“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 away.  Chances are, he wonders what boys wearing khaki suits like the regular soldiers of the army are up here for.  You agree with me, don’t you, Hugh?”

“It looks that way,” admitted the other.  “Here you see one of the impressions has stopped short only a few inches from the door, as if the man stood here listening before stepping in, after carefully removing the piece of rope we fixed to hold the door shut from the outside.”

“But how do you know that that impression wasn’t made twenty-four hours ago?” asked Bud, who was sitting cross-legged close by and listening to their talk, even while he worked at his broken wing tip.

“Nothing could be easier,” replied Hugh.  “Here’s the foot mark I purposely made when I came out of the cabin last, and you can see that he stepped into the same place.  That tells us he was here afterward.  Get that, don’t you, Bud?”

“I’d be a donkey if I didn’t, and not worthy of being called a scout,” the other boy remarked with scathing emphasis.  “Fact is, if my mind wasn’t so much wrapped up with this aeroplane stability device, I couldn’t have missed seeing that little trick myself if I’d looked the ground over; because that happens to be one of the first things I ever learned about tracking and trailing.”

“Even if the sneak didn’t get anything worth while,” Ralph continued indignantly, “the fact that they seem to be hanging out around here seems to tell that they must have a good reason for it all.  The more I get to thinking about it, the less I feel like saying we’d better let the thing alone because it doesn’t concern us.  When things come to such a pass that unknown persons even sneak into your cabin in your absence and steal what they believe to be valuable papers, it’s high time to take a hand in the business.  And if while we’re wandering around here we happen to run across those two men, I’ll feel like asking what they mean by poking their noses into Bud’s private business.”

“Hear! hear!” said Bud, pretending to clap his hands; “that’s the sort of a chum to stack up with.  Ralph’s the kind to stick to a fellow through thick and thin.  And please inform that taller walking mystery for me, Ralph, that I feel like telling him to his face that he’s a thief.  Will, too, if ever I get half a chance.”