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.

s 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.

hielding 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!

s 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.

lancing 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.

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.

hough 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."

is 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.

or 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?"

endrick 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."

e 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.

he 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.

he 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!"

endrick 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.

endrick 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.

uicide 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.

earning 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.

he 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.

e 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.

arjorie 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.

he 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.

endrick was thinking fast.

"There's just a chance," he said. "It all depends upon how wide the zone of these escalator rays is, and whether we can tune in on them. At least, I can probably answer the latter question."

Pushing the protrudent round button on his mysterious bracelet as he spoke, he leaned over the edge of the trap-door and awaited results.

They were not long in coming. The vibration he was already under from the invisibility rays seemed to double. Alternate waves of giddiness and depression, of push and pull, swept over him.

A minute of it was enough. He pressed the round button that now protruded, ending this influence, and faced Marjorie, stating:

"I'm positive now that these things control descent and ascent. As nearly as I can figure, the rays work on the principle of an endless belt. If you're up here, you get carried down, and vice versa. As to how wide the belt is, and whether you can move sideways on it, remains to be seen. Anyway, I'm going to take a chance. I'll go first. If my guess is wrong, you—well, needn't follow."

"No, I'm going with you!" she declared resolutely. "We've come this far together. I shan't be left alone now. Let's go!"

And again her soft, warm hand was in his.

Lord, what a girl! How many would be brave enough to take a gamble like that, on a fellow's mere supposition?

"All right—go it is!" he said. "Push your round button, like this." He showed her the way he thought was right, pushed his own. "Ready?"

"Ready!"

heir voices were grave. It was a grim prospect, stepping off into space like that, with only a guess between them and death.

"Then jump!"

They jumped, gripping each other's hands tightly—and instead of dropping like plummets were caught in a powerful field of force and whirled gently downward.

"Oh, you were right!" gasped Marjorie, awed. "See, we—"

Then she paused, horror-stricken, for it was obvious that they were to descend within that lake of molten glass, unless they could change their course at once.

"Quick!" he called. "Hold fast! Now—run!"

Breathless, they raced to the left, across that invisible descending belt.

Too far, Kendrick knew, and they would plunge outside its zone, fall crushed and mangled. Not far enough, and they would meet cremation. It was a fearful hazard, either way, but it had to be taken.

They were almost down, now, and still not quite far enough to the left. The heat of that yawning crater rose toward them.

"Faster—faster!" he cried, fairly dragging her along with him.

A last dash—a breathless instant—and they stood there on the ground, not three feet from the edge of doom.

Swooning with the heat, Marjorie swayed against him, murmured an incoherent prayer.

"Take heart!" he whispered, lifting her bodily and bearing her some yards away. "We're down—safe!"

heir safety was but relative, however, Kendrick well knew. Until they could put miles between them and this monstrous disc, they were not really safe. No telling how soon their escape might be discovered. No telling what terrible means Cor might take of curbing their flight.

So as soon as Marjorie had recovered sufficiently to proceed, they headed off across the desert at a fast walk toward Ajo, where he hoped to catch the afternoon train for Gila Bend. From there, they could board the limited for Tucson and points east, when it came through from Yuma that night.

They had tuned out on the escalator rays, but continued on still invisible—for the disc hung above them in plain view and it would have been suicide to let themselves be seen.

Even so, Kendrick soon began to have an uneasy feeling of being followed. He looked around from time to time, but could see nothing. Were some of those invisible little creatures on their trail?

He said nothing to Marjorie of his anxiety, but presently she too began glancing backward uneasily, every few steps.

"They are near us!" she said at length, in a whisper. "I can sense them."

It was more than sense, they soon discovered. Little paddings became quite audible, and once or twice they saw the sand scuffed up, not twenty feet away, as though by a foot passing over it.

eanwhile they were climbing a rise of ground, broken by many small hummocks and dotted with thorny shrubs. On the other side, at the foot of a long down-slope, lay Ajo.

Once they reached the summit, Kendrick felt sure they could outdistance their pursuers on the descent. Already, if his watch was right, the train was preparing to pull out. It would be a breathless dash, but he was confident they could make it.

So he reassured Marjorie as best he could, and helped her on up the slope.

They were practically on the summit and already in view of the little railroad station and huddle of shacks below—when suddenly he felt himself tripped and flung violently to the ground. At the same instant, his companion emitted a scream, as she felt herself seized by invisible hands.

