[Footnote 52:  It is probable that the ice is on the increase in this glaciere, and that an archway, now filled up by the growing ice, has at one time existed in the wall on this side of the care, through which the ice and water used to pour into the subterranean depths of which the old woman had told us.  At the time of our visit, we could find no outlet.]

[Footnote 53:  The following remarks may give some explanation of the phenomenon of alternating currents in this cave, I should suppose that during the night there is atmospheric equilibrium in the cave itself, and in the three pits A, B, C. When the heat of the sun comes into operation, the three pits are very differently affected by it, C being comparatively open to the sun’s rays, while A is much less so, and B is entirely sheltered from radiation.  This leads naturally to atmospheric disturbance.  The air in the pit C is made warmer and less heavy than that in A and B, and the consequence is, that the column of air in C can no longer balance the columns in A and B, which therefore begin to descend, and so a current of air is driven from the cave into the pit C. Owing to the elasticity of the atmosphere, even at a low temperature, this descent, and the consequent rush of air into C, will be overdone, and a recoil must take place, which accounts for the return current into the cave from the pit C. The sun can reach A more easily than B, and thus the air is lighter and more moveable in the former pit, so that the recoil will make itself more felt in A than in B:  accordingly, we found that the main currents alternated between A and C, with very slight disturbance in the neighbourhood of B. B will, however, play its part, and the weighty column of air contained in it will oscillate, though with smaller oscillations than in the case of A. Probably, when the sun has left A, while acting still upon C, the return current from C will be much slighter, and there will be a general settling of the atmosphere in the pits A and B, until C also is freed from the sun’s action, when the whole system will gradually pass into a state of equilibrium.

With respect to the action of the more protected pits, the principle of the hydraulic ram not unnaturally suggests itself.

In considering the minor details of the currents, such elements as the refrigeration of the air in its passage across the face of the ice must be taken into account.  It may be observed that the candle did not occupy an intermediate position with respect to two opposing currents, for it was practically on the floor of the cave, owing to the continuity of the slope of snow on which it stood, as shown in the vertical section on p. 108.]

* * * * *

CHAPTER VIII.

THE GLACIERE AND NEIGIERE OF ARC-SOUS-CICON.