So the Human Beings had been put put there. Adda had no doubt about the basic truth of the old story - d.a.m.n it, you only had to watch humans in flight to see the blinding self-evidence of it - but on the other hand, he thought as he watched the flock of Human Beings soar across the sky, he wouldn't really there. Adda had no doubt about the basic truth of the old story - d.a.m.n it, you only had to watch humans in flight to see the blinding self-evidence of it - but on the other hand, he thought as he watched the flock of Human Beings soar across the sky, he wouldn't really want want to be built like an Air-pig. Fat and round and flying by farts? to be built like an Air-pig. Fat and round and flying by farts?

Mind you, flatulence was one skill he had had bettered as he had got older. Maybe it wouldn't have been such a bad idea to have been an Air-pig after all. bettered as he had got older. Maybe it wouldn't have been such a bad idea to have been an Air-pig after all.

Adda was the oldest surviving Human Being. He knew what the others thought of him: that he was a sour old fool, too gloomy for his own good. But he didn't care much about that. He hadn't survived longer than any of his contemporaries by accident. But he was, and always had been, essentially a simple man, not gifted with the power over people and language shown by, say, a Logue. Or even a Dura, he thought, even though she mightn't realize it yet. So if he irritated folk with anecdotes of his boyhood... but, even as they laughed at him, if they soaked up any one of the small lessons which had kept him alive... well, that was all right by Adda.

Of course, there were fragments from the past he didn't share with anyone. He'd no doubt, for instance, that the Glitches were changing.

There had always been Glitches, spin storms. He even knew what caused them, in an abstract sort of way: the slowing of the Star's rotation, and the consequent explosive equalizations of spin energy. But over the last few years the Glitches had got worse... far worse, and much more frequent.

Something else was causing Glitches now. Something unknowably powerful, disrupting the Star...

Of course, his crotchety exterior had a major advantage - one he'd never admitted to anyone else, and only half-allowed to himself. By acting so sour he never had to show the unbearable love he felt for his fellow humans as he watched their alien, vulnerable, impossibly beautiful flight across the Magfield, or the heartbreak he endured at the loss of even the most wasted, most spoiled life.

Hefting his dragging spear in tiring fingers, Adda kicked on toward the treetops of the Crust with renewed vigor.

Farr hovered in the Air, his knees tucked against his chest. With four or five brisk pushes he emptied his bowels. He watched the pale, odorless pellets of s.h.i.t sail sparkling into the empty Air and sink toward the underMantle. Dense with neutrons, the waste would merge into the unbreathable underMantle and, perhaps, sink at last into the Quantum Sea.

He'd never been so high.

The treetops were only a few minutes' Waving above him now: only a score of mansheights or so. The round, bronzed leaves of the trees, all turned toward the Quantum Sea, formed a glimmering ceiling over the world. As he Waved he stared up at that ceiling longingly, as if the leaves somehow represented safety - and yet he looked nervously too. For beyond the leaves were the tree trunks, suspended in darkness; and beyond the trunks lay the Crust itself, where all manner of creatures prowled... At least according to old Adda, and some of the other kids.

But still, Farr realized, he'd rather be up there amidst the trees than - suspended suspended - out here. - out here.

He pushed at the Magfield and shimmered upward.

Farr, young as he was, was used to the feeling of fear. Of mortal terror, even. But he was experiencing a kind of fear new to him - a novelty - and he probed at it, trying to understand.

The nine adults around him Waved steadily upward, their faces turned up to the trees like inverted leaves. Their bodies moved efficiently and with varying degrees of grace, and Farr could smell the musky photons they exuded, hear the steady rhythm of their breathing as they worked, wordless. His own breath was rapid; the Air up here felt thin, shallow. And he was growing colder, despite the hard work of Waving.

Somehow, without realizing it, Farr had gotten himself to the center of the Waving group, so they formed a protective barrier around him. In fact, he realized, he was Waving close to his sister, Dura, as if he were some little kid who needed his hand holding.

How embarra.s.sing.

Discreetly, without making it too obvious, he leaned forward so that he slid out toward the edge of the group, away from Dura. And at the edge that strange new flavor of fear - a feeling of exposure - a.s.sailed him again. Shaking his head as if to clear out musty Air, he forced himself to turn away from the group, twisting in the Air, so that he faced outward, across the Mantle.

