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 efficient cause of the motion.
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