These and some similar measurements brought to light a very important fact. The reader will observe that the middle numbers, corresponding to the central portion of the glacier, are the largest: hence it was obvious that the centre of a glacier, like that of a river, moves more rapidly than the sides.
Owing to the greater central motion of a glacier, its crevasses invariably assume a curved outline, of which the convexity advances towards the bottom of the valley.
It has also been ascertained that the superficial part of a glacier moves more rapidly than its base.
Again: Tyndall and Hirst, by employing instruments of great precision, have demonstrated that the maximum of motion is not to be found exactly in the centre, but that, according to the windings of the valley through which the glacier flows, it moves sometimes to the right of the centre, and sometimes to the left. Now, the progression of a river exhibits all the characters we have just enumerated, and the truth foreshadowed by Rendu has been confirmed in every detail. The glacier is a “river of ice.”
The reader will naturally ask, How can a substance of such apparent rigidity as ice obey, as it does obey, the same laws which regulate the movement of fluids? I can understand, he may say, how water flows in such and such a manner: it is a liquid, and its molecules are deficient in the property of cohesion; but that so solid, and firm, and unimpressible a substance as ice should be capable of motion seems impossible. I can understand very easily that a mass of ice, when loosened or detached from its resting-place, will glide downwards until arrested by some adequate obstacle; but this is not the kind of motion you are describing. According to your explanations, every constituent portion of the glacier moves, and the central faster than the lateral, and the surface faster than the base.
These objections were advanced by men of science when the motion of glaciers was first put forward as a theory; and the answer given by Scheuchzer was, that a glacier might be compared, in the summer season, to a sponge saturated with water, which, when afterwards congealed by the cold temperature of autumn and winter, expanded, and produced a dilatation of the mass in every direction. Then, as it could not recede, as it could not reascend its valley-slope, the augmentation of size would necessarily take place in its lower portion.
It is unnecessary for us to explain why this answer was unsatisfactory. Subsequent observations, however, proved its impossibility, and Professor Forbes then put forward his ideas of the viscous character of ice. But these, too, did not meet the conditions of the phenomenon; and the view now adopted is that of Professor Tyndall, who has shown that it is the result of the regelation we have already described.
Professor Forbes enunciated his theory in words to the following effect: “A glacier is an imperfect fluid or viscous body, which is urged down slopes of certain inclination by the natural pressure of its parts.” But we know the exceeding brittleness of ice, and how is viscosity compatible with brittleness? We know, too, that crevasses and fissures will suddenly form on a glacier, like the cracks on a pane of glass. But if ice were viscous, and could expand, dilate, or stretch as viscous substances do, these crevasses would be impossible. They would gradually close up, like an indent in a mass of jelly. And yet it cannot be denied that a glacier does move like a viscous body; the centre flowing past the sides, the top flowing over the bottom, while the motion through a curved valley corresponds to fluid motion. How are we to reconcile these apparently conflicting circumstances?
By Professor Tyndall’s regelation theory, which is founded on a fact already mentioned; namely, that when two pieces of thawing ice are brought in contact, they freeze together.
This fact, and its application irrespective of the cause of regelation, may be thus illustrated: “Saw two slabs from a block of ice, and bring their flat surfaces into contact; they immediately freeze together. Two plates of ice, laid one upon the other, with flannel round them overnight, are sometimes so firmly frozen in the morning that they will rather break elsewhere than along their surface of junction. If you enter one of the dripping ice-caves of Switzerland, you have only to press for a moment a slab of ice against the roof of the cave to cause it to freeze there and stick to the roof.