Farr knew that the Mantle was tens of millions of mansheights deep. But humans could survive only in a band about two million mansheights thick. Farr knew why... or some of it anyway. The complex compounds of heavy tin nuclei which composed his body (so his father had explained earnestly) could remain stable - remain bonded by exchanges of neutron pairs - only within this layer. It was all to do with neutron density: too far up and there weren't enough neutrons to allow the complex bonding between nuclei; too far down, in the cloying underMantle, there were too many too many neutrons - in the underMantle the very nuclei which composed his body would begin to dissolve, liquefying at last into smooth neutron liquid. neutrons - in the underMantle the very nuclei which composed his body would begin to dissolve, liquefying at last into smooth neutron liquid.

And here - close to the treetops, nearing the top of the habitable band - he was tens of thousands of mansheights above the site of the ruined Net.

Farr looked down, beyond his Waving feet, back the way he had climbed. The vortex lines crossed the enormous sky, hundreds of them in a rigid parallel array of blue-white streaks which melted into misty vanishing-points to left and right. The lines blurred below him, the distance between them foreshortening until the lines melted into a textured blue haze above the Quantum Sea. The Sea itself was a purple bruise below the vortex lines, its surface mist-shrouded and deadly.

...And the surface of the Sea curved downward.

Farr had to suppress a yell by gulping, hard. He looked again at the Sea and saw how it fell subtly away in every direction; there seemed no doubt that he was looking down at a huge sphere. Even the vortex lines dipped slightly as they arced away, converging, toward the horizons of the Sea. It was as if they were a cage which encased the Sea.

Farr had grown up knowing that the world - the Star - was a multilayered ball, a neutron star. neutron star. The Crust was the outer surface of the ball, with the Quantum Sea forming an impenetrable center; the Mantle, including the levels inhabited by humans, was a layer inside the ball filled with Air. But it was one thing to know such a fact; it was quite another to The Crust was the outer surface of the ball, with the Quantum Sea forming an impenetrable center; the Mantle, including the levels inhabited by humans, was a layer inside the ball filled with Air. But it was one thing to know such a fact; it was quite another to see see it with your own eyes. it with your own eyes.

He was high. high. And he And he felt felt it. He stared down now, deep down, past his feet, at the emptiness which separated him from the Sea. Of course, the Net was long since lost in the Air, a distant speck. But even that, had he been able to see it, would have been a comforting break in this looming immensity... it. He stared down now, deep down, past his feet, at the emptiness which separated him from the Sea. Of course, the Net was long since lost in the Air, a distant speck. But even that, had he been able to see it, would have been a comforting break in this looming immensity...

A break from what?

Suddenly he felt as if his stomach were turning into a ma.s.s of Air, and the Magfield he was climbing seemed - not just invisible - but intangible, almost irrelevant. It was as if there was nothing keeping him up...

He shut his eyes, tight, and tried to retreat into another world, into the fantasies of his childhood. Perhaps once more he could be a warrior in the Core Wars, the epic battles with the Colonists at the dawn of time. Once humans had been strong, powerful, with magical four-walled "wormhole Interfaces" which let them cross thousands of mansheights in a bound, and great machines which allowed them to fly through the Star and beyond.

But the Colonists, the mysterious denizens of the heart of the Star, had emerged from their glutinous realm to wage war on humanity. They had destroyed, or carried off, the marvelous Interfaces and all the rest - and would have sc.r.a.ped mankind out of the Mantle altogether if not for the wily cunning of Farr: Farr the Ur-human, the giant G.o.d-warrior...

At length he felt a touch on his shoulder; he opened his eyes to see - not a Colonist - but Dura hovering before him, a look of careful neutrality on her face. She pointed upward. "We're there."

Farr looked up.

Leaves - six of them arranged in a neat, symmetrical pattern - hung down just above his head. With a surge of absurd grat.i.tude Farr pulled himself up into the darkness beyond the leaves.

A branch about the thickness of his waist and coated with slick-dark wood led from the leaf into a misty, blue-glowing darkness above him... no, he thought, that was the wrong way round; somewhere up there was the trunk of the tree, suspended from the Crust, and from it grew this branch, and from that in turn grew the leaves which faced the Sea. He ran a hand along the wood of the branch; it was hard and smooth, but surprisingly warm to the touch. A few twigs dangled from the main stem, and tiny leaves sought c.h.i.n.ks of light between their larger cousins.

