The body went twisting and turning into the gulf below.

Raiders Invisible

By D. W. Hall

T he muffled, helmeted figure of a pilot climbed down the spider ladder, nestled into the foremost scout's cockpit and pressed the starting button. The motor spat out a wisp of smoke, then burst into its full-throated roar: the automatic clamp above loosened: the scout dropped plummet-like, bobbed to the flagship below, straightened out and zoomed six thousand feet up into the morning blue, where it hovered for a few moments like an eagle on taut wings. Lieutenant Christopher Travers, the pilot, glanced around.

Behind and below him was spread a magnificent panorama. Across the plate of scintillating glass that was the sea moved rows of toy ships, tipped by the gleaming, one-fifth-mile long shape of a dirigible, of whose three scout planes Chris's was the leader. As he watched, the second scout dropped from the plane rack beneath the dirigible's sleek underside and went streaking away, followed by the third, in response to the Admiral's order of: "Proceed ahead to locate the enemy's position."

A grin relaxed Chris Travers' tanned, boyish face. His narrowed gray eyes swept the horizon. Below it somewhere lay hidden the ranks of the Black Fleet, complete with its own destroyers, submarines, cruisers, battleships, aircraft carriers and the ZX-2, sister dirigible of the Blue Fleet's ZX-1. Chris spurted the scout ahead and murmured:

"This war game's goin' to be a big affair—the biggest yet!"

It was. The Atlantic Fleet of the United States Navy, termed "Blue" for convenience, had been assigned to guard the Panama Canal; the Pacific Fleet, "Black," to attack it. The cream of America's sea forces had been assembled for that week of March, 1935, all the way from crabby little destroyers to the two newly completed monarchs of the air, the twin dirigibles, fresh from the hangars at Akron, a thousand feet each in length and loaded with the latest offensive and defensive devices developed by Government laboratories.

The war game around the Canal was planned for more than practice, however. The eyes of the whole world were on that array of America's ocean might—the eyes of one foreign nation in particular. Washington knew of the policies of that nation, and wished to impress upon it the hopelessness of them. More than a game, this concentration of sea and air-borne fighting power was a gesture for the continued peace of the world—a gesture strong with the hint of steel.

Chris Travers was vaguely aware, through the rumors of the mess-room, of the double meaning of the game he was playing his part in, but this morning he didn't give a single thought. He was too wrapped up in his job of spotting the van of the Black Fleet, radio-telephoning latitude and longitude to the bridge of the Blue Fleet flagship, and getting home to his dirigible without being declared destroyed by one of the war game umpires.

Therefore, half an hour later, his heart thrilled as he glimpsed, wraith-like on the steely horizon, a wisp of smoke.

H e catapulted forward, eyes steady on that hint of ships. The smoke grew to a cloud of black pouring from the funnels of a V-shaped squad of destroyers, rolling through the lazy swells of the Pacific waters. Behind them came the bulldogs, larger warships, hazy blurs in the distance.

Chris struck fist in palm to the tune of a gleeful chortle. He was first! He hauled the microphone from its cubby in the dashboard and spoke the code words. Latitude, longitude and steaming direction of the Black Fleet he gave rapidly, and the information knifed back to the bridge of the Blue Fleet flagship, a hundred miles behind, where a white-haired admiral said: "Ah! Good boy! Get those bombers up—pronto!"

Chris commanded a superb view of the ZX-2, whose gleaming shape, showering rays of sunlight, hung like a thing in a painting over the Black Fleet. He stared at the far-off dirigible, lost in admiration of her trim lines, pausing a minute before returning to his own ZX-1. At that distance, the mammoth craft seemed no more than four inches long, yet, through his telescopic sight, he could discern her markings, machine-gun batteries and the airplane rack along her belly plainly. One plane, he saw, was suspended from the rack; the others were scouting for the Blue Fleet, even as he had scouted for the Black. He wondered if something were wrong with the plane left behind. Somehow, it did not look quite familiar.

But, even as he watched, it dropped from the automatic rack, then straightened and soared dizzily up. And, from one of the airplane carriers' broad decks, he saw two pursuit craft begin to rise. He grinned. They'd seen him, were coming after him!

He gripped the stick, prepared to swerve around. He had already raised a spread-fingered hand for a derisive parting gesture, when suddenly he stiffened. The hand dropped as if paralyzed.

"Good Lord!" he gasped. "What—"

The mighty thousand-foot dirigible ZX-2, pride of the Navy and all America, had wobbled drunkenly in her path. She stuck her nose down, and then her whole vast frame shivered like a wind-whipped leaf as the dull roar of an explosion rolled over the sea. A huge sliver of hide was stripped from her as if by magic, revealing the skeleton of girders inside—revealing a tongue of crimson that licked out and welled into a hell of flame.

Chris's blood froze. He watched the ZX-2 wallow in her death throes, writhe in the fiery doom that had struck her in seconds, that was devouring her with awful rapidity while thousands of men, blanched and trembling, gazed on helplessly. He saw her plunge, a blazing inferno, into the sea beneath....

There were old pals on her—buddies, gone in a flash of time!

This wasn't a war game. This was tragedy, stark before his eyes.

T he Black Fleet forgot its mimic battle. Radio telephone messages winged over the horizon to the approaching Blue Fleet. The Black dreadnoughts hove to; launches with ashen-faced men in white manning them dropped overboard; a dozen destroyers rolled in the swells around a crumbled, charred egg-shell that but minutes before had been an omnipotent giant of the sky.

Chris Travers, aloft in sunlight suddenly bereft of its beauty, jammed the stick of the scout full over. He could do nothing, he knew. He could only return to the ZX-1 and tell the story of its sister as he had seen it.

But why, he wondered as he flew almost blindly, had the ZX-2 so quickly flamed to oblivion? The helium of its inner bags had been uninflammable, as had the heavy oil of its fuel tanks; the ten engines were Diesels, and hence without the ordinary ignition system and gasoline. Safety devices by the score had been installed on board; nothing had been overlooked. And the weather, perfect.

It was uncanny. It seemed totally unexplainable.

Swarms of planes droned between sea and sky, all speeding in the one direction, west, to where the crumpled remnants of a dirigible were slipping quickly beneath the billows, beyond the sight of man. Planes of war game umpires, of officials, of newspaper correspondents and photographers. And soon a spectral, gleaming wisp of silver nosed out of the east, and the lone scout flying east dropped in altitude to meet its mother.

Mechanically, his mind elsewhere, Chris shoved the button which reared the automatic clamp behind the cockpit in preparation for affixing the scout to the plane rack beneath the ZX-1. The dirigible, far in advance of the Blue Fleet, was roaring along at its full one hundred and fifty to hover over the grave of its sister. Chris eyed its course and changed his. To jockey into the rack, he had to pass the dirigible and come up underneath from its rear.

T he air giant roared closer. As the distance between then loosened, Chris's brow wrinkled and he swore softly in puzzlement.

"Now, just what's wrong with them?" he exclaimed, "The darned zep isn't flying straight! She's wobbling in her course!"

It was hardly apparent, but true. Ever so slightly, the snub nose of the ZX-1 was swaying from side to side as it sped through the air; ever so slightly, her massive stern directional-rudders were wavering.

She was less than a mile away now. At that time, there were no other planes in sight; none flying in that vicinity save Chris's. He glued his eyes to the telescopic sight. A moment later, sheer horror swept his face.

