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.
“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.”