Mr. HOPKINS, ON THE MOTION OF GLACIERS. 53 



It may perhaps be answered, that the best way of making such experiments is by observing 

 the glaciers themselves, or in other words, that it is better to make our theory depend on obser- 

 vation than on direct experiment; and, undoubtedly, it is thus that we do arrive at the highest 

 order of evidence which the greatest problems of physical science admit of. We set out with 

 some determinate hypothesis, of which we calculate the consequences. These calculated results are 

 then compared with the results of observation, and the degree of accordance between them will 

 constitute the evidence in favour of our original hypothesis. The conclusiveness, however, of this 

 inductive process of reasoning must depend on the rigorousness with which we can calculate 

 our results, and the accuracy with which the phenomena to be accounted for can be observed. If 

 our methods possess, in both these particulars, the requisite degree of exactness, we shall be certain 

 of demonstrating the truth or detecting the fallacy of our original hypotheses, and of thus elimi- 

 nating, as it were, all but the true one. In the case before us, however, the required exactness 

 is not attainable, for it will appear, in the course of this paper that the particular phenomena to 

 which Professor Forbes would seem to appeal in evidence of the truth of his theory, are equally 

 consistent with that which I shall offer. Consequently, the necessity of direct experimental proof 

 of the viscosity of glacial ice assumed in this theory cannot be superseded, in the present state of 

 our knowledge of the motion of viscous fluids and of glacial movements, by an appeal to phenomena 

 which those movements themselves present to us. 



This review of the existing state of glacial theories is sufficient to shew how imperfect a 

 solution of the problem of glacial motion has yet been offered. All the above theories repose 

 more or less on hypotheses unsupported by the direct evidence of experiment or observation. The 

 theory of De Saussure is apparently in opposition to the ascertained facts respecting the motion 

 of sliding bodies ; in the theories of dilatation and expansion, the alternations of thawing and freez- 

 ing is an unsupported assumption, and the mechanical adequacy of the causes assigned by these 

 theories (supposing them to be real causes) a pure hj'pothesis; and in the last-mentioned of the 

 above theories, the viscosity of the glacial mass necessary to give effectiveness to the moving force 

 of gravity, seems to be opposed to the evidence of our senses. It would be difficult perhaps to 

 conceive the solution of any mechanical problem in a much more unsatisfactory state than the 

 one before us; for, of the different solutions which have been proposed, each involves some difficulty, 

 which, if not removed, must ensure its ultimate rejection. 



In considering these difficulties it occurred to me, that the motion down an inclined plane 

 of a mass of ice having its lower surface in a state of disintegration, might take place according 

 to laws different from those observed in the sliding motion of rigid bodies, and, without forming 

 any very definite conception of the manner in which the motion might be modified under this new 

 condition, I determined to try the experiment. The results were such as to remove entirely, 

 I conceive, what appeared to be an insuperable objection to the sliding theory, by shewing that 

 ice, under the condition above stated, is capable of descending with a slow unaccelerated motion, 

 by the action of gravity alone, down planes of much smaller inclination than those over which 

 known glaciers are observed to move. In the next section I shall describe the experiments whicli 

 leave no doubt, in my estimation, as to the real cause of glacial motion. 



SECTION II. 



On the Cause of Glacial Motion. 



1. Experiments. — A slab of sandstone was so arranged that the inclination of its surface 

 to the horizon could be slowly and continuously varied by the elevation of one edge. The sur- 

 face was in the state in which it had been sent from the quarry, and in which such stones 

 are sometimes laid down as paving stones, retaining the marks of the pick with which the 



