Page:Africa by Élisée Reclus, Volume 3.djvu/28

12 WEST AFEIOA. Marine Currents of the Azorian Basin. The more salient features of the marine, like those of the aerial currents, in the Azorian Atlantic are already known ; but many obscure and doubtful points still remain to be cleared up. It is aU the more difficult to follow the course of the circulating waters, that certain currents move too slowly to be directly measured. They canlbe detected only by means of the thermometer, when their temperature differs from that of the circumambient liquid. In this way has been determined the existence of a deep stream flowing from the Antarctic seas to the equatorial waters and even to the neighbourhood of the Azores ; by means of the thermometer the presence of corresponding cool currents from the Arctic Ocean has been revealed in the same region. But as a rule the waters occupying nearly the whole of the Central Atlantic basin have a very perceptible velocity, in some places reaching one or two miles per hour. Altogether the section of the Atlantic comprised between the telegraph plateau and the equator, between the west coast of Africa and the Antilles, is filled by a vast vortex incessantly rotating, and constantly influenced by the same forces. The current, deflected from the Senegambian coast, bends across the ocean in the direction of the West Indies. Here it ramifies into two branches, one penetrating into the Caribbean Sea, the other skirting the east side of the Bahamas, beyond which it joins the American Gulf Stream, flowing thence east and north-east. The current returning from America towards the Old AVorld traverses the Azorian Atlantic, and in the neighbourhood of the Portuguese and Maroccan coasts bends southwards, thus completing the vast circuit. These oceanic streams flow nearly parallel with those of the atmosphere above them, from which they differ only in their more sluggish motion, and in the deflec- tions imposed upon them by the sudden obstacles of insular and continental barriers. The surface waters being directly exposed to the action of the wind, necessarily move in the same direction, lashed into crested billows under high gales, gently rippled beneath the soft zephyrs. The casual winds produce only a passing effect, their action never reaching far below the surface. But regular currents, such as the trade winds, acting from century to century throughout countless ages, have gradually penetrated to great depths, thus largely contributing to determine their general movement. Till recently physicists supposed that the chief cause of the equatorial current flowing westwards in the contrary direction to the globe itself, was the terrestial rotation, a movement necessarily outstripping that of the encircling oceanic waters. The transverse currents would then be explained in the same way by the greater velocity of planetary rotation acquired by the waters under the equa- torial latitudes. According to Miihry, the centrifugal force of the globe, being greater on the equator than elsewhere, is the primary cause of the general oceanic movement. But in any case the varying degrees of salinity and heat between the liquid layers must also tend to produce these currents, although the effects produced by them cannot be determined with the most delicate observations con- tinued for many years by skilled observers.