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1911 Encyclopedia Britannica

Ocean And Oceanography

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AND OCEANOGRAPHY. " Ocean " is the name applied to the great connected sheet of water which covers the greater part of the surface of the Earth. It is convenient to divide the subject-matter of physical geography into the atmosphere, hydrosphere and lithosphere, and in this sense the ocean is less than the hydrosphere in so far as the latter term includes also the water lying on or flowing over the surface of the land. The conception of an encompassing ocean bounding the habitable world is found in the creation myths of the most ancient civilizations. The Babylonians looked on the world as a vast round mountain rising from the midst of a universal sheet of water. In the Hebrew scriptures the waters were gathered together in one place at the word of 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 Oceanus, the name of the primeval god, father of gods and men, and thus the bond of union between heaven and earth. The Greek word c'eiceavos is related to the Sanskrit arayanas, " the encompassing." Philologists do not know of any related word in Semitic languages.

Pictet, however, recognizes allied forms in Celtic languages, e.g. the Irish aigean and Cymric eigiawn. Since the Pythagorean school of philosophy upheld the spherical as against the disk-shaped world, some of the ancient geographers, including Eratosthenes and Strabo, looked upon the hydrosphere as forming two belts at right angles to each other, one belt of ocean following the equator, the other surrounding the earth from pole to pole as in the terra quadrifida of Macrobius; while others, including Aristotle and Ptolemy, looked upon the inhabited land, or oikumene, as occupying the greater part of the earth's surface, so that the Indian Ocean was an enclosed sea and India (i.e. eastern Asia) was only separated from Europe by the Atlantic Ocean. The latter view prevailed and was as a rule held by the Arab geographers of the middle ages, so that until the discovery of America and of the Pacific Ocean the belief was general that the land surface was greater than the water surface, or that at least the two were equal, as Mercator and Varenius held. Thus it was that a great South Land appeared on the maps, the belief in the prodigious extension of which certainly received a severe shock by Abel Tasman's voyage of circumnavigation, but was only overthrown after Cook's great voyages had proved that any southern land which existed could not extend appreciably beyond the polar circle. Only in our own day has the existence of the southern continent been demonstrated within the modest limits of Antarctica.

Oceanography is the science which deals with the ocean, and since the ocean forms a large part of the earth's surface oceanography is a large department of geography. The science is termed talassografia by the Italians, and attempts have been made without success to introduce the name " thalassography." Of recent years the use of " hydrography " as the equivalent of physical oceanography has acquired a certain currency, but as the word is also used with more than one other meaning (see 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 common to the whole ocean, and special oceanography, which has to do with the individual characteristics of the various divisions of the ocean. This article is restricted to general oceanography in its physical aspects, the closely-related meteorological,, biological and economic aspects being dealt with elsewhere.

1 Methods of Research

2 Extent of the Ocean

3 Surface of the Ocean

4 Dissolved Gases of Sea-water

5 Temperature of the Oceans

6 Sea Ice

7 Oceanic Circulation

8 AuTxoRrrIES

Methods of Research

When research in oceanography began, the conditions of the sea were of necessity observed only from the coast and from islands, the information derived from mariners as to the condition of parts of the sea far from land being for the most part 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 century, observations at sea with accurate instruments have become common, and the ships' logs of to-day are provided with headings for entering daily observations of the phenomena of the seasurface. The contents of the sailors' scientific logs were brought together by the American enthusiast in the study of the sea, Matthew Fontaine Maury (1806-1873), whose methods and plans were discussed and adopted at international congresses held in Brussels in 1853 and in London in 1873. By 1904 more than 6800 of these meteorological logs with 7,000,000 observations had been accumulated by the Meteorological Office in London; 20,000 with io,600,000 observations by the German Marine Observatory at Hamburg; 4700 with 3,300,000 observations by the Central Institute of the Netherlands at de Bilt near Utrecht. The Hydrographic Office of the United States had collected 3800 meteorological logs with 3,200,000 entries before 1888; but since that time the logs have contained only one observation daily (at Greenwich noon) and of these 2,380.000 entries had been received by 1904. In the archives of the French Marine in Paris there were 3300 complete logs with 830,000 entries and II ,000 abstract logs from men-of-war. The contents of these logs, it is true, refer more to maritime meteorology than to oceanography properly so-called, as their main purpose is to promote a rational system of navigation especially for sailing ships, and they are supplied by the voluntary co-operation of the sailors themselves.

