THE FAUNA OF THE GREAT AFRICAN LAKES. 629 



The larger African lakes are for the most part situated 

 in the tropics north aiid south of the Equator, the bulk of 

 Nyassa lying between 10' and 14° south. This lake is 1500 

 feet above the level of the sea, some 340 miles from north to 

 south, and from twenty-five to forty miles across. Its water 

 is profoundly deep, soundings of more than 300 fathoms, no 

 bottom, having been obtained over a great portion of its 

 length. The water of this lake, therefore, goes below the 

 level of the oceans on each side of the continent, and from 

 its immense size it acquires all the superficial characters of 

 a stretch of sea such as the Baltic or the English Channel. 

 The tiuna of such a lake is exposed to the full force of 

 storms and currents and to the effects of heavy surf, while 

 owing to its altitude the superficial layers of water are at a 

 considerably lower atmospheric pressure than those of the 

 lakes and rivers on or about the level of the sea. Nyassa, 

 moreover, has a broad outlet to the ocean to the east, and 

 the water of the lake is thereby kept peculiarly sweet and 

 fresh. These conditions are very different from those 

 obtainino- in Lake Shirwa, which stands about the same 

 level at a short distance to the south. Here the water is 

 nowhere more than about fourteen or fifteen feet deep, the 

 lake is small in superficial extent, filled with reed patches 

 which effectually prevent all surf, and, lasdy, it is without an 

 outlet, and consequently the organisms living in it have to 

 endure a continually increasing salinity as the water year 

 by year dries up. 



Lake Bangweolo, which lies between Lake Nyassa and 

 Tanganyika to the north-west, is a broad, shallow basin of 

 water which is fresh and pure, but it stands at an elevation 

 of 3000 feet, its basin being really a slight depression in the 

 interior plateau of the continent, and ovv^ing to its greater 

 altitude, the average temperature is far lower than that of 

 the Nyassa Valley to the south. Except that it is deep, 

 Lake Mwero is under the same physical conditions as Lake 

 Bangweolo. Tanganyika is at a considerably lower level, 

 2700 feet, but it is so shut in by hills that the average daily 

 temperature of the air and of the water of this lake are 

 higher than those of Nyassa, although, as I have said, the 



