“What craft is that?” inquired Dick, looking toward a sailboat that was moving lazily along about a half-mile to the eastward.

“I don’t know,” Tom answered, after a look.  “Never saw the boat before.  Regular cabin cruiser, isn’t she, about forty feet long?”

“About that,” nodded Dick.  “What interested me in her was the fact that a fellow on board has been watching us with a marine glass.  I caught the glint of the sun on the lenses.”

“Why should he want to be watching us?” demanded Hazelton.

“That’s just what made me curious,” replied Prescott.  “As an army officer, if this were a fort that I commanded in troublous times, I’d want to look into any strange craft that I caught cruising lazily in the offing and holding a marine glass on us.”

“I wonder if that boat can be in the service of those who are annoying us?” Tom muttered.

“It’s an even chance that it is a ‘hostile ship,’” Prescott suggested.  “You have a motor boat here.  I’m inclined to think you ought to use it in overhauling that suspicious craft.  Of course you’d have no right unless there was a police officer along.  Can you get one?”

“The authorities in Blixton would send a policeman on request.”

“Then send a messenger to request them to send over a policeman in citizen’s clothes,” proposed Dick.

Tom promptly despatched Foreman Dill on that errand.

“Now don’t let the men on the boat see that you’re paying any more attention,” Prescott advised.  “Leave it to me, and I’ll contrive to keep the boat and its people under observation without looking too plainly in their direction.”

In due time the plain clothes policeman arrived.  He, the young engineers and the army lieutenant boarded the “Morton,” which put out from the landing as though on a trip of inspection of the wall.

“Don’t anyone look over at the sloop,” Prescott urged.  “I’ll do the watching.  A fellow on that craft is holding the glasses on us right now.  Officer, do you demand the assistance of all present in any police duty that may come up?”

“I do,” replied the Blixton policeman, a man named Carnes, returning Prescott’s wink.

“All right, then,” laughed Dick.  “That demand makes policemen of us all.  Tom, you can turn, now, when ready, and put on full speed in going after that craft.”

Reade gave the order for full speed, then took the steering wheel himself.

“Guilty conscience!” laughed Prescott.  “There’s the sloop putting about at once and heading away from us.”

“They can’t get away from us, in this light wind,” chuckled the young chief engineer.

A few minutes later the “Morton” came up within easy hailing distance of the sloop, aboard which only one man now appeared.

“Sloop ahoy!” called the policeman.  “What are you doing in these waters?”

“Looking for a good fishing ground,” answered the dark-faced man at the tiller.