West 12-3/4 15 17-1/4
East 14-3/4 17-1/2 19
Here we find that in each case the _eastern_ stake moved faster than
its fellow. The point of maximum motion has therefore once more crossed
the axis of the glacier.
Determining the point of maximum motion for a great number of
transverse sections of the Mer de Glace, and uniting these points, we
have what is called the _locus_ of the point. The dotted line in the
annexed figure represents the centre of the Mer de Glace; the hard
line which crosses the axis of the glacier at the points A A is then
the locus of the point of swiftest motion. It is a curve more deeply
sinuous than the valley itself, and it crosses the central line of the
valley at each point of contrary flexure. The position of towns upon
the banks of rivers is usually on the convex side of the stream, where
the rush of the water renders silting-up impossible; and the same law
which regulated the flow of the Thames, and determined the position
of the towns upon its banks, is at this moment operating with silent
energy among the Alpine glaciers.
[Illustration: Fig. 1.]
Another peculiarity of glacier motion is now to be noticed.
Before any observations had been made upon the subject, it was surmised
by Prof. Forbes that the portions of a glacier near its bed were
retarded by friction against the latter. This view was afterwards
confirmed by his own observations, and by those of M. Martins.
Nevertheless the state of our knowledge upon the subject rendered
further confirmation of the fact highly desirable. A rare opportunity
for testing the question was furnished in 1857 by an almost vertical
precipice of ice, constituting the side of the Glacier du Géant,
exposed near the Tacul. The precipice was about 140 feet in height.
At the top and near the bottom stakes were fixed, and by hewing steps
in the ice I succeeded in fixing a stake in the face of the precipice
at a point about forty feet above the base.[31] After the lapse of
a sufficient number of days, the progress of the three stakes was
measured; reduced to the diurnal rate, the motion was as follows:
[31] It was here that my prudent guide, Édouard Simon, demanded,
‘Est-ce que vous avez une femme?’ and, when I replied in the negative,
added, ‘Vous serez tué tout de même.’
Top stake 6.00 inches.
Middle stake 4.59 „
Bottom stake 2.56 „
We thus see that the top stake moved with more than twice the velocity
of the bottom one, while the velocity of the middle stake lies between
the two. But it also appears that the augmentation of velocity upwards
is not proportional to the distance from the bottom, but increases
in a quicker ratio. At a height of 100 feet from the bottom, the
velocity would undoubtedly be practically the same as at the surface.
Measurements made upon an adjacent ice-cliff proved this. We thus see
the perfect validity of the reason assigned by Forbes for the continued
verticality of the walls of transverse crevasses. Indeed a comparison
of the result with his anticipations and reasonings will prove alike
their sagacity and their truth.
The most commanding view of the Mer de Glace and its tributaries is
obtained from a point above the remarkable cleft in the mountain-range
underneath the Aiguille de Charmoz, which is sure to attract the
attention of an observer standing at the Montanvert. This point, marked
G on the map of Forbes, I succeeded in attaining. A Tübingen Professor
once visited the glaciers of Switzerland, and seeing these apparently
rigid masses enclosed in sinuous valleys, went home and wrote a book,
flatly denying the possibility of their motion. An inspection from
the point now referred to would have doubtless confirmed him in his
opinion; and indeed nothing can be more calculated to impress the mind
with the magnitude of the forces brought into play than the squeezing
of the three tributaries of the Mer de Glace through the neck of the
valley at Trélaporte.
But let me state numerical results. Previous to its junction with its
fellows, the Glacier du Géant measures 1,134 yards across. Before it
is influenced by the thrust of the Talèfre, the Glacier de Léchaud has
a width of 825 yards; while the width of the Talèfre branch across the
base of the cascade, before it joins the Léchaud, is approximately
638 yards. The sum of these widths is 2,597 yards. At Trélaporte
those three branches are forced through a gorge 893 yards wide, with
a central velocity of 20 inches a day! The result is still more
astonishing if we confine our attention to one of the tributaries--that
of the Léchaud. This broad ice-river, which before its junction with
the Talèfre has a width of 825 yards, at Trélaporte is squeezed to
a driblet of less than 88 yards in width, that is to say, to about
one-tenth of its previous horizontal transverse dimension.
Whence is the force derived which drives the glacier through the
gorge? No doubt pressure from behind. Other facts also suggest that
the Glacier du Géant is throughout its length in a state of forcible
longitudinal compression. Taking a series of points along the axis of
this glacier--if these points, during the descent of the glacier,
preserved their distances asunder perfectly constant, there could be
no longitudinal compression. The mechanical meaning of this term, as
applied to a substance capable of yielding like ice, must be that the
hinder points are incessantly advancing upon the forward ones. I was
particularly anxious to test this view, which first occurred to me on
_à priori_ grounds. Three points, A, B, C, were therefore fixed upon
the axis of the Glacier du Géant, A being the highest up the glacier.