Leaping to his feet, Kendrick flailed out with solid fists at their attackers. Groans answered the impacts and he knew his blows were taking effect.

ree for a moment he dashed to Marjorie, felt for the midgets who swarmed around her. Seizing one of the invisible forms, he lifted it and flung it crashing to the ground. Another, likewise, and another.

Then he threshed his legs, where two of the creatures clung, trying to drag him down again. They flew through the air, with cries of fright.

"Well, so far, so good!" he exclaimed. "We won't wait to see if there are any more. Come on—let's go!"

"Right!"

Reaching for each other's hands, they raced down the slope.

Halfway there they saw a warning blast of steam rise from the engine, followed by a whistle.

"They'll be pulling out in a minute now!" he gasped, increasing speed. "We've got to make it!—our only chance!"

"We will make it!" she sobbed through clenched teeth, meeting his pace.

Glancing over his shoulder, after another fifteen seconds, Kendrick saw that the disc was no longer visible. Since there was no vibration he realized with relief that it was now hidden behind the slope they were descending.

"Quick—push your button!" he said, pushing his own.

They came out of the influence of the invisibility rays, raced breathless on down the slope—gained the station platform just as the train was getting under way.

Helping the exhausted girl aboard, he mounted the steps himself, led her through the vestibule into its single passenger coach.

Dropping into a seat, they sat there panting as the train gathered speed.

y the time the decrepit but life-saving little local drew into Gila Bend they had somewhat recovered from their harrowing experience.

Marjorie was still pale, however, as Kendrick helped her from the train.

"I may recover," she said with a wan smile, "but I'll never look the same! An old saying, but I know what it means now."

He thought better of a sudden impulse to tell her she looked quite all right to him. Instead, he said grimly:

"I know now what a lot of things mean!"

The Tucson limited would not be through for over an hour, they learned. That would give them time to hunt up the authorities and sound a warning of the ominous invader that was in the vicinity. Perhaps, by prompt military action, it might be destroyed, or at least crippled.

But first they went to the telegraph office, where Marjorie got off a message that would bring joy to her grieved family.

While standing there outside the barred window, odors of food wafting to them from a nearby lunch-room.

"Um-m!" she sniffed. "That smells good to me! I haven't tasted any earthly cooking for ages. Everything on that horrible disc was synthetic."

"Then I suggest we have ham and eggs, at once," he said. "Or would you prefer a steak?"

"I think I'll have both!"

s they walked into the lunch-room, Kendrick told her of the banquet in his honor Cor had promised for that night.

"I guess I didn't miss much," he ended.

"You certainly didn't!" she assured him, with a smile. "It would have opened with a purÉe of split-molecule soup, continued with an entrÉe of breaded electrons, and closed with an ionic cafÉ."

He laughed.

"I'm just as well satisfied. I was unable to attend! Humble as it is, I think this will prove to be much more wholesome food."

Night had fallen by the time they left the lunch-room. Glancing at his watch, Kendrick saw that they still had better than a half-hour before the limited was due, so they betook themselves to the police station.

It was only a block away and in consequence they weren't long reaching it.

The chief had gone home, the officer at the desk informed them, but if there was anything they cared to report, he would be glad to make note of it.

A big raw-boned westerner, he shifted his quid as he spoke and spat resoundingly in a cuspidor at his feet.

"All right, then—get your pencil ready!" said Kendrick with a smile. "This is Miss Marjorie Blake, daughter of Henderson Blake, of New York. Perhaps you read of her disappearance, a few weeks ago. And I...."

As he introduced himself and told briefly of their astounding experience, the officer's eyes bulged with amazement.

"Say, what yuh-all tryin' to hand me?" he snorted finally. "D'yuh think I was born simple?"

"Press your button!" whispered Marjorie. "Show him how the invisibility ray works. It'll save a lot of argument."

"Right!"

e held up his wrist.

"See this? Now watch!"

Whereupon he pressed the button. But to their dismay, nothing happened.

"Wa-al. I'm still watchin'!" drawled the officer. "Who's loony now?"

Kendrick examined the mechanism in impatience, pressed that little button repeatedly: but still nothing happened.

"Try yours!" he told Marjorie finally.

She did so, with similar results—or lack of them, rather.

"Something's wrong," he said at length. "The ray isn't working."

"Wrong is right!" declared the officer with a contemptuous flood of tobacco juice. "Yuh folks better go catch yuhr train 'fore yuh ferget where it is."

