In the burning solitude of the great Arizona desert, some two miles south of Ajo, a young scientist was about to perform an experiment that might have far-reaching results for humanity.

The scientist was Gordon Kendrick--a tall, tanned, robust chap who looked more like a prospector in search of gold than a professor of physics from the State University of Tucson.

Indeed, he was in a way, a prospector, since it was gold he sought--some practical method of tapping the vast radio-energetic treasure of the sun--and it was an apparatus designed to accomplish just this that he was about to test.

The primary unit of the mechanism comprised a spheroidal vacuum-tube measuring a little over a foot across its long axis, mounted in a steel bracket that held it horizontal with the ground. Down through its short axis ran a shaft on which was centered a light cross of aluminum wire, carrying four vanes of mica, one face of each coated with lampblack. A flexible cable led from the bottom of this shaft to the base of the bracket, where it was geared to a small electric motor driven by two dry cells. A rheostat-switch for delivering and controlling the current was mounted nearby.

At the wide arc of the egg-shaped tube was a concave platinum cathode, at the narrow arc a nib of some sort, ending in a socket. From this socket, two heavy insulated wires extended sixty feet or so across the sand to the secondary unit of the mechanism, which was roughly a series of resistance coils, resembling those in an ordinary electric heater.

As Kendrick prepared to test this delicate apparatus that represented so much of his time and thought, held so much of his hope locked up in it, a turmoil was in his heart, though his brown face was calm.

If his theories were right, that revolving cross would tap and draw into its vanes radio-energetic waves of force, much as the whirling armature of a dynamo draws into its coils electro-magnetic waves of force. For the blackened sides of the vanes, absorbing more radiation than the bright sides, would cause the molecules to rebound from the warmer surfaces with greater velocity, setting up an alternate pressure and bringing the rays to a focus on the cathode, where they would be reflected to the nib as waves of heatricity, to use the word he had coined.

Those were Kendrick’s theories, and now he moved to put them to the supreme test. Switching on the current, he set the motor going. In response, the cross began to revolve, slowly at first--then faster, faster, as he opened the rheostat wider.

Eyes fixed on his resistance coils, he gave a sudden cry of triumph. Yes, there was no doubt about it! They were growing red, glowing brightly, whitely, above the intense desert sunlight.

Here was a means of convening solar radiation into heat, then, that offered tremendous commercial possibilities!

But even as he exulted, there came a blinding flash--and the overtaxed coils burst into flame.

Shielding his eyes from the glare, he reached for the rheostat, shut off the current, rushed to his secondary unit--where he beheld an amazing sight. Not only had this part of the apparatus completely disintegrated, but the sand of the desert floor under it as well. On the spot quivered a miniature lake of molten glass!

As Kendrick stood ruefully beside that fiery pool, meditating on the spectacular but not altogether gratifying results of his experiment, a peculiar low humming sound reached his ears. Rushing back to his primary unit, with the thought that perhaps by some chance he had not fully closed the rheostat, he looked at the cross. But no, the vanes were still.

The humming increased, however--grew into a vibration that made his eardrums ache.

Puzzled, he looked around. What on earth could it be? Had his unruly experiment called into play some tremendous, unsuspected force of the universe. Was he to bring the world to ruin, as a result of his blind groping after this new giant of power?

Such predictions had often been made by the ignorant, to be dismissed by scientists as the veriest nonsense. But was there some truth in the universal fear, after all? Was he to be the Prometheus who stole fire from Olympus, the Samson who toppled down the temple?

Chilled, dizzied with the pain of the ever-increasing vibration, he gritted his teeth, awaiting he knew not what.

Then it came--a spectacle so staggering that he went rigid with awe as he regarded it, all power of motion utterly numbed for the moment. The vibration ceased. The thing appeared.

It was a city--a city in the air--a flying city!

As Kendrick stood staring at this phenomenon, he could scarcely credit his senses.

