The astronomer takes exception to the term 'underground glacier'[120]
which had been applied to this cavern. He represents that the mountain
is abundantly covered each winter with snow, in the neighbourhood of the
ice-cave, which is nearly within the snow-line, and the stores of snow
thus accumulated in the cave have no great difficulty in resisting the
effects of summer heat, since all radiation is cut off by the roof of
rocks. The importance of this protection may be understood from the fact
that in the middle of July the thermometer at this altitude gave 130 deg. in
the sun, but fell to 47 deg. when relieved from the heat due to radiation.
At the time of this observation, there were still patches of snow lying
on the mountain-side, exposed to the full power of direct radiation;
and, therefore, there is not anything very surprising in the permanence
of snow under such favourable circumstances as are developed in the
cave. Mr. Airy, a few summers ago, found the rooms of the Casa Inglese,
on Mount Etna, half filled with snow, which had drifted in by an open
door, and had been preserved from solar radiation by the thick
roof.
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