He found himself clinging to the branch, his arms wrapped around it as if around the arm of his mother. The warmth of the wood seeped through his chilled body. Embarra.s.sment flickered through his mind briefly, but he ignored it; at last he felt safe.

Dura slid through the leaves and came to rest close to him. The subdued shadow-light of the tree picked out the curves of her face. She smiled at him, looking self-conscious. "Don't worry about it," she said, quietly enough that the others couldn't hear. "I know how you feel. I was the same, the first time I came up here."

Farr frowned. Reluctantly he released the branch and pushed himself away. "You were? But I feel as if - as if I'm about to be pulled out of this tree..."

"It's called being frightened of falling."

"But that's ridiculous. Isn't it?" To Farr, "falling" meant losing your grip on the Magfield when Waving. It was always over in a few mansheights at the most - the tiny resistance of the Air and the currents induced in your skin soon slowed you down. Nothing to fear. And then you could just Wave your way around the Magfield to where you wanted to get to.

Dura grinned. "It's a feeling as if..." She hesitated. "...as if you could let go of this tree, right now, and not be able to stop yourself sliding down, across the Magfield and across the vortex lines, faster and faster, all the way to the Sea. And your belly clenches up at the prospect."

"That's exactly it," he said, wondering at how precise her description was. "What does it mean? Why should we feel like that?"

She shrugged, plucking at a leaf. The heavy plate of flesh came free of its attaching branch with a sucking sound. "I don't know. Logue used to say it's something deep inside us. An instinct we carried with us, when humans were brought to this Star."

Farr thought about that. "Something to do with the Xeelee."

"Perhaps. Or something even older. In any event, it's not something you need to worry about. Here." She held out the leaf toward him.

He took it from her cautiously. It was a bronze-gold plate, streaked radially with purple and blue, about as wide as a man's hand. It was thick and pulpy - springy between his fingers - and, like the wood, was warm to the touch, although, away from its parent branch, it seemed to be cooling rapidly. He turned it over, prodding it with a fingertip; its underside was dry, almost black. He looked up at Dura. "Thanks," he said. "What shall I do with it?"

She laughed. "Try eating it."

After a cautious inspection of her face to make sure this wasn't some kind of joke - Dura didn't usually play tricks on him; she was a little too serious for that... but you never knew - Farr lifted the leaf to his lips and bit into it. The flesh of the leaf was thin, surprisingly insubstantial, and it seemed to melt against his tongue; but the taste it delivered was astonishingly sweet, like the meat of the youngest Air-piglet, and Farr found himself cramming his mouth.

Within seconds he was swallowing the last of the leaf, savoring the lingering flavor on his tongue. It had been delicious but really quite light, and had done little but whet his hunger further. He looked around avidly. Here on the upper side of the treetop ceiling he could see the leaves turned downward toward the Quantum Sea, like a layer of broad, flattened child-faces. Farr reached down to pluck another leaf.

Dura, laughing, restrained him. "Take it easy. Don't strip the whole d.a.m.n tree."

Around a full mouth Farr said, "It's delicious."

She nodded. "I know. But it won't fill your belly. Not unless you really do strip the tree... That's why we have to hunt the Air-pigs, who eat the leaves - and the gra.s.s - for us." She pursed her lips. Then, in a tone suddenly and, to Farr, shockingly similar to their lost father's, she said, "Let's have a little lesson. Why do you think the leaves are so tasty?"

Farr thought about that. "Because they're full of protons."

Dura nodded seriously. "Near enough. Actually they are laced with proton-rich isotopes - of krypton, strontium, zirconium, molybdenum... even a little heavy iron. Each nucleus of krypton, for instance, has a hundred and eighteen protons, while the tin nuclei of our bodies have just fifty each. And our bodies need protons for their fuel." The heavy nuclei fissioned in human stomachs. Protons combined with neutrons from the Air to make more tin nuclei - tin was the most stable nucleus in the Air - and gave off energy in the process. "Now. Where does the proton-rich material come from?"

"From the Crust." He smiled. "Everyone knows that."

The Crust, no more substantial than Air, was a gossamer solid. Its outermost layer was composed of iron nuclei. Further in, steepening pressures drove neutrons into the nuclei of the solid, forming increasingly heavy isotopes... until the nuclei became so soft that their proton distributions began to overlap, and the neutrons dripped out to form the Air, a superfluid of neutrons.