The distance between A and B was 545 yards, and that between B and C
was 487 yards. The daily velocities of these three points, determined
by the theodolite, were as follows:
A 20.55 inches.
B 15.43 „
C 12.75 „
The result completely corroborates the foregoing anticipation. The
hinder points are incessantly advancing upon those in front, and
that to an extent sufficient to shorten a segment of this glacier,
measuring 1,000 yards in length, at the rate of 8 inches a day. Were
this rate uniform at all seasons, the shortening would amount to 240
feet in a year. When we consider the compactness of this glacier, and
the uniformity in the width of the valley which it fills, this result
cannot fail to excite surprise; and the exhibition of force thus
rendered manifest must be mainly instrumental in driving the glacier
through the jaws of the granite vice at Trélaporte.
When the Glacier du Géant is observed from a sufficient distance, a
remarkable system of seams of white ice appears to sweep across it,
in the direction of the ‘dirt-bands.’ These seams are more resistant
than the ordinary ice of the glacier, and sometimes protrude above
the surface to a height of three or four feet. Their origin was for
some time a difficulty, and it was at the base of the ice-cascade
which descends from the basin of the Talèfre that the key to their
solution first presented itself. It was well known that the ice of a
glacier is not of homogeneous structure, but that the general more
or less milky mass is traversed by blue veins of a more compact and
transparent texture. In the upper portions of the Mer de Glace these
veins sweep across the glacier in gentle curves, leaning forward--to
which leaning forward Prof. Forbes gave the name of the ‘frontal dip.’
A case of ‘backward dip’ has never been described. But at the base of
the ice-cascade referred to I had often noticed the veins exposed upon
the walls of a longitudinal crevasse leaning backwards and forwards on
both sides of a vertical line, like the joints of stones used to turn
an arch.
This fact was found to connect itself in the following way with the
general state of the glacier. At the base of the ice-fall a succession
of protuberances, with steep frontal slopes, followed each other, and
were intersected by crevasses. Let the hand be placed flat upon the
table, with the palm downwards; let the fingers be bent so as to render
the space between the joints nearest the nails and the ends of the
fingers nearly vertical. Let the second hand be now placed upon the
back of the first, with its fingers bent as in the former case, and
their ends resting upon the roots of the first fingers. The crumpling
of the hands fairly represents the crumpling of the ice, and the spaces
between the fingers represent the crevasses by which the protuberances
are intersected. On the walls of these crevasses the change of dip of
the veined structure above referred to was always observed, and _at the
base of each protuberance a vein of white ice was found firmly wedged
into the mass of the glacier_.
The next figure represents a series of these crumples with the veins of
white ice _i i i_ at their bases.
[Illustration: FIG 2.]
It was soon observed that the water which trickled down the
protuberances, and gushed here and there from glacier orifices,
collected at the bases of the crumples, and formed streams which cut
for themselves deep channels in the ice. These streams seemed to be the
exact matrices or moulds of the veins of white ice, and the latter
were finally traced to the gorging up of the channels of glacial
rivulets by winter snow. The same explanation applies to the system
of bands upon the Glacier du Géant. I was enabled to trace the little
arms of white ice which once were the tributaries of the streams, to
see a trunk vein of the ice dividing into branches, and uniting again
so as to enclose glacial islands. I finally traced them to the region
of their formation, and by sketches of existing streams taken near the
base of the _séracs_, and of bands of white ice taken lower down, a
resemblance so striking was exhibited as to leave no doubt of their
relationship. On the walls of some deep crevasses, moreover, which
intersected the white ice-seams, I found that the latter penetrated the
glacier only to a limited depth, having the appearance of a kind of
glacial ‘trap’ intruded from above.
[Illustration: FIG. 3.]
But how is the backward dip of the blue veins to be accounted for?
Doubtless in the following way: At the base of the cascade the glacier
is forcibly compressed by the thrust of the mass behind it; besides
this, it changes its inclination suddenly and considerably; it is
bent upwards, and the consequence of this bending is a system of
wrinkles, such as those represented in the next figure. The interior
of a bent umbrella-handle sometimes presents wrinkles which are the
representatives, in little, of the protuberances upon the glacier. The
coat-sleeve is an equally instructive illustration: when the arm is
bent at the elbow the sleeve wrinkles, and as the places where these
wrinkles occur in the cloth are determined, to some extent, by the
previous creasing, so also the places where the wrinkles are formed
upon the glacier are determined by the previous scarring of the ice
during its descent down the cascade. The manner in which these crumples
tend to _scale off_ speaks strongly in favour of the explanation
given. The following figure represents a type of numerous instances
of scaling off. By means of a hydraulic press it is easy to produce
a perfectly similar scaling in small masses of ice. One consequence
of this crumpling of the glacier would be the backward and forward
inclination of the veins as actually observed. The same appearance was
noticed on the wrinkles of the Glacier du Géant. It was also proved, by
measurements, that these wrinkles _shorten_ as they descend.
