248 Mr. Hopkins on the Mechanism of Glacial Motion. 



some motion may result from the plasticity of glacial ice. A 

 horizontal beam loaded with a heavy weight at its middle 

 point will bend, and thus be put in motion without immediate 

 fracture ; but after a certain time the motion will cease or the 

 beam will be broken. With respect to glacial ice, however, 

 I can conceive that the change of form and consequent mo- 

 tion may proceed for a much longer time without fracture 

 than in ordinary cases, for the reasons already stated in my 

 second letter (art. 27). The two theories may agree in ad- 

 mitting both these kinds of motion ; but while one of them 

 claims, for instance, nine-tenths of the whole motion as due 

 to sliding, the other claims the same proportion as due to the 

 plasticity of glacial ice. The object of my first letter, more 

 especially, was to place the sliding theory in its true position, 

 and to meet those objections to it which have been urged by 

 Prof. Forbes and others, in the advocacy of their own views. 

 But for the final determination of the claims of the two theo- 

 ries, an appeal must, and doubtless will, be made to further 

 observation. Each theory has its leading difficulty. That of 

 the sliding theory has always been considered to consist in the 

 improbability of a glacier's sliding over a bed of such small 

 inclination as that of some glaciers ; that of the viscous theory, 

 in the apparent contradiction offered by the assumed plasti- 

 city of glacial ice to the evidence of our senses. The first 

 difficulty has been met by my experiments on the descent of 

 ice along inclined planes, as far as it can be met by experi- 

 mental evidence. No similar evidence has been brought for- 

 ward to meet the other difficulty. No experiments have been 

 adduced to show that the plasticity of glacial ice is really 

 greater than common inspection might lead us to suppose; 

 and I maintain that no direct evidence whatever has been 

 offered to show that the compressions and flexures of glaciers 

 are really indicative of a degree of plasticity in which the mo- 

 tion originates, and are not the consequences of a motion due 

 to independent causes. The reasoning by which this de- 

 ficiency of evidence is attempted to be supplied, calls for a 

 few remarks. 



If it be contended that the greater part of the motion of a 

 glacier, as nine-tenths for example, be independent of the 

 sliding over its bed, then, if there were no such sliding, i. e. 

 if the lower surface of the ice were firmly frozen to its bed, 

 nine-tenths of the actual motion would still exist. This is 

 perhaps the best form of presenting the leading difficulty of 

 the viscous theory. According to that theory, the upper sur- 

 face must move much faster than the lower one; but no proof 

 whatever, founded on observation, has been offered to show 



