The more rarified the lower external air, the more rapid
will be the current of cool air; and, therefore, the greater the
evaporation. The winter phenomenon is to be explained by the fact that
the current of air will be about the mean annual temperature of the
district, taking its temperature, in fact, from the rocks through which
it passes; and, therefore, by contrast the grotto will appear warm.
The same writer mentions a similar example of summer ice in
Auvergne.[151] There is a natural grotto in the basalt near Pont Gibaud,
some miles to the north-west of Clermont, in which a small spring is
found partly frozen during the greatest heats of summer, while the water
is said to be warm in winter; probably, Scrope observes, only seeming to
be warm by contrast with the external temperature. The water is
apparently frozen by means of the powerful evaporation produced by a
current of very dry air proceeding from some long fissures or arched
galleries which communicate with the cave. In this case also the writer
suggests that the air owes its dryness to the absorbent qualities of the
lava through which it passes: he repeats, too, the remark that the
phenomenon is of common occurrence in caverns in volcanic
districts.
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