^{page 98 note 1} The exact depth deduced from the Swiss Contour Map is 47 metres (154 ft.), while that measured by Prof. Ramsay in 1858 was 157 ft.

^{page 98 note 2} Ph. Gosset, Jahrbuch S.A.C. 1887–8.

^{page 98 note 3} This colour appears to correspond approximately to shade v of Prof. F. A. Forel's scale of eleven shades of lake waters.—F. A. Forel, “Le Léman,” vol. ii, p. 464. Lausanne, 1895.

^{page 99 note 1} A short notice of this occurrence is given by F. v. Salis in the Jahrbuch of the Swiss Alpine Club, 1878 to 1879. Prof. T. G. Bonney (“Nature,” 1867, xxxvi, p. 612) in August, 1858, also saw the lake full one day and empty the next, the emptying having taken place during the night. On the other hand, in 1890, according to Prince Ronald Bonaparte, the emptying of the lake did not take place till five days after the initial fall (“Archives de Genive,” xxiv, 1890, p. 401).

^{page 100 note 1} The lake is said to have emptied itself formerly every seven years, latterly every three years.

^{page 100 note 2} Ph. Gosset, Jahrbuch S.A.C. 1887 and 1888, p. 350.

^{page 100 note 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 the lake on the porous glacier wall is no less than tons per square metre, or 37 tons per square foot, viz. five times the pressure allowed for the walls of large reservoirs.

^{page 101 note 1} The sectional area is given by the volume divided by the velocity, and the velocity is given by being the acceleration due to gravity = 10 (metric), and h the mean head of water.

^{page 101 note 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 1873. In 1893 it receded five metres or seventeen feet, according to Prof. F. A. Forel's “Variations Périodiques des Glaciers des Alpes” for that year.

^{page 102 note 1} The data up to 1890 are derived from the lists given by M. Ph. Gosset and by Prof. F. A. Forel (Jahrbuch Swiss Alpine Club, 1887 to 1890, p. 356, and 1890 to 1891, p. 358); those up to 1895 are collected by myself.

^{page 102 note 2} Formerly, the cowherd who first arrived in the Rhone valley with the news of the lake having emptied itself (the distance from the lake to Brieg being about nine miles) was rewarded with a pair of new shoes.

^{page 102 note 3} This actually happened on the 23rd September last, when the lake had risen to within 50 centimetres (1.6 foot) of the floor of the overflow tunnel, and then, as if to show its contempt for this work of man's hand, emptied itself again through its natural channel, the Aletsch glacier. The committee of inspection which was on its way from Brieg, expecting to see the overflow tunnel in operation, arrived on the 24th September only to find the lake empty.