Africa

periodic fluctuations in the level of Lake Tanganyika are such that its

outllow is intermittent. Besides the East African lakes the principal are:—-

Lake Chad, in the northern area of inland drainage; Bangweulu and Mweru,

traversed by the head-stream of the Congo; and Leopold II. and Ntomba

(Mantumba), within the great bend of that river. All, exceot possibly

Mweru, are more or less shallow, and Chad appears to by drying up. The

altitudes of the African lakes have already been stated.

Divergent opinions have been beld as to the mode of origin of the East

African lakes, especially Tanganyika, which some geologists have considered

to represent an old arm of the sea, dating from a time when the whole

central Congo basin was under water; others holding that the lake water has

accumulated in a depression caused by subsidence. The former view is based

on the existence in the lake of organisms of a decidedly marine type. They

include a jelly-fish, molluscs, prawns, crabs, &c., and were at first

considered to form an isolated group found in no other of the African

lakes; but this supposition has been proved to be erroneous.

Islands.—With one exception—-Madagascar—the African islands are small.

Madagascar, with an area of 229,820 sq. m., is, after New Guinea and

Borneo, the largest island of the world.

It lies off the S.E. coast of the continent, from which it is separated

by the deep Mozambique channel, 250 m. wide at its narrowest point.

Madagascar in its general structure, as in flora and fauna, forms a

connecting link between Africa and southern Asia. East of Madagascar are

the small islands of Mauritius and Reunion. Sokotra lies E.N.E. of Cape

Guardafui. Off the north-west coast are the Canary and Cape Verde

archipelagoes. which, like some small islands in the Gulf of Guinea, are of

volcanic origin.

Climate and Health.—-Lying almost entirely within the tropics, and

equally to north and south of the equator, Africa does not show excessive

variations of temperature. Great heat is experienced in the lower plains

and desert regions of North Africa, removed by the great width of the

continent from the influence of the ocean, and here, too, the contrast

between day and night, and between summer and winter, is greatest. (The

rarity of the air and the great radiation during the night cause the

temperature in the Sahara to fall occasionally to freezing point.) Farther

south, the heat is to some extent modified by the moisture brought from the

ocean, and by the greater elevation of a large part of the surface,

especially in East Africa, where the range of temperature is wider than in

the Congo basin or on the Guinea coast. In the extreme north and south the

climate is a warm temperate one, the northern countries being on the whole

hotter and drier than those in the southern zone; the south of the

continent being narrower than the north, the influence of the surrounding

ocean is more felt. The most important climatic differences are due to

variations in the amount of rainfall. The wide heated plains of the Sahara,

and in a lesser degree the corresponding zone of the Kalahari in the south,

have an exceedingly scanty rainfall, the winds which blow over them from

the ocean losing part of their moisture as they pass over the outer

highlands, and becoming constantly drier owing to the heating effects of

the burning soil of the interior; while the scarcity of mountain ranges in

the more central parts likewise tends to prevent condensation. In the inter-

tropical zone of summer precipitation, the rainfall is greatest when the

sun is vertical or soon after. It is therefore greatest of all near the

equator, where the sun is twice vertical, and less in the direction of both

tropics. The rainfall zones are, however, somewhat deflected from a due

west-to-east direction, the drier northern conditions extending southwards

along the east coast, and those of the south northwards along the west.

Within the equatorial zone certain areas, especially on the shores of the

Gulf of Guinea and in the upper Nile basin, have an intensified rainfall,

but this rarely approaches that of the rainiest regions of the world. The

rainiest district in all Africa is a strip of coastland west of Mount

Cameroon, where there is a mean annual rainfall of about 390 in. as

compared with a mean of 458 in. at Cherrapunji, in Assam. The two distinct

rainy seasons of the equatorial zone, where the sun is vertical at half-

yearly intervals, become gradually merged into one in the direction of the

tropics, where the sun is overhead but once. Snow falls on all the higher

mountain ranges, and on the highest the climate is thoroughly Alpine. The

countries bordering the Sahara are much exposed to a very dry wind, full of

fine particles of sand, blowing from the desert towards the sea. Known in

Egypt as the khamsin, on the Mediterranean as the sirocco, it is called on

the Guinea coast the harmattan. This wind is not invariably hot; its great

dryness causes so much evaporation that cold is not infrequently the

result. Similar dry winds blow from the Kalahari in the south. On the

eastern coast the monsoons of the Indian Ocean are regularly felt, and on

the south-east hurricanes are occasionally experienced.

While the climate of the north and south, especially the south, is

eminently healthy, and even the intensely heated Sahara is salubrious by

reason of its dryness, the tropical zone as a whole is, for European races,

the most unhealthy portion of the world. This is especially the case in the

lower and moister regions, such as the west coast, where malarial fever is

very prevalent and deadly; the most unfavourable factors being humidity

with absence of climatic variation (daily or seasonal). The higher

plateaus, where not only is the average temperature lower, but such

variations are more extensive, are more healthy; and in certain localities

(e.g. Abyssinia and parts of British East Africa) Europeans find the

climate suitable for permanent residence. On tablelands over 6500 ft. above

the sea, frost is not uncommon at night, even in places directly under the

equator. The acclimatization of white men in tropical Africa generally is

dependent largely on the successful treatment of tropical diseases.

Districts which had been notoriously deadly to Europeans were rendered

comparatively healthy after the discovery, in 1899, of the species of

mosquito which propagates malarial fever, and the measures thereafter taken

for its destruction and the filling up of swamps. The rate of mortality

among the natives from tropical diseases is also high, one of the most

fatal being that known as sleeping sickness. (The ravages of this disease,

which also attacks Europeans, reached alarming proportions between 1893 and

1907, and in the last-named year an international conference was held in

London to consider measures to combat it.) When removed to colder regions

natives of the equatorial districts suffer greatly from chest complaints.

Smallpox also makes great ravages among the negro population.

Flora.—The vegetation of Africa follows very closely the distribution of

heat and moisture. The northern and southern temperate zones have a flora

distinct from that of the continent generally, which is tropical. In the

countries bordering the Mediterranean are groves of oranges and olive

trees, evergreen oaks, cork trees and pines, intermixed with cypresses,

myrtles, arbutus and fragrant tree-heaths. South of the Atlas range the

conditions alter. The zones of minimum rainfall have a very scanty flora,

consisting of plants adapted to resist the great dryness. Characteristic of

the Sahara is the date-palm, which flourishes where other vegetation can

scarcely maintain existence, while in the semidesert regions the acacia

(whence is obtained gum-arabic) is abundant. The more humid regions have a

richer vegetation —dense forest where the rainfall is greatest and

variations of temperature least, conditions found chiefly on the tropical

coasts, and in the west African equatorial basin with its extension towards

the upper Nile; and savanna interspersed with trees on the greater part of

the plateaus, passing as the desert regions are approached into a scrub

vegetation consisting of thorny acacias, &c. Forests also occur on the

humid slopes of mountain ranges up to a certain elevation. In the coast

regions the typical tree is the mangrove, which flourishes wherever the

soil is of a swamp character. The dense forests of West Africa contain, in

addition to a great variety of dicotyledonous trees, two palms, the Elaeis

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