"All right," said Dura. "So how do the isotopes get all the way from the Crust to these leaves?"

"That's easy," Farr said, reaching to pluck another succulent leaf. "The tree pulls them down, inside its trunk."

"Using veins filled with Air. Right."

Farr frowned, feeling his cheeks bulge around the leaf. "But why? What's in it for the tree?"

Dura's mouth opened and closed, and then she smiled, her eyes half-closed. "That's a good question," she said. "One I wouldn't have thought of at your age... The isotopes make the leaves more opaque to the neutrinos shining out of the Quantum Sea."

Farr nodded, chewing.

A flood of neutrinos, intangible and invisible, shone continually from the Sea - or perhaps from the mysterious Core deep beneath the Sea itself - and sleeted through the vortex lines, through the bodies of Farr and the other humans as if they were ghosts, and through the Crust to s.p.a.ce. The trees turned slightly neutrino-opaque leaves to that unseen light, absorbing its energy and turning it into more leaves, branches, trunk. Farr pictured trees all over the interior of the Crust, straining toward the Sealight with their leaves of krypton, strontium and molybdenum.

Dura watched him eat for a moment; then, hesitantly, she reached out to ruffle his hair-tubes. "I'll tell you a secret," she said.

"What?"

"I'm glad you're here."

Briefly he considered pushing her hand away, of saying something funny, or cruel, to break up the embarra.s.sing moment. But something made him hold back. He studied her face. It was a strong face, he supposed, square and symmetrical, with small, piercing eyes and shining yellow nostrils. Not beautiful, but with something of the strength of their father; and now with the first lines of age it was acquiring a bit more depth.

But there was uncertainty in that face. Loneliness. Indecision, a need for comfort.

Farr thought about it. He felt safe with Dura. Not as safe as when Logue was alive... But, he thought ruefully, as safe as he would ever feel again. Dura wasn't really all that strong, but she did her best.

And this moment, as the others moved away from them, being together and talking quietly and tasting the leaves, seemed to be important to her. So he said, gruffly: "Yes. Me too."

She smiled at him, then bent to pluck a leaf for herself.

Adda slid silently through the treetops, following the circ.u.mference of a rough circle twenty mansheights wide. Then he moved a little further up into the suspended forest, working parallel to the lines of the trunks. The trees grew along the Magfield flux lines, and he kept his spear pointing along the Magfield as he worked his way along the smooth bark.

Save for the low, tinkling rustle of the leaves, the subdued talk of his companions, he found only silence.

He pulled himself back along the length of the tree trunk to the inverted canopy of leaves. None of the Human Beings - except, maybe, Logue's boy Farr, who was looking a little lost - had even noticed he'd been absent. Adda relaxed a little, munching on the thin, deceptively tasty meat of a leaf. But he kept his good eye wide open.

The Human Beings were bunched together around one trunk, nibbling leaves desultorily and clinging, one-handed, to branchlets. They were huddled together for warmth. Here, where the Air was attenuated by height, it was cold and hard to breathe: so hard, in fact, that Adda felt his reflexes - his very thinking - slowing down, turning sluggish. And it wasn't as if he had a lot of margin in that area, he reflected. It was as if the very Air which drove his bones was turning to a thin, sour soup.

The boy Farr was crouched against a section of bark a mansheight or so from everyone else. He looked as if he were suffering a bit: visibly shivering, his chest rising and falling rapidly in the attenuated Air, his hands pushing leaves into his downturned mouth with an urgency that looked more like a craving for comfort than for food.

Adda, with a single flip of his legs, Waved briskly over to the boy; he leaned toward Farr and winked with his good eye. "How are you doing?"

The boy looked up at him, lethargic despite the shivering, and his voice, when he spoke, was deepened by the cold. "I can't seem to get warm."

Adda sniffed. "That's the way it is, up here. The Air's too thin for us, see. And if you go higher, toward the Crust, it gets thinner still. But there's no need to be cold."

Farr frowned. "What do you mean?"

For answer Adda grinned. He raised his spear of hardened wood and aligned it parallel to the tree trunk, along the direction of the Magfield flux lines. He hefted it for a few seconds, feeling its springy tension. Then he said, "Watch and remember."

The boy, eyeing the quivering spear with wide eyes, scrambled back out of the way.

