ure--a weight amounting to about fifteen
pounds to the square inch--the temperature at which the ice will melt
is lowered to the amount of sixteen thousandths of a degree
centigrade. If we take a piece of ice at the temperature of freezing
and put upon it a sufficient weight, we inevitably bring about a small
amount of melting. Where we can examine the mass under favourable
conditions, we can see the fluid gather along the lines of the
crystals or other bits of which the ice is composed. We readily note
this action by bringing two pieces of ice together with a slight
pressure; when the pressure is removed, they will adhere. The adhesion
is brought about not by any stickiness of the materials, for the
substance has no such property. It is accomplished by melting along
the line of contact, which forms a film of water, that at once
refreezes when the pressure is withdrawn. When a firm snowball is
made by even pressing snow, innumerable similar adhesions grow up in
the manner described. The fact is that, given ice at the temperature
at which it ordinarily forms, pressure upon it will necessarily
develop melting.

The consequences of pressure melting as above described are in
glaciers extremely complicated. Because the ice is built into the
glacier at a temperature considerably below the freezing point, it
requires a great thickness of the mass before the superincumbent
weight is sufficient to bring about melting in its lower parts. If we
knew the height at which a thermometer would have stood in the surface
ice of the ancient glacier which covered the northern part of North
America, we could with some accuracy compute how thick it must have
been before the effect of pressure alone would have brought about
melting; but even then we should have to reckon the temperature
derived from the grinding of the ice over the floor and the crushing
of rocks there effected, as well as the heat which is constantly
though slowly coming forth from the earth's interior. The result is
that we can only say that at some depth, probably less than a mile,
the slowly accumulating ice would acquire such a temperature that,
subjected to the weight above it, the material next the bottom would
become molten, or at least converted into a sludgelike state, in which
it could not rub against the bottom, or move stones in the manner of
ordinary glaciers.

As fast as the ice assumed this liquid or softened state, it would be
squeezed out toward the regio