In some cases it has
been suggested that the length of time required for external heat or
cold to penetrate through the earth and rock which lie above the caves
is sufficient to account for the phenomenon of summer frost and winter
thaw. Thus, it is said, the thickness of the superincumbent bed may be
such that the heat of summer only gets through to the cave at Christmas,
and then produces thaw, while in like manner the greatest cold will
reach the cave in mid-summer. But there is a fatal objection to this
idea in the fact that the invariable stratum--i.e., the stratum beyond
which the annual changes of external temperature are not felt--is
reached about 60 feet below the surface in temperate latitudes,[194]
while at the tropics such changes are not felt more than a foot below
the surface. Humboldt calculated that in the latitude of central France
the whole annual variation in temperature at a depth of 30 feet would
not amount to more than one degree.[195]
FOOTNOTES:
[Footnote 174: As Gollut's phraseology is peculiar, it may be as well
to reproduce his account of the cave:--'Je ne veux pas omettre
toutefois (puisque je suis en ces eaux) de mettre en memoire la
commodite que nature hat done a quelques delicats, puis qu'au fond
d'un montagne de Leugne, la glace (_glasse_ in the index), se treuve
en este, pour le plaisir de ceux qui aim[~e]t a boire frais.
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