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.

The beauties of the Val de Travers end only w