onge, or the cavities reach the
bottom of the glacier. Probably the two conditions are generally
combined.

In direct connection with the narrower fissures are the so-called
_moulins_,--the circular wells on the glacier. We will suppose that a
transverse, narrow fissure has been formed across the glacier, and that
one of the many rivulets flowing longitudinally along its surface
empties into it. As the surface-water of the glacier, producing these
rivulets, arises not only from the melting of the ice, but also from the
condensation of vapor, or even from rain-falls, and flows over the
scattered dust-particles and fragments of rock, it has always a
temperature slightly above 32 deg., so that such a rivulet is necessarily
warmer than the icy edge of the fissure over which it precipitates
itself. In consequence of its higher temperature it melts the edge,
gradually wearing it backward, till the straight margin of the fissure
at the spot over which the water falls is changed to a semicircle; and
as much of the water dashes in spray and foam against the other side,
the same effect takes place there, by which a corresponding semicircle
is formed exactly opposite the first. This goes on not only at the upper
margin, but through the whole depth of the opening as far down as the
water carries its higher temperature. In short, a semicircular groove is
excavated on either side of the fissure for its whole depth along the
line on which the rivulet holds its downward course. After a time, in
consequence of the motion of the glacier, such a fissure may close
again, and then the two semicircles thus brought together form at once
one continuous circle, and we have one of the round deep openings on the
glacier known as _moulins_, or wells, which may of course become
perfectly dry, if any accident turns the rivulet aside or dries up its
source. The most common cause of the intermittence of such a waterfall
is the formation of a crevasse higher up, across the watercourse which
supplied it, and which now begins another excavation.

These wells are often very profound. I have lowered a line for more than
seven hundred feet in one of them before striking bottom; and one is by
no means sure even then of having sounded the whole depth, for it may
often happen that the water meets with some obstacle which prevents its
direct descent, and, turning aside, continues its deeper course at a
different angle. Such a well may be like a crooked shaft i