Galder winked at him and raised his arms again.

'To me, o spirits of—'

There was a thunderclap, an implosion of light and a moment of complete physical uncertainty during which even the walls seemed to turn in on themselves. Trymon heard a sharp intake of breath and then a dull, solid thump.

The room was suddenly silent.

After a few minutes Trymon crawled out from behind a chair and dusted himself off. He whistled a few bars of nothing much and turned towards the door with exaggerated care, looking at the ceiling as if he had never seen it before. He moved in a way that suggested he was attempting the world speed record for the nonchalant walk.

The Luggage squatted in the centre of the circle and opened its lid.

Trymon stopped. He turned very, very carefully, dreading what he might see.

The Luggage seemed to contain some clean laundry, smelling slightly of lavender. Somehow it was quite the most terrifying thing the wizard had ever seen.

'Well, er,' he said. 'You, um, wouldn't have seen another wizard around here, by any chance?'

The Luggage contrived to look more menacing.

'Oh,' said Trymon. 'Well, fine. It doesn't matter.'

He pulled vaguely at the hem of his robe and took a brief interest in the detail of its stitching. When he looked up the horrible box was still there.

'Goodbye,' he said, and ran. He managed to get through the door just in time.

'Rincewind?'

Rincewind opened his eyes. Not that it helped much. It just meant that instead of seeing nothing but blackness he saw nothing but whiteness which, surprisingly, was worse.

'Are you all right?'

'No.'

'Ah.'

Rincewind sat up. He appeared to be on a rock speckled with snow, but it didn't seem to be everything a rock ought to be. For example, it shouldn't be moving.

Snow blew around him. Twoflower was a few feet away, a look of genuine concern on his face.

Rincewind groaned. His bones were very angry at the treatment they had recently received and were queuing up to complain.

'What now?' he said.

You know when we were flying and I was worried we might hit something in the storm and you said the only thing we could possibly hit at this height was a cloud stuffed with rocks?'

'Well?'

'How did you know?'

Rincewind looked around, but for all the variety and interest in the scene around him they might as well have been in the inside of a pingpong ball.

The rock underneath was – well, rocking. He ran his hands over it, and felt the scoring of chisels. When he put an ear to the cold wet stone he fancied he could hear a dull, slow thumping, like a heartbeat. He crawled forward until he came to an edge, and peered very cautiously over it.

At that moment the rock must have been passing over a break in the clouds, because he caught a dim but horribly distant view of jagged-edged mountain peaks.

They were a long way down.

He gurgled incoherently and inched his way backwards.

'This is ridiculous,' he told Twoflower. 'Rocks don't fly. They're noted for not doing it.'

'Maybe they would if they could,' said Twoflower. 'Perhaps this one just found out how.'

'Let's just hope it doesn't forget again,' said Rincewind. He huddled up in his soaking robe and looked glumly at the cloud around him. He supposed there were some people somewhere who had some control over their lives; they got up in the mornings, and went to bed at night in the reasonable certainty of not falling over the edge of the world or being attacked by lunatics or waking up on a rock with ideas above its station. He dimly remembered leading a life like that once.

Rincewind sniffed. This rock smelt of frying. The smell seemed to be coming from up ahead, and appealed straight to his stomach.

'Can you smell anything?' he said.

'I think it's bacon,' said Twoflower.

'I hope it's bacon,' said Rincewind, 'because I'm going to eat it.' He stood up on the trembling stone and tottered forward into the clouds, peering through the wet gloom.

At the front or leading edge of the rock a small druid was sitting crosslegged in front of a small fire. A square of oilskin was tied across his head and knotted under his chin. He was poking at a pan of bacon with an ornamental sickle.

'Um,' said Rincewind. The druid looked up, and dropped the pan into the fire. He leapt to his feet and gripped the sickle aggressively, or at least as aggressively as anyone can look in a long wet white nightshirt and a dripping headscarf.

'I warn you, I shall deal harshly with hijackers,' he said, and sneezed violently.

'We'll help,' said Rincewind, looking longingly at the burning bacon. This seemed to puzzle the druid who, to Rincewind's mild surprise, was quite young; he supposed here had to be such things as young druids, theoretically, it was just that he had never imagined them.

'You're not trying to steal the rock?' said the druid, lowering the sickle a fraction.

'I didn't even know you could steal rocks,' said Rincewind wearily.

'Excuse me,' said Twoflower politely, 'I think your breakfast is on fire.'

The druid glanced down and flailed ineffectually at the flames. Rincewind hurried forward to help, there was a fair amount of smoke, ash and confusion, and the shared triumph of actually rescuing a few pieces of rather charred bacon did more good than a whole book on diplomacy.

'How did you get here, actually?' said the druid. 'We're five hundred feet up, unless I've got the runes wrong again.'

Rincewind tried not to think about height. 'We sort of dropped in as we were passing,' he said.

'On our way to the ground,' Twoflower added.

'Only your rock broke our fall,' said Rincewind. His back complained. Thanks,' he added.

'I thought we'd run into some turbulence a while back,' said the druid, whose name turned out to be Belafon. That must have been you.' He shivered. 'It must be morning by now,' he said. 'Sod the rules, I'm taking us up. Hang on.'

