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OCEAN AND OCEANOGRAPHY
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OCEAN AND OCEANOGRAPHY . " Ocean " is the name applied to the See also:great connected See also:sheet of See also:water which covers the greater See also:part of the See also:surface of the See also:Earth. It is convenient to See also:divide the subject-See also:matter of See also:physical See also:geography into the See also:atmosphere, See also:hydrosphere and See also:lithosphere, and in this sense the ocean is less than the hydrosphere in so far as the latter See also:term includes also the water lying on or flowing over the surface of the See also:land. The conception of an encompassing ocean bounding the habitable See also:world is found in the creation myths of the most See also:ancient civilizations. The Babylonians looked on the world as a vast See also:round See also:mountain rising from the midst of a universal sheet of water. In the See also:Hebrew scriptures the See also:waters were gathered together in one See also:place at the word of See also:God, and the dry land appeared. The Ionian geographers looked on the circular disk of the habitable world as surrounded by a mighty stream named See also:Oceanus, the name of the primeval god, See also:father of gods and men, and thus the See also:bond of See also:union between See also:heaven and earth. The See also:Greek word d. Keavos is related to the See also:Sanskrit acdyanas, " the encompassing." Philologists do not know of any related word in Semitic See also:languages. Pictet, however, recognizes allied forms in See also:Celtic languages, e.g. the Irish aigean and Cymric eigiawn. Since the See also:Pythagorean school of See also:philosophy upheld the spherical as against the disk-shaped world, some of the ancient geographers, including Eratosthenes and See also:Strabo, looked upon the hydrosphere as forming two belts at right angles to each other, one See also:belt of ocean following the See also:equator, the other surrounding the earth from See also:pole to pole as in the terra quadrifida of See also:Macrobius; while others, including See also:Aristotle and See also:Ptolemy, looked upon the inhabited land, or oikumene, as occupying the greater part of the earth's surface, so that the See also:Indian Ocean was an enclosed See also:sea and See also:India (i.e. eastern See also:Asia) was only separated from See also:Europe by the See also:Atlantic Ocean. The latter view prevailed and was as a See also:rule held by the Arab geographers of the See also:middle ages, so that until the See also:discovery of See also:America and of the Pacific Ocean the belief was See also:general that the land surface was greater than the water surface, or that at least the two were equal, as See also:Mercator and See also:Varenius held. Thus it was that a great See also:South Land appeared on the maps, the belief in the prodigious See also:extension of which certainly received a severe See also:shock by See also:Abel See also:Tasman's voyage of circumnavigation, but was only overthrown after See also:Cook's great voyages had proved that any See also:southern land which existed could not extend appreciably beyond the polar circle. Only in our own See also:day has the existence of the southern See also:continent been demonstrated within the modest limits of Antarctica. Oceanography 'is the See also:science which deals with the ocean, and since the ocean forms a large part of the earth's surface oceanography is a large See also:department of geography. The science is termed talassografia by the Italians, and attempts have been made without success to introduce the name " thalassography." Of See also:recent years the use of " See also:hydrography " as the See also:equivalent of physical oceanography has acquired a certain currency, but as the word is also used with more than one other meaning (see See also:SURVEYING) it ought not to be used for oceanography. Like geography, oceanography may be viewed in two different ways, and is conveniently divided into general oceanography, which deals with phenomena See also:common to the whole ocean, and See also:special oceanography, which has to do with the individual characteristics of the various divisions of the ocean. This See also:article is restricted to general oceanography in its physical aspects, the closely-related meteorological, biological and economic aspects being dealt with elsewhere. Methods of See also:Research.—W See also:hen research in oceanography began, the conditions of the sea were of See also:necessity observed only from the See also:coast and from islands, the See also:information derived from mariners as to the See also:condition of parts of the sea far from land being for the most part See also:mere anecdotes bearing on the marvellous or the frightful. In recent times, especially since the rapid increase in the study of the exact sciences during the 19th See also:century, observations at sea with accurate See also:instruments have become common, and the See also:ships' logs of to-day are provided with headings for entering daily observations of the phenomena of the sea-surface. The contents of the sailors' scientific logs were brought together by the See also:American enthusiast in the study of the sea, See also:Matthew See also:Fontaine See also:Maury (1806-1873), whose methods and plans were discussed and adopted at See also:international congresses held in See also:Brussels in 1853 and in See also:London in 1873. By 1904 more than 6800 of these meteorological logs with 7,000,000 observations had been accumulated by the Meteorological See also:Office in London; 20,000 with so,600,000 observations by the See also:German Marine See also:Observatory at See also:Hamburg; 4700 with 3,300,000 observations by the Central See also:Institute of the See also:Netherlands at de Hilt near See also:Utrecht. The Hydrographic Office of the See also:United States had collected 3800 meteorological logs with 3,200,000 entries before 1888; but since that See also:time the logs have contained only one observation daily (at See also:Greenwich See also:noon) and of these 2,380,000 entries had been received by 1904. In the archives of the See also:French. Marine in See also:Paris there were 3300 See also:complete logs with 830,000 entries and 11,000 abstract logs from men-of-See also:war. The contents of these logs, it is true, refer more to maritime See also:meteorology than to oceanography properly so-called, as their See also:main purpose is to
promote a rational See also:system of See also:navigation especially for sailing ships, and they are supplied by the voluntary co-operation of the sailors themselves.
While the sailors' logs See also:supply the greater part of the scientific See also:evidence available for the study of the surface phenomena of the ocean, they have been supplemented by the records of numerous scientific expeditions and latterly by publications embodying systematic observations on a permanent basis. Valuable observations were made in oceanography during the expeditions of See also:Captain See also: Speaking generally, we may say with confidence that water predominates in the unexplored north polar area, and that it is very unlikely that new land of any great extent exists there. On the other See also:hand, recent See also:Antarctic exploration makes it practically certain that a great continent surrounds the south pole with a total area : considerably more than Sir John See also: The centre of the so-called land-hemisphere lies near the mouth of the See also:Loire (49a° N. and 2a° W.), while the centre of the so-called water-hemisphere lies to the S.E. of New See also:Zealand and eastward of See also:Antipodes See also:Island. Even in the land hemisphere the water area (134.5 million sq. km.) is in excess of the land area (121 million sq. km.), while in the water-hemisphere the amount of land is quite insignificant, ' being only 24.5 million sq. km. compared with 230.5 million sq. km. of water. The outline of the water surface depends on the outline of the basins in which it is contained. The four great See also:continental masses therefore give the ocean a distinctly tripartite See also:form, the three great divisions being known as the Atlantic, the Indian and the Pacific Oceans, all three See also:running together into one around Antarctica. Thus the connecting belt of water is narrow as compared with the extent of the oceans from north to south—See also:Drake Strait south of South America is barely 400 M. wide, from Cape Agulhas to Enderby Land, 2200 m., and from See also:Tasmania to Wilkes Land, 1550 m., while the meridianal extension of the Indian Ocean is 6200 m., of the Pacific, 9300 m., and of the Atlantic, 12,500 m., measuring across the North Pole to See also:Bering Strait. These proportions are not readily grasped from a See also:map of the world on Mercator's See also:projection, and must be studied on a globe. A See also:simple, practical boundary between the three oceans can be obtained by prolonging the See also:meridian of the southern