Page:Africa by Élisée Reclus, Volume 3.djvu/33

 two insular groups of Tristam da Cunha and Goncalo Alvarez, forms the median limit between the two sections of the South Atlantic. A straight line drawn along the meridian from Sierra Leone to Tristam da Cunha indicates exactly the "great divide " between the Guinea and Brazilian basins.

The somewhat quadrilateral section comprised between this divide and the African seaboard, and stretching north and south from Cape Palmas to the Cape of Good Hope, is by no means of uniform depth. It may in fact be subdivided into two secondary basins with cavities of over 2,800 fathoms, one extending west and east parallel with the Gold and Slave Coasts, the other of nearly oval form, with its greatest depression to the south-east of St. Helena. The greatest depth hitherto revealed in this section of the African waters is 3,250 fathoms; and the whole basin, presenting a general depth of over 2,200 fathoms off the south-west coast of Africa, has an area of about 2,800,000 square miles—that is, over twice that of the Mediterranean Sea. South of a line running from the mouth of the Orange River to Tristam da Cunha there stretches a second basin also of 2,200 fathoms, limited southwards by the submarine heights on which stands the island of Bouvet, and which slope gently towards the coasts of the Antarctic lands.

In this vast cauldron of the African seas the waters are in continual motion, the mean result of all the shifting and ever opposing currents being a general movement running parallel with the coast from the Cape of Good Hope to Cape Lopez, then trending westwards in the direction of the New World, and returning by the south and east to complete this vast circuit. Thus this southern vortex corresponds with that of the North Atlantic, of which the Gulf Stream forms the western branch. But its general movement is reversed, while also presenting more uniform outlines, thanks to the greater regularity of its basin. Its mean diameter may be estimated at 2,400 miles, with a varying velocity which, however, is never very great except under the influence of high winds. During her voyage from the Cape Verd Islands by Ascension to the mouth of the Congo, the Gazelle found a part of the equatorial current south of the equator moving westwards with a velocity of 1| mile per hour, whereas most other observations had recorded a speed of little over 1¼ half a mile, and in some cases not more than 500 feet. In many parts of these oceanic regions there is in fact no perceptible motion at all, the whole mass accomplishing its vast circuit by a slow movement of translation, while here and there the obstruction of the coastline or the local winds produce secondary currents running in the opposite direction to the main drift.

The most powerful of these backwaters is that which skirts the continental seaboard between Cape Palmas and the Bight of Biafra, and which sets from west to east with a mean velocity of a little over two-thirds of a mile an hour. But off Cape Palmas it attains an occasional speed of 3½ miles, or nearly 90 miles a day. This "Guinea Stream," as sailors call it, intervenes between the two sections