" Good God! "

The scout leaped as its throttle rammed down. The gleaming, thousand-foot shell of the ZX-1 roared by it at equal altitude, making it a puny fly-speck in the sky. But the fly-speck was faster. It turned in a screaming bank; it straightened; it lunged back after the swaying, retreating mammoth like a whippet, lower, now, than its quarry. It maneuvered expertly as it gained, for one of the best pilots of the service was at its controls, and there were deep lines graven in his face, lines of anguish and intolerable suspense.

Through the telescopic sight, Chris had not seen a single white-clad figure standing beside the glass ports of the dirigible's control car. But he had seen, slung from the rack along her belly, a single plane—the same rather peculiar-looking plane he had seen hanging beneath the rack of the ZX-2 a few minutes before she had gone down in flames!

And in that plane, he knew surely, was the answer to the mystery.

S peed cut to just a trifle more than the dirigible's. Chris passed a few feet underneath the huge expanse of her lower directional rudder. From so close, its uncontrolled wavering was terrifying.

His faculties were concentrated on the task of sliding the scout's clamp into the groove of the plane rack, but he was also surveying the lone airplane hanging from it. A powerful machine, painted in Navy colors, a peculiar knob on the upper side of each half of the top wing gave it its unfamiliar appearance. Its pilot was obviously aboard the dirigible, working....

Closer and closer the scout crept, quarter-way now along from the stern of the massive bulk that loomed above it, and within fifty feet of the third clamp in the rack. Touchy work, maneuvering into it, with the ZX-1 yawing as she was, and the need for haste desperate. Chris's hands were glued to the stick: his nerves were as tight as violin strings. Then, when only ten feet from the rack clamp, he gave a startled jump of uncomprehending amazement.

The propeller of the mysterious plane ahead had roared over. Its clamp had left the rack; it had dropped down in a perfectly controlled dive and flattened out as if a master pilot were at its controls.

But the plane's cockpit was still empty, Chris could see; nor had he seen any figure pass down the ladder from the dirigible into it!

Devoid of all emotion save bewilderment, he sat stupidly in the scout. A moment later, so well had he aimed it, its clamp nestled snugly into the groove of the rack, and the regular automatic action took place. A tiny door slid open directly above in the dirigible's hull: a thin ladder craned down—and Chris's nostrils caught a faint whiff of something that cleared his mind of its confusion instantly.

Just a whiff, but it registered. Gas, with an odor resembling carbon monoxide.

He stared up. Over the edge of the automatic trap-door above, a white, contorted face was hanging. The dirigible swung; white-clad shoulders and body slumped into view. Then, with a rush, the body slipped through, jarred against the connecting ladder, slithered off and went twisting and turning into the gulf below.

"God!"

Gassed! How, by what, Chris had no idea. A moment before he had been about to follow the uncannily piloted plane; but now his duty was plain. He knew with awful certainty that in minutes, seconds perhaps, the giant ZX-1 was scheduled to roar into flames like its sister and plunge into the Pacific.

He jerked out a gas mask. He was fitting it on with one hand as, with the other, he hauled himself up the spider ladder into the hull of the thundering, yawing dirigible.

He did not see, hovering a few hundred yards behind the ZX-1, the mystery plane; he did not see it now begin to approach the rack once more.

T he crew of that dirigible of death, Chris discovered, had not had a chance. White-clad bodies lay sprawled throughout the cabin which contained the mechanism of the plane rack, stricken down silently at their posts. There was no life, no sound save the booming of the motors and the whip of the wind screaming past the uncontrolled air titan.

But he did not pause there. He did not know what he was grappling with—it seemed black magic—but he darted to a ladder which angled up from the lowermost entrance cabin to the cat-walk that stretched from the nose to the stern of the ship. If any infernal contrivance had been planted aboard, it would be in the most vital spot.

Heart pumping from the artificial air he was breathing and from the consciousness that each second might well be his last, he sprinted along the interior gangway. Above was the vasty gloom of the gas bags and the interweaving latticework of the supporting girders; the drum of power-car motors and the strained creakings of cables and supports echoed weirdly throughout. Outside was the sun and the sea and the clean air, but this realm of mammoth shapes and dimness seemed apart from the world. Once he stumbled against something soft and yielding—a body flung down there in death, fingers at its throat. And there were other white-clad figures, grimly marking off the length of the cat-walk....

Chris's nerves were raw and his face sopping with sweat beneath its mask when suddenly he stopped at sight of something that lay on the cat-walk, with the main fuel tanks on the girders just above it and the entrance to the control car just below.

I t was a black box, perhaps two feet square and a foot in depth, made of dull metal that did not reflect the rays of the light bulb placed at the head of the ladder leading down in the control car. There were three curious little dials on its face, and the trembling finger of each one was mounting.

It had been strategically placed. An explosion at that point would rip open the fuel tanks, split the largest gas bag, wreak havoc on an intricate cluster of main girders, and destroy the control car with its mechanism.

"No wonder the ZX-2 crashed!" Chris muttered.

Then his hands swept down. The next instant he was hugging the thing tight to his chest and stumbling down into the control car, hearing only a high-pitched, impatient whine that was coming from the box as the fingers of its dials crept slowly upward.

The ZX-1 was wavering wildly as her rudders flopped from side to side, and with every swing the bodies that lay in her control car, strangled by gas, stirred slightly. The gray-haired commander was stretched there, one arm limply rolling as his ship, which had gone so suddenly from him, rolled. Subordinate officers were tumbled around him. Death rode the control car.

But down to it and through it now came one who was alive, a figure made grotesque by the mask it wore and the pack of the parachute strapped to it, who threaded past the littered bodies, an ever-rising whine wailing from the box clasped in his arms.

With a leap, he was at one of the car's port-holes, fingers fumbling at the heavy bolts. The seconds seemed eternal, and the box's whine had become a shattering, sinister scream when at last the bolts loosened. The round pane of glass teetered back, swung open—and the masked man slung his metal burden out, out from the ZX-1 into the gulf between sea and sky.

It arced through the sunlight, went spinning down, became a dot, its screaming faded. Then something synchronized within it, and it was gone—in a burst of weird, bluish light, whose fangs forked upwards for a second, their unearthly flash dimming even the sunlight, and then were gone, too....

C hris found that his whole body was shaking. For a moment he stood there with his masked face through the port.

"Damn close," he muttered. "But what was it that left the box here?"

Then he jarred against the side of the car as the ship swung and came back to realization of what was needed to be done, and done at once. He shifted his gaze, drew his head back, and thrust it forth again, staring.

"Good Lord!" he cried. "That plane's come back!"

His own craft was not alone under the rack. The same mysterious machine hung there again, its cockpit empty, and the automatic spider ladder was stretched down to it from the trap-door in the dirigible above.

"Whatever flies it is aboard now." Chris thought aloud. "But it got back too late to stop me. Well, this time—"

He felt uneasy, however, almost powerless. What was this thing that had wiped out the crews of two dirigibles with deadly gas, and wrecked one of them? He spun around. The control car looked the same. But what might be moving in it?...

Chris carried no gun; but he extracted the service repeater from the holster of a body at his feet. Gripping it, he leaped to the helm of the dirigible. It was the work of a moment to clamp on the mechanical "iron mike," which steadied the ZX-1's mad swaying and leveled her ahead in a dead straight course. He could not cut down her speed, unless he went to each one of the hull-enclosed engine stations, and more urgent work awaited before he could afford to do that—work of sending out an S.O.S. before the weird, unseen killer and wrecker came to grips with him.

Though seeming hours, only minutes had passed since he had tooled his scout into the rack. Ahead, he could see the smudge of the Black Fleet's smoke on the horizon. Not so very far away, but a lot could happen in the distance still separating dirigible and surface craft.

