of the earth.

Busy days followed, for there was still much to be done to the Flying Mermaid. The machinery, which was only partly completed, had to be finished. Besides this the professor was working on some apparatus, the use of which he did not disclose to any one. It was stored aboard the ship at the last minute.

Plenty of provisions had to be taken aboard, and many supplies needed to work the Mermaid and insure that it would go to the end of the voyage. The materials for generating the gas and negative gravity, spare parts, records for the automatic piano and other things were stored away.

Some guns and ammunition were taken along as were a few revolvers, since old Andy had said it was best to prepare for any thing in the shape of enemies or wild beasts that might be met with in the interior regions.

It was decided to make the start by sailing along the surface of the sea for several days, as in the event of any weakness in the machinery being discovered there would be less danger. If, at the end of four days, no trouble developed, the professor said he would send the Mermaid into the air and make the rest of the voyage through the sky.

The night before the start was to be made the professor, with the boys, Washington and the other helpers, went about through the various shops and buildings, locking them up securely. For they could not tell how long they would be away, and they had to leave behind much valuable material.

As there were several things that needed attention they divided the work up. Mark had finished his share and was walking back toward the living cabin where they were all quartered, when, down at the shore, near where the boat was moored, he fancied he saw, in the gathering darkness, a moving figure.

“I wonder who that can be,” he thought. “All the others are near the machine shop, for I just left them there. Perhaps it’s some one trying to spy out how the Mermaid is built.”

Knowing the professor wanted his secret well guarded, Mark walked softly toward the little dock that served as a place whence the Mermaid could be easily boarded. As he approached he saw the figure moving. Something struck the boy as peculiar.

Though the object had some of the characteristics of a man it did not walk like a human being, but shuffled along more like a huge ape or monkey. It seemed bent over, as if it stooped toward the ground.

“Who are you?” called Mark suddenly.

For an instant the figure halted and then hurried on faster than before, with a curious, shuffling walk. It was approaching the ship.

Somehow it struck Mark as if it was an uncanny being; an inhabitant of some other world. Then he laughed at his half-fear, and started on a run toward the dock.

“If it’s some tramp trying to find a place to sleep he’d better not go aboard the ship, he might do some damage,” the boy thought.

He could hardly see the figure now as it had passed into the shadow cast by the boat. He was about to summon the professor to make an investigation, when Washington started going the search light which was placed just over the door of the living cabin. It was kept there as a sort of beacon light, as, near the island was a dangerous ledge of rocks.

Then, in the blinding white glare from the big lantern as Washington accidentally swung it toward the Mermaid, Mark beheld a strange sight.

The figure he had been watching stood out in bold relief. Though it was shaped like a human being it was not like any person the boy had ever seen. It seemed covered with a skin twice too large for it; a skin, which, in spite of the clothes that concealed it, hung in folds about the arms and legs, dropping pendent like from the neck like a big garment, and flapping in the wind.

For an instant Mark was so startled he cried out, and the professor and the others ran to see what was the matter.

“There—by the ship! A horrible creature!” exclaimed Mark.

Shouting to Washington to keep the light steady in the direction of the dock, Mr. Henderson ran toward the moored Mermaid. Jack, Andy, Bill and Tom, with Mark in the rear followed him.

“Nothing here,” said the scientist, after a careful search about. “Are you sure you saw something, Mark?”

“Positively,” replied the lad with a shudder. He described the vision of the darkness.

“I guess it was a big otter, or maybe an enormous turtle,” the professor said.

CHAPTER V

ATTACKED BY A WHALE

But Mark was certain it was nothing like that, though a careful search failed to reveal anything or any person near the ship. It was too dark to examine for footprints, and even Mark, after taking a look all about, felt he might have been deceived by shadows. Still he was a little nervous, and could hardly sleep for imagining what the thing he saw could have been.

The next day every one was so busy that no one, not even Mark, recalled the little excitement of the night before. Shortly after noon, final preparations having been made, they all got aboard the Mermaid and started off.

It was a bright sunshiny day, and the craft, speeding away from the island where it had been constructed, over the dancing blue waves, must have presented a strange sight had there been any spectators. For surely no such ship had ever before sailed those waters.

However, there was no other vessel in sight, and the island, as far as the professor and his friends knew, had never been inhabited.

“We will not try for any great speed,” Mr. Henderson remarked as he, with Mark and Jack, stood in the conning tower managing the Mermaid. “We don’t want to strain any joints at the start or heat any engine bearings. There will be time enough for speed later.”

“Yes, and we may need it more when we get into the centre of the earth than we do now,” observed Mark.

“Why so?” asked Jack.

“No telling what we may run up against underneath the ground,” went on Mark. “We may have to fight strange animals and stranger beings. Besides, the atmosphere and water there can’t be the same as up here; do you think so, Professor?”

For a few minutes the scientist was silent. He seemed to be thinking deeply.

“I will tell you what I believe,” he said at length. “I have never spoken of it before, but now that we are fairly started and may eventually have a chance to prove my theory, I will say that I think the centre of this earth on which we live is hollow. Inside of it, forming a core, so to speak, I believe there is another earth, similar to ours in some respects which revolves inside this larger sphere.”

They were well out to sea now, as they could observe when they emerged on the little deck. Above their heads was the aluminum gas holder, which served as a sort of protection from the sun that was quite warm. The Mermaid rode with an easy motion, being submerged just enough to make her steady, yet not deep enough to encounter much resistance from the water. In fact it could not have been arranged better for speed or comfort.

“I think we will sail well to the eastward before making our course south,” Mr. Henderson said. “I do not care to meet too many ships, as those aboard will be very curious and I do not want too much news of this venture to get out. We will take an unfrequented route and avoid delays by being hailed by every passing vessel whose captain will wonder what queer craft he had met with.”

The boys enjoyed the sail, for the weather could not have been better. Even old Andy, who seldom said much, seemed delighted with the prospect of having strange adventures. He had his rifle with him, and, indeed, he seldom went anywhere that he did not carry it.

“For there’s no telling when you may see something you want to shoot or that ought to be shot,” he used to say, “and it’s always the man without a gun who needs it