extremity of'each of the three southern continents to the See also:Ant-See also:arctic circle. A See also:committee of the Royal See also:Geographical Society—the deliberations of which were interrupted by the departure on his last voyage of Sir John See also:Franklin, one of the members—suggested these meridians as boundaries; the north and south boundaries of the Atlantic and Pacific Oceans being the polar circles, leaving an Arctic and an Antarctic Ocean to complete the hydrosphere. We now know, however, that the Antarctic circle runs so See also:close to the coast of Antarctica that the Antarctic Ocean may be See also:left out of See also:account. It has been found more convenient to take as northern boundaries the narrowest part of the straits near the Arctic circle, Bering Strait on the Pacific See also:side, and on the Atlantic side the narrowest part of See also:Davis Strait, and of Denmark Strait, then the shortest See also:line from See also:Iceland to the Faeroes, thence to the most northerly island of the Shetlands and thence to Cape Statland in Norway. It has also been found convenient to take the boundary between the Atlantic and Pacific, as the shortest line across Drake Strait, from Cape See also:Horn through See also:Snow Island to Cape Gunnar, instead of the meridian of Cape Horn. Possibly ridges of the sea-See also:bed running southward from the southern continents may yet be discovered which would form more natural boundaries than the meridians. The committee of the Royal Geographical Society settled the existing nomenclature of the three great oceans. Some authors include the Arctic Sea in the Atlantic Ocean, and some prefer to consider the southern part of the Atlantic, Indian and Pacific Oceans as a Great Southern Ocean. Sir John See also:Herschel took as the northern boundary of the southern ocean the greatest circle which could See also:touch the southernmost extremities of the three southern continents. Such a circle, however, runs so near the coast of Antarctica as to make the southern ocean very small. Others, like Make Brun (1803) and Supan (1903), take the loxodromes between the three capes and See also:call the ocean to the south the Antarctic Ocean. G. v. Boguslawski suggested the parallel of 550 S. and Ratzel that of 4o° S. as limits; but in none of these schemes has the coast of Antarctica been adequately considered, and they have all been too much influenced by the Mercator map. Each of the three oceans, Atlantic, Indian and Pacific, possesses an Antarctic facies in the southern part and a tropical facies between the tropics, and the Atlantic and Pacific an Arctic facies in their northern parts. Where the ocean touches the continents the margin is in places deeply indented by peninsulas and islands marking off portions of the water surface which from all antiquity have been known as " seas." These seas are entirely dependent on the ocean for their regime, being filled with ocean water, though subject to See also:influence by the land, and the tides and currents of the ocean affect them to a greater or less extent. They owe their origin to depressions of the earth's crust of no very wide extent and not running very far into the continental See also:mass, and geologically they are of recent See also:age and still subject to See also:change. In these respects they contrast with the great oceans which owe their origin to the most extensive and the profoundest depressions of the crust, date back at least to Mesozoic times, and have perhaps remained permanently in their See also:present position from still remoter ages. Seas may be classified according to their form either as " en-closed " or as " partially enclosed " (or " fringing "). Enclosed seas extend deeply into the land and originate either by the breaking through of the ocean or by the overflowing of a subsiding area. They are connected with the ocean by narrow straits, the salinity of the water contained in them differs in a marked degree from that of the ocean, and the tidal waves are of small See also:amplitude. Four great intercontinental enclosed seas are included between adjacent continents—the Arctic Sea, the Central American or See also:West Indian Sea, the Australo-See also:Asiatic or See also:Malay Sea and the Mediterranean Sea. There are also four smaller continental enclosed seas each with a single channel of communication with the ocean, viz. the Baltic Sea and See also:Hudson See also:Bay with very See also:low salinity, the Red Sea and See also:Persian Gulf with very high salinity. The fringing or partially enclosed seas adjoin the great land masses and are only separated from the oceans by islands or peninsulas. Hence their tidal conditions are quite oceanic, though their salinity is usually rather See also:lower than that of ocean water. The four fringing seas of eastern Asia, those of Bering, See also:Okhotsk, See also:Japan and See also:East See also:China, are arranged parallel to the main lines of dislocation in the neighbouring land-masses, and so are the Andaman Sea and the Gulf of See also:California. On the contrary, the North Sea, the See also:British fringing seas (See also:English Channel, Irish Sea and Minch), and the Gulf of St See also:Lawrence See also:cross the main lines of dislocation. In addition to these seas See also:notice must be taken of the sub-See also:ordinate marginal features, such as gulfs and straits. Gulfs may be classified according to their origin as due to fractures of the crust or overflowing of depressed lands. The former are either the extensions of oceanic depressions, e.g. the Arabian Sea, Bay of See also:Bengal and Gulf of See also:Arica, or such caldron-depressions as the Gulfs of See also:Genoa and See also:Taranto, or rift-depressions like,the Gulfs of See also:Aden and See also:Akaba. See also:Compound gulfs are formed sea-wards by fracture and landwards by the overflowing of depressed land, e.g. the Bay of See also:Biscay, Gulf of See also:Alaska and Gulf of the See also:Lion. Gulfs formed by the overflowing of depressed lands See also:lie upon the continental shelf, e.g. the Gulf of See also:Maine, Bay of See also:Fundy, Bay of See also:Odessa, Gulf of See also:Martaban. Straits have been formed (I) by fracture across isthmuses, and such may be by See also:longitudinal fracture as in the Strait of Bab-el-Mandeb, or transverse fracture as in the Strait of See also:Gibraltar or Cook Strait; (2) by erosion, e.g. the Strait of See also:Dover, the See also:Dardanelles and See also:Bosporus; (3) by overflowing through the subsidence of the land, as in the straits of Bering, Torres and See also:Formosa. Surface of the Ocean.—If the whole globe were covered with a uniformly deep ocean, and if there were no difference of See also:density between one part and another, the surface would form a perfect See also:ellipsoid of revolution, that is to say, all the meridians would be exactly equal ellipses and all See also:parallels perfect circles. At any point a See also:sounding line would hang in the line of the See also:radius of curvature of the water surface. But as things are the water-surface is broken by land, and the mean density of the substance of the land is 2.6 times as great as that of sea-water, so that the gravitational attraction of the land must necessarily cause a heaping up of the sea around the coasts, forming what has been called the continental See also:wave, and leaving the sea-level lower in See also:mid-ocean. Hence the See also:geoid or figure of the sea-surface is not part of an ellipsoid of rotation but is irregular. The See also:differences of level between different parts of the geoid have been greatly overestimated in the past; F. G. Helmert hasshown that they cannot exceed 65o ft. and are probably much less. Recent pendulum observations have shown that it is incorrect to assume a uniform density of 2.6 in the elevated part of the earth's crust, that on the contrary there are great See also:local differences in density, the most important being a See also:confirmation of See also:Airy's discovery that there is a marked deficiency of mass under high mountains and a marked excess under the bed of the ocean. The intensity of gravity at the surface of the sea far from land has been measured on several occasions. During See also:Nansen's expedition on the " Fram " in 1894-1895, See also:Scott See also:Hansen made observations with a Sterneck's See also:half-seconds pendulum on the See also:ice where the sea was more than 1600 fathoms deep and found only an insignificant deviation from the number of swings corresponding to a normal ellipsoid. In 1901 O. See also:Hecker took the opportunity of a voyage from Hamburg to La See also:Plata, and in 1904 and 1905 of voyages in the Indian and Pacific Oceans to determine the local attraction over the ocean by comparing the atmospheric pressure measured by means of a See also:mercurial See also:barometer and a boiling-point thermometer, and obtained results similar to Scott Hansen's. The inequalities of the geoid in no See also:case exceed 300 ft. Distortion of the ocean surface may