H e ran back into the radio-telephone cubby, which was a division of the control car. The operator was sprawled there, limp in his seat before the shining, switch-studded panel. Chris removed the head-gear of ear-phones: then he hauled one of the cubby's port-holes open, letting in a rush of cleansing air. His fingers sped quickly over the panel; a row of tubes glowed; the machinery hummed. Chris jerked off his mask.

A last faint odor was present, but he hardly noticed it, for his lips were at the mouthpiece and he was thrusting out a call for help.

"ZX-1 calling ... ZX-1 calling ... ZX-1—Hello!"

An answer from the flagship of the Black Fleet ahead had sounded.

"This is Travers, pilot on the ZX-1, speaking. We're coming dead for you; full speed; you'll see us in minutes. Get some planes with men capable of handling the dirigible up here immediately. The whole crew's been laid out by gas; there was a contrivance planted aboard to blow up the ship and send it down in flames as the ZX-2 was. The thing that did it—"

Crack!

A gun barked out from behind; something crashed and splintered on the radio panel. Chris felt a white-hot needle sear along the side of his head. His brain reeled; with everything dancing queerly before him in splotches of gray and black he toppled down off the seat, knowing the radio-telephone had been put out of commission by the cessation of sound in the ear-phones clamped to him.

He gripped his consciousness hard. It was like a delirium: he was lying sprawled beside the seat, twisted round so that he saw, hanging in the cubby's entrance door, an automatic, dribbling a wisp of smoke—the automatic that had just fired, but hanging there by itself, held by something he could not see!

He was only half conscious, for the scorching pain along his head was throbbing his brain dizzily, but he realized that the service repeater he had taken from the control car lay by his side, within easy reach. But, while on the verge of risking a wild grab for it, he heard a voice, speaking very softly and with a slight thickness of accent.

"Do not move," it said. "I fire if you do. Now, listen: What did you do with the box that you found? Tell me quick, or die."

It was fantastic, unreal. There was—nothing, and yet a man, living, breathing, but invisible, was speaking! Chris could not understand; but it was at least a little relief to know he had a human to deal with. For with humans, strategy can be used....

H e groaned. He saw plainly that the unseen marauder had been aboard when he had thrown the box over, and thus had not seen it explode in midair: did not know whether it had been tossed out or merely rendered harmless by being tampered with. If only the latter, it could be quickly repaired and set again. That must be the invisible man's reasoning.

Again Chris groaned. He moved an arm weakly and whispered:

"Can't speak much. Come closer."

The service repeater was very close now to his right hand. And he felt a thrill when he saw the automatic come forward through the air, descend, and pause right next to his head. He sensed a man close behind him, and he heard:

"Well? Tell me, quick. Did you throw it over, or—?"

"Don't shoot!" Chris groaned. "I'll tell you. I didn't—throw it over. I took it apart to get the secret of it. I put it—there."

He pointed feebly with his right hand, thus leading the invisible man to turn his head. His legs braced imperceptibly. And then:

"Like hell!" roared Chris Travers, and shot his whole weight backwards, grasping the service gun, whipping it around and yanking the trigger three times at the same instant.

Shooting at nothing! But, even above the bunched roar of the explosions, there pierced out a howl of agony that died quickly to a sobbing moan. Chris saw the automatic drop to the floor, felt the invisible body he had crashed into jerk away. He jumped to his feet, clutched at that body, and caught thin air. He swung around, listening, the service repeater in his hand.

Out of the air somewhere before him there came the sound of low, racking gasps, and also the slow noise of feet dragging heavily towards the cubby's door, towards the ladder that led up to the fore-and-aft cat-walk.

Chris sprang, slashing the butt of the gun downwards. The lead was false. He hurtled jarringly into the door jamb, the gun thumping against the floor. The wind was knocked from him; the nausea of his wound swept him again with a surge of dizziness. But the painful scuffle of unseen feet ahead pulled him up once more; like a punch-drunk fighter he staggered out from the cubby to the ladder and hauled himself up the steps. He half-fell at the top, but his mind was clearing; and as he swayed there he knew what he had to do—saw the duty that lay before him....

More slowly, he crawled after the dragging footsteps and the gasps of the invisible raider, following them through the vast dimness of the interior of the dirigible ZX-1.

T he chief operator on duty in the flagship of the Black Fleet swung round in his seat and yelled through into the bridge of the massive battleship:

"Urgent, sir! From the ZX-1!"

A moment later the captain of the ship, for the fleet's admiral was out in a launch inspecting what little of the fallen ZX-2 was still floating on the surface, was at the operator's side, listening amazedly.

The operator read off, word for word, what Chris Travers had sent. "... There was a contrivance planted aboard to blow up the ship and send it down in flames as the ZX-2 was. The thing that did it is—" he finished, and fell silent on that uncompleted sentence.

The captain's lined face expressed incredulity. "My God!" he burst out. "First the ZX-2, now— That all?"

"Yes, sir. I can't get any answer or connection."

They stared at each other. Finally the captain spluttered:

"Is some maniac loose in this fleet? Don't sit there like a fool, man! Get in touch with the Saratoga ; tell 'em what you received; tell 'em to send some men up to that dirigible, wherever she is. We can't lose both of them!"

The operator's fingers skipped nimbly; even while he was speaking into the microphone, the red-faced captain had rushed back into the control bridge and was roaring:

"Signal the Admiral back here! Hurry!"

T hings moved quickly then; small things, but significant. A casual eye glancing over the ranks of the Black Fleet as it lay around the scene of the tragedy, waiting for orders, would not have noticed any difference. The launch containing the fleet's admiral, which had been fussing about with its load of officers and various dignitaries, suddenly wheeled and pointed back for the mammoth flagship, in response to swift signals from the arms of a gob on her bridge; and, on the broad landing deck of the carrier, Saratoga , two three-seater planes, equipped with automatic clamps for a dirigible's rack, were wheeled up to the line.

Their props were spun over. But even before their cockpits had been filled, an officer on the bridge of the flagship, and a dozen others throughout the fleet, cried:

"There she is!"

Over the eastern horizon, a gleaming sliver in the sunlight, thundered the ZX-1, straight for the array of the Black Fleet. Only a few men were aware of the drama-fraught message which had come down from her radio cubby, but her growing shape commanded the eyes of every sailor and officer alike who had time to watch. A few telescopic sights were trained on her as she bellowed ahead; the keen old eyes of a very perplexed and puzzled admiral were at one of them.

"Two planes hanging from her rack," he muttered, half to himself and half to the officers standing around him. "Both Navy. Say, they're dropping off! Not coming this way, either. Going northeast. Fast, too. Can't see 'em any more.... Those men getting up from the Saratoga ? Good. We'll find out something soon. Here she comes!"

Closer and closer roared the dirigible. Two planes from the Saratoga were swooping up to enter her rack, but the other two planes that shortly before had been suspended from it were gone—already vanished into the northeast.

"Don't understand this at all!" said the Admiral of the Black, or Pacific, Fleet of the United States Navy.

T hings had broken well, Chris Travers considered. He had only wounded the invisible raider; but, luckily, had wounded him badly, so that, evidently, just one object was in the man's mind: to get back to where he came from, to where he could find help. He seemed oblivious of the scout that was following behind at the full speed of its mighty rotary motor, following him to his base, wherever it was.

"Just as well I didn't kill him," Chris muttered.

The rush of wind had cleared his brain; his faculties were steady and normal. Not so with the man in the plane he pursued. It was flying crazily, but clinging to one course, nevertheless—into the northeast, towards land, some two hundred and fifty miles over the horizon.

