1858.] on the Mer -de- Glace. 553 



Further, the marginal crevasses of a glacier are known to be a 

 consequence of the swifter flow of its central portions, which throws 

 the sides into a state of strain from which they relieve themselves 

 by breaking. Now it is easy to calculate the amount of stretch- 

 ing demanded of the ice in order to accommodate itself to the 

 speedier central flow. Take the case of a glacier, half a mile 

 wide. A straight transverse element, or slice, of such a glacier, 

 is bent in twenty-four hours to a curve. The ends of the slice 

 move a little, but the centre moves more : let us suppose the 

 versed side of the curve formed by the slice in twenty-four hours to 

 be a foot, which is a fair average. Having the chord of this arc, 

 and its versed side we can calculate its length. In the case of the 

 Mer-de-Glace which is about half-a-mile wide, the amount of 

 stretching demanded would be about the eightieth of an inch in 

 twenty-four hours. Surely, if the glacier possessed a property 

 which could with any propriety be called viscosity, it ought to be 

 able to respond to this moderate demand ; but it is not able to do 

 so ; instead of stretching as a viscous body, in obedience to this slow 

 strain, it breaks as an eminently fragile one, and marginal crevasses 

 are the consequence. It may be urged that it is not fair to distri- 

 bute the strain over the entire length of the curve : but reduce the 

 distance as we may, a residue must remain which is demonstrative 

 of the non- viscosity of the ice. 



To sum up then, two classes of facts present themselves to the 

 glacier investigator, one class in harmony with the idea of vis- 

 cosity, and another as distinctly opposed to it. Where pressure 

 comes into play we have the former, where tension comes into play 

 we have the latter. Both classes of facts are reconciled by the 

 assumption, or rather the experimental verity, that the fragility of 

 ice* and its power of regelation, render it possible for it to change 

 its form without prejudice to its continuity; and no doubt was 

 entertained that the motion of the parts of a glacier was aided by 

 the partial liquefaction of the mass by pressure, as pointed out by 

 Mr. James Thomson, and proved experimentally by Prof. Wm. 

 Thomson, and the speaker himself. 



A detailed account of the observations referred to in the fore- 

 going pages is contained in a paper recently presented to the Royal 

 Society. It now remains for the author to express his grateful 

 sense of the able and unremitting assistance rendered him by his 

 friend Mr. T. A. Hirst, during his six weeks* residence at the 

 Montenvert. f J T 1 



♦ Wherever the compressed ice is surrounded by a resistant mass, the yield- 

 ing is so gradual as to resemble plasticity ; I should have no objection to the 

 use of this term, but the term " viscous " has undoubtedly led to erroneous con- 

 ceptions of the physical qualities of glacier ice. 



