y, frowning at the same time.

"Nobody else," replied Andy, impressively. "They must have swung
around, passed up to the old mill on that side road, and from the upper
windows have been watching us all the time through the fieldglasses
they carry!"

CHAPTER V

FIGURING IT ALL OUT

"It begins to look as though you were right, Andy, and that these
strangers certainly feel an uncommon interest in what we've been doing
up here," said Frank, seriously.

"Oh! I don't take much of the credit for hitting on that idea, Frank,"
declared the other Bird boy, quickly. "You kept watching that Marsh
right from the start. I could see a question in your eye every time
you looked at him. And it spurred me on to keeping closer tab over his
ways."

"Are they still up there, d'ye think?" queried Frank; while Larry,
Elephant, and Stuttering Nat hung around, saying nothing, but listening
for all they were worth.

"No," replied Andy; "I've got an idea they began to suspect some of you
were looking that way. Anyhow, I saw Marsh duck his head, and think
they came down. No use going in to take a shy at 'em now."

"I'd give a fit to know what they are up to?" mused Frank, a thoughtful
look on his face.

"Well, perhaps we can hit somewhere near the facts if we start
guessing," remarked Andy, with a knowing nod.

"Look here, you've been turning it over your mind, then?" asked his
cousin.

"Sure I have," grinned Andy, promptly. "Never could bear to let
anything puzzle me long. Used to lie awake half the night trying to
clinch a name that had just slipped a cog in my memory."

"All right. Suppose you give us the benefit of what you decided might
be the answer to this problem. Who are these two men, Andy?"

"You know they admit being well up in aviation?" the other remarked as
a preliminary.

"So Larry and Elephant said," Frank replied.

"And that not only had they attended many meets but admitted being well
acquainted with a lot of people whose names we see in the papers every
day--men who have done things along the line of aviation. Get that,
Frank?"

"I have. Now go on with your answer," nodded the other, encouragingly.

"These gentlemen have been sent up here for a purpose! Perhaps they
are in the pay of some unscrupulous manufacturer of aeroplanes, who
would not be above stealing the ideas of two boys, and applying them to
his up-to-date machines, placed on the market, and for sale to the
public!"

"Go