But even now, in the security of their hiding places, the Germans were meditating a bold stroke.  Submarines were being coaled and victualed in preparation for a dash across the Atlantic.  Already, one enemy submarine—­a merchantman—­had passed the allied ships blocking the English channel and had crossed to America and returned.  Some months later, a U-Boat of the war type had followed suit.  A cordon of ally ships had been thrown around American ports to snare this venturesome submarine on its return, but it had eluded them and returned safely to its home port.

But soon—­very soon, indeed—­German undersea craft were to strike a more severe blow at allied shipping, carrying, for the moment, the war in all its horrors to the very door of America.  While the United States was arming and equipping its millions to send across the sea to destroy the kaiser and German militarism, these enemy undersea craft were crossing the Atlantic determined to reap a rich harvest upon American, allied and neutral shipping off the American coast.

And the blow was to be delivered without warning—­almost.

When the U.S.S.  Plymouth, under Jack’s command, returned to Liverpool, the captain of the vessel, having somewhat recovered, came aboard and relieved Jack of command.

“I’m obliged for your services, Captain,” he said, “but I’ll take charge of the old scow again myself, with your leave.”

Jack and Frank went ashore, where, at their hotel, they received a brief telegram from Lord Hastings.  It read as follows: 

    “Return to Dover at once.  Important.”

“Now I wonder what is up,” said Frank after reading the message.

“The simplest way to find out,” replied Jack, “is to go and see.”

CHAPTER III

OFF FOR AMERICA

“Then everything went first rate your first trip, Captain?” questioned Lord Hastings.

“First rate, sir,” Jack replied.

The lads were back in Dover where, the first thing after their arrival, they sought an audience with their former commander.

“Yes, sir,” Frank agreed, “Jack makes an A-1 captain.”

“I’m glad to hear it,” was Lord Hastings’ comment.  “I’ve other work in hand and I wouldn’t want to trust it to a man who is nervous under fire.”

“But we were not under fire this time, sir,” said Jack.

“You mustn’t always take me literally, Jack,” smiled Lord Hastings.  “It was your first venture in your present rank and you acquitted yourself creditably.  That is what I meant.”

“And what is the other venture, sir?” Frank asked eagerly.

“There you go again, Frank,” said Lord Hastings.  “How many times have I told you that you must restrain your impatience.”

Frank was abashed.

“Your warnings don’t seem to do much good, I’ll admit, sir.  Nevertheless, I’ll try to do better.”