INDIAN OCEAN, that body of water which has Asia on the north, the East Indian Islands. Nicobar and the Andaman Islands, Australia and Tasmania on the east, Africa on the west, and the Antartic continent on the south. The Cape of Good Hope and the south ern extremity of Tasmania may be considered its extreme limits from east to west. Its length from north to south somewhat exceeds 6,300 miles, its breadth varies from 6,000 to 4,000 miles. Its gulfs are the Red Sea, the Gulf of Aden, the Persian Gulf, the Gulf of Oman, the Arabian Sea, the Bay of Bengal and the Great Australian Bight. Its islands are Ceylon, Madagascar, the Laccadives, Maldives, Socotra, Andamans, Nicobar, Mauritius, Bourbon, Ker guelen's Land, etc. The Ganges, Brahmaputra, Irrawaddy, Indus and Euphrates, empty into the Indian Ocean from Asia, and the Zambezi and Limpopo from Africa. The southeast tradewind blows between the 10th and 28th parallels of south latitude from April to Octo ber, after which date its limits are contracted; south of these are the northwest winds which prevail almost in the same latitude, in the Atlantic and Pacific. The monsoons are mainly in the north, from the continent of Asia to about latitude 8° S., and from the Mozambique Channel on the west to the western shores of Australia and the Sea of China. They blow for six months, changing about the equinoxes. North of the equator the northeast monsoon prevails from October to April, the southwest from April to October; while south of that the northwest monsoon blows while the north east is blowing on the north side; and the south east prevails during the time of the southwest monsoon north of the equator. In the hot season, likewise, when the southeast trade-wind recedes south, the northwest monsoon blows between the equator and the 12th south parallel. The hurricanes of this ocean usually range be tween lat. 9° and 35° S., extending from Mada gascar to the Island of Timor. They usually come from the northeast, and travel southwest and south, returning again east. Their season is from December to April.
According to the most recent soundings the mean depth of the Indian Ocean is 2,300 fath oms, or somewhat greater than that of the Atlantic. The greatest depths are in the east
ern part to the south of the equator, where it is estimated that there are fully 50,000 square miles with a depth of over 3,000 fathoms. The deepest sounding has been recorded off the southeast coast of Java, 20,340 feet. Over 13,000,000 square miles lie between the depths of 2,000 and 3,000 fathoms.
The area of land draining into the Indian Ocean is estimated at 6,813,600 square miles, and the rainfall on this land amounts to 4.379 cubic miles of water annually. The rivers flow ing from the Asiatic continent are by far the most important, and they carry a vast amount of detritus into the Bay of Bengal and Arabian Sea, these forming immense deposits of blue mud. Along the African coasts, in depths from 100 to 1,000 fathoms, there are glauconitic sands and muds, and on these as well as other coasts, coral muds and sands, and blue and green muds in the shallower depths. In the deeper parts of the ocean, far from land, there are deposits of red clay, radiolarian-ooze, and glo bigerina-ooze. Toward the Antarctic continent the ocean bed is covered with a diatom-ooze.
The temperature of the surface waters varies much in different parts of the ocean, at different seasons, and under the influence of different winds. In tropical regions the tem perature usually varies from 70° to 80° F., and the yearly range is 7° or 8° F. Off the Cape of Good Hope and Cape Guardafui, the annual range may be from 20° to 30° F. Sudden changes of temperature are often noticed off Cape Guardafui when the wind blows off shore.
The cold and deep water is thus drawn up along the coast to take the place of the warm surface water which is driven east by the wind.
The temperature of the water at the bottom is very uniform and subject to little, if any, annual variation. In the Bay of Bengal and Arabian Sea temperatures of 33.7° F. and 34.2° F. have been recorded; these are only very slightly higher than those recorded by the Challenger in lat. 50° S. It is certain, there fore, that this deep cold water is. slowly drawn into the Indian Ocean from the Antartic to supply the place of the warm surface currents that are driven south by the winds.