le, and
descending into the water with a vicious plunge.

No sooner had it struck the sea than it seemed to raise the surface of
the water like the foaming mass in a boiling pot. The explosion was
dull, vibrant, ominous.

"They are shooting another one," shouted Alfred, although he tried to
suppress his voice.

"Boom!" came the sound, as he uttered the words.

The second shot struck the water not fifty feet distant from the first
one.

"Do you think they will fire another?" asked Alfred.

"Probably not," answered the captain.

"What is that little boat going over there for?" asked Ralph, as one of
the torpedo boats boldly advanced over the spot where the two shells had
entered the water.

The captain nodded his head for a few moments before speaking.

"The shots were successful."

"I can see that now," said Ralph. "Look at the oil coming up and
covering the sea."

It was, indeed, a sad sight to witness, knowing that the shots meant the
death of thirty or more human beings.

"Well, I am awfully sorry for them, even if they had no sympathy for us,
and didn't wait to see whether or not we were put into safety before
they sent our ship down," said Alfred reflectively, as he turned and
entered the conning tower.

The scene had its fascination for Ralph, although he felt the horror of
it all as he stood leaning over the railing, gazing at the patrol boats
which were sailing back and forth in and around the spot where the
petroleum was fast covering the surface of the water in all directions.

"You can understand now, can't you, why flying machines are such good
spotters for submarines?" remarked the captain.

"Do you mean the oil that comes on top of the water?" asked Ralph.

"Yes," was the reply.

"But does oil arise at all times when a submarine is submerged?" asked
Ralph.

"More or less oil is constantly detaching itself from the body of the
hull, at the discharge ports, and it can't be helped because all of the
gas discharge ports are under water at all times, whether the vessel is
running on or under the water, hence, as it moves along it will leave a
trail of oil which can be easily detected by a machine in flight above
the surface of the water," said the captain.

"But doesn't a machine, when it is under the water, leave a ripple that
is easily seen by a flying machine?" asked Ralph.

"Yes; I was going to refer to that," replied the captain. "An aviator
has a great advantage over an observer