t cause of the motion. 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. 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. It is
admitted by all, that the waste of a glacier at its surface, in
consequence of evap