“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.

He could hardly see the figure now as it had passed into the shadow cast by the boat.  He was about to summon the professor to make an investigation, when Washington started going the search light which was placed just over the door of the living cabin.  It was kept there as a sort of beacon light, as, near the island was a dangerous ledge of rocks.

Then, in the blinding white glare from the big lantern as Washington accidentally swung it toward the Mermaid, Mark beheld a strange sight.

The figure he had been watching stood out in bold relief.  Though it was shaped like a human being it was not like any person the boy had ever seen.  It seemed covered with a skin twice too large for it; a skin, which, in spite of the clothes that concealed it, hung in folds about the arms and legs, dropping pendent like from the neck like a big garment, and flapping in the wind.

For an instant Mark was so startled he cried out, and the professor and the others ran to see what was the matter.

“There—­ by the ship!  A horrible creature!” exclaimed Mark.

Shouting to Washington to keep the light steady in the direction of the dock, Mr. Henderson ran toward the moored Mermaid.  Jack, Andy, Bill and Tom, with Mark in the rear followed him.

“Nothing here,” said the scientist, after a careful search about.  “Are you sure you saw something, Mark?”

“Positively,” replied the lad with a shudder.  He described the vision of the darkness.

“I guess it was a big otter, or maybe an enormous turtle,” the professor said.

CHAPTER V

 Attacked by A whale

But Mark was certain it was nothing like that, though a careful search failed to reveal anything or any person near the ship.  It was too dark to examine for footprints, and even Mark, after taking a look all about, felt he might have been deceived by shadows.  Still he was a little nervous, and could hardly sleep for imagining what the thing he saw could have been.

The next day every one was so busy that no one, not even Mark, recalled the little excitement of the night before.  Shortly after noon, final preparations having been made, they all got aboard the Mermaid and started off.

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.