At the end of that week Gordon's strength had fully returned, and by that time he was able to speak the language fluently.

“We are on the planet Earth?” was the first eager question he had put to Vel Quen.

The old scientist nodded. “Yes, this tower is located amid the highest mountains of Earth.”

So it was the Himalayas whose snowy peaks rose around the tower, as Gordon had guessed. They looked as wild and lonely and grand as when he had flown over them in war days long ago.

“But aren't there any cities or people left on Earth?” he said.

“Certainly there are. Zarth Arn chose this lonely spot on the planet, simply so that his secret experiments would not be disturbed.

“From this tower, he has been exploring the past by going back into the bodies of many men in various epochs of human history. Yours is the remotest period of the past that Zarth Arn has yet tried to explore.”

It was a little overwhelming to John Gordon to realize that other men had found themselves in his own uncanny present position.

“Those others-they were able to return without trouble to their own bodies and times?”

“Of course-I was here to operate the mind-transmitter, and when the time came I effected the re-exchange just as I will do with you later.”

That was reassuring. Gordon was still wildly excited by this unprecedented adventure into a future age, but he hated to think that he might be marooned indefinitely in a stranger's body.

Vel Quen explained to Gordon in detail the amazing scientific method of contacting and exchanging minds across time.

He showed him the operation of the telepathic amplifier that could beam its thought-message back to any selected mind in the past. And then he outlined the operation of the mind exchange apparatus itself.

“The mind is an electric pattern in the neurones of the brain. The forces of this apparatus detach that pattern and embody it in a network of nonmaterial photons.

“That photon-mind can then be projected along any dimension. And since time is the fourth dimension of matter, the photon-mind can be hurled into past time. The forces operate in a two-way channel, simultaneously detaching and projecting both minds so as to exchange them.”

“Did Zarth Arn himself invent this method of exchanging minds?” Gordon asked wonderingly.

“We invented it together,” Vel Quen said. “I had already perfected the principle. Zarth Arn, my most devoted scientific pupil, wanted to try it out and he helped me build and test the apparatus.

“It has succeeded beyond our wildest dreams. You see those racks of thought-spools? In these is the vast mass of information brought back by Zarth Arn from past ages he has explored thus. We've worked secretly because Arn Abbas would forbid his son to take the risk if he knew.”

“Arn Abbas?” repeated Gordon questioningly. “Who is he, Vel Quen?”

“Arn Abbas is sovereign of the Mid-Galactic Empire, ruling from its capital world at the sun Canopus. He has two sons. The oldest is his heir, Jhal Arn. The second son is Zarth Arn.”

Gordon was astounded. “You mean that Zarth Arn, the man whose body I now inhabit, is son of the greatest ruler in the galaxy?”

The old scientist nodded. “Yes, but Zarth is not interested in power or rule. He is a scientist and scholar, and that is why he leaves the court at Throon to carry on his exploration of the past from this lonely tower on Earth.”

Gordon remembered now that Zarth Arn had said he was high in the Empire. But he had had no suspicion of his true exalted position.

“Vel Quen, what exactly is the Mid-Galactic Empire? Does it take in all the galaxy?”

“No, John Gordon. There are many star-kingdoms in the galaxy, warlike rivals at times. But the Mid-Galactic Empire is the largest of them.”

Gordon felt a certain disappointment. “I had thought the future would be one of democracy, and that war would be banished.”

“The star-kingdoms are really democracies, for the people rule,” Vel Quen explained. “We simply give titles and royal rank to our leaders, the better to hold together the widely separated starsystems and their human and aboriginal races.”

Gordon could understand that. “I get it. Like the British democracy in my own day, that kept up the forms of royalty and rank to hold together their realm.”

“And war was banished on Earth, long ago,” Vel Quen went on. “We know that from traditional history. The peace and prosperity that followed were what gave the first great impetus to space-travel.

“But there have been wars between the star-kingdoms because they are so widely separated. We are now trying to bring them together in union and peace, as you unified Earth's nations long ago.”

Vel Quen went to the wall and touched a switch beside a bank of lenses. From the lenses was projected a realistic little image of the galaxy, a flat, disk-shaped swarm of shining sparks.

Each of those little sparks represented a star, and their number was dizzying to John Gordon. Nebulae, comets, dark clouds-all were faithfully represented in this galactic map. And the map was divided by zones of colored light into a number of large and small sections.

“Those colored zones represent the boundaries of the great star-kingdoms,” Vel Quen explained. “As you see, the green zone of the Mid-Galactic Empire is much the largest and includes the whole north and middle of the galaxy. Here near its northern border is Sol, the sun of Earth, not far from the wild frontier star-systems of the Marches of Outer Space.

“The little purple zone south of the Empire comprises the Baronies of Hercules, whose great Barons rule the independent star-worlds of Hercules Cluster. Northwest lies Fomalhaut Kingdom, and south of it stretch the kingdoms of Lyra, Cygnus, Polaris and others, most of these being allied to the Empire.

“This big black blot southeast of the Empire is the largest dark cloud in the galaxy, and within it lies the League of Dark Worlds, composed of suns and worlds engulfed in the perpetual dimness of that cloud. The League is the most powerful and jealous rival of the Empire.

“The Empire is dominant and has long sought to induce the star-kingdoms to unite and banish all war in the galaxy. But Shorr Kan and his League have intrigued against Arn Abbas' policy of unification, by fomenting the jealousies of the smaller star-kingdoms.”

It was all a little overwhelming for John Gordon, man of the 20th Century. He looked in wonder at that strange map.

Vel Quen added, “I shall teach you how to use the thought-spools and then you can learn that great story.”

In the following days while he learned the language, Gordon had thus learned also the history of two thousand centuries.

It was an epic tale that the thought-spools unfolded of man's conquest of the stars. There had been great feats of heroism in exploration, disastrous wrecks in cosmic clouds and nebulae, bitter struggles against stellar aborigines too alien for peaceful contact.

Earth had been too small and remote to govern all the vast ever-growing realm of man. Star-systems established their own governments, and then banded into kingdoms of many stars. From such a beginning had grown the great Mid-Galactic Empire which Arn Abbas now governed.

Gordon's study of the history of two hundred thousand years showed him how the entire structure of galactic civilization was based upon the epochal discovery of sub-spectrum rays.

The era of space-travel had really dawned in 1945 and '46, with the first release of atomic energy and the discovery that radar could function efficiently in space. By the end of the 20th Century, atomic-powered rockets guided by radar had reached the Moon, Mars and Venus.

Interplanetary exploration and exploitation had increased rapidly. But the vast distances to other stars remained unconquerable until late in the 22nd Century, when three great inventions made interstellar travel possible.