"I certainly heard something," he said to himself. "It sounded just as
it did the other night. I wonder if I ought to investigate."

He thought over the matter carefully as he sat upright in his bunk in
the darkness. True the noise might be a natural one, due to the
vibration of the engine, or to some echo from the machinery. As Mark
listened he heard it again.

This time he realized it was the slow movement of some heavy body. He
felt a cold shiver run over him and his hair evinced an uncomfortable
tendency to stand upright. But he conquered his feelings and resolved
to keep cool and see if he could discover what had awakened him.

He got up and moved softly about the little room that contained his
bunk. He could hear better now, and knew it was no echo or vibration
that had come to his ears.

Once again he heard the strange sound. It was exactly the same as
before; as if some big creature was pulling itself over the floor.

"Maybe it's a snake; a water snake!" thought Mark. "It may have
crawled aboard when we did not notice it."

Then he remembered that the ship had not been open in any way that
would enable a serpent to come on it, since it had been started on its
ocean trip. Before that, he was sure no snake had entered the Flying
Mermaid. Still it sounded more like a snake than anything else.

"I'm going to make a search," decided the boy.