Hans laid out some provisions on a lump of lava, and we each supped with keen relish. One thing, however, caused us great uneasiness—our water reserve was already half exhausted. My uncle had full confidence in finding subterranean resources, but hitherto we had completely failed in so doing. I could not help calling my uncle’s attention to the circumstance.
“And you are surprised at this total absence of springs?” he said.
“Doubtless—I am very uneasy on the point. We have certainly not enough water to last us five days.”
“Be quite easy on that matter,” continued my uncle. “I answer for it we shall find plenty of water—in fact, far more than we shall want.”
“But when?”
“When we once get through this crust of lava. How can you expect springs to force their way through these solid stone walls?”
“But what is there to prove that this concrete mass of lava does not extend to the center of the earth? I don’t think we have as yet done much in a vertical way.”
“What puts that into your head, my boy?” asked my uncle mildly.
“Well, it appears to me that if we had descended very far below the level of the sea—we should find it rather hotter than we have.”
“According to your system,” said my uncle; “but what does the thermometer say?”
“Scarcely fifteen degrees by Reaumur, which is only an increase of nine since our departure.”
“Well, and what conclusion does that bring you to?” inquired the Professor.
“The deduction I draw from this is very simple. According to the most exact observations, the augmentation of the temperature of 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 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 Kitzbühl, in the Tirol, and those of Württemberg.
The temperature, which should have been eighty-one, was in this place only fifteen. This was a matter for serious consideration.
XVI
The Eastern Tunnel
The next day was Tuesday, the 2d of July—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 center 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 entertained any feeling of hesitation it might have been prolonged indefinitely, for there was no indication by which to determine on a choice. It was absolutely necessary to trust to chance and good fortune!