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