Had the magic carpet of Bagdad suddenly materialized before him, he would not have been more astounded. And indeed, it was in a way a magic carpet--a great disclike affair, several miles in diameter, its myriad towers and spires glinting like gold under the noonday sun, while its vast shadow fell athwart the desert like the pall of an eclipse.

The lower portion, he noted, was in the main flat, though a number of wartish protuberances jutted down from it, ejecting a pale violet emanation. Whatever this was it seemed to have the effect of holding the thing motionless in the air, for it hovered there quite easily, a hundred yards or so above the ground.

But what was it? Where was it from? What had brought it?

Those were the questions he wanted answered; and they were to be, sooner than he knew.

As he stood there speculating, a device like a trap-door opened in the base of the disc, and creatures resembling human beings began descending. Began floating down, rather.

Whereupon Kendrick did what any sensible man would have done, under similar circumstances. He reacted into motion. In short, he ran.

Glancing back over his shoulder after a minute or two, however, he drew up sheepishly. Of that strange apparition and those who had descended from it there was not a trace, not a shadow!

But the peculiar humming had recommenced, he realized in the next breath--and at the same instant he felt himself seized by invisible hands.

There was a struggle, but it was brief and futile. When it was over his captors became visible once more. They were singular little beings about four feet tall, with strange, wise, leathery faces, their heads grotesquely bald.

The humming had ceased again. The disc, too, was once more visible.

What happened next was something even more astounding, if there could be any further degrees of wonder possible for the utterly baffled young scientist. He felt himself lifted up, leaving the desert floor, whirling away toward that incredible phenomenon hovering there.

Another moment or two and he had been borne up through its trap-door opening, was standing in a dark space bounded by solid metal walls. Then he was thrust into a cylinder with several of his tiny guards, shot swiftly upward.

A door opened as they came to rest, and he was led out into a vast court of gleaming amber crystal. Something like a taxi slid up, with irridescent planes, and he was bundled into it, whirled away again.

Down broad, gleaming avenues they passed, where similar traffic flowed densely, but under marvelous control. Towering skyscrapers loomed to right and left. Tier on tier of upper and lower boulevards revealed themselves, all crowded with automotive and pedestrian activity.

At length a stupendous concourse was reached. Thousands of these taxis and similar vehicles were parked along its broad flanks, while literal swarms of diminutive individuals circulated to and fro.

Assisted from the vehicle that had brought him to this obvious center of the disc’s activities. Kendrick was led into a monumental structure of jade-green stone that towered a full hundred, stories above the street level. There he was escorted into another of those projectilelike elevators, shot up, up--till at length it came to rest. The door opened and he was led out into a small lobby of the same amber crystal he had observed before.

By now his guards had diminished to two, but he no longer made any effort to escape. Wherever this amazing adventure might lead, he was resolved to follow it through.

One of the guards had advanced to a jewelled door and was pressing a button. In response, the door opened. A golden-robed, regal creature stood there.

Though dwarfed to four feet, like his fellow, he was obviously their mental superior to a prodigious degree. Not only was his symmetrical bald head of large brain content, but the finely-cut features of his parchment face bore the unmistakable stamp of a powerful intellect.

“Ao-chaa!“ commanded this evident monarch of the disc, addressing the guards.

They bowed and departed, abruptly.

“My dear Kendrick!” the regal personage now said, in thin, precise English. “It is indeed a pleasure to welcome you to my humble quarters. Pray enter and make yourself comfortable.”

Whereupon he ushered him into a dazzling apartment that was one vast mosaic of precious gems, indicated a richly carved chair, into which the young scientist dropped wonderingly.

“Now then, Professor,” continued the mighty little dwarf, when he was seated in a chair even more sumptuous, “suppose we have a friendly little discussion. I have been much interested in your experiments on heat radiation. What you demonstrated this morning, in particular, was most absorbing. You have hit upon a rather profound scientific principle, yes?”

“Possibly,” Kendrick admitted, quite conscious that he was being patronized.

