The Science of Discworld II

hear—’
    â€˜ Listen to the voice of people who aren’t there?’ said Niklias. His faceclouded. ‘Listen to the voices of dead men ?’
    There was silence.
    â€˜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.
    â€˜â€¦ finally my master came up with a new idea,’ said Niklias.
    â€˜Another one?’ said the Dean. ‘Was it better than his idea about dropping a horse from a sling to see if it fell over?’
    â€˜Dean!’ snapped Ridcully.
    â€˜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.
    â€˜Eureka!’ he said.
    â€˜I’ve got a towel in my—’ Rincewind began.
    â€˜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?’
    â€˜Yes!’ beamed Niklias.
    â€˜And you raked the sand so that the prints showed up?’
    â€˜Yes!’
    â€˜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—’
    â€˜You’d be doing it wrong,’ said Ponder.
    â€˜Yes!’ said Niklias, delightedly. ‘That’s what we found!’
    â€˜No, of course it’s right,’ said the Dean. ‘Listen: when the hoof is stationary—’
    â€˜It’s moving backwards relative to the horse at the same speed that the horse is moving forward,’ said Ponder. ‘Sorry.’
    â€˜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 …
    â€˜Are you telling us parts of the horse are actually going backwards? ’
    â€˜Perhaps if we pulled the cart in the opposite direction—’
    â€˜The hoof would definitely be stationary, look, because if the ground was moving forward—’
    â€˜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—’
    â€˜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.
    â€˜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 …’
    â€˜How?’ said Ponder.
    â€˜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 bargeforward 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