Or, referring to actual numbers:--the stake No. 15 on our 5th line, page 284, stood on the lateral moraine of the Mer de Glace; and between it and No. 14 a distance of 190 feet intervened. Let A B, Fig. 29, be the side of the glacier, moving in the direction of the arrow, and let _a b c d_ be a square upon the glacier with a side of 190 feet. The whole square moves with the ice, but the side _b d_ moves quickest; the point _a_ moving 10 inches, while _b_ moves 14.75 inches in 24 hours; the differential motion therefore amounts to an inch in five hours. Let _a b' d' c_ be the shape of the figure after five hours' motion; then the line _a b_ would be extended to _a b'_ and _c d_ to _c d'_.
[Illustration: Fig. 29. Diagram illustrating test of viscosity.]
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.