1845.] EXPERIMENTS ON THE PLASTICITY OF ICE. 103 



5. EXPERIMENTS AT CHAMOUNI ON THE PLASTICITY OF ICE. 



It has been shown that in order to reconcile De Saussure's 

 theory of sliding motion with the ascertained fact that the centre 

 of the glacier moves faster than the sides, it had been assumed 

 that solutions of continuity or longitudinal crevasses were formed 

 parallel to the length of the glacier, by means of which the 

 central portion slides past that adjacent to it, and so on for 

 successive strips as we approach the sides, the more rapid 

 retardation near the sides being rendered mechanically possible 

 by the increased number of these longitudinal dislocations. 



The result was therefore predicted to be, that the glacier 

 would be found to move by echelons, or that strips of ice of a 

 certain number of feet, or yards, or fathoms, would move either 

 suddenly or by gradual sliding, but at all events so as to mark 

 by an abrupt separation at the longitudinal fissure, that the one 

 portion of ice has slipped past the other by a distinct measurable 

 quantity. 



When I first learnt at Geneva, in August 1844, from Mr. 

 Hopkins's published papers,* that this was really the author's 

 meaning, it occurred to me that the proof between the rival 

 theories was easy, and that it was only necessary to place a 

 series of marks in a right line transversely to the glacier, and 

 observe whether they were displaced by an imperceptible flexure, 

 or whether they slid past one another by sudden dislocations. 



Such a proof was independent of any assertion as to the 

 existence or not of such fissures as those contended for, about 

 which different opinions might be formed, especially as they 

 might be asserted to exist although invisible to the eye. Being 

 satisfied in my own mind of the non-existence of such fissures 

 wherever the ice is not violently dislocated and descends a steep 

 place in a tumultuous manner (which, as already mentioned, is 

 not the case which we consider), I had no hesitation in predicting 

 that the result of the experiment would be confirmatory of my 

 theory, and contradictory of the other ; that the transverse line 



* Cambridge Transactions, vol. viii. 



