He pushed open a door.

It was a large room, heavily outfitted with the usual badly ventilated furnaces, rows of bubbling crucibles, and one stuffed alligator. Things floated in jars. The air smelled of a limited life expectancy.

A lot of equipment had been moved away, however, to make room for a billiard table. Half a dozen alchemists were standing around it in the manner of men poised to run.

'It's the third this week,' said Sendivoge, gloomily. He nodded to a figure bent over a cue.

'Er, Mr Silverfish—' he began.

'Quiet! Game on!' said the head alchemist, squinting at the white ball.

Sendivoge glanced at the score rail.

'Twenty-one points,' he said. 'My word. Perhaps we're adding just the right amount of camphor to the nitro-cellulose after all—'

There was a click. The cue ball rolled away, bounced off the cushion—

—and then accelerated. White smoke poured off it as it bore down on an innocent cluster of red balls.

Silverfish shook his head.

'Unstable,' he said. 'Everybody down!'

Everyone in the room ducked, except for the two Watchmen, one of whom was in a sense pre-ducked and the other of whom was several minutes behind events.

The black ball took off on a column of flame, whiffled past Detritus' face trailing black smoke and then shattered a window. The green ball was staying in one spot but spinning furiously. The other balls cannoned back and forth, occasionally bursting into flame or caroming off the walls.

A red one hit Detritus between the eyes, curved back on to the table, holed itself in the middle pocket and then blew up.

There was silence, except for the occasional bout of coughing. Silverfish appeared through the oily smoke and, with a shaking hand, moved the score point one notch with the burning end of his cue.

'One,' he said. 'Oh well. Back to the crucible. Someone order another billiard table—'

' 'Scuse me,' said Cuddy, prodding him in the knee.

'Who's there?'

'Down here!'

Silverfish looked down.

'Oh. Are you a dwarf?'

Cuddy gave him a blank stare.

'Are you a giant?' he said.

'Me? Of course not!'

'Ah. Then I must be a dwarf, yes. And that's a troll behind me,' said Cuddy. Detritus pulled himself into something resembling attention.

'We've come to see if you can tell us what's on this paper,' said Cuddy.

'Yur,' said Detritus.

Silverfish looked at it.

'Oh, yes,' he said, 'some of old Leonard's stuff. Well?'

'Leonard?' said Cuddy. He glared at Detritus. 'Write this down,' he snapped.

'Leonard of Quirm,' said the alchemist.

Cuddy still looked lost.

'Never heard of him?' said Silverfish.

'Can't say I have, sir.'

'I thought everyone knew about Leonard da Quirm. Quite barmy. But a genius, too.'

'Was he an alchemist?'

Write this down, write this down . . . Detritus looked around blearily for a burnt bit of wood and a handy wall.

'Leonard? No. He didn't belong to a Guild. Or he belonged to all the Guilds, I suppose. He got around quite a bit. He tinkered, if you know what I mean?'

'No, sir.'

' He painted a bit, and messed about with mechanisms. Any old thing.'

Or a hammer and chisel even, thought Detritus.

'This,' said Silverfish, 'is a formula for . . . oh, well, I might as well tell you, it's hardly a big secret . . . it's a formula for what we called No. 1 Powder. Sulphur, saltpetre and charcoal. You use it in fireworks. Any fool could make it up. But it looks odd because it's written back to front.'

'This sounds important,' hissed Cuddy to the troll.

'Oh, no. He always used to write back to front,' said Silverfish. 'He was odd like that. But very clever all the same. Haven't you seen his portrait of the Mona Ogg?'

'I don't think so.'

Silverfish handed the parchment to Detritus, who squinted at it as if he knew what it meant. Maybe he could write on this, he thought.

'The teeth followed you around the room. Amazing. In fact some people said they followed them out of the room and all the way down the street.'

'I think we should talk to Mr da Quirm,' said Cuddy.

'Oh, you could do that, you could do that, certainly,' said Silverfish. 'But he might not be in a position to listen. He disappeared a couple of years ago.'

. . . then when I find something to write with, thought Detritus, I have to find someone teach me how write . . .

'Disappeared? How?' said Cuddy.

'We think,' said Silverfish, leaning closer, 'that he found a way of making himself invisible.'

'Really?'

'Because,' said Silverfish, nodding conspiratorially, 'no-one's seen him.'

'Ah,' said Cuddy. 'Er. This is just off of the top of my head, you understand, but I suppose he couldn't . . . just have gone somewhere where you couldn't see him?'

'Nah, that wouldn't be like old Leonard. He wouldn't disappear. But he might vanish.'

'Oh.'

'He was a bit . . . unhinged, if you know what I mean. Head too full of brains. Ha, I remember he had this idea once of getting lightning out of lemons! Hey, Sendivoge, you remember Leonard and his lightning lemons?'

Sendivoge made little circular motions alongside his head with one finger. 'Oh, yes. “If you stick copper and zinc rods in the lemon, hey presto, you get tame lightning.” Man was an idiot!'

'Oh, not an idiot,' said Silverfish, picking up a billiard ball that had miraculously escaped the detonations. 'Just so sharp he kept cutting himself, as my granny used to say. Lightning lemons! Where's the sense in that? It was as bad as his “voices-in-the-sky” machine. I told him: Leonard, I said, what are wizards for, eh? There's perfectly normal magic available for that kind of thing. Lightning lemons? If 11 be men with wings next!

