The view which Deluc adopted was one which I have myself independently
formed; and he would probably have written with more force if he had
been acquainted with various small details relating to the position and
surroundings of many of the caves. The heavy cold air of winter sinks
down into the glacieres, and the lighter warm air of summer cannot on
ordinary principles of gravitation dislodge it, so that heat is very
slowly spread in the caves; and even when some amount of heat does reach
the ice, the latter melts but slowly, for ice absorbs 60 deg. C. of heat in
melting; and thus, when ice is once formed, it becomes a material
guarantee for the permanence of cold in the cave.
For this explanation to hold good, it is necessary that the level at
which the ice is found should be below the level of the entrance to the
cave; otherwise the mere weight of the cold air would cause it to leave
its prison as soon as the spring warmth arrived. In every single case
that has come under my observation, this condition has been emphatically
fulfilled. It is necessary, also, that the cave should be protected from
direct radiation, as the gravitation of cold air has nothing to do with
resistance to that powerful means of introducing heat.
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