For a long minute Dick, Larry and Sandy stood in a compact group, feeling rather stunned by the sudden springing of the trap, as they considered the closed hangar.

Larry, calm and cool in an emergency, was first to recover.

“Even if Jeff did want to catch us and demand ransom to let us go,” he remarked quietly, “he wasn’t outside that rolling door—and I don’t think he could pull it down anyhow.”

“No,” Dick agreed, seeing no fun in the situation for once. “See! There is a motor connected to a big drum up in the top of the hangar, and the door is counterbalanced so that turning the drum winds up the cable that pulls it up. I suppose the motor reverses to run it down and——”

“What was that?”

Sandy’s voice was tense and strained.
14

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!”
15

“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.
16

“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 and grew chilly with apprehension.

At once he saw Larry’s amazed, relieved expression and followed the older comrade’s eyes.

With an instant return of his old amused self he threw back his head and let out a deep howl of delight.

“Oh—ho-ho-ho-ha-ha! Oh, my!—ho-ho——”

“What’s the matter with you?” demanded Sandy. “Have you gone silly?”

“Oh—ho-ho! Suspicious Sandy!—ho-ho!”

Larry explained.

“You got us all worked up and worried,” he told Sandy, “with your suspicions. And all the time——”

“Ho-ho-ha-ha! All the t-time, we were like mice racing around a treadmill.” Dick had to speak between chuckles. “All the time we ran around in circles so fast we didn’t see the end of the cage. Sus—suspicious Sandy! Thinking we would be trapped and held for ransom! Ho, golly-me! Look around you, Sandy!”
17

Sandy looked.

His face slowly changed, gradually became red.

“Oh!” His voice was sheepish. “You mean the switch for the motor over by that small metal door they use when they don’t want to run up the big one?”

“That runs the motor,” Larry agreed. “The cable must have slipped on the drum and let the door go down——”

“But,” Sandy clung obstinately to his theories, “why did Jeff pick this haunted place and cut the ignition—and why was the door up in the first place?”

“What do we—ho-ho—care?” Dick chuckled. “Another thing—even if the electric current is off and the motor doesn’t work—look at that small, hinged door—do you see that the knob of the spring lock—is on—our—side!” He broke out in a fresh cackle of laughter.

“But—those raps——”

For reply Larry strode over to the metal door set in the wall for use when anyone chose to enter or leave the hangar.

Throwing it open, he faced Jeff.
18

“Took you long enough to answer!” grumbled Jeff. “What made you fool with that door and shut yourselves in?”

“What made you cut the ignition!” snapped Sandy, working on the idea he had read in so many detective stories that a surprise attack often caused a person to be so startled as to reveal facts.

Larry and Dick turned their eyes to Jeff.

The older pilot, staring at his accuser for an instant, as though hesitating about some sharp response, suddenly began to chuckle.

“That-there is one on me!” he admitted. “You must have mighty quick eyes.”

“I don’t miss much!” Sandy said meaningly.

“None of us do!” Dick caught the spirit of Sandy’s accusing manner. “I know you’ve been here before, too. There are lots of chunks of old chewing gum stuck around in that front compartment of the amphibian—and someone has been working on it, too. I saw the signs.”

“Chewing gum?” Jeff was startled. Swiftly he strode across the dimly sunlit floor, got onto the forward step, peered into the cockpit.

“That-there certainly is queer,” he commented. “You’re right. Gum is stuck every place, wads of it.”
19

“And you chew gum!” snapped Sandy, unwilling to be left out of the suddenly developing “third degree” he had begun. Jeff made a further inspection, touched a bit of the dried gum curiously, stepped down and stood with a thoughtful face for a moment.

Presently he walked to an old soap box holding metal odds and ends, washers, bolts and so on. This he up-ended. He sat down, his lean jaws working as he chewed his own gum slowly. Around him, like three detectives watching the effect of a surprise accusation, stood the chums.

Presently Jeff looked up at them.

“Looks bad, this-here, don’t it?” He grinned.

Dick, Larry and Sandy were silent.

“I guess I better explain,” Jeff decided. “I didn’t think you was so suspicious and quick or I’d of done different.”

“You can’t trap us!” challenged Sandy.

“Trap you?——”

“Well, didn’t you make friends with us and let us work on your crate and help get passengers that you never took up? Didn’t you say you’d give us a joy-ride, then come straight here, cut out your ignition and make believe you had a dead stick, land and then try to get us into this haunted hangar?” Sandy ran out of breath and stopped.

“I do think you ought to explain!” Larry said quietly.
20

“Yes, I did all that—and I guess I will explain. I meant to, anyhow—or I wouldn’t have brought you here.”

They waited, neither convinced nor satisfied.

Fixing accusing eyes on Sandy, Jeff spoke:

“I never dreamed you’d be suspicious of me! I made friends with you all and tried you out to be sure you were dependable and honest and all that—and I did bring you to this place because it is so far from telephones and railroads. But I didn’t think you’d get the wrong idea. I only wanted you in a place it would take time to get away from if you refused to help me.”

“Help you—help you with what?”

Speaking seriously, Jeff replied to Larry’s challenge.

“Help me save the most valuable set of emeralds in the world from being—destroyed!”