“Oh, don’t be modest, my dear fellow!” smiled the dwarf. “I am the last one to belittle your achievement. Indeed, it is because of it that I have invited you here to-day. Permit me to introduce myself, and to make clear one or two possibly perplexing matters. Then I am sure we shall have a most agreeable chat.”

His name was Cor, he said, and he was in truth the monarch of this strange realm. His people had come from the one-time planet of Vada, far distant in the universe. A thousand years ago, this planet had been doomed by the approach of an alien star. Their great scientist, Ravv, had met the emergency by inventing the disc, into whose construction they had poured all their resources. The pick of their populace had been salvaged on this giant life-raft. The rest had perished when that destroying star had crashed down on the doomed Vada.

Since then these survivors and their descendants had been voyaging through space on their marvelous disc. For hundreds of years they had given no thought to the future, content to drift on and on in the interstellar void, breathing an atmosphere produced artificially. But at length the inevitable had happened. This superb piece of mechanism devised by their super-genius, Ravv, was beginning to show signs of wear. Some of its mighty engines were nearing the exhaustion point. Either they must soon find a planet comparable with the one they had once known, where they could pause and rehabilitate their machinery, or they must disintegrate and pass into oblivion.

Faced with that crisis, Cor had long been seeking such a planet. He had found it, at last, in the earth--and had resolved that this was where they were going to alight and transplant the civilization of ancient Vada, pending such time as they could take to space again.

For some months now they had been hovering over various portions of the earth, studying its geography and its peoples, with the result that they had concluded the United States offered the most logical point for launching the attack. Once this country was subdued, they were in possession of the richest and most advanced section of the planet. The conquest of the rest of it could await their leisure.

With such an invasion in view, their scientists had mastered the language of the country. This had been accomplished very easily, since in addition to their power of mingling with the populace in an invisible form, they had the principles of radio developed to a high degree and were able to tune in on any station they wanted.

Kendrick sat there, stunned, as Cor followed his astounding revelation of their origin with this calm plan for the conquest of America, of the world. Why, of all people on earth, had he alone been singled out for this disclosure?

He asked the question now.

“My dear Professor, can’t you really guess?” replied Cor, with that leathery smile. “Hasn’t it dawned that you were a little too near our own field with that machine of yours? A trifle more research, a slightly different application--and you would have become a dangerous enemy.”

“You--you mean--?”

“I mean there isn’t a great deal of difference between the experiments you have been making and those our great Ravv once made. For instance, had you broadcast your heatricity, as you call it, instead of trying to transmit it on wires--well, picture a receiving apparatus in each home of the land, like your commercial radio sets. You would have become a billionaire, don’t you see?”

Kendrick saw indeed. It was simple, so simple! Fool--why hadn’t he thought of it?

“But your invention will never make you wealthy now, my dear fellow,” Cor went on, tauntingly. “You will be our guest, here, until we have taken over your interesting country. After that, if there is any need for the broadcasting of heat, we will furnish it ourselves. We have those facilities, among others, fully developed. Would you care to see our plant?”

Kendrick naturally admitted that he would, so the dwarf led him through a rear door and up a winding flight of stairs. They emerged presently into a great laboratory housed in the glass-roofed pinnacle of the tower.

There he beheld a sight that left him breathless. Never before had he seen such an assemblage of scientific apparatus. Its vastness and strangeness were fairly overpowering, even to a man as well versed in physio-chemical paraphernalia as he was.

Before his eyes could take in a tenth part of the spectacle, Cor had led him to the left wall.

“There,” he said, “you will observe a development of your heat generator.”

Kendrick looked--to see a long bank of large vacuum-tubes, each about three feet high and a foot wide, connected by a central shaft that caused series of little vanes in each of them to revolve at lightning speed.

Around the apparatus moved numerous small attendants, oiling, wiping, adjusting its many delicate parts.

“Well, what do you think now?” asked Cor.

