“We are at the earth’s surface,” the scientist said.  “Now we are below it.  Now we are fairly within the big hole!  Boys, we may be on the verge of a great discovery!”

An instant later it seemed as if a hot wave had struck the Mermaid, or as if the craft had been plunged into boiling water.

“It’s going to be hot!” cried the professor.  “Lucky I provided the water jackets!”

Then the lights in the interior of the ship went out, leaving the whole craft in darkness.

“What has happened?” cried Mark.

CHAPTER XIV

 Many miles below

“Don’t be alarmed,” spoke the calm voice of the professor.  “I have only turned off the electrics.  I want to switch on the search lights, to see if we can learn anything about our position.”

As he spoke he turned a switch, and, the gloom below the ship, as the boys could see by glimpses from the floor-window, was pierced by a dazzling glare.  In the bottom of the Mermaid were set a number of powerful electric arc lights with reflectors, constructed to throw the beams downward.  The professor had built them in for just this emergency, as he thought that at some time they might want to illuminate what was below the craft.

Not that it was of much avail on this occasion, for, though the lights were powerful, they could not pierce the miles of gloom that lay below them.  The beams only served to accentuate the darkness.

“I guess we’ll have to trust to luck,” the professor said, after a vain attempt, by means of powerful glasses, to distinguish something.  “There is too much fog and vapor.”

“What makes it so warm?” asked Mark, removing his coat.

“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.”