Examples of such currents are met with near Rome (in the _Monte
Testaceo_), at Lugano, Lucerne (the caves of Hergiswyl), and in various
other districts. It is found that the hotter the day, the stronger is
the current of cold air; in winter the direction of the current is
changed, and it blows into the rock instead of out from it.[190] De
Saussure's theory, as developed by M. Pictet, was no doubt satisfactory,
so far as it was used to account for the phenomenon of 'cold-caves,' but
it seems to be insufficient as an explanation of the existence of large
masses of subterranean ice; of which, by the way, De Saussure must have
been entirely ignorant, for he makes no allusion to such ice, and the
temperatures of the coldest of his caves were considerably above the
freezing point.
Pictet represents the case of a cave with cold currents of air to be
much the same as that of a mine with a vertical shaft, ending in a
horizontal gallery of which one extremity is in communication with the
open air, at a point much lower, of course, than the upper extremity of
the shaft.
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