“Why, Jack told me, sir,” replied Frank.

“And how did you know it?” demanded Lord Hastings of Jack.

“I heard you and Mr. Churchill discussing it, sir,” replied Jack.

Lord Hastings drew a long breath, evidently of relief.

“I didn’t know we had been so indiscreet,” he said, half to himself.  “However, there is no harm done, for I know you boys are to be trusted not to repeat what you overhear.  I’ll tell you this, you two are among the very few who know that any of the German submarines have been accounted for.”

“Then it is true?” asked Frank.

“Oh, it’s true enough,” replied Lord Hastings.  “Perhaps a hundred of them have disappeared.”

“And where are they, sir?” asked Frank.  “At the bottom?”

“That,” said Lord Hastings with a slow smile, “is the mystery the German government would like to solve.”

“But surely you know, sir.”

“If I did, I would not repeat it within these four walls,” declared Lord Hastings.  “Walls have ears, you know, as is proven by the fact that Jack overheard my conversation with Mr. Churchill.”

“I didn’t mean to listen, sir,” interrupted Jack.

“Oh, I know that,” replied Lord Hastings.  “But now take my advice, and keep what you know locked close within you.”

“We shall, sir,” replied both lads.

“Good!  Now I have a piece of news for you.”

The two lads stepped forward eagerly.

“Are we to go on active service again, sir?” asked Frank anxiously.

“It’s about time we did,” mumbled Jack, half to himself.

Lord Hastings smiled as he saw the eager looks upon the faces of both.

“Well, we have a little work cut out for us,” he replied quietly.

“Hooray!” cried Frank.

A pleased expression fluttered across Jack’s face, but he gave voice to no exclamation; he was never as effusive as his chum.

“I’m glad you’re pleased,” returned Lord Hastings.  “Yes, we shall see active service, at once.”

“When do we start, sir?” asked Frank, his face shining.

“In the morning.”

Frank’s face fell.

“I was in hopes it was to-night,” he replied.

“Scouting, submarine or what?” demanded Jack.

“You will have to wait for an answer to that question,” said Lord Hastings.  “In the meantime, it would be well this afternoon to get whatever equipment you may need.  Your other things, together with mine, are at the bottom of the sea with the old D-16.”

“And perhaps,” said Frank slyly, glancing at Lord Hastings, “before our present work is over we may know something of the mystery”—­he lowered his voice—­“of the vanishing submarines.”

Lord Hastings eyed him somewhat coldly.

“Perhaps,” he said, and, turning on his heel, left the room.

“You shouldn’t have said that, Frank,” declared Jack, when they were left alone.  “You remember what he said about the walls having ears.”