had
ceased to rise in the crater, yet plainly it existed somewhere in an
incandescent state, and was still transmitting considerable heat to
inferior strata.

Lieutenant Procope had brought in his hand a mercurial thermometer,
and Servadac carried an aneroid barometer, by means of which he could
estimate the depth of their descent below the level of the Gallian Sea.
When they were six hundred feet below the orifice the mercury registered
a temperature of 6 degrees below zero.

"Six degrees!" said Servadac; "that will not suit us. At this low
temperature we could not survive the winter. We must try deeper down. I
only hope the ventilation will hold out."

There was, however, nothing to fear on the score of ventilation.
The great current of air that rushed into the aperture penetrated
everywhere, and made respiration perfectly easy.

The descent was continued for about another three hundred feet, which
brought the explorers to a total depth of nine hundred feet from their
old quarters. Here the thermometer registered 12 degrees above zero--a
temperature which, if only it were permanent, was all they wanted. There
was no advantage in proceeding any further along the lava-course; they
could already hear the dull rumblings that indicated that they were at
no great distance from the central focus.

"Quite near enough for me!" exclaimed Ben Zoof. "Those who are chilly
are welcome to go as much lower as they like. For my part, I shall be
quite warm enough here."

After throwing the gleams of torch-light in all directions, the
explorers seated themselves on a jutting rock, and began to debate
whether it was practicable for the colony to make an abode in these
lower depths of the mountain. The prospect, it must be owned, was not
inviting. The crater, it is true, widened out into a cavern sufficiently
large, but here its accommodation ended. Above and below were a few
ledges in the rock that would serve as receptacles for provisions; but,
with the exception of a small recess that must be reserved for Nina,
it was clear that henceforth they must all renounce the idea of
having separate apartments. The single cave must be their dining-room,
drawing-room, and dormitory, all in one. From living the life of rabbits
in a warren, they were reduced to the existence of moles, with the
difference that they could not, like them, forget their troubles in a
long winter's sleep.

The cavern, however, was quite capable of being lighted