The great silver shape of the ZX-1, barren, now, of life, dropped away, speeding ever due west; the hazy dots and blur of smoke which denoted the motionless Black Fleet vanished. But Chris was in contact with the fleet's flagship once more, through the compact radio-telephone set of his scout. As he flew, his eyes fixed steadily on the plane ahead, he was rapping into the microphone the story of what had happened. He told of the invisibility of the strange marauder, of how accurately he had judged the time of his raids; of how he, Chris, had managed to prevent the destruction of the ZX-1.

"He uses a tremendously expansive gas resembling carbon monoxide," he went on. "It seeps into every cranny of the dirigible, killing everything. The crews got no warning; they didn't know what was happening; couldn't see him! Well, I managed to wound him on the ZX-1. He beat it. I'm following him. If he lasts out, he'll go to where he came from, and we'll find out who's in back of all this. Let you know where his base is soon as I get there. Keep listening. Okay? Right; signing off."

Silence, then, between the scout and the flagship far behind....

O n—on Time passed. The scout's gas was down below the half-way mark. They had covered two hundred miles now at a speed just bordering three hundred. The plane ahead looked uncanny with its apparently empty cockpit, but Chris could see all too well that death was pressing at its invisible pilot. The big machine was literally staggering in its course as the hands on its control stick grew weaker; was yawing wildly, even as the ZX-1 had yawed after her crew had been slain by vapors they could not see.

"He's got to last out!" Chris muttered. "Got to!"

At that moment land appeared, and the fleeing plane altered its northeast course to due east with an abrupt jerk.

First it was a mere hazy line on the horizon; then it rose to a thrust of land, jutted with cloud-misted hill-tops. Then, as the two roaring specks that were airplanes came closer, heavy tropical foliage became distinct, and white slashes of surf breaking on the shore. This was the Azuero Peninsula, most western point of the Republic of Panama.

Aside from one small cluster of wretched huts, it was practically uninhabited. Guarded by dense growth, only one or two of the dusty paths which passed for roads wandered aimlessly through its tangled creepers, trees and bush. To the southeast was the broad Gulf of Panama, doorway to the Canal; on the other sides this thumb of land was surrounded by the reaches of the Pacific.

The plane was obviously nearing its eyrie—dropping lower and lower, losing speed and altitude; and also threatening each moment to tumble down out of control into the smothering welter of olive-green below, with a dead, unseen body in its cockpit.

But where was the landing field? They were now over the very heart of the Peninsula, and still Chris, searching through his telescopic sight, could see nothing but the monotonous roll of jungle. They must come to it soon, or be over to the Caribbean Sea and the Mosquito Gulf.

Then suddenly he started forward, staring. Of course there was no landing field in sight. The mystery plane needed none. It possessed the power of the helicopter: it could rise straight up or sink straight down.

From each one of the two knob-like projections on its upper wing that had puzzled him previously, a propeller had risen and unfolded into long, flat blades. They whirled in circles of light in the sun; and the airplane beneath them poised, all but motionless, its main propeller swinging idly, and then began slowly to drop downwards.

But Chris, swooping nearby, was still perplexed. Dropping down to what? There was only the dense tropical growth beneath. He could see no trace of men, no clearing, however small, no base—nothing but the jungle.

"How in the dickens—" he began; and then stopped. At that moment the jungle's secret was revealed.

A s the helicopter-plane dropped to within a few hundred feet of it, a strip of the sea verdure split in two and reared up. It looked, at first, like magic. But from aloft Chris saw the trick and how the camouflage was worked. What appeared to be a slice of the jungle roof was, in reality, a metal framework cunningly plastered with layers of green growth. An oblong, some fifty by a hundred feet, it parted in the middle like a bridge that opens to let a steamer through, revealing the lair of the plane.

Soon more was revealed. Two tiny, green-painted huts stood in the minute clearing, and a few white-clad figures were by them, staring up at the plane sinking down and at the other plane which soared above like a buzzing mosquito.

One of the dwarfed figures in white waved an arm. The others around him darted into the left-side hut. Then the helicopter-plane's wheels touched the small space allotted for it in the clearing, and the whirling propellers halted.

"So that's the secret!" Chris muttered. He pulled the microphone of the radio-telephone to his lips and angled with the dials for connection with the fleet hundreds of miles behind, meanwhile noting his exact position on Azuero Peninsula. But before he spoke, some sixth sense bade him glance below once more.

An icy shiver gripped his body.

A thin slit had appeared in the roof of the left-side hut. A spot of bright blue light was winking evilly inside it. And, though he could not hear it, Chris knew with terrible certainty that a shrill, impatient whining was piercing from the machinery of a weapon inside that hut—a weapon whose fangs had forked close to him once before—a weapon which the winking eye of blue presaged.

It struck. But at the same instant Chris leaped desperately from the cockpit of the scout.

H e leaped almost into the teeth of the blue-tinged ray which knifed up with uncanny accuracy from the slit in the roof of the hut. He was conscious of a flash of unearthly light, of terrible heat which came with it. Only the force of his jump saved him. He pulled the ripcord of the 'chute strapped to him and jerked to a pause; then he was swinging beneath a mushroom of white, trembling as he stared at the fate he had missed by a hair's breadth.

A web of spectral blue light had enveloped the abandoned scout. The plane appeared to shudder, hanging almost motionless in the wraith-like mist. Then, with a crackle, the wings and tail shivered into countless fragments; the stripped fuselage nosed over and plunged earthward, a roaring mass of flames. A fiery comet, it screamed past the man who swayed beneath his 'chute, coming within a few hundred feet of him and searing him with its hot breath. Then it drove into the dense flanks of the jungle growth.

Soon only a charred skeleton marked the last landing field of a scout of the dirigible ZX-1.

"And now, I guess," Chris whispered, "they'll turn that ray on me...."

But he had only been a thousand feet up when he jumped. Already he was close to the top of the jungle. The clearing and its huts disappeared from view; he was out of range of the swift-striking ray. And, he reflected, though the scout was gone, he was still free—and could get to the Canal....

But tropical growth is difficult to land in.

A moment later his swinging body crashed through the branches of a tree, and he pitched forward, unable to control the impetus. A sudden shock of pain stabbed through his head and everything spun dizzily before him. He knew he was falling, jerking down as the parachute ripped on the boughs. There was another impact which drove all remaining consciousness from him.

Darkness washed over Chris Travers, lying limp beneath the shreds of a silky white shroud....

E lectric light. A strong glare of it somewhere. A dull throbbing in his head. Then, a voice, with queer, hissing s's, speaking very close to him.

"Ah, yess. Look you, Kashtanov. He will be conscious soon, I think."

"You're a damned fool, Istafiev, to let him wake up," said another voice, cool and of easy correctness. "He'll see the machines. And these Americans are tricky—one can never tell."

"Tricky? Bah! This fellow is a service man; there are things I can learn from him. Come, now, wake yourself properly, you! That glass of water, throw it on his face."

Kashtanov—Istafiev. Names that could belong to only one country, to that huge power overseas which was hovering, so said rumors, on the brink of war, waiting only for a favorable opportunity to strike—the country which the war game around the Canal had been designed to impress. Chris Travers' mind cleared just then with complete comprehension of who had schemed to send both dirigibles down and who had built this secret lair on Azuero Peninsula.

