An analogy between the motion of a glacier through a sinuous valley and of a river in a sinuous channel has been already pointed out. But the analogy fails in one important particular: the river, and much more so a mass of flowing treacle, honey, tar, or melted caoutchouc, sweeps round its curves without rupture of continuity. The viscous mass stretches, but the icy mass breaks, and the ‘excessive crevassing’ pointed out by Prof. Forbes himself is the consequence. The inclinations of the Mer de Glace and its three tributaries were, moreover, taken, and the association of transverse crevasses with the changes of inclination were accurately noted. Every traveller knows the utter dislocation and confusion produced by the descent of the Mer de Glace from the Chapeau downwards. A similar state of things exists in the ice-cascade of the Talèfre. Descending from the Jardin, as the ice approaches the fall, great transverse chasms are formed, which at length follow each other so speedily as to reduce the ice-masses between them to mere plates and wedges, along which the explorer has to creep cautiously. These plates and wedges are in some cases bent and crumpled by the lateral pressure, and some large pyramids are turned 90° round, so as to have their veins at right angles to the normal position. The ice afterwards descends the fall, the portions exposed to view being a fantastic assemblage of frozen boulders, pinnacles, and towers, some erect, some leaning, falling at intervals with a sound like thunder, and crushing the ice-crags on which they fall to powder. The descent of the ice through this fall has been referred to as a proof of its viscosity; but the description just given does not harmonise with our ideas of a viscous substance.
But the proof of the non-viscosity of the substance must be sought at places where the change of inclination is very small. Nearly opposite l’Angle there is a change from four to nine degrees, and the consequence is the production of transverse fissures which render the glacier here perfectly impassable. Further up the glacier transverse crevasses are produced by a change of inclination from three to five degrees. This change of inclination is protracted in fig. 5; the bend occurs at the point B; it is scarcely perceptible, and still the glacier is unable to pass over it without breaking across.
Fig. 5.
Again, the crevasses being due to a state of strain from which the ice relieves itself by breaking, the rate at which they widen may be taken as a measure of the amount of relief demanded by the ice. Both the suddenness of their formation and the slowness with which they widen are demonstrative of the non-viscosity of the ice. For were the substance capable of stretching, even at the small rate at which they widen, there would be no necessity for their formation.
Further, the marginal crevasses of a glacier are known to be a consequence of the swifter flow of its central portions, which throws the sides into a state of strain from which they relieve themselves by breaking. Now it is easy to calculate the amount of stretching demanded of the ice in order to accommodate itself to the speedier central flow. Take the case of a glacier half a mile wide. A straight transverse element, or slice, of such a glacier, is bent in twenty-four hours to a curve. The ends of the slice move a little, but the centre moves more: let us suppose the versed side of the curve formed by the slice in twenty-four hours to be a foot, which is a fair average. Having the chord of this arc, and its versed side, we can calculate its length. In the case of the Mer de Glace, which is about half a mile wide, the amount of stretching demanded would be about the eightieth of an inch in twenty-four hours. Surely, if the glacier possessed a property which could with any propriety be called viscosity, it ought to be able to respond to this moderate demand; but it is not able to do so: instead of stretching as a viscous body, in obedience to this slow strain, it breaks as an eminently fragile one, and marginal crevasses are the consequence. It may be urged that it is not fair to distribute the strain over the entire length of the curve: but reduce the distance as we may, a residue must remain, which is demonstrative of the non-viscosity of the ice.
To sum up, then, two classes of facts present themselves to the glacier investigator—one class in harmony with the idea of viscosity, and another as distinctly opposed to it. Where pressure comes into play we have the former; where tension comes into play we have the latter. Both classes of facts are reconciled by the assumption, or rather the experimental verity, that the fragility of ice and its power of regelation render it possible for it to change its form without prejudice to its continuity.
[Very interesting experiments upon the bending of ice have been recently made by Mr. Matthews and Mr. Froude. In these experiments the temperature of the ice, I believe, was some degrees below the freezing point: it would be important to repeat these experiments with ice at the temperature which it actually possesses in glaciers, namely, at 32°.—April 1871.]