Part 24 (2/2)

s.h.i.+rwell Mer de Glace 300 ”

M. Rendu Mer de Glace 365 ”

Saussure's Ladder Mer de Glace 375 ”

... Such was the state of our knowledge when Professor Forbes undertook the investigation of the subject.”

I am persuaded that the writer of this article will be the first to applaud any attempt to remove an error which, advanced on his great authority, must necessarily be widely disseminated. The numbers in the above table certainly differ widely, and it is perhaps natural to conclude that such discordant results can be of no value; but the fact really is that _every one of them may be perfectly correct_. This fact, though overlooked by Professor Forbes, was clearly seen by Rendu, who pointed out with perfect distinctness the sources from which the discrepancies were derived.

[Sidenote: DISCREPANCIES EXPLAINED.]

”It is easy,” he says, ”to comprehend that it is impossible to obtain a general measure,--that there ought to be one for each particular glacier. The nature of the slope, the number of changes to which it is subjected, the depth of the ice, the width of the couloir, the form of its sides, and a thousand other circ.u.mstances, must produce variations in the velocity of the glacier, and these circ.u.mstances cannot be everywhere absolutely the same. Much more, it is not easy to obtain this velocity for a single glacier, and for this reason. In those portions where the inclination is steep, the layer of ice is thin, and its velocity is great; in those where the slope is almost nothing, the glacier swells and acc.u.mulates; the ma.s.s in motion being double, triple, &c., the motion is only the half, the third, &c.

[Sidenote: LIQUID MOTION ASCRIBED TO GLACIER.]

”But this is not all,” adds M. Rendu: ”_Between the Mer de Glace and a river, there is a resemblance so complete that it is impossible to find in the latter a circ.u.mstance which does not exist in the former._ In currents of water the motion is not uniform, neither throughout their width nor throughout their depth; _the friction of the bottom, that of the sides_, the action of obstacles, cause the motion to vary, _and only towards the middle of the surface is this entire...._”[J]

In 1845 Professor Forbes appears to have come to the same conclusion as M. Rendu; for after it had been proved that the centre of the Aar glacier moved quicker than the side in the ratio of fourteen to one, he accepted the result in these words:--”The movement of the centre of the glacier is to that of a point five metres from the edge as FOURTEEN to ONE: such is the effect of plasticity!”[K] Indeed, if the differences exhibited in the table were a proof of error, the observations of Professor Forbes himself would fare very ill. The measurements of glacier-motion made with his own hands vary from less than 42 feet a year to 848 feet a year, the minimum being less than _one-twentieth_ of the maximum; and if we include the observations made by Balmat, the fidelity of which has been certified by Professor Forbes, the minimum is only _one-thirty-seventh_ of the maximum.

[Sidenote: NORTH BRITISH REVIEW.]

There is another point connected with Rendu's theory which needs clearing up:--”The idea,” writes the eminent reviewer, ”that a glacier is a semifluid body is no doubt startling, especially to those who have seen the apparently rigid ice of which it is composed. M. Rendu himself shrank from the idea, and did not scruple to say that 'the rigidity of a ma.s.s of ice was in direct opposition to it;' and we think that Professor Forbes himself must have stood aghast when his fancy first a.s.sociated the notion of imperfect fluidity with the solid or even the fissured ice of the glacier, and when he saw in his mind's eye the glaciers of the Alps flowing like a river along their rugged bed. A truth like this was above the comprehension and beyond the sympathy of the age; and it required a moral power of no common intensity to submit it to the ordeal of a shallow philosophy, and the sneers of a presumptuous criticism.”

These are strong words; but the fact is that, so far from ”shrinking”

from the idea, Rendu affirmed, with a clearness and an emphasis which have not been exceeded since, that all the phenomena of a river were reproduced upon the Mer de Glace; its deeps, its shallows, its widenings, its narrowings, its rapids, its places of slow motion, and the quicker flow of its centre than of its sides. He did not shrink from accepting a difference between the central and lateral motion amounting to a ratio of ten to one--a ratio so large that Professor Forbes at one time regarded the acceptance of it as a simple absurdity. In this he was perhaps justified; for his own first observations, which, however valuable, were hasty and incomplete, gave him a maximum ratio of about one and a half to one, while the ratio in some cases was nearly one of _equality_. The observations of Aga.s.siz however show that the ratio, instead of being ten to one, may be _infinity_ to one; for the lateral ice may be so held back by a local obstacle that in the course of a year it shall make no sensible advance at all.

[Sidenote: THE ICE AND THE GLACIER.]

From one thing only did M. Rendu shrink; and it is _the_ thing regarding which we are still disunited. He shrank from stating the physical quality of the ice in virtue of which a glacier moved like a river. He demands experiments upon snow and ice to elucidate this subject. The very observations which Professor Forbes regards as proofs are those of which we require the physical explanation. It is not the viscous flow, if you please to call it such, of the glacier as a whole that here concerns us; but it is the quality of the _ice_ in virtue of which this kind of motion is accomplished. Professor Forbes sees this difference clearly enough: he speaks of ”fissured ice” being ”flexible” in hand specimens; he compares the glacier to a mixture of ice and sand; and finally, in a more matured paper, falls back for an explanation upon the observations of Aga.s.siz regarding the capillaries of the glacier.[L]

FOOTNOTES:

[A] Expressions such as ”last summer,” ”last autumn,” ”recently,” will be taken throughout in the sense which they had in the early half of 1860, when this book was first published.--L. C. T.

[B] 'Memoir,' p. 77.

[C] P. 75.

[D] P. 71.

[E] 'Philosophical Magazine,' 1859.

[F] 'Memoir,' p. 69.

[G] Page 80.

[H] Page 95.

[I] At page 38 of the 'Travels' the following pa.s.sage also occurs:--”I believe that I may safely affirm that not one observation of the rate of motion of a glacier, either on the average or at any particular season of the year, existed when I commenced my experiments in 1842.”

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