From the bridge came commands to the lookouts stationed in various parts of the French steamer. Orders flashed to the engine room, and the vessel lost way and floated under her momentum. As yet she was shrouded in darkness, the only lights glowing being those actually required to enable persons to see their way about. Below, of course, as long as the incandescents were shaded, they could be turned on, and many passengers, awakened by the concussion and the following sounds, illuminated their staterooms.

The lights that gleamed across the billows came from the convoying destroyers, and signals flashed from one to the other, though the meaning of them the moving picture boys could only guess at.

Immediately following the explosion, which seemed to come from the side of the Jeanne where Labenstein had flashed his signal, the German and the Frenchman had subsided into silence. Each one had given voice to an exclamation in his own tongue and then had hurried away.

And so occupied were Blake and his chums with what had gone on out there on the ocean—trying to guess what had happened—that they did not notice the departure of the two men.

"What's that you said it was?" asked Joe of his partner. "I mean the explosion."

"I think it was a depth charge," answered Blake. "One of the destroyers must have sighted a submarine and let go a bomb, with a heavy charge of explosive, which didn't go off until after it got to a certain depth below the surface. That's the new way of dealing with submarines, you know."

"I only hope they got this one, with a depth charge or any other way," remarked Charles Anderson. "Look, we're lighting up! I guess the danger must be over."

Lights were flashing on the deck of the Jeanne, and signals came from the destroyers. It was evident that messages were being sent to and fro.

And then, as passengers crowded up from their staterooms, some in a state of panic fearing a torpedo had been launched at the ship, another muffled explosion was heard, and in the glare of the searchlights from one of the convoying ships a column of water could be seen spurting up between the French steamer and the war vessel.

"That's caused by a depth charge," Blake announced. "They must be making sure of the submarine."

"If they haven't, we're a good target for her now," said Joe, as he noted the lights agleam on their steamer. "They're taking an awful chance, it seems to me."

"I guess the captain knows what he's doing," stated Blake. "He must have been signaled from the destroyers. We'll try to find out."

An officer went about among the passengers, calming them and telling them there was no danger now.

"But what happened?" asked Blake, and he and his chums waited eagerly for an answer.

"It was a submarine," was the officer's reply. "She came to attack us, trying to slip around or between our convoying ships. But one of the lookouts sighted her and depth charges were fired. The submarine came up, disabled, it seemed, but to make sure another charge was exploded beneath the surface. And that was the end of the Hun!" he cried.

"Good!" exclaimed Blake, and his chums also rejoiced. There was rejoicing, too, among the other passengers, for they had escaped death by almost as narrow a margin as before. Only the sharp lookout kept had saved them—that and the depth charge.

"But how does that depth charge work?" asked Charlie Anderson, when the chums were back in their cabin again, discussing what they had better do in reference to telling the captain of the conduct of Labenstein and Secor.

"It works on the principle that water is incompressible in any and all directions," answered Blake. "That is, pressure exerted on a body of water is transmitted in all directions by the water. Thus, if you push suddenly on top of a column of water the water rises.

"And if you set off an explosive below the surface of water the force goes up, down sidewise and in all directions. In fact, if you explode gun-cotton near a vessel below the surface it does more damage than if set off nearer to her but on the surface. The water transmits the power.

"A depth charge is a bomb timed to go off at a certain depth. If it explodes anywhere near a submarine, it blows in her plates and she is done for. That's what happened this time, I imagine."

And that is exactly what had happened, as nearly as could be told by the observers on the destroyer. The submarine had risen, only to sink disabled with all on board. A few pieces of wreckage and a quantity of oil floated to the surface but that was all.

Once more the Jeanne resumed her way in the midst of the protecting convoys, the value of which had been amply demonstrated. And when all was once more quiet on board, Blake and his chums resumed their talk about what was best to do regarding what they had observed just before the setting off of the depth charge.

"I think we ought to tell the captain," said Charlie.

"So do I," added Joe.

"And I agree with the majority," said Blake. "Captain Merceau shall be informed."

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.

Watchful guard was maintained night and day against an attack by submarines, and though there were several alarms, they turned out to be false. And in due season, the vessel arrived at "an English port," as the papers stated.

"Let's go and see if Captain Merceau wants us to give any evidence against those fellows," suggested Joe; and this seemed a good plan to follow.

"Ah, yes, my American friends!" the commander murmured, as the boys were shown into his cabin. "What can I do for you?"

"We thought we'd see if you wanted us in relation to the arrest of Secor and Labenstein," answered Blake.

"Ah, yes! The two men who signaled the submarine. I have had them under surveillance ever since you made your most startling disclosures. I sent a wireless to the war authorities here to come and place them under arrest as soon as the vessel docked. I have no doubt they are in custody now. I'll send and see."

He dispatched a messenger who, when he returned, held a rapid conversation with the captain in French. It was evident that something unusual had taken place.

The captain grew more excited, and finally, turning to the boys, said in English, which he spoke fluently:

"I regret to tell you there has been a mistake."

"A mistake!" cried Blake.

"Yes. Owing to some error, those men were released before the war authorities could apprehend them. They have gone ashore!"