Kendrick made no reply, though he was thinking plenty.

“You see, it is your invention, my dear Professor,” the dwarf went on in his taunting voice, “only anteceded by a thousand years--and rather more perfected, you must admit.”

He walked now to the center of the laboratory, where stood a huge dial of white crystal, ranked with many levers and switches, all capped with the same material.

“Behold!” he said, throwing over one.

Instantly there came again that peculiar low humming that had so puzzled him a few minutes before--and the entire room, its engines, its attendants, Cor himself, leapt into invisibility. Only Kendrick remained, facing the faintly visible crystal dial.

Then he saw a switch move, as though automatically. But no, for the dwarf’s hand was on it now. Visibility had returned. The vibration ceased.

“That is the central control,” said Cor. “Our city and all its inhabitants become invisible when that switch is thrown. Only the dial remains, for the guidance of the operator, and even that cannot be seen at a distance of more than fifty feet. But now behold!”

He raised his hand, touched a watch-like device strapped to his wrist--and was instantly invisible. But the laboratory and every machine and person in it remained in plain view. Nor was there any vibration now.

The next moment, having touched that curious little device again, Cor reappeared.

“That is the local control,” he said. “Every one of our inhabitants, except those under discipline, has one of these little mechanisms. It enables us to make ourselves invisible at will. A convenience at times, you must admit.”

“Decidedly,” Kendrick agreed. “And the principle?”

“Quite simple. One of those, in fact, that lies behind your researches. Doubtless you would have hit upon it yourself in time. Your own scientist, Faraday, you may recall, held the opinion that the various forms under which the forces of matter manifest themselves have a common origin. We of the disc, thanks to our great Ravv, have found that common origin.”

It was the origin of matter itself, Cor said, which lay in the ether of interstellar space--energy, raw, cosmic--vibrations, rays.

By harnessing and controlling these various rays, his people had been able to accomplish their seeming miracles--miracles that the people of earth, too, were beginning to achieve--as in electricity, for instance, and its further application, radio.

But the people of Vada had long since mastered such simple rays, and now, in possession of vastly more powerful ones, had the elemental forces of the universe at their disposal.

The disc was propelled through space by short rays of tremendously high frequency, up above the ultra-violet. The same rays, directed downward instead of outward, enabled them to overcome the pull of gravity when in a planet’s influence, as at present. And the escalator rays, by which they could proceed to and from the disc, were also of high frequency, as were their invisibility rays.

“But you, Professor, are more interested in low frequency rays, the long ones down below infra-red,” continued Cor. “You have seen our development of the heat-dynamo principle. It utilizes, I might add, not only solar radiation but that of the stars as well. There being a billion and a half of these in the universe, many of them a thousand times or more as large as your own sun, we naturally have quite an efficient little heating plant here. It provides us with our weapon of warfare, as well as keeping us warm. Permit me to demonstrate.”

He led the way to a gleaming circle of glass like an inverted telescope, about a yard in diameter, mounted in the floor.

“Look!” said the dwarf.

Kendrick did so--and there, spread below him, lay the floor of the desert. His camp, his apparatus, were just as he had left them.

Cor now moved toward the dial.

“Behold!” he said, pulling a lever.

Instantly the scene below was an inferno. Stricken by a blast of stupendous heat, the whole area went molten, lay quivering like a lake of lava in the crater of an active volcano.

“Suppose, my dear Professor,” smiled the dwarf, strolling back from the dial, “just suppose, for instance, that instead of the lonely camp of an obscure scientist, your proud city of New York had been below there!”

Kendrick shuddered.

Well he knew now the terrible power, the appalling menace of this strange invader.

“I would prefer not to make such a supposition,” he said, quietly, with a last thoughtful glance at that witches’ caldron below.

“Then let us think of pleasanter things. You are my guest of honor, sir--America’s foremost scientist, though she may never realize it,” with a piping chuckle. “To-night there will be a great banquet in your honor. Meanwhile, suppose I show you to your quarters.”

