INDIAN OCEAN 821 Depth and Islands. The main basin of the Indian Ocean has an average depth of about 2500 fathoms, increasing to 3000 fathoms in the angle between Java and north-western Australia, which is the deepest part of it yet sounded. Its southern border is formed by a submarine plateau, which, rises in some parts to within 1500 fathoms of the surface, and which forms the common foundation, not only of the islands already mentioned, but also of the Crozets, the Kerguelen group, Prince Edward s Islands, and the Heard Islands, all of which seem to have had a volcanic origin. This plateau, however, does not shut in the south-eastern portion of the basin ; for a southward extension of the depression already described follows the trend of the western and southern coasts of Australia and the western coast of Tasmania, and is continuous with the deep channel (in some parts exceed ing 2500 fathoms) between Australia and New Zealand. The western and north-western parts of the basin, on the other hand as the number of their island-groups would lead us to anticipate have a much less uniform depth. In the first place, the western border of the basin is encroached on by the great island of Madagascar, which must be considered as an outlying extension of the conti nental platform of South Africa, the Mozambique channel being comparatively shallow ; and, although the bottom, at Plan showing Depths of the Indian Ocean. To 1000 fathoms, white ; 1000 to 2000 fathoms, light shading more than 2000 fathoms, dark shading. no great distance from its eastern coast, rapidly deepens to 2000 fathoms or more, yet this is only in a channel that separates Madagascar from a platform .of about half that depth, on which are based the volcanic Mascarene Islands (Mauritius, Bourbon, and Rodriguez), and of which a northern extension forms the base of the Seychelles group. This platform then curves to the south-west, so as to pass round the north of Madagascar, forming the base of several coral islands, and thus comes into continuity with the bed of the Mozambique Channel, from which the Comoro group arises. To the north of this platform, the 2500 fathom line follows the trend of the African coast as far as Cape Gardafui, keeping outside the island of Socotra ; and a bottom of more than 2000 fathoms (crossed by the tele graph-cable between Aden and Bombay) extends into the Arabian Sea as far as 15 N. lat. On the eastern side of that gulf, however, the declivity from the Indian coast-line to the deepest part of the basin is much more gradual ; the Maldive and Laccadive groups of coral islands rising from a comparative shallow, which extends itself a little to the south of the equator. And about half way between this platform and that of the Seychelles the bottom rises into the bank which bears the Chagos archipelago, and which divides the communication between the deeper portion of the general basin and that of the Arabian Sea into two channels of no great width. Though the 2500 fathom line does not enter the Bay of Bengal, a considerable portion of it has a depth exceeding 2000 fathoms. Here, again, the declivity is more gradual along the eastern margin of the gulf ; and the Andaman and Nicobar Islands arise from a comparatively shallow platform that .stretches between the delta of the Irawadi and the north end of Sumatra. Surface and Bottom Temperature. The surface-temperature of the Indian Ocean is higher than that of either the Atlantic or the Pacific ; and this difference shows itself especially in its northern division, on which the proximity of tropical land exerts an import ant thermal influence. For the mean annual temperature of the portion which lies between the equator and the Tropic of Cancer, including the Arabian Sea and the Bay of Bengal, is considerably above 80, whilst that of the corresponding part of the southern division lying between the equator and the Tropic of Capricorn ranges from 80 to 70, the average maximum temperature in the centre of the Arabian Sea being 87. In July the thermal equator moves considerably to the north, and the surface-temperature sometimes rises in the Arabian Gulf and the Bay of Bengal to above 90. In January, when the thermal equator lies to the south of the geographical, the temperature of these two gulfs falls below 80, while that of the vast expanse which lies between the parallel of 10" N. and 25 S. , has a temperature of 80 or upwards. In the southern hemisphere the January (summer) isotherm of 70 and the July (winter) isotherm of 60 correspond pretty closely with the border of the Indian Ocean, the range of its temperature being thus very moderate. The bathymetrical isotherms of the Indian Ocean have not yet been systematically worked out by temperature-soundings ; but there is adequate evidence of the extension of the Antarctic underflow over the deeper portion of its sea-bed, even to the north of the equator. For the "Hydra " line of soundings between Aden and Bombay gave a bottom-temperature of 36 5 at a depth of 1800 fathoms, the surface- temperature being 75, while in the deep depression on the eastern side of the basin, almost immediately beneath the equator, Commander Chiinmo met with a bottom-temperature but little above 32. Surface-Level. A very remarkable effect is produced upon the coast-level of part of the northern division of the Indian Ocean, by the attraction of the great mountain-masses and high table-lands of Central Asia, uncompensated by that of any elevated land-mass to the southward, nearer than that which may lie behind the Antarctic ice-barrier. From the results of the great geodetical survey of India Archdeacon Pratt was able to deduce the very remarkable fact that the level of the sea at the mouth of the Indus is no less than 515 feet higher than at Cape Comorin. 1 Currents. -The current-system of the Indian Ocean is clearly dependent upon the winds which prevail over its several parts, the seasonal reversal of the monsoons in the northern part of its area producing a corresponding modification in the direction of the surface-movement of its water, whilst in the southern division the constancy of the south-east trade-wind keeps up through the whole year a strong westerly equatorial current. The north-east monsoon has, of course, while it lasts, the same eifect as a north-east trade- wind would exert, in producing a general south -westerly drift over the northern division of the Indian Ocean, which manifests itself in a southerly ilow along all the shores it meets, viz., the south east coast of the Indian peninsula, the south-east coast of Arabia, and the east coast of Central Africa. Besides this, a special current- movement is produced by the action of the north-cast monsoon on the surface-water of the China Sea, by the drift of which to the south-west it is forced into the channel between the Malay Penin sula and Sumatra, whence it issues into the Indian Ocean, through the Strait of Malacca, as a current that crosses the Bay of Bengal and impinges against the Coromandel coast of India. By this it is deflected southwards, along with the general drift already mentioned, and then courses round the southern angle of the great peninsula, partly between Ceylon and the mainland, nml partly along the outer coast-line of Ceylon, into the Arabian Sea, where it merges into the general drift of the surface-water towards the African coast. The average rate of this current, as it issues from the Strait of Malacca, is 30 miles per day ; along the south-east coast of India, 24 miles ; on the east coast of Ceylon, 40 miles ; and along the Arabian coast, 18 miles. But when the north-cast gives place to the south-west monsoon, about the vernal equinox, the whole of this movement is reversed. The drift then commences from the African and Arabian coasts, and sets across the Arabian Sea, at tha rate of about 24 miles a day, to the Malabar coast of India, along which a current flows in a southerly direction at the rate of about
Transactions, 1859, p. 795.
