GLACIERS. 



681 



glaciers, on the principle that the ice moves like a viscous fluid, is fully elucidated. His 

 later writings on the subject are contained in a volume entitled "Occasional papers on 

 the Theory of Glaciers." Later, Tyndall (from whom these historical notes are taken) 

 made a further series of measurements and observations in the Alps, demonstrating the 

 influence of bends in a glacier, and explaining other glacial phenomena. His views 

 are contained in "The Glaciers of the Alps," 1860, and "The Forms of Water," 1872. 



The rate of descent in the mass of a glacier varies from one or two 

 inches, to over fifty a day ; and the rate is about half less in winter 

 than in summer. Ten to twenty inches a day in the warm season is 

 most common ; twelve inches corresponds to three hundred and sixty- 

 five feet a year, or one mile in about fourteen and a half years. It 

 takes the ice of the Col du Geant one hundred and twenty years to 

 reach the lower end of the Mer de Glace. 



Opposite Montanvert, where there is a bend in the stream, Tyndall found the move- 

 ment per day, at eleven stakes, from the east to the west side, 20, 23, 29, 30, 34, 28, 25, 

 25, 18, 9 inches, the first and last being near the opposite sides. Descending from Tre- 

 laporte to Montanvert, the rate increases from twenty to thirty-four inches a day. At 

 Trelaporte, the three tributary glaciers of the Col du Geant, Lechaud, and Talefre have 

 become one; and the ice moves in a channel but half as wide as the sum of the widths 

 of these three tributaries. The rate of movement above this narrowing is hence slow; 

 Tyndall found the movement per day, across the lower part of the Col du Geant, 11, 

 10, 12, 13, 12, 13, 11, 10, 9, 5 inches; across the lower part of the Lechaud glacier, 5, 8, 

 10, 9, 9, 8, 6, 9, 7, 6. 



Forbes deduced, from his measurements, made at two stations on each of the Bois and 

 Bossons Glaciers, the following results. The first station on the Bois Glacier was near 

 its upper part, where the rapidity is unusually great, and the other near its lower ex- 

 tremitv. 



Motion from November, 1844, to Novem- 

 ber, 1845 



Mean daily motion 



Mean daily motion in summer, April to 

 October 



Mean daily motion in winter, October to 

 April , 



Bois I. 



847-5 ft. 

 27-8 in. 



37-7 in. 



19-1 in. 



Bois II. 



220-8 ft, 

 7-3 in. 



9-9 in. 



4-7 in. 



I. 



657-8 ft. 

 21-6 in. 



28-0 in. 



15-8 in. 



Boss. II. 



489-1 ft. 

 16-1 in. 



22-2 in. 



10-7 in. 



The winter movement of the Mer de Glace is not over half that of the summer. 

 Forbes found for the maximum in July, at his upper station on the Bois Glacier, 52-1 

 inches a day, and in December 11-5 inches. 



(6.) The capability of motion in a glacier is attributed to — 

 (a.) A kind of plasticity in ice. Ice may be made, through simple 

 pressure, to copy a seal or mould, like wax ; or to take the form of a 

 long cylinder, by pressing it through holes ; and, if the ice. in such an 

 experiment, is added in fragments, it comes out solid. The ice, when 

 thus under pressure, is somewhat clouded, by the incipient fractures 

 in it ; but, when the pressure ceases, it is quite clear, owing to rege- 

 lation along all such microscopic fractures. Kane mentions, in his 

 " Arctic Explorations," the case of a table of ice, eight feet thick and 

 twenty or more wide, supported only at the sides, which, between the 



