bottom on which it rests
and by the state of the atmosphere. Under a certain temperature, the
snow may only be glazed at the surface by the formation of a thin, icy
crust, an outer membrane, as it were, protecting the mass below from a
deeper transformation into ice; or it may be rapidly soaked throughout
its whole bulk, the snow being thus changed into a kind of soft pulp,
what we commonly call slosh, which, upon freezing, becomes at once
compact ice; or, the water sinking rapidly, the lower layers only may be
soaked, while the upper portion remains comparatively dry. But, under
all these various circumstances, frost will transform the crystalline
snow into more or less compact ice, the mass of which will be composed
of an infinite number of aggregated snow-particles, very unequal in
regularity of outline, and cemented by ice of another kind, derived from
the freezing of the infiltrated moisture, the whole being interspersed
with air. Let the temperature rise, and such a mass, rigid before, will
resolve itself again into disconnected ice-particles, like grains more
or less rounded. The process may be repeated till the whole mass is
transformed into very compact, almost uniformly transparent and blue
ice, broken only by the intervening air-bubbles. Such a mass of ice,
when exposed to a temperature sufficiently high to dissolve it, does not
melt from the surface and disappear by a gradual diminution of its bulk,
like pond-ice, but crumbles into its original granular fragments, each
one of which melts separately. This accounts for the sudden
disappearance icebergs, which, instead of slowly dissolving into the
ocean, are often seen to fall to pieces and vanish at once.

Ice of this kind may be seen forming every winter on our sidewalks, on
the edge of the little ditches which drain them, or on the summits of
broad gateposts when capped with snow. Of such ice glaciers are
composed; but, in the glacier, another element comes in which we have
not considered as yet,--that of immense pressure in consequence of the
vast accumulations of snow within circumscribed spaces. We see the same
effects produced on a small scale, when snow is transformed into a
snowball between the hands. Every boy who balls a mass of snow in his
hands illustrates one side of glacial phenomena. Loose snow, light and
porous, and pure white from the amount of air contained in it, is in
this way presently converted into hard, compact, almost transparent ice.