"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."