Shuddering, terrified, Chick clung to Garry’s steady arm as he gazed upward.

One of the clustered clouds seemed to be picked out from the others by a phosphorescent glow: it was luminous but not fiery; whitish in tint rather than ruddy.

Out of it came a silent, gliding, dark shape—an airplane!

For a brief interval Garry felt his own blood chilling. That spectral shape was very much like the mental pictures he had visualized after he had listened to the story of the pilot who had cracked up because of a similar apparition.

Then the real explanation flashed into his mind.

He gave a relieved laugh.

“Hooray!” he cried in the still, dark cockpit seat, “the ghost of the skies is explained.”

“So it is!” agreed Scott, the pilot.

“Don’t—” began Chick; but his own words died as he saw that they were not facing any supernatural appearance.

The light died out of the cloud as the airplane, a lightly-built and fast-moving craft, came steadily lower, closer. It was real!

“It’s Don!” said Garry, reassuringly.

“Yes, it is Don, all right,” agreed Chick, his own fears gone.

Garry watched the light ship make its approach, silent but genuine and then gave Chick a brief lecture.

“I’m glad you came, after all—aren’t you?” he remarked. “Now you can see for yourself that every scare that seems to be started by spooks is all in the way you judge what you see.”

“It’s that way just this time,” admitted Chick grudgingly. “The darkness, and the swamp, and all the talk made me think I saw a ghost ship coming out of a lighted cloud.”

“Certainly,” agreed Garry, “and you thought that, because you heard somebody else say that was how the ghost appeared. But it turns out to be Don in the Dart, coming down out of the sky just when the control man at the airport had his searchlight switched on and turned it past the clouds.”

“For my part,” Scott informed the two chums, “I don’t think the first crack-up happened because the pilot saw a real ’bus.”

“I do,” argued Garry.

The talk ceased as the light ship came swiftly down, across the marsh, dropping lower, leveling off, setting its pontoon body lightly into the water.

If not as experienced, in point of years, as Scott, the seventeen-year-old junior flyer at the Dart’s controls was as expert. Landings in daylight, night conditions, or in darkness, were easy for Don: because of a season of timidity concerning “getting down,” at the start of his flying practice, the youth had determined to break himself of his timidity before it interfered with his rapid progress. Alone in his uncle’s Dart, he had made practice take-offs and landings in every sort of weather and under all imaginable conditions, until he was so sure of his ship that he had no uneasiness about setting down. He realized that the modern airplane is so well stabilized, so well designed, that it does just what its pilot wants it to do—that in every case where some part has not failed, the pilot’s mental condition and its resulting reaction on the handling of the ship is what makes the difference between safe flying and accidents that result in injury or worse.

The small, wide-winged craft sent out a split crest of foam, coming swiftly closer to the Dragonfly; but it lost speed and Don maneuvered it to a point close alongside the larger craft and with his own wings just a little behind those of the biplane. Gliding up to its stop, the Dart rested quietly in the still, rather murky water.

“Hello!” its pilot greeted the others. “Did I give you a solution of the Mystery of Mystery Airport!”

“You certainly did!” Garry admitted. “Chick thought you were the flying phantom——”

“Just as the first pilot to crash thought some chance ship, lighted up by a flash of some beacon, was the ghost,” Don interrupted.

“I’m not so sure of that,” Scott spoke, taking up the thread of a statement he had been about to make before the Dart came down. “I’ve been interested in the mystery—I like spooks, you know——”

“More than I do!” broke in Chick, gloomily. Scott, laughing, agreed.

“Every fellow to his taste,” he quoted. “Anyhow, I’ve been reading up on ghosts, and talking to some of the ‘old inhabitants’ around the marsh. Want to know what I dug up?”

All three eagerly chorused agreement. “Away back in the days when airplanes were tricky to handle and the pilots knew less about aerodynamics than they do today,” he stated, “a flyer was over this swamp, on just about this sort of night,” he indicated the clustered, slow-moving, fleecy groups of clouds, some assuming the pyramid shape of thunderheads, “one of the clam-diggers at the edge of the swamp recalls it very plainly. He was out at low tide after clams when—it happened!”

“What!” asked Chick, forgetting his uneasiness and the gloomy, spooky environment in his suspense.

The aviator had appeared suddenly, coming down, through a cloud, as Scott repeated the tale told him by an old man who earned his meagre living with a clam-hoe and bucket; at the same instant another ship, diving swiftly in apparent oblivion of the first, came into view.

“It must have happened in the flick of an eyelid,” Scott went on. “As old Ike tells it, he heard the engines, looked up, saw one ship for a split second, saw the other, and then—saw them come together!”

“Oh!” exclaimed Garry, “collided, did they!” Scott completed his story quickly, after admitting that Garry had diagnosed the accidental smash correctly.

“Right-o! And they never found one of the ships. It must have gone down in Devil’s Sink.” He referred to a portion of the marsh either of the quicksand bottom sort or very similar in the softness of its muddy shallows. “And—-”

“That’s—why they found—a skeleton, there!” Chick shivered as he spoke in a hushed voice.

“Maybe.”

“But—” Don objected, “what connection is there between an accident years ago and the excitement that has gotten into some of the newspapers and made a reporter call our new development ‘Mystery Airport?’”

“Ever read the ‘Proceedings’ and other books of the Society for Psychical Research?” Scott questioned in turn.

“I saw some of them in a bookstore,” Garry admitted. “They were too dull and prosy for me. Just old stories collected by scientific men who were trying to find out whether ghosts existed or not.”

