But
while this is probably true in the main, other elements tending to the
same result, and generally overlooked by investigators, ought to be
taken into consideration; and before leaving the subject, I would add a
few words upon infiltration in this connection.
The weight of the glacier, as a whole, is about the same all the year
round. If, therefore, pressure, resulting from that weight, be the
all-controlling agency, its progress should be uniform daring the whole
year, or even greatest in winter, which is by no means the case. By a
series of experiments, I have ascertained that the onward movement,
whatever be its annual average, is accelerated in spring and early
summer. The average annual advance of the glacier being, at a given
point, at the rate of about two hundred feet, its average summer
advance, at the same point, will be at a rate of two hundred and fifty
feet, while its average rate of movement in winter will be about one
hundred and fifty feet. This can be accounted for only by the increased
pressure due to the large accession of water trickling in spring and
early summer into the interior through the net-work of capillary
fissures pervading the whole mass.
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