While the sailors' logs supply the greater part of the scientific 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 Captain James Cook and the polar explorers, especially those of Sir John Ross in the north and Sir James Ross in the south, but the voyage of H.M.S. " Challenger " in1872-1876formed an epoch marking the end of the older order of things and the beginning of modern oceanography as a science of precision. The telegraph cable companies were quick to apply and to extend the oceanographical methods useful in cable-laying, and to their practical acuteness many of the most important improvements in apparatus are due. A second epoch comparable to that of the " Challenger " and resulting like it in a leap forward in the precision of the methods previously employed was marked by the institution in 1901 of the International Council for the Study of the Sea. This council was nominated by the governments of Norway, Sweden, Denmark, Finland, Russia, Germany, Great Britain, Holland and Belgium, with headquarters in Copenhagen and a central laboratory at Christiania, and its aim was to furnish data for the improvement of the fisheries of the North Sea and surrounding waters. In the course, of investigating this special problem great improvements were made in the methods of observing in the deep sea, and also in the representation and discussion of the data obtained, and a powerful stimulus was given to the study of oceanography in all the countries of Europe. The efforts of individual scientific workers cannot as a rule produce such results in oceanography as in other sciences, but exceptions are found in the very special services rendered by the prince of Monaco, who founded the Oceanographical Institute in Paris and the Oceanographical Museum in Monaco; and by Professor Alexander Agassiz in the investigation of the Pacific.

Extent of the Ocean

The hydrosphere covers nearly threequarters of the earth's surface as a single and continuous expanse of water surrounding four great insular land-masses known as the continents of the Old World (Europe, Asia, Africa), America, Australia and Antarctica. As we are still ignorant of the proportions of land and water in the polar regions, it is only possible to give approximate figures for the extent of the ocean, for the position of the coast-lines is not known exactly enough to exclude possible errors of perhaps several hundred thousand square miles in estimates of the total area. 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 hand, recent Antarctic exploration makes it practically certain that a great continent surrounds the south pole with a total area considerably more than Sir John Murray's estimate in 1894, when he assigned to it an area of 9,000,000 sq. km. (3,500,000 sq. statute miles). It is probable that the Antarctic continent measures about 13,000,000 sq. km. (5,000,000 sq. statute miles); and thus if we accept Bessel's figure of 509,950,000 sq. km. (196,900,000 sq. m.) for the whole surface of the sphere, there is a total land area of 148,820,000 sq. km. (57,460,000 sq. m.), and a total water area of 361,130,000 sq. km. (139,435,000 sq. m.), 29% of land and 7 1% of water, or a ratio of 1: 2.43 Divisions of the Ocean. - The arrangement of the water surface on the globe is far from uniform, the ocean forming 61% of the total area of the northern and 8, % of that of the southern hemisphere. Of the whole ocean only 43% (1 54.9 million sq. km.) lies in the northern hemisphere and 57% (206.2 million sq. km.) in the southern. If the globe is divided into hemispheres by the meridians of 20° W. and 160° E., as is usual in atlases, the eastern hemisphere, to which the Old World belongs, has 62% of its surface made up of water, while the western hemisphere, including America, has 81%. A great circle can be drawn upon a terrestrial globe in such a way as to divide it into two hemispheres, one of which contains the greatest amount of land and the other the greatest amount of sea of any possible hemispheres. The centre of the so-called land-hemisphere lies near the mouth of the Loire (471° N. and 21° W.), while the centre of the so-called water-hemisphere lies to the S.E. of New Zealand and eastward of Antipodes 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 continental masses therefore give the ocean a distinctly tripartite form, the three great divisions being known as the Atlantic, the Indian and the Pacific Oceans, all three 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 - Drake Strait south of South America is barely 400 m. wide, from Cape Agulhas to Enderby Land, 2200 m., and from 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 Bering Strait. These proportions are not readily grasped from a map of the world on Mercator's projection, and must be studied on a globe. A simple, practical boundary between the three oceans can be obtained by prolonging the meridian of the southern extremity of each of the three southern continents to the Antarctic circle. A committee of the Royal Geographical Society - the deliberations of which were interrupted by the departure on his last voyage of Sir John 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 close to the coast of Antarctica that the Antarctic Ocean may be left out of 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 side, and on the Atlantic side the narrowest part of Davis Strait, and of Denmark Strait, then the shortest line from 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 Horn through Snow Island to Cape Gunnar, instead of the meridian of Cape Horn. Possibly ridges of the sea-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 Herschel took as the northern boundary of the southern ocean the greatest circle which could 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 Malte Brun (1803) and Supan (1903), take the loxodromes between the three capes and call the ocean to the south the Antarctic Ocean. G. v. Boguslawski suggested the parallel of 55° S. and Ratzel that of 40° 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 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 mass, and geologically they are of recent age and still subject to 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 present position from still remoter ages.