Inwardly, he groaned. It was all too plain. The destruction of the ZX-2 and the thwarted destruction of her sister had only been the first step of some gigantic plan which was to provide the opportunity for the mighty fighting machine overseas to strike. And he, who might have balked it, had made a rotten landing from the scout and delivered himself, helpless by his own clumsiness, into the hands of these men. The self-accusation was bitter.

With their secret of invisibility, their deadly blue rays, what havoc couldn't they wreak, working from their cunningly concealed base?

And now they were waiting for him to recover consciousness—waiting to question him before killing him....

But as he lay there, apparently still senseless, Chris was grappling with the seemingly hopeless problem. So, even when he felt the tingling coldness of a spray of water on his cheeks, not one line of his face moved, nor did the tiniest flutter of eyelids betray him.

Although the fumbled landing in the jungle had been a catastrophe, it had granted him his only weapon. He was believed to be genuinely unconscious.

"Another—he iss stubborn," hissed the voice of the man called Istafiev. "His senses will soon come. I can bring them back—oh, yess!"

"Enough of this!" complained the suave, beautifully modulated voice. "Darkness is coming; there's a lot to be done. Shoot him and throw him out!"

"It iss I who am in command here, comrade Kashtanov. Remember that. I desire to speak to this man. There! No? No sign yet? Well! We will see if this helps those eyes of yours to open, my American!"

Then began sheer torture.

I t was an ordeal of silence. By no motion, sound or slightest sign of consciousness could he seek relief. Inanimate Chris Travers lay, holding his pose sturdily, although it seemed that the sweat was spurting from the pores, while a thin, cruel knife-blade drove into the quivering nerves beneath his left thumb-nail.

Deeper and deeper it inched, accompanied by the soft breathing of the man who guided it, until Chris felt one great sob of pain welling up inside him, struggling to break past his lips; felt a tremendous urge to writhe, to break away from the digging steel. His tongue seemed to be trembling, shivering; but no other part of his body, not even the smallest flicker of eyelash, betrayed him. At long last there came a voice, sounding as if from miles away, and the disgust in it was very good to Lieutenant Christopher Travers.

"Bah! It iss no use. His thick skull must be fractured. I could cut him open and he would not awake. He might be conscious for minutes after some hours—no, do not shoot him. I shall learn a few details from him then. Throw him over there. Now—Zenalishin iss dead, but the mask and cylinder on him should be returned to visibility. Well, we will return him, too. Then, Kashtanov, to your instructions and your work."

Hands gripped Chris's body. He felt himself thud against a wall, and slumped into a heap, head lolling over. The cessation of pain was sweet, though his thumb was raw, but sweeter still was the knowledge that he had won the first tussle: that he was deemed to be harmlessly unconscious for hours.

And carefully, through his lashes, he permitted himself a glimpse of the room he lay in, and the men whom he had heard and felt but not yet seen.

I t seemed more like the belly of a submarine than a room, that maze of tubes, levers, wheels, switchboards and queer metallic shapes; and the blur cast upon his vision by barely raised eyelashes made it appear doubly unreal and grotesque. It might have been another world.

Some of it was recognizable. A massive radio-telephone set, by which, he judged, all communications between the fleets in the Pacific were overheard; a squat dynamo; a set of huge cylinders, from which, probably, had come the highly expansive gas that had snuffed out the crews of the two dirigibles. But there were other things—strange, monstrous. One of them, the tapered tube of metal that angled up to the hut's ceiling, its base a mass of wheels and dials and tubing, was evidently the weapon of the ray that had struck the scout down.

There were three men visible in the room, and Chris switched his attention now to them.

Two were standing by a table in the center of the room, directly under a shaft of light from a powerful electric bulb. The shorter of them was saying to a third man, who knelt in front of the dynamo:

"On full." Then, as a full-throated drone pulsed from it: "Zenalishin iss there? Yess. Put him in."

The voice of the hissing s's—that was Istafiev. Short, stocky, black-haired, he was a direct contrast to the tall figure next him of one whose pointed black beard gave elegance to sharp, thin features. He carried a gun at his waist, and he identified himself as Kashtanov by saying languidly:

"Better strap him in. He'll fall, otherwise. Get some cord; I'll lift him."

The other man, by the dynamo, apparently a subordinate mechanic, dull-faced, drew a loop of cord from a box nearby, while Kashtanov went through actions that seemed fantastic. He stooped, groped along the floor, and then gripped what looked like thin air with his fingers and lifted upwards. But it wasn't air, Chris knew; it was the invisible body of a man—the man who had destroyed the ZX-2, the man whom he had shot at in the cubby of the ZX-1—whose invisibility was now to be stripped from him.

By what? Carefully Chris swivelled his gaze around until it caught on an object which dwarfed Istafiev, now waiting by its side with one hand on the small panel of a switchboard.

A strange thing, truly, to find in a little hut on Azuero Peninsula! Row upon row of slender curved tubes, describing a three-quarter ovoid so that there was an opening for entrance in front, rose to a height of some eight feet, the whole topped by a curious glassy dome which was filled with creamy substance. There was room inside the layers of tubes for a man's body to stand upright—and a man's body was upright in it now, held by cords strapped to his unseen arms.

Invisibility! The dream of scientists for years! Here created, here taken away—by the simple manipulation of two levers on the control panel.

Intently Chris watched Istafiev pull down the right-side lever.

As it came down, the creamy liquid in the dome above the cage began to swirl slowly, then to froth and boil and whip round and round, while thick, dropsical bubbles slid up from its heaving surface and burst, discharging a kind of grayish mist, under which the white substance sank, until there was nothing left in the dome but drab-colored vapor. On the completion of this stage, the layers of tubes below warmed into life. They glowed with a soft vari-colored brightness that filled the cage with a golden splendor and seemed to rim each one of the watching men with fire.

"See you, Kashtanov," came Istafiev's voice. "The refractive index, lowered to that of air to produce invisibility, iss being raised again—all through a simple adaptation of Roentgen's theories! The substance above, mark, in the dome, which this morning you saw affect Zenalishin's blood and the pigment of his hair so that the vibrations would render his colorless tissues transparent, iss now reversing. Soon—see!—already he becomes visible!"

Something was growing in the heart of the ribbons of color, and Chris strained his shrouded eyes to discern what it was.

Black lines, standing out in the dazzling welter of light—lines that grew and became more solid as he peered at them—lines that were shaping into a recognizable form, the form of a man's skeleton!

The effect was that of an X-ray. A skeleton hung in the cage, held steady by the cords around its arms, its naked skull with yawning eye-pits grinning out at the four men in the room. Soon other details became visible: black lumps that were organs, the web of fine thin lines that were veins; and then a hazy, ghostly outline of flesh that quickly assumed solidity, burying the bones and veins and organs which had been first apparent.

A nd all the time the dynamo was filling the hut with its sweeping drone, and the million points of light flung from the intercrossing flame-tongues inside the cage were dancing madly on the walls and floor and ceiling, making the whole scene unreal, fantastic, as from a dream....

"There! That iss enough," said Istafiev.

The lever went back. The streaks of blue-white that threaded the cage died; the grayish vapor in the dome above faded away, leaving more of the creamy, bleaching substance than had been there originally; the dynamo was shut off, and silence fell in the room. A naked man with a very white, peaked face and a blotch of blood encrimsoning his neck hung inside the cage, his head pitched over lifelessly to one side.

