naval situation was practically at a standstill,
where indications pointed to its remaining until the main German fleet,
bottled up in Heligoland, and the main Austrian fleet in the Adriatic
should summon sufficient courage to sally forth and give battle; and
there had been nothing to indicate any sudden action on the part of
either.

On several occasions British submarines had penetrated the mine fields
and created considerable havoc, and aircraft had dropped bombs from the
air. But along these lines the German submarines had been more
successful and now were the one real menace confronting the naval
supremacy of the Entente powers.

Hundreds of ships, large and small, had fallen easy prey to these
under-sea terrors. Big ocean liners, crowded with passengers,
non-combatants, had been sent to the bottom with terrible loss of
innocent lives. Chief among these tragedies laid to the door of the
German submarines was the sinking of the Cunard liner _Lusitania_, in
which more than a thousand men, women and children had been drowned.

And, so far as the British public knew, England had taken no steps to
combat this under-sea peril. However, as Lord Hastings had told the boys
at the opening of this story, Great Britain had taken such steps, and
that they were effective was evident from his additional statement that
in the neighborhood of a hundred submarines had "vanished."

But this warfare was not to end until the submarine evil had been
eradicated. The German under-sea craft must be disposed of so
effectively as to preclude further danger to British shipping. And it
was in this work that Jack and Frank were soon to play a prominent part.

CHAPTER II.

ON ACTIVE SERVICE AGAIN.

For some reason unknown to Jack and Frank, when morning came, Lord
Hastings announced that the start would not be made until after
nightfall, at which both lads showed keen disappointment.

"I'll tell you what you can do," said Lord Hastings. "I'll give you an
order for my motorboat and you can go to Gravesend during the day if you
care to. I'll meet you there at the Lion Inn to-night at 10 o'clock."

Frank was delighted.

"That's better than hanging around here all day, waiting for night to
come," he said. "What do you say, Jack?"

"Anything to humor you," replied the latter with a smile.

"Take all your things with you," said Lord Hastings; "and, above all,
hang on to that motorboat. Don't let anybody get it away from you."