They heard the strange, hollow sound again, seeming to come from the metal wall, but impossible to locate at once because of the echo.
Rap—tap—tap!
“Somebody’s knocking,” Dick gasped.
“Not somebody—something!” corrected Sandy. “The same ‘something’ that worked the door and shut it!”
“Gracious-to-gravy!” exclaimed Larry, “you don’t believe in ghosts, do you, Sandy? Not really!”
“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!”