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.”

“We’ll hang on to it, never fear,” replied Frank.  “Come on, Jack.”

“Wait a minute,” ordered Lord Hastings.  “You’ll need this written order to get the boat.”

“I’d forgotten, sir,” said Frank.

Lord Hastings scribbled rapidly on a piece of paper, which he passed to Jack.

“This will fix you up,” he said.  “Now remember, 10 o’clock sharp.”

The boys nodded their understanding of this order, saluted and left.  Getting their things together, they hurried to the river, where Lord Hastings kept his motorboat; and an hour and a half later they were proceeding slowly down the river.

“Guess none of the enemy will ever get in here,” declared Frank, after a careful survey of the river.