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.]