Slow and methodical in his mental processes, Garry was rather stunned by the situation the chums were confronting.

Don, climbing, listened for information. None came through the Gossport tube.

Chick, alert, with an impulsive quick flash, got an inspiration.

To dive meant going in line with the flight of bomb or rocket, or—if that strange Thing in the helicopter really was deadly in its intent—bullets.

A side-slip would be no better for them.

Their climb did not get them away quickly enough. Perhaps the Dragonfly had superior climbing ability, better maneuvering capacity; but no ship could outfly those messengers of fire—the rockets!

“Garry!” Chick prodded his chum.

The older youth turned.

“Tell Don—a lot of barrel rolls!”

Garry saw the utility of the maneuver. They had already executed a half-roll, turning from upside down to level. One after another, in succession, a series of barrel rolls would get them, sidewise, away.

They would lose very little altitude.

Best of all, the stunt would send them off in an evolution that would make it almost impossible to catch them, by angular direction of the missiles, because Chick knew a variation—and gave it.

“Tell Don—barrel rolls, and reverse rolls!”

Garry realized the value of Chick’s wit, relayed the idea.

Down went a wing. Up went the other.

Over and over, the Dragonfly revolved on its tail-and-nose line.

Three rolls followed one another.

At the end of the third, Don checked the ship.

Down went the nose. Short was the dive, to gain speed. Up came the nose.

With the wing that had swung upward before now down, he went into a triple reverse roll.

On the descent of their dive the dying flare had shown them the helicopter still climbing. Its surprised occupant evidently had the large tube they saw at one side trained at a certain angle. To change it took time.

Don’s reverse maneuver sent them almost directly underneath the ship.

But there their wings were in danger.

Checking the ship, Don began the disconcerting, but very useful stunt termed a “falling leaf,” in which by alternately dropping one and then the opposite wing, the ship descended in a sort of zig-zag drop, much like the erratic course of a leaf falling from a tree.

Twelve hundred feet above the swamp, Don leveled, and with full-gun on sent the Dragonfly unerringly down-wind and straight at the runway approach lights a mile ahead.

They must get the mail in, he knew.

Swift, at its best, the craft sped toward its goal.

Don cut the gun, began the approach glide.

Far behind, unable to keep up, or, perhaps, giving up the chase, the Demon in the helicopter—human or otherwise—stayed aloft.

Down to the runway in a well-calculated glide Don swung his ship.

“Hooray!” exulted Chick. “Don—Garry—we win!”

The trucks leveled with the tail. The ship lost speed. Its wheels set their tires on the concrete and the ship, rumbling, ran forward.

Instantly Don cut the gun.

Mr. McLeod, the control room chief, and Doc Morgan ran up.

Garry tossed out the mail pouches.

“The ship-to-shore record is just tied!” cried McLeod. “Don—I owe you a lot for this—and your friends, too!”

“All right, Uncle!” Don swung about in his cockpit. “Doc—Mr. Vance—has anything been done about the Dart?”

“Why—ah—” Doc rubbed his chin as the older executive, with the control room chief at his side, raced for the waiting car in which the mail, still in its sacks, would be rushed to the New York Post Office, “yes, Don. They got a new prop set!”

“Good enough.”

“Why?” asked Garry.

Don had not loosened the chin strap of his helmet. He clambered hastily out of his cockpit, onto a wing, to the earth.

“Who’ll go with me?” he cried, as Garry and Chick came tumbling out of their places. “Garry—Chick! Which one?”

“Where? For what?” asked Garry.

“Back to the swamp—to find that Thing and make sure that it never molests any more pilots.”

“I will!” cried Chick.

“I will!” Garry’s words were shouted in the same instant.

“No! Only one can go in the two-place Dart!” said Don. “She’s all fueled up for the flight, you know, but there aren’t three places.” “But—” began Chick.

“I need one of you for ground work,” Don cried. “I have a plan! Two of us go aloft, fly back to the swamp. Maybe we can get there before the Thing has a chance to set down. Maybe we can fly around until we locate it. But somebody has to stay here in the control tower!”

“Why?” asked Garry.

Don spoke earnestly.

“Because—the control room chief took back a tracing, claiming he owned it—and because there was the beam of that searchlight on us tonight—and there isn’t another searchlight that I know of near!”

“Oh!” Garry saw light as Don spoke in his clipped, alert way.

“Yes!” Chick cried, “you think the control chief is working with the one in the helicopter—they are the ones who are trying to ruin your uncle.”

“Yes!” Don was already hurrying into the wide open hangar, where he saw several mechanicians adjusting the hub bolts on the small, swift Dart.

“But the chief of control has gone with your uncle!” Garry objected.

They were inside the hangar as Don answered, putting his words in low, swift earnest sentences.

“Only to the car. He’ll slip back here. We mustn’t leave him free to signal——”

“I’ll stay!” Chick declared. “I’ll stick to him as tight as his skin, Don. I’ll be a part of the Airlane Guard—and if he signals, I’ll be there—and we can get Doc——”

“No! Trust nobody!” Garry whispered. “Not even Doc! Don’t forget he was the one in the boathouse when you found the tracing.”

On swift feet Don raced away to the men completing adjustments on the propeller assembly.

“I’m taking the Dart aloft,” he said quickly. “Come on, Garry—run her out!”

In ten minutes, while Chick watched, and kept an eye on Doc and on the control chief, who stood watching in the hangar doorway, Don revved up the newly warmed engine, lifted the tailskid from the concrete, the chocks were dragged from under the wheels, and up from the runway leaped the Dart, with Garry, adjusting the Gossport, transferred from the other craft. They would scotch the Demon of the Swamps—or do their best to end his unexplained but menacing career.

Stars twinkled in the night sky as they roared straight for the haunt of mysteries. And the Demon was ready for them!