In the Glaciere of La
Genolliere I carried a large block of perfectly prismatic ice into the
outer cave, where it was exposed to the free currents of air passing
from the pit of entrance to the hole newly opened by the falling in of
the ground; and, two days after, the external lines were scarcely
perceptible, while on the occasion of our third visit I found that they
had entirely disappeared, and the whole block was rapidly following
their example. This disappearance of the surface-lines under the action
of atmospheric thaw is probably the same thing as their absence when the
flooring of ice is thinly covered with water. Wherever the flooring rose
slightly towards the edges of the sea of ice, the usual structure
appeared again.
There were no currents of air in the cave, the candles burning steadily
through the whole time of our visit. Excepting for the purpose of
detecting disturbance in the air, there is no need of candles, as the
two holes in the roof supply sufficient light. Some account of the
careful observations made here by M. Thury, at different seasons of the
year, will be found in other parts of this book.
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