‘Er… what did you use to do?’ said Moist.

The postmen looked at one another.

‘Get one penny off ‘em for the stamping, bring it back here to be stamped with the official stamp,’ said Groat promptly. ‘Then it gets sorted and delivered.’

‘So… people have to wait until they see a postman? That seems rather—’

‘Oh, in the old days there was dozens of smaller offices, see?’ Groat added. ‘But when it all started going bad we lost ‘em.’

‘Well, let’s get the mail moving again and we can work things out as we go along,’ said Moist. ‘I’m sure ideas will occur. And now, Mr Groat, you have a secret to share… ’

Groat’s key ring jingled as he led Moist through the Post Office’s cellars and eventually to a metal door. Moist noted a length of black and yellow rope on the floor: the Watch had been here, too.

The door clicked open. There was a blue glow inside, just faint enough to be annoying, leave purple shadows on the edge of vision and make the eyes water.

‘Voil-ah,’ said Groat.

‘It’s a… is it some kind of theatre organ?’ said Moist. It was hard to see the outlines of the machine in the middle of the floor, but it stood there with all the charm of a torturer’s rack. The blue glow was coming from somewhere in the middle of it. Moist’s eyes were streaming already.

‘Good try, sir! Actually it is the Sorting Engine,’ said Groat. ‘It’s the curse of the Post Office, sir. It had imps in it for the actual reading of the envelopes, but they all evaporated years ago. Just as well, too.’

Moist’s gaze took in the wire racks that occupied a whole wall of the big room. It also found the chalk outlines on the floor. The chalk glowed in the strange light. The outlines were quite small. One of them had five fingers.

‘Industrial accident,’ he muttered. ‘All right, Mr Groat. Tell me.’

‘Don’t go near the glow, sir,’ said Groat. ‘That’s what I said to Mr Whobblebury. But he snuck down here all by hisself, later on. Oh, dear, sir, it was poor young Stanley that went and found him, sir, after he saw poor little Tiddles dragging something along the passage. A scene of carnage met his eyes. You just can’t imagine what it was like in here, sir.’

‘I think I can,’ said Moist.

‘I doubt if you can, sir.’

‘I can, really.’

‘I’m sure you can’t, sir.’

‘I can! All right?’ shouted Moist. ‘Do you think I can’t see all those little chalk outlines? Now can we get on with it before I throw up?’

‘Er… right you are, sir,’ said Groat. ‘Ever heard of Bloody Stupid Johnson? Quite famous in this city.’

‘Didn’t he build things? Wasn’t there always something wrong with them? I’m sure I read something about him—’

‘That’s the man, sir. He built all kinds of things, but, sad to say, there was always some major flaw.’

In Moist’s brain, a memory kicked a neuron. ‘Wasn’t he the man who specified quicksand as a building material because he wanted a house finished fast?’ he said.

‘That’s right, sir. Usually the major flaw was that the designer was Bloody Stupid Johnson. Flaw, you might say, was part of the whole thing. Actually, to be fair, a lot of the things he designed worked quite well, it was just that they didn’t do the job they were supposed to. This thing, sir, did indeed begin life as an organ, but it ended up as a machine for sorting letters. The idea was that you tipped the mail sack in that hopper, and the letters were speedily sorted into those racks. Postmaster Cowerby meant well, they say. He was a stickler for speed and efficiency, that man. My grandad told me the Post Office spent a fortune on getting it to work.’

‘And lost their money, eh?’ said Moist.

‘Oh no, sir. It worked. Oh yes, it worked very well. So well that people went mad, come the finish.’

‘Let me guess,’ said Moist. ‘The postmen had to work too hard?’

‘Oh, postmen always work too hard, sir,’ said Groat, without blinking. ‘No, what got people worried was finding letters in the sorting tray a year before they were due to be written.’

There was a silence. In that silence, Moist tried out a variety of responses, from ‘Pull the other one, it’s got bells on’ to ‘That’s impossible’, and decided they all sounded stupid. Groat looked deadly serious. So instead he said: ‘How?’

The old postman pointed to the blue glow. ‘Have a squint inside, sir. You can just see it. Don’t get right above it, whatever you do.’

Moist moved a little closer to the machine and peered into the machinery. He could just make out, at the heart of the glow, a little wheel. It was turning, slowly.

‘I was raised in the Post Office,’ said Groat, behind him. ‘Born in the sorting room, weighed on the official scales. Learned to read from envelopes, learned figuring from old ledgers, learned jography from looking at the maps of the city and history from the old men. Better than any school. Better than any school, sir. But never learned jommetry, sir. Bit of a hole in my understanding, all that stuff about angles and suchlike. But this, sir, is all about pie.’

‘Like in food?’ said Moist, drawing back from the sinister glow.

‘No, no, sir. Pie like in jommetry.’

‘Oh, you mean pi the number you get when… ’ Moist paused. He was erratically good at maths, which is to say he could calculate odds and currency very, very fast. There had been a geometry section in his book at school, but he’d never seen the point. He tried, anyway.

‘It’s all to do with… it’s the number you get when the radius of a circle… no, the length of the rim of a wheel is three and a bit times the… er… ’

‘Something like that, sir, probably, something like that,’ said Groat. ‘Three and a bit, that’s the ticket. Only Bloody Stupid Johnson said that was untidy, so he designed a wheel where the pie was exactly three. And that’s it, in there.’

‘But that’s impossible!’ said Moist. ‘You can’t do that! Pi is like… built in! You can’t change it. You’d have to change the universe!’

‘Yes, sir. They tell me that’s what happened,’ said Groat calmly. ‘I’ll do the party trick now. Stand back, sir.’

Groat wandered out into the other cellars and came back with a length of wood.

‘Stand further back, sir,’ he suggested, and tossed the piece of wood on top of the machine.

The noise wasn’t loud. It was a sort of ‘clop’. It seemed to Moist that something happened to the wood when it went over the light. There was a suggestion of curvature—

Several pieces of timber clattered on to the floor, along with a shower of splinters.

‘They had a wizard in to look at it,’ said Groat. ‘He said the machine twists just a little bit of the universe so pie could be three, sir, but it plays hob with anything you put too near it. The bits that go missing get lost in the… space-time-continuememememem, sir. But it doesn’t happen to the letters because of the way they travel through the machine, you see. That’s the long and short of it, sir. Some letters came out of that machine fifty years before they were posted!’

‘Why didn’t you switch it off?’

‘Couldn’t, sir. It kept on going like a siphon. Anyway, the wizard said if we did that terrible things might happen! ‘cos of, er, quantum, I think.’

‘Well, then, you could just stop feeding it mail, couldn’t you?’

‘Ah, well, sir, there it is,’ said Groat, scratching his beard. ‘You have positioned your digit right on the nub or crux, sir. We should’ve done that, sir, we should’ve, but we tried to make it work for us, you see. Oh, the management had schemes, sir. How about delivering a letter in Dolly Sisters thirty seconds after it had been posted in the city centre, eh? Of course, it wouldn’t be polite to deliver mail before we’d actually got it, sir, but it could be a close run thing, eh? We were good, so we tried to be better… ’

And, somehow, it was all familiar…

Moist listened glumly. Time travel was only a kind of magic, after all. That’s why it always went wrong.