We were able to stretch a string in a straight line from the lowest
point we reached, through the interstices of the blocks of stone, and
up to the entrance-hole, and this measurement gave 50 feet.
Considering the inclination of the upper ice-floor, and the sharpness
of the angle between the wall of ice and the line of our descent to
this lowest point, I believe that 50 feet will fairly represent the
height of the ice-wall from this point to the foot of the slope from
the upper wall; so that 72 feet will be the whole depth of ice, from
the top of the third ladder to the point where our further progress
downwards was arrested. The correctness of this calculation depends
upon the honesty of Mignot, who had charge of the farther end of the
string, and was proud of the wonders of his cave. A dishonest man
might easily, under the circumstances, have pulled up a few feet more
of string than was necessary, but 50 feet seemed in no way an
improbable result of the measurement.

[Illustration: SECTION OF THE LOWER GLACIÈRE OF THE PRÉ DE S. LIVRES.]

The ice was as solid and firm as can well be conceived. The horizontal
bands would seem to prove conclusively that it was no coating of greater
or less thickness on the face of a vertical wall of rock, an idea which
might suggest itself to anyone who had not seen it, and I think it
probable that the amount of ice represented in the section of the cave
is not an exaggeration. We were unable to measure the whole length of
the wall in the lower cave, from the large number of blocks of stone
which had fallen at one end, and lay against its face. Probably, from
the nature of the case, it was not so long as the 72 feet of wall above;
but we measured 50 feet, and could see it still passing on to the right
hand as we faced it. In trying to penetrate farther along the face, I
found a wing of the brown fly we had seen in considerable abundance on