Without wasting an instant, when he saw the silhouette of the spectre in the cloud, Don fired the Verey pistol set at the side of his airplane.

Arranged for the discharge of the Verey lights, the implement, fixed at one side of the fuselage, sent out into the air a bright, white flash.

The smoke bomb that Don used was such as pilots employ to show them wind direction. The light was almost instantly gone, being succeeded by the liberation of a dense volume of smoke that drifted in the light Summer breeze. But Don was not concerned with the smoke: he knew that watchful eyes had been ready to catch the flash, through the dark.

“They know, in the control room, that I got what I came for,” he told himself. “Now they’ll shut off the light and get everything put away before the control man returns from his late supper.”

With quick hands he set the controls to swing back, and made the return trip in as brief a space of time as the Dart’s power permitted.

At the runway, as he came to rest, Chick ran up.

“We got your flash!” he said, keeping his voice low. “Garry’s putting back the things. Let’s get the Dart back. You’ll have to explain the flight to the control man. He must have heard the take-off and landing.”

“Right. Well, Chick, one thing is settled, anyhow.”

“One thing? You mean——”

Don, unsnapping his helmet chin strap, put his lips close to Chick’s ear and spoke very earnestly.

“The spectre appeared. But!—it wasn’t from your projector!”

If he expected a cry of surprise, as his grip on Chick’s arm for a warning seemed to indicate, he was, himself, surprised.

“We know it,” Chick, to Don’s amazement remarked.

“You do? Then you saw it?”

“No, Don. Come on. Let’s not talk till we get back to Garry.”

He led the way to the stairs, and instead of going on to the control tower, turned aside at the door to the lower corridor.

“Let’s go into the designing department,” he suggested. “As soon as the control man returns, Garry will meet us there.”

“Just where I meant to go.”

Together they entered the room, lighting its dome bulbs.

“As soon as I saw the picture on the cloud,” Don stated, “I knew it didn’t come from the control room.”

“How did you find that out!”

Don, at the table, took pencil and paper.

While he sketched rapidly Garry entered. Chick put him in possession of Don’s news.

Watching, Garry nodded.

“Don knew, from the light angle, I guess,” he whispered.

The sketch Don made was proof of his accuracy of judgment.

It showed a small airplane, as though viewed from above. Its nose was directed toward a sketchy line that indicated the shore of the bay.

A little in front of its nose Don had made a small indication of a cloud. On that he put a straight line, that the others saw was meant to represent the “screen,” or place where the picture had been seen.

And the airport control room when he sketched it in, lay at exact right angles to that screen line!

“As the nose pointed West,” Don said, excitedly, “the light from your projector, coming from the South, would have been on the South part of the cloud. But the picture was on the East side, the one I faced. That’s how I knew you didn’t throw the picture. Besides, as I saw earlier, the diffused light from your beam, as it touched a cloud before the picture appeared, was very faint!”

“Q. E. D.” Chick quoted his school algebraic phrase.

“But if I saw the picture facing toward the West, how could you see it from the South!” asked Don.

“We didn’t!”

“Then how did you know, Garry? What proved you didn’t project it?”

Garry answered slowly.

“The film we had,” he explained, “started off with a couple of ‘shots’ of airplanes—flying over our swamp. But then it became a series of moving pictures, taken from the air, of water and marsh.”

“And that was all,” Chick added.

“The more things I see,” Don said after a long moment of thought, “the more I begin to think that Indian, John, had the right idea.”

“About Smith—the mail flyer?” Garry asked.

“Yes.”

“We will see a little later,” Chick stated. “His ’plane comes in after awhile.”

“Don’t forget,” Don argued, “that a man clever enough to do all the things we have seen done is bright enough to have somebody else fly his mail close to this airport, set down, and let Smith take it over and bring it in. For money, and with a man far enough away, it would be possible—and we could never check it up.”

“He’s still in that swamp, close by,” Don argued. “He is as brazen as they come, too!” Chick wondered audibly why Don had not flown straight up “to catch the man.”

“Alone?” Garry defended Don from a hint of caution. “Don did the right thing, coming back here. The stores haven’t reported a call for spare carburetor parts. The man is clever.”

“Maybe he got spare parts at Bennett Field, or Roosevelt Field,” Chick suggested.

Don held up a hand and shook his head.

“It isn’t important, just now,” Don declared. “Let’s make sure how the picture was thrown, tonight, while I flew around. Then we can work out why there is this extra projector head and a misfit airplane crash picture afterward, and about the carburetor.”

