" exclaimed Andy firmly. "This is a public place, and
we have as much right here as you have. Besides, you haven't any
cannon now, and we're two to one."

"Oh, you are; eh?" demanded the man in an ugly voice. "We'll see about
that. Once more I order you to stop following me; do you hear?"

"We're not going to let you get away until you answer our questions!"
declared Frank. "We demand to know what you are doing with Paul's
boat, and we want to know what his full name is, so that we can
communicate with his friends."

"You'll never know from me!" fairly shouted the man. "And I defy you
to get anything out of me. I'm not going to be bothered with you.
Come on, men, here are these two bothersome boys! Let's get rid of
them!" he suddenly cried, waving his hand as if at someone approaching
Andy and Frank from the rear.

Involuntarily they turned, but the next instant they heard a triumphant
laugh, and when they turned back, having seen no one, they beheld the
mysterious man racing across the sands toward the interior of the
island.

"Quick! After him!" cried Frank.

"Yes, we mustn't let him get away again!" added his brother.

They set off after the stranger at full speed. He was running rapidly,
now and then glancing over his shoulder at them.

All at once he changed his course, and darted around a small rocky
promontory. The tide was rising and he had to step into the water to
make the turn.

"After him!" yelled Frank again.

The two brothers made the turn, and just far enough behind the man to
see him dart into the black entrance of a small cave. It was one they
had looked into, but into which they had not penetrated far.

"Now we've got him!" yelled Andy. "There's no way out of that! Come
on, Frank!"

Together the two brothers entered the dark cavern. The change from the
glaring sunlight on the sands to intense gloom made them pause for a
moment, and they heard from somewhere in the blackness of the rear a
sinister chuckle.

"He's in here," declared Andy. "We have him now."

The two pressed forward resolutely in the darkness. Of what lay before
them--the danger from a desperate man and the danger of the cavern they
knew not--they only resolved to end the mystery if possible.

CHAPTER XX

THE RISING TIDE

"Where are you, Frank?" called Andy.

"Right here. Give me your hand. It won't do to get lost in this
darkness. Where are you?"

The two brothers groped about in the da