52 



Mh. HOPKINS, ON THE MOTION OF GLACIERS. 



and a repetition of the process to which the motion is referred is perhaps still more inexpli- 

 cable than in the dilatation theory. 



If, however, we chose to allow the alternations of congelation and dissolution required by 

 these theories, it might still be shewn (as I have done elsewhere*) that the effectiveness of the 

 causes of glacial motion assigned by them must probably be very much less than that of gravity 

 whenever the inclination of the bed of the glacier is not much less than that of any known 

 glacier. I think it unnecessary, however, to repeat such investigations in this communication, 

 or to insist on other difficulties involved in these theories, because there is an obvious and con- 

 clusive test to which they will doubtless be soon subjected. It is manifest, that, according to 

 either theory, the velocity with which any proposed point of a glacier will move must be approxi- 

 mately proportional to its distance from the upper and fixed extremity. If, therefore, it should 

 be found, on the contrary, that the motion near the two extremities of a glacier is nearly the same, 

 the refutation of both these theories will be complete. M. Agassiz has been engaged in the most 

 careful determination of all the circumstances connected with the motion of the glacier of the Aar, 

 and Professor Forbes has in like manner been occupied with the Mer de Glace of Mont Blanc. 

 The results in the latter case are already partially known through Professor Forbes's letters to 

 Professor Jamesonj", and appear to be totally inconsistent with both the theories of which we 

 are now speaking. The full details of the surveys of these two glaciers will form most important 

 additions to our knowledge of glacial phenomena. In the mean time sufficient has been said to 

 indicate the great, and, as I believe, insuperable difficulties both of the expansion and dilatation 

 theories. 



A conviction of the inadequacy of any of the three theories above mentioned to account for 

 the motion of glaciers, has led Professor Forbes to suggest another theory. In common with that 

 of De Saussure, it attributes the motion of a glacier to the action of gravity ; but whereas, according 

 to the sliding theory, gravity is enabled to act effectively in communicating motion to the glacial 

 mass in consequence of the facility with which the lower surface of the glacier moves over the 

 bed on which it rests, the theory now alluded to attributes the efficiency of gravity to the facility 

 with which contiguous particles of the ice itself may move with reference to each other. Such 

 at least is my conception of the theory, and it is only in this sense that I can understand it as a 

 mechanical theory : for if it be merely meant to assert that certain phenomena of glacial motion 

 are similar to those which would present themselves if the glacial mass were really a viscous fluid, 

 the assertion is only equivalent to a particidar geometrical representation of the phenomena in ques- 

 tion. In this sense the theory asserts nothing respecting mechanical causes, and therefore cannot 

 be classed with the theories already mentioned. 



Regarding this view of glacial motion, however, (in the absence of its more complete development) 

 according to my conception of it as a mechanical theory, it may be asked, what reason have we 

 to suppose that the adhesion of contiguous particles of glacial ice is much less than that of a 

 particle of ice in the lower surface of the mass to the contiguous particle of the bed of the glacier ? 

 The general mass of glacial ice is extremely hard and compact, and has unquestionably a great 

 cohesive power, so that when we consider the probable effects of terrestrial heat and subglacial 

 currents in destroying the adhesion between the glacier and its bed, it would appear the more 

 probable that this adhesion should be much less instead of being much greater than that between 

 contiguous portions of the ice itself. I do not insist on the absolute conclusiveness of this reasoning, 

 but on its sufficiency to shew the necessity of proving, by independent experimental evidence, that 

 glacial ice does possess this property of semi-JluidHy or viscosity, if we would attribute to that 

 property the effectiveness of gravity in setting a glacier in motion. 



* The investigations alluded to were printed and privately 

 circulated among most persons interested in glacial researches. 

 The object was to compare the degrees of efficiency of the causes 



of glacial motion assigned respectively by the three theories men- 

 tioned in the text. 



+ Edinburgh Quarterly Journal of Science. 



