then, to be considered with reference to the
motion of the glacier are as follows. First that the rate of advance
between the axis and the margins of a glacier differs in the ratio of
about ten to one and even less; that is to say; when the centre is
advancing at a rate of two hundred and fifty feet a year, the motion
toward the sides may be gradually diminished to two hundred, one hundred
and fifty, one hundred, fifty feet, and so on, till nearest the margin
it becomes almost inappreciable. Secondly, the rate of motion is not the
same throughout the length of the glacier, the advance being greatest
about half-way down in the region of the _neve_, and diminishing in
rapidity both above and below; thus the onward motion in the higher
portion of a glacier may not exceed twenty to fifty feet a year, while
it reaches its maximum of some two hundred and fifty feet annually in
the _neve_ region, and is retarded again toward the lower extremity,
where it is reduced to about one-fourth of its maximum rate. Thirdly,
the glacier moves at different rates throughout the thickness of its
mass; toward the lower extremity of the glacier the bottom is retarded,
and the surface portion moves faster, while in the upper region the
bottom seems to advance more rapidly. I say _seems_, because upon this
latter point there are no positive measurements, and it is only
inferred from general appearances, while the former statement has been
demonstrated by accurate experiments. Remembering the form of the
troughs in which the glaciers arise, that they have their source in
expansive, open fields of snow and _neve_, and that these immense
accumulations move gradually down into ever narrowing channels, though
at times widening again to contract anew, their surface wasting so
little from external influences that they advance far below the line of
perpetual snow without any sensible diminution in size, it is evident
that an enormous pressure must have been brought to bear upon them
before they could have been packed into the lower valleys through which
they descend.

Physicists seem now to agree that pressure is the chief agency in the
motion of glaciers. No doubt, all the facts point that way; but it now
becomes a matter of philosophical interest to determine in what
direction it acts most powerfully, and upon this point glacialists are
by no means agreed. The latest conclusion seems to be, that the weight
of the advancing mass is itself the efficien