Careless of the attention they might attract, Don and Chick rode the low altitudes toward the sheet of water before the boathouse. Chick had a parachute flare ready.

Don signaled.

Overside went the flare, to ignite and throw its fierce, white glare over the approach.

As it settled Don spiraled down, far enough away to make his pass at the water, power-stalling to a safe drop onto the surface.

He gunned the engine enough to bring them close to the old wharf and then let the incoming tide drift them, while Chick, out on a pontoon, sidewise to the piling, caught the rope they had cut and left hanging some nights earlier.

To draw the ship closer was no task.

Securing it, and taking the precaution of pocketing the parts previously removed from the helicopter carburetor, Don passed them up to Chick, whose agility had enabled him to reach the planking of the dock.

“Now,” Don helped by Chick, made the level and drew a small flashlamp out of his coat, “let’s see, first, if anybody’s up here.”

The light of the small torch danced to and fro as they stood in the open door of the old building.

“Nobody at home!” Chick declared, following the light into corners, behind the table, still lying on its side where it had been overthrown in the former struggles, and lifting the trap in the dark corner.

“Down we go!” Don whispered.

“Nothing to stop us.”

Their light, showing a rusting iron ladder, also revealed the surface of stagnant water, around a small landing stage, built to float up and down with the tide. Around it, thickly clustered, were the dozen dories owned by Toby Tew.

Boards, on the lee side, nailed to the string-pieces, served to keep wind out in storms, and since the boarding was carried down below the tide marks, disturbances from wind drift did not much affect the tethered crabbing boats.

“If we find anything,” Don held the light while Chick descended, head bent to screen his eyes, “if we find a projector, and a battery, it will narrow down our suspicions to Toby, and point to him after all.”

“It will!” Chick agreed, reaching up to take the light, then jumping from the lower rungs of the hanging ladder which did not quite go down to the platform, allowing for its rise with the tide.

“Here I come!”

Don, with Chick lighting his way, made the climb and jump.

“Nothing on the platform,” remarked Chick sending the beam to and fro.

“Put it on the dories—that’s it. There!”

A note of triumph was in Don’s voice.

They hurried to the edge of the platform, drew a dory close, and were quickly within its cluttered hull.

A tarpaulin, dragged aside, revealed, in the light, a good-sized box-like metallic contrivance, its sides rounded, with a sort of chimney on top: there was, besides, a large, circular tank, and a smalled metal case.

“Here’s a portable projector,” Don identified the metallic object, “and there is the ‘head’ and probably film, in that smaller case. But where does he get his light?”

“Maybe that tank holds acetylene gas,” suggested Chick.

Don, unfastening the projector lamp-house, exclaimed in elation.

“I know!” he cried.

The round, pure white object set in a holder within the lamp-house, identified by Don as a calcium disk, told him the source of light.

“This calcium gives the whitest, most brilliant light there is,” he declared. “See, Chick! The tank probably contains oxygen, under a strong pressure. Yes—there’s a gauge, and a pet-cock to regulate the gas flow. The tank connects, by this rubber hose, to the base of the burner, and the thing on the lamp, like a bent finger, pointing toward the calcium disk, is to throw the oxygen jet onto its surface. Then it glares like all get-out!”

“Let’s row the dory, and never mind the Dart: she’ll stay put,” Chick found oars on the staging in a big box.

They found quite a direct channel, along the shore line from the boathouse to the position in which the helicopter still lay tethered.

Expertly, as Chick obeyed his orders, Don assembled the parts of the apparatus in the cockpit of the helicopter. Iron, or perhaps aluminum, pieces, set into the coaming, enabled them to attach the portable projector, and to swing it to and fro, and direct it up and down.

“Did you ever fly a helicopter before?” Chick asked, as they perfected the connections between tank and the lamp base.

“No. But it’s simple! I mean—I can do it! You see, Chick, my flying experience will let me handle the tractor propeller, just the same as in the Dart.”

“I suppose so! And I see that all you have to do about the top set of blades is to throw in a clutch that meshes the gears on the upright mast. The mast is set in a step and bearing in the body frame. It is squared into the gear that turns it—I guess you can manage it.”

Don agreed.

Nevertheless, being a cautious youth who believed in being forehanded, he went over the curious, squat fuselage, tracing gas, oil, water-cooling and other feeds and piping. Then he examined the engine. Except that it was of a make he had not handled, it offered no difficulties.

