Dr Wallace laid the palm of her hand upon the plate, then removed it after a few seconds. She glanced at Poole, and said smilingly: 'Come and look at this.'

The inscription that had suddenly appeared made a good deal of sense, when he read it slowly:WALLACE, INDRA [F2970.03.11 :31.885 / /HIST.OXFORD] 'I suppose it means Female, date of birth 11 March 2970 – and that you're associated with the Department of History at Oxford. And I guess that 31.885 is a personal identification number. Correct?'

'Excellent, Mr Poole. I've seen some of your e-mail addresses and credit card numbers – hideous strings of alpha-numeric gibberish that no one could possibly remember! But we all know our date of birth, and not more than 99,999 other people will share it. So a five-figure number is all you'll ever need... and even if you forget that, it doesn't really matter. As you see, it's a part of you.'

'Implant?'

'Yes – nanochip at birth, one in each palm for redundancy. You won't even feel yours when it goes in. But you've given us a small problem...'

'What's that?'

'The readers you'll meet most of the time are too simple-minded to believe your date of birth. So, with your permission, we've moved it up a thousand years.'

'Permission granted. And the rest of the Ident?'

'Optional. You can leave it empty, give your current interests and location – or use it for personal messages, global or targeted.'

Some things, Poole was quite sure, would not have changed over the centuries. A high proportion of those 'targeted' messages would be very personal indeed.

He wondered if there were still self or state-appointed censors in this day and age – and if their efforts at improving other people's morals had been more successful than in his own time.

He would have to ask Dr Wallace about that, when he got to know her better.

'Frank – Professor Anderson thinks you're strong enough to go for a little walk.'

'I'm very pleased to hear it. Do you know the expression "stir crazy"?'

'No – but I can guess what it means.'

Poole had so adapted to the low gravity that the long strides he was taking seemed perfectly normal. Half a gee, he had estimated – just right to give a sense of well-being. They met only a few people on their walk, all of them strangers, but every one gave a smile of recognition. By now, Poole told himself with a trace of smugness, I must be one of the best-known celebrities in this world. That should be a great help – when I decide what to do with the rest of my life. At least another century, if I can believe Anderson.

The corridor along which they were walking was completely featureless apart from occasional numbered doors, each bearing one of the universal recog panels. Poole had followed Indra for perhaps two hundred metres when he came to a sudden halt, shocked because he had not realized something so blindingly obvious.

'This space-station must be enormous!' he exclaimed. Indra smiled back at him.

'Didn't you have a saying – "You ain't seen anything yet"?'

'"Nothing",' he corrected, absent-mindedly. He was still trying to estimate the scale of this structure when he had another surprise. Who would have imagined a space-station large enough to boast a subway – admittedly a miniature one, with a single small coach capable of seating only a dozen passengers.

'Observation Lounge Three,' ordered Indra, and they drew silently and swiftly away from the terminal.

Poole checked the time on the elaborate wrist-band whose functions he was still exploring. One minor surprise had been that the whole world was now on Universal Time: the confusing patchwork of Time Zones had been swept away by the advent of global communications There had been much talk of this, back in the twenty-first century, and it had even been suggested that Solar should be replaced by Sidereal Time. Then, during the course of the year, the Sun would move right round the clock: setting at the time it had risen six months earlier.

However, nothing had come of this 'Equal time in the Sun' proposal – or of even more vociferous attempts to reform the calendar. That particular job, it had been cynically suggested, would have to wait for somewhat major advances in technology. Some day, surely, one of God's minor mistakes would be corrected, and the Earth's orbit would be adjusted, to give every year twelve months of thirty exactly equal days.

As far as Poole could judge by speed and elapsed time, they must have travelled at least three kilometres before the vehicle came to a silent stop, the doors opened, and a bland autovoice intoned, 'Have a good view. Thirty-five per cent cloud-cover today.'

At last, thought Poole, we're getting near the outer wall. But here was another mystery – despite the distance he had gone, neither the strength nor the direction of gravity had altered! He could not imagine a spinning space-station so huge that the gee-vector would not be changed by such a displacement... could he really be on some planet after all? But he would feel lighter – usually much lighter – on any other habitable world in the Solar System.

When the outer door of the terminal opened, and Poole found himself entering a small airlock, he realized that he must indeed be in space. But where were the spacesuits? He looked around anxiously: it was against all his instincts to be so close to vacuum, naked and unprotected. One experience of that was enough...

'We're nearly there,' said Indra reassuringly.

The last door opened, and he was looking out into the utter blackness of space, through a huge window that was curved both vertically and horizontally. He felt like a goldfish in its bowl, and hoped that the designers of this audacious piece of engineering knew exactly what they were doing. They certainly possessed better structural materials than had existed in his time.

Though the stars must be shining out there, his light-adapted eyes could see nothing but black emptiness beyond the curve of the great window. As he started to walk towards it to get a wider view, Indra restrained him and pointed straight ahead.

'Look carefully,' she said 'Don't you see it-'

Poole blinked, and stared into the night. Surely it must be an illusion – even, heaven forbid, a crack in the window...

He moved his head from side to side. No, it was real. But what could it be? He remembered Euclid's definition 'A lie has length, but no thickness'.

For spanning the whole height of the window, and obviously continuing out of sight above and below, was a thread of light quite easy to see when he looked for it, yet so one-dimensional that the word 'thin' could not even be applied. However, it was not completely featureless; there were barely visible spots of greater brilliance at irregular intervals along its length, like drops of water on a spider's web.

Poole continued walking towards the window, and the view expanded until at last he could see what lay below him. It was familiar enough: the whole continent of Europe, and much of northern Africa, just as he had seen them many times from space. So he was in orbit after all – probably an equatorial one, at a height of at least a thousand kilometres.

Indra was looking at him with a quizzical smile.

'Go closer to the window,' she said, very softly. 'So that you can look straight down. I hope you have a good head for heights.'

What a silly thing to say to an astronaut! Poole told himself as he moved forward. If I ever suffered from vertigo, I wouldn't be in this business...

The thought had barely passed through his mind when he cried 'My God!' and involuntarily stepped back from the window, Then, bracing himself, he dared to look again.

He was looking down on the distant Mediterranean from the face of a cylindrical tower, whose gently curving wall indicated a diameter of several kilometres. But that was nothing compared with its length, for it tapered away down, down, down – until it disappeared into the mist somewhere over Africa. He assumed that it continued all the way to the surface.