The cave corresponds to the horizontal gallery, and the
various fissures in the rock take the place of the vertical shaft, and
communicate freely with the external air. In summer, the columns of air
contained in these fissures assume nearly the temperature of the rock in
which they rest, that is to say, the mean temperature of the district,
and therefore they are heavier than the corresponding external columns
of air which terminate at the mouth of the cave; for the atmosphere in
summer is very much above the mean temperature of the soil, or of the
interior of the earth at moderate depths. The consequence is, that the
heavy cool air descends from the fissures, and streams out into the
cave, appearing as a cold current; and the hotter the day is--that is,
the lighter the columns of external air--the more violent will be the
disturbance of equilibrium, and therefore the more palpable the cold
current. Naturally, in this last case, the air which enters by the upper
orifices of the fissures is more heated, to begin with, than on cooler
days; but external heat so very slightly affects the deeper parts of the
fissures, that the columns of air thus introduced are speedily impressed
with the mean temperature of the district.
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