On questioning the girl he would be apt to learn how curious Frank and Andy had seemed about him; and Sallie might even admit that they had asked to see his wonderful collection of rare and costly butterflies.

Well, if such a thing did occur, of course the keen-witted man would immediately know that the cat was out of the bag.  Realizing that there must be a great hue and cry throughout the entire county just then, with reference to the yeggs who had looted the bank, he could easily imagine what had brought these boys here.

Through association with Todd Pemberton, Casper must have learned a whole lot with regard to Frank and his cousin.  Being an aviator himself he would naturally take an immediate interest in boys who had given such a good account of themselves in the field of aeronautics.  The attempt to steal the hydroplane in the first place before they turned to Percy Carberry’s biplane proved that they knew all about the Bird boys.  And so, learning of their presence would immediately give Casper warning that his hideout was no longer a secret, but that the net of the law must be closing around him.

What then?

Would he, like a desperate man, attempt to capture these venturesome lads, so as to keep them from informing the authorities at Bloomsbury?  Either that, or else he would think that, since the game was up, and they could no longer loiter in the neighborhood of the aroused district in order to carry out the second part of the great scheme, they had better take to the aeroplane and vanish from view, leaving no trail behind by means of which they could be followed.

Frank had said all this in his mind when he lay there and waited to see what would turn up.  He felt that they could surely afford to linger for some time, if there was any chance of learning whether the yeggmen meant to change their plans, or proceed to carry out their original scheme.

All seemed quiet at the farmhouse.

Sallie had come out on the porch, and looked rather disappointed to find that the two boys had strangely vanished.  She stood there glancing around in a puzzled manner for several minutes, and then with a pretty shrug of her shoulders, and a pout of her lips whirled about and went back into the house again.

“Wow!” said Andy in a low tone, “she’s got it in for you, Frank, because you dropped out of sight without even so much as saying goodbye.”

But the other was thinking of weightier matters than the humor of a little coquette.  He wondered whether Sallie would run across the professor and ask him if he had met two boys down the lane; which remark would excite his suspicions, and lead to other questions, now on his part.

If nothing happened inside of half an hour.  Frank was of a mind to try the plan that had come to him—­sending Andy off to try and reach some other farm where they would have a telephone; while he himself remained to keep watch.