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.

"I do not want it, if it is the only one you have, but they are a great convenience in one's berth, for the lights must be kept turned off, now that we are in the danger zone made by those terrible Germans. Ah, how I hate them!" and his anger seemed very real and earnest.

"Did you say you wanted to borrow a pocket electric flash lamp?" asked Blake, wishing to make the caller repeat his request. As he asked this question Blake looked at his chums, as though to ask them to take particular note of the reply.

"I should like to, yes, if you have one to spare. There are three of you, and, I presume, like most travelers, you each have one. I am alone in a single stateroom, and I may have need of a light. I will return it to you at the end of the voyage, or buy it of you at a good price. You see, I have a little Jew in me. I will make a bargain with you. And I will pay you well, something a Jew proverbially does not like to do. But I realize the value of what I want, and that the market is not well supplied, so you may take advantage of my situation. My battery is either worn out or the light is broken. It will not flash."

He shoved down the little sliding catch, but there was no glow in the tiny tungsten bulb.

"You have me at your mercy if you wish to sell me a lamp," he went on, with a smile and a shrug of his shoulders, not unlike that of Lieutenant Secor.

"Hasn't your friend a spare light?" asked Joe quickly.

"My friend?" repeated the German, as though surprised. "You mean——?"

"I mean Lieutenant Secor."

"Oh, him!" and again came the deprecatory shrug of the shoulders. "He is an acquaintance, not a friend. Besides, he has but one lamp, and he needs that. So, also, will you need yours. But as there are three of you together, I thought perhaps——"

"We each have a light," said Blake, interrupting the rather rapid talk of Labenstein. "In fact, I have two, and I'll let you take one."

"That is very kind of you. Ah, it is like mine!"

The visitor was watching Blake eagerly as he brought forth one of the flat, three-cell nickel-plated holders of tiny batteries, with the white-backed and tungsten-filamented incandescent light set in a depressed socket.

"Yes, this is the best type," Blake said. "You may have this."

"And the price?" asked Labenstein, as his hand quickly went into his pocket.

"Is nothing," answered Blake. "It is a gift."

"Ah, but, my dear sir, that is too much! I could not think of taking it without pay!" insisted Mr. Labenstein, as he flashed on the light and then slipped the switch back in place again. "I protest that I must pay you."

"Please don't insist on paying," begged Blake, "for I shall only have to refuse to take any money. Please consider the light a gift. I have a spare one."

"You are very kind, I'm sure," said the other, bowing with some exaggeration, it seemed to the boys. "I appreciate it, I assure you, and I shall look for a chance to repay the favor."

"That's all right," said Blake, and he tried to make his voice sound hearty. "You are welcome to the light."

"A thousand thanks," murmured Mr. Labenstein, as he bowed himself out.

And then, when the door had closed on him and they had taken the precaution of closing their transom, Joe burst out in a cautious whisper with:

"What in the world did you let him take it for, Blake Stewart? Don't you see what his game is?"

"Yes," was Blake's quiet answer; "I think I do."

"Well, then——"

"What is his game?" asked Charlie.

"I presume he wants to use the flash lamp to give a signal at night to some German submarine," said Blake quietly—very quietly, under the circumstances, it might seem.

"And you let him take a light for that?" cried Joe.

"Wait a bit!" advised Blake, and he smiled at his chum. "Do you know anything about these flashlights, Joe?"

"A little—yes. I know a powerful one, like that you gave Labenstein, can be seen a long way on a dark night."

"Well, then maybe you know something else about them, or you may have forgotten it. Like the proverb which says 'blessings brighten as they vanish,' so the light of these lamps sometimes glows very strong just before the battery goes on the blink and douses the glim."

Joe looked at his chum for a moment, uncomprehendingly, and then a smile came over his face.

"Do you mean you gave him a light with a battery in it that was almost played out?" he asked.

"Exactly," answered Blake, with another smile. "This is a light I have had for some time. I noticed, only last night, that it was brighter than usual. Just as a fountain pen—at least, the old-fashioned kind—used to flow more freely when there were only a few drops of ink left, so this battery seems to be strongest just before it gives out altogether.

"I suspected this was going to happen, but I tested the battery with a galvanometer to-day and I found out it has about ten flashes left. After that the light will be dead."

"Is that why you gave it to him?" asked Charlie.

"The very reason. As soon as he asked for a light it occurred to me that he wanted to use it—or might use it—to give a signal at night to some watching submarine commander waiting for a chance to torpedo us. I thought if I let him do it with this failing light he might do the Huns more damage than he could us."

"How?" asked Joe.

"By not being able to give the proper signals. He'll need to flash a light for some little 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.

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.