Poole joined in the laughter, partly in shame-faced reaction to his own fright. To change the subject, he asked Indra the question that was still worrying him.

'All this,' he said, 'it's wonderful – but why go to so much trouble, when anyone in the Tower can reach the real thing, just as quickly?'

Indra looked at him thoughtfully, weighing her words. 'That's not quite true. It's uncomfortable – even dangerous – for anyone who lives above the half-gee level to go down to Earth, even in a hoverchair. So it has to be this -or, as you used to say, Virtual Reality.'

(Now I begin to understand, Poole told himself bleakly. That explains Anderson's evasiveness, and all the tests he's been doing to see if I've regained my strength. I've come all the way back from Jupiter, to within two thousand kilometres of Earth – but I may never again walk on the surface of my home planet. I'm not sure how I will be able to handle this...)

His depression quickly passed: there was so much to do and see. A thousand lifetimes would not have been enough, and the problem was to choose which of the myriad distractions this age could offer. He tried, not always successfully, to avoid the trivia, and to concentrate on the things that mattered – notably his education.

The Braincap – and the book-sized player that went with it, inevitably called the Brainbox – was of enormous value here. He soon had a small library of 'instant knowledge' tablets, each containing all the material needed for a college degree. When he slipped one of these into the Brainbox, and gave it the speed and intensity adjustments that most suited him, there would be a flash of light, followed by a period of unconsciousness that might last as long as an hour. When he awoke, it seemed that new areas of his mind had been opened up, though he only knew they were there when he searched for them. It was almost as if he was the owner of a library who had suddenly discovered shelves of books he did not know he possessed.

To a large extent, he was the master of his own time. Out of a sense of duty – and gratitude – he acceded to as many requests as he could from scientists, historians, writers and artists working in media that were often incomprehensible to him. He also had countless invitations from other citizens of the four Towers, virtually all of which he was compelled to turn down.

Most tempting – and most hard to resist – were those that came from the beautiful planet spread out below. 'Of course,' Professor Anderson had told him, 'you'd survive if you went down for short time with the right life-support system, but you wouldn't enjoy it. And it might weaken your neuromuscular system even further. It's never really recovered from that thousand-year sleep.'

His other guardian, Indra Wallace, protected him from unnecessary intrusions, and advised him which requests he should accept – and which he should politely refuse. By himself, he would never understand the socio-political structure of this incredibly complex culture, but he soon gathered that, although in theory all class distinctions had vanished, there were a few thousand super-citizens. George Orwell had been right; some would always be more equal than others.

There had been times when, conditioned by his twentyfirst-century experience, Poole had wondered who was paying for all this hospitality – would he one day be presented with the equivalent of an enormous hotel bill? But Indra had quickly reassured him: he was a unique and priceless museum exhibit, so would never have to worry about such mundane considerations. Anything he wanted – within reason – would be made available to him: Poole wondered what the limits were, never imagining that one day he would attempt to discover them.

All the most important things in life happen by accident, and he had set his wall display browser on random scan, silent, when a striking image caught his attention.

'Stop scan! Sound up!' he shouted, with quite unnecessary loudness.

He recognized the music, but it was a few minutes before he identified it; the fact that his wall was filled with winged humans circling gracefully round each other undoubtedly helped. But Tchaikovsky would have been utterly astonished to see this performance of Swan Lake – with the dancers actually flying...

Poole watched, entranced, for several minutes, until he was fairly confident that this was reality, and not a simulation: even in his own day, one could never be quite certain. Presumably the ballet was being performed in one of the many low-gravity environments – a very large one, judging by some of the images. It might even be here in Africa Tower.

I want to try that, Poole decided. He had never quite forgiven the Space Agency for banning one of his greatest pleasures – delayed parachute formation jumping – even though he could see the Agency's point in not wanting to risk a valuable investment. The doctors had been quite unhappy about his earlier hang-gliding accident; fortunately his teenage bones had healed completely.

'Well,' he thought, 'there's no one to stop me now unless it's Prof. Anderson...'

To Poole's relief, the physician thought it an excellent idea, and he was also pleased to find that every one of the Towers had its own Aviary, up at the one-tenth-gee level.

Within a few days he was being measured for his wings, not in the least like the elegant versions worn by the performers of Swan Lake. Instead of feathers there was a flexible membrane, and when he grasped the hand-holds attached to the supporting ribs, Poole realized that he must look much more like a bat than a bird. However his 'Move over, Dracula!' was completely wasted on his instructor, who was apparently unacquainted with vampires.

For his first lessons he was restrained by a light harness, so that he did not move anywhere while he was taught the basic strokes – and, most important of all, learned control and stability. Like many acquired skills, it was not quite as easy as it looked.

He felt ridiculous in this safety-harness – how could anyone injure themselves at a tenth of a gravity! – and was glad that he needed only a few lessons; doubtless his astronaut training helped. He was, the Wingmaster told him, the best pupil he had ever taught: but perhaps he said that to all of them.

After a dozen free-flights in a chamber forty metres on a side, criss-crossed with various obstacles which he easily avoided, Poole was given the all-clear for his first solo – and felt nineteen years old again, about to take off in the Flagstaff Aero Club's antique Cessna.

The unexciting name 'The Aviary' had not prepared him for the venue of this maiden flight. Though it seemed even more enormous than the space holding the forests and gardens down at the lunar-gee level, it was almost the same size, since it too occupied an entire floor of the gently tapering Tower. A circular void, half a kilometre high and over four kilometres wide, it appeared truly enormous, as there were no features on which the eye could rest. Because the walls were a uniform pale blue, they contributed to the impression of infinite space.

Poole had not really believed the Wingmaster's boast, 'You can have any scenery you like', and intended to throw him what he was sure was an impossible challenge. But on this first flight, at the dizzy altitude of fifty metres, there were no visual distractions, Of course, a fall from the equivalent altitude of five metres in the ten-fold greater Earth gravity could break one's neck; however, even minor bruises were unlikely here, as the entire floor was covered with a network of flexible cables The whole chamber was a giant trampoline; one could, thought Poole, have a lot of fun here – even without wings.

With firm, downward strokes, Poole lifted himself into the air. In almost no time, it seemed that he was a hundred metres in the air, and still rising.