e's rising now," cried Bob.

Without his headpiece, Frank could not hear the words and kept his
eyes to the fore, as he swung now above the line of the shore. Jack,
however, also was straining his eyes to the rear, and he snatched the
glasses from Bob and trained them on the plane.

True enough, Higginbotham was rising.

CHAPTER VII

A CALL FROM HEADQUARTERS

It was not yet five o'clock when, the airplane safely stowed away and
the doors of the hangar closed and locked, the boys once more stood on
the skidway.

"What say to a plunge before we go up to the house?" proposed Frank.
"There's nobody to see us. We can strip down at the beach, splash
around for ten minutes, and then head home. It's a hot, sticky day and
that trip to the city left me with the feeling that I wanted to wash
something away."

The others agreed to the proposal and they started making their way to
the shore, discussing the latest turn of events on the way.

"It certainly looks as if your hunch about Higginbotham, when we met
him in his office, was justified," said Jack, clapping Frank on the
shoulder.

"The boy's a wonder," agreed Bob. Then, more seriously, he added:

"But, I say. Higginbotham isn't the man who flew the radio-controlled
plane before. I mean the fellow whose tracks I found in the sand. That
chap was peg-legged."

"That's right," agreed Jack. "And where does Higginbotham figure in
this matter, anyhow? It's some mystery."

"Well, let's see what we do know so far," suggested Frank. "It's
little enough that we have found out. But I like mysteries. First of
all, Bob finds a secret radio plant, and----"

"No," interrupted Jack. "First of all, I discover interference in the
receivers at a 1,375-meter wave length."

"Yes, that's right," said Frank. "Well, second is Bob's find of the
radio plant to which he is led by tracks in the sand made by a
peg-legged man. Look here. Bob thought at the time that man had
arrived in a boat. He saw marks on the sand indicating a boat had been
pulled up on the shore. Might not that have been the indentation made
by the radio plane?"

"Just what I was thinking to myself a minute ago," said Bob.

"Anyhow," continued Frank, "we then discovered the radio plane in
Starfish Cove. From Uncle George we learned a mysterious stranger had
recently bought the Brownell place, the 'haunted house,' and had
built a fence about the property and set armed guards to keep out
intruders. The plot wa