“Well, if you looked around, you must have seen the crate that the ‘ghost’ used,” Chick inferred.

“But I didn’t.”

They knew that he had not been careless: had a ship been in sight his sharp eyes, looking for just that, would have noted it.

“Listen,” Garry drew up a chair by the table, “Don, your knowledge of angles, and the things you had to study about angle of attack of a wing, and angles of incidence of air and wing, and all that, ought to help out here. This seems to be a question of angles.”

“It does,” Chick agreed. “What’s more, Garry, you’ve studied about light, because I know the control chief gave you some books when he saw that you took an interest in his work.”

“Maybe we can both get something out of what we’ve learned,” Don admitted. “Now—how?”

“Well,” Chick offered an opinion, “the old Indian gave us passes that showed us ‘how’ the ghost could be worked. Maybe there is a clue to ‘where from.’”

“Yes—I think there is!” Don caught a fresh sheet of paper, and began to draw a rough diagram of the theatre stage, sketching in the position of the pillar of smoke broadly.

“From what we’ve proved, about tonight, the stage picture couldn’t have come from the wings,” he stated. “It would have to fall on the smoke from the front, almost, or else the people in side seats might see it and not those in direct line, from in front of it.”

Garry drew the sheet to him, made an addition, showing the projection room of the Palace, up on its balcony.

“The theatre was made very dark,” he said, “and all the light on the stage was adjusted so that the sunset died out when the pillar of smoke went upward. Then the man at the film projectors in the balcony ‘faded in’ the picture—from in front, and at an angle ‘above’ the audience.”

Don jumped up, upsetting his chair in his excitement.

“Knowledge is Power!” he cried, excitedly. “Study of angles has given us the answer to Chick’s ‘where from!’ That shows why there is a helicopter hidden in the marsh!”

“I see it!” Chick was equally animated. “With the helicopter, the ‘ghost’ projector could hover above the clouds, well hidden.”

“Yes, and ‘throw down’ from that makeshift ‘projection room’ onto a cloud,” exclaimed Garry.

“He could hover very high,” Don contributed. “There he could see an airplane, coming, at a distance, gauge its direction, swing his own ship and descend to a point over a cloud. Hidden there, with his light on, and his film going through, the spectre would appear on smoke or clouds right in front of the coming airplane.”

“That’s exactly how, and from where, the ghost comes!” Garry agreed. “Now, here’s a suggestion, Don! Let’s ‘show’ everybody!”

“I don’t quite see—” began the young pilot.

“It’s almost midnight.” Garry consulted his wrist watch. “The Palace has finished the second show. The control chief and the others will all come here to see that everything is right, and for the arrival of the midnight passenger ’plane from the Maine Summer resorts. I’ll stay here and you, Don, and Chick, take the Dart, fly to where the helicopter is, with the projector and film, and when I give a beam signal that they are here, you two, in the helicopter, pick a cloud they can all see, and ‘put on your show.’ The minute that everybody sees how simply it is done, the ghost’s claws will be pulled—no pilot will be afraid and maybe—maybe your uncle will get a whale of a lot of business.”

“Yes!” Chick was enthused. “And Garry can see whose face betrays guilty knowledge, when the actual ‘spook’ is projected.”

“But—” Don saw the difficulties, “this isn’t the same film, hidden in the locker. Besides—where will we get the light?”

“The ‘ghost’ must get light from somewhere—” Chick began.

A flash of inspiration made him bang his palm on the table.

“The boathouse!” he exclaimed. “Don—Garry! I saw the Man who Never Lived come up from under that boathouse. That’s where he stores all his real stuff—light, and film, and maybe another projector, complete! This one is just in case he is suspected—to mix up the trails!”

“I believe Chick has the right idea!” Garry conceded.

“So do I! Come on, Chick! We’ll ‘put on a show’ and clear up the airport mystery of the spectre in the clouds for once and all!”

Again the faithful Dart, with two youthful occupants, took to the air.

And someone, behind the screen at the wash basin in the designing room, smiled, waited until Garry left for the control room, and then strolled nonchalantly back to the cottage where he roomed, and went peacefully to his quarters.

“There won’t be any more need for the ghost,” remarked the quietly smiling person to his shaving mirror. “Tomorrow the boys will be busy getting out of this little experiment—the engineers won’t be working, and it ought to be easy to find the chest that must have been buried when the mud in Crab Channel sucked down the brigantine, Lady O’ Fortune.”

Don and Chick, in the Dart, drove on, full-gun, to help his prophecy come true.