The extension of these lines does not however express the maximum strain to which the ice is subjected. Mr. Hopkins has shown that this takes place along the line a d; in five hours then this line, if capable of stretching, would be stretched to a d'. From the data given every boy who has mastered the 47th Proposition of the First Book of Euclid can find the length both of a d and a d'; the former is 3224.4 inches, and the latter is 3225.1, the difference between them being seven-tenths of an inch.
This is the amount of yielding required from the ice in five hours, but it cannot grant this; the glacier breaks, and numerous marginal crevasses are formed. It must not be forgotten that the evidence here adduced merely shows what ice cannot do; what it can do in the way of viscous yielding we do not know: there exists as yet no single experiment on great masses or small to show that ice possesses in any sensible degree that power of being drawn out which seems to be the very essence of viscosity.
I have already stated that the crevasses, on their first formation, are exceedingly narrow rents, which widen very slowly. The new crevasse observed by our guide required several days to attain a width of three inches; while that observed by Mr. Hirst and myself did not widen a single inch in three days. This, I believe, is the general character of the crevasses; they form suddenly and open slowly. Both facts are at variance with the idea that ice is viscous; for were this substance capable of stretching at the slow rate at which the fissures widen, there would be no necessity for their formation.
STRETCHING OF ICE NOT PROVED.
It cannot be too clearly and emphatically stated that the proved fact of a glacier conforming to the law of semi-fluid motion is a thing totally different from the alleged fact of its being viscous. Nobody since its first enunciation disputed the former. I had no doubt of it when I repaired to the glaciers in 1856; and none of the eminent men who have discussed this question with Professor Forbes have thrown any doubt upon his measurements. It is the assertion that small pieces of ice are proved to be viscous[A] by the experiments made upon glaciers, and the consequent impression left upon the public mind—that ice possesses the "gluey tenacity" which the term viscous suggests—to which these observations are meant to apply.
FOOTNOTES:
[A] "The viscosity, though it cannot be traced in the parts if very minute nevertheless exists there, as unequivocally proved by experiments on the large scale."—Forbes in 'Phil. Mag.,' vol. x., p. 301.