the interior
of the earth is one degree for every hundred feet. But certain local
causes may considerably modify this figure. Thus at Yakoust in Siberia,
it has been remarked that the heat increases a degree every thirty-six
feet. The difference evidently depends on the conductibility of certain
rocks. In the neighborhood of an extinct volcano, it has been remarked
that the elevation of temperature was only one degree in every
five-and-twenty feet. Let us, then, go upon this calculation--which is
the most favorable--and calculate."

"Calculate away, my boy."

"Nothing easier," said I, pulling out my notebook and pencil. "Nine
times one hundred and twenty-five feet make a depth of eleven hundred
and twenty-five feet."

"Archimedes could not have spoken more geometrically."

"Well?"

"Well, according to my observations, we are at least ten thousand feet
below the level of the sea."

"Can it be possible?"

"Either my calculation is correct, or there is no truth in figures."

The calculations of the Professor were perfectly correct. We were
already six thousand feet deeper down in the bowels of the earth than
anyone had ever been before. The lowest known depth to which man had
hitherto penetrated was in the mines of Kitzbuhel, in the Tirol, and
those of Wurttemberg.

The temperature, which should have been eighty-one, was in this place
only fifteen. This was a matter for serious consideration.

CHAPTER 16

THE EASTERN TUNNEL

The next day was Tuesday, the 30th of June--and at six o'clock in the
morning we resumed our journey.

We still continued to follow the gallery of lava, a perfect natural
pathway, as easy of descent as some of those inclined planes which, in
very old German houses, serve the purpose of staircases. This went on
until seventeen minutes past twelve, the precise instant at which we
rejoined Hans, who, having been somewhat in advance, had suddenly
stopped.

"At last," cried my uncle, "we have reached the end of the shaft."

I looked wonderingly about me. We were in the centre of four cross
paths--somber and narrow tunnels. The question now arose as to which it
was wise to take; and this of itself was no small difficulty.

My uncle, who did not wish to appear to have any hesitation about the
matter before myself or the guide, at once made up his mind. He pointed
quietly to the eastern tunnel; and, without delay, we entered within its
gloomy recesses.

Besides, had he entertai