Assuring himself that the gas gauge indicated at least several hours of fuel supply, and that he understood the controls for the lifting mechanism, Don operated the momentum starter. Its handle, rapidly rotated, gave a big, heavy wheel considerable momentum. Then, applying its control, he transmitted the power thus achieved to the engine and after several attempts the starter caused the charge in a cylinder to be ignited as the flywheel turned onto a point where a firing current passed into a charge of fuel mixture.

At once the engine took up its revolutions.

Don manipulated the throttle until he became fairly conversant with the power response, then, carefully, being sure that all was well, and that they had the film already threaded properly in the projector and that Chick comprehended the handling of the fuel for the lamp jet, he eased down the engine, let in the upper blade clutch, and saw the mast whirl its fan-like top slowly.

Gradually, as Chick cast loose the ropes, Don increased the speed of the upper blades, leaving the forward propeller idle.

The speed of the rotating fan soon began to be felt; but they did not rise.

“Don!” Chick, sensed a solution, having studied a good deal about the various points of airplane design, “remember that when a pontoon is in water, as it lifts, the suction of the surface increases and has a strong pull to keep it from leaving: that’s why they design a pontoon with a ‘step’ so the contact is with the top of the water, and not down in it.”

“That’s right,” Don agreed. “We’ll have to go forward a little as we lift. On land that wouldn’t be necessary. In water it may.”

The experiment was tried: he used the thrust-propeller, and in short order they were rising: then he cut out the forward speed, until he had made some tests of lifting speed with the horizontal blades.

“Here we go!” he cried finally. Go they did—up and forward.

“Aside from the handling of the upper fan,” he mused, “this isn’t much different from a slow cruising airplane. The tail and propeller control are similar—rudder and elevators; but I’ll have to remember the turn is made without ailerons to bank. We don’t have to bank on a turn. We just rudder around, and the upper blades keep us on a stable keel as we turn—here, we do it!”

Around swung the forward propeller, and Don directed the craft toward the higher levels on a slanting line that climbed it and also progressed it toward the airport.

He saw, when they came close to the open space, figures on the illuminated tower balcony, watching upward.

Someone blinked a flash lamp.

“That means ‘O.K.’” he murmured.

“We can put on our show!” cried Chick, also misinterpreting the signal that they did not pause to spell out in full.

Swiftly, with the engine gunned in, Don lifted the helicopter above the small groups of fluffy, white cloud that gave him excuse for his experiment. The airport vanished beneath the mist and the shrouding clusters of dense, smoky-white vapor.

Don swung the nose, as they hovered, drifting only slightly.

Thus he maneuvered into a position where his understanding of the angles they had worked out enabled Chick to train the projector on a mass of white vapor just over the edge of the bay.

He threw up his arm.

The beam of the white light glowed, and Chick quickly maneuvered it, through a threaded-up section of transparent, non-inflammable film, into the cloud. He began to turn the crank. Darkness ensued in the cloud as part of an opaque film covered the light.

Suddenly Don screamed.

“Stop!”

He threw up his arm, trying to signal Chick.

But the younger chum, intent on his handling of the intense light and the focusing tube of the lenses, as well as the proper course of the film as it jerked downward, paid no attention, failed to hear the cry and did not see the signal, his eyes being turned downward and away.

From the airport came screeches, as of warning, terror or distress.

The crash siren was going!

Don, from their high point, looking alertly around the horizon, had observed that the midnight mail ’plane, behind schedule, was coming, low and fast, over the swamp.

It all happened in a few instants.

On came the mail ’plane.

Up above the clouds, hidden from the mail ship by vapor, Chick cranked his projector.

As the mail ship approached, near the edge of the swamp, out on the cloud leaped the glow that suffused it, went through it, made of it a weird, terrifying set of illuminated atoms of moisture. Onto that plane of light leaped the black silhouette of an oncoming ship. The swing Don tried to make, to turn the vision away from its screen, did not help, since they had no forward speed to cause the rudder to work.

Hideous terrors gripped the young pilot. He knew what was about to happen. The pilot of the mail ship, already superstitious, and aware of the ghostly stories that had been flung far and wide, would look toward that cloud as he dropped the nose toward the airport approaches.

It couldn’t be helped. The spectre in the clouds was flying right at him as his ship disappeared from Don’s sight under the cloud over which they hovered.

Swiftly he cut the speed of the upper blades. They began to settle.

“What will we find on the ground?” Don muttered.