Seas may be classified according to their form either as " enclosed " 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 amplitude. Four great intercontinental enclosed seas are included between adjacent continents - the Arctic Sea, the Central American or West Indian Sea, the Australo-Asiatic or 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 Hudson Bay with very low salinity, the Red Sea and 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 lower than that of ocean water. The four fringing seas of eastern Asia, those of Bering, Okhotsk, Japan and East 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 California. On the contrary, the North Sea, the British fringing seas (English Channel, Irish Sea and Minch), and the Gulf of St Lawrence cross the main lines of dislocation.

In addition to these seas notice must be taken of the subordinate 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 Bengal and Gulf of Arica, or such caldron-depressions as the Gulfs of Genoa and Taranto, or rift-depressions like the Gulfs of Aden and Akaba. Compound gulfs are formed seawards by fracture and landwards by the overflowing of depressed land, e.g. the Bay of Biscay, Gulf of Alaska and Gulf of the Lion. Gulfs formed by the overflowing of depressed lands lie upon the continental shelf, e.g. the Gulf of Maine, Bay of Fundy, Bay of Odessa, Gulf of Martaban.

Straits have been formed (I) by fracture across isthmuses, and such may be by longitudinal fracture as in the Strait of Bab-el-Mandeb, or transverse fracture as in the Strait of Gibraltar or Cook Strait; (2) by erosion, e.g. the Strait of Dover, the Dardanelles and Bosporus; (3) by overflowing through the subsidence of the land, as in the straits of Bering, Torres and Formosa.

Surface of the Ocean

If the whole globe were covered with a uniformly deep ocean, and if there were no difference of density between one part and another, the surface would form a perfect ellipsoid of revolution, that is to say, all the meridians would be exactly equal ellipses and all parallels perfect circles. At any point a sounding line would hang in the line of the radius of curvature of the water surface. But as things are the watersurface 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 wave, and leaving the sea-level lower in mid-ocean. Hence the geoid or figure of the sea-surface is not part of an ellipsoid of rotation but is irregular. The differences of level between different parts of the geoid have been greatly overestimated in the past; F. G. Helmert has shown that they cannot exceed 650 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 local differences in density, the most important being a confirmation of 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 Nansen's expedition on the " Fram " in 1894-1895, Scott Hansen made observations with a Sterneck's half-seconds pendulum on the 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. Hecker took the opportunity of a voyage from Hamburg to La 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 mercurial barometer and a boiling-point thermometer, and obtained results similar to Scott Hansen's. The inequalities of the geoid in no 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. Solar 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 annual range of temperature between summer and winter of a surface layer of water about 25 fathoms thick in the Baltic is as much as 20° F., but this only corresponds to a difference of level of 14 in. due to expansion or contraction.

Atmospheric precipitation poured into the sea by the great rivers must necessarily create a permanent rise of the sea-level at their mouths, and from this cause the level round the coasts of rainy lands must be greater than in mid-ocean. H. Mohn has shown how the inequalities of what he terms the densitysurface 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 Norwegian Sea between Jan Mayen and the Lofoten Islands. The level of the Gulf of Finland at Kronstadt and of the Gulf of Bothnia at Haparanda should similarly be 15 in. higher than that of the Skagerrak. Recent levellings along the 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 Azores the atmospheric pressure in 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 monsoon region, where the barometer rises o

