“I will tell you,” the aged inventor said.  “It is something about which I have kept silent, as I did not want to frighten any of you.  It was my latest invention, and I had only perfected it when we started off on this voyage.  Consequently I had no chance to try it.  The machine works in theory, but whether it does in practice is another question.  That is why I say there is a risk.  But we may have to take this risk.  I have placed aboard this ship a——­”

The professor was interrupted in what he was about to say by a curious tremor that made the whole ship shiver as though it had struck some obstruction.  Yet there was no sudden jolt or jar such as would have been occasioned by that.

At the same time Washington, who was out in the kitchen, came running into the dining room, crying: 

“We’re droppin’ into a ragin’ fire, Perfesser!”

“What do you mean?” asked Mr. Henderson.

“I jest took a look down through th’ hole in th’ bottom of the ship!” cried Washington.  “It’s all flames an’ smoke below us!”

“I wonder if it is the end,” the professor muttered in a low voice.

Followed by the boys, the inventor hastened to the floor-window.  The lights were turned off to enable a better view to be had of what was below them.

Leaning over the glass protected aperture the boys and the professor saw, far, far down, a bright light shining.  It was as if they were miles above a whole town of blast furnaces, the stacks of which were belching forth flames and smoke.  The rolling clouds of vapor were illuminated by a peculiar greenish light, which, at times, turned to red, blue, purple and yellowish hues.

The effect was weird and beautiful though it was full of terror for the travelers.  It seemed as if they were falling into some terrible pit of fire, for the reflection of what they feared were flames, could plainly be seen.

“I wish I’d never come on this terrible voyage!” wailed Washington.  “I’d rather freeze to death than be burned up.”

“Washington, be quiet!” commanded the professor sternly.  “This is no time for foolishness.  We must work hard to save our lives, for we are in dire peril.

“Mark, you and Washington, with Jack, start the engines.  Turn on every bit of power you can.  Fill the gas holder as full as it will hold, and use extra heavy pressure.  I will see if I can not work the negative gravity apparatus to better advantage than we did before.  We must escape if possible!”

The boys, as was also Washington, were only too glad to have something to do to take their mind off their troubles.  All three were much frightened, but Mark and Jack tried not to show it.  As for Washington he was almost crying.

Soon the whirr and hum of the machinery in the Mermaid was heard.  The craft, which was rushing in some direction, either downward, ahead or backwards within the unknown depths, shivered from the speed of the dynamos and other apparatus.  Soon the boys could hear the professor starting the negative gravity engine, and then began a struggle between the forces of nature and those of mankind.