Chagrined, embarrassed, they took their leave, headed back toward the railroad station.

"Of all the utterly silly things!" declared Marjorie, as they walked along. "Why do you suppose it didn't work?"

Kendrick didn't reply at once. When he did, his voice was grave.

"Because the disc has gone!" he said. "We are outside its zone of influence. That's my hunch, at least, and I think we'd better act on it."

"You mean...?"

"I mean our escape has probably caused them to hurry their plans. They're probably over New York right now. I think we'd better get there the quickest possible way."

he result was that when the train came, they remained on it only to Tucson. There they chartered a fast plane and started east at once.

At sunset the following day the plane swooped out of the sky and slid to rest on the broad grounds of the Blake estate at Great Neck.

As Kendrick stepped from the cabin and helped Marjorie down, a tall, distinguished-looking man with graying hair and close-cropped mustache came hurrying toward them.

"Daddy!" she cried, rushing into his arms. "Oh, Daddy—Daddy!"

Even without this demonstration. Kendrick would have recognized Henderson Blake from pictures he had seen recently in the papers.

Now he was introduced, and Blake was gripping his hand warmly.

"I don't quite know what this is all about, Professor," he heard the great financier say. "Marjorie's telegram last night was as cryptic as it was over-joying. But I do know that I owe you a deep debt of gratitude."

"Yes, and you owe our pilot about a thousand dollars, too!" put in the daughter of the house, clinging to her father's arm. "Please give him a check—then we'll go inside and I'll explain all about it."

"A matter very much easier dispatched than my debt to Professor Kendrick," said Blake, complying.

The check was for two thousand, not one, the pilot saw when he received it.

"Thank you very much, sir!" he said, saluting.

"Don't mention it. Good night—and good luck to you!"

he pilot returned to his plane, it lifted from the lawn, droned off into the twilight.

Then they approached the cool white villa that stood invitingly a hundred yards or so away beyond sunken gardens.

As they neared it, a handsome, well-preserved woman whose face reflected Marjorie's own beauty came toward them. Lines of suffering were still evident around her sensitive mouth, but her dark eyes were radiant.

"Mother!"

"My poor darling!"

They rushed into each other's arms, clung, sobbing and laughing.

Kendrick was glad when these intimate greetings were over and he had met Mrs. Blake.

They were in the drawing-room now, listening to a somewhat more lucid account of their daughter's experiences and those of her rescuer. Marjorie was doing most of the talking, but every now and again she would turn to Kendrick for verification.

"Heavens!" gasped Mrs. Blake, finally. "Can such things be possible?"

"Almost anything seems possible nowadays, my dear," her husband told her. "And you say, Professor, that you have brought back samples of this invisibility device?"

"Yes, we have, but I can't promise they'll work. I'll try, however."

Whereupon, sceptically, he pressed that little square button—and instantly faded out of sight.

"Good Lord!" cried Blake, leaping to his feet. "That proves it! Why, this is positively—"

is remarks were cut short by a scream of terror from his wife.

"Marjorie—Marjorie!" she shrieked.

Wheeling, he faced the chair where his daughter had sat. It was empty, so far as human eyes could see.

"Don't worry Mother—Daddy!" came a calm voice from it. "I'm quite all right—coming back—steady."

And back she came, as did Kendrick, from the empty chair beside her.

His face was grave. The success of the demonstration, which had proved their story to practical-minded Henderson Blake, had proved to him something altogether more significant. The disc, as he had surmised, had rushed eastward immediately on learning of their escape, and was now probably hovering right over New York.

"Marvelous—marvelous!" declared Blake. "But that heat ray, Professor. That sounds bad. You are convinced it is as powerful as they make out!"

"Positively! That blast they let go in the desert would have utterly destroyed New York."

"Hm! Yes, no doubt you're right. I fully realize how the fearful menace of this thing. Do you think the military authorities will be able to cope with it?"

"I don't know. Perhaps, if they are prompt enough."

"And is there no other way—no scientific way?"

endrick grew thoughtful.

"I wonder," he said at last. "There's just a possibility—something running through my mind—an experiment I'd like to make, if I had the facilities of some large electrical laboratory."

"You shall have them to-morrow!" Blake promised. "I'm one of the directors of Consolidated Electric. Their experimental laboratory in Brooklyn is the finest of its kind in America. I'll see that you have the run of it."

"That will be very kind," said Kendrick. "But don't expect anything to come from it, necessarily. It's just a theory I want to work out."

