“But he’d be signaling for his own death warrant!” cried Joe.

“Not necessarily,” answered Blake.  “He may have had some understanding with the submarine that he was to be saved first.  Perhaps he was going to jump overboard before the torpedo was fired and was to be picked up.  Anyhow, I saw him draping a white cloth over the side, and I’m sure it was a signal.”

“Well, I guess you’re right,” said Joe.  “The next question is, what’s to be done?  This fellow is a spy and a traitor, and we ought to expose him.”

“Yes,” agreed Blake.  “But we’d better have a little more evidence than just my word.  You fellows didn’t see what I saw, that’s plain, and perhaps no one else did.  So it would only make a big fuss and not result in anything if I told the captain.”

“Then what are you going to do?” asked Charlie.

“Just keep watch,” Blake answered.

“What about Lieutenant Secor?” asked Joe.

“Well, I didn’t see him do anything,” admitted Blake.  “Though I have my suspicions of him also.  He and Labenstein weren’t talking so earnestly together for nothing.  We’ll watch that Frenchman, too.”

“And if he tries any more games in spoiling films I’ll have my say!” threatened Macaroni.

The boys talked the situation over at some length as they put away the films they had taken of the submarine attack, and agreed that “watchful waiting” was the best policy to adopt.  As Blake had said, little could be gained by denouncing Labenstein with only the word of one witness to rely on.

“If all three of us catch him at his traitorous work, then we’ll denounce him,” suggested Blake.

“Yes, and the Frenchman, too!” added Charlie, in a louder voice, so that Blake raised a cautioning hand.

At that moment came a knock on their door, and a voice said: 

“I am Mr. Labenstein!”

CHAPTER X

THE FLASHLIGHT

Almost like conspirators themselves, the boys looked at one another as the voice and knock sounded together.  Blake was the first to recover himself.

“Come in!” he called, in as welcoming a tone as he could muster under the circumstances.  Then as the knob of the door was ineffectually tried, he added: 

“Oh, I forgot it was locked!  Wait a moment!”

A moment later he had swung the door open, and the man who, the boys believed, was a German spy confronted them, smiling.

“You are locked in as if you feared another submarine,” he said.  “It is not the best way to do.  You should be on deck!”

“But not on deck as you were, with a flag to signal to the Huns,” thought Joe; and he wished he dared make the accusation.

Blake motioned to the caller to seat himself on a stool.

“I came to see if I might borrow something,” began the caller.  “I find that mine is out of order for some reason,” and he held out a small, but powerful, electric flash lamp, of the sort sold for the use of soldiers.  “Have you, by any chance, one that you could spare me?” asked Mr. Labenstein.