100 Dr. Du Riche Prellcr—The Merjelen Laic. 



The emptying of the Merjelen lake recurs at irregular, and not 

 at all, as is popularly assumed, at regular intervals. 1 It lias been 

 explained by the existence of a tunnel in the glacier, 2 which is 

 supposed to widen, narrow, or close entirely, according as the 

 glacier shifts its position, the total emptying, the partial emptying, 

 or the rise to overflow level of the lake being the result. But this 

 explanation is far from satisfactory. During a period of growth of 

 the glacier, the supposed tunnel would move downward at the rate 

 of about 50 metres per annum, or half a foot per day (viz. at about 

 half the speed of the glacier at its axis) ; hence, in ten years it 

 would travel the whole length of the bar of 500 metres, and the 

 supposed tunnel would then become inoperative so far as the lake 

 is concerned. 



The theory of this mysterious tunnel, whose connection with the 

 stream flowing at the bottom of the glacier has, moreover, never 

 been explained, may therefore be discarded ; and, in my view, 

 the phenomenon of the lake rising to its maximum level is 

 due essentially to meteorological causes, while the emptying is 

 produced by mecbanical agencj'. In other terms, the rise to the 

 maximum level is due to the rate of ablation or melting of the ice 

 at the glacier wall being, at certain periods, e.g. during a hot and 

 dry summer or during the prevalence of hot southerly winds 

 (Foehn), largely in excess of the ordinary outflow through small 

 fissures in the glacier wall ; while the emptying of the lake in the 

 space of ten or twelve hours is due to the largely increased pressure 

 of the volume of water against the glacier wall, 3 whose porous 

 condition, produced by the process of ablation, facilitates the forma- 

 tion of large fissures, through which the efflux of the water takes 

 place. The fact of the level of the lake at first falling comparatively 

 slowly, say about six feet in twelve hours, and then rapidly, or about 

 160 feet in the same period, is easily explained by the gradual 

 yielding of the ice to the pressure of the water, until the rent or 

 orifice enlarges and the rate of efflux is thereby accelerated. 



The drainage area of the lake is about 3 square kilometres, or 1*15 

 square mile, which, at an annual rainfall of 3 metres (10 feet), and 

 allowing 45 per cent, for absorption and evaporation, yields about 

 5 million cubic metres or tons per annum. The lake may therefore 

 be said to contain its normal volume when it is about half full. To 

 keep it at this normal level, the requisite outflow is only 004 cubic 

 metre, or 1-4 cubic foot, per second, and this, at a mean head of 



1 The lake is said to have emptied itself formerly every seven years, latterly every 

 three years. 



2 Ph. Gosset, Jahrbuch S.A.C. 1887 and 1888, p. 350. 



3 The pressure of a head of water of 50 metres (164 feet) is equal to five 



atmospheres or 73 lbs. per square inch; and the pressure of the whole volume of 



+ , , , ,, , . „ . , 10,000,000 



tne lake on the porous glacier wall is no less than —±- ,„ = 400 tons per square 

 r oUO x 00 l l 



metre, or 37 tons per square foot, viz. five times the pressure allowed for the walls 



of large reservoirs. 



