“Say, that’s great!” cried Charlie, as soon as he understood the plan Blake had so quickly evolved.

“If it works,” conceded Joe.  “But how are we going to know when that German spy signals the submarine and fails to convey his full meaning, Blake?”

“We’ll have to watch him, of course.  Catch him in the act, as it were.  The defective lamp will help.”

“So it will!” exclaimed Joe.  “Blake, I take back all I thought of you.  I imagined you were making a mistake to let that lamp go out of your possession; but now I see your game.  It’s a good one!  But we’ve got to be on the watch for this spy!”

“Oh, yes,” agreed his chum.  “And not only him but the Frenchman as well.  I didn’t believe it possible that Secor could be in with this German, but perhaps he is, and maybe he’ll betray his own countrymen.  Either one may give the signal, but if they do we’ll be ready for them.  No more moving pictures for us, boys, until we get to the war front.  We’ve got to be on this other job!”

“But hadn’t we better tell Captain Merceau?” asked Charlie.

“Yes, I think so,” assented Blake.  “We’ll tell him what we think, and what we have done.”

But they did not get a chance that day, for there was a submarine scare toward evening—­a lookout thinking he saw a periscope—­and the consequent confusion made it impossible to have a talk with the commander.  The boys did not want to report to any subordinate officer, and so concluded to wait until the next day.

“But we’ll keep watch to-night on our friend across the corridor,” Blake said.  “And on Lieutenant Secor as well.  His stateroom is next to Labenstein’s, and we can tell when either of them goes out after dark—­that is, if we keep watch.”

“And we’ll keep it, all right!” declared Joe “Now that we know something about what to look out for, we’ll do it!”

And so, as evening came on and the lights of the ship were darkened and as she sped along in company with her convoy, the three boys prepared to divide the night into watches, that they might be on guard against what they regarded as an attempt at black treachery.

For somewhere under or on that waste of waters they believed a deadly submarine was lurking, awaiting the favorable moment to send a torpedo at the ship.

CHAPTER XI

THE DEPTH CHARGE

Charlie Anderson, who had taken the earliest watch, roused Blake at the appointed time, and reported: 

“All quiet so far.”

“Then you haven’t seen anything of our friends across the hall?”

“Not a thing.  Just as we arranged, I’ve had my eye at the hole, but their doors have both been closed.  Maybe you’ll have better luck.”

“I don’t think it will be good luck at all to see one of them sneak out to flash a signal to a waiting submarine, or one that may be following us all the while, waiting for a chance to strike.  But I will call it exceedingly good luck if we can stop it,” said Blake.