“No human hand touched the switch that ran that door down!”

“I think it did!” challenged Larry. “We thought we saw somebody at the back of the hangar—that’s why we came in! I’m going to see where he is, what he’s doing and why he’s trying to fright—frighten us!”

He broke his sentence in the middle of a word because the queer knocking repeated itself, but with quick presence of mind he completed his phrase to steady Sandy, whose face was growing drawn with dismay.

Larry took a swift, sharp look around the enclosure.

“There’s a big, closed can for waste and oily rags,” he commented, “but anyone would suffocate who hid in that!”

“Well, there’s a clothes cupboard—in the back corner,” Dick said. “Let’s look in that, you and I. Sandy, you stay back and keep watch.” Dick, quick to see Larry’s attitude toward Sandy, wanted to have a dependable chum at his side as he investigated while he hoped to give Sandy more confidence by leaving him in the lighted part of the building, under the smudged, dusty skylight.

“Come on!” agreed Larry.

With Dick he walked boldly enough to the built-in wooden cupboard, protected from dust by a heavy burlap hanging.

Throwing the curtain aside sharply, both youths peered in.

“Nothing but old overalls and some tools on the floor,” Dick commented.

“It’s peculiar,” Larry said doubtfully. “Nobody here—but—” a new idea struck him. Quietly he gestured toward the amphibian, old, uncared for, looking almost ready to fall apart, its doped wings stained with mould, its pontoons looking as if the fabric was rotting on them.

Dick, instantly catching Larry’s notion, went to the forward seat, while Larry took the second compartment behind the big fuel tank.

“Nobody here,” he reported, and investigated, by climbing in the vacant part of the fuselage toward the tail.

“This place is empty, too,” Dick agreed. “Where could?——”

“Oh!”—Sandy almost screamed the word as the dull, hollow knocks came again.

Larry leaped from the wing-step, sent his sharp gaze rapidly around the enclosure and, of a sudden, gripped Dick’s arm so tightly that the plump youth winced 

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