Nettled, fuming, though outwardly calm, Kendrick permitted himself to be escorted from the laboratory to an ornate apartment on one of the lower floors.

There Cor left him, with the polite hint that he would find plenty of attendants handy should he require anything.

Alone now, in the midst of this vast, nightmarish metropolis, he paced back and forth, back and forth--knowing the hideous fate that threatened the world but powerless to issue one word of warning, much less avert it.

Kendrick was still thinking and brooding along these lines when he saw the door of the apartment swiftly open and close again.

Someone had entered, invisible!

Backing away, he waited, tense. Then, suddenly, his visitor materialized. With a gasp, he saw standing before him a beautiful girl.

She was a young woman, rather, in her early twenties. Not one of these pigmies of the disc either, but a tall, slender creature of his own world.

Her hair was dark, modishly bobbed. Her eyes were a deep, clear brown, her skin a warm olive. And she was dressed as though she had just stepped off Fifth Avenue--which indeed she had, not so long ago, as he was soon to learn.

“I hope I haven’t startled you too much, Mr. Kendrick,” she said, in a rich, husky murmur, “but--well, there wasn’t any other way.”

“Oh, I guess I’ll get over it,” he replied with a smile. “But you have the advantage of me, since you know my name.”

Hers was Marjorie Blake, she told him then.

“Not the daughter of Henderson Blake?” he gasped.

“Yes,” with a tremor, “his only daughter.”

Whereupon Kendrick knew the solution of a mystery that had baffled the police for weeks. The newspapers had been full of it at the time. This beautiful girl, whose father was one of America’s richest men and president of its largest bank, had disappeared as though the earth had swallowed her. She had left their summer estate at Great Neck, Long Island, on a bright June morning, bound for New York on a shopping tour--and had simply vanished.

Suicide had been hinted by some of the papers, but had not been taken seriously, since she had no apparent motive for ending her life. Abduction seemed to be the more logical explanation, and huge rewards had been offered by her frantic parents--all to no avail.

What had happened was, she now explained, that after visiting several shops and making a number of purchases, she had stepped into Central Park at the Plaza for a breath of fresh air before lunching at the Sherry-Netherlands, where she planned to meet some friends.

But before advancing a hundred yards along the secluded path, she had been seized by invisible hands--had felt something strapped to her wrist, before anyone came in sight--and then, invisible too, had been lifted up, whirled away into a vast, humming vibration that sounded through the air.

Once on the disc, it had swept off into space at incredible speed, pausing only when some hundreds of miles above the earth and invisible from below without mechanical aid. When its vibration finally ceased that amazing city had leapt before her eyes.

Then, her own visibility restored, she had been led into the presence of that mighty little monarch, Cor, who explained that she had been seized as a hostage and would be held as an ace in the hole, pending conquest of her country. Since when she had been a prisoner aboard the disc.

Learning of Kendrick’s capture, from gossip among the women, she had taken the first opportunity of coming to him, in the hope that between them they might devise some means of escape.

Indeed, that was his own fondest hope--their imperative need, if the people of America and of the earth were to be saved from this appalling menace. But what basis was there for such a fantastic hope? Just one, that he could see.

“That thing on your wrist,” he said, voicing it. “I’m surprised they let you wear one of those.”

“They don’t,” she smiled. “I stole it!--from one of the maids in my apartment. It was the only way I could get here without being seen. I felt I must see you at once. We’ve got to do something, soon, or it’ll be too late. I felt that, as a scientist, you might have some idea how we could get off.”

“How do the people themselves get off?” he asked. “That escalator ray--do you know how they use it?”

“No, I’ve never been able to find out. They don’t let me go near that part of the city.”

Kendrick reflected a moment.

“Let’s have a look at that invisibility affair,” he said.

She removed it from her wrist, handed it to him. Somewhat in awe, he examined it.