also arise from meteorological causes, and be periodic or unperiodic in its occurrence, but it does not amount to more than a few feet at the utmost. See also:Solar See also:radiation warms the tropical more than the polar waters, but, assuming equal salinity, this cause would not account for a difference of level of more than 20 ft. between tropical and polar seas. The See also:annual range of temperature between summer and See also:winter of a surface layer of water about 25 fathoms thick in the Baltic is as much as 2o° F., but this only corresponds to a difference of level of r; in. due to expansion or contraction. Atmospheric precipitation poured into the sea by the great See also:rivers must necessarily create a permanent rise of the sea-level at their mouths, and from this cause the level round the coasts of See also:rainy lands must be greater than in mid-ocean. H. Mohn has shown how the inequalities of what he terms the density-surface can be found from the salinity and temperature; and he calculates that the level of the Skagerrak should be about 2 ft. higher than that of the open See also:Norwegian Sea between See also:Jan See also:Mayen and the Lofoten Islands. The level of the Gulf of Finland at See also:Kronstadt and of the Gulf of See also:Bothnia at See also:Haparanda should similarly be 15 in. higher than that of the Skagerrak. Recent levellings along the See also:Swedish and Danish coasts have confirmed the higher level of the Baltic; and the level of the Mediterranean has also been determined by exact measurements to be from 15 to 24 in. lower.than that of the Atlantic on account of evaporation. Apart from the effects of varying precipitation and evaporation the atmosphere affects sea-level also by its varying pressure, the difference in level of the sea-surface from this cause between two given points being thirteen times as great as the difference between the corresponding readings of the mercurial barometer. In the north tropical belt of high pressure south of the See also:Azores the atmospheric pressure in See also:January is o•87 in. higher than in the Irminger Sea; hence the sea-level near the Azores is almost 1 ft. lower than in the northern sea. In the See also:monsoon region, where the barometer rises o-38 in. between See also:July and January, the level of the sea falls in consequence by 5 in. See also:Wind also gives rise to differences of level by See also:driving the water before it, and the prevailing See also:westerly wind` of the southern Baltic is the See also:chief cause of the sea-level at See also:Kiel being 51 in. lower than at Arkona on See also:Rugen. Periodic See also:variations of level due to meteorological causes account for the Baltic being See also:fuller in the time of the summer rains than in winter, when the rivers and lakes are frozen and most of the precipitation on the land is in the form of snow. The range on the Arkona See also:gauge is from 3.5 in. below mean level in See also:April to 2.75 in. above the mean level in See also:August. A similar range occurs on the Dutch coast in the North Sea, where the maximum level is reached in See also:October, the See also:month of highest rainfall, and there is a range of 8 in. to the minimum level at the time of least rainfall in See also:early See also:spring. In the monsoon regions the half-yearly change from on-See also:shore to off-shore winds produces noticeable differences in level; thus fifteen years' observations at Aden show a maximum in May at the end of the north-east monsoon, and a rapid falling off after the beginning of the south-west monsoon to a minimum in August, the total range being 9i in. The influence of wind on water-level is most remarkable in heavy storms on the See also:flat coasts of the North Sea and Baltic, when the rise may amount to very many feet. In the region of tropical hurricanes the converging wind system of a circular See also:storm causes a heaping up of water capable of devastating the low See also:coral islands of the Pacific. On the 1st of See also:November 1876 a See also:cyclone acting in this way submerged a great area of the level See also:plain of the See also:Ganges See also:delta to a See also:depth of 46 ft.; here the influence of the difference -f pressure within and without the cyclone acted in the same direction as the wind. The old speculations as to a great difference of level between the Mediterranean and the Red Sea, and on the two sides of the See also:Isthmus of See also:Panama, which hindered the projects for canals connecting those waters, have been proved by modern levelling of high precision to be totally erroneous. Deep-sea Soundings.—The hand-See also:lead attached to a line divided into fathoms was a well-known aid to navigation even in high antiquity, and its use is mentioned in See also:Herodotus (ii. 5) and in the Acts of the Apostles (See also:xxvii. 29). Greater depths than those usually sounded by a hand-line may possibly not have been beyond the reach of the earlier navigators, for Strabo says " of measured seas the Sardonian is the deepest with full one thousand fathoms " (i. 3, p. 53 Cas.). Yet we find that the great discoverers of the modern See also:period were only See also:familiar with the hand-lead, and the lines in use did not exceed 200 fathoms in length. Ingenious devices had indeed been tried in the 17th century and earlier, by which a lead thrown into the sea without a line detached a See also:float on striking the bottom, and it was proposed to calculate the depth by the time required for the float to re-appear. The earliest deep-sea sounding on See also:record is that of Captain Phipps on the 4th of See also:September 1773 in the Norwegian Sea, in 65° N. 3° E., on his return from his expedition to See also:Spitsbergen. He spliced together all the sounding-lines on See also:board, and with a See also:weight of 150 lb attached he found bottom in 683 fathoms and secured a See also:sample of See also:fine soft See also:blue mud. He detected the moment of the lead touching the bottom by the sudden slackening in the See also:rate at which the line ran out. Polar explorers frequently repeated those experiments in deep-sea soundings, both See also: The See also:honour of first sounding really oceanic depths belongs to Sir James See also:Clark Ross, who made some excellent measurements in very deep water, though in a few instances he overestimated the depth by failing to detect the moment at which the lead touched bottom. The pursuit of these isolated investigations received a great impetus from the See also:enthusiasm of the great American oceanographer Captain Matthew Fontaine Maury, U.S.N., who directed the whole impetuous strength of his See also:character to the task of compelling the silent depths of the ocean to tell their See also:tale. Instead of the expensive mile-See also:long stout See also:hemp lines used by Ross, Maury introduced a See also:ball of strong twine attached to a See also:cannon shot, which ran it out rapidly; when the bottom was reached the twine was cut and the depth deduced from the length of See also:string left in the ball on board. The time of touching bottom was judged by timing each roo-See also:fathom See also:mark and noting the sudden increase in the time See also:interval when the shot reached the bottom. Maury, however, recognized that in great depths the surest See also:guarantee of bottom having been reached was to bring up a sample of the See also:deposit. To do this with a heavy lead attached required a very strong hemp line, and the twine used in the American method was useless for this purpose. In 1854 J. M. See also:Brooke, a See also:midshipman of the U.S.N., invented the principle already foreshadowed by Nicolaus See also:Cusanus in the 15th century and by See also:Robert See also:Hooke in the 17th, of using a heavy weight so hung on the sounding-See also:tube that it was automatically released on striking the bottom and left behind, while the See also:light See also:brass tube containing a sample of the deposit was easily hauled up. This principle has been adopted universally for deep soundings, and is now applied in many forms. In 1855 Maury publishedthe first See also:chart of the depths of the Atlantic between 520 N. and ro° S. At this period an exact knowledge of the depths of the ocean assumed an unlooked-for practical importance from the daring project for laying a telegraph cable between See also:Ireland and See also:Newfoundland. Deep soundings were made in the Atlantic for this purpose by vessels both of the British and of the American navies, while in the Mediterranean and in the Indian Ocean many soundings were made in connexion with submarine cables to the East. Another stimulus came from the biologists, who began to realize the importance of a more detailed investigation of the See also:life conditions of organisms at great depths in the sea. The lead in this direction was taken by British biologists, beginning with See also:Edward See also:Forbes in 1839, and in 1868 a party on board H.M.S. " See also:Lightning " pursued researches in the waters to