een 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