Dr. Whewell's Additional Remarks on Glacier Theories. 219 



of the glacier moves onward without much relative motion of 

 its parts. But still it is to be supposed that a theorist who 

 had accepted the horizontal plasticity, would not long continue 

 to reject the vertical plasticity ; and thus, beginning from the 

 hypothesis of solid glaciers, he would be led to the conviction 

 of plastic glaciers. 



I have traced these changes of hypothesis, and the final re- 

 sult, of a plastic glacier, as if any theorist might have been led 

 to make these successive steps by the obvious necessity of the 

 case. But we know well, from the history of science, that the 

 progress from wrong to right is not usually made in this direct 

 and simple manner; and that, till some discoverer has firmly 

 seized and clearly presented the ultimate truth, theorists mani- 

 fest a wonderful ingenuity in taking the wrong turn at every 

 step of their advance. In the case of glaciers, the view of 

 them as plastic, and the application of that view in a very 

 striking and extensive manner to the facts of their motion and 

 structure, is, as all your readers know, due to Prof. Forbes. 

 He is the discoverer on this subject; and to him the scientific 

 world is indebted for a theory of glaciers so convincing, that 

 to it the views of other theorists are steadily converging. 

 This being the obligation for which speculators respecting 

 glaciers are indebted to him, it would be a very unworthy 

 return for such a benefit, if they were to write as if they owed 

 nothing to him ; and were to present the above-noticed steps 

 of the transition from the solid to the plastic hypothesis, as if 

 they were the necessary results of their own reasonings, and 

 not merely modified forms of expressing views suggested by 

 the writings of Prof. Forbes. 



There is one point on which I have as yet said nothing, — 

 the condition and motion of the lower surface of the glacier. 

 This point, though not unimportant, offers nothing of novelty ; 

 the glacier slides by the constant melting of its lower surface 

 in virtue of the heat of the subjacent soil. This was distinctly 

 taught by De Saussure, and has long been the view familiar 

 to the minds of geologists, English, French and German; as 

 it would be easy to show if it were necessary. If we combine 

 this doctrine with that of the plasticity of the mass, we have a 

 complete view of the nature of glacier motion. Prof. Forbes's 

 theory, of course, includes what is true in that of Saussure. 



Since these views are thus combined, it may be asked what 

 is the principal cause of glacier motion? Is it gravity? or the 

 melting of the lower surface? or the plasticity of the mass? 

 But it is plain that if this theory be adopted in fact, the ques- 

 tion among these forms of expression is quite unimportant; 

 just as it would be if, in the case of a river, we were to ask 



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