“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 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 decided 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 bunk several times. He was nearest the storeroom where spare parts, and the curious cylinder 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 disturbance.