that pressure is, of course, greater at its bottom than at its centre or
surface. It is therefore plain, that, inasmuch as the snow can be
compressed by its own weight, it will be more compact at the bottom of
such an accumulation than at its surface, this cause acting most
powerfully at the upper part of a glacier, where the snow has not yet
been transformed into a more solid icy mass. To these two agencies, the
downward tendency and the vertical pressure, must be added the pressure
from behind, which is most-effective where the mass is largest and the
amount of motion in a given time greatest. In the glacier, the mass is,
of course, largest in the centre, where the trough which holds it is
deepest, and least on the margins, where the trough slopes upward and
becomes more shallow. Consequently, the middle of a glacier always
advances more rapidly than the sides. Were the slope of the ground over
which it passes, combined with the pressure to which the mass is
subjected, the whole secret of the onward progress of a glacier, it is
evident that the rate of advance would be gradually accelerated,
reaching its maximum at its lower extremity, and losing its impetus by
degrees on the higher levels nearer the point where the descent begins.
This, however, is not the case. The glacier of the Aar, for instance, is
about ten miles in length; its rate of annual motion is greatest near
the point of junction of the two great branches by which it is formed,
diminishing farther down, and reaching a minimum at its lower extremity.
But in the upper regions, near their origin, the progress of these
branches is again gradually less. Let us see whether the next cause of
displacement, the infiltration of moisture, may not in some measure
explain this retardation, at least of the lower part of the glacier.
This agency, like that of the compression of the snow by its own weight
and the pressure from behind, is most effective where the accumulation
is largest. In the centre, where the body of the mass is greatest, it
will imbibe the most moisture. But here a modifying influence comes in,
not sufficiently considered by the investigators of glacial structure.
We have already seen that snow and ice at different degrees of
compactness are not equally permeable to moisture. Above the line at
which the annual winter snow melts, there is, of course, little
moisture; but below that point, as soon as the temperature rises in
summer sufficiently to melt