Chris stared, almost forgetting the pose of unconsciousness in his bewilderment. A queer mechanism shaped in the form of a cylinder from some oddly sparkling, almost transparent material, was clasped to the nude body's chest: over the nose and mouth was another small attachment of the same substance. A nozzle midway in the large cylinder's front side gave him the clue: from it, obviously, had come the gas which had strangled the crews of the dirigibles, and the covering over nose and mouth was a novel gas mask. The material they were made of could, obviously, be rendered invisible—a virtue not possessed by ordinary inorganic substances. Invisible death from an invisible container, carried by an invisible man!

"Yess, dead," hissed Istafiev, probing the motionless, naked body. "He just got here, told what had happened, and died. He was hurt too badly to think of taking off the gas cylinder or putting on a coat. Well, it makes no difference.... Here, Grigory, take off the mask and cylinder and bury him. And you, Kashtanov, look well at this."

From the table, he picked up a large white piece of cardboard and tapped it meaningly. There were two broad lines on it, running side by side through other smaller lines and shaded patches, and there was also a thick black arrow pointing to one particular place on it.

The chart was easy to understand. Chris Travers recognized it immediately, and his heart seemed to stop for a moment as he did.

Their first step had been the dirigibles: their second was a blow which paled the other into insignificance. And Chris told himself desperately:

"It can't go through! It can't!"

The lines on the cardboard were a detailed map of the Panama Canal; and the black arrow pointed unerringly to its most vulnerable, unguarded and vital point, the Gatun Spillway, which, if wrecked, would put the whole intricate Canal hopelessly out of commission.

I stafiev was speaking again, in low, terse tones, oblivious of the desperate resolve forming in Chris's brain.

"Only one of the dirigibles had been destroyed. Well, it iss too bad, but not fatal to the plan. The ZX-1 can hamper our country's operations when she strikes, but if the ZX-2 were also in action, they would be hampered much more—perhaps fatally. It iss not serious. So we go ahead. Now, Kashtanov, for the last time, the scheme of wrecking Gatun Spillway iss this:

"Note, here, the small golf course. That iss your landing space. You know its location: a mile, perhaps, from Gatun Dam and the spillway. At night, there iss no one near it or on it. You drop down to the golf course from seven thousand feet: the helicopter motors are muffled, and no one will hear you come. Some of the stretches of the course are secluded and hidden by the surrounding jungle; choose one of these to land on. Well, that iss easy.

"The spillway iss about midway in Gatun Dam: its channel has been cut through a hill. You come along the side of this channel right up close to the spillway—close, remember!—and leave the box there. The range of the rays, you know, iss two hundred feet: set them to fire one minute after you leave the box. They will destroy the seven gates of the spillway and also part of the dam and the hydro-electric station. Gatun Lake will then empty itself; the canal will be half drained; the power will be gone—it will take half a year to repair it all. The ZX-1 can fly up to the east coast, thanks to Zenalishin's fumbling—yess; but these American fleets are massed in the Pacific; they will have to go around South America to reach the Atlantic—and that will take weeks.

"And in that time the Soviet has crossed the Atlantic uncontested and has paralyzed the heart of America, her eastern states. Ah, it iss magnificent!"

B ut Kashtanov's thoughts were elsewhere. Peering hard at the chart, he said:

"I have a minute to get clear, eh? Well, I can do that; but won't the water sweeping through from Gatun Lake after the spillway is wrecked catch me?"

"No. You run up the hill the spillway channel is cut through; it iss high ground, and the golf course iss on high ground. No one will see you coming or going, naturally, and the box iss not big enough to be noticed at night. The noise of its equalizers will be covered by the water coming through the spillway. It iss—what they say?—fool-proof. You cannot fail, Kashtanov. And—" he broke into swift-flowing, liquid Russian, his swarthy face lighting up, his arms waving, one of them slapping the other's back.

"Stop the dramatics," said Kashtanov, "and speak in English. I've worked so long in America, Russian is hard to understand. Time to begin?"

Istafiev glanced at a watch on his wrist. "A few minutes. Look you." He went to a side locker in the room, opened it, hauled out with both hands a box of plain dull metal, and put it on the table. It was larger than the one Chris Travers had seen on the ZX-1, but otherwise similar.

"A double charge of nitro-lanarline iss in this," murmured Istafiev complacently. "Imagine it, when released! You know the working well, do you not? Yess. Well, I put it in the plane, ready." He stepped to the hut's single door and passed out. Through it Chris could see the tiny clearing, dark under the camouflaged framework, now closed once more; the light from the hut showed him the wings of the helicopter-plane standing there. He heard, moreover, the sound of a shovel from somewhere, and knew that a lonely grave was being dug in the wilderness. Then Istafiev shouted:

"Grigory! That grave, make it wide, make room for two." He came back and peered sidewise at Chris. "Not conscious yet?" A foot thudded into the American's side. "No. Well, I see to him when you are gone, Kashtanov. Yess, thick darkness iss here. Time to begin. Take off your clothes."

C hris was now keenly alert, poised, ready for any chance that might come. The odds were two or three to one, and a gun into the bargain, but the stakes were higher than any ever played for before; and a stroke had to be made, no matter how seemingly hopeless. Through his lashes he watched, turned things over in his mind—and something leaped within him when he saw Kashtanov unbuckle the gun around his waist and lay it down, meanwhile taking off the clothes he was wearing: and when he heard the question which followed, and Istafiev's answer.

Naked, lean-muscled and sinewy, Kashtanov paused before the door of the cage. "How will this affect me?" he asked. "Painful?"

"You will be conscious of no sensation. You will see me, yess, and the room, but you will be paralyzed completely while the power is on."

"Paralyzed, eh?" murmured Kashtanov. "Well, let's go," and he placed himself inside the cage.

Paralyzed, when the power was on! In effect, that left only Istafiev in the room: the man Grigory was outside, and the noise of the dynamo would drown any shouts for help. And Kashtanov's gun was on the table....

Imperceptibly, Chris's muscles tensed as he judged the distance to the table and reckoned out each movement after the first leap. One sweeping blow with the gun would put Istafiev safely out of action; then he could be bound and Grigory summoned and tied also at the point of the gun. If, by that time, Kashtanov was invisible inside the cage, the levers could be reversed and his body brought back to visibility and bound.

Then—a call broadcast from the hut's radio-telephone to headquarters at the Canal and the fleets in the Pacific!

"It'll work," Chris told himself. "It's damn well got to!"

But a certain part of the invisibility machine did not enter his plans.

T he creamy liquid in the glassy dome began, as before, to swirl slowly: but apart from that its action was different. The white mass, instead of discharging the vapor-laden bubbles, became a whipping, highly agitated whirlpool as the tubes below glowed softly and ribbons of golden light snaked out and laced through the nude body of Kashtanov. The liquid above flowed rapidly in a complete circle, its center sucked hollow, exactly as a glass quarter-filled with water behaves when rotated quickly. Thus the outer surface of the dome, coated inside with the milky liquid, gleamed and scintillated as the whirl of light struck it and danced off it: and it even became dimly reflective.

In seconds Kashtanov's figure lost definite outline and assumed a ghostly transparency that bared the internal organs and veins: and then his skeleton appeared.

Istafiev was facing the control panel. As he gathered his limbs for the decisive leap, Chris's eyes were on his stocky back. But Istafiev was watching keenly the gleaming, glassy dome above.

He was surveying the action of the white substance and judging the time of the process by it. Then suddenly his vision centered on something that had seemed to move on the surface of the dome.

Something had moved. Chris, lying against the wall behind, had opened his eyes fully, had dragged back his legs beneath him and balanced himself for his leap. And, in distorted perspective, his actions were reflected on the dome.

Just for a second he poised—then sprang.

The speed Istafiev showed seemed foreign to the build of his body. In an instant he had whirled from the switchboard, fingers not lingering to release Kashtanov, and leaped.

