1885.] 



and the Motion of Glaciers, 



103 



The supposition that, while ice at 0° C. is sensibly viscous, the vis- 

 cosity diminishes rapidly with the temperature, is in complete 

 accordance with the facts of the changes which take place in a 

 glacier during the winter. The terminal melting ceases, but the 

 advance of the end of the glacier into the valley is very slow, and 

 possibly ceases altogether in the depth of winter. Higher up the 

 forward movement of the surface continues, though at a slower rate 

 than in summer, and though the glacier does not lengthen much in 

 winter, it thickens considerably, and the surface rises, often through 

 many feet, so as to make up (in a glacier which is neither increasing 

 nor diminishing) for the enormous surface waste of the summer. 

 This is exactly what we should expect if the great working mass of 

 the glacier retained the same mobility in winter as in summer, while 

 the surface layers and the extremity had their resistance to change of 

 form very greatly increased. The whole movement of a glacier in 

 winter is closely parallel to that of a lava stream when it is beginning 

 to cool, when the outer crust and the terminal portion have become 

 solidified while the great mass remains semifluid; the stream con- 

 tinues to advance, but only slowly, while the lower portions increase 

 in thickness like those of a glacier in winter. 



V. Mr. Browne's Argument from Ice Cliffs. 



Mr. Browne in the paper which has been above referred to (" Proc. 

 Roy. Soc," vol. 34, p. 210) brings forward an additional argument 

 in favour of a large shearing strength of ice. He calculates the 

 shearing strength necessary in order that a vertical ice cliff of a 

 certain height may be able to stand, and finds about 30 lbs.. per square 

 inch to be the minimum strength consistent with the existence of ice 

 cliffs 300 feet high. The argument would be perfectly sound if for 

 " stand " we read " stand permanently," but as it is put it is liable to 

 the same objections as Canon Moseley's direct experiments upon 

 shearing. 



Crevasses 300 feet deep are said to exist, but an individual crevasse 

 is by no means a long-lived structure. When the bed of a glacier 

 widens suddenly below a projecting vertical face of rock, we some- 

 times find a cliff bounding the glacier for some little distance below 

 the projection. It never, however, extends very far, and when the 

 glacier-bed widens gradually the ice spreads out so as to fill the wider 

 bed without any cliff being formed at all. 



The spreading out of a glacier like the Rhone Glacier where ifc 

 emerges from a gorge on to a comparatively open space, which Mr. 

 Browne strangely enough quotes (loc. cit., p. 215) as favourable to 

 Canon Moseley's view, is in itself a convincing proof that ice at 0° C. 

 will not stand permanently in a vertical cliff of any considerable 

 height ; it gives way gradually, but still it gives way. 