the north of See also:Scotland. In 1869 and 187o this See also:work was extended to the Irish Sea and Bay of Biscay in H.M.S. See also:Porcupine," and to the Mediterranean in H.M.S. " See also:Shearwater." The last-named See also:vessel secured 157 trustworthy deep soundings, with samples of the deposits, and also observations of temperature and salinity in different depths, as well as dredgings for the collection of the organisms of the deep sea. These preliminary trips of scientific marine investigation were followed by the greatest purely scientific expedition ever under-taken, the voyage of H.M.S. " Challenger " round the world under the scientific direction of Sir Wyville See also:Thomson and the See also:naval command of Sir See also:George See also:Nares. This epoch-making expedition lasted from See also:Christmas 1872 to the end of May 1876, and gave the first wide and general view of the physical and biological conditions of the ocean as a whole. Almost simultaneously with the " Challenger," a German expedition in S.M.S. " Gazelle " conducted observations in the South Atlantic, Indian and South Pacific Oceans; and the U.S.S. " See also:Tuscarora " made a cruise in the North Pacific, sounding out lines for a projected Pacific cable. The successor of Sir Wyville Thomson in the editorship of the " Challenger " Reports, Sir John Murray, has rightly said that since the days of See also:Columbus and See also:Magellan no such See also:revelation regarding the surface of our See also:planet had been made as in that eighth See also:decade of the 19th century. Since that time the British cable-ships have been busy in all the oceans making sections across the great expanses of water with ever-increasing accuracy, and in that work the See also:government surveying ships have also been engaged, vast stretches of the Indian and Pacific Oceans having been opened up to knowledge by H.M.SS. " See also:Egeria," " Waterwitch," " Dart," " See also:Penguin," " See also:Stork," and " Investigator." American scientific enterprise, mainly under the guidance of Professor Alexander Agassiz, has been active in the North Atlantic and especially in the Pacific Ocean, where very important investigations have been made. The eastern part of the North Atlantic has been the See also:scene of many expeditions, often purely biological in their purpose, amongst which there may be mentioned the cruises of the " Travailleur " and " See also:Talisman " under Professor Milne-See also:Edwards in 188o-1883, and since 1887 those of the prince of Monaco in his yachts, as well as numerous Danish vessels in the sea between Iceland and See also:Greenland, conspicuous amongst which were the expeditions in 1896-1898 on board the " Ingolf." The Norwegian Sea was studied by the Norwegian expedition on board the " Voringen " in 1876-1878, and the north polar See also:basin by Nansen and See also:Sverdrup in the " Fram " in 1893-1896, the Mediterranean by the Italians on the " See also:Washington " and by the Austrians on the " See also:Polo. " in 189o-1893, the latter carrying the investigations to the Red Sea in 1895-1898, while the Russians investigated the See also:Black Sea in 189o-1893. For high southern latitudes special value attaches to the soundings of the German deep-sea expedition on the " See also:Valdivia " in 1898-1899, and to those of the " Belgica " in 1897-1898, the " See also:Gauss " in 1902-1903, and the " See also:Scotia " in 1903-1904. The soundings of the Dutch expedition on the " Siboga " during 1899-1900 in the eastern part of the Malay seas and those of the German surveying See also:ship " Planet " in 1906 in the South Atlantic, Indian and North Pacific Oceans were notable, and Sir John Murray's expedition on the " See also:Michael Sara " in the Atlantic in 19ro obtained important results. Modern surveying ships no longer make use of hempen lines with enormously heavy sinkers, such as were employed on the " Challenger," but they See also:sound instead with See also:steel piano See also:wire not more than a' to - of an See also:inch in'See also:diameter and a detachable lead seldom weighing more than 701b. The soundings are made by means of a special See also:machine fitted with a See also:brake so adjusted that the revolution of the See also:drum is stopped automatically the instant the lead touches the bottom, and the depth can then be read directly from an See also:indicator. The line is hauled in by a See also:steam or electric winch, and the sounding-tube containing a sample of the bottom deposit is rapidly brought on board. The sounding See also:machines most frequently employed are those of See also:Admiral C. D. Sigsbee, U.S.N., of See also:Lucas, which was perfected in the Telegraph Construction and See also:Maintenance See also:Company's ships, and of the Prince of Monaco, constructed by Leblanc of Paris. All attempts to dispense with a lead and line and to measure the depth by determining the pressure at the bottom have hitherto failed when applied to depths greater than 200 fathoms; a new See also:hydraulic See also:manometer has been tried on board the German surveying ship " Planet." A. See also:Siemens has pointed out that a See also:profile of the sea-bed can be delineated by taking account of the varying See also:strain on a submarine cable while it is being laid, and the See also:average depth of a See also:section can thus be ascertained with some accuracy. All deep-sea measurements are subject to uncertainty because the sounding machine merely measures the length of wire which runs out before the lead touches bottom, and this agrees with the depth only when the wire is perpendicular throughout its run. It is improbable, however, that the smooth and slender wire is much influenced by currents, and the best deep-sea soundings may be taken as accurate to within 5 fathoms. See also:Relief of the Ocean See also:Floor.—Recent soundings have shown that the floor of the ocean on the whole lies some 2 or 3 M. beneath the surface, and O. Krummel has calculated the mean depth to be 2010 fathoms (12,060 ft.), while the mean See also:elevation of the surface of the continents above sea-level is only 2300 ft. Viewed from the floor of the ocean the continental See also:block would thus appear as a great See also:plateau rising to a height of 14,360 ft. Nevertheless, the greatest depths of the ocean below sea-level and the greatest heights of the land above it are of the same order of magnitude, the See also:summit of See also:Mount See also:Everest rising to 29,000 ft. above the sea-level, while the See also:Nero Deep near See also:Guam sinks to 31,600 ft. (5268 fathoms) below sea-level. Of course the area at great heights is very much less than the area at corresponding depths. Above the height of 15,000 ft. there are 800,000 sq. km. (310,000 sq. m.), and below the depth of 15,000 ft. there are 120,000,000 sq. km. (46,300,000 sq. m.); above the height of 20,000 ft. there are on the whole surface of the earth only 33,000 sq. km. (12,800 sq. m.), while below the depth of 20,000 ft. there are no less than 5,400,000 sq. km. (2,100,000 sq. m.). According to Krummel's calculation the areas of the ocean beyond various depths are as follows: Fathoms. sq. km. sq. st. m. More than 350,500,000 135,300,000 too 500 319,500,000 123,400,000 I 000 304,000,000 I I 7,400,000 1500 276,500,000 106,800,000 2000 215,000,000 83,000,000 2500 120,000,000 46,300,000 3000 22,500,000 8,7oo,000 3500 3,000,000 1,200,000 4000 I,200,000 460,000 On the whole the floor of the ocean is very smooth in its contours, and great stretches can almost be called level. Modern orometry has introduced the calculation of the mean See also:angle of the slope of a given uneven surface provided that maps can be prepared showing equidistant See also:contour lines. If the distance between the contour lines is h and the length of the individual contour lines 1, the sum of their lengths (1), and A the areaof the surface under investigation, then the mean angle of slope is obtained from the See also:equation tan a =hl(l)A Calculating from sheet A I of the Prince of Monaco's See also:Atlas of Ocean Depths,' Krummel obtained a mean angle of slope of 0° 27' 44" or an average fall of 1 in 124 for the North Atlantic between o° and 470 N., the enclosed seas being left out of account. In the same way a mean angle of slope of approximately half a degree was found for the Adriatic and the Black Sea. Large angles of slope may, however, occur on the flanks of oceanic islands and the continental See also:borders. On the submarine slopes leading up to isolated volcanic islands angles of 15° to 20° are not uncommon, at St See also:Helena the slopes run up to 382° and even 40°, at See also:Tristan d'Acunha to 332°. E. See also:Hull found a mean angle of slope of 13° to 14° for the edge of the continental shelf off the west coast of Europe, and off Cape Torinana (43°4'N.) as much as 340. Where the French telegraph cable between See also:Brest and New See also:York passes from the continental shelf of the Bay of Biscay to the depths of the Atlantic the angle of slope