of the atmosphere, and enable the local vapours to condense and precipitate themselves on the spot. In the northern hemisphere a zone of two wet seasons stretches from the equator to the fifteenth degree of latitude. In summer, copious rains are caused by the moisture-bearing south-west winds; in winter, those blowing from the north-west become in their turn the bearers of heavy rain-charged clouds to the southern hemisphere. But on both sides of the torrid zone, which comprises about seven-tenths of the whole continent, the difference in the disposition of the winds
Fig. 4.—Isothermal Lines of Africa.
Scale 1 : 75,000,000.
causes a corresponding contrast in the rainfall. Here the trade winds maintain their normal direction constanttly, or with but slight temporary deviations; blowing from the north-east in the northern, from the south-east in the southern hemisphere, they divert to the equator most of the vapours crossing their path, leaving elsewhere clear skies and arid lands. Thus it happens that Africa has two almost completely barren zones of rocks, gravels, marls, clay and sand—the Sahara and Libyan desert in the north, Kalahari and other wastes in the south. This