"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!"

Somewhat crestfallen at the outcome of his investigations, but
realizing that the professor could do what he wanted to aboard his own
ship, Mark went back to bed. But he could not sleep. All the rest of
the night he was wondering whether Mr. Henderson had some strange
creature hidden aboard the Mermaid. He feared lest the old scientist's
mind might be affected and, in his wildness he had made some infernal
machine that would, in time, blow the whole ship apart.

But tired nature asserted itself at last, and, weary with vain
imaginings, Mark fell into a slumber. The next morning he awoke with a