Adda braced himself against the Magfield. With a single movement - he remained lithe in spite of everything, Adda congratulated himself - Adda thrust the spearpoint deep into the bulk of the tree. The first stab took the spearpoint through the bark and perhaps a hand's length into the wood. By working the haft of the spear, twisting it in his hands, Adda was able to drive the spear further into the flesh of the branch, to perhaps half an arm's length.

That done, feeling his chest drag at the thin Air, Adda turned to make sure Farr was still watching. "Now," he rasped. "Now comes the magic."

He twisted in the Air and placed his feet against the branch, close to the line of his half-buried spear. Then he bent and wrapped both hands around the protruding shaft of the spear, squatted so that his legs were bent and his back was straight, and heaved upward, using the spear as a lever to prize open the wood of the branch.

...Actually it was a long time since he'd done this, he realized a few heartbeats after starting. His palms grew slick with superfluid sweat, a steady ache spread along his back, and for some reason the vision of his good eye was starting to tremble and blur. And, though the spear bowed upward a bit as he strained, the branch did little more than groan coldly.

He let go of the spear and wiped his palms against his thighs, feeling the breath rattle in his chest. He carefully avoided eye contact with the boy.

Then he bent to the spear again.

This time, at last, the branch gave way; a plate of it the size of his chest yielded and lifted up like a lid. Adda felt his aching legs spring straight, and he tumbled away from the branch. Quickly recovering his dignity, he twisted in the Air, ignoring the protests from his back and legs, and Waved back to Farr and the opened branch. He looked down at his handiwork appraisingly and nodded. "Not as difficult as it looks," he growled at the boy. "Used to do that one-handed... But trees have got tougher since I was your age. Maybe something to do with this d.a.m.n spin weather."

But Farr wasn't listening; he crept forward to the wound in the branch and stared into it with fascination. Close to the rim of the ripped bark the wood was a pale yellow, the material looking much like that of the spear Adda had used. But further in, deeper than a hand's length, the wood was glowing green and emitting a warmth which - even from half a mansheight away - Adda could feel as a comforting, tangible presence against his chest. The glow of the wood sparkled against Farr's face and evoked verdant shadows within his round eyes.

Dura, Logue's ungainly daughter, joined them now; she shot a brief smile of thanks to Adda as she crouched beside her brother and raised her palms to the warmth of the wood. The green fire scattered highlights from her limbs and face which made her look, Adda thought charitably, half-attractive for once. As long as she didn't move about too much and reveal her total lack of grace, anyway.

Dura said to Farr, "Another lesson. What's making the wood burn?"

He smiled at her, eyecups full of wood-glow. "Heavy stuff from the Crust?"

"Yes." She leaned toward Farr so that the heads of brother and sister were side by side over the glowing wood, their faces shining like two leaves. Dura went on, "Proton-rich nuclei on their way to the leaves. The tree branch is like a casing, you see, enclosing a tube where the pressure is lower than the Air. But when the casing is breached the heavy nuclei inside fission, decaying rapidly. What you're seeing is nuclei burning into the Air..."

Adda saw how Farr's smooth young face creased with concentration as he absorbed this new bit of useless knowledge.

Useless?

Well, maybe, he thought; but these precious, abstract facts, polished by retelling and handed down from the earliest days of the Human Beings - from the time of their expulsion from Parz City, ten generations ago - were treasures. Part of what made them human.

So Adda nodded approvingly at Dura and her attempts to educate her brother. The Human Beings had been thrust into this upflux wilderness against their will. But they were not savages, or animals; they had remained civilized people. Why, some of them could even read; a handful of books sc.r.a.ped painfully onto scrolls of pigskin with styli of wood were among the Human Beings' princ.i.p.al treasures...

He leaned toward Dura and said quietly, "You'll have to go on, you know. Deeper into the forest, toward the Crust."

Dura started. She pulled away from the trunk-wound, the light of the burning nuclei shining from the long muscles of her neck. The other Human Beings, a few mansheights away, were still cl.u.s.tered about the treetops; most of them, having crammed their bellies full, were gathering armfuls of the succulent leaves. She said, "I know. But most of them want to go back to the camp already, with their leaves."

Adda sniffed. "Then they're d.a.m.n fools, and it's a shame the spin weather didn't take them instead of a few with more sense. Leaves taste good but they don't fill a belly."

"No. I know." She sighed and rubbed the bridge of her nose, ran a finger around the rim of one eyecup absently. "And we have to replace the Air-pigs we lost in the spin storm."