38 in. between July and January, the level of the sea falls in consequence by 5 in. Wind also gives rise to differences of level by driving the water before it, and the prevailing westerly wind of the southern Baltic is the chief cause of the sea-level at Kiel being 51 in. lower than at Arkona on Riigen. Periodic variations of level due to meteorological causes account for the Baltic being 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 gauge is from 3 . 5 in. below mean level in April to 275 in. above the mean level in August. A similar range occurs on the Dutch coast in the North Sea, where the maximum level is reached in October, the month of highest rainfall, and there is a range of 8 in. to the minimum level at the time of least rainfall in early spring. In the monsoon regions the half-yearly change from on-shore to off-shore winds produces noticeable differences in XIX. 31a level; thus fifteen years' observations at Aden show a maximum the first chart of the depths of the Atlantic between 52° N. and in May at the end of the north-east monsoon, and a rapid falling io S. At this period an exact knowledge of the depths of the ocean off after the beginning of the south-west monsoon to a minimum assumed an unlooked-for practical importance from the daring in August, the total range being 92 in. The influence of wind project for laying a telegraph cable between Ireland and on water-level is most remarkable in heavy storms on the flat Newfoundland. Deep soundings were made in the Atlantic coasts of the North Sea and Baltic, when the rise may amount for this purpose by vessels both of the British and of the American to very many feet. In the region of tropical hurricanes the navies, while in the Mediterranean and in the Indian Ocean converging wind system of a circular storm causes a heaping many soundings were made in connexion with submarine up of water capable of devastating the low coral islands of the cables to the East. Another stimulus came from the biologists, Pacific. On the 1st of November 1876 a cyclone acting in this who began to realize the importance of a more detailed investigaway submerged a great area of the level plain of the Ganges tion of the life conditions of organisms at great depths in the delta to a depth of 46 ft.; here the influence of the difference sea. The lead in this direction was taken by British biologists, of pressure within and without the cyclone acted in the same beginning with Edward Forbes in 1839, and in 1868 a party on direction as the wind. The old speculations as to a great differboard H.M.S. " Lightning " pursued researches in the waters ence of level between the Mediterranean and the Red Sea, and to the north of Scotland. In 1869 and 1870 this work was on the two sides of the Isthmus of Panama, which hindered the extended to the Irish Sea and Bay of Biscay in H.M.S. " Porprojects for canals connecting those waters, have been proved cupine," and to the Mediterranean in H.M.S. " Shearwater." by modern levelling of high precision to be totally erroneous. The last-named vessel secured 157 trustworthy deep soundings, Deep-sea Soundings. - The hand-lead attached to a line with samples of the deposits, and also observations of temperature divided into fathoms was a well-known aid to navigation even and salinity in different depths, as well as dredgings for the in high antiquity, and its use is mentioned in Herodotus (ii. 5) collection of the organisms of the deep sea. and in the Acts of the Apostles (xxvii. 29). Greater depths These preliminary trips of scientific marine investigation were than those usually sounded by a hand-line may possibly not have followed by the greatest purely scientific expedition ever underbeen beyond the reach of the earlier navigators, for Strabo taken, the voyage of H.M.S. " Challenger " round the world says " of measured seas the Sardonian is the deepest with full under the scientific direction of Sir Wyville Thomson and the one thousand fathoms " (i. 3, p. 53 Cas.). Yet we find that the naval command of Sir George Nares. This epoch-making great discoverers of the modern period were only familiar with expedition lasted from Christmas 1872 to the end of May 1876, the hand-lead, and the lines in use did not exceed 200 fathoms and gave the first wide and general view of the physical and in length. Ingenious devices had indeed been tried in the 17th biological conditions of the ocean as a whole. Almost simulcentury and earlier, by which a lead thrown into the sea without taneously with the " Challenger," a German expedition in S.M.S. a line detached a float on striking the bottom, and it was proposed " Gazelle " conducted observations in the South Atlantic, Indian to calculate the depth by the time required for the float to reand South Pacific Oceans; and the U.S.S. " Tuscarora " made a appear. The earliest deep-sea sounding on record is that of cruise in the North Pacific, sounding out lines for a projected Captain Phipps on the 4th of September 1773 in the Norwegian Pacific cable. The successor of Sir Wyville Thomson in the Sea, in 65° N. E., on his return from his expedition to Spitseditorship of the " Challenger " Reports, Sir John Murray, has bergen. He spliced together all the sounding-lines on board, rightly said that since the days of Columbus and Magellan no and with a weight of 1501b attached he found bottom in 683 such revelation regarding the