10 



NOETH-EAST AFEICA. 



of the atmosphere, and enable the local vapours to condense and precipitate them- 

 selves 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. 



1,200 Miles. 



causes a corresponding contrast in the rainfall. Here the trade winds maintain 

 their normal direction constantly, 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 else- 

 where clear skies and arid lands. Thus it happens that Africa has two almost com- 

 pletely 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 