'What to?' said Rincewind.

'Well, just indicate a general unwillingness to fall off,' said Belafon. He took a large iron pendulum out of his robe and swung it in a series of baffling sweeps over the fire.

Clouds whipped around them, there was a horrible feeling of heaviness, and suddenly the rock burst into sunlight.

It levelled off a few feet above the clouds, in a cold but bright blue sky. The clouds that had seemed chillingly distant last night and horribly clammy this morning were now a fleecy white carpet, stretching away in all directions; a few mountain peaks stood out like islands. Behind the rock the wind of its passage sculpted the clouds into transient whirls. The rock—

It was about thirty feet long and ten feet wide, and blueish.

'What an amazing panorama,' said Twoflower, his eyes shining.

'Um, what's keeping us up?' said Rincewind.

'Persuasion,' said Belafon, wringing out the hem of his robe.

'Ah,' said Rincewind sagely.

'Keeping them up is easy,' said the druid, holding up a thumb and squinting down the length of his arm at a distant mountain, The hard part is landing.'

'You wouldn't think so, would you?' said Twoflower.

'Persuasion is what keeps the whole universe together,' said Belafon. 'It's no good saying it's all done by magic.'

Rincewind happened to glance down through the thinning cloud to a snowy landscape a considerable distance below. He knew he was in the presence of a madman, but he was used to that; if listening to this madman meant he stayed up here, he was all ears.

Belafon sat down with his feet dangling over the edge of the rock.

'Look, don't worry,' he said. 'If you keep thinking the rock shouldn't be flying it might hear you and become persuaded and you will turn out to be right, okay? It's obvious you aren't up to date with modern thinking.'

'So it would seem,' said Rincewind weakly. He was trying not to think about rocks on the ground. He was trying to think about rocks swooping like swallows, bounding across landscapes in the sheer joy of levity, zooming skywards in a—

He was horribly aware he wasn't very good at it.

The druids of the Disc prided themselves on their forward-looking approach to the discovery of the mysteries of the Universe. Of course, like druids everywhere they believed in the essential unity of all life, the healing ower of plants, the natural rhythm of the seasons and the burning alive of anyone who didn't approach all this in the right frame of mind, but they had also thought long and hard about the very basis of creation and had formulated the following theory:

The universe, they said, depended for its operation on the balance of four forces which they identified as charm, persuasion, uncertainty and bloody-mindedness.

Thus it was that the sun and moon orbited the disc because they were persuaded not to fall down, but didn't actually fly away because of uncertainty. Charm allowed trees to grow and bloody-mindedness kept them up, and so on.

Some druids suggested that there were certain flaws in this theory, but senior druids explained very pointedly that there was indeed room for informed argument, the cut and thrust of exciting scientific debate, and basically it lay on top of the next solstice bonfire.

'Ah, so you're an astronomer?' said Twoflower.

'Oh no,' said Belafon, as the rock drifted gently around the curve of a mountain, I'm a computer hardware consultant.'

'What's a computer hardware?'

'Well, this is,' said the druid, tapping the rock with a sandalled foot.'Part of one, anyway. It's a replacement. I'm delivering it. They're having trouble with the big circles up on the Vortex Plains. So they say, anyway; I wished I had a bronze tore for every user who didn't read the manual.' He shrugged.

'What use is it, then, exactly?' asked Rincewind. Anything to keep his mind off the drop below.

'You can use it to – to tell you what time of year it is,' said Belafon.

'Ah. You mean if it's covered in snow then it must be winter?'

'Yes. I mean no. I mean, supposing you wanted to know when a particular star is going to rise —'

'Why?' said Twoflower, radiating polite interest.

'Well, maybe you want to know when to plant your crops,' said Belafon, sweating a little, 'or maybe—'

'I'll lend you my almanac, if you like,' said Twoflower.

'Almanac?'

'It's a book that tells you what day it is,' said Rincewind wearily. 'It'd be right up your leyline.'

Belafon stiffened. 'Book?' he said. 'Like, with paper?'

'Yes.'

That doesn't sound very reliable to me,' said the druid nastily. 'How can a book know what day it is? Paper can't count.'

He stamped off to the front of the rock, causing it to wallow alarmingly. Rincewind swallowed hard and beckoned Twoflower closer.

'Have you ever heard of culture shock?' he hissed.

'What's that?'

'It's what happens when people spend five hundred years trying to get a stone circle to work properly and then someone comes up with a little book with a page for every day and little chatty bits saying things like “Now is a good time to plant broad beans” and “Early to rise, early to bed, makes a man healthy, wealthy and dead,” and do you know what the most important thing to remember about culture shock.' Rincewind paused for breath, and moved his lips silently trying to remember where the sentence had got to, 'is?' he concluded.

'What?'

'Don't give it to a man flying a thousand ton rock.'

'Has it gone?'

Trymon peered cautiously over the battlements of the Tower of Art, the great spire of crumbling masonry that loomed over Unseen University. The cluster of students nd instructors of magic, far below, nodded.

'Are you sure?'

The bursar cupped his hands and shouted.