le of?" asked the
naval officer.

"Yes, sir," responded Rob respectfully; "they can't do another
revolution."

The officer looked anxious.

"In that case, we shall have to resort to my forlorn hope," he said.

"And what is that, sir?" asked Rob, his heart beating uncomfortably
fast.

"Come forward and you shall see."

The ensign turned and swung out of the engine room, followed closely by
two anxious boys, Rob having waited only to shut off the engines.

In the main cabin Mr. Barr, his face white and strained, sat on one of
the leather divans.

He looked up as the boys and the naval officer entered.

"The engines won't back her out?" he asked in a voice harsh and rough
from anxiety.

"No. I'm sorry, Barr, but we're in a mighty bad fix. This submarine
cliff must be of a sort of blue clay formation that is common off this
coast. We have apparently driven into it so far that nothing short of an
earthquake would dislodge us."

"An earthquake?"

"Yes; such a spasm of nature alone can set us free."

"Then we are doomed to remain here."

"Not of necessity; we have still a chance of escape."

"What do you mean?"

"That my plan offers a mere chance."

"Then let us not delay in putting it into execution."

"But it is a dangerous one!"

"Never mind that. Nothing could be more serious than our present
predicament."

"Very well then, we will try out my idea. It's our last chance."

"Our last chance!" The words sounded to the boys almost like a requiem.
Plainly enough, whatever Ensign Hargreaves' plan might be, there were
dangers attached to it, and no light dangers, either, to judge from his
grave tones. Eagerly they awaited his next words.

"My plan is nothing more nor less than this," he said; "I propose to
create an earthquake."

"To _create_ an earthquake!" Mr. Barr echoed the words, staring at the
ensign as if he thought he had gone suddenly insane.

"Precisely. I intend to produce by artificial means an eruption which
will destroy enough of this cliff to set us free, or else blow the
_Peacemaker_ herself into atoms."

Mr. Barr buried his head in his hands. Skillful inventor and scientific
expert though he was, the last words of the naval officer had sapped
even his iron courage.

"Is there no other way?"

"No other way. It's a gamble for our lives."

"What do you propose doing?" asked Mr. Barr in a strange, broken voice.

"As I said, to create an artificial earthquake."

"I am unable t