your precious plan behind when you went to
try the model out."

Bud scratched his tousled head as though endeavoring to collect his
wits.

"Let me see," he started to say, slowly; "don't blame me if I'm all
mixed up over this business. I've waited so long for this time to
come that I'm sure rattled, that's what's the matter. What did I
lay down here? Some parts of the plans for the model, that goes
without saying; but seems like I've got a paper in my pocket right
now. Let's see, I honestly believe that it's the key to the
invention. Just what it is, boys; and without this, nobody'd ever
know what all those markings on the other papers were meant for."

"Then it's all right, after all?" asked Hugh, smiling again when he
saw that suggestive grin on his comrade's face.

"It seems that I can make an aeroplane fool proof, when all the
time the fool killer ought to get hold of me," Bud confessed contritely.
"It was certainly careless of me to leave any part of my precious
papers floating around, and only good luck has saved me from being
left high and dry. But perhaps those fellows will think they've
got hold of some remarkable design. They might crack their brains
trying to make it out, and then not learn a single thing. When
a fellow laughs last, he's going to laugh best, believe me."

"Well, next time be more careful, that's all," advised Ralph. "You
can't always count on being so lucky. An inventor ought to keep his
secrets locked up, and not let them lie around loose. You'll find
that there are plenty of sly thieves ready to steal ideas, as well
as money and jewels. An idea that's worth a fortune can be cribbed
a heap easier than the coin itself."

"Only a part of the morning's gone," announced Hugh. "How shall we
put in the rest of the day?"

"Oh! I'll be pretty busy making repairs to my model," Bud replied.
"You two fellows can find plenty to interest you both. Only don't
roam too far afield in looking up interesting things. With that
mysterious pair hanging out around here, I won't feel any too easy
in my mind. My idea may be worth a cool million, you know."

While Bud was working diligently, the other two amused themselves
first of all by closely examining the ground just before the door of
the cabin.

"Here you can see several imprints of that fine pair of shoes," Ralph
said almost immediately; "and that makes it look as though the man
who wore them really did slip inside while we were