Page:Africa by Élisée Reclus, Volume 3.djvu/26

 water as well as on dry land the zone of equilibrium between the northern and southern climates falls within the northern hemisphere.

But whatever be the local differences of temperature, the gradual normal fall from the surface towards the bottom down to 2° or 8° above zero, or even lower, is constant for every part of the oceanic basin. The case is different for the nearly landlocked basin of the Mediterranean, which from the neighbouring ocean receives _ only surface waters at a temperature always above 58° or 54°F. M. Faye's well-known theory regarding the density of the terrestrial crust is largely based on the fact that the lower oceanic waters are nearly always icy cold. Being exposed for long geological ages to this cooling influence, the rocky bed itself has become cooler down to a certain depth. It has thus become contracted, with a corresponding increase of thickness and density, so that, bulk for bulk, the submarine are heavier

than the continental masses, the latter consequently exercising a less relative influence on the vibrations of the pendulum.

As regards its aërial currents, the Azorian Atlantic partakes of two different zones, in the north coming within the European zone of westerly winds, in the south within that of the trade winds, that is, the oceanic zone properly so called. The African waters are further distinguished by a special atmospheric system, the neighbourhood of the mainland reversing the normal disposition in the development of the local daily breezes and the periodical monsoons. The Azores lie nearly on the limit between the trade-winds and the opposing currents, which descending from the upper regions, take a normal south-westerly or westerly direction. The winds which set towards the coasts of Iberia, France, and the British Isles have their origin in this central part of the oceanic basin. M. Brault's exhaustive studies on the direction and intensity of the North Atlantic winds show that in summer, Flores, one of the western Azores, forms the focus of a regular aërial rotation. The waters around this island are the only Atlantic region where the northern blow as