And you know what he said? You know what he said? He said: Funny you should say that . . . Poor old chap.'

Even Cuddy joined in the laughter.

'And did you try it?' he said, afterwards.

'Try what?' said Silverfish.

'Har. Har. Har,' said Detritus, toiling behind the others.

'Putting the metal rods in the lemons?'

'Don't be a damn: fool.'

'What dis letter mean?' said Detritus, pointing at the paper.

They looked.

'Oh, that's not a symbol,' said Silverfish. 'That's just old Leonard's way. He was always doodling in margins. Doodle, doodle, doodle. I told him: you should call yourself Mr Doodle.'

'I thought it was some alchemy thing,' said Cuddy. 'It looks a bit like a crossbow without the bow. And this word Ennogeht. What does that mean?'

'Search me. Sounds barbarian to me. Anyway . . . if that's all, officer . . . we've got some serious research to do,' said Silverfish, tossing the fake ivory ball up in the air and catching it again. 'We're not all daydreamers like poor old Leonard.'

'Ennogeht,' said Cuddy, turning the paper round and round. 'T-h-e-g-o-n-n-e—'

Silverfish missed the ball. Cuddy got behind Detritus just in time.

'I've done this before,' said Sergeant Colon, as he and Nobby approached the Fools' Guild. 'Keep up against the wall when I bangs the knocker, all right?'

It was shaped like a pair of artificial breasts, the sort that are highly amusing to rugby players and anyone whose sense of humour has been surgically removed. Colon gave it a quick rap and then flung himself to safety.

There was a whoop, a few honks on a horn, a little tune that someone somewhere must have thought was very jolly, a small hatch slid aside above the knocker and a custard pie emerged slowly, on the end of a wooden arm. Then the arm snapped and the pie collapsed in a little heap by Colon's foot.

'It's sad, isn't it?' said Nobby.

The door opened awkwardly, but only by a few inches, and a small clown stared up at him.

'I say, I say, I say,' it said, 'why did the fat man knock at the door?'

'I don't know,' said Colon automatically. 'Why did the fat man knock at the door?'

They stared at each other, tangled in the punchline.

'That's what I asked you,' said the clown reproachfully. He had a depressed, hopeless voice.

Sergeant Colon struck out towards sanity.

'Sergeant Colon, Night Watch,' he said, 'and this here is Corporal Nobbs. We've come to talk to someone about the man who . . . was found in the river, OK?'

'Oh. Yes. Poor Brother Beano. I suppose you'd better come in, then,' said the clown.

Nobby was about to push at the door when Colon stopped him, and pointed wordlessly upwards.

'There seems to be a bucket of whitewash over the door,' he said.

'Is there?' said the clown. He was very small, with huge boots that made him look like a capital L. His face was plastered with flesh-coloured make-up on which a big frown had been painted. His hair had been made from a couple of old mops, painted red. He wasn't fat, but a sort of hoop in his trousers was supposed to make him look amusingly overweight. A pair of rubber braces, so that his trousers bounced up and down when he walked, were a further component in the overall picture of a complete and utter twerp.

'Yes,' said Colon. 'There is.'

'Sure?'

'Positive.'

'Sorry about that,' said the clown. 'It's stupid, I know, but kind of traditional. Wait a moment.'

There were sounds of a stepladder being lugged into position, and various clankings and swearwords.

'All right, come on in.'

The clown led the way through the gatehouse. There was no sound but the flop-flop of his boots on the cobbles. Then an idea seemed to occur to him.

'It's a long shot, I know, but I suppose neither of you gentlemen'd like a sniff of my buttonhole?'

'No.'

'No.'

'No, I suppose not.' The clown sighed. 'It's not easy, you know. Clowning, I mean. I'm on gate duty 'cos I'm on probation.'

'You are?'

'I keep on forgetting: is it crying on the outside and laughing on the inside? I always get it mixed up.'

'About this Beano—'Colon began.

'We're just holding his funeral,' said the little clown. 'That's why my trousers are at half-mast.'

They stepped out into the sunlight again.

The inner courtyard was lined with clowns and fools. Bells tinkled in the breeze. Sunlight glinted off red noses and the occasional nervous jet of water from a fake buttonhole.

The clown ushered the guards into a line of fools.

'I'm sure Dr Whiteface will talk to you as soon as we've finished,' he said. 'My name's Boffo, by the way.' He held out his hand hopefully.

'Don't shake it,' Colon warned.

Boffo looked crestfallen.

A band struck up, and a procession of Guild members emerged from the chapel. A clown walked a little way ahead, carrying a small urn.

'This is very moving,' said Boffo.

On a dais on the opposite side of the quadrangle was a fat clown in baggy trousers, huge braces, a bow tie that was spinning gently in the breeze, and a top hat. His face had been painted into a picture of misery. He held a bladder on a stick.

The clown with the urn reached the dais, climbed the steps, and waited.

The band fell silent.

The clown in the top hat hit the urn-carrier about the head with the bladder – once, twice, three times . . .

The urn-bearer stepped forward, waggled his wig, took the urn in one hand and the clown's belt in the other and, with great solemnity, poured the ashes of the late Brother Beano into the other clown's trousers.

A sigh went up from the audience. The band struck up the clown anthem 'The March of the Idiots', and the end of the trombone flew off and hit a clown on the back of the head. He turned and swung a punch at the clown behind him, who ducked, causing a third clown to be knocked through the bass drum.

Colon and Nobby looked at one another and shook their heads.