“And Cadger saw it all, did he?” asked practical Frank.

“A good lot of it, by twisting his head from time to time,” replied Chief Waller.  “And after the thing had been successfully done, he could watch the two thieves gathering the swag together, and putting it in a satchel they found in the cashier’s room.  Then, just at a quarter to three they doused the glim, which was only an electric torch one of them carried, and skipped out, locking the door on poor Cadger.  It was hours afterwards when the day watchman came on duty and the discovery followed.”

Frank and Andy had somehow turned, and exchanged a significant look about this time; and the expression of astonishment on the face of the latter deepened.

“Did you say an electric torch, Chief?” demanded Frank, immediately.

“Yes, one of the handy kind that are used so commonly now,” the other replied.

“Tell us, did Cadger say anything about the thieves wearing masks over their faces; or did they use handkerchiefs to hide them from him?”

“I didn’t mention that matter, but it was just as you say, Frank; both men had on masks all the time,” answered the police officer.

“Black ones too, I expect?” ventured Andy.

“That’s what they were; but see here, are you two just guessing this, or do you happen to know something about those men?” asked the other, quickly; for he could not help seeing from the manner of the Bird boys that they were on some sort of a scent; and he knew from past experiences that their sagacity could always be trusted to do the right thing.

“Well,” Frank went on to say, drily, “while Andy was watching our new hydroplane out in the Quackenboss pasture, I worked until eleven o’clock in our shop, and then went home.  This morning, early, after a bite to eat, I hurried over there to do some finishing touches and carry the thing out to apply to our broken plane, when to my astonishment I found that the shop had been broken into later in the night, as well as our hangar, where the aeroplane is usually kept.  And here’s what I discovered lying on the work-bench, where the men had forgotten them.”

With these words he held up the flashlight torch, and the twin black masks; and they produced an immediate shock upon the Chief of Police.

“And you found those things in your workshop this morning, you say?” he cried, reaching out to take hold of the torch, and the bits of black muslin.

“Yes, and whoever was there, they must have been mad because they didn’t find the aeroplane, for they smashed a few things, just for spite, it looked like,” was what Frank added.

“Then, if it was the same men who robbed the bank they must have known about you boys having a brand new machine.  And say, that must mean one of the robbers was something of a birdman himself; because no greenhorn would ever think of making his getaway in an aeroplane.  Don’t you see that’s a pretty good clue, Frank?  I’ll remember that when I’m getting in touch with other points, and find out if there’s any aviator who’s gone crooked of late.  Yes, that’s worth knowing, now; and I’m glad you mentioned it to me.”