"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."
"Well?"
"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. We had already attained a depth of six thousand feet beyond that hitherto reached by the foot of man, such as the mines of Kitz Bahl in Tyrol, and those of Wuttembourg in Bohemia.