At which point, another sound became apparent. It was a rhythmical cracking, a khss ... khss ...
khss behind which it was just possible to hear voices raised in song; the singers sounded as if they were far away and at the bottom of a tin bath.
Ridcully stopped outside the largest shack, from which the sound appeared to be issuing.
'Rincewind?' he said, beckoning. 'One for you, I think.'
'Yes, yes, all right,' said Rincewind, and entered with extreme caution.
It was dark inside, but he could see workbenches and a few tools, with a forgotten look about them. The shack must have been thrown up quickly. There wasn't even a floor; it had been built directly on the sand.
The singing was coming from a large horn attached to a device on a bench. Rincewind wasn't very good at technical things, but there was a large wheel projecting over the edge of the bench and it was turning slowly, probably because of the small weight, attached to it by string, which was gently descending towards the sand.
'Is everything okay?' said Ridcully, from outside.
'I've found a kind of voice mill,' said Rincewind.
'That's amazing,' said a voice from the shadows. 'That's exactly what my master called it.'
His name, he said, was Niklias the Cretan, and he was very old. And very pleased to see the wizards.
'I come up here sometimes,' he said. 'I listen to the voice mill and remember the old days. No one else comes here. They say it's the abode of madness. And they are right.'
The wizards were sitting around a fire of driftwood, that burned blue with the salt. They were tending to huddle, although they'd never admit it. They wouldn't have been wizards if they couldn't sense the strangeness in the place. It had the same depressing effect on the senses as an old battlefield. It had ghosts.
'Tell us,' said Ridcully.
'My master was Phocian the Touched,' said Niklias, and he said it the way of a man telling a story he'd told many times before. 'He was a pupil of the great philosopher Antigonus, who one day declared that a trotting horse must at all times have at least one foot on the ground, lest it fall over.
'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.'