rivers have their origin on the interior slopes of the highlands, and therefore discharge into the Atlantic Ocean, while the Zambezi drainage basin, lying largely on the outer slopes, falls off toward the Indian Ocean. The longest river system is that of the Nile, which rises in the lake region of Equatorial Africa and flows northward through the mountainous divide to the plateau region of eastern Sudan, where it receives an important affluent from the west in the Bahr-el-Ghazal. and is joined further north by the Bahr-el-Azrek (Blue Nile) and by the Atbara, both from the plateau of Abyssinia. In the middle portion of its course the Nile practically completes its vertical descent by numerous cataracts, after which it flows through a valley that is but little above the level of the sea. The drainage basin of the Nile includes an area of about 1,500,000 square miles. Next to the Nile in length and superior to it and to all other rivers of the world excepting the Amazon in volume is the Congo, which rises in the equatorial lake region and drains an area probably exceeding that of the Nile. The Congo flows northwest, then describes a great arc, with its chord formed by the equator, and finally turns southwest, and pierces the coastal barrier of lower Guinea to enter the Atlantic. The tributaries of the Congo include many great rivers, such as the Ubangi, Kassai, and Kuango. South of the Congo are the drainage basins of the Zambezi and Orange rivers, which extend nearly across the lower limb of the continent, and have an eastward and westward slope respectively. The great land-mass composing the western limb of the continent is poorly watered, the Niger being the only river of first importance lying wholly within the area. This river drains the northern slopes of the coastal highlands of Guinea, through which it breaks after being joined by an important tributary from the east, the Benue, and enters the Gulf of Guinea. Of lesser rivers may be mentioned the Limpopo, Rovuma, Sabi, Tana, and Jub, which enter the Indian Ocean, and the Kunene, Kuanza, Ogowe, Volta, Gambia, Senegal, and Draa on the western coast. Owing to the mountainous barrier through which they must pierce to reach the sea, the smaller rivers of Africa generally are unnavigable in their lower courses.
Between the drainage basins of the Nile, Niger, and Congo, and west of the north and south range of highlands of Sudan, is the interior basin of Lake Chad. This lake is fed chiefly by the Shari and Waube, and is subject to great variations of level. It is at the present time a shallow body of fresh water, with an area that is said to range at various times from 10,000 to 20,000 square miles. This phenomenon of sudden variations in level and consequently in area is peculiar to all the rivers and lakes of Africa within the equatorial regions, and is due to the seasonal distribution of rainfall. Between Abyssinia and the Zambezi River and within the bounds of the north and south highland region there is another inland drainage basin with several large lakes, which together constitute one of the most striking physiographical features of Africa. Apparently the lakes lie along a line of rifts or fissures which have been formed by sudden displacements of the earth's crust. Some of the lakes are, Margherita, Abaya, Stephanie, Rudolf, Manyara, Natron, Baringo, Eyassi, and Leopold (Rikwa), all but Rudolf being small bodies of water. The largest lakes (Victoria, Albert, Albert Edward, Kivu, Tanganyika, and Nyassa) drain into the Nile, the Congo, or the Zambezi, and are fresh water bodies. Victoria, Tanganyika, and Nyassa rival in extent the great lakes of North America. For further details, see articles on Congo, Victoria Nyanza, etc.
Climate. Of all the great land divisions of the globe, Africa is characterized by the greatest uniformity of climate. It stretches into both the north temperate and south temperate zones, but the greater part of its area is included within the tropics; there is consequently a successive decrease of average annual heat northward and southward of the equatorial belt, but the regularity of the decrease is modified by certain other factors, so that the region of greatest average heat for the year is located not at the equator but considerably north of it, between the parallels of 10° and 20°. These modifying factors are mainly the direction of the winds and the distribution of the mountains. It is, of course, cooler here in certain seasons than in others; but the average temperature of any given season shows little fluctuation. In summer the isotherm of 80° F. incloses the whole of the Sahara Desert, and over a considerable portion of this area the average summer temperature is 97° or more. This region of extreme heat, which is the largest in the world, may be delimited by a line drawn from Khartum west to Timbuktu, thence north to El-Golea in the Algerian Sahara, thence southeast to Murzuk and thence to Berber on the Nile. The mountain regions of Algeria and Morocco, and parts of British South Africa and of German South-West Africa have a subtropical or temperate climate. Throughout a large portion of Africa, especially in the mountains of the east, and in the Sahara and Kalahari deserts, the temperature varies widely between summer and winter and between day and night, as is characteristic of all desert regions. (See Desert.) In the Kalahari Desert the extreme seasonal fluctuation reaches 113°, and in the Sahara Desert the temperature during the night often approaches the freezing point. In general, the western coast of Africa is cooler than the eastern coast, owing to the conditions heretofore stated, and to the influence of the drift northward along that coast (south of the equator) of the cool water from the Antarctic Ocean. (See article on Climate.) Winds.—Trade winds are characteristic of nearly the whole continent. The Sahara Desert is a region of high barometric pressure during the winter months, thus causing outward blowing winds, while in the summer season the pressure is lowered, and there is an indraught from the surrounding territory. In the western part of the Sahara Desert and Sudan, north and northeast winds prevail during the greater part of the year, alternating with northwest and west winds for a few months in winter. The eastern Sahara region and Egypt have prevailing north and northeast winds. A devastating wind called the “khamsin” blows from the southeast across this region at times, carrying dust and sand and causing sudden rises of temperature. A similar dust wind, but usually cooler, blows from the interior of the Sahara over Senegambia and Upper Guinea, and is called the “harmattan.” During the summer, in the lower limb of Africa, an area of low pressure occurs in the interior, and the prevailing winds are from the east and