[Illustration: FIG. 4.]
In virtue of what quality, then, can ice be bent and squeezed, and have
its form changed in the manner indicated in the foregoing observations?
The only theory worthy of serious consideration at the present day is
the celebrated Viscous Theory of glacial motion. Numerous appearances,
as we have seen, favour the idea that ice is a viscous or ‘semi-fluid’
substance, and that it flows as such in the glaciers of the Alps.
The aspect of many glaciers, as a whole--their power of closing
up crevasses, and of reconstructing themselves after having been
precipitated down glacial gorges--the obvious bendings and contortions
of various portions of the ice, are all in harmony with the notion.
The laminar structure of the glacier has also been regarded by eminent
authorities as a crucial test in favour of the viscous theory, and
affirmed to be impossible of explanation on any other hypothesis.
Nevertheless, this theory is so directly opposed to our ordinary
experience of the nature of ice as to leave upon the mind a lingering
doubt of its truth. Can we imitate the phenomena without invoking the
explanation? We can. Moulds of various forms were hollowed out in
boxwood, and pieces of ice were placed in these moulds and subjected
to pressure. In this way spheres of ice were flattened into cakes, and
cakes formed into transparent lenses. A straight bar of ice, six inches
long, was passed through a series of moulds augmenting in curvature,
and was finally bent into a semiring. A small block of ice was placed
in a hemispherical cavity, and was pressed upon by a hemispherical
protuberance, not large enough to fill the cavity; the ice yielded and
filled the space between both, thus forming itself into a transparent
cup. The specimens of ice here employed were so exceedingly brittle
that a pricker driven into the ice was competent to split blocks of the
substance eight cubic feet in volume, the surface of fracture being in
all cases as clean and sharp as that of glass.
These experiments, then, demonstrate a capacity on the part of small
masses of ice which they have not been hitherto known to possess. They
prove, to all appearance, that the substance is much more plastic than
it was ever imagined to be. But the real germ from which these results
have sprung is to be found in a lecture given at the Royal Institution
in June 1850, and reported in the ‘Athenæum’ and ‘Literary Gazette’
for that year. Faraday then showed that when two pieces of ice, at a
temperature of 32° Fahr., are placed in contact with each other, they
freeze together, by the conversion of the film of moisture between
them into ice. The case of a snowball is a familiar illustration of
the principle. When the snow is below 32°, and therefore _dry_, it
will not cohere, whereas when it is in a thawing condition it can be
squeezed into a hard mass. During one of the hottest days of July 1857,
when the thermometer was upwards of 100° Fahr. in the sun, and more
than 80° in the shade, I observed a number of blocks of ice, which had
been placed in a heap, frozen together at their places of contact; and
I afterwards caused them to freeze together under water as hot as the
hand could bear. Facts like these suggested the thought that if a piece
of ice--a straight prism, for example--were placed in a bent mould and
subjected to pressure it would break, but that the force would also
bring its ruptured surfaces into contact, and thus the continuity
of the mass might be re-established. Experiment, as we have seen,
completely confirmed this surmise: the ice passed from a continuous
straight bar to a continuous bent one, the transition being effected,
not by a viscous movement of the particles, but _through fracture and
regelation_.
Let the transition from curve to curve be only gradual enough, and we
have the exact case of a transverse slice of a glacier.
All the phenomena of motion, on which the idea of viscosity has been
based, are brought by such experiments as the above into harmony with
the demonstrable properties of ice. In virtue of this property, the
glacier accommodates itself to its bed while preserving its general
continuity, crevasses are closed up, and the broken ice of a cascade,
such as that of the Talèfre or the Rhone, is recompacted to a solid
continuous mass.
* * * * *
The very essence of viscosity is the ability of yielding to a force
of tension, _the texture of the substance, after yielding, being in a
state of equilibrium_, so that it has no strain to recover from; and
the substances chosen by Prof. Forbes as illustrative of the physical
condition of a glacier possess this power of being drawn out in a very
eminent degree. But it has been urged, and justly urged, that we
ought not to conclude that viscosity is absent because hand specimens
are brittle, any more than we ought to conclude that ice is not blue
because small fragments of the substance do not exhibit this colour.
To test the question of viscosity, then, we must appeal to the glacier
itself. Let us do so.
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.
[Illustration: Fig. 5.