This can not be done, the shoot replied. We can not interact.

I know it! Cub gestured irritably, though for a moment he was tempted to challenge OX to make a circuit for the attempt. Let's go on.

They went on -- but after that Cub's attention was on his memory of his kind, on the bare-fleshed females. If only there were some way to get across the barrier physically!

Suddenly Mach appeared, rising up out of the storage cave. All of them were caught off guard. Once more the machine had been too cunning for them and had arranged to come on their special frame-trip, after all!

The thing came whirling its blade and spinning its treads, forcing the physical beings out of its way, and its pattern-disruptive emanations were so strong that OX had to move explosively to avoid nonsurvival effects. Cub could see the sparkles flying out like a stellar display on a chill night.

Then the group mobilized, as it had so many times before. Ornet served as decoy, flapping his wings and squawking just outside the range of the blade. Dec swooped by, flicking his tail at the perceptor bulbs. Cub stood back and threw stones into the blade. And OX formed shoots that spun across the elements in the machine's vicinity, distracting its alternate-frame perspective.

They could hardly damage Mach, let alone destroy it; it was invulnerable to their attack. But their combined hara.s.sment made it uncomfortable and always drove it back.

This time it persisted for an extraordinary time. It was undeniably strong. But finally the stones and sand that Cub shoveled at its blade and into its hopper discouraged it. Sand did not hurt it, but it was unable to disgorge it while under attack. And so it retreated -- just far enough to abate their defensive action.

Through the years they had come to a kind of understanding with Mach. Once the machine retreated, they would let it alone -- and it would not attack again that day. Truce, while both sides recuperated. Neither side had ever broken that tacit agreement; that temporary security was too important. Mach actually seemed to be honest; perhaps the mechanical circuits prevented dishonesty in any form. This was one of the things about the machine that Cub respected. Sometimes he and Dec and Ornet sought out Mach and attacked it merely to invoke the truce so that they could be a.s.sured it would not attack them while something important was going on.

Cub threw himself down, panting, as the machine became quiescent. It remained within the area of OX's influence -- but Cub had no desire to drive it outside. They had brought it along on this trip, and they would have to return it to the normal enclave. It would not be right to leave it stranded.

Once he had wished for some way to rid the enclave of this constant menace. Now he had the chance -- and would not take it. Not merely because of his interpretation of their truce; because he was even more certain that Mach was a sentient ent.i.ty, too, and deserved a certain measure of respect.

But then he remembered what he had seen beyond the enclave, in the cave of the musicians, and forgot the machine.

Chapter 13.

DREAMS.

Aquilon wiped her eyes with her fists. "This R Pentomino is a menace!" she complained. "I'm getting a headache! It just goes on and on."

Cal pulled his head out of the innards of the machine. "I told you it was an impressive dead end after eleven hundred and three moves."

"I know. I wanted to see for myself."

"Try the glider," he suggested.

"The what?"

"You have been dealing with stationary forms. There are others. Here." He extricated himself and came over. "This is the glider." He made the pattern of dots on her canvas-sheet:

"That's another pentomino!" she said indignantly.

He shrugged and returned to his work. "I hope to convert this machine to a specialized oscilloscope, or facsimile thereof, so that we can translate our signals into pattern-language. I have the feeling that the pattern-ent.i.ties are as eager to talk with us as we are to talk with them. Think how confusing we must be to them!"

"But we are solid and visible!" she said, working on the new figure. It had gone from [1] to [2] to [3] In fact, it was now a mirror image of its original form, turned endwise. Funny.

"Precisely. An ent.i.ty whose system is based on patterns of points would find our mode of operation virtually incomprehensible."

She made the next figure, jumping straight from one to the next without such laborious additions and erasures. [4] "Do you think Veg is all right?"

"I doubt I ever get used to the caprices of female thought," he remarked. "Veg is with Tamme."

"That's what I meant."

"Jealousy -- at your age?"

She looked at the next figure: [5] "Hey -- this thing repeats itself on new squares! It's like a blinker -- only it moves!"

"Precisely. Patterns can travel. The glider moves diagonally at a quarter the speed of light."

"Speed of light?"

"An advance of one square per move is the maximum possible velocity in this game, so we call it the speed of light. The glider takes four moves to repeat itself, one square across and one down, so that is one quarter light-speed."

She looked at it, nodding. "Beautiful!"

Veg would have said, "So are you." Not Cal. He said: "A variant of that formation is called the s.p.a.ceship. s.p.a.ceships of various sizes can move at half the speed of light. As they go, they fire off sparks that vanish, like propulsion."

"The sparkle cloud did that!" she cried.

"Yes. We also know of a 'glider gun' that fires off gliders regularly. And another figure that consumes gliders. In fact, it is possible to fire several gliders to form new figures at the point of convergence -- even another glider gun that shoots back at its parent guns, destroying them."

"If I were a pattern, I'd be very careful where I fired my gliders!" Aquilon said. "That game plays a rough game!"

"It does. As does all nature. I should think a.s.sorted defensive mechanisms would appear by natural selection, or the game would be unstable -- a.s.suming it were self-willed. The possibilities are obvious."

"Especially when you get into three dimensions!"

"Yes. It is a three-dimensional computerized grid I am working on now. I wish I were a more experienced technician!"

"I think you're a genius," she said sincerely. And she felt a flare of emotion.

"You can help me now if you will. I'll need some figures for my three-dimensional grid."

"What's wrong with the ones we have? The R Pentomino, the glider -- "

"They won't be the same. A line of three points would manufacture four new ones, not two -- because of the added dimension. That would form a short cross, which would in turn form a kind of hollow cube. I believe that's an infinitely expanding figure -- and that is not suitable for our purpose. We need figures that are approximately in balance -- that neither fade out too rapidly nor expand to fill the whole framework."

"Hm, I see," she murmured, trying to trace the three-dimensional permutations of the figure on her two-dimensional canvas. She compromised by using color to represent the third dimension. "Your line becomes an indefinitely expanding three-dimensional figure, as you said. Looks like two parallel caterpillar treads with eight cleats in each, if I haven't fouled it up. But almost any figure expands; there are just too many interactions."

"Agreed. So we must modify the rules to do for three dimensions what 'Life' does for two. Perhaps we must require four points to generate a fifth and let a point be stable with three or four neighbors. Perhaps some other combination. If you can suggest viable rules and figures, it will save me time, once I have this equipment modified."

"I'll try!" she said, and bent to it. They both had difficult, intricate jobs, and from time to time they had to break off. They also chatted intermittently during the work.

"Say -- did you ever find the missing earthquake?" Aquilon asked suddenly.

Cal paused momentarily at his labor. She knew he was finding his mental place, as she had just made another momentous leap of topic. To her surprise, he placed her reference accurately. "We were separated three days on Paleo, during which time there were two tremors, a minor and a strong one. I remember them clearly."

"For a genius, you have a poor memory," she said, smiling over her complex dot-pattern. "We were separated four days, and there were quakes on the first three. You must really have been absorbed with that dinosaur not to notice."