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” in 1872-1876 formed 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 three-quarters 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 71% 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 81% of that of the southern hemisphere. Of the whole ocean only 43% (154.9 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 (47½° N. and 2½ 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 meridional 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
