The Scientist Second Class stood in the doorway of the Bridge. He watched the new Third Class approach and tried to hide a smile. The young man's uniform was so obviously new, he stared with such awe at the Bridge's silver hull, and his pallor was undisputable evidence of his Thousandth Shift celebration, which had finished probably mere hours earlier… The Second Class felt quite old as he remembered his own Thousandth Shift, his own arrival at the Bridge, a good three thousand shifts ago.

At least this boy had a look of inquiry about him. So many of the apprentices the Second had to deal with were sullen and resentful at best, downright contemptuous at worst; and the rates of absenteeism and dismissal were worsening. He reached out a hand as the young man approached.

"Welcome to the Bridge," said Scientist Second Class Rees.

The boy — blond, with a premature streak of gray — was called Nead. He smiled uncertainly.

A bulky, grim-faced security guard stood just inside the door. He fixed Nead with a threatening stare; Rees saw how the boy quailed. Rees sighed. "It's all right, lad; this is just old Forv; it's his job to remember your face, that's all." It was only recently, Rees realized a little wistfully, that such heavy-handed security measures had come to seem necessary; with the continuing decline in food supplies, the mood on the Raft had worsened, and the severity and frequency of the attacks of the "discontents" were increasing. Sometimes Rees wondered if—

He shook his head to dismiss such thoughts; he had a job to do. He walked the wide-eyed boy slowly through the Bridge's gleaming corridors. "It's enough for now if you get an idea of the layout of the place," he said. "The Bridge is a cylinder a hundred yards long. This corridor runs around its midriff. The interior is divided into three rooms — a large middle chamber and two smaller chambers toward the ends. We think that the latter were once control rooms, perhaps equipment lockers; you see, the Bridge seems to have been a part of the original Ship…"

They had reached one of the smaller chambers; it was stacked with books, piles of paper and devices of all shapes and sizes. Two Scientists, bent in concentration, sat cocooned in dust. Nead turned flat, brown eyes on Rees. "What's this room used for now?"

"This is the Library," Rees said quietly. "The Bridge is the most secure place we have, the best protected from weather, accident — so we keep our records here. As much as we can: one copy of everything vital, and some of the stranger artefacts that have come down to us from the past…"

They walked on following the corridor to a shallow staircase set into the floor. They began to descend toward a door set in the inner wall, which led to the Bridge's central chamber. Rees thought of warning the boy to watch his step — then decided against it, a slightly malicious humor sparkling within him.

Nead took three or four steps down — then, arms flailing, he tipped face-forward. He didn't fall; instead he bobbed in the stairwell, turning a slow somersault. It was as if he had fallen into some invisible fluid.

Rees grinned broadly.

Nead, panting, reached for the wall. His palms flat against the metal he steadied himself and scrambled back up the steps. "By the Bones," he swore, "what's down there?"

"Don't worry, it's harmless," Rees said. "It caught me the first time too. Nead, you're a Scientist now. Think about it. What happened when you went down those steps?"

The boy looked blank.

Rees sighed. "You passed through the plane of the Raft's deck, didn't you? It's the metal of the deck that provides the Raft's gravity pull. So here — at the center of the Raft, and actually in its plane — there is no pull. You see? You walked into a weightless zone."

Nead opened his mouth — then closed it again, looking puzzled.

"You'll get used to it," Rees snapped. "And maybe, with time, you'll even understand it. Come on."

He led the way through the doorway to the central chamber, and was gratified to hear Nead gasp.

They had entered an airy room some fifty yards wide. Most of its floor area was transparent, a single vast window which afforded a vertiginous view of the depths of the Nebula. Gaunt machines taller than men were fixed around the window. To Nead's untutored eye, Rees reflected, the machines must look like huge, unlikely insects, studded with lenses and antennae and peering into some deep pool of air. The room was filled with a clean smell of ozone and lubricating oil; servomotors hummed softly.

There were perhaps a dozen Scientists working this shift; they moved about the machines making adjustments and jotting copious notes. And because the plane of the Raft passed over the window-floor at about waist height, the Scientists bobbed in the air like boats in an invisible pond, their centers of gravity oscillating above and below the equilibrium line with periods of two or three seconds. Rees, looking at the scene as if through new eyes, found himself hiding another grin. One small, round man had even, quite unselfconsciously, turned upside down to bring his eyes closer to a sensor panel. His trousers rode continually toward the equilibrium plane, so that his short legs protruded, bare.

They stood on a low ledge; Rees took a step down and was soon floating in the air, his feet a few inches from the window-floor. Nead lingered nervously. "Come on, it's easy," Rees said. "Just swim in the air, or bounce up and down until your feet hit the deck."

Nead stepped off the ledge and tumbled forward, slowly bobbing upright. He reminded Rees of a child entering a pool for the first time. After a few seconds a slow smile spread across the young man's face; and soon he was skimming about the room, his feet brushing at the window below.

Rees took him on a tour around the machines.

Nead shook his head. "This is amazing."

Rees smiled. "This equipment is among the best preserved of the Ship's materiel. It's as if it were unloaded only last shift… We call this place the Observatory. All the heavy-duty sensors are mounted here, and this is where — as a member of my Nebular physics team — you'll be spending most of your time." They stopped beside a tube ten feet long and encrusted with lenses. Rees ran a palm along the instrument's jewelled flank. "This baby's my favorite," he said. "Beautiful, isn't she? It's a Telescope which will work at all wavelengths — including the visual. Using this we can see right down into the heart of the Nebula."

Nead thought that over, then glanced at the ceiling. "Don't we ever need to look outwards?"

Rees nodded approvingly. Good question. "Yes, we do. There are ways of making the roof transparent — in fact, we can opaque the floor, if we want to." He glanced at the instrument's fist-sized status panel, "We're in luck; there are no observations currently running. I'll give you a quick guided tour of the Nebula. You should know most of what I'll tell you from your studies so far, and don't worry about the details for now…" Slowly he punched a sequence of commands into the keypad mounted below the sensor. He became aware that the lad was watching him curiously. Maybe he's never seen anyone with such rusty keyboard skills, Rees reflected, here on this Raft of a hundred supply machines—

The stab of old resentment shocked him. Never mind…

A disc of ceiling faded to transparency, revealing a red sky. Rees indicated a monitor plate mounted on a slim post close to the Telescope. The plate abruptly filled with darkness punctuated by fuzzy lens shapes; the lenses were all colors, from red through yellow to the purest blue. Once more Nead gasped.

"Let's review a few facts," Rees began. "You know we live in a Nebula, which is an ellipse-shaped cloud of gas about five thousand miles across. Every particle of the Nebula is orbiting the Core. The Raft is in orbit too, embedded in the Nebula like a fly on a spinnng plate; we circle the Core every twelve shifts or so. The Belt mine is further in and only takes about nine shifts to complete its orbit. When the pilots fly between mine and Raft their trees are actually changing orbits…! Fortunately the gradients in orbital speeds are so shallow out here that the velocities the trees can reach are enough for them easily to fly from one orbit to another. Of course the pilots must plan their courses carefully, to make sure the Belt mine isn't on the other side of the Core when they arrive at the right orbit…