128
AFRICA
[geography
derives some support from the existence in the lake of which occupy portions of the great rift-valleys have steep organisms of a decidedly marine type, the investigation of sides and are very deep. This is the case with the v o which has been the work of Mr J. E. S. Moore. They largest of the type, Tanganyika and ^yasa, the latter of include a jelly-fish, molluscs, prawns, crabs, &c., and form which was found in 1899 to have a depth of 430 fathoms. an isolated group found in no other of the African lakes. Others, however, are shallow, and hardly reach the steep M. J. Cornet, a Belgian geologist, has expressed a doubt sides of the valleys in the dry season. Such are Lake whether these organisms furnish evidence of a former conRukwa, in a subsidiary depression north of Lake N yasa, nexion of Tanganyika with the sea. and Eiassi and Manyara, in the system of the eastern nttLying almost entirely within the tropics, Africa does valley. Lakes of the broad type, such as the ictona not show excessive variations of temperature. Great Nyanza, are probably of intermediate depth. Apart from summer heat is experienced in the lower plains C/imate, the seasonal variations of level, most of the lakes show of North Africa, removed by the great width of periodic fluctuations, while some have supposed that a the continent from the influence of the ocean, and here too progressive desiccation of the whole region is traceable, the contrast between day and night, and between summer tending to the ultimate disappearance of the lakes, buck and winter, is greatest. Farther south, the heat is to some a drying up has no doubt been in pro- extent modified by the moisture brought from the ocean,, gress during long and by the greater elevation of a large part of the surface, geologic ages, but is especially in East Africa, where the range of temperature probably of no prac- is also wider than in the western basin. In the extreme tical importance at north and south the climate is a warm temperate one. The the present time. most important climatic differences are due to variations in The periodic fluctu- the amount of precipitation. The wide heated plains of ations in the level North Africa, and in a lesser degree a corresponding zone of Lake Tanganyika of latitude in the south, have an exceedingly scanty rainare such that its out- fall, the winds which blow over them from the ocean flow appears to be losing part of their moisture as they pass over the outer intermittent. After highlands, and becoming constantly drier , owing to the rising steadily for heating effects of the burning soil of the interior; while some years after the scarcity of mountain ranges in the more central parts 1871, a fall seems likewise tends to prevent condensation. . In the interto have set in about tropical zone of summer precipitation, this is greatest when 1879, which before the sun is vertical or soon after. It is therefore greatest the end of the cen- of all near the equator, where the sun is twice vertical, and tury had carried the less in the direction of both tropics. The rainfall zones lake back within its are, however, somewhat deflected from a due west-to-east natural bed. Within direction, the drier northern conditions extending southabout the same time wards along the east coast, and those of the south noiththe neighbouring wards along the west. Within the equatorial zone certain Lake Rukwa (the areas, especially on the shores of the Gulf of Guinea and in level of which seems the Upper Nile basin, have an intensified, rainfall, but this to be almost iden- rarely approaches that of the rainiest regions of the “world. tical with that of The rainiest district in all Africa, and (in the present state Tanganyika) has in of knowledge) the second rainiest in the world, is a little great part dried up. west of Mt. Cameroon, where at the Debunja station the Sketch Map of the Lake Region of Africa. Others of the East following results have been recorded :— African lakes have, 1895 • 353‘04 inches ^ 1896 . 384-96 „ 372 "40 inches. Mean on the contrary, risen in level, Xyasa having been unusually 1897 . 372-72 ,, high in 1896 and Rudolf in 1896-98 ; so that, if the fluctua1898 • 379-20 ,, tions are due to variations of rainfall, these do not affect as compared with a mean of 4/5 inches at Gherrapunji, in the whole lake region simultaneously in the same. direcAssam. The two distinct rainy seasons of the equatorial tion. In the case of the Victoria Nyanza a variation to the extent of 5 feet has been thought to recur in periods of zone, where the sun is vertical at half-yearly intervals, eighteen to twenty-five years. Since 1896 records of the become gradually merged into one in the direction of the seasonal variations have been kept at stations north of the tropics, where the sun is overhead but once. The queslake; the maximum in the year having been so far about tion of a progressive desiccation of the continent has been 15 inches. They may be accounted for in part by the action touched upon in speaking of the lakes. While the climate of the north and south, especially the of winds. Besides the East African lakes the principal latter, is eminently healthy, and even the intensely heated —Lake Chad, in the northern area of inland drainage
are : Eano'weulu and Mweru, traversed by the head-stream of Sahara is salubrious by reason of its dryness, CUmateaad the *Congo; and Leopold II. and Mantumba, within the the tropical zone as a whole is the most Health great bend of that river. All, except possibly Mweru, are unhealthy portion of the world. . This is more or less shallow. The altitudes of the African lakes especially the case in the lower and moister regions, such as the west coast, where malarial fever is very prevalent, and have already been stated. Divergent opinions have been held as to the mode of deadly; the most unfavourable factors being humidity origin of the Central African lakes, especially Tanganyika, with absence of climatic variation (daily or seasonal). The which some geologists have considered to repre- higher plateaux, where not only is the average temperature Origin of sent an old arm of the sea, dating from a time lower, but such variations are more extensive, are more Lakes. when the whole central Congo basin was under healthy; but even here, with the exception of a few water ; others holding that the lake water has accumulated specially favoured localities, Europeans find it difficult to in a depression caused by subsidence. The former view maintain their health permanently. Opinions still differ
