But that which formed steps under our feet became stalactites overhead. The lava, which was porous in many places, had formed a surface covered with small rounded blisters; crystals of opaque quartz, set with limpid tears of glass, and hanging like clustered chandeliers from the vaulted roof, seemed as it were to kindle and form a sudden illumination as we passed on our way. It seemed as if the genii of the depths were lighting up their palace to receive their terrestrial guests.
"It is magnificent!" I cried spontaneously. "My uncle, what a sight! Don't you admire those blending hues of lava, passing from reddish brown to bright yellow by imperceptible shades? And these crystals are just like globes of light."
"Ali, you think so, do you, Axel, my boy? Well, you will see greater splendours than these, I hope. Now let us march: march!"
He had better have said slide, for we did nothing but drop down the steep inclines. It was the facifs _descensus Averni_ of Virgil. The compass, which I consulted frequently, gave our direction as southeast with inflexible steadiness. This lava stream deviated neither to the right nor to the left.
Yet there was no sensible increase of temperature. This justified Davy's theory, and more than once I consulted the thermometer with surprise. Two hours after our departure it only marked 10° (50° Fahr.), an increase of only 4°. This gave reason for believing that our descent was more horizontal than vertical. As for the exact depth reached, it was very easy to ascertain that; the Professor measured accurately the angles of deviation and inclination on the road, but he kept the results to himself.
About eight in the evening he signalled to stop. Hans sat down at once. The lamps were hung upon a projection in the lava; we were in a sort of cavern where there was plenty of air. Certain puffs of air reached us. What atmospheric disturbance was the cause of them? I could not answer that question at the moment. Hunger and fatigue made me incapable of reasoning. A descent of seven hours consecutively is not made without considerable expenditure of strength. I was exhausted. The order to 'halt' therefore gave me pleasure. Hans laid our provisions upon a block of lava, and we ate with a good appetite. But one thing troubled me, our supply of water was half consumed. My uncle reckoned upon a fresh supply from subterranean sources, but hitherto we had met with none. I could not help drawing his attention to this circumstance.
"Are you surprised at this want of springs?" he said.
"More than that, I am anxious about it; we have only water enough for five days."
"Don't be uneasy, Axel, we shall find more than we want."
"When we have left this bed of lava behind us. How could springs break through such walls as these?"
"But perhaps this passage runs to a very great depth. It seems to me that we have made no great progress vertically."
"Why do you suppose that?"
"Because if we had gone deep into the crust of earth, we should have encountered greater heat."
"According to your system," said my uncle. "But what does the thermometer say?"
"Hardly fifteen degrees (59° Fahr), nine degrees only since our departure."
"Well, what is your conclusion?"
"This is my conclusion. According to exact observations, the increase of temperature in the interior of the globe advances at the rate of one degree (1 4/5° Fahr.) for every hundred feet. But certain local conditions may modify this rate. Thus at Yakoutsk in Siberia the increase of a degree is ascertained to be reached every 36 feet. This difference depends upon the heat-conducting power of the rocks. Moreover, in the neighbourhood of an extinct volcano, through gneiss, it has been observed that the increase of a degree is only attained at every 125 feet. Let us therefore assume this last hypothesis as the most suitable to our situation, and calculate."
"Well, do calculate, my boy."
"Nothing is easier," said I, putting down figures in my note book. "Nine times a hundred and twenty-five feet gives a depth of eleven hundred and twenty-five feet."
"Very accurate indeed."
"By my observation we are at 10,000 feet below the level of the sea."
"Is that possible?"
"Yes, or figures are of no use."
The Professor's calculations were quite correct.