_OBSERVATIONS ON THE MER DE GLACE._
The law established by Forbes and Agassiz, that the central portions
of a glacier moved faster than the sides, was amply illustrated and
confirmed by the deportment of lines of stakes placed across the
Mer de Glace and its tributaries in 1857. The portions of the trunk
glacier derived from these tributaries were easily traceable throughout
the glacier by means of the _moraines_. Thus, for example, the
portion of the trunk stream derived from the Glacier du Géant might
be distinguished in a moment from the other portions by the absence
of _débris_ upon its surface. Attention was drawn by Prof. Forbes
to the fact that the eastern side of the Mer de Glace in particular
is ‘excessively crevassed;’ and he accounted for this crevassing by
supposing that the Glacier du Géant moves most swiftly, and in its
effort to drag its more sluggish companions along with it tears them
asunder, thus producing the fissures and dislocation for which the
eastern side of the glacier is remarkable. Too much weight must not be
attached to this explanation. It was one of those suggestions which
are perpetually thrown out by men of science during the course of an
investigation, and the fulfilment or non-fulfilment of which cannot
materially affect the merits of the investigator. Indeed, the merits of
Forbes must be judged on far broader grounds. The qualities of mind and
the physical culture invested in his ‘Travels in the Alps’ are such as
to make it, in the estimation of the physical investigator at least,
outweigh all other books upon the subject.
While thus acknowledging its merits, however, let a free and frank
comparison of its statements with facts be instituted. To test whether
the Glacier du Géant moved more quickly than its fellows, _five
different lines_ were set out across the Mer de Glace, in the vicinity
of the Montanvert. In each case it was found that the point of swiftest
motion did not lie upon the Glacier du Géant at all, but was displaced
so as to bring it comparatively close to the eastern side of the
glacier. But though the special opinion of Forbes just referred to here
falls to the ground, the deviation of the point of swiftest motion from
the centre of the glacier will probably, when its cause is pointed out,
be regarded as of special importance to his theory.
At the place where these five lines were run across it the glacier
turns its convex curvature to the eastern side of the valley, being
concave towards the Montanvert. Let us then take a bolder analogy than
even that suggested in the explanation of Forbes, where he compares the
Glacier du Géant to a strong and swiftly flowing river. Let us enquire
how a river, would behave in sweeping round a curve similar to that
here existing. The point of swiftest motion would undoubtedly lie on
that side of the centre of the stream towards which it turns its convex
curvature. Can this be the case with the trunk of the Mer de Glace? If
so, then we ought to have a shifting of the point of maximum motion
towards the eastern side of the valley, when the curvature of the
glacier so changes as to turn its convexity to the western side.
Now, such a change of flexure actually occurs opposite the passages
called _Les Ponts_, and at this place the view just enunciated was
tested. It was immediately ascertained that the point of swiftest
motion here lay at a different side of the axis from that observed
lower down. But to confer strict numerical accuracy upon the result,
stakes were fixed at certain distances from the western side of the
glacier, and others _at equal distances_ from the eastern side. The
velocities of these stakes were compared with each other, two by two,
a stake on the western side being always compared with a second one
which stood at the same distance from the eastern side. The results of
this measurement are given in the following table, the numbers denoting
inches: