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."

He and Jack went to the room where there was an opening in the floor of the ship, covered by heavy gla.s.s. They slid back the steel shutter and there, down below them, was the strange new, world they had come to, stretched out like some big map.

They could see mountains, forests, plains, and rivers, the water sparkling in the colored light. Over green fields they flew, then across some stretches where only sand and rocks were to be seen.

Faster and faster the ship went, as the professor found the machinery was once more in perfect order. Jack was idly watching the play of tinted lights over the surface of the ground.

"I wonder what makes it," he said.

"I have tried to account for it in several ways," said the professor, who had called Washington to the conning tower and come to join the boys. "I have had first one theory and then another, but the one I am almost sure is correct is that hidden volcanic fires cause the illumination.

"I think they flare up and die away, and have become so regular that they produce the same effect as night and day with us. Probably the fires go out for lack of fuel, and when it is supplied they start up again. Perhaps it is a sort of gas that they burn."

"Well, it's queer enough, whatever it is," Jack remarked. "What strikes me as funny, though, is that we haven't seen a single person since we came here. Surely this place must be inhabited."

Mark thought of the strange shadow he had seen, but said nothing.

"I believe it is," the professor answered. "We will probably come upon the inhabitants soon. I only hope they are a people who will do us no harm."

"If they tried any of their tricks we could mount up in our ship and escape them," said Andy.

"Provided they gave us the chance," Mr. Henderson put in. "Well, we'll not worry about that now."

For several hours the ship traveled on, until it had come to a different sort of country. It was wilder and not so level, and there were a number of streams and small lakes to be seen.

"Are you going to sail all night?" asked Jack.

"No," replied the professor. "I think we'll descend very soon now, and camp out for a while. That lake just ahead seems to offer a good place," and he pointed to a large sheet of water that sparkled in the distance, for by this time they had all gone back to the conning tower.

The lake was in the midst of a wood that extended for some distance on all sides, and was down in a sort of valley. The ship headed toward it, and in a short time a landing was made close to sh.o.r.e.

"Maybe we can have some fresh fish for supper," exclaimed Jack as he ran from the ship as soon as the sliding door in the side was opened.

"Looks as if that lake had some in it. It is not thick water like in that stream we stopped at," he added.

"I believe you're right," old Andy put in, as he turned back to look for some lines and hooks among his traps. He soon found what he wanted, and gave them to the boys, taking his trusty gun along for himself.

While the professor, Washington, Tom and Bill remained behind to make some adjustments to the machinery, and to get things in shape for the night, which, they calculated would soon be upon them, Jack, Mark and Andy went down to the sh.o.r.e of the lake. The boys cut some poles from the trees, and baiting the hooks with some fat worms found under the bark, threw in.

"Let's see who'll get the first bite," spoke Jack. "I'm pretty generally lucky at fishing."

"Well, while you're waiting to decide that there contest, I think I'll take a stroll along sh.o.r.e and see if I can see anything to shoot,"

Andy remarked.

For several minutes the boys sat in silence on the bank of the lake, watching the play of the vari-colored lights on the water. Suddenly Jack felt a quiver on his line, and his pole began to shake.

"I've got something!" he cried. Then his pole bent almost double and he began to pull for all he was worth. "It's a whopper!" he cried.

"Come and help me, Mark!"

Mark ran to his friend's aid. Whatever was on the other end of the line was strong enough to tax the muscles of both boys. They could hear the pole beginning to break. But for the excellent quality of Andy's line that would have parted some time before.

All at once there came a sudden slacking of the pull from whatever was in the water. And so quickly did it cease that both boys went over backward in a heap.

"He's got away!" cried Jack, getting up and brushing some of the dirt from his clothes.

"There's something that didn't get away!" cried Mark, who had risen to his knees, and was pointing at the lake. Jack looked and what he saw made him almost believe he was dreaming.

For, emerging from the water, dragging the pole and line the boys had dropped along with it, was a most curious creature. It was a big fish, but a fish with four short legs on which it was walking, or rather waddling along as much as a duck, with a double supply of feet, might do.

"Say, do I see that or is there something the matter with my eyes?"

sung out Jack, making ready to run away.

"It's there all right!" exclaimed Mark. "Hi! Andy! Here's something to shoot!" he yelled, for indeed the creature was big enough to warrant attack with a gun. It was about five feet long and two feet through.

On and on it came, straight at the boys, as if to have revenge for the pain the fish hook must have caused it, for the barb could be seen dangling from its lip. On and on it came, waddling forward, the water dripping from it at every step. It had the body and general shape of a fish, save that the tail was rather large in proportion. As it came nearer the boys noted that the feet were webbed, like those of a water fowl.

"Come on!" cried Jack. "It may attack us!"

At that moment the creature opened its mouth, showing a triple row of formidable teeth, and gave utterance to a sort of groan and grunt combined.

This was enough to send Jack and Mark off on a run up the bank, and did they stop until they heard Andy's voice hailing them.

"What's the matter, boys?"

"Come here! Quick!" answered Jack.

The fish-animal had halted and seemed to be taking an observation. To do this, as it could not turn its neck, it had to shift its whole body. Old Andy came up on the run, his gun held in readiness.

"Where is it?" he asked, and the boys pointed silently.

The hunter could not repress a start of astonishment as he saw the strange creature. But he did not hesitate a second. There was a crack of the rifle, and the thing, whatever it was, toppled over, dead.

Andy hurried up to it, to get a closer view.

"Well, this is the limit!" he exclaimed. "First we have gra.s.shoppers that can roll peaches as big as hogsheads, and now we come across fish that walk. I wonder what we will see next."