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 s.p.a.ce--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 s.p.a.ce 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 gla.s.s 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.