A butler entered at this moment and announced dinner.

"Well, theories are mighty these days!" beamed Blake, as they rose, clapping the younger man on the shoulder. "You go ahead with your theories—and I'll bring a few facts to bear. To-morrow noon I'll escore some military men and others of my friends over to the laboratory to hear and see something of this menace direct. Meanwhile, and during this crisis, it will honor me to have you as my guest."

"Our guest!" amended Marjorie, with a warm smile.

ext morning Blake motored Kendrick out to the Brooklyn Laboratory of the Consolidated Electric Utilities Corporation and installed him there.

Then he left—to return at noon with the promised delegation of generals, admirals, statesmen and financiers.

They were all frankly sceptical, though realizing that Henderson Blake was not a man given to exaggeration. Nor did their scepticism altogether vanish when Kendrick had ended his bizarre story with a demonstration of the invisibility device.

Murmurs of amazement ran around the laboratory, it is true, but the more hard-headed of his spectators charged him with having invented the apparatus himself. Though they didn't come right out and say so, they seemed to imply that he was seeking publicity.

Annoyedly, Kendrick tried to refute their charges. But even as he was summoning words, refutation utter and complete came from the air.

A low, humming vibration sounded, grew in volume till it filled the room—and as suddenly ceased: The light of midday faded to twilight.

"The disc!" gasped Kendrick, rushing to the west windows.

They followed, tense with awe. And there, between earth and sun, its myriad towers and spires refracting a weird radiance, hovered that vast flying city.

"My God!" muttered a famous general, staring as though he had seen a ghost.

A great statesman opened his lips, but no words came.

"Appalling! Incredible!" burst from others of that stunned assemblage.

heir comments were cut short by a broadcast voice, thin and clear, tremendously amplified, a voice Kendrick recognized at once as that of Cor.

"People of America!" it said. "We of the planet Vada have come to conquer your country. You will be given forty-eight hours to lay down your arms. If complete surrender has not been made by high noon, two days from now, New York will be destroyed."

The voice ceased. The humming recommenced—waned in volume till it died away. Twilight turned once more to midday.

Peering fixedly through the west windows of the laboratory, the little assemblage saw the disc swallowed up in the clear blue sky.

Then they turned, faced one another gravely.

Outside, on the streets, confusion reigned. In newspaper plants, presses were whirling. In telegraph and cable offices, keys were ticking. From radio towers, waves were speeding.

Within an hour, the nation and the world knew of this planetary invader and its staggering ultimatum.

Naturally, the government at Washington refused to meet these shameful terms. Military and naval forces were rushed to the threatened metropolis. The Atlantic Fleet steamed up from Hampton Roads under forced draught and assembled in the outer harbor. Thousands of planes gathered at Mitchell Field and other nearby aerodromes.

ut where was the enemy? He must be miles up in space, Kendrick knew, as he toiled feverishly in the laboratory over his experiment after a sleepless night. For had that flying city been nearer earth, it could not have maintained invisibility without that peculiar humming vibration.

Scout planes urged on by impatient squadron commanders, climbed till they reached their ceilings, searching in vain. They could encounter nothing, see nothing of the invader.

Thus passed a morning of growing tension.

But by noon of that day, with a bare twenty-four hours left before the expiration of the ultimatum, the disc came down, showed itself boldly.

There followed stunning disasters.

One salvo, and the ray shot down—the Atlantic Fleet, the pride of America, burst and melted in flaming hell. Squadrons of planes, carrying tons of bombs, frizzled like moths in the air. Mighty projectiles hurled by land batteries were deflected off on wild trajectories.

Appalled, the nation and the world followed in lurid extras these crushing defeats.

By nightfall of that day, all seemed lost. All opposition had been obliterated. America must capitulate or perish. It had until the next noon to decide which.

eanwhile, in that great Brooklyn laboratory, Kendrick was working against time, besieged by frantic delegations of the nation's leaders. They knew now that their one hope lay in him. Was he succeeding? Was there even any hope?

Face haggard, eyes bloodshot from lack of sleep, he waved them away, went on with his work.

"I will tell you—as soon as I know."

That was all he would say.

Followed a night that was the blackest in all history, though the myriad stars of heaven shone tauntingly brilliant in the summer sky.

At length, as dawn was breaking. Kendrick paused in his labors.

"There!" he said, grimly, surveying an apparatus that seemed to involve the entire facilities of the laboratory. "It is done! Now then—will it work?"

