'That's impossible! Six is the maximum they ever gave me in the centrifuge, and I didn't enjoy weighing half a ton. I know we haven't moved since we stepped inside.'

Poole had raised his voice slightly, and suddenly became aware that the other passengers were pretending not to notice.

'I don't understand how it's done, Frank, but it's called an inertial field. Or sometimes a Sharp one – the "S" stands for a famous Russian scientist, Sakharov – I don't know who the others were.'

Slowly, understanding dawned in Poole's mind – and also a sense of awe-struck wonder. Here indeed was a 'technology indistinguishable from magic'.

'Some of my friends used to dream of "space drives" – energy fields that could replace rockets, and allow movement without any feeling of acceleration, Most of us thought they were crazy – but it seems they were right! I can still hardly believe it... and unless I'm mistaken, we're starting to lose weight.'

'Yes – it's adjusting to the lunar value. When we step out, you'll feel we're on the Moon. But for goodness' sake, Frank – forget you're an engineer, and simply enjoy the view.'

It was good advice, but even as he watched the whole of Africa, Europe and much of Asia flow into his field of vision, Poole could not tear his mind away from this astonishing revelation. Yet he should not have been wholly surprised: he knew that there had been major breakthroughs in space propulsion systems since his time, but had not realized that they would have such dramatic applications to everyday life – if that term could be applied to existence in a thirty-six-thousand-kilometre-high skyscraper.

And the age of the rocket must have been over, centuries ago. All his knowledge of propellant systems and combustion chambers, ion thrusters and fusion reactors, was totally obsolete. Of course, that no longer mattered – but he understood the sadness that the skipper of a windjammer must have felt, when sail gave way to steam.

His mood changed abruptly, and he could not help smiling, when the robovoice announced, 'Arriving in two minutes. Please make sure that you do not leave any of your personal belongings behind.'

How often he had heard that announcement, on some commercial flight? He looked at his watch, and was surprised to see that they had been ascending for less than half an hour So that meant an average speed of at least twenty thousand kilometres an hour, yet they might never have moved. What was even stranger – for the last ten minutes or more they must actually have been decelerating so rapidly that by rights they should all have been standing on the roof, heads pointing towards Earth!

The doors opened silently, and as Poole stepped out he again felt the slight disorientation he had noticed on entering the elevator lounge. This time, however, he knew what it meant: he was moving through the transition zone where the inertial field overlapped with gravity – at this level, equal to the Moon's.

Indra and Danil followed him, walking carefully now at a third of their customary weight, as they went forward to meet the next of the day's wonders.

Though the view of the receding Earth had been awesome, even for an astronaut, there was nothing unexpected or surprising about it. But who would have imagined a gigantic chamber, apparently occupying the entire width of the Tower, so that the far wall was more than five kilometres away? Perhaps by this time there were larger enclosed volumes on the Moon and Mars, but this must surely be one of the largest in space itself.

They were standing on a viewing platform, fifty metres up on the outer wall, looking across an astonishingly varied panorama. Obviously, an attempt had been made to reproduce a whole range of terrestrial biomes. Immediately beneath them was a group of slender trees which Poole could not at first identify: then he realized that they were oaks, adapted to one-sixth of their normal gravity. What, he wondered, would palm frees look like here? Giant reeds, probably...

In the middle-distance there was a small lake, fed by a river that meandered across a grassy plain, then disappeared into something that looked like a single gigantic banyan tree. What was the source of the water? Poole had become aware of a faint drumming sound, and as he swept his gaze along the gently curving wall, he discovered a miniature Niagara, with a perfect rainbow hovering in the spray above it.

He could have stood here for hours, admiring the view and still not exhausting all the wonders of this complex and brilliantly contrived simulation of the planet below. As it spread out into new and hostile environments, perhaps the human race felt an ever-increasing need to remember its origins. Of course, even in his own time every city had its parks as – usually feeble – reminders of Nature. The same impulse must be acting here, on a much grander scale. Central Park, Africa Tower!

'Let's go down,' said Indra. 'There's so much to see, and I don't come here as often as I'd like.'

Followed by the silent but ever-present Danil, who always seemed to know when he was needed but otherwise kept out of the way, they began a leisurely exploration of this oasis in space. Though walking was almost effortless in this low gravity, from time to time they took advantage of a small monorail, and stopped once for refreshments at a cafe´, cunningly concealed in the trunk of a redwood that must have been at least a quarter of a kilometre tall.

There were very few other people about – their fellow passengers had long since disappeared into the landscape – so it was as if they had all this wonderland to themselves.

Everything was so beautifully maintained, presumably by armies of robots, that from time to time Poole was reminded of a visit he had made to Disney World as a small boy. But this was even better: there were no crowds, and indeed very little reminder of the human race and its artefacts.

They were admiring a superb collection of orchids, some of enormous size, when Poole had one of the biggest shocks of his life. As they walked past a typical small gardener's shed, the door opened – and the gardener emerged.

Frank Poole had always prided himself on his self-control, and never imagined that as a full-grown adult he would give a cry of pure fright. But like every boy of his generation, he had seen all the 'Jurassic' movies – and he knew a raptor when he met one eye to eye.

'I'm terribly sorry,' said Indra, with obvious concern. 'I never thought of warning you.'

Poole's jangling nerves returned to normal. Of course, there could be no danger, in this perhaps too-well-ordered world: but still...!

The dinosaur returned his stare with apparent total disinterest, then doubled back into the shed and emerged again with a rake and a pair of garden shears, which it dropped into a bag hanging over one shoulder. It walked away from them with a bird-like gait, never looking back as it disappeared behind some ten-metre-high sunflowers.

'I should explain,' said Indra contritely. 'We like to use bio-organisms when we can, rather than robots – I suppose it's carbon chauvinism! Now, there are only a few animals that have any manual dexterity, and we've used them all at one time or another.'

'And here's a mystery that no one's been able to solve. You'd think that enhanced herbivores like orangutans and gorillas would be good at this sort of work. Well, they're not; they don't have the patience for it.'

'Yet carnivores like our friend here are excellent, and easily trained. What's more – here's another paradox! -after they've been modified they're docile and good-natured. Of course, there's almost a thousand years of genetic engineering behind them, and look what primitive man did to the wolf, merely by trial and error!'

Indra laughed and continued: 'You may not believe this, Frank, but they also make good baby-sitters – children love them! There's a five-hundred-year-old joke: "Would you trust your kids to a dinosaur?" "What – and risk injuring it?"'