is from 30° to 41°. Such gradients are of a truly mountainous character, the angle of slope from the Eibsee to the Zugspitze is 3o°, and that from Alpiglen station to the summit of the Eiger is 42°. Particularly steep slopes are found in the case of submarine domes, usually incomplete volcanic cones, and there have been cases in which after such a See also:dome has been discovered by the soundings of a surveying ship it could not be found again as its whole area was so small and the deep floor of the ocean from which it See also:rose so flat that an See also:error of 2 or 3 M. in the position of the ship would prevent any irregularity of the bottom from appearing. While such steep mountain walls are found in the bed of the ocean it must be remembered that they are very exceptional, and except where there are great dislocations of the submarine crust or volcanic outbursts the forms of the ocean floor are incomparably gentler in their outlines than those of the continents. Being protected by the water from the rapid subaerial erosion which sharpens the features of the land, and subjected to the See also:regular See also:accumulation of deposits, the whole ocean floor has assumed some approach to uniformity. Still there are everywhere See also:gentle inequalities on the smoothest ocean floor which give to its greater feattres a distinct relief. In spite of the increase of deep-sea soundings in the last few decades, they are still very irregularly distributed in the open ocean, and the See also:attempt to draw isobaths (lines of equal depth) on a chart of the world is burdened with many difficulties which can only be evaded by the widest generalizations. Bearing this caution in mind the existing bathymetrical charts, amongst which that of the prince of Monaco stands first, give a very See also:fair See also:idea of the great features of the bed of the oceans. A definite terminology for the larger forms of sub-oceanic relief was put forward by the International Geographical See also:Congress at See also:Berlin in 189g and adopted by that at Washington in 1004. Equivalent terms, which are not necessarily identical or literal See also:translations, were adopted for the English, French and German languages, the equivalence being closest and most systematic between the English and German terms. The larger forms designated by special generic terms include the following. The continental shelf is the gentle slope which extends from the edge of the land to a depth usually about too, though in some cases as much as 300 fathoms, and is there demarcated by an abrupt increase in the steepness of the slope to ocean depths. In the deep sea two types of feature are recognized under the general names of depression and elevation. The depression is distinguished according to form and slope as (I) a basin when of a roughly round outline, (2) a trough when wide and elongated, or (3) a See also:trench when narrow and elongated lying along the edge of a continent. The extension of a basin or trough stretching towards the continent is termed an embayment when relatively wide and a See also:gully when narrow. The elevation includes (I) the gently swelling rise which separates See also:Carte generale bathymetrique See also:des oceans dressee See also:par ordre de S.A.S. le Prince See also:Albert de Monaco, 24 sheets (Paris, 1904). troughs and basins in the middle of the ocean, (2) the steeply sloping See also:ridge which interposes a narrower barrier between two depressions, and (3) the plateau or wide elevation rising steeply on all sides from a depression. The deepest part of a depression is termed a deep, and the highest part of an elevation when not reaching the surface a height. In addition to these larger forms a few See also:minor forms must be recognized. Amongst these are the dome, an isolated elevation rising steeply but not coming within too fathoms of the surface; the See also:bank, an elevation coming nearer the surface than too fathoms, but not so near as 6 fathoms; and finally the shoal or See also:reef, which comes within 6 fathoms of the surface, and so may constitute a danger to See also:shipping. Similarly we may See also:note the caldron or small steep depression of a round out-line, and the furrow or long narrow groove in the continental shelf. According to the resolutions of the International Geographical Congress the larger individual forms which have been described by generic terms shall have specific names of a purely geographical character; but in the case of the minor forms the names of ships and persons are considered applicable. In 1899 A. Supan published a chart of the oceans with a suggested nomenclature based on these principles; and the larger forms in the Prince of Monaco's great chart also are named in accordance with the rule. Although put forward by the highest inter-See also:national authority recognized by geographers the system of nomenclature has not been adopted universally. In particular Sir John Murray considers that only deeps exceeding 3000 fathoms in depth should be named, and in his charts he has named these deeps after persons whether the individuals thus honoured had themselves discovered or explored the deeps in question or not. Some of the " deeps " to which names have been given disappear or are divided into two or three smaller deeps when the contour lines representing hundreds of fathoms are translated into contour lines representing hundreds of metres. A similar change in the contour lines may result from the substitution of lines in fathoms for those originally drawn in metres, and hence it is extremely desirable that specific names should only be given to such features as are pronounced enough to appear on maps drawn with either unit. For the See also:sake of uniformity it is to be hoped that the system of nomenclature recommended by the International Geographical Congress will ultimately be adopted.
The continental shelves are parts of the great continental blocks which have been covered by the sea in comparatively recent times, and their surface consequently presents many similarities to that of the land, modified of course by the destructive and constructive work of the waters. Waves and tidal currents produce their full effects in that region, and in high latitudes the effect of transport of materials by ice is very important; while in the warm water of the tropics the reef-See also:building animals and See also:plants (See also:corals and calcareous See also:algae) carry on their work most effectively there. The continental shelves include not only the oceanic border of the continents but also great areas of the enclosed seas and particularly of the fringing seas, the origin of which through See also:secular subsidence is often very clearly apparent, as for instance in the North Sea and the See also:tract lying off the mouth of the English Channel. A closer investigation of the numerous long, narrow See also:banks which lie off the Flemish coast and the See also:Thames See also:estuary shows that they are composed of fragments of See also:rock abraded and transported by tidal currents and storms in the same way that the See also:chalk and See also:lime-See also: Such valleys are very clearly indicated in the belts of the western Baltic by furrows a thousand yards wide and twenty to See also:thirty fathoms deeper than the neighbouring sea-bed. Amongst the best known of the furrows of the continental shelf are the Cape See also:Breton Deep, in the Bay of Biscay, the Hudson Furrow, southward of New York, the so-called See also:Congo See also:Canon, the Swatch of No Ground off the Ganges delta, the Bottomless See also:Pit off the See also:Niger delta, and numerous similar furrows on the west coast of North America and outside the fjords of Norway, Iceland and the west of Scotland, as well as in the See also:Firth of Forth and See also:Moray Firth. The seaward edge of the continental shelf often falls steeply to the greatest depths of the ocean, and not infrequently forms the slope of a trench, a form of depression which has usually a steep slope towards a continent or an island-bearing rise on one side and a gentler slope towards the general level of the ocean on the other. All the greatest depths of ocean, i.e. all soundings exceeding 4000 fathoms, occur in trenches, and there are only a few small trenches known (on the west coast of Central America) in which the maximum depth is less than 3000 fathoms. Most trenches are narrow, but of considerable length, and their steeper side is believed to be due in every case to a great fracture of the earth's crust. Strong evidence of this is afforded by the association of some of the depressions, notably the Japan Trench and the See also:Atacama Trench, with the origin of frequent submarine earthquakes. Troughs and rises are features of more frequent occurrence and are best described as they occur in the particular oceans. In the Atlantic the prevailing meridianal direction of the shore lines extends to the submarine features also. Captain Sherard See also:Osborn in 187o was the first to recognize that the North