“When?”

“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?”