surface of our planet had been fathoms and secured a sample of fine soft blue mud. He detected made as in that eighth decade of the 19th century. Since that the moment of the lead touching the bottom by the sudden time the British cable-ships have been busy in all the oceans slackening in the rate at which the line ran out. Polar explorers making sections across the great expanses of water with everfrequently repeated those experiments in deep-sea soundings, increasing accuracy, and in that work the government surveying both William Scoresby and Sir John Ross obtaining notable ships have also been engaged, vast stretches of the Indian and results, though not reaching depths of more than 1200 fathoms. Pacific Oceans having been opened up to knowledge by H.M.SS. The honour of first sounding really oceanic depths belongs to " Egeria," " Waterwitch," " Dart," " Penguin," " Stork," Sir James Clark Ross, who made some excellent measurements and " Investigator." American scientific enterprise, mainly in very deep water, though in a few instances he overestimated under the guidance of Professor Alexander Agassiz, has been the depth by failing to detect the moment at which the lead active in the North Atlantic and especially in the Pacific Ocean, touched bottom. The pursuit of these isolated investigations where very important investigations have been made. The received a great impetus from the enthusiasm of the great Amerieastern part of the North Atlantic has been the scene of many can oceanographer Captain Matthew Fontaine Maury, U.S.N., expeditions, often purely biological in their purpose, amongst who directed the whole impetuous strength of his character to which there may be mentioned the cruises of the " Travailleur " the task of compelling the silent depths of the ocean to tell their and " Talisman " under Professor Milne-Edwards in 1880-1883, tale. Instead of the expensive mile-long stout hemp lines used and since 1887 those of the prince of Monaco in his yachts, as by Ross, Maury introduced a ball of strong twine attached to a well as numerous Danish vessels in the sea between Iceland and cannon shot, which ran it out rapidly; when the bottom was Greenland, conspicuous amongst which were the expeditions reached the twine was cut and the depth deduced from the length in1896-1898on board the " Ingolf." The Norwegian Sea was of string left in the ball on board. The time of touching bottom i studied by the Norwegian expedition on board the " VOringen " was judged by timing each loo-fathom mark and noting the in 1876-1878, and the north polar basin by Nansen and Sverdrup sudden increase in the time interval when the shot reached the in the " Fram " in 1893-1896, the Mediterranean by the Italians bottom. Maury, however, recognized that in great depths on the " Washington " and by the Austrians on the " Pola " the surest guarantee of bottom having been reached was to bring in 1890-1893, the latter carrying the investigations to the Red up a sample of the deposit. To do this with a heavy lead Sea in 1895-1898, while the Russians investigated the Black attached required a very strong hemp line, and the twine used Sea in 1890-1893. For high southern latitudes special value in the American method was useless for this purpose. In 1854 attaches to the soundings of the German deep-sea expedition J. M. Brooke, a midshipman of the U.S.N., invented the principle on the " Valdivia " in 1898-1899, and to those of the " Belgica " already foreshadowed by Nicolaus Cusanus in the 15th century in 1897-1898, the " Gauss " in 1902-1903, and the " Scotia " and by Robert Hooke in the 17th, of using a heavy weight so in 1903-1904. The soundings of the Dutch expedition on hung on the sounding-tube that it was automatically released the " Siboga " during1899-1900in the eastern part of the on striking the bottom and left behind, while the light brass tube Malay seas and those of the German surveying ship " Planet " containing a sample of the deposit was easily hauled up. This in 1906 in the South Atlantic, Indian and North Pacific Oceans principle has been adopted universally for deep soundings, and were notable, and Sir John Murray's expedition on the " Michael is now applied in many forms. In 1855 Maury published Sars " in the Atlantic in 1910 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 sound instead with steel piano wire not more than 310 to 215 of an inch in diameter and a detachable lead seldom weighing more than 70 lb. The soundings are made by means of a special machine fitted with a brake so adjusted that the revolution of the drum is stopped automatically the instant the lead touches the bottom, and the depth can then be read directly from an indicator. The line is hauled in by a steam or electric winch, and the sounding-tube containing a sample of the bottom deposit is rapidly brought on board. The sounding machines most frequently employed are those of Admiral C. D. Sigsbee, U.S.N., of Lucas, which was perfected in the Telegraph Construction and Maintenance 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 hydraulic manometer has been tried on board the German surveying ship " Planet." A. Siemens has pointed out that a profile of the sea-bed can be delineated by taking account of the varying strain on a submarine cable while it is being laid, and the average depth of a 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.