“What did they decide?” Chick spoke eagerly.

“Nothing very definite,” Scott informed him. “But I’ve gone over a lot of the dry ‘case-histories’ and I firmly believe that if somebody has done something wrong, he has to haunt and stay around the place.”

“Like a criminal ‘haunting’ the scene of his crime,” chuckled Don. “I’m surprised at you, Scott. I believe, in every case, if you could get to the bottom of it you’d find that the ghost is either produced by fraud, or else some perfectly natural things are misjudged——”

“Chick thought you were the sky spook,” broke in Garry.

“I believe that’s so in most of the cases,” Scott agreed. “But this time I think the ghost is restless, because he was careless in coming along through the clouds where he couldn’t see ahead far enough to be able to avoid other ships—and he may have caused the other ship to go down into the Sink. That makes his spirit hang around, and of course whenever it appears, it lives over all the terrible scenes of the smash!”

“But I just proved—” began Don.

“Yes, you proved that people can be mistaken,” Scott was serious. “You didn’t prove that any ship was near at the other times that pilots have claimed they saw the ghost.”

“One caught the fever from another,” argued Garry. “The first one saw something—or he tried to get out of culpability for carelessness in making his crack-up, by saying a spook put him out of control. The rest were all superstitious and the story got headway. The next pilot to see a flicker of Summer lightning and a bird flying or anything at all, was quick to twist it into a spectre, and come down to tell his story and give everybody chills and shivers.”

“I think we’ll soon find out,” Scott spoke quietly.

Surprised, the others clamored for his reason.

“This is just the sort of night that the three other pilots had, when they claimed to see the ghost of an airplane coming out of luminous clouds,” Scott stated. “It’s close, humid—storm-breeding July weather.

“Well, then, for another thing, if you check up you’ll find that the spook has appeared every seven days—and this is the seventh night since this last time!”

“Let’s go home, Don,” whispered Chick, across the narrow span of water. Don laughed.

“No, sirree!” he retorted. “In the first place, if it is pure chance, nothing will happen, because it isn’t reasonable that a beam of light from the control room search-lamp would strike a cloud every seventh night and four successive weeks. Besides, it isn’t possible that an airplane would be flying around just at the same time that light came, and that no other ship would be noticed.”

“No,” declared Garry. “My opinion is that it’s some real person who flies out of the clouds, after seeing a ship coming. Then he goes up into another cloud and is lost, and because of the first fib the pilot told to protect himself from censure by the Board of Inquiry sitting about the crack-up, all the rest believe they see a spook.”

“I think he is trying to use the ghost scare to drive business away from Uncle,” Don asserted. “Uncle has several people he can name who are none too fond of him. Any one of them might be doing the ‘spooking.’”

“In that case,” Garry was practical, “if we go up, scouting, that person will know it, and won’t ‘appear’ tonight.”

“That’s why I liked Scott’s plan when he suggested guarding the sky,” Don agreed. “It’s important, too—because Uncle Bruce is expecting to get a big airline to contract for space for its ships, servicing and all that, take-off and landing, and fuel and oil. It will mean a lot to him not to lose that contract. If we prevent any ‘spooking’ tonight, there won’t be any newspaper scarehead stories tomorrow to make the men hesitate about signing up.”

“Then let’s get up out of this stagnant water!” urged Chick, fired by the realistic explanation of the spectre. “We’ll be a sort of Sky Watchman.”

“An Airlane Guard!” suggested Garry.

“That’s it—an Airlane Guard!” Scott agreed. “Well, come in here, Don, take this Dragonfly aloft and cruise around. If you see signs of any other ship than the mail ’plane—it’s due soon—let Garry send over a green flare if it’s in the air, or have Chick fire a red Verey if it goes up off the earth or water—and you go around on wingtip to point to it and start after it, and I’ll come up on a slanting course, and we can corner the fellow, and end the mystery of the Spectre in the Clouds.”

“Why not come up in the Dragonfly, and let Don fly the Dart, too?” Garry suggested.

“The Dragonfly isn’t fast. The Dart is. If the ‘spook’ pilot sees you young lads cruising around, he’ll think it’s just a joy-hop. If he happened to see you start out—with me—he’ll suppose we are testing the visibility of the new airport lighting system—and he might try to scare up a little excitement for us, as he’d suppose. Then, if Don flew the Dart, taking off first, to surprise him, the ‘spook’ might do stunts and I’d rather be the one to handle the Dart in the night time if stunting is in order. As far as both ships flying around is concerned, what self-respecting ghost, or sensible enemy of Mr. McLeod’s, would give us a chance to drive him down and capture him if he saw two ships in the airlane waiting for him?”

They saw the logic in his reasoning and agreed to abide by Scott’s original plan.

The Dragonfly was warmed up.

Don, in its pilot’s seat, waved a hand to Scott who had shifted to the other craft, opened his throttle carefully to avoid unnecessary air disturbance as he drove away from the Dart, and then got his pontoons “on the step,” so that take-off would be easy, and lifted the three-place Dragonfly into the night.

Garry felt a thrill of expectancy. He loved the mysterious, but of the practical, worldly brand; he had no belief in supernatural things. This would be a chase against a human enemy of Bruce McLeod, airport designer and airways development specialist. Don, steady but hopeful, felt much the same.

Chick, for his part, snapped his safety belt with a little tremble of his fingers. He anticipated something fearful.

His premonition was fulfilled.