T hey met at the table. Two hands shot out for the gun lying on it. Chris grabbed it first. But he paid for his speed. The swipe he had aimed with his left arm went wild; a fist thudded into his stomach and belted the wind from him, and he felt his gun-wrist seized and wrenched back.

Gasping for breath, dizzy, only the fighting instinct enabled him to crane a leg behind the other and throw his whole weight forward. The planks of the floor shivered under the two bodies that toppled onto them.

There was a melee on the floor, furious, savage, mad. In cold fact, it lasted merely for seconds; but Chris was grappling with a man whose strength was as desperate as his own, and who had not been weakened by a solar plexus blow or a cramping wait of hours in one position: the American had passed through an eternity of physical and mental agony when Istafiev, hunching up, strained the finger of his right hand upward, searching for the gun trigger.

One stubby finger found it. Istafiev grunted. The gun trembled from the force of the hands disputing its direction; then its ugly snout, stuck out parallel to the floor, and began to creep slowly downwards as Istafiev bore on it with all his might.

"So!" he hissed. "It was clever, your little game, but it iss finished!"

But Chris, undermost, had braced his elbow on the floor. The gun held. Every ounce of his strength went into holding that one position, into keeping the weapon's muzzle away; he was therefore not prepared for Istafiev's sudden strategy.

There was a quick pull, a tug. Istafiev had wrenched himself over, reversing their positions and dragging Chris uppermost—and, as the American's balance was destroyed, the gun whipped up and fired.

A bullet sang past his head. It missed by inches. But from behind came a sound as of rending cloth. The glassy dome above the cage of the machine had splintered into countless fragments.

The effect was amazing. The shafts of light from the machine's tube ceased; creamy liquid dribbled out from the cracked dome, and, as it met the air, frothed into billows of dense gray smoke. In seconds, the room was choked with a thick, foggy vapor that obscured every object in it as well as the blackest of moonless nights.

I stafiev had not fired again, could not. With a quick, frantic wrench and twist Chris had knocked the gun from his hand, and it had slithered away, now lost in the bunching smoke. But Istafiev's other hand, steel-ribbed with tense muscles, had darted like a snake into the American's throat, and under that iron, relentless grip Chris was weakening. His head was whirling; the old wound throbbing waves of nausea through him. Desperately he tried to struggle loose, flailing with his legs—but useless. He knew he was slipping; slipping....

Then, out of the gray, all-hiding mist, came a voice.

"Istafiev! Where are you? Call! The machine's broken; I'm out and invisible. Where is the American?"

Kashtanov!

Istafiev hissed:

"It iss all right. He will be finished in a moment. But you—go! The box iss aboard the plane; don't wait! You must not take chance of being hurt. Go to your work. Call Grigory in. Go, Kashtanov!"

"I go, Istafiev."

"No, you don't!" Chris Travers croaked almost inaudibly. " You don't! "

Thought of the Canal lying there defenseless, of Kashtanov speeding towards it on his wrecker's errand, kindled within him a strength that was unnatural, superhuman. Like a wildcat he tore loose from the choking grip on his throat; Istafiev tried to subdue that sudden, unlooked-for surge of power, but could not. Five piston-like, jabbing blows crunched into him from Chris's hurtling fist, and with the fifth Istafiev faded quietly out of the picture....

Chris sprang up and started a leap for the door he could not see. A body brushed against him; dimly through the smoke he saw the man called Grigory, and Grigory saw him, but not for long. A whaling swing lifted him two inches clear of the floor, and then he went down onto the peacefully recumbent Istafiev; and Chris Travers, fighting mad, stormed from the hut into the clearing outside.

The camouflaged framework had been raised; soft motors were purring helicopter propellers around and lifting a plane up towards the stars hanging high above.

The airplane was already feet off the ground and sweeping straight up. A sane man wouldn't have thought of it, but Chris wasn't quite sane just then. With a short sprint, he launched himself into a flying leap, grabbed out desperately—and felt the bar of the undercarriage between his hands.

The plane jolted. Then it steadied; rose with terrific acceleration. And Chris hauled himself up onto the undercarriage and clung to one of the wheel-stanchions, breathing, hard, hidden by the fuselage from the invisible pilot.

The clearing and the hut, with smoke billowing from it, dropped into nothingness. The night enclosed the helicopter-plane.

F rom the air, Panama Canal at night is a necklace of lights strung across the thin neck of land that separates sea from sea. Then, as a high-flying plane drops lower, the beams of light loosen into widely separated patches, which are the locks; between them the silky black ribbon of water runs, now widening into a dim, hill-girt lake, now narrowing as it passes through massive Culebra Cut, then widening again as it comes to the artificial Gatun Lake, at the far end of which stands Gatun Dam and its spillway.

Silence hung close over the Canal. The last ship had passed through; the planes that daily maneuver over it had returned to their hangars; the men who shepherd ships through the locks had gone either to bed or to Panama City or Colon. The Canal, as always at night, seemed almost deserted.

To Chris, clutching tight to his hazardous perch, it looked utterly deserted. The ride had been nightmare-like, fraught every second with peril. Several times the whip of wind had come near tearing him loose; the cold air of the upper layers had numbed his fingers, his whole body; he was chilled and, experiencing the inevitable let-down which comes after a great effort, miserable. Just then, the task ahead appeared well-nigh impossible.

The only thing in his favor was that Kashtanov apparently did not know he was aboard, since the plane had flown evenly, steadily, not trying to shake off the man hanging to its landing gear by somersaulting in the sky. Evidently the jolt as it was rising hadn't warned the unseen pilot; the fog from the broken machine had obscured Chris's wild leap.

But what, he thought, of that? The element of surprise was in his favor—but how to gain advantage by it? He had no weapon, nothing save bare hands with which to subdue a foe as elusive as the wind that was now hurtling by him. Clinging there, slipping now and again, drenched with cold, the odds looked hopeless.

Then, suddenly, the booming of the main motor stopped. Only a quiet purring from the wings took its place. The helicopter-plane hovered almost motionless, quiet and deadly like a sinister bird of prey. It began to drop straight down through the dark. Chris Travers glanced below.

T here, misty, fainty, small as the toy of a child, lay Gatun Dam, with the spillway in its center.

Chris stared. So small the dam looked—this dream of an engineer, this tiny outpost of man's genius thrust boldly into the breast of the tropics, holding back a whole lake with its cement flanks, enabling ocean to be linked to ocean! It was the heart of the Canal; if burst, the veins would be drained.

Something that cannot be caught in words seemed to seize the lone American then. As in a trance, he saw more than the dam; he saw what it symbolized. He saw the Frenchmen who had tried to thrust the Canal through first, and who had failed, dying in hundreds. He saw the men of his own race who had carried that mighty work on; saw them gouging through the raw earth and moving mountains, tiny figures doing the work of giants; saw them stricken down by fever and disease, saw others fill the empty files and go on, never wavering. He saw them complete it and seal the waters in captivity with the dam that lay below....

And with that vision of stupendous achievement, cold, weariness, hopelessness passed from Chris Travers and swept clean away. The odds that had loomed so large fell into insignificance.

The golf course spread out and became dimly visible as the plane dropped cautiously down. Away to the left there were the few twinkling lights of Gatun Dam, whitening the crests of the waters that tumbled through the spillway. Their drone was dully audible. On every other side dark rolling hills stretched, covered in untamed jungle growth. The golf course was shrouded by them. Its smooth sward made a perfect landing place; an ordinary plane might alight there.