The mechanism portion, which was linked in a strap of elastic metal, resembled only superficially a watch, he now saw. Rather it had the appearance of some delicate electric switch. Rectangular in shape, it was divided into two halves by a band of white crystal. In each of these halves were two little buttons of the same material, those on one side round, on the other square.

“Which buttons control the invisibility?” he asked.

“The square ones,” she replied. “One’s pushed in now, you see. If you should push the other, the first would come out--and you’d pass out of the picture, so to speak.”

Kendrick was half tempted to try the thing then and there, but deferred the impulse.

“What are the round buttons for?” he inquired instead.

Marjorie didn’t know, but thought they were probably an emergency pair, in case something went wrong with the square ones. In any event, nothing happened when you pushed them.

Kendrick pushed one, just to see. It was true. Nothing happened--but he seemed to sense a faint, peculiar vibration and a wave of giddiness swept over him. On pushing the other, which released the first, it stopped.

He handed the device back to Marjorie.

“There’s your bracelet. Now, if I can just get one like it, I think we’ll get down to earth all right.”

“Oh, Mr. Kendrick!” Her eyes lit up eagerly. “Then you’ve thought of a way?”

“Not exactly. I think I’ve discovered their own way. I can’t be certain, but I’m willing to gamble on it, if you are.”

“Then you--you think those round buttons are connected with the escalator rays?”

“Exactly! I think they control individual descent and ascent, just as the square ones control individual visibility and invisibility. At any rate, it’s the hunch I’m going to act on right now, if you’re with me.”

“Oh, I’m with, you!” she breathed. “Anything, death almost, would be preferable to this.”

“Then stand by, invisible. I’m going to get one of my jailors in here and relieve him of his wrist-watch.”

Marjorie touched that little square button on her own. She instantly became invisible.

Kendrick touched a button too, a button he had noticed beside the door. As he had supposed, it brought one of the Vadans.

Shutting the door quietly, he seized the fellow before he could move his hand to his wrist. Thwarted in his attempt to vanish from sight, the diminutive guard attempted an outcry. But Kendrick promptly throttled him.

Marjorie had reappeared by now and together they bound him to a chair with a gilded cord torn from the drapery.

Removing the precious mechanism from his wrist, Kendrick slipped it on his own.

“Now let’s go!” he said, pressing the protruding square button of the device. “We haven’t a minute to--my golly, what a peculiar sensation!”

“It is rather odd, isn’t it?” she laughed, pressing her own and joining him in that invisible realm.

“Feels like a combination electric massage and cold shower! Where are you, anyway? I can’t see you.”

“Of course you can’t!” came an unseen tinkle. “Here!”

He felt her brush him.

“Better hold hands,” he suggested, then gave an invisible flush he was glad she couldn’t see.

“All right. A good idea.”

Her delicate hand came into his, soft, warm. Heart vibrating even faster than his body, his whole being a-quiver with a strange exaltation, Kendrick opened the door, and they left the apartment.

The next half-hour was the tensest either of then had ever experienced. Every foot of the way was fraught with peril.

Not only did they have to carefully avoid the visible swarms of little people who hurried everywhere, but had to be on their guard as well against any who might be moving about like themselves under cover of invisibility.

Nor could they use any elevator or public conveyances, but were obliged to make their way down to the concourse by heaven knew how many flights of stairs, and cross heaven knew how many teeming streets on foot, before they reached the amber court, below which the trap-door and their hope of freedom.

They got there at last, however, descended, and peered down from that yawning brink upon the desert floor--to draw back with gasps of dismay. For the area still gleamed semi-molten from the stupendous blast that had wiped out Kendrick’s camp.

“W-what is it?” she gasped.

Swiftly he told her.

“But isn’t there any way around it? Look, over there to the left. One edge of the crater seems to end almost underneath us.”

It was true that the center of the caldron was far to the right of where they stood, and that its left rim was only a little within their direct line of descent. But to land even one foot inside that inferno would be as fatal as to alight in its very midst.