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 Mermaid¹ 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 vessel² 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 Jack³, stood in the conning⁴ tower managing the Mermaid. “We don’t want to strain any joints⁵ 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 underneath⁶ 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 revolves⁸ 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 aluminum⁹ gas holder¹⁰, 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 eastward¹¹ 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 prospect¹² 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 most. So I’m taking no chances.”
 
They sailed all that afternoon without meeting with a craft of any kind. Straight to the east they went, and when night began to settle down Washington got supper. It was decided¹³ to run slowly after dark until all hands were more familiar with the ship.
 
Morning found the Mermaid about a hundred miles from the island where she had been launched. The night had been uneventful, except that Mark told Jack he heard some strange noise near his bunk¹⁴ several times. He was nearest the storeroom where spare parts, and the curious cylinder¹⁵ the professor had brought aboard, were kept.
 
“I guess it was rats,” said Jack. “They are always in ships.”
 
“Old wooden ships, yes,” admitted Mark. “But I’ll bet there’s not a rat aboard the Mermaid.”
 
“Then you were dreaming,” said Jack, as if that settled it.
 
Mark did not speak further of the noise, but he did considerable thinking. However, the next night there was no further disturbance¹⁶.
 
The fourth day out, when everything had passed off well, the engines doing their best, the professor decided to speed them up a bit, since he was satisfied they had “found” themselves as mechanics term it.
 
“We’ll see how fast we can go through the water,” said Mr. Henderson, “and then I think we can safely turn our course south. We are well beyond the ordinary lines of travel now.”
 
Having oiled the bearings well, and seen that everything was in place and properly adjusted, the professor and the boys took their places in the conning tower, while Washington, Tom and Bill remained in the engine room. Andy stayed on deck with his gun.
 
“I might see a big fish, and we could vary our bill of fare,” he said with a laugh.
 
“Here we go!” exclaimed the professor as he shifted the levers and turned some wheels and valves. “Now we’ll see how fast we can travel.”
 
As he spoke⁷ the Mermaid responded to the added impulse of the compressed air and shot through the water at a terri............