The question of ice in summer where thaw prevails in winter, may fairly be considered to have been eliminated from the discussion of such caves as I have seen, in spite of the persistent assertions of some of the peasantry. The observations, however, in caverns in volcanic formations, and in basaltic debris, are so circumstantial that it is impossible to reject them; and in such cases a theory similar to that enunciated by Mr. Scrope seems to be the only one in any way satisfactory, though I have not heard of such marvellous results being produced elsewhere by evaporation. One observer, for instance, of the cavern near the village of Both, in the Eiffel, found a thickness of 3 feet of ice; and in that case it was melting in summer, instead of forming. In some cases it has been suggested that the length of time required for external heat or cold to penetrate through the earth and rock which lie above the caves is sufficient to account for the phenomenon of summer frost and winter thaw. Thus, it is said, the thickness of the superincumbent bed may be such that the heat of summer only gets through to the cave at Christmas, and then produces thaw, while in like manner the greatest cold will reach the cave in mid-summer. But there is a fatal objection to this idea in the fact that the invariable stratum—i.e., the stratum beyond which the annual changes of external temperature are not felt—is reached about 60 feet below the surface in temperate latitudes, while at the tropics such changes are not felt more than a foot below the surface. Humboldt calculated that in the latitude of central France the whole annual variation in temperature at a depth of 30 feet would not amount to more than one degree.
[Footnote 174: As Gollut’s phraseology is peculiar, it may be as well to reproduce his account of the cave:—’Je ne veux pas omettre toutefois (puisque je suis en ces eaux) de mettre en memoire la commodite que nature hat done a quelques delicats, puis qu’au fond d’un montagne de Leugne, la glace (glasse in the index), se treuve en este, pour le plaisir de ceux qui aim[e]t a boire frais. Neanmoins dans ce t[e]ps cela se perd, no pour autre raison (ainsi que ie pense) que pour ce que lon hat depouille le dessus de la motagne d’une epoisse et aulte fustaie de bois, qui ne permettoit pas que les raions du soleil vinsent echauffer la terre et deseicher les distillations, que se couloi[e]t iusques au plus bas et plus froid de la montagne: ou (par l’antiperistase) le froid s’epoississoit, et se reserroit, contre les chaleurs, entornantes et environnantes le long de l’este, toute la circonference exterieure du mont.’—_Histoire_, &c., p. 87.]
[Footnote 175: Hist. de l’Acad., t. ii., p. 2.]
[Footnote 176: Hist. de l’Acad., an 1712, p. 20.]
[Footnote 177: C’est a dire—M. Billerez explains—a 10 degres au-dessous du tres-grand froid. What the 60 deg. may be worth, I cannot say.]