A little thought suggested to M. Thury
that the cold in the cave in mid-winter would most probably be greater
than the external cold of the day, and less than that of the night; so
that there should be a time in the later evening when a column of colder
and heavier air would begin, to descend through the hole in the roof. To
test the correctness of this supposition, he took up his abode in the
cavern for the evening of the 10th January, 1858, with a lighted candle.
The flame burned steadily for some time; but at 7.16 P.M. it began to
flicker, and soon inclined downwards through an angle of about 45 deg.; and
when M. Thury placed himself under the principal opening, the flame was
forced into an almost horizontal position. At 8 P.M. the current of air
had all but disappeared. This violent and temporary disturbance of
equilibrium was a matter of much surprise to M. Thury; for he had
naturally expected a quiet current downwards, continuing through the
greater part of the night.
At 7.16 P.M. the external temperature was 23.9 deg. F., and the temperature
of the atmosphere in the cave at the same time was 30 deg.
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