Atlantic Basin was divided by a central rise running generally from north to south into two parallel depressions. In 1876 the " Challenger " expedition found that a similar configuration exists in the South Atlantic also. As the result of all the deep-sea surveys now available we know that the central rise of the Atlantic starts from Iceland as the Reykjanes Ridge with less than loon fathoms of water over it in most parts and runs south-westward until in 51° N. it widens into what was called by Maury the Telegraph Plateau. Continuing southwards the rise joins the Azores Plateau, which has in parts a very marked relief, and runs thence southward almost exactly in the middle of the ocean, becoming gradually lower as it goes. As far as 29° N. the depth over it is less than 1500 fathoms, thence to See also:r2° N. the depths are between 1500 and 2000 fathoms, and then it rises again to about 15oo fathoms and runs eastward under the name of the See also:Equatorial Ridge. See also:Crossing the equator in 13° W. the rise resumes a southerly direction and from See also:Ascension to Tristan d'Acunha, the depth is in many places less than 1500 fathoms. The soundings of See also:Bruce's Antarctic expedition in the " Scotia " showed that the rise cannot be traced beyond 55° S. where the depths increase rapidly to over 2000 fathoms. The whole length of the rise which divides the Atlantic into an eastern and a western basin may be taken as 7500 nautical miles. Between 3o and 4o° S. two lateral ridges diverge from the great Atlantic rise, the Rio Grande ridge towards the north-west and the Walfisch ridge towards the north-east. The existence of the latter, which extends to the See also:African continent, was announced by Sir Wyville Thomson in 1876 as a result of his discussion of the deep-sea temperature observations of the " Challenger " expedition, though the fact was not confirmed by soundings until many years later. The West Atlantic Trough lying on the western side of the Central Rise widens in the north into the North American Basin, and its greatest depths appears to be in the See also:Porto Rico Trench, where in 1882 Capt. W. H. See also:Brownson, U.S.N., obtained a sounding of 4561 fathoms in 19° 36' N., 66° 26' W. The Brazilian Basin has also a large area lying at a depth greater than 2500 fathoms and culminates in the Romanche Deep close to the Equatorial Ridge in o° t x' S., r8° 15' W. with a depth of 4030 fathoms. The Eastern Atlantic Trough cannot boast of such great depths though the Peake Deep with 3284 fathoms sinks abruptly from the Azores Plateau in 43° 9' N., 19° 45' W., and several soundings exceeding 2700 fathoms have been obtained in the Bay of Biscay east of the meridian of 5° E. The North African Basin has several deeps with more than 3300 fathoms to the north-west and the south-west of the Cape Verde Islands, but the South African Basin is less deep. In the South Atlantic there is no connexion between the Central Rise and the Antarctic Shelf, for the Indo-Atlantic Antarctic Basin stretches from near the The trenches of Yap (4122 fathoms) and Palau (Pelew) (445o South See also:Sandwich Islands towards Kerguelen with depths exceeding 2500 fathoms and reaching in places 3100. The Cape Trough runs northward from this basin. It was long believed on the strength of a sounding of " 4000 fathoms, no bottom " reported by Sir James Ross in 68° 22' S., 12° 49' W., that the Indo-Atlantic Basin was of enormous depth, but W. S. Bruce, in the " Scotia," showed in 1904 that the real depth at that point is only 266o fathoms. In the Indian Ocean the Kerguelen Rise stretches broadly southward, east of the island which gives it a name, to the Antarctic Shelf with the greatest depths upon it usually less than 2000 fathoms, and it stretches northward beyond New See also:Amsterdam to 3o° S. This rise is separated from the Crozet Rise by a depression extending to 2675 fathoms, through which the Kerguelen Trough (which lies north of Kerguelen) is brought into See also:free communication with the Indo-Atlantic Antarctic Basin. The greater part of the Indian Ocean is occupied by the great Indian Basin, which covers 35,000,000 sq. km. (13,500,000 sq. m.) and extends from the See also:Chagos Islands eastward to Australia and south-eastward to Tasmania. The Australian Shelf rises steeply as a rule from depths of 2500 to 3000 fathoms. A broad depression with depths of from 3300 to 3500 fathoms lies to the east of the Cocos Islands and extends into the angle between the Malay See also:Archipelago and Australia. On the north this depression sinks into the long and narrow Sunda Trench south of See also:Java, and here in ro° 15' S., 108° 5' E., the German surveying-ship " Planet " obtained a sounding of 3828 fathoms in 1906. The Sunda Trench is distinguished by the wave-like configuration of its floor, and this wave-like character is continued to the westward of See also:Sumatra with islands rising from the higher portions. The western part of the Indian Ocean has been shown by the surveys cf H.M.S. " Sealark " and the German surveying-ship " Planet " to have a somewhat complicated. configuration, the island See also:groups and banks of atolls which occur there rising abruptly as a rule from depths of about 2000 fathoms or more. Between the See also:Seychelles and See also:Sokotra (o°—9° N.) there are great stretches of the ocean floor forming an almost level expanse at a depth of 2800 fathoms. The Arabian Gulf and Gulf of Aden are also very uniform with depths of about 'goo fathoms, while the floor of the Bay of Bengal rises very gradually northwards and is loon fathoms deep close up to the Ganges Shelf. The Pacific Ocean consists mainly of one enormous basin bounded on the west by New Zealand and the See also:Tonga, See also:Marshall and Marianne ridges, on the north by the festoons of islands marking off the North Pacific fringing seas, on the east by the coast of North America and the great See also:Easter Island Rise and on the south by the Antarctic Shelf. The total area of this basin is about 80,000,000 sq. km. (30,000,000 sq. m.), its surface being almost twice that of Asia. Half of this basin lies deeper than 2750 fathoms, and the greater part of it belongs to the northern hemisphere. From the floor of this vast and profound depression numerous isolated volcanic cones rise with abrupt slopes, and even between the islands of the Hawaiian See also:group there are depths of more than 2000 fathoms. The Society Islands and See also:Tahiti See also:crown a rise coming within 1500 fathoms of the surface, two similar rises form the See also:foundation of the See also:Paumotu group where Agassiz found soundings of 2187 fathoms between Marokau and Hao. This greatest of ocean basins contains also the largest and deepest trenches. The Tuscarora Deep of the Japan Trench (4655 fathoms in 44° 55' N., 152° 26' E.) was famed for many years as the deepest depression of the earth's crust. This great trench is continued along the Luchu Islands where the cable-steamer " See also:Stephan " sounded in 4080 fathoms, and through the Bonin Trench with a maximum of 3595 fathoms) to the famous Marianne Trench in which the U.S.S. " Nero " in 1899 found 5269 fathoms in 12° 43' N., 145° 49' E., the greatest depth yet measured. The northern part of the Marianne Trench leads to a wave-like configuration of the ocean floor, the depth to the east of Saipan being over 4300 fathoms, followed by a rise to 1089 fathoms and then a descent to 3167 fathoms.fathoms) are not immediately connected with that of Marianne. To the east of the Philippines a sounding of 3490 fathoms is found close to the Strait of St Bernardino and north-east of Talaut there is a trench with 4648 fathoms. To the north-east the Japan Trench adjoins the Aleutian Trench, where a depth of 4038 fathoms has been found south-west of Attu. Trenches of great See also:size also occur south of the equator. The Tonga and See also:Kermadec trenches, both deeper than 4000 fathoms, stretch from the See also:Samoa Islands southwards toward New Zealand for a distance of 1600 nautical miles. The deepest sounding obtained in the Tonga Trench is 5022 fathoms in 23° 39.4' S., 175° 4' W., and in the Kermadec Trench, 5155 fathoms, 3o° 27.7' S., 176° 39' W. The steep western sides of these trenches often show an angle of slope of 7°. The south-western part of the Pacific Ocean has a very See also:rich and diversified submarine relief, abounding in small basins separated by ridges and rises. There are no depths, however, much exceeding 2500 fathoms amongst these depressions. The south-eastern part of the Pacific is mainly occupied by the Easter Island Rise with depths rarely so great as 2000 fathoms; but close to the continent of South America the Atacama Trench is a typical example of the deepest form of depression culminating with 4175 fathoms in 25° 42' S., 71° 31.5' W. The Pacific Antarctic Basin occupies the vast region south of 5o° S. right up to the Antarctic Shelf, with depths ranging down to 2500-3000 fathoms, and communicating with the main Pacific Basin to the east of New Zealand. The