directly or indirectly on the life-conditions of food fishes.

.—General: M. F. Maury, The Physical Geography of the Sea and its Meteorology (New York and London, 1860); J. J. Wild, Thalassa: an Essay on the Depths, Temperature and Currents of the Ocean (London, 1877); C. D. Sigsbee, Deep-sea Sounding and Dredging (Washington, 1880); O. Krümmel, Handbuch der Ozeanographie (2 vols., Stuttgart, 1907); O. Krümmel, Der Ozean (Leipzig, 1902); J. Thoulet, Océanographie (2 vols., Paris), vol. i. Statique (1890), vol. ii. Dynamigue (1896); J. Thoulet, L’Océan, ses lois et ses problèmes (Paris, 1904); J. Thoulet, Guide de l’océanographie pratique (Paris, 1895); J. Walther, Allgemeine Meereskunde (Leipzig, 1893); Luigi Hugues, Oceanografia (Turin, 1904); Sir J. Prestwich, “Tables of Temperatures of the Sea at Different Depths made between the years 1749 and 1868,” ''Phil. Trans.'' clxv. (1876), 639-670; A. Buchan, “Specific Gravities and Oceanic Circulation,” ''Trans. Roy.'' ''Soc. Edinburgh'', xxxiv. (1896), 317-342; Sir John Murray, “Presidential Address to Section E (Geography),” British Association Report (Dover), 1899; M. Knudsen, Hydrographical Tables (Copenhagen, 1901); Sir John Murray, “Deep-Sea Deposits and their Distribution in the Pacific Ocean,” ''Geogr. Journal'', 1902, 19, pp. 691-711, chart; “On the Depth, Temperature of the Ocean Waters and Marine Deposits of the South Pacific Ocean,” ''R. Geogr. Soc. of'' Australia, Queensland, 1907, pp. 71-134, maps and plates; J. Thoulet, Instruments et opérations d’océanographie pratique (Paris, 1908); Précis d’analyse des fonds sous-marins actuels et anciens (Paris, 1907); T. Richard, L’Océanographie (Paris, 1907); List of Oceanic Depths and Serial Temperature Observations, received at the Admiralty in the year 1888 (et seq.) from H.M. Surveying Ships, Indian Marine Survey and British Submarine Telegraph Companies (Official).

Important current and temperature charts of the ocean and occasional memoirs are published for the Admiralty by the Meteorological Office in London, by the U.S. Hydrographic Office in Washington, the Deutsche Seewarte in Hamburg, and also at intervals by the French, Russian, Dutch and Scandinavian Admiralties. Pilot Charts of the North Atlantic and North Pacific are issued monthly by the U.S. Hydrographic Office, and of the North Atlantic and of the Indian Ocean and Red Sea by the British Meteorological Office, giving a conspectus of the normal conditions of weather and sea.

Reports of Important Expeditions.—Sir C. Wyville Thomson, The Depths of the Sea (cruises of “Porcupine” and “Lightning”) (London, 1873); The Atlantic (cruise of “Challenger”) (London, 1877); Die Forschungsreise S.M.S. “Gazelle” in den Jahren 1874 bis 1876 (5 vols., Berlin, 1889–1890); Report of the Scientific Results of the Voyage of H.M.S. “Challenger” in the years 1872–1876 (50 vols., London, 1880–1895); A. Agassiz, Three Cruises of the U.S. Coast and Geodetic Survey Steamer “Blake” from 1877 to 1880 (2 vols., Boston, Mass., 1888); S. Makaroff, Le Vitiaz et l’Océan Pacifique, 1886–1889 (St Petersburg, 1894); S. Makarofi, The Yermak in the Ice (in Russian) (St Petersburg, 1901); The Norwegian North Atlantic Expedition (on the “Vöringen”), 1876–1878 (Christiania, 1880–1900); Expéditions scientifiques du “Travailleur” et du “Talisman,” 1880–1883 (Paris, 1891 et seq.); Die Ergebnisse der Plankton-Expedition, 1889 (Kiel, 1892 et seq.); Résultats des campagnes scientifiques accomplies sur son yacht par Albert Iᵉʳ Prince Souverain de Monaco (Monaco, from 1889); The Danish “Ingolf” Expedition, 1806 (Copenhagen, 1900); Prof. Luksch, Expeditionen S.M. Schiff “Pola” in das Mittelmeer und in das Rote Meer, Kais. Akad. Wissenschaften (Vienna, 1891–1904); Die Deutsche “Valdivia” Tief-See Expedition, 1898–1899 (Berlin, 1900); M. Weber, “Siboga Expedition,” Petermanns Mitteilungen (1900); Siboga Expeditié (Leiden, 1902 et seq.); F. Nansen, The Norwegian North Polar Expedition, 1893–1896 (Christiania and London, 1900); R. S. Peake, “On the Results of a Deep-sea Sounding Expedition in the North Atlantic Ocean during the Summer, 1899” (Extra Publ. Geogr. Soc., London); Bulletin des résultats acquis pendant les courses périodiques (Conseil permanent international pour l’exploration de la mer) (Copenhagen, 1902 seq.).

Reports of many minor expeditions and researches have appeared in the Reports of the Fishery Board for Scotland; the Marine Biological Association at Plymouth; the Kiel Commission for the Investigation of the Baltic; the Berlin Institut für Meereskunde; the bluebooks of the Hydrographic Department; the various official reports to the British, German, Russian, Finnish, Norwegian, Swedish, Danish, Belgian and Dutch governments on the respective work of these countries in connexion with the international co-operation in the North Sea; the Bulletin du musée océanographique de Monaco (1903 seq.); the Scottish Geographical Magazine; the Geographical Journal; Petermanns Mitteilungen; Wagner’s Geographisches Jahrbuch; the Proceedings and Transactions of the Royal Societies of London and Edinburgh; the Annalen der Hydrographie; and the publications of the Swedish Academy of Sciences.

 OCEAN CITY, a city and seaside resort of Cape May county, New Jersey, U.S.A., in the S.E. part of the state, about 10 m. S.W. of Atlantic City. Pop. (1890), 452; (1900), 1307; (1905), 1835; (1910), 1950. It is served by the Atlantic City and the West Jersey & Seashore railways. The city is laid out to face both the ocean and Great Egg Harbor Bay, and is a popular resort during the summer months. Ocean City was incorporated as a borough in 1884, and was chartered as a city in 1897. 