“Well, you must remember you are approaching the interior of the earth,” the professor answered. “It has been calculated that the heat increases one degree for every fifty-five feet you descend. We have come down several hundred feet and of course it is getting warmer.”
“Then if we go down very far it will get so hot we will not be able to stand it,” Jack put in.
“I do not believe we will suffer any great inconvenience,” Mr. Henderson went on. “I believe that after we pass a certain point it will become cooler. I think the inner fires of the earth are more or less heated gas in a sort of inner chamber between two shells. If we can pass the second shell, we will be all right.”
“But aren’t we liable to hit something, going down into the dark this way?” asked Mark.
“We will guard ourselves as far as possible,” the scientist answered.
The Mermaid seemed to be going down on a side of the immense shaft a good way distant from the strange waterfall. When they had first dropped into the hole the travelers could hear the rush of waters, but now the noise was not audible.
“I think the hole must widen out the farther down we go,” the professor said. “We are probably many miles from the fall now.”
“I’m sure I hope so,” put in Jack. “It would be no fun to have to take a shower bath in this place.”
After a meal, the boys and the professor took some more observations, but with all their efforts nothing could be seen below the ship but a vast black void, into which they were steadily descending.
“I wonder when we’re going to stop,” asked Mark. “It’s like playing the game ‘Going to Jerusalem,’ you keep wondering when the music will cease and you will have a chance to grab a chair. I only hope we have a chair or something else to sit on, in case we go to smash.”