greatest of the intercontinental seas, the Arctic, comes nearest to oceanic conditions in the extent and depth of its depressions. The soundings of Nansen and Sverdrup on the " Fram " expedition indicate that northward from the Siberian Shelf the great North Polar Basin has an area of about 4,000,000 sq. km. (1,500,000 sq. m.) with depths down to 2200 fathoms. A rise between Spitsbergen and Greenland separates the Norwegian Trough (greatest depth 2005 fathoms in 68° 21' N., 2° 5' W.) which in turn is divided from the Atlantic by the Wyville Thomson Ridge which runs between the See also:Faeroe and See also:Shetland islands and is covered by only 314 fathoms of water at the deepest point. The ridge across Denmark Strait west of Iceland nowhere exceeds 300 fathoms in depth, so that the deeper water of the North Polar Basin is effectively separated from that of the Atlantic. A third small basin occupies See also:Baffin Bay and contains a maximum depth of 1050 fathoms. Depths of from roo to 300 fathoms are not uncommon amongst the channels of the Arctic Archipelago north of North America, and Bering Strait, through which the surface water of the Arctic Sea meets that of the Pacific, is only 28 fathoms deep. The Central American Sea communicates with the Atlantic through the channels between the See also:Antilles, none of which is quite r000 fathoms deep, and it sinks to a depth of 2843 fathoms in the Caribbean Basin, 3428 fathoms in the Cayman Trench and 2080 fathoms in the Gulf of See also:Mexico. The Austral-Asiatic or Malay Sea is occupied by a great shelf in the region west of See also:Borneo and north of Java, while in the east there are eight abruptly sunk basins of widely different size. The China Sea on the north has a maximum depth of 2715 fathoms off the Philippines, the Sulu Basin reaches 2550 fathoms, and the See also:Celebes Basin 2795 fathoms. Some of the channels between the islands are of very great depth, See also:Macassar Strait exceeding r000 fathoms, the Molucca Passage exceeding 2000 fathoms, and the See also:Halmahera Trough sinking as deep as 2575 fathoms. The deepest of all is the See also:Banda Basin, a large area of which lies below 2500 fathoms and reaches 3557 fathoms in the Kei Trench. A depth of 2789 fathoms also occurs north of See also:Flores. The borders of the Malay Sea are everywhere shallower on the side of the Indian Ocean than on that of the Pacific, and consequently water from the Pacific preponderates in the depths. The Mediterranean Sea, the best-known member of the inter-continental class, is separated from the Atlantic Ocean by a ridge running from Cape Spartel to Cape See also:Trafalgar on which Name. Depth. Area. See also:Volume. -- cb. km. cb. st. m. Fathoms. sq. km. sq. st. m. Atlantic Ocean . 2110 81,657,800 31,529,390 314,821,680 75,533,900 Indian Ocean . 2148 73,441,960 28,357,150 288,527,610 69,225,200 Pacific Ocean 2240 165,715,490 63,985,370 678,837,190 162,870,600 I. Oceans . 2186 320,815,250 123,871,910 1,282,186,480 307,629,700 Arctic Sea . 640 14,352,340 5,541,630 16,794,140 4,029,400 Malay Sea . 595 8,125,060 3,137,210 8,848,110 2,122,900 Central American Sea 1143 4,584,570 1,770,170 9,579,490 2,298,400 Mediterranean Sea 782 2,967,570 1,145,830 4,249,020 1,019,400 Intracontinental Seas 718 30,029,540 11,595,840 39,470,760 9,470,100 Baltic Sea . 30 406,720 157,040 22,360 5,360 Hudson Bay 70 1,222,610 472,070 156,690 37,590 Red Sea 267 458,480 177,030 223,810 53,700 Persian Gulf 14 232,850 89,910 5,910 1,420 Smaller Enclosed Seas 96 2,320,660 896,050 408,770 98,070 II. Enclosed Seas 674 32,350,200 12,490,890 39,879,530 9,568,170 Bering Sea . 790 2,274,800 878,340 3,286,230 788,500 Okhotsk Sea 694 1,507,610 582,110 1,895,100 454,700 Japan Sea 837 1,043,820 403,040 1,597,040 383,200 East China Sea . 97 1,242,480 479,740 219,820 52,700 Andaman Sea 426 790,550 305,240 615,910 147,770 Californian Gulf 540 166,790 64,400 164,590 39,490 North Sea . 51 571,910 220,820 53,730 12,890 Irish Sea 34 213,380 82,390 13,320 3,200 Laurentian Sea . 70 219,300 84,670 28,100 6,740 See also:Bass Sea . 39 83,170 32,110 6,020 1,440 Seas (Enclosed and Fringing) . 645 40,464,010 15,623,750 47,759,390 11,458,800 Hydrosphere 2013 361,279,160 139,495,660 1,329,945,870 319,087,500 lies the extremely shallow Gulf of See also:Azov; but the greater part of the sea consists of a deep basin, the central part of which is an almost flat expanse at a uniform depth of 1220 fathoms. The smaller enclosed seas are for the most part very shallow. The Persian Gulf nowhere exceeds 5o fathoms, the southern part of Hudson Bay does not exceed too fathoms except at one spot, though in the less-known fjords of the northern part depths up to 200 fathoms have been reported. The Baltic Sea exceeds 5o fathoms in few places except the broad central portion, though small caldron-like depressions here and there may sink below 200 fathoms. The Red Sea on the other hand, though shut off from the Indian Ocean by shallows of the Strait of Bab-el-Mandeb with little more than too fathoms, sinks to a very considerable depth in its central trough, which reaches 1209 fathoms in 20° N. The fringing seas as a rule show little variety of submarine relief. The Bass Sea (Bass Strait), Irish Sea and North Sea lie on the continental shelf. In the North Sea the depth of too fathoms is only exceeded to any extent in the Norwegian gully, which has a maximum depth of 383 fathoms in the See also:Skagerrack. the greatest depth is only 175 fathoms. The depth increases so rapidly towards the east that soundings exceeding 500 fathoms occur off Gibraltar. The Balearic Basin, between See also:Spain and the rise bearing See also:Corsica and See also:Sardinia, has a maximum depth of 1742 fathoms, and the Tyrrhenian Basin between that rise, See also:Italy and See also:Sicily deepens to 2040 fathoms. The larger Eastern Mediterranean Basin stretches eastward from Sicily with large tracts more than 2000 fathoms below the surface, and the greatest depth ascertained during the detailed researches of the See also:Austrian expedition on board the " See also:Pola " was 2046 fathoms in 350 44'8' N., z 1° 46.8' E. The Adriatic Sea though very shallow in the north deepens southward to about goo fathoms, and the See also:Aegean Sea has a maximum depth of 1230 fathoms north of See also:Crete. The Black Sea, connected with the Mediterranean by long and narrow channels, is occupied in the north by an extensive shelf on which Mean Depths of Oceans and Seas. Most of the other seas of this class are formed on a common See also:plan. Towards the continent there is a broad shelf, and just before the See also:chain of islands separating them from the ocean runs a narrow and deep trough. In the Bering Sea the trough north of Buldir in the Aleutian Islands sinks to 2237 fathoms, and in the Sea of Okhotsk, north-west of the See also:Kuriles, to 1859 fathoms. Similar conditions prevail in the East China Sea and the Andaman Sea. The Sea of Japan has a wide shelf only in the north, the central part forms a broad basin with depths of 165o fathoms. The Laurentian Sea (Gulf of St Lawrence) has a narrow branching gully running between wide shelves, in which a depth of 312 fathoms is found south of See also:Anticosti. The area, general depth and total volume of the oceans and See also:principal seas have been recalculated by Krummel, and the accompanying table presents these figures. Oceanic Deposits.-It has long been known that the deposits which See also:carpet the floor of the ocean differ in different places, and See also:coasting sailors have been accustomed from time immemorial to use the lead not only to ascertain the depth of the water but also to obtain samples of the bottom, the See also:appearance of which is often characteristic of the locality. In depths down to too fathoms the old-fashioned hand-lead, hollow below and " armed " with See also:tallow, suffices to bring up a sample large enough to be recognizable. Captain Phipps in 1773 secured samples of soft blue See also:clay in this manner from a depth of 683 fathoms, but as a rule when sounding in great depths the sample is washed off the tallow before it can be brought on board. Various devices have consequently been attached to leads intended to catch and hold the material when soft enough to be penetrated. One of the most effective early forms was the snapper or " deep-sea clamm " of Sir John Ross, a pair of powerful spring jaws held apart by an arrangement which when released on striking the bottom allowed the jaws to close, biting out and holding securely a substantial portion of the ground. A simpler form of See also:collector, now almost universally used, is a plain brass tube which is driven into the bottom of the sea by the weight of the sounding lead, and in which the deposit may be retained by a See also:valve or other contrivance, though in many cases See also:friction alone suffices to hold the punched-out core. Larger quantities of deposit may be conveniently collected by means of the dredge, which can be worked in any depth and brings up large stones, concretionary