Terry Pratchett - The Science of Discworld II - The Globe

'There was much debate about this and my master, being very rich and also being a keen pupil, decided to prove that the philosopher was correct. Oh, dreadful day! For it was then the troubles began!

The old slave pointed to some derelict woodwork at the far end of the beach.

'That was our test track,' he said. 'The first of four. I helped him build it with my own hands.

There was a lot of interest at that time, and many people came to watch the tests. We had hundreds, hundreds of slaves lying in rows, peering through little slits at just one tiny area of the track each. It didn't work. They argued about what they had seen.'

Niklias sighed. 'Time, said my master, was important. So I told him about work gangs, and how songs helped us keep time. He was very excited about that, and after some thought we built the voice mill which you have heard. Do not be afraid. There is no magic in it. Sound makes things shake, does it not? Sound in the big parchment horn, which I stiffened with shellac, writes the pattern of the sounds it hears on a warm wax cylinder. We used the weighted wheel to spin the cylinder, and it worked quite well after we devised the rocking-trap mechanism. After that, we used it to inscribe the perfect song, and every dawn before we began work we would sing it with the machine. Hundreds of slaves, all singing in perfect time on this beach. The effect was amazing.'

'I bet it was,' said Ridcully.

'But still it did not work, no matter what we devised. A trotting horse travels too fast. My master told me that we must be able to count in tiny parts of time, and after much thinking we built the toc-toc machine. Would you care to see it?'

It was like the voice mill, but had a much bigger wheel. And a pendulum. And a big pointer. As the big wheel turned very slowly, smaller wheels inside the mechanism spun in a blur, and caused a long pointer to revolve against a white-painted wooden wall, along an arc covered in tiny markers. The whole device was mounted on wheels, and had probably taken four men to move.

'I come and grease it occasionally,' said Niklias, patting the wheel. 'For old time's sake.'

The wizards looked at one another with a tame surmise, which is a wild surmise that had been thought about for a while.

'It's a clock,' said the Dean.

'Pardon?' said Niklias.

'We have something like them,' said Ponder. 'We use them for telling the time.'

The slave looked puzzled. 'For telling the time what?' he said.

'He means, so that we know what time it is,' said Ridcully.

'What ... time ... it ... is ... ' muttered the slave, as if trying a square thought in a round mind.

'What hour of the day it is,' said Rincewind, who had run into minds like this before.

'But we can see the sun,' said the slave. 'The toc-toc mechanism does not know where the sun is.'

'Oh, I know ... supposing a baker needed to know how long he should bake his loaves,' said Rincewind. 'Well, with a clock he—'

'How could he be a baker if he did not know how long it takes to bake a loaf?' said Niklias, smiling nervously. 'No, this is a special thing, sirs. It is not for uncursed men.'

'But, but ... you've got a device for recording sound, too!' Ponder burst out. 'You could record the speeches of great thinkers! Why, even after they were dead you could still hear—'

'Listen to the voice of people who aren't there?' said Niklias. His face clouded. 'Listen to the voices of dead men?

There was silence.

'Do tell us more about the fascinating project to find out if a trotting horse is ever entirely airborne,' said Rincewind, loudly and brightly.

The sun drifted down the sky or, rather, the horizon gradually rose. The wizards hated to think about that. You could lose your balance if you thought about it too much.

'... finally my master came up with a new idea,' said Niklias.

'Another one?' said the Dean. 'Was it better than his idea about dropping a horse from, a sling to see if it fell over?'

'Dean!' snapped Ridcully.

'Yes, it was,' said the old slave, who didn't seem to notice the sarcasm. 'We still used the sling, but this time we put it in a very large cart. The bottom of the cart was open, so that the horse's hooves just touched the ground. Are you following me? And then -and this is the clever part, I felt - my master arranged that the cart was pulled by four trotting horses'

He sat back, giving them a pleased look, as if expecting praise.

The Dean's expression slowly changed.

'Eureka!' he said.

'I've got a towel in my—' Rincewind began.

'No, don't you see? If the cart is being pulled forward then whatever the horse does, the ground is disappearing backwards. So if you've got a trained horse and you can get it to trot while it's in the harness ... you designed the cart so that the pulling horses were offset, so that the supported horse was trotting over unmarked sand?'

'Yes!' beamed Niklias.

'And you raked the sand so that the prints showed up?'

'Yes!'

Then whenever the horse touched the ground and the hoof was stationary relative to the ground, the ground would in fact be moving, and you'd get a smeared print, and if you carefully measured the total length of the ground covered during the trot, and added up the total of all the smears, and found that they were less than the total length of the track, then—'

'You'd be doing it wrong,' said Ponder.

'Yes!' said Niklias, delightedly. 'That's what we found!'

'No, of course it's right,' said the Dean. 'Listen: when the hoof is stationary—'

'It's moving backwards relative to the horse at the same speed that the horse is moving forward,'

said Ponder. 'Sorry.'

'No, listen,' the Dean protested. 'It must work, because when the ground isn 't moving—'

Rincewind groaned. Any minute now all the wizards would express an opinion, and none of them would listen to anyone else. And here it came ...

'Are you telling us parts of the horse are actually going backwards?'

'Perhaps if we pulled the cart in the opposite direction—'

'The hoof would definitely be stationary, look, because if the ground was moving forward—'

'It's no different than it would be if the horse was trotting all by itself! Look, supposing the cart and all the other horses were invisible—'

'You're all wrong, you're all wrong! If the horse was ... no, wait a moment—'

Rincewind nodded to himself. The wizards were entering the special fugue state known as Hubbub, where no-one was going to be allowed to finish a sentence because someone else would drown them out. It was how the wizards decided things. In all likelihood, in this case it would result in them deciding that the horse should, logically, end up at one end of the beach, while all its feet were up at the other end.

'My master Phocian said we should try it, and the hooves just left hoofprints,' said Niklias the Cretan, when the argument had died away through lack of breath. 'Then we tried moving the beach under the horse ...'

'How?' said Ponder.

'We built a long flat barge, filled it full of sand and tried it in the lagoon,' said the slave. 'We suspended the horse from a gantry. Phocian felt we were getting somewhere when we moved the barge forward at twice the speed of the horse, but the beast kept trying to keep up ... and then there was the night of the big storm and the barge was sunk. Oh, those were a few busy months.

We lost four horses and Nosios the Carpenter was kicked in the head.' The smile faded. And then

... and then ...'

'Yes?'

'... something terrible happened.'

The wizards leaned forward.

'... Phocian designed the fourth test. It's over there. Not much to see now, of course. People stole all the heavy cloth of the Endless Road and a lot of the woodwork, too.' The slave sighed. 'It was Hades to build and took many months to get right but, in short, it worked like this. We used a huge roll of heavy white cloth, which we rolled off one huge spindle and on to the other. Believe me, sirs, even that took some doing, and the work of forty slaves. At the place where the horse was to be suspended, we stretched the cloth tight over a shallow trough of powdered charcoal, so that a little weight on the cloth would press it down on to the stuff…'