The delegation were called to witness the test.

Henderson Blake was among them, as was Marjorie. She stepped forward, as he prepared to make the demonstration.

"I know, somehow, you're going to be successful!" she murmured, pressing his hand, meeting his eyes with a smile of confidence.

"I hope you're right—Marjorie!" he replied, letting slip the last word almost unconsciously.

Her face colored warmly as the stepped back and rejoined her father.

Kendrick's heart was beating fast as he turned to his instruments. How could he fail, with faith like that behind him?—love, even, perhaps! He mustn't fail—nor would he, if his theories were sound.

ddressing the assemblage, he explained briefly the complicated apparatus.

"These towers," he said, pointing to four steel structures about ten feet high, arranged at the corners of a square roughly twenty feet across, "are miniature radio masts. The area enclosed by them, we will assume, is the city of New York. That metal disc suspended above the area represents the invader. It contains a miniature heat-generator such as I was experimenting with recently in the Arizona desert."

He paused, threw a switch. Somewhere in the laboratory a dynamo began to whir.

"I am now sending electro-magnetic waves from the four towers," he resumed. "But instead of broadcasting them in every direction. I am bending them in concave cathode of force over the city. You may picture this cathode as an invisible shield, if you choose, but it is more than that. It it a reflector. If my theories are right, the radio-energetic ray I am about to project upon it from my miniature disc will be flung back to its source as though it had been a ray of light falling on a mirror. The success of the experiment depends upon what the result will be."

endrick ceased, moved toward a rheostat.

As he made ready to touch it, a breathless tension settled upon the assemblage. Upon the outcome of what was now to happen rested the fate of America—and the world.

Calmly, though every fiber of his being was at breaking stress, the young scientist opened the rheostat.

For an instant, the ray seared down—then, as it boomeranged back, the disc burst into flame, dissolved, disintegrated. A thin dust, like carbon, slowly settled to the laboratory floor.

Cutting off the current from the radio towers, Kendrick faced them, a light of triumph in his tired eyes.

"You see—it works," he said.

They saw. Beyond a doubt, it worked!

And what Kendrick saw, as his eyes met Marjorie's, made him forget his fatigue.

he rest was a mad scramble of preparation. Only a few brief hours remained, and much was to be done.

The application of the principle that had just been demonstrated involved a hook-up from the Consolidated Electric laboratory with every broadcasting station in the metropolitan area, power being supplied by commandeering every generating plant within a radius of fifty miles.

The city, moreover, had to be evacuated of all but the few brave hundreds who volunteered to stand by their posts at radio stations and generating plants.

As for Kendrick, it was the busiest, most hectic morning he had ever experienced. Only the realization of a girl's love and a nation's trust enabled him to overcome the exhaustion of two sleepless nights.

At length, a little before eleven, all was in readiness. Just two questions troubled the young scientist's mind. Had the people of the disc learned of their preparations to counter the attack? And would the improvised broadcasting apparatus of the area stand the stupendous strain that would be placed upon it if the ray came down?

The first of these questions was answered, staggeringly, at a quarter after eleven.

"Kendrick—oh, my God!" cried Blake, bursting into the laboratory. "Marjorie—they've got her again! Look! Read this!"

He thrust out a piece of paper. Kendrick took it, read:

Your daughter will be my queen, after this noon.

"Where'd you get it?" he gasped.

"One of the invisible devils thrust it into my hand right out in the street, not five minutes ago," Blake explained, trembling with anguish. "Do you realize what this means, Kendrick? She's on the disc now—and in a scant three-quarters of an hour...."

"Yes, I realize!" his voice came grimly. "And I realize, too, that they don't know their fate. They'll stay. There's forty-five minutes yet. We can't abandon our defense against the ray, not even for Marjorie. But I'll go, I'll rescue her—or die with her!"

And even as Blake mutely reached out his hand to grip that of the determined young man who stood before him. Kendrick touched his wrist mechanism and went invisible.

nce on the street, he pressed the escalator button as well—and by the strength of the vibrations that followed, he knew he must be very close within that mysterious lifting zone.

Running west a block, he found it growing stronger.

Fairly racing now, he continued on toward the river, progress unhampered in the deserted streets. Suddenly, with a thrill of exultation, he felt himself swept up, whirled away toward that great shimmering hulk against the sun.

"What hope?" he was thinking. "What possible hope?" And the answer came: Cor!

