time to make sure to attract the attention of the
submarine, won't he?"

"Probably," agreed Joe.

"Well, then, if, while he's in the midst of signaling, his light goes
out, the submarine won't know what to make of it, and will come up
closer to find out what's wrong. Then our own guns, or those of the
destroyers, can bang away and catch the Germans napping."

"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.

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