. . .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 wal. . . Read More