1885.] 



and the Motion of Glaciers. 



101 



carefully made, and were perfectly concordant, and experience had 

 shown me the precautions necessary to obtain an accurate result. 



The shearing force employed was, indeed, rather more than double 

 that which, according to Canon Moseley's calculations, is exerted by 

 gravity in the Mer de Glace, near the Tacul ("Phil. Mag.," xxxvii, 

 p. 369) ; bat it is about one twenty-fifth of his smallest value of the 

 shearing strength of ice, and the amount of shear is larger than is 

 implied in any of the ordinary cases of glacier motion. 



I may mention that in spite of the low temperature of the grotto 

 the exposed ends of the bar were rendered partially opaque by a 

 number of fine cracks, showing a tendency in the ice to break up into 

 angular fragments such as may be observed in the partially disinte- 

 grated ice on the surface of a glacier (the Gletscherkdrner of Heim 

 and others) ; the parts, however, which were inclosed within the 

 wooden blocks and had been effectually protected from radiation, 

 had preserved their transparency quite unimpaired. 



I think then that there is little doubt that under conditions closely 

 resembling those of the interior of a glacier, and under the influence 

 of forces comparable with those which gravity is capable of exerting 

 in a glacier, hand specimens of ice shear in the same manner as a truly 

 viscous solid would do. 



IV. Objections from the supposed Inextensibility of Ice. 



It has been objected to the " viscous " theory of glacier motion that 

 if ice is viscous it must be extensible, and that the fact of the 

 formation of crevasses is inconsistent with this supposition. There 

 is, however, some direct evidence that ice is extensible. Pfaff found 

 ("Phil. Mag.," vol. 1, pp. 335—336) that a bar of ice 52x2'5 

 X 1'3 cm. was drawn out to the extent of 1 mm. by the action of 

 a weight of 3 kilos, for five days. Without laying too much stress 

 upon a single measurement of a somewhat difficult kind, it may 

 safely be said that there is no reason for denying such extensibility. 



The formation of crevasses in a glacier is no argument against it. 

 When a viscous body is extended, its density is not diminished, but 

 simply its form altered. It cannot extend in one direction unless it 

 is at liberty to contract in a direction at right angles to that of its 

 extension. It is true that a glacier has always one free surface, but 

 the changes of form of ice under the influence of moderate forces are 

 avowedly very slow. The formation of crevasses shows that they are 

 too slow to allow the glacier to take its new shape in all cases without 

 rupture. Nor is this surprising. An ordinary icefall presents no diffi- 

 culty when we consider that in order that a glacier may pass without 

 a transverse fissure over a place in which the slope of its bed is 

 increased by 1° (a change which when protracted on paper is hardly 



