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APPENDIX.

M. Thury’s observations during his winter visit to the Glaciere of S. Georges are so curious and valuable, that I give the principal results of them here.

It will be remembered that this glaciere consists of a roomy cave, 110 feet long and 60 feet high, with two orifices in the higher part of the roof, one of which is kept covered with the trunks of trees to shut out the direct radiation of the sun.  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..88 F.;[217] so that there is no wonder the current of air should be strong.  It is very difficult to say, however, why it did not commence much earlier, considering that the external air must have been heavier than that in the cave long before 7 o’clock.  M. Thury refers to the mirage as a somewhat similar instance, that phenomenon being explained by the supposition that atmospheric layers of different temperatures lie one above another in clearly-defined strata.  He suggests, also, that as the heavier air tends to pass down into the cave, the less cold air already in the cave tends to pass out; and the narrow entrance confining the struggle between the opposing tendencies to a very small area, the weaker initial current is able for a time to hold its own against the intruder.  On this supposition, it is easy to see that when the rupture does occur it will be violent.

The next day, M. Thury arrived at the glaciere at 9.50 A.M.  He had determined, in the summer, that the temperature of the cave was invariable, at any rate through the 3-1/2 hours of his visit (from 7.30 to 11 A.M.); but his winter experience was very different.  The following are the results of his observations.

In the cave:—­

January 9, at 7.16 P.M.[218]... 30 deg..884 Fahr.
   " " 7.20 " ... 29 deg..75 "
   " " 7.27 " ... 27 deg..5 "
   " " 7.50 " ... 26 deg..834 "