“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.”