Fathoms.

sq. km.

sq. st. m.

More than

100

350,500,000

135,300,000

500

319,500,000

123,400,000

1000

304,000,000

117,400,000

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

8,700,000

3500

3,000,000

1,200,000

4000

1,200,000

460,000

Relief of the Ocean 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. Kriimmel has calculated the mean depth to be 2010 fathoms (12,060 ft.), while the mean elevation of the surface of the continents above sea-level is only 2300 ft. Viewed from the floor of the ocean the continental block would thus appear as a great 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 summit of Mount Everest rising to 29,000 ft. above the sea-level, while the Nero Deep near 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 Kriimmel's calculation the areas of the ocean beyond various depths are as follows: - 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 angle of the slope of a given uneven surface provided that maps can be prepared showing equidistant 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 area of the surface under investigation, then the mean angle of slope is obtained from the equation h? (l) tan a = A .

Calculating from sheet A I of the Prince of Monaco's Atlas of Ocean Depths,' Kriimmel obtained a mean angle of slope of 0° 2 7 ' 44" or an average fall of i in 124 for the North Atlantic between o° and 47° 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 borders. On the submarine slopes leading up to isolated volcanic islands angles of 15° to eo are not uncommon, at St Helena the slopes run up to 382° and even 40°, at Tristan d'Acunha to 331°. E. 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 34°. Where the French telegraph cable between Brest and New 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 30°, 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 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 rose so flat that an 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 regular accumulation of deposits, the whole ocean floor has assumed some approach to uniformity. Still there are everywhere gentle inequalities on the smoothest ocean floor which give to its greater features 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 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 fair 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 Congress at Berlin in 1899 and adopted by that at Washington in 1004. Equivalent terms, which are not necessarily identical or literal 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 loo, 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 (r) a basin when of a roughly round outline, ( 2) a trough when wide and elongated, or (3) a 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 gully when narrow. The elevation includes (i) the gently swelling rise which separates 1 Carte generale bathymetrique des oceans dressee par ordre de S.A.S. le Prince Albert de Monaco, 24 sheets (Paris, 1904).

troughs and basins in the middle of the ocean, (2) the steeply sloping 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 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 bank, an elevation coming nearer the surface than too fathoms, but not so near as 6 fathoms; and finally the shoal or reef, which comes within 6 fathoms of the surface, and so may constitute a danger to shipping. Similarly we may note the caldron or small steep depression of a round outline, 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 international 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 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 reefbuilding animals and plants (corals and calcareous 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 secular subsidence is often very clearly apparent, as for instance in the North Sea and the tract lying off the mouth of the English Channel. A closer investigation of the numerous long, narrow banks which lie off the Flemish coast and the Thames estuary shows that they are composed of fragments of rock abraded and transported by tidal currents and storms in the same way that the chalk and limestone worn off from the eastern continuation of the island of Heligoland during the last two centuries has been reduced to the coarse gravel of the off-lying Dune. Numerous old river valleys and furrows entrenched in. the continental shelf bear witness to its land origin. Such valleys are very clearly indicated in the belts of the western Baltic by furrows a thousand yards wide and twenty to thirty fathoms deeper than the neighbouring sea-bed. Amongst the best known of the furrows of the continental shelf are the Cape Breton Deep, in the Bay of Biscay, the Hudson Furrow, southward of New York, the so-called Congo Canon, the Swatch of No Ground off the Ganges delta, the Bottomless Pit off the 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. Firth of Forth and 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. Jlost 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 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 Osborn in 1870 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 150o fathoms, thence to 12° N. the depths are between 1500 and 2000 fathoms, and then it rises again to about 150o fathoms and runs eastward under the name of the Equatorial Ridge. Crossing the equator in 13 6 ° W. the rise resumes a southerly direction and from Ascension to Tristan d'Acunha, the depth is in many places less than r50o fathoms. The soundings of 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 30 and 40° S. two lateral ridges diverge from the great Atlantic rise, the Rio Grande ridge towards the north-west and the Walfisch ridge towarls the north-east. The existence of the latter, which extends to the 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 Porto Rico Trench, where in 1882 Capt. W. H. 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° rr' S., 18° 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., 1 9° 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 northwest 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 South 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 2660 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 Amsterdam to 30° 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 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 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 Archipelago and Australia. On the north this depression sinks into the long and narrow Sunda Trench south of Java, and here in 10 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 Sumatra with islands rising from the higher portions. The western part of the Indian Ocean has been shown by the surveys of H.M.S. " Sealark " and the German surveying-ship " Planet " to have a somewhat complicated configuration, the island groups and banks of atolls which occur there rising abruptly as a rule from depths of about 2000 fathoms or more. Between the Seychelles and Sokotra (0° - 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 1900 fathoms, while the floor of the Bay of Bengal rises very gradually northwards and is 1000 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 Tonga, Marshall aid 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 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 group there are depths of more than 2000 fathoms. The Society Islands and Tahiti crown a rise coming within 150o fathoms of the surface, two similar rises form the foundation of the Paumotu group where Agassiz found soundings of. 2187 fathoms between Marokar 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 " 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., 1 45° 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.

