At 7.16 P.M. the external temperature was 23·9° F., and the temperature of the atmosphere in the cave at the same time was 30°·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 glacière 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°·884 Fahr. " " 7.20 " ... 29°·75 " " " 7.27 " ... 27°·5 " " " 7.50 " ... 26°·834 " January 10, at 10.12 A.M. ... 23°·684 " " " 10.0 " ... 23°·9 " " " 11.20 " ... 24°·022 " " " 12.14 P.M. ... 24°·134 " " " 1.30 " ... 24°·35 " " " 2.30 " ... 24°·584 " " " 3.14 " ... 24°·8 " " " 4.0 " ... 25°·142 " Supposing the weather to have been much the same on the 9th and 10th of January, as M. Thury's account seems to say, there is something very strange in the great difference between the temperatures registered at 4 P.M. on the one day, and at 7.16 P.M. on the other. The external temperatures at the mouth of the cave were as follows:-- January 10, at 10.53 A.M. 25°·934 Fahr. " " 11.14 " 26°·384 " " " 11.45 " 28°·04 " " " 12.32 P.M. 27°·944 " " " 1.12 " 30°·644 " " " 3.3 " 26°·834 " " " 3.56 " 25°·7 " " " 4.26 " 25°·25 " The minimum temperature of the external air during the night of January 10-11 was 18°·392 F., and that of the glacière 19°·76 F.[219] During the preceding night, the minimum in the cave was 22°·442 F., which may throw some light upon the difference between the temperatures at 7.16 P.M. on the 9th, and at 4 P.M. on the 10th.