Fortunately for the Scientists, most of the other Classes' apprentices were quite happy with this state of affairs. The Officer cadets in particular sat through their lectures with every expression of disdain, clearly eager to abandon this dry stuff for the quick of life, the exercise of power.

So the Scientists went unchallenged, but Rees wasn't sure about the wisdom of their policy. The Raft itself, while still comfortable and well-supplied compared to the Belt, was now riven by shortages. Discontent was widespread, and — since the people did not have the knowledge to understand the (more or less) genuine contribution to their welfare made by the more privileged Classes — those Classes were more often than not the target of unfocused resentment.

It was an unstable mixture.

And the enslaving of knowledge had another adverse effect, Rees realized. Turning facts into precious things made them seem sacred, immutable; and so he saw Scientists pore over old printouts and intone litanies of wisdom brought here by the Ship and its Crew, unwilling — or unable — to entertain the idea that there might be facts beyond the ageing pages, even — breathe it quietly — inaccuracies and mistakes!

Despite all his doubts and questions, Rees found the shifts following his acceptance the happiest of his life. As a fully fledged apprentice he was entitled to more than Grye's grudging picture-book sessions; now he sat in classes with the other apprentices and learned in a structured and consistent way. For hours outside his class time he would pore over his books and photographs — and he would never forget an ageing picture buried in one battered folder, a photograph of the blue rim of the Nebula.

Blue!

The magical color filled his eyes, every bit as clear and cool as he had always imagined.

At first, Rees sat, awkwardly, with apprentices some thousands of shifts younger than himself; but his understanding progressed rapidly, to the grudging admiration of his tutors, and before long he had caught up and was allowed to join the classes of Hollerbach himself.

Hollerbach's style as a teacher was as vivid and captivating as the man himself. Abandoning yellowing texts and ancient photographs the old Scientist would challenge his charges to think for themselves, adorning the concepts he described with words and gestures.

One shift he had each member of the class build a simple pendulum — a dense metal bob attached to a length of string — and time its oscillation against the burning of a candle. Rees set up his pendulum, limiting the oscillations to a few degrees as Hollerbach instructed, and counted the swings carefully. A few benches along he was vaguely aware of Doav languidly going through the motions of the experiment; whenever Hollerbach's fierce eye was averted Doav would poke at the swinging bob before him, elaborately bored.

It didn't take long for the students to establish that the period of the pendulum's swing depended only on the length of the string — and was independent of the mass of the bob.

This simple fact seemed wonderful to Rees (and that he had found it out for himself made it still more so); he stayed in the little student lab for many hours after the end of the class extending the experiment, probing different mass ranges and larger amplitudes of swing.

The next class was a surprise. Hollerbach entered grandly and eyed the students, bade them pick up the retort stands to which their pendulums were still fixed, and beckoned. Then he turned and marched from the lab.

The students nervously followed, clutching their retorts; Doav rolled his eyes at the tedium of it all.

Hollerbach led them on a respectable hike, out along an avenue beneath the canopy of turning trees. The sky was clear of cloud today and starlight dappled the plates of the deck. Despite Ms age Hollerbach kept up a good pace, and by the time he paused, under open sky a few yards beyond the edge of the flying forest, Rees suspected that his weren't the only young legs that ached a little. He looked around curiously, blinking in the direct starlight; since beginning classes he had scarcely had a chance to come out this way, and the apparent tilt of the riveted deck under his feet felt strange.

Solemnly Hollerbach lowered himself to the deck plates and sat cross-legged, then bade his students do the same. He fixed a series of candles to the plates. "Now, ladies and gentlemen," he boomed, "I would like you to repeat your experiments of our last class. Set up your pendulum."

There were stifled groans around the class, presumably inaudible to Hollerbach. The students began work, and Hollerbach, restless, got up and paced among them. "You are Scientists, remember," he told them. "You are here to observe, not judge; you are here to measure and understand…"

Rees's results were… odd. As Hollerbach's supply of candles burned through he went over his results carefully, repeating and testing.

At last Hollerbach called them to order. "Conclusions, please? Doav?"

Rees heard the cadet's breathy groan. "No difference," he said languidly. "Same result curve as last time.»

Rees frowned. That was wrong; the periods he had measured had been greater than yesterday's — by a small amount, granted, but greater consistently.

The silence gathered. Doav shifted uneasily.

Then Hollerbach let him have it. Rees tried not to grin as the old Scientist tore into the cadet's sloppy methods, his closed mind, his laziness, his lack of fitness to wear the golden braids. By the end of it Doav's cheeks burned crimson.

"Let's have the truth," Hollerbach muttered, breathing hard. "Baert…?"

The next apprentice supplied an answer consistent with Rees's. Hollerbach said, "Then what has happened ? How have the conditions of this experiment changed?"

The students speculated, listing the effect of the starlight on the pendulum bobs, the greater inaccuracy of the timing method — Hollerbach's candles flickered far more out here than in the lab — and many other ideas. Hollerbach listened gravely, occasionally nodding.

None of it convinced Rees. He stared at the simple device, willing it to offer up its secrets.

At last the student Baert said hesitantly, "What about gravity?"

Hollerbach raised his eyebrows. "What about it?"

Baert was a slender, tall boy; now he rubbed his thin nose uncertainly. "We're a little further from the Raft's center of gravity here, aren't we? So the pull of gravity on the pendulum bob will be a bit less…"

Hollerbach eyed him fiercely, saying nothing. Baert flushed and went on, "It's gravity that makes the bob swing, by pulling at it. So if gravity's less, the period will be longer… Does that make sense?»

Hollerbach rocked his head from side to side. "At least that's a little less dubious than some of the other proposals I've heard. But if so, what precisely is the relationship between the strength of gravity and the period?"

"We can't say," Rees blurted. "Not without more data."

"Now that," Hollerbach said, "is the first intelligent thing any of you have said this shift. Well, ladies and gentlemen, I suggest you proceed to gather your facts. Let me know what you find out." He stood, stiffly, and walked away.

The students dispersed to their task with varying degrees of enthusiasm. Rees went at it with a will, and for the next few shifts scoured the deck, armed with his pendulum, notepad and supply of candles. He recorded the period of the pendulum, made careful notes and drew logarithmic scale graphs — and more; carefully he observed how the plane of the pendulum's swing formed various angles with the surface, showing how the local vertical was changing as he moved across the face of the Raft. And he watched the slow, uncertain oscillations of the pendulum at the Rim itself.

At last he took his findings to Hollerbach. "I think I have it," he said hesitantly. "The period of the pendulum is proportional to the square root of its length… and also inversely proportional to the square root of the acceleration due to gravity."