This condition,
also, is fulfilled by nature in all the glacieres I have visited,
excepting that of S. Georges; and there art has replaced the protection
formerly afforded by the thick trees which grew over the hole of
entrance. The effect of the second hole in the roof of this glaciere is
to destroy all the ice which is within range of the sun. A third and
very necessary condition is, that the wind should not be allowed access
to the cave; for if it were, it would infallibly bring in heated air, in
spite of the specific weight of the cold air stored within. It will be
understood from my descriptions of such glacieres as that of the Grand
Anu, of Monthezy, and the Lower Glaciere of the Pre de S. Livres, how
completely sheltered from all winds the entrances to those caves are.
There can be no doubt, too, that the large surfaces which are available
for evaporation have much to do with maintaining a somewhat lower
temperature than the mean temperature of the place where the cave
occurs. This had been noticed so long ago as Kircher's time; for among
the answers which his questions received from the miners of Herrengrund,
we find it stated that, so long as mines are dry, the deeper they are
the hotter; but if they have water, they are less warm, however deep.
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