Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/278

Rh 256 AFRICA [PHYSICAL Belonging to the drainage system of the Indian Ocean are, Lake Nyassa, 1500 feet above the sea, and stretching meridionally over an area of nearly 9000 square miles in the basin of the Zambeze ; and Lake Samburu, a reported lake of great extent, lying in the plateau edge north of Mount Keuia, and probably belonging to the basin of the Juba river. The great Lake Tanganyika, upwards of 10,000 square miles in area, and united by a broad channel with Lake Liemba in the south, occupies a deep longitudinal basin, girt with mountains ; it is 2800 feet above the sea level As yet no outlet has been discovered for this vast lake, and the question whether it has or has not an over flowing river, is still undecided ; but its waters are not perfectly fresh, the drainage to it is small, and the proba bility is that the Tanganyika is a continental lake. Lake Shirwa, enclosed by mountains on the plateau edge south east of Lake Nyassa, and 2000 feet above the sea, has brackish water, and no outlet. Lake Chad, the greatest lake of the continental system of North Africa, is a shallow lagoon of very variable extent, with numerous islands : it lies at about 1100 feet above the sea; its waters are fresh and clear, and its over flow is carried off to north-eastward by the wady named Bahr-el-Ghazal. Lake Ngami, the corresponding lake in the southern continental system, at an elevation of about 2900 feet, is also a shallow reedy lagoon, varying in extent according to the season. The Zuga river carries off its surplus water to eastward. Salt lakes are of frequent occurrence in the areas of continental drainage; perhaps the most remarkable of these is the Assal lake, which lies in a depression east of Abyssinia comparable with that of the Dead Sea, 600 feet beneath the level of the Red Sea; the Sebka-el-Faroon or Schott Kebir, south of Tunis, is a great salt lagoon, 100 miles in length, dried up in summer, when its bed is found to be thickly encrusted with salt, and in winter covered with water to a depth of two or three feet. It lies several feet beneath the level of the Mediterranean. Africa lies almost entirely in the torrid zone, and is the hottest continent of all. The greatest heat, however, is not found under the equator, since the whole of the central belt of the continent is protected by a dense covering of forest vegetation, supported by the heavy rainfall, and has in consequence a more equable climate, but in the dry, bare exposed desert belts, which lie on the margins of the tropics, the Sahara in the north and the Kalahari in the south, where the climate is extreme. The highest tempera ture is found throughout the Sahara, particularly in its eastern portions towards the Red Sea. In Upper Egypt and Nubia eggs may be baked in the hot sands ; and the saying of the Arabs is, &quot; in Nubia the soil is like fire and the wind like a flame.&quot; The regions along the Mediterranean and Atlantic coasts are rendered more tem perate by the influence of the sea. To the south of the Great Desert the temperature decreases, chiefly on account of the increasing moisture and protection of the land sur face from extreme heating by its tree growth, but also because of the greater elevation of the land as the great southern plateau is approached. Both on account of its elevation and its narrower form, which gives greater access to the equalising influence of the surrounding ocean, the southern half of the African continent has a less high temperature than the northern, though the same gradations of climate outward from the centre belt are clearly marked in each division. Regular snowfall does not occur even in the most southern or northern regions; and this pheno menon is only known in the most elevated points of the continent, as in the Atlas Mountains in the north, the summits of which retain patches of snow even in summer, in the Abyssinian peaks, in the highest points of the mountains of the Cape Colony, and most remarkably in the lofty summits of Mounts Kenia and Kilima-njaro, which rise on the plateau directly beneath the equator. The intensity of radiation and its influence upon the tempera ture are very great in Northern Africa; while in the day time the soil of the Sahara rapidly absorbs the solar rays, during the night it cools so rapidly that the formation of ice has often been known to occur. The observed average temperatures of the extreme months of the year at various points of Africa, from N. to S., are given in the following table: Jan. July. Jan. July. Las Palmas, Can- ) 61 9 73 6 Kobbe, Darfur, . 67-1 87-8 ary Islands, . j Ankobar, Abyssinia, 52-0 58-1 Santa Cruz, Tcuc- I rifFe, . . . j 637 77-2 Elniina, Gold Coast, Christiansborg,, , 797 81-0 76-7 76-5 Funchal, Madeira, 63-5 72-5 Niger Mouth (5 ) Qfl.A RO-O Casa Blanca, Ma- ) 57-4. 77-Q 9 N.), . . . j OJ J ou & rocco, ... | 1 / Gondokoro (5 N.), 89-3 78-5 La Calle, Algeria, 54-6 78-4 Zanzibar, . 83-3 77-1 Algiers, ) (37 K) 55-8 76-3 Ascension I. (7 30 S.),. . . 77-0 75-0 Oran, ,, 56-2 76-9 St Helena,. . 73-6 65-8 Constantino,, , 44-6 81-0 Tcte, on the Zain- ) Of) ,f A-n. t L Aghouat, ,, 54-2 98-9 beze (16S.),. | o- y 1 2, 4 Tunis, .... Alexandria, Egypt, 57-2 57-4 77 2 78-5 Port Louis, Mau- ) ritius, . . . ) 81-7 71-8 Cairo, ) ^K fl Sfi-O St Denis, Bourbon, 797 71-8 (80 N.) | 00 o OO U Durban, Natal, . 74-2 62-4 Kenneli,, , Freetown, Sierra ) 62-4 QO -n 94-3 77 Pietermaritzburg ) (30 S.), . . 1 71-4 55-2 Leone, . o^ U 1 1 O Cape Town (34S.), 74-3 57-6 Kuka, Bornu j if* -R QQ.C Stellenbosch, . 77-0 57-0 (13 N.), . . j / O oo o Swellendam, . 727 59-9 Africa is not much under the influence of the regular Wim winds, except the monsoons of the Indian Ocean, the great movement of the atmosphere depending chiefly on the oscillation of the continent beneath the sun during the sea sons, as will be afterwards explained. The wind currents over the whole continent have a prevailing direction from the east. There are the trade winds, modified by inter ruptions of changing heat and elevation of the land sur face. In the northern part of the Indian Ocean the year is divided between the south-west monsoon, blowing from March till September, away from Africa, towards the then heated continent of Asia; and the north-east monsoon, or rather the normal trade wind, blowing towards the African coasts, from October till February. It will be seen in the next paragraph, that the monsoons, although they extend only to about a third portion of the East African shores, have an extremely important bearing upon the physical economy of the whole African continent. From hurricanes Africa is nearly exempt, except in its south-eastern extremity, to which at times the Mauritius hurricanes extend. At rare intervals these have visited the east coast as far as Zanzibar. Northern Africa is much exposed to the hot winds and storms from the Sahara, which are called in Egypt Khamsin, in the Mediterranean Scirocco, Shume or Asshurne in Marocco, and Harmattan on the west coasts of the Sahara and in the countries bordering on the Gulf of Guinea. These always blow directly across the coast from the interior, and seem to move round the compass during the year, beginning in Egypt in April, in Algeria in July, in Marocco in August, in Senegambia in November. Similar dry electrical winds are experienced in the Kalahari desert in the south. Whirlwinds, frequently carrying sand up into the atmosphere, are of frequent occurrence in these deserts, and are also known in the dry region of Unyaniuezi, between Zanzibar and the Tanganyika, and in the Limpopo basin farther south. Extreme heat and dry- ness are the characteristics of these winds, which, raising the sand, filling the air with dust, and prodigiously favour-
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