.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.
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