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!