The temperatures at 1.25 P.M. and 2.12 P.M. respectively were as follows:—In the sun, between 3 and 4 feet above the snow, 72 deg..1 and 70 deg..5; in the shade, outside the cave, 36 deg..7 and 35 deg..8; at the Observatory of Geneva, in the shade, 27 deg..3 and 28 deg..2, having risen from 24 deg..5 since noon. In the cave, 1 foot above the surface of the ice-floor, the thermometer stood at 24 deg..8; and in a hole in the ice, some few inches below the surface, 24 deg..1. In the large fissure, which has been already mentioned as the source of the summer currents of air, the temperature at various points was from 29 deg..3 to 27 deg..5. The circumstances of these currents of air were now of course changed. Instead of a steady current passing from the fissure into the cave, and so out by the main entrance into the open air, strong enough to incline the flame of a candle 45 deg., M. Thury found a gentle current passing from the cave into the fissure, sufficient only to incline the flame 10 deg., and near the entrance 8 deg., while in the entrance itself no current was perceptible at 4 P.M.
M. Thury remarks that less current was to be expected in winter than in summer, because the upper ends of the fissures would be probably choked with snow, and their lower ends with ice. It is evident that the current which passes up into the fissure in winter, is favourable to the introduction of the colder air from without; while the opposite current in summer keeps up a supply of cold air in the cave, and so increases its powers of resisting the attempts of the heated external air to make a partial entrance. Both these currents, then, favour the glacial conditions of the cave, and to some extent counterbalance the disadvantages of its situation: viz., its aspect, towards the south-east; the large size of its opening to the air, and the absence of all shelter near the mouth, such as is so often provided by trees or rocks. The small depth of the cave, scarcely amounting to 18 feet below the level of the entrance, is also a great disadvantage.
The people of Pralong asked, on the return of the party, what had been found in the grand’ cave, and the answer reduced them to silence for a few moments. Their prejudices, however, were invincible, and they persisted in their belief that a true glaciere ought to have no ice in it in the winter. M. Thury did not enquire from what source they drew their ideas of a true glaciere.
There is a book, in three volumes, on the ‘Glacieres of the Alps,’ by M. Bourrit, dedicated to Buffon, in which is a description of the Valley of Reposoir; but no mention whatever is made of the grand’ cave. Indeed, M. Bourrit merely meant by glaciere, a glacial district, something more extensive than a glacier, and he had evidently no knowledge of the existence of caves containing ice.
[Footnote 77: Premiere Serie, t. xx. pp. 261, &c.]