“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.”