Page:Africa by Élisée Reclus, Volume 3.djvu/27

 frequently as the southern gales, and where the western are balanced by the eastern currents. East of this point the prevailing breezes are northerly, westwards southerly, northwards mainly westerly, southwards easterly. Hence round this central region revolves the great atmospheric ocean of the Azorian Atlantic, a fact which will add greatly to the importance of the submarine cable about to connect the Azores with all the European meteorological stations. The chief station will be established at the point of intersection of the great aerial currents, whence more or less trustworthy weather forecasts can be announced some days in advance for the west of Europe.

The normal movement of the winds in the Azorian Atlantic has been well known since the early navigators began to frequent these waters. All were struck by the regularity of the currents blowing off the coasts of Madeira and the Canaries, to which they gave names betraying their knowledge of the law regulating the circulation of the winds in this region. For the Portuguese these currents setting regidarly from the north-east to the south-west are the "geraes" or "general;" for the French the "alizes," that is, "uniform" or "regular;" while for the English they are at first the " tread winds," that is the "steady," or " constant," afterwards by an unconscious but easily understood play of words, changed to the "trade winds." But notwithstanding their general regularity, these sea breezes are subject to certain changes of velocity from season to season, as well as to deflections to right and left of the normal direction. The main features of this atmospheric system may be studied in Maury's pilot-charts, in those of Brault and Toynbee, which give the results of many hundred thousand observations, and which continue the labours of previous meteorologists in this field. During the summer of the northern hemisphere the whole space stretching from the Azores southwards to the fourteenth degree N. ]at. is swept by the trade winds, which in winter are deflected much farther south. Thus, while the Azorian waters are temporarily brought within the influence of the variable western breezes, the Central Atlantic as far as 3° or 4° S. lat. is exposed to the action of the trade winds.

Seafarers have also to study the zones of calm or less intense aerial currents, one of which lies about the equator, the other to the south of the Azores, both forming elliptical spaces round which are developed the curves of equal force first described by Brault, and by him named "isanemonic curves." Lastly there remains to be considered the thickness of the aerial curves constituting the trade winds, above which set the counter-winds which, after rising vertically into the zone of equatorial calms turn northward in the direction of the pole, gradually falling towards the surface of the earth. At the Peak of Teyde, in the Canaries, the intervening zone between the trade and counter-winds rises in summer and descends in winter on the upper slopes of the mountain, and Piazzi Smith has been able several times to measure the exact thickness of the lower current blowing in the direction from north-east to south-west. But the Teyde Peak is a mere islet in this atmospheric ocean, and there still remain to be studied in the same systematic way the heights of Madeira and the Cape Verd Islands, as well as the general movement of all the counter-winds.