But I tell you what I think, Frank," he went on, "either that man was hiding in some cave farther back than we looked, or he has just arrived."

"The motor boat has just arrived, anyhow, or at least since night before last," a.s.sented the elder lad. "We couldn't have overlooked that. Say, Andy, he is getting inside! Now's our chance!"

They saw, by peering over the edge of their craft, that the mysterious man had climbed over the half-burned rail of the damaged motor boat.

His back was toward them, and they could not see his head. He appeared to be tearing the interior of the craft apart.

Cautiously the two brothers crept out from behind their shelter and made their way softly over the sand toward where the man was. What they intended to do when they confronted him they hardly knew. Frank was sure that he wanted to ask the queer stranger certain questions, and he hoped to be able to plead with him to tell what he knew of Paul Gale. The question was, whether or not the man would answer.

It was lucky that their footsteps made no sound on the soft sand, for they were thus enabled to approach to within a short distance of the wreck as it rested on the beach. The man was still in it, and they could hear him pounding and splitting wood in the interior. Evidently he was not aware of their presence.

For the first time since they had begun their surprising series of adventures, the boys were able to make out the name of the strange craft. It was the _Swallow_, and as they had a chance to look at her graceful lines they realized that, before the fire, wreck, and explosion the boat had been a powerful one.

"I think we have him this time," whispered Andy, as they came nearer and nearer, and the man was still bending over with his back toward them.

Frank laid his finger across his lips as a sign of caution. At that moment an unfortunate thing happened. Andy stepped on a sh.e.l.l, not seeing it, and it broke under his weight with a sharp, crackling sound.

Like a flash the man leaped up, and fairly sprang out of the boat. He stood confronting the Racer boys.

"Who are you? What do you want?" he demanded sharply. Then, as he recognized them, he added: "Oh, it's you two again. Didn't I warn you to stop following me?"

"We didn't choose to," retorted Frank calmly. "We've found you after a good deal of trouble, and we intend to end this mystery now. A boy's life--the life of Paul Gale--hangs in the balance."

"As if I cared," sneered the man. "You have had your trouble for your pains. I shall tell you nothing, and I order you off this island."

"We're not going!" 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 darkness until they had found each other.

"Listen," whispered the older one. "Do you hear him?"

In the silence and blackness there came to them the sound of retreating footsteps, and of small stones and particles of earth falling.

"He must be climbing up," said Andy. "This cave is bigger than we thought, and he must know the place, even in the dark.

"It _is_ as dark as a pocket," complained Frank. "I can't see anything."

"Wait!" suddenly exclaimed Andy. "Why didn't we think of them before?

Our pocket electrics. They'll do the trick!"

"Sure enough."

An instant later two small but powerful gleams of light cut the blackness of the cavern, and the boys were enabled to see so they could hurry ahead. They could still hear the man retreating before them.

"We're coming!" cried Andy in reckless bravado.

"Hush! He'll hear you," cautioned his mother.

"What of it? I want him to. He'll see our lights anyhow. But I think we have him trapped."

"If there isn't another outlet to the cave. But come on."

Forward they pressed. They could still hear the noise made by the man, and once they were startled by his mocking laugh. So close was it that they knew he must have doubled on his tracks returned toward them.

"There are several pa.s.sages in this cave, I'm sure of it," declared Frank. "We'll have to be careful not to get lost."

"That's right. This fellow must be at home here. But the floor is beginning to slope upward. Say, it's damp in here, all right," Andy added, as he stepped into a little puddle of water.

"From the rain, I guess," replied Frank.