“There is nothing more to be done here,” said Lord Hastings at this juncture.  “We’ll get back to our boat.”

Accordingly they took their leave of the house, and half an hour later were again moving down the waters of the Thames.

“Where to now, sir?” asked Frank.

Lord Hastings smiled slightly.

“Can’t wait until you find out, eh?” he replied.  “Well, I’ll tell you.  We are about to do a little submarine chasing.”

“Submarine chasing?” exclaimed both lads.

“Exactly.”

“Good!” ejaculated Frank.  “And where is our vessel, sir?”

“If you will look about a little you will see it,” replied Lord Hastings.

Frank and Jack let their eyes roam over the broad expanse of the Thames, but they could see nothing but a few small boats of various sorts—­nothing bearing the slightest resemblance to a ship of war.

“I don’t see anything that looks like it, sir,” declared the boy.

“That,” said Lord Hastings, “is because you insist on looking too far away.  You don’t see it for the reason that you are sitting in it right now.”

“What, sir?” exclaimed Frank.  “Sitting in it now?  You mean we are going submarine chasing in this motorboat?”

“Exactly,” replied Lord Hastings.

“But, sir——­” began Jack.

“You’ll find,” interrupted Lord Hastings, “that for submarine warfare there is nothing to equal the motorboat—­particularly a swift motorboat such as this; and we are now on our way to join the fleet.”

“Fleet?  Fleet of what?” asked Jack, with some sarcasm.  “Fleet of motorboats, perhaps?”

“Precisely,” said Lord Hastings with a smile, and added:  “You don’t seem to think much of the idea.”

“No, I don’t, sir,” was the reply.  “I was in hopes that we were to feel a real vessel beneath our feet once more.  What good is a motorboat against a submarine, anyway?”

“That’s what I would like to know,” agreed Frank.

“I’ll tell you,” replied Lord Hastings.  “But first let me ask you something.  Do you remember, the other day, of asking me to explain the mystery of the vanishing submarines?”

“Yes, sir,” replied both lads.

“Very well.  The solution of this mystery is, primarily, motorboats.”

“What do you mean, sir?” exclaimed Jack.

“Just what I say.  In the main, the possible hundred German submarines that have disappeared recently have been accounted for by high-speed, powerfully armed motorboats.  The government has discovered, after much experimenting, that the one craft with an advantage over a submarine is a powerful motorboat; and England now has a fleet of several hundred scouring the seas in the proximity of the British Isles.”

“But I can’t see where they would do any good,” said Jack.

“In the first place,” said Lord Hastings, “they are so small that they escape the notice of a submarine until the motorboat is almost upon them; and then it is too late for them to act.  Also, the motorboat, being small, is a much more difficult object to hit with a torpedo—­it is, in fact, a very poor target.  Then again, a motorboat is so much swifter than a submarine that the advantage is all with the motorboat.”