irregularity of the surface over which the ice is flowing.

If the observer is fortunate, he may in his journey over the glacier
have a chance to see and hear what goes on when crevasses are formed.
First he will hear a deep, booming sound beneath his feet, which
merges into a more splintering note as the crevice, which begins at
the bottom or in the distance, comes upward or toward him. When the
sound is over, he may not be able to see a trace of the fracture,
which at first is very narrow. But if the break intersect any of the
numerous shallow pools which in a warm summer's day are apt to cover a
large part of the surface, he may note a line of bubbles rushing up
through the water, marking the escape of the air from the glacier,
some remnant of that which is imprisoned in the original snow. Even
where this indication is wanting, he can sometimes trace the crevice
by the hissing sound of the air streams where they issue from the ice.
If he will take time to note what goes on, he can usually in an hour
or two behold the first invisible crack widen until it may be half an
inch across. He may see how the surface water hastens down the
opening, a little river system being developed on the surface of the
ice as the streams make their way to one or more points of descent. In
doing this work they excavate a shaft which often becomes many feet in
diameter, down which their waters thunder to the base of the glacier.
This well-like opening is called a _moulin_, or mill, a name which, as
we shall see, is well deserved from the work which falling waters
accomplish. Although the institution of the _moulin_ shaft depends
upon the formation of a crevice, it often happens that as the ice
moves farther on its journey its walls are again thrust together,
soldered in the manner peculiar to ice, so that no trace of the
rupture remains except the shaft which it permitted to form. Like
everything else in the glacier, the _moulin_ slowly moves down the