er various pressure, may take the
appearance it presents in each fragment with every compressed air-bubble
trending in the same direction, while their divergence in the different
fragments is owing to a change in the respective position of the
fragments resulting from the movement of the whole glacier. I have
further assumed, that throughout the glacier the change of the snow and
porous ice into compact ice is the result of successive freezing,
alternating with melting, or at least with the resumption of a
temperature of 32 deg. Fahrenheit in consequence of the infiltration of
liquid water, to which the effects of pressure must be added, the
importance of which in this connection no one could have anticipated
prior to the experiments of Dr. Tyndall. Of course, if the interior
temperature of the glacier never falls below 32 deg., the changes here
alluded to could not take place. But if the _vacuous spaces_ observed by
Dr. Tyndall are really identical with the spaces I have described as
_extremely flattened air-bubbles_, I think the arrangement of these
spaces as above described proves that it freezes in the interior of the
glacier to the depth at which these crosswise fragments have been
observed: that is, at a depth of two hundred feet. For, since the
experiments of Dr. Tyndall show that the vacuous spaces are parallel to
the surface of crystallization, and as no crystallization of water can
take place unless the surrounding temperature fall below 32 deg., it follows
that these vacuous spaces could not exist in such large continuous
fragments, presenting throughout the fragments the same trend, if there
had been no frost within the mass, affecting the whole of such a
fragment while it remained in the same position.

The most striking evidence, in my opinion, that at times the whole mass
of the glacier actually freezes, is drawn from the fact, already alluded
to, that, while the surface of the glacier loses annually from nine to
ten feet of its thickness by evaporation and melting, it swells, on the
other hand, in the spring, to the amount of about five feet. Such a
dilatation can hardly be the result of pressure and the packing of the
snow and ice, since the difference in the bulk of the ice brought down,
during one year, from a point above to that under observation, would not
account for the swelling. It is more readily explained by the freezing
of the water of infiltration during spring and early summer, when the
infi