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.

He took a small portable electric light, run by a storage battery, and, slipping on a pair of shoes and a bath robe, he left his stateroom.

He had decided that the noise came from the storage compartment and so made for that.  The door he knew was not locked, since he had seen Mr. Henderson go in late that afternoon, and the professor had used no key.

Moving softly, Mark left his room and soon found himself in a corridor, on either side of which were located the sleeping quarters of the others.  He did not want to awaken them, and, perhaps, be laughed at for his curiosity.

To get to the storeroom Mark had to go first from the corridor into the dining room.  He soon reached the door that guarded what he thought might be a strange secret.  Trying the knob softly he found it giving under his fingers.

“I wonder if I had better go in,” he thought.  “Perhaps, after all, it was only rats, as Jack said.”

But, even as he listened he again heard the odd sound coming from the room.  This determined him.  He would solve the mystery if possible.

Cautiously he turned the knob.  The door was slowly swinging open when Mark was startled by a noise from behind him.  He turned suddenly to see Professor Henderson confronting him.

“What is it, Mark?  Is the ship on fire?  What’s the matter?  Is any one hurt?”

“I was just going in this room to——­” began Mark.

“Don’t do it!  Don’t do it!” exclaimed the professor in an excited whisper.  “No one must go in that room.  I forgot to tell you and Jack about it.  No one must enter.  It contains a secret!”

“I heard a strange noise and——­” Mark began again.

“It could make no noise!  It would be impossible for it to make a noise!” the professor exclaimed.

“I heard something,” the boy insisted.

“You were dreaming!” said the professor.  “Now go back to bed, Mark, and don’t let this happen again.  Remember, no one must enter that room unless I give permission!”