ds that have been mentioned as being
scattered pretty closely in the vicinity. Rob was just in time to catch
sight of a motor boat, seemingly a fast one, slipping behind the spot of
land.

At the same instant a figure rose from the grass almost in front of
them. It was Barton the machinist. He had apparently been concealed in
the grass, and had not calculated on the boys discovering him.

"Well, what are you after now? Spying on me again, eh?" he snarled
angrily.

"I don't know why you should say we were spying on you," rejoined Rob,
"unless you are up to something wrong."

"What do you mean?" asked Barton, stepping quickly toward him as if to
strike him.

The man's hand was upraised, but the determined way in which Rob met his
angry glare caused him to drop it.

[Illustration: ROB WAS JUST IN TIME TO CATCH SIGHT OF A MOTOR BOAT,
SLIPPING BEHIND THE SPOT OF LAND.]

"I want you to quit followin' me around, that's all," he said.

"I guess this island's big enough to hold all of us," snapped Merritt,
"and as for following you around, we have other and better occupations
on our hands."

The machinist made an angry reply and set off across the island at a
rapid pace. As he did so an odd incident took place on the island behind
which the motor boat had vanished.

On a staff which the boys had not previously noticed a red flag began to
glide up and down. Sometimes it was hoisted quickly and then again
slowly.

"What in the world are they doing over there?" wondered Merritt.

"I can't make out unless they are a party of crazy campers amusing
themselves," rejoined Rob; and then he suddenly burst out: "By Jove, I
have it; they are signalling."

"But signalling what or who?"

"I haven't decided yet; but I wonder if the same thought has occurred to
you as to me?"

"Namely, what?"

"Why, that Barton was watching those signals."

"You mean that they were intended for him?"

"That's what."

"But what would they be signalling him for?"

"Only one thing that I can think of. You know what the ensign said about
spies; well, if that fellow Barton isn't up to some crooked work, why
should he sneak off like this and be so anxious to hide from us when we
happen along accidentally? Then, too, there's that paper covered with
drawings that I found in the engine room."

"By ginger, I see what you are driving at. You think that Barton is a
traitor, and is in league with those spies?"

"It's a grave accusation to make