I Awoke on Sunday morning without any sense of hurry and bustle attendant on an immediate departure. Though the day to be devoted to repose and reflection was spent under such strange circumstances, and in so wonderful a place, the idea was a pleasant one. Besides, we all began to get used to this kind of existence. I had almost ceased to think of the sun, of the moon, of the stars, of the trees, houses, and towns; in fact, about any terrestrial necessities. In our peculiar position we were far above such reflections.  
The grotto was a vast and magnificent hall. Along its granitic soil the stream flowed placidly and pleasantly. So great a distance was it now from its fiery source that its water was scarcely lukewarm, and could be drunk without delay or difficulty.
 
After a frugal breakfast, the Professor made up his mind to devote some hours to putting his notes and calculations in order.
 
"In the first place," he said, "I have a good many to verify and prove, in order that we may know our exact position. I wish to be able on our return to the upper regions to make a map of our journey, a kind of vertical section of the globe, which will be, as it were, the profile of the expedition."
 
"That would indeed be a curious work, Uncle; but can you make your observations with anything like certainty and precision?"
 
"I can. I have never on any occasion failed to note with great care the angles and slopes. I am certain as to having made no mistake. Take the compass and examine how she points."
 
I looked at the instrument with care.
 
"East one quarter southeast."
 
"Very good," resumed the Professor, noting the observation, and going through some rapid calculations. "I make out that we have journeyed two hundred and fifty miles from the point of our departure."
 
"Then the mighty waves of the Atlantic are rolling over our heads?"
 
"Certainly."
 
"And at this very moment it is possible that fierce tempests are raging above, and that men and ships are battling against the angry blasts just over our heads?"
 
"It is quite within the range of possibility," rejoined my uncle, smiling.
 
"And that whales are playing in shoals, thrashing the bottom of the sea, the roof of our adamantine prison?"
 
"Be quite at rest on that point; there is no danger of their breaking through. But to return to our calculations. We are to the southeast, two hundred and fifty miles from the base of Sneffels, and, according to my preceding notes, I think we have gone sixteen leagues in a downward direction."
 
"Sixteen leagues—fifty miles!" I cried.
 
"I am sure of it."
 
"But that is the extreme limit allowed by science for the thickness of the earth's crust," I replied, referring to my geological studies.
 
"I do not contravene that assertion," was his quiet answer.
 
"And at this stage of our journey, according to all known laws on the increase of heat, there should be here a temperature of fifteen hundred degrees of Reaumur."
 
"There should be—you say, my boy."
 
"In which case this granite would not exist, but be in a state of fusion."
 
"But you perceive, my boy, that it is not so, and that facts, as usual, are very stubborn things, overruling all theories."
 
"I am forced to yield to the evidence of my senses, but I am nevertheless very much surprised."
 
"What heat does the thermometer really indicate?" continued the philosopher.
 
"Twenty-seven six-tenths."
 
"So that science is wrong by fourteen hundred and seventy-four degrees and four-tenths. According to which, it is demonstrated that the proportional increase in temperature is an exploded error. Humphry Davy here shines forth in all his glory. He is right, and I have acted wisely to believe him. Have you any answer to make to this statement?"
 
Had I chosen to have spoken, I might have said a great deal. I in no way admitted the theory of Humphry Davy—I still held out for the theory of proportional increase of heat, though I did not feel it.
 
I was far more willing to allow that this chimney of an extinct volcano was covered by lava of a kind refractory to heat—in fact a bad conductor—which did not allow the great increase of temperature to percolate through its sides. The hot water jet supported my view of the matter.
 
But without entering on a long and useless discussion, or seeking for new arguments to controvert my uncle, I contented myself with taking up facts as they were.
 
"Well, sir, I take for granted that all your calculations are correct, but allow me to draw from them a rigorous and definite conclusion."
 
"Go on, my boy—have your say," cried my uncle goodhumoredly.
 
"At the place where we now are, under the latitude of Iceland, the terrestrial depth is about fifteen hundred and eighty-three leagues."
 
"Fifteen hundred eighty-three and a quarter."
 
"Well, suppose we say sixteen hundred in round numbers. Now, out of a voyage of sixteen hundred leagues we have completed sixteen.&q............