Part 26 (2/2)

[Ill.u.s.tration: Fig. 28. Diagram ill.u.s.trating the creva.s.sing of Convex Sides of glacier.]

Here then we have the elements, so to speak, of glacier-creva.s.sing, and through their separate or combined action the most fantastic cutting up of a glacier may be effected. And see how beautifully these simple principles enable us to account for the remarkable creva.s.sing of the eastern side of the Mer de Glace. Let A B, C D, be the opposite sides of a portion of the glacier, near the Montanvert; C D being east, and A B west, the glacier moving in the direction of the arrow; let the points _m n_ represent the extremities of our line of stakes, and let us suppose an elastic string stretched across the glacier from one to the other. We have proved that the point of maximum motion here lies much nearer to the side C D than to A B. Let _o_ be this point, and, seizing the string at _o_, let it be drawn in the direction of motion until it a.s.sumes the position, _m_, _o'_, _n_. It is quite evident that _o' n_ is in a state greater tension than _o' m_, and the ice at the eastern side of the Mer de Glace is in a precisely similar mechanical condition. It suffers a greater strain than the ice at the opposite side of the valley, and hence is more fissured and broken. Thus we see that the creva.s.sing of the eastern side of the glacier is a simple consequence of the quicker motion of that side, and does not, as. .h.i.therto supposed, demonstrate its slower motion. The reason why the eastern side of the glacier, as a whole, is much more fissured than the western side is, that there are two long segments which turn their convex curvature eastward, and only one segment of the glacier which turns its convexity westward.

[Sidenote: CREVa.s.sING OF CONVEX SIDE.]

The lower portion of the Rhone glacier sweeps round the side of the valley next the Furca, and turns throughout a convex curve to this side: the creva.s.ses here are wide and frequent, while they are almost totally absent at the opposite side of the glacier. The lower Grindelwald glacier turns at one place a convex curve towards the Eiger, and is much more fissured at that side than at the opposite one; indeed, the fantastic ice-splinters, columns, and minarets, which are so finely exhibited upon this glacier, are mainly due to the deep creva.s.sing of the convex side. Numerous other ill.u.s.trations of the law might, I doubt not, be discovered, and it would be a pleasant and useful occupation to one who takes an interest in the subject, to determine, by strict measurements upon other glaciers, the locus of the point of maximum motion, and to observe the a.s.sociated mechanical effects.

[Sidenote: BERGSCHRUNDS.]

The appearance of creva.s.ses is often determined by circ.u.mstances more local and limited than those above indicated; a boss of rock, a protuberance on the side of the flanking mountain, anything, in short, which checks the motion of one part of the ice and permits an adjacent portion to be pushed away from it, produces creva.s.ses. Some valleys are terminated by a kind of mountain-circus with steep sides, against which the snow rises to a considerable height. As the ma.s.s is urged downwards, the lower portion of the snow-slope is often torn away from its higher portion, and a chasm is formed, which usually extends round the head of the valley. To such a creva.s.se the specific name _Bergschrund_ is applied in the Bernese Alps; I have referred to one of them in the account of the ”Pa.s.sage of the Strahleck.”

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The phenomena described and accounted for in the last chapter have a direct bearing upon the question of viscosity. In virtue of the quicker central flow the lateral ice is subject to an oblique strain; but, instead of stretching, it breaks, and marginal creva.s.ses are formed. We also see that a slight curvature in the valley, by throwing an additional strain upon one half of the glacier, produces an augmented creva.s.sing of that side.

But it is known that a substance confessedly viscous may be broken by a sudden shock or strain. Professor Forbes justly observes that sealing-wax at moderate temperatures will mould itself (with time) to the most delicate inequalities of the surface on which it rests, but may at the same time be s.h.i.+vered to atoms by the blow of a hammer. Hence, in order to estimate the weight of the objection that a glacier breaks when subjected to strain, we must know the conditions under which the force is applied.

The Mer de Glace has been shown (p. 287) to move through the neck of the valley at Trelaporte at the rate of twenty inches a day. Let the sides of this page represent the boundaries of the glacier at Trelaporte, and any one of its lines of print a transverse slice of ice. Supposing the line to move down the page as the slice of ice moves down the valley, then the bending of the ice in twenty-four hours, shown on such a scale, would only be sufficient to push forward the centre in advance of the sides by a very small fraction of the width of the line of print. To such an extremely gradual strain the ice is unable to accommodate itself without fracture.

[Sidenote: NUMERICAL TEST OF VISCOSITY.]

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'_.

[Ill.u.s.tration: Fig. 29. Diagram ill.u.s.trating 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 creva.s.ses 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 ma.s.ses 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 creva.s.ses, on their first formation, are exceedingly narrow rents, which widen very slowly. The new creva.s.se 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 creva.s.ses; 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.

[Sidenote: 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. n.o.body 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 a.s.sertion 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.

HEAT AND WORK.

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