The Patrician moved a stack of drawings and sat down.

'These are good,' he said. 'What are they?'

'My cartoons,' said Leonard.

'This is a good one of the little boy with his kite stuck in a tree,' said Lord Vetinari.

'Thank you. May I make you some tea? I'm afraid I don't see many people these days, apart from the man who oils the hinges.'

'I've come to . . .'

The Patrician stopped and prodded at one of the drawings.

'There's a piece of yellow paper stuck to this one,' he said, suspiciously. He pulled at it. It came away from the drawing with a faint sucking noise, and then stuck to his fingers. On the note, in Leonard's crabby backward script, were the words: 'krow ot smees sihT: omeM'.

'Oh, I'm rather pleased with that,' said Leonard. 'I call it my “Handy-note-scribbling-piece-of-paper-with-glue-that-comes-unstuck-when-you-want”.'

The Patrician played with it for a while.

'What's the glue made of?'

'Boiled slugs.'

The Patrician pulled the paper off one hand. It stuck to the other hand.

'Is that what you came to see me about?' said Leonard.

'No. I came to talk to you,' said Lord Vetinari, 'about the gonne.'

'Oh, dear. I'm very sorry.'

'I am afraid it has . . . escaped.'

'My goodness. I thought you said you'd done away with it.'

'I gave it to the Assassins to destroy. After all, they pride themselves on the artistic quality of their work. They should be horrified at the idea of anyone having that sort of power. But the damn fools did not destroy it. They thought they could lock it away. And now they've lost it.'

'They didn't destroy it?'

'Apparently not, the fools.'

And nor did you. I wonder why?'

'1 . . . do you know, I don't know?'

'I should never have made it. It was merely an application of principles. Ballistics, you know. Simple aerodynamics. Chemical power. Some rather good alloying, although I say it myself. And I'm rather proud of the rifling idea. I had to make a quite complicated tool for that, you know. Milk? Sugar?'

'No, thank you.'

'People are searching for it, I trust?'

'The Assassins are. But they won't find it. They don't think the right way.' The Patrician picked up a pile of sketches of the human skeleton. They were extremely good.

'Oh, dear.'

'So I am relying on the Watch.'

'This would be the Captain Vimes you have spoken of.'

Lord Vetinari always enjoyed his occasional conversations with Leonard. The man always referred to the city as if it was another world.

'Yes.'

'I hope you have impressed upon him the importance of the task.'

'In a way. I've absolutely forbidden him to undertake it. Twice.'

Leonard nodded. 'Ah. I . . . think I understand. I hope it works.'

He sighed.

'I suppose I should have dismantled it, but . . . it was so clearly a made thing. I had this strange fancy I was merely assembling something that already existed. Sometimes I wonder where I got the whole idea. It seemed . . . I don't know . . . sacrilege, I suppose, to dismantle it. It'd be like dismantling a person. Biscuit?'

'Dismantling a person is sometimes necessary,' said Lord Vetinari.

'This, of course, is a point of view,' said Leonard da Quirm politely.

'You mentioned sacrilege,' said Lord Vetinari. 'Normally that involves gods of some sort, does it not?'

'Did I use the word? I can't imagine there is a god of gonnes.'

'It is quite hard, yes.'

The Patrician shifted uneasily, reached down behind him, and pulled out an object.

'What,' he said, 'is this?'

'Oh, I wondered where that had gone,' said Leonard. 'It's a model of my spinning-up-into-the-air machine.'[20]

Lord Vetinari prodded the little rotor.

'Would it work?'

'Oh, yes,' said Leonard. He sighed. 'If you can find one man with the strength of ten men who can turn the handle at about one thousand revolutions a minute.'

The Patrician relaxed, in a way which only then drew gentle attention to the foregoing moment of tension.

'Now there is in this city,' he said, 'a man with a gonne. He has used it successfully once, and almost succeeded a second time. Could anyone have invented the gonne?'

'No,' said Leonard. 'I am a genius.' He said it quite simply. It was a statement of fact.

'Understood. But once a gonne has been invented, Leonard, how much of a genius need someone be to make the second one?'

'The rifling technique requires considerable finesse, and the cocking mechanism that slides the bullette assembly is finely balanced, and of course the end of the barrel must be very . . .' Leonard saw the Patrician's expression, and shrugged. 'He must be a clever man,' he said.

'This city is full of clever men,' said the Patrician. 'And dwarfs. Clever men and dwarfs who tinker with things.'

'I am so very sorry.'

'They never think.'

'Indeed.'

Lord Vetinari leaned back and stared at the skylight.

'They do things like open the Three Jolly Luck Take-Away Fish Bar on the site of the old temple in Dagon Street on the night of the Winter solstice when it also happens to be a full moon.'

'That's people for you, I'm afraid.'

'I never did find out what happened to Mr Hong.'

'Poor fellow.'

'And then there's the wizards. Tinker, tinker, tinker. Never think twice before grabbing a thread of the fabric of reality and giving it a pull.'

'Shocking.'

'The alchemists? Their idea of civic duty is mixing up things to see what happens.'

'I hear the bangs, even here.'

'And then, of course, along comes someone like you—'

'I really am terribly sorry.'

Lord Vetinari turned the model flying machine over and over in his fingers.

