GLACIERS. 



695 



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

 tremity. 



Boss. II. 



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. 



Bois II. 



Boss. I. 



847-5 ft. 



220-8 ft. 



657-8 ft. 



27-8 in. 



7-3 in. 



21-6 in. 



37-7 in. 



9-9 in. 



28-0 in. 



19-1 in. 



4-7 in. 



15-8 in. 



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 

 middles of the months of March and May, became so deeply bent that 

 the centre was depressed five feet. The temperature during the in- 

 terval was at all times many degrees below the freezing-point. 



(b.) The facility with which the ice breaks. It thus easily accom- 

 modates itself to new forms of surface. It also mends its fractures 

 easily by a freezing together again of the surfaces in contact, — a pro- 

 cess called regelation. On breaking a piece of ice and then pressing 

 lightly the parts together again, the surfaces, if moist, will become 

 firmly united. Having supported a block of ice at its two ends, if a 

 fine wire is passed around it at middle and weighted below, the wire 

 will slowly melt its way completely through the ice ; but when the 

 cut is completed, the mass will be as solid as at the outset, regelation 

 having gone forward above the wire. The multitudes of fractures 

 made in a glacier on steep slopes may all disappear where the motion 

 becomes slow, and the ice feels the pressure from above. 



The fragility of glacier-ice is owing not only to the brittleness of ice, 

 but very largely (1) to the innumerable an-cells throughout the ice- 

 mass, and (2) to the water which permeates it everywhere, especially 

 during the warm season. Helland found the specific gravity of some 

 Greenland glacier-ice to be only 0.866, owing to the air-cells within 

 it. The air-cells are the occasion of melting about them during the 



