Pallis shrugged. "I thought, maybe just until the tree was ready…"

"You thought wrong. I'm a busy man, tree-pilot."

Pallis turned to the boy. "Tell him why you're here. Tell him what you told me, on the tree."

Rees was staring at Hollerbach. "I left the Belt to find out why the Nebula is dying," he said simply.

The Scientist sat forward, intrigued despite himself. "Oh, yes? We know why it's dying. Hydrogen depletion. That's obvious. What we don't know is what to do about it."

Rees studied him, apparently thinking it over. Then he asked: "What's hydrogen?"

Hollerbach drummed his long fingers on the desk top, on the point of ordering Pallis out of the room… But Rees was waiting for an answer, a look of bright inquiry in his eyes.

"Hmm. That would take more than a sentence to explain, lad." Another drum of the fingers. "Well, maybe it wouldn't do any harm — and it might be amusing—"

"Sir?" Pallis asked.

"Are you any good with a broom, lad? The Bones know we could do with someone to back up that useless article Gover. Yes, why not? Pallis, take him to Grye. Get him a few chores to do; and tell Grye from me to start him on a bit of basic education. He may as well be useful while he's eating our damn food, just until the tree flies, mind."

"Hollerbach, thanks—"

"Oh, get out, Pallis. You've won your battle. Now let me get on with my work. And in future keep your damn lame skitters to yourself!"

A handbell shaken somewhere told him that the shift was over. Rees peeled off his protective gloves and with an expert eye surveyed the lab; after his

efforts its floor and walls now gleamed in the light of the globes fixed to the ceiling.

He walked slowly out of the lab. The light from the star above made his exposed skin tingle, and he rested for a few seconds, drinking in gulps of antiseptic-free air. His back and thighs ached and the skin of his upper arms itched in a dozen places: trophies of splashes of powerful cleaning agents.

The few dozen shifts before the next tree departure seemed to be flying past. He drank in the exotic sights and scents of the Raft, anticipating a return to a lifetime in a lonely cabin in the Belt; he would pore over these memories as Pallis must treasure his photograph of Sheen.

But what he'd been shown and taught had been precious little, he admitted to himself — despite Hollerbach's vague promises. The Scientists were an unprepossessing collection — mostly middle-aged, overweight and irritable. Brandishing the bits of braid that denoted their rank they moved about their strange tasks and ignored him. Grye, the assistant who'd been assigned the task of educating him, had done little more than provide Rees with a child's picture book to help him read, together with a pile of quite incomprehensible lab reports,

Although he'd certainly learned enough about cleaning, he reflected ruefully.

But occasionally, just occasionally, his skitter-like imagination would be snagged by something. Like that series of bottles, set out like bar stock in one of the labs, filled with tree sap in various stages of hardening—

"You! What's your name? Oh, damn it, you, boy! Yes, you!"

Rees turned to see a pile of dusty volumes staggering towards him. "You, the lad from the mine. Come and give me a hand with this stuff…" Over the volumes appeared a round face topped by a bald scalp, and Rees recognized Cipse, the Chief Navigator. Forgetting his aches he hurried towards the puffing Cipse and, with some delicacy, took the top half of the pile.

Cipse panted with relief. "Took your time, didn't you?"

"I'm sorry…"

"Well, come on, come on; if we don't get these printouts to the Bridge sharpish those buggers in my team will have cleared off to the bars again, you mark my words, and that'll be another shift lost." Rees hesitated, and after a few paces Cipse turned. "By the Bones, lad, are you deaf as well as stupid?"

Rees felt his mouth working. "I… you want me to bring this stuff to the Bridge?"

"No, of course not," Cipse said heavily. "I want you to run to the Rim and dump it over the side, what else…? Oh, for the love of — come on, come on!"

And he set off once more.

Rees stood there for a full half-minute. The Bridge…!

Then he ran after Cipse towards the heart of the Raft.

The city on the Raft had a simple structure. Seen from above — without its covering deck of trees — it would have appeared as a series of concentric circles.

The outermost circle, closest to the Rim, was fairly empty, studded by the imposing bulks of supply machines. Within that was a band of storage and industrial units, a noisy, smoky place. Next came residential areas, clusters of small cabins of wood and metal. Rees had come to understand that the lower-placed citizens occupied the cabins closest to the industrial region. Within the housing area was a small region containing various specialist buildings: a training unit, a crude hospital — and the labs of the Scientist class where Rees was living and working. Finally, the innermost disc of the Raft — into which Rees had not previously been allowed — was the preserve of the Officers.

And at the center, at the hub of the Raft itself, was embedded the gleaming cylinder which Rees had spotted on his first arrival here.

The Bridge… And now, perhaps, he might be allowed to enter it.

The Officers' cabins were larger and better finished than those of the ordinary crew; Rees stared with some awe at the carved door frames and curtained windows. Here there were no running children, no perspiring workers; Cipse slowed his bustle to a more stately walk, nodding to the gold-braided men and women they encountered.

Pain lanced through Rees's foot as he stubbed his toe on a raised deck plate. His load of books tumbled to the surface, yellowed pages opening tiredly to reveal tables of numbers; each page was stamped with the mysterious letters "IBM."

"Oh, by the Bones, you useless mine rat!" Cipse raged. Two young Officer cadets walked by; the braid in their new caps glittered in the starlight and they pointed at Rees, laughing quietly.

"I'm sorry," Rees said, face burning. How had he tripped? The deck was a flat mosaic of welded iron plates… or was it? He stared down. The plates here were curved and studded with rivets, and their sheen was silvery, a contrast to the rusty tinge of the iron sheets further out. On one plate, a few feet away, was a blocky, rectangular design; it was tantalizingly incomplete, as if huge letters had once been painted on a curving wall, and the surface cut up and reassembled.

Cipse muttered, "Come on, come on…"

Rees picked up the books and hurried after Cipse. "Scientist," he said nervously, "why is the deck here so different?"

Cipse gave him a glance of exasperation. "Because, lad, the innermost part of the Raft is the oldest. The areas further out were added later, constructed of sheets of star metal; this part was built of hull sections. All right?"

"Hull? The hull of what?"

But Cipse, bustling along, would not reply.

Rees's imagination whirled like a young tree. Hull plates! He imagined the hull of a Mole; if that were cut up and reassembled then that, too, would be an uneven thing of broken curves.

But the shell of a Mole would be much too small to provide all this area. He imagined a huge Mole, its mighty walls curving far above his head…

But that wouldn't be a Mole. A Ship, then? Were the children's tales of the Ship and its Crew true after all?

He felt frustration well up inside him; it was almost like the ache he sometimes felt to reach out to Sheen's cool flesh… If only someone would tell him what was going on!

At last they passed through the innermost rank of dwellings and came to the Bridge. Rees found his pace slowing despite his will; he felt his heart pump within his chest.