Reaching the disc, he switched out the escalator influence and hastened across the city to that monumental structure of jade-green stone.

The mighty little dwarf would be up there in his glittering mosaic apartment, or in his pinnacle laboratory, perhaps, ready to pull the lever that would release that stupendous blast of heat.

Gaining the jewelled door of the monarch's quarters at last, after escaping detection by a hair's breadth more than once, he pressed the button outside, just as the guard had done that first time.

In response, the door opened—and there stood Cor.

e stood there an instant, that is, while the expression on his leathery face went from inquiry to alarm. Then, as Kendrick burst into the room and shut the door, he went invisible.

In that same instant, the young scientist's eyes beheld a sight that caused his heart to leap. There sat Marjorie, bound in a chair, an expression half of hope, half of dejection, on her face.

"It's I—Gordon!" he called. "Take courage!"

"Oh, I prayed so you'd come—and you came!" she murmured as her face lighted. Then, tensely, she added, "The door—look out!"

Kendrick wheeled, and just in time. The door was opening.

"Not so fast!" he called, lunging.

His hands gripped the dwarf, yanked him back, throttled him before he could emit a cry, pushed the door shut.

Cor struggled like a madman, but it was futile. Kendrick's hands cut into his throat like a vice. After a moment or two, he gasped, relaxed.

Releasing his grip then, Kendrick felt for his wrist, stripped off his bracelet—whereupon the dwarf became visible. His face was putty-white. He was either dead or unconscious.

Restoring his own visibility then, he advanced to Marjorie, swiftly freed her.

"Take this!" he said, handing her Cor's bracelet.

She slipped it on.

"Now let's tie him and get out of here. He may be dead, but we can't take any chances."

he dwarf wasn't dead, however, for he groaned and opened his eyes as they lifted him into the chair.

"You win, Professor—but it avails you nothing!" He smiled maliciously. "My capture, my death even, will not prevent the ray. The orders have been given. It will be projected sharp at twelve. You but go to your doom!"

"That," said Kendrick, "is a matter of opinion."

Swiftly they bound him, gagged him.

"And now," he added, "we wish you good day—and such fate as you deserve!"

Then, turning to Marjorie:

"Your hand again!"

There was a new tenderness in its soft warmth that thrilled him.

They touched their buttons, went invisible.

Silently, then, they stole from the apartment. Swiftly they made their way down to the concourse, raced across the city to the amber court, descended to the trap-door.

It must be nearly twelve, Kendrick knew. He couldn't look at his watch, for it as well as himself was invisible. Indeed, even as they stood there, poised for the plunge, a faint whistle rose from below.

Marjorie trembled.

"Steady!" he spoke. "Some of them always blow a minute or two before. Are you ready?"

"Yes!"

"Then press your button—jump!"

Even as they leapt, the sickening thought came that perhaps the escalator ray was no longer running. But the fear was unwarranted. They were caught up, whirled gently downward.

Moving along laterally, as they descended, they were able to land without difficulty in the middle of a deserted street near the Consolidated Electric laboratory.

"Thank heaven!" she sighed, as their feet touched solid ground. They pressed off both buttons, becoming visible once more.

"Echo!" he agreed. "So let's—"

ut Kendrick never completed that sentence—for now whistles all over the metropolitan area, rising from the generating plants, announced the ominous hour.

It was high noon. The ultimatum had expired.

Lifting tense faces to the disc, they waited. Would that stupendous ray be hurled back upon itself? Or would it sear through their makeshift defense, plunging them and the whole great metropolis into oblivion?

Suddenly, cataclysmically, the answer came.

There burst a withering whirlwind from the disc. It struck that mighty concave cathode of interlaced waves above the city. There followed an instant's clash of titanic forces. Then the cathode triumphed, hurled it back.

Rocked by a concussion as of two worlds in impact, blinded by a glare that made the sunlight seem feeble in comparison. Marjorie and Kendrick clung together, while the disc grew into a satellite of calcium fire in the sky.

Presently, as the conflagration waned, they opened their eyes. Gravely, but with deep thanksgiving, they searched each other's faces. In them they read deep understanding, too, and a new hope.

"I think we'd better go and find father," she said at length, quietly.

"I think so too!" he agreed.

As they headed toward the laboratory, a fine, powdery dust, like volcanic ash was falling.

It continued to fall until the city streets were covered to a depth of an inch or more.

Thus passed the menace of Vada.

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