nodules or fossils, of the existence of which a sounding-tube could give no indication. The voyage of the " Challenger " supplied for the first time the See also:nucleus of a collection of deep-sea deposits sufficient to serve as the basis for comprehensive See also:classification and mapping. The " Challenger " collections supplemented by those of other expeditions and of many telegraph and surveying-ships were studied in detail by Sir John Murray and Professor A. See also:Renard, whose monograph,' published in 1891, laid the See also:foundations and ' " Challenger " Reports, " Deep Sea Deposits." reared the greater part of the structure of our present knowledge on the subject. The classification adopted was a See also:double one, taking account both of the origin and of the See also:distribution in depth of the various deposits, thus: I. Red Clay. 2. Radiolarian See also:Ooze A. PELAGIC D E P O S I T S 3. Diatom Ooze (formed in deep water 4. See also:Globigerina Ooze remote from land) I. DEEP SEA DEPOSITS 5. Pteropod Ooze (beyond See also:loo fathoms) 6. Blue Mud 7. Red Mud 8. See also:Green Mud Q. Volcanic Mud to. Coral Mud H. SHALLOW WATER} DEPosrrs (in less Sands, gravels, muds, &c. than xoo fathoms) (between high and Sands, gravels, muds, &c. low-water marks) Rrummel prefers to simplify this by grouping the deposits in a single See also:category arranged according to their position into: (a) Littoral (including Murray and Renard's littoral and shallow water deposits [II. and III.]). (~) Hemi pelagic (including Nos. 6-Io of Deep Sea Deposits). (7) Eupelagic (including Nos. 1-5 of Deep Sea Deposits). As so defined the hemipelagic deposits are those which occur in general on the slope from the continental shelves to the ocean depths and also in the deep basins of enclosed and fringing seas. The eupelagic deposits are subdivided by Krummel into two main groups; (a) epilophic,i including the pteropod, globigerina and diatom oozes occurring on the rises and ridges and in the less deep troughs. (b) Abyssal, including the radiolarian ooze and red clay of the deepest abysses. The littoral deposits include those of the actual shore on the See also:wash of the waves and of the surface of the continental shelf. Shore Deposits are the product of the See also:waste of the land arranged and bedded by the See also:action of currents or tidal streams. On the rocky coast of high latitudes blocks of stone detached by See also:frost fall on the See also:beach and becoming embedded in ice during winter are often drifted out to sea and so carry the shore deposits to some distance from the land. Similar effects are produced along the See also:boulder-clay cliffs of the Baltic. Where the force of the waves on the beach produces its full effect the coarser material gets worn down to gravel, See also:sand and silt, the finest particles remaining long suspended in the water to be finally deposited as mud in quiet bays. A particularly fine-grained mud is formed on the low coasts of the eastern border of the North Sea by a mixture of the finest sediment carried down by the slow-running rivers with the calcareous or siliceous remains of See also:plankton. Pure calcareous sand and calcareous mud are formed by wave action on the shores of coral islands where the only material available is coral and the accompanying calcareous algae, See also:crustacea, molluscs and other organisms secreting carbonate of lime. Recent limestones are being produced in this way and also in some places by the precipitation of See also:calcium carbonate by See also:sodium or ammonium carbonate which has been carried into the sea or formed by organisms. The precipitated carbonate may See also:agglomerate on See also:mineral or organic grains which serve as nuclei, or it may form a sheet of hard deposit on the bottom as occurs in the Red Sea, off See also:Florida, and round many coral islands in the Pacific. ' Only the sand and the finest-grained sediments of the shore See also:zone are carried outwards over the continental shelf by the tides or by the reaction-currents along the bottom set up by on-shore winds. The very finest sediment is kept in a See also:state of See also:movement until it drops into the gulleys or furrows of the shelf, where it can come to See also:rest together with the finer fragments of the remains of littoral or bank vegetation. Thus are formed the " mud-holes " of the Hudson Furrow so welcome as guides telling their position to ship captains making New York See also:harbour in a See also:fog. Sand may be taken as the predominating deposit on the continental shelves, often with a large admixture of remains of calcareous organisms, for instance the deposits of maerl made up of nullipores off the coasts of See also:Brittany and near Belle Isle. Amongst the most widely distributed of the I Errl )t600rs--on the See also:threshold.deposits actually formed on the continental shelf are phosphatic nodules; these are especially abundant on the east coast of the United States and on the Agulhas Bank, where the amount of calcium phosphate in the nodules is as much as 5o%. Sir John Murray finds the source of the phosphoric See also:acid to be the decomposition of large quantities of See also:animal matter, and he illustrates this by the well-known circumstance of the See also:death of vast shoals of See also:fish when warm Gulf-Stream water displaces the See also:cold current which usually extends to the American coast. Glacial detritus naturally plays a great part in the deposits on the polar continental shelves. Hemipelagic deposits are a mixture of deposits of See also:terrigenous and pelagic origin. The most abundant of the terrigenous materials are the finest particles of clay and calcium carbonate as well as fragments derived from land vegetation, of which twigs, leaves, &c., may form a perceptible proportion as far as 200 M. from land. Blue mud, according to Murray and Renard, is usually of a blue or slaty or See also:grey-green See also:colour when fresh, the upper surface having, however, a reddish tint. The blue colouring substance is ferrous sulphide, the upper reddish layer contains more ferric See also:oxide, which the predominance of decomposing organic matter in the substance of the mud reduces to ferrous oxide and subsequently by further action to sulphide. The proportion of calcium carbonate varies greatly according to the amount of See also:foraminifera and other calcareous organisms which it contains. Blue mud prevails in large areas of the Pacific Ocean from the Galapagos Islands to Acapulco. In the Indian Ocean it covers the Bay of Bengal, the Arabian Gulf, the See also:Mozambique Channel and the region to the south-west of See also:Madagascar. In the Atlantic it is the characteristic deposit of the slopes of continental shelves of western Europe and of New See also:England, being largely mixed with ice-See also:borne material to the south of Newfoundland. It is particularly in evidence round the whole of the Antarctic Shelf, where it occurs down to depths of 2 500 fathoms. It is the chief deposit, according to Nansen, of the North Polar Basin and, according to Schmelck and Boggild, of the Norwegian Sea also, where it is largely mixed with the shells of the bottom-living foraminifer Biloculina. Max See also:Weber states that' blue mud occurs in the deep basins of the eastern part of the Malay Sea. In the form of volcanic mud it is common round the high volcanic islands of the South-Western Pacific. Red mud may be classed as a variety of blue mud, from which it differs on account of the larger proportion of ochreous sub-stance and the See also:absence of sufficient organic matter to reduce the whole of the ferric oxide. This variety surrounds the tropical parts of the continental shelves of South America, South Africa and eastern China. Green mud differs to a greater extent from the blue mud, and owes its characteristic nature and colour to the presence of See also:glauconite, which is formed inside the cases of foraminifers, the spines of echini and the spicules of See also:sponges in a manner not yet understood. It occurs in such abundance in certain See also:geological formations as to give rise to the name of green-sand. Green mud abounds off the east coast of North America seawards of Cape Hatteras, also to the north of See also:Cuba, and on the west off the coast of California. The " Challenger " expedition found it on the Agulhas Bank, on the eastern coasts of Australia, Japan, South America and on the west coast of See also:Portugal. When the pro-portion of calcium carbonate in the blue mud is considerable there results a calcareous ooze, which when found on the continental slope and in enclosed seas is largely composed of remains of deep-sea corals and bottom-living foraminifera, pelagic organisms including pteropods being less frequently represented. The floors of the Caribbean, Cayman and Mexican Basins in the Central American Sea are covered with a See also: Additional information and CommentsThere are no comments yet for this article.
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