372 



MOTION OF GLACIERS. 



[Ch. XVI. 



discontinuity, arising from the different rates at which the 



semi 



each other. Many 



pass 



a 



model itself to the form 



it is forced, exactly as would happen if it possessed a certain 

 ductility, and this power of yielding under intense pressure, 

 was supposed not to be irreconcilable with the idea of 

 the ice being sufficiently compact to break into fri 

 when the strain upon its parts is excessive ; as where the 

 glacier turns a sharp angle, or descends upon a rapid or 

 convex slope. The increased velocity in summer was attri- 

 buted partly to the greater plasticity of the ice, when not 

 exposed to intense cold, and partly to the hydrostatic pressure 

 of the water in the capillary tubes, which imbibe more of this 

 liquid in the hot season. 



Mr. Hopk 



mass, attributed the more 



in the centre to the unequal rate at which the broad stripes 

 of ice, intervening between longitudinal fissures, advance; 

 bnt besides that there are parts of the glacier where no such 

 fissures exist, such, a mode of progression, said Mr. Forbes, 

 would cause the borders of large transverse rents or 'cre- 

 vasses, 5 to be jagged like a saw, instead of being perfectly 

 even and straight-edged.* An experiment made in 1853 by 

 Mr. Christie, secretary to the Koyal Society, demonstrated 



sufficient 



moulding 



and self-adapting power to allow it to be acted 

 upon, as if it were a pasty or viscous substance. A hollow 

 shell of iron an inch and a half thick, the interior being ten 

 inches in diameter, was filled with water, in the course of a 

 severe winter, and exposed to the frost, with the fuze-hole 

 uppermost. A portion of the water expanded in freezing? so 

 as to protrude a cylinder of ice from the fuze-hole ; and tins 

 cylinder continued to grow inch by inch in proportion as tne 

 central nucleus of water froze. As we cannot doubt that an 



* See Mr. Hopkins on Motion of rence between him and Principal Forbes 



Glaciers, Cambridge Phil. Trans. 1844, little more than one of degree. (For 



and Phil. Mag. 1845. Some of the con- summary of Principal J. D. For 



cessions of this author as to a certain views, see Phil. Trans. 1846, pt. H 

 plasticity in the mass, made the diffe- 



