INDIAN OCEAN. One of the five grand divisions of the hydrosphere, hounded on the west by Africa and the 20th meridian east of Ore•nwieh (running through Cape Agulhas. the southern extremity of Africa), on the north by Asia, on the east by the East Indian Archipelago, Australia. and the 147th meridian (running through the southern of Tasmania) (Map: World, Eastern 'Hemisphere, K 28). The southern boundary is in an indefinite way as sumed to be approximately the Antarctic Circle, although ninny geographers prefer to call what is south of latitude 40° S. the Southern Ocean.
in its broader sen,e it covers about 27.500,000 square tulle.. Gradually narrowing front south to north, the Indian Ocean fork, at Cape Comorin into the Bay of Bengal on the east, and the Ara bian Sea on the west, the latter again branch ing .d1 into two arms, the Persian Gulf and the Red Net, t‘hieli reach respectively the mouth t be Enplirate.-Tigris and the neighbor hood of the Alediterranean. These limitations exclude the waters of the Indian Archipelago, as belonging rather to the Pacific Oven ti. From Atrim it receive, the water, of the Limpopo and the Zambezi, and from Asia those of the Irra waddy, Br:Om:Tiara, Ganges, Indus, and the Shat-el-Arab. It contains numerous islands, chiefly confined to the northern and western re gions. Of these Madagascar and Ceylon are the only ones of considerable magnitude, the others being mostly small groups of volcanic or coral formation, the volcanic islands being usually sur rounded by coral reefs.
The greatest depth of the Indian Oeea.n is found in the northeastern part, where, close to the southeast of .1a Va , s011nilings have bean made of 20,340 feet, and a considerable basin of nearly this average depth lies to the northwest of the Australian continent. An elevated ridge run
ning along the 40th parallel has an average depth of 12,000 feet. 1:rititimel estimates the average depth of the ocean to he 10,970 feet. The surface temperature of this ocean is higher than that of the others, the mean temperature of the northern portion being considerably over SO°, while the bottom temperature, falling Is• low 40° at the equator, indicates an undercur rent of cold water from the Antarctic region. The currents of the Indian Ocean depend to a great extent on the winds. North of the equator they reverse their direction with the annual change of the northeast trade wind to the south west monsoon, while under the equatorial palm belt the equatorial countercurrent flows east w:tril. Smith of the equator, under the southeast trade winds a current tills west, bending south along the coast of Africa through the :Mozambique Channel. and meeting the Atlantic westward current at the Cape, where it is turned back to the east toward the south coast of Australia. The winds over the Indian Ocean are, as a rule, gentle, with frequent protracted calms, though burrieanes occur oecasionally. This 'wean was the first to find a place in the history of eommerce. As a commercial channel it, Nirturilly maintained its superiority during two thousand years. being habitually traversed in a direct line between Arabia and 'Hindustan, coast ing voyages alone were known in the Atlantic. This comparatively bold navigation of the In dian Ocean was suggested and facilitated by the alternating monsoons of the north ern part..