Lakes 307 



In all lakes there are changes of level corresponding with 

 periods of rain and of drought. They are the more considerable 

 the greater the extent of country draining into them, and the 

 more constrained the outflow. In the great American lakes, 

 which occupy nearly one-third of their drainage area, the 

 fluctuations of level are quite insignificant ; for Lake Michigan 

 the United States surveyors give as the maximum and minimum 

 yearly range 1-64 and 0-65 feet. In the Lake of Geneva the 

 mean annual oscillation is five feet, and the difference between 

 the highest and the lowest waters of this century is 9-3 feet. 

 Tin- most rapid rise has been 3-23 inches (82 mm.) in 24 hours. 

 A very remarkable exception to the rule that large freshwater 

 lakes are subject to small variations of level is furnished by 

 Lake Tanganyika in Central Africa. Since its discovery 

 travellers have been much perplexed by the evidence and 

 reports of considerable oscillations of level of uncertain period, 

 and also by the apparent absence of visible outlet, while 

 the freshness of its waters was of itself convincing evidence 

 of the existence of an outlet. By the careful observations of 

 successive explorers the nature of this phenomenon has been 

 fully explained, and is very instructive. It has recently been 

 vi-ited by Captain Hore of the London Missionary Society, and 

 it appears from his reports that the peculiar phenomena observed 

 depend on the fact that the area of country draining into the 

 lake is very limited, so that in the dry seasons the streams 

 running into it dry up altogether, and its outlet gets choked 

 by the rapid growth of vegetation in an equatorial climate. 

 A dam or dyke is thus formed which is not broken down until 

 the waters of the lake have risen to a considerable height. 

 A catastrophe of this kind happened whilst Captain Hore was 

 in the neighbourhood, and he noted the height of the water at 

 :<-nt times near hi- -tati-m at t'jiji. and observed it tall 

 two feet in two months. It continued to fall until in seventeen 

 months it had fallen over ten fee 1 n- the length of tin- 



lake at 330 miles, and the mean breadth at 30 mile^. it- -urface 

 is 9900 square nautical mil. . It tin be reduced two 



feet in 60 days, the water will have to escape at the rate of 

 I 37.5oo cubic feet per second. The MI.MM rate of discharge of 



202 