'You dream of flying,' he said.

'Oh, yes. Then men would be truly free. From the air, there are no boundaries. There could be no more war, because the sky is endless. How happy we would be, if we could but fly.'

Vetinari turned the machine over and over in his hands.

'Yes,' he said, 'I daresay we would.'

'I had tried clockwork, you know.'

'I'm sorry? I was thinking about something else.'

'I meant clockwork to power my flying machine. But it won't work.'

'Oh.'

'There's a limit to the power of a spring, no matter how tightly one winds it.'

'Oh, yes. Yes. And you hope that if you wind a spring one way, all its energies will unwind the other way. And sometimes you have to wind the spring as tight as it will go,' said Vetinari, 'and pray it doesn't break.'

His expression changed.

'Oh dear,' he said.

'Pardon?' said Leonard.

'He didn't thump the wall. I may have gone too far.'

Detritus sat and steamed. Now he felt hungry – not for food, but for things to think about. As the temperature sank, the efficiency of his brain increased even more. It needed something to do.

He calculated the number of bricks in the wall, first in twos and then in tens and finally in sixteens. The numbers formed up and marched past his brain in terrified obedience. Division and multiplication were discovered. Algebra was invented and provided an interesting diversion for a minute or two. And then he felt the fog of numbers drift away, and looked up and saw the sparkling, distant mountains of calculus.

Trolls evolved in high, rocky and above all in cold places. Their silicon brains were used to operating at low temperatures. But down on the muggy plains the heat build-up slowed them down and made them dull. It wasn't that only stupid trolls came down to the city. Trolls who decided to come down to the city were often quite smart – but they became stupid.

Detritus was considered moronic even by city troll standards. But that was simply because his brain was naturally optimized for a temperature seldom reached in Ankh-Morpork even during the coldest winter . . .

Now his brain was nearing its ideal temperature of operation. Unfortunately, this was pretty close to a troll's optimum point of death.

Part of his brain gave some thought to this. There was a high probability of rescue. That meant he'd have to leave. That meant he'd become stupid again, as sure as

10-3(Me/Mp)a6aG – N = 10N.

Better make the most of it, then.

He went back to the world of numbers so complex that they had no meaning, only a transitional point of view. And got on with freezing to death, as well.

Dibbler reached the Butchers' Guild very shortly after Cuddy. The big red doors had been kicked open and a small butcher was sitting just inside them rubbing his nose.

'Which way did he go?'

'Dat way.'

And in the Guild's main hall the master butcher Gerhardt Sock was staggering around in circles. This was because Cuddy's boots were planted on his chest. The dwarf was hanging on to the man's vest like a yachtsman tacking into a gale, and whirling his axe round and round in front of Sock's face.

'You give it to me right now or I'll make you eat your own nose!'

A crowd of apprentice butchers was trying to keep out of the way.

'But—'

'Don't you argue with me! I'm an officer of the Watch, I am!'

'But you—'

'You've got one last chance, mister. Give it to me right now!'

Sock shut his eyes.

'What is it you want?'

The crowd waited.

'Ah,' said Cuddy. 'Ahaha. Didn't I say?'

'No!'

'I'm pretty sure I did, you know.'

'You didn't!'

'Oh. Well. It's the key to the pork futures warehouse, if you must know.' Cuddy jumped down.

'Why?'

The axe hovered in front of his nose again.

'I was just asking,' said Sock, in a desperate and distant voice.

'There's a man of the Watch in there freezing to death,' said Cuddy.

There was quite a crowd around them when they finally got the main door open. Lumps of ice clinked on the stones, and there was a rush of supercold air.

Frost covered the floor and the rows of hanging carcasses on their backwards journey through time. It also covered a Detritus-shaped lump squatting in the middle of the floor.

They carried it out into the sunlight.

'Should his eyes be flashing on and off like that?' said Dibbler.

'Can you hear me?' shouted Cuddy. 'Detritus?'

Detritus blinked. Ice slid off him in the day's heat.

He could feel the cracking up of the marvellous universe of numbers. The rising temperature hit his thoughts like a flamethrower caressing a snowflake.

'Say something!' said Cuddy.

Towers of intellect collapsed as the fire roared through Detritus' brain.

'Hey, look at this,' said one of the apprentices.

The inner walls of the warehouse were covered with numbers. Equations as complex as a neural network had been scraped in the frost. At some point in the calculation the mathematician had changed from using numbers to using letters, and then letters themselves hadn't been sufficient; brackets like cages enclosed expressions which were to normal mathematics what a city is to a map.

They got simpler as the goal neared – simpler, yet containing in the flowing lines of their simplicity a spartan and wonderful complexity.

Cuddy stared at them. He knew he'd never be able to understand them in a hundred years.

The frost crumbled in the warmer air.

The equations narrowed as they were carried on down the wall and across the floor to where the troll had been sitting, until they became just a few expressions that appeared to move and sparkle with a life of their own. This was maths without numbers, pure as lightning.

They narrowed to a point, and at the point was just the very simple symbol: '='.

'Equals what?' said Cuddy. 'Equals what?'

The frost collapsed.

Cuddy went outside. Detritus was now sitting in a puddle of water, surrounded by a crowd of human onlookers.