The commander was greatly astonished when told what the boys had seen.  He questioned them at length, and made sure there could have been no mistake.

“And they gave a signal,” mused the captain.  “It hardly seems possible!”

“It was Labenstein who actually flashed the light,” said Blake.  “Do you know anything about him, Captain Merceau?”

“Nothing more than that his papers, passport, and so on are in proper shape.  He is a citizen of your own country, and appeared to be all right, or he would not have been permitted to take passage with us.  I am astounded!”

“What about the Frenchman?” asked Joe.

“Him I know,” declared the captain.  “Not well, but enough to say that I would have ventured everything on his honor.  It does not seem possible that he can be a traitor!”

“And yet we saw him with the German while Labenstein was signaling the submarine,” added Blake.

“Yes, I suppose it must be so.  I am sorry!  It is a blot on the fair name of France that one of her sons should so act!  But we must be careful.  It is not absolute proof, yet.  They could claim that they were only on deck to smoke, or something like that.  To insure punishment, we must have absolute proof.  I thank you young gentlemen.  From now on these two shall be under strict surveillance, and when we reach England I shall inform the authorities.  You have done your duty.  I will now be responsible for these men.”

“That relieves us,” said Blake.  “We shan’t stay in England long ourselves, so if you want our testimony you’d better arrange to have it taken soon after we land.”

“I shall; and thank you!  This is terrible!”

The boys realized that, as the captain had said, adequate proof would be required to cause the arrest and conviction of the two plotters.  While it was morally certain that they had tried to bring about the successful attack on the French steamer, a court would want undisputed evidence to pronounce sentence, whether of death or imprisonment.

“I guess we’ll have to leave it with the captain,” decided Blake.  “We can tell of his borrowing the light, and that we saw him flash it.  Of course he can say we saw only his lighted cigarette, or something like that, and where would we be?”

“But there was the signal with the white cloth,” added Joe.

“Yes, we could tell that, too; but it isn’t positive.”

“And there was Secor’s running into me and spoiling our other films,” said Charlie.

“That, too, would hardly be enough,” went on Blake.  “What the authorities will have to do will be to search the baggage of these fellows, and see if there is anything incriminating among their papers.  We can’t do that, so we’ll have to wait.”

And wait they did.  In spite of what Captain Merceau had said, the boys did not relax their vigilance, but though, to their minds, the two men acted suspiciously, there was nothing definite that could be fastened on them.