He left the room and they followed him through a mighty corridor to another room filled with elaborate machinery.

The Engineer strode to a control panel and worked with dials and studs. Intense blue power surged through long tubes and flashed in dizzy whirls through coils of glass.

Tubes boomed into sudden brilliance and the deep hum of power surged into the room.

They could hear the probing fingers of the Engineer's thoughts, thrusting out, calling to those other people in another universe. The power of thought being hurled through the very warp and weave of twisted time and space.

Then came another probing thought, a string of thoughts that were impossible to understand, hazed and blurred and all distorted. But apparently perfectly clear to the Engineer, who stood motionless under the inverted cone of glass that shimmered with blue fire of power.

Two entities talking to one another and the queer, challenging unknown of five-dimensional inter-space separating them!

The power ebbed and the blue fire sank to a glimmer in the tubes.

The Engineer turned around and faced the Earthlings.

"They will come," he said, "but only on one condition."

Suddenly a shiver went through Gary. Condition! That was something he hadn't thought about — that these other things might exact terms, might want concessions, might seek to wring front another universe some measure of profit for a service done.

He had always thought of them as benevolent beings, entities like the Engineers, living a life of service, establishing themselves as guardians of their universe. But that was it. Would they go out of their way to save another universe? Or would they fight only for their own? Was there such a thing as selflessness and universal brotherhood? Or must the universes, in time to come, be forever at one another's throats, as in ancient times nations had torn at one another in savage anger, in more recent times planets had warred for their selfish interests?

"What condition?" asked Kingsley.

"That we or they find out something concerning the nature of the inter-space and of the energy which will be generated when the universes rub," said the Engineer. "They are willing to come and fight for us, but they are not willing to deliberately invite disaster to themselves. No one knows what the inter-space is like. No one knows what laws of science it may hold. There may be laws that are utterly foreign to both our universes, laws that would defy our every bit of knowledge. They are afraid that the budding of a smaller universe from the surface of their own might serve to generate the energy they know will result when two four-dimensional frames draw close to one another."

"Now, wait," said Gary. "There is something I didn't consider when I proposed this thing. It just occurred to me now. When you said the word "condition," it came to my mind that they might want concessions or promises. I was wrong, interpreted the thought wrong. But the idea is still there. We don't know what these things in the other universe might be. We don't know what they look like or what their philosophy is or what they can do. If we allowed them to come here, we'd be giving them a key to this universe. Just opening the door for them. They might be all right and they might not. They might take over the universe."

"There's something to that," said Tommy. "We should have thought of it before."

"I do not believe it," said the Engineer. "I have some reason to believe they would not be a menace to us."

"What reason?" rumbled Kingsley.

"They notified us of the danger," said the Engineer.

"They wanted help," said Tommy.

"We have been of little help to them," said the Engineer.

"What difference does it make?" asked Herb. "Unless we can do something about this energy, we're going to be goners, anyhow. And that goes for the other universe as well. If they could save themselves by ruining us, maybe they'd do it, but it's a cinch that if we puff out they go along with us."

"That's right," agreed Kingsley. "It would be to their interest to help us beat off the Hellhounds on the chance that we might find something to save the universes. They wouldn't be very likely to turn on us until somebody had figured out something about this energy."

"And we can't control something we don't understand," said Caroline. "We have to find out what that energy is, what it's like, what form it is apt to take, something about it, so we will know how to handle it."

"How much more time have we to find some way to save us from the big explosion?" asked Gary.

"Very little time," said the Engineer. "Very little time. We are perilously close to the danger point. Shortly the two space-time frames of the two universes will start reacting upon one another, creating the lines of force and stress that will set up the energy fields in the inter-space."

"And you say there is another race that can tell us about this inter-space?"

"One other race I know of," said the Engineer. "There may be others, but I know only of this one. And it is hard to reach. Perhaps impossible to reach."

"Listen," said Gary, "it is our only chance. We might as well fail in reaching them as waiting here for the energy to come and wipe us out. Let a couple of us try. The others may find something else before it is too late. Caroline's hyperspheres might take care of the energy, but we can't be sure. And we have to be sure. The universe depends upon us being sure. We can't just shoot in the dark.

We have to know."

"And if we find out," said Herb, "those guys over in the other universe can come over and help us hold the Hellhounds off while we rig up the stuff we have to have."

"I'm afraid," said Kingsley, "we have to take the chance."

"Chance," said the Engineer. "It's a whole lot more than chance. The place I have in mind may not even exist."

"May not even exist?" asked Caroline and there was an edge of terror in her words.

"It is far away," said the Engineer. "Not far in space — perhaps even close to us in space. But far away in time."

"In time?" asked Tommy. "Some great civilization of the past?"

"No," said the Engineer. "A civilization of the future. A civilization which may never exist. One that may never come to be."

"How do you know about it, then?" flared Gary.

"I followed its world line," said the Engineer. "And yet not its actual world line, but the world line that was to come. I traced it into the realm of probability. I followed it ahead in time, saw it as it is not yet, as it may never be. I saw the shadow of its probability."

Gary's head reeled. What talk was this? Following of probable world lines. Tracing the course of an empire before it had occurred! Seeing a place that might not ever exist. Talking of sending someone to a place that might never be!

But Caroline was talking now, her cool voice smooth and calm, but with a trace of excitement tinging the tenor of her words.

"You mean you used a geodesic tracer to follow the world line into probability. That you established the fact that in some future time a certain world may exist under such conditions as you saw. That barring unforeseen circumstances it will exist as you saw it, but that you cannot be certain it ever will exist, for the world line you traced could not take into account that factor of accident which might destroy the world or divert it from the path you charted, the path that it logically would have to take."

"That is correct," said the Engineer. "Except for one thing. And that is that the world will exist as I saw it in some measure. For all probabilities must exist to some extent. But its existence might be so tenuous that we could never reach it… that for us, in hard, solid fact, it would have no real existence. In other words, we could not set foot upon it. For every real thing there are infinite probabilities, all existing, drawing some shadow of existence from the mere fact that they are probable or have been probable or will be probable. The stress and condition of circumstance selects one of these probabilities, makes it an actuality. But the others have an existence, just the same. An existence, perhaps, that we could not perceive."