h mystery," he told himself. "I must unravel it if I can."

"I am going to return home now," said St. John presently, when the storm
seemed to be clearing away. "If I don't get back, my mother will be
wondering what has become of me."

"All right," answered the doctor. "But let me hear from you by to-morrow
night, sure."

"I will."

"And don't mention my name to Colonel Stanton."

"But if I have him arrested you will furnish those witnesses to the fact
that he is a spy?"

"I will, rest assured on the point."

A little later St. John hurried off in the darkness. Dr. Mackey watched
him go, and then began to pace the floor nervously.

Jack touched Old Ben on the shoulder.

"Wot am it, Massah Jack?" whispered the faithful old negro.

"Ben, we must make the surgeon a prisoner."

"All right, Ise ready to do my share."

"I am going to jump down on his back. You follow me with the gun."

"I will, Massah Jack."

The surgeon continued to pace the floor of the cabin, and, watching his
chance, Jack crawled to the edge of the loft opening.

Just as Dr. Mackey swung around on his heel our hero gave a nimble leap
and landed squarely on his shoulders, sending the surgeon to his knees.

"Hi, what's this?" spluttered the rascal, and tried to throw Jack off.
But our hero clung as fast as grim death.

"It means that you are now my prisoner, Dr. Mackey."

"You!" ejaculated the astonished man. "Let me go, I say!" And he began
to struggle more violently than ever.

But by this time Old Ben was on the floor, and the negro lost no time in
poking the muzzle of the gun under the surgeon's nose. This brought Dr.
Mackey to a standstill, and he glared at his opponents in amazement.

"Don't--don't shoot!" he gasped.

"Then keep quiet."

"How did you escape from the cave?"

"That is our business, Dr. Mackey. Will you submit, or not?"

"I suppose I'll have to submit. You are three to one." Columbus
Washington was now beside Ben.

"Columbus, see if you can find a rope or a strap. We'll bind his hands
behind him," went on Jack.

"What are you going to do with me?" questioned the surgeon anxiously.

"Put you where you deserve to be--behind the bars," was our hero's
quiet, but firm, answer.

CHAPTER XXXI.

FATHER AND SON--CONCLUSION.

"Jack, do you mean to say you would put your own father in prison?"
asked Dr. Mackey reproachfully, after Old Ben had tied his hands behind
him.

"I would--were he such a fraud