The trenches of Yap (4122 fathoms) and Palau (Pelew) (4450 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 size also occur south of the equator. The Tonga and Kermadec trenches, both deeper than 4000 fathoms, stretch from the 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 2 3° 39.4' S ., 1 75° 4' W., and in the Kermadec Trench, 5155 fathoms, 30 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 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 50° 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. (1,50o,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 Faeroe and 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 Baffin Bay and contains a maximum depth of 1050 fathoms. Depths of from loo 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 Antilles, none of which is quite 1000 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 Mexico.

The Austral-Asiatic or Malay Sea is occupied by a great shelf in the region west of 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 Celebes Basin 2795 fathoms. Some of the channels between the islands are of very great depth, Macassar Strait exceeding 1000 fathoms, the Molucca Passage exceeding 2000 fathoms, and the Halmahera Trough sinking as deep as 2575 fathoms. The deepest of all is the Banda Basin, a large area of which lies below 2500 fathoms and reaches 3557 fathomsin the Kei Trench. A depth of 2789 fathoms also occurs north of 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 intercontinental class, is separated from the Atlantic Ocean by a ridge running from Cape Spartel to Cape Trafalgar on which 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 Spain and the rise bearing Corsica and Sardinia, has a maximum depth of 1742 fathoms, and the Tyrrhenian Basin between that rise, Italy and 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 Austrian expedition on board the " Pola " was 2046 fathoms in 35° 44' 8' N., 21° 46.8' E. The Adriatic Sea though very shallow in the north deepens southward to about 9 00 fathoms, and the Aegean Sea has a maximum depth of 1230 fathoms north of 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. lies the extremely shallow Gulf of 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 50 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 50 fathoms in few places except the broad central portion, though small caldron-like depressions here and there may sink below zoo fathoms. The Red Sea on the other hand, though shut off from the Indian Ocean by shallows of the Strait of Bab-elMandeb with little more than ioo 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 ioo fathoms is only exceeded to any extent in the Norwegian gully, which has a maximum depth of 383 fathoms in the Skagerrack.

Most of the other seas of this class are formed on a common plan. Towards the continent there is a broad shelf, and just before the 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 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 1650 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 Anticosti.

The area, general depth and total volume of the oceans and principal seas have been recalculated by Krt mmel, and the accompanying table presents these figures.

Oceanic Deposits.-It has long been known that the deposits which carpet the floor of the ocean differ in different places, and 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 appearance of which is often characteristic of the locality. In depths down to ico fathoms the old-fashioned hand-lead, hollow below and " armed " with tallow, suffices to bring up a sample large enough to be recognizable. Captain Phipps in 1773 secured samples of soft blue 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 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 valve or other contrivance, though in many cases 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.

Name.

Depth.

p

Fathoms.

Area.

Volume.

sq. km.

sq. st. m.

.

cb. km.

cb. st. m.

'Atlantic Ocean. .. .

2110

81,657,800

31,529,390

314,821,680

75,533,900

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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Ocean And Oceanography'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/bri/o/ocean-and-oceanography.html. 1910.

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