Lower, lower, ever so slowly. A bare one hundred feet, now. Chris scanned the lay of the land. Right close to the spot Kashtanov had chosen to set the plane down on was a deep sand-trap, put there to snare unskilful golfers. Chris steadied himself on the cross-bar.

"I'll have to go up over the side and grab him," he planned. "Then hold on to his throat till I feel him go limp."

The wheels of the plane touched gently, and she settled to rest.

I n one furious movement Chris was off and springing up the side of the fuselage into the single cockpit, his hands clutching for the invisible man who sat there.

He heard a croak of alarm; then his fingers thumbed into bare flesh and slid up over a nude shoulder to the throat. They tightened, bored in, held with terrible pressure. Sprawled over the cockpit, he clung grimly, to what seemed nothing more than air.

Spattering noises came from somewhere. An unseen body thrashed frantically. Transparent hands clawed over the American's frame, worried at him. But he held his grip, tightening it each second. There was a gasping, choking sound, a desperate writhe, another scratching of the invisible hands—and then came what Chris had feared, what he could not guard against since his eyes could not forewarn him. A heavy monkey-wrench appeared to rise of its own accord from the floor of the cockpit and come swinging at his head.

He ducked at the last second. But it clipped him; his brain whirled dizzily. The next moment he slithered off the plane and fell to the ground, dragging the unseen Kashtanov with him. And as he pitched into the damp grass, the shock dislodged his grip.

He was up in a flash, but the damage was done. The monkey-wrench curved through the darkness in a vicious swipe that landed it flush against his jaw; swung back, pounded again like a trip-hammer—again and again and again....

Chris reeled back, teetered on the edge of nothingness, then went tumbling crazily down into the sand-trap behind. One leg was doubled underneath him as he crashed.

A voice floated down out of the darkness. "That is the end of you!" it said. But Chris Travers did not hear it....

P ain. Agonizing pain. The whole lower side of his face was a burning, throbbing, aching lump of flesh, and his left leg seemed on fire. What had happened? Where was he?

Then came remembrance, and it was far worse than the fangs of pain that were gnawing him. Chris cried out—a cracked, twisted cry. Kashtanov, the dam—the box of the ray! How much time had passed?

He hunched his body over and stared up. Limned against the starlight were the wings of a plane, still standing where it had landed beside the sand-trap. He clutched his thoughts. The plane meant—it meant Kashtanov had gone on his errand, had not yet returned? Only minutes had gone by since the blows from the monkey-wrench. But was the box placed yet? Was Kashtanov already hurrying back?

He listened. From far away came a drone that was separate from the throbbing of his head. The drone of waters, controlled waters. The normal sound of the spillway of Gatun Dam. The box had not yet unleashed its disintegrating bolt of blue.

"I've got to stop it!" he whispered.

He tried to rise. Only one leg held. The other twisted awrily with a rasp of broken bones. A spearing pain tore through him. Useless! His fall had broken it. He could not rise, could not walk, much less run. He was no more than a cripple.

"Oh, God!" he groaned, "How can I, how can I?"

Then his eyes fell on the plane resting above him.

"I've got one leg," he muttered, "and two hands and two eyes.... They're left me. Yes!"

He rolled over. He shoved with his right leg and clawed at the bank of the sand-trap. Inch by inch he wormed up, slipping, scraping. The sand grated into his battered face and seeped through onto his tongue; he coughed and spluttered, groaning from the effort and his feebleness. Spots of blood showed black against the crazy course he left behind him; ages seemed to pass before he thrust his head over the top of the bank, dug his chin into it and pulled onto level ground. Ages, but in reality only seconds, and the whole Canal—America—lying at the mercy of what each one of those seconds might unloose!

B ut the plane was near now, and it almost seemed that some unseen force mightier than the strength of men hauled Chris's broken body to it and up the stretch of its fuselage-side into the cockpit.

Ordinarily, he should have been delirious from the pain of jaw and leg, but the controls of the plane were before him and he saw nothing else. Wings and propeller were better than legs! He was in his element: by the sixth sense of born airmen, he knew and could handle any flying machine, no matter how foreign.

In a second, his fingers had fumbled onto the starting button. The choke of the motor and then its full-throated roar were sweet to his ears.

The lonely golf course and the night re-echoed with the bellow of twelve pistons thrusting in line; watching, one would not have dreamed that a cripple was at the controls of the plane that now swung around with a blast of power, leveled its nose down the course and raced smoothly over close-clipped grass. Its wheels bumped, spun on the ground and lifted into air.

A mile to the dam! Istafiev's words came back to him. It would take Kashtanov twenty minutes at least, for he would go cautiously. But how long had passed—how long? That was the agonizing question.

Staring forward through the hurtling prop, the night rushed at him; the dark hills melted away to either side; clear ground swept into view and then a long black thread that was the spillway channel. Behind was the bubbling, leaping flow of the spillway itself, and Gatun Dam. The smooth cement sides were as yet unharmed.

"Thank God!" Chris muttered. "Now, where—where?"

A stream of light flowed out from the hydro-electric station on the left side of the spillway channel. The opposite bank was bare, running right up to the face of the dam beneath the spillway's seven gates. There the box was to be placed. But from the air, the light was uncertain, deceptive—and a little two-foot-square box was all he had to go by!

"I can't see!" Chris said hoarsely. "I can't see!"

L ike a roaring black meteor the plane hurtled over the banks of the spillway, the eyes of its pilot scouring the ground. It zoomed just in time to miss the wall of the dam, banked, doubled like a scared jack-rabbit, dove down again, coming within feet of the spillway channel. Mad, inspired flying! But what good could it do?

Then from its cockpit came a yell.

"There! There! By heaven, I can make it!"

Two or three hundred feet—it was not clear just how far—from the face of the dam, on the bare right bank of the channel, a tiny pin-prick of black was moving slowly along. It seemed to move by itself through the air. And now, as the screaming plane banked again and came rushing closer, the pin-prick grew into a black box that suddenly stopped its advance, held motionless some four feet off the ground. Though the man who held it was not visible, Chris could fancy him staring up at the plane, could fancy the look of consternation on his unseen face.

Two hundred feet was the range of the rays! Was Kashtanov that close? Obviously the controls had not yet been set, for he still held the box. But he could switch them on in a second and fling the deadly machine up toward the dam, if he were at present just out of range. A second—a second!

"Damn you, here goes!" roared Chris.

He wrenched the stick way over. The plane appeared to hang crazily on one wing. Then it leveled off and stuck its nose down, flipping its tail up, and down—down—down it bellowed; with no hope in the world of ever coming out of its insane plunge.

What he saw in that last momentary glimpse was burned forever into Chris Travers' memory. There was the black box, hanging in the air straight before the plane's thundering nose; there, behind it, the black tide of the spillway waters; and, still further behind, he could see the other bank and the hydro-electric station, and a few tiny figures that rushed out from it just then to see what some fool flyer was doing.

All this flashed into his sight, etched against the sable night as if in flame. Then the plane's snout smashed into the black box hanging before it, and the propeller crunched through a naked, invisible body. A ragged scream that marked the passing of Kashtanov split through the air for a flash of time, and the dark, blurred mass that was an airplane teetered clean over and flopped into the rushing spillway channel.

T he men who had scrambled out from the hydro-electric station stared at each other blankly. One of them stuttered:

"But—he did that deliberately! Nothing went wrong with his ship! I saw him! He seemed to be diving at something!"

"Come on!" snapped another. "We might be able to get him out. A mad fool like that's just the kind who'll live through it."