Dr. Du Riche Preller — The Merjelen Lake. 101 



water of 14: metres (46 feet), requires an orifice of only 23 square 

 centimetres, 1 or 3 6 square inches, so that a very small fissure in the 

 ice-wall suffices for the ordinary drainage of the lake through the 

 glacier. When the lake, swelled hy water derived from the glacier, 

 reaches its maximum level (viz. contains 10 million cubic metres or 

 tons, or double its normal volume), and then empties itself, the 

 discharge of about 1 million cubic metres during the first twelve 

 hours corresponds to 23 cubic metres (805 cubic feet) per second, or, 

 at a mean head of 25 metres (82 feet), to an orifice of 1 square metre 

 (10-7 square feet) ; and the discharge of 9 million cubic metres in 

 the following twelve hours corresponds to nine times as much — that 

 is, to 208 cubic metres per second, and to an orifice of 9 square 

 metres, or about 100 square feet. The outflow of about 200 cubic 

 metres per second is larger than the average volume of the Rhone at 

 its inflow into the lake of Geneva (150 cubic metres per second) ; 

 but the pressure, and hence the velocity, of the discharge from the 

 Merjelen lake is, of course, so much greater, that a much smaller 

 sectional ai*ea is required. It is thus seen that the fissures or rents 

 through which the lake empties itself are by no means of extra- 

 ordinary dimensions. The whole process is, in fact, precisely like 

 that of an ordinary reseiwoir discharging its contents through a rent 

 gradually formed and widening in a saturated dam ; in both cases, 

 the discharge takes place at the point of least resistance. 



There are no exact data to show how much time the lake requires 

 to refill to its normal level (about half full) ; but it is evident that 

 this operation is performed very gradually, and chiefly in winter and 

 early spring, when, owing to the stoppage of ablation, the glacier 

 wall becomes again compact, and the lake is fed by increased 

 precipitation in its drainage area. The greater frequency of the 

 partial or complete emptying of the lake within the last twenty 

 years coincides with the ascertained general recession or shrinkage 

 of the Great Aletsch glacier since 1873 s ; and there is, possibly, a 

 relation between the two phenomena, since both are, in the main, 

 the effects of meteorological causes. 



Owing to the conflict of evidence, it is extremely difficult to 

 obtain authentic information respecting the partial or complete 

 emptying of the lake and the exact dates of the phenomenon. This 

 much, however, is certain, that a partial emptying produced by the 

 forcing of a passage at a point above the base of the glacier wall 

 is of much more frequent occurrence than a total discharge of the 

 lower basin. Even in the latter case a pool always remains in the 

 upper or shallow basin. Moreover, the mere fact of the lake having 



1 The sectional area is given by the volume divided by the velocity, and the 

 velocity is given by "V 2 g h, g being the acceleration due to gravity = 10 (metric), and 

 h the mean head of water. 



2 Like the majority of Alpine glaciers, the Aletsch glacier increased during the 

 first half of this century ; its decrease began about 1860, and has become more 

 marked since 187 3. In 1893 it receded five metres or seventeen feet, according to 

 Prof. F. A. Forel's " Variations Periodiques des Glaciers des Alpes" for that year. 



