The Glaciers of the Alps / Being a narrative of excursions and ascents, an account of the origin and phenomena of glaciers and an exposition of the physical principles to which they are related

by John Tyndall

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 circumstances, must produce variations in the velocity of the glacier, and these circumstances 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 accumulates; the mass in motion being double, triple, &c., the motion is only the half, the third, &c.

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 circumstance 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 mètres 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.

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 mass of ice was in direct opposition to it;' and we think that Professor Forbes himself must have stood aghast when his fancy first associated 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 Agassiz 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.

THE ICE AND THE GLACIER.