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.