“What is that?” asked Jack.

“We may be held down, as were those stones,” was the grave answer.

CHAPTER XXI

 The fish that walked

It was with no little apprehension that the professor prepared to take his first flight aboard the ship in the realms of the new world.  He knew little or nothing of the conditions he might meet with, the density of the atmosphere, or how the Mermaid would behave under another environment than that to which she was accustomed.

Yet he felt it was necessary to make a start.  They would have to attempt a flight sooner or later, and Mr. Henderson was not the one to delay matters.  So, the last adjustment having been made to the repaired machinery, they all took their places in the ship.

The boys and the professor went to the conning tower to direct matters, while Washington and the others were in the engine room to see that the machinery worked properly.  Mark gave a last look outside as he closed the big steel cover over the hole through which admission was had to the craft.  He thought he might catch a glimpse of the queer shadow, but nothing was in sight.  It was like a beautiful summer’s day, save for the strange lights, shifting and changing.  But the travelers had become somewhat used to them by this time.

The professor turned the valve that allowed the gas to enter the holder.  There was a hissing sound and a sort of trembling throughout the entire ship.  The dynamos were whizzing away and the negative gravity machine was all ready to start.

For several minutes the travelers waited until the big lifting tank was filled with the strong vapor.  They watched the gages which indicated the pressure to be several hundred pounds.

“I think we can chance it now,” remarked Mr. Henderson, as he threw over several levers.  “We’ll try, at any rate.”

With a tremor the Mermaid left the surface of the inner earth and went sailing upward toward the—­ well it wasn’t exactly the sky, but it was what corresponded to it in the new world, though there were no clouds and no blue depths such as the boys were used to.  At all events the Mermaid was flying again, and, as the adventurers felt themselves being lifted up they gave a spontaneous cheer at the success which had crowned their efforts.

The ship went up several hundred feet, and then, the professor, having brought her to a stop, sent her ahead at a slow pace.  He wanted to be sure all the apparatus was in good working order before he tried any speed.

The Mermaid responded readily.  Straight as an arrow through the air she flew.

“Well, this is almost as good as being on the regular earth!” exclaimed Jack.

“It’s better,” put in Mark.  “We haven’t seen half the wonders yet.  Let’s open the floor shutter, and see how it looks down below.”