The unusually large infiltration of
water at that season is owing to the melting of the winter snow. Careful
experiments made on the glacier of the Aar, respecting the water thus
accumulating on the surface, penetrating its mass, and finally
discharged in part at its lower extremity, fully confirm this view.
Here, then, is a powerful cause of pressure and consequent motion, quite
distinct from the permanent weight of the mass itself, since it operates
only at certain seasons of the year. In midwinter, when the infiltration
is reduced to a minimum, the motion is least. The water thus introduced
into the glacier acts, as we have seen above, in various ways: by its
weight, by loosening the particles of snow through which it trickles,
and by freezing and consequent expansion, at least within the limits and
during the season at which the temperature of the glacier sinks below
32 deg. Fahrenheit. The simple fact, that in the spring the glacier swells
on an average to about five feet more than its usual level, shows how
important this infiltration must be. I can therefore only wonder that
other glacialists have given so little weight to this fact.
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