They followed him towards the shop containing the big furnaces. 'See how you obtain such hot fires now. Your waterworks drive the bellows faster than any number of puffers ever could, and the heat of the fire is accordingly higher. Now, Aristotle says fire is trapped in wood, and released by heat. Fair enough, but why does more air inake the fire burn hotter? Why does wind drive a wildfire so? Does it mean air is essential to fire? Could we find out? If we built a chamber in which the air was pulled out by the bellows rather than pushed in, would the fire burn less?'

'Suck air out of a chamber?' Khalid said.

'Yes. Arrange a valve that lets air out but won't let it back in. Pump out what's there, and then hold any replacement air out.'

'Interesting! But what would remain in the chamber then?'

Iwang shrugged. 'I don't know. A void? A piece of the original void, perhaps? Ask the lamas about that, or your sufis. Or Aristotle. Or just make a glass chamber, and look in it.'

' I will,' Khalid said.

'And motion is easiest of all to study,' Iwang said. 'We can try all manner of things with motion. We can time this attraction of things to the Earth. We can see if the speed is the same up in the hills and down in the valleys. Things speed up as they fall, and this might be measurable too. Light itself might be measurable. Certainly the angles of refraction are constant, I've measured those already.'

Khalid was nodding. 'First this reverse bellows, to empty a chamber. Although surely it cannot be a true void that results. Nothingness is not possible in this world, I think. There will be something in there, thinner than air.'

'That is more Aristotle,' Iwang said. Natureabhors the void." But what if it doesn't? We will only know when we try.'

Khalid nodded. If he had had two hands he would have been rubbing them together.

The three of them walked out to the waterworks. Here a canal brought a hard flow from the river, its gliding surface gleaming in the morning light. The water powered a mill, which geared out to axles turning a bank of heavy metal working hammers and stamps, and finally the rotating bellows handles that powered the blast furnaces. It was a noisy place, filled with sounds of falling water, smashed rock, roaring fire, singed air; all the elements raging with transmutation, hurting their cars and leaving a burnt smell in the air. Khalid stood watching the waterworks for a while. This was his achievement, he was the one who had organized all the artisans' skills into this enormous articulated machine, so much more powerful than people or horses had ever been. They were the most powerful people in all the history of the world, Bahram thought, because of Khalid's enterprise; but with a wave Khalid dismissed it all. He wanted to understand why it worked.

He led the other two back to the shop. 'We'll need your glassblowing, and my leather and iron workers,' he said. 'The valve you mention could perhaps be made of sheep intestines.'

'It might have to be stronger than that,' said Iwang. 'A metal gate of some sort, pressed into a leather gasket by the suck of the void.'

'Yes.'

No Jinn in This Bottle

Khalid set his artisans to the task, and Iwang did the glassblowing, and after a few weeks they had a two part mechanism: a thick glass globe to be emptied, and a powerful pump to empty it. There were any number of collapses, leaks and valve failures, but the old mechanists of the compound were ingenious, and attacking the points of failure, they ended up with five very similar versions of the device, all very heavy. The pump was massive and lathed to newly precise fits of plunger, tube and valves; the glass globes were thick flasks, with necks even thicker, and knobs on the inside surfaces from which objects could be hung, to see what would happen to them when the air in the globe was evacu ated. When they solved the leakage problems, they had to build a rackand pinion device to exert enough force on the pump to evacuate the final traces of air from the globe. Iwang advised them not to create such a perfect void that they ended up sucking in the pump, the compound, or perchance the whole world, like jinni returning to their confinement; and as always, Iwang's stone face did not give them any clue as to whether he was joking or serious.

When they had the mechanisms working fairly reliably (occasionally one would still crack its glass, or break a valve), they set one on a wooden frame, and Khalid began a sequence of trials, inserting things in the glass globes, pumping out the air, and seeing what resulted. All philosophical questions on the nature of what remained inside the globe after the air was removed, he now refused to address. 'Let's just see what happens,' he said. 'It is what it is.' He kept big blank paged books on the table beside the apparatus, and he or his clerks recorded every detail of the trials, timing them on his best clock.

After a few weeks of learning the apparatus and trying things, he asked Iwang and Bahram to arrange a small party, inviting several of the qadi and teachers from the madressas in the Registan, particularly the mathematicians and astronomers of Sher Dor Madressa, who were already involved in discussions of ancient Greek and classical caliphate notions of physical reality. On the appointed day, when all those invited had gathered in the open walled workshop next to Khalid's study, Khalid introduced the apparatus to them, describing how it worked and indicating what they could all see, that he had hung an alarm clock from a knob inside the glass globe, so that it swung freely at the end of a short length of silk thread. Khalid cranked the piston of the rack and pinion down twenty times, working hard with his left arm. He explained that the alarm clock was set to go off at the sixth hour of the afternoon, shortly after the evening prayers would be sung from Samarqand's northernmost minaret.

'To be sure the alarm is truly sounding,' Khalid said, 'we have exposed the clapper, so that you can see it hitting the bells. I will also introduce air back into the globe little by little, after we have seen the first results, so you can hear for yourself the effect.'

He was gruff and direct. Bahram saw that he wanted to distance himself from the portentous, magical style he had affected during his alchemical transmutations. He made no claims, spoke no incantations. The memory of his last disastrous demonstration – his fraud must have been in his mind, as it was in everyone else's. But he merely gestured with his hand at the clock, which advanced steadily towards six.

Then the clock began to spin on its thread, and the clapper was visibly smashing back and forth between the little brass bells. But there was no sound coming from the glass. Khalid gestured: 'You might think that the glass itself is stopping the sound, but when the air is let back into the flask, you will see that it isn't so. First I invite you to put your car to the glass, so you can confirm that there is no sound at all.'

They did so one by one. Then Khalid unscrewed a stopcock that released a valve set in the side of the flask, and a brief penetrating hiss was joined by the muted banging of the alarm, which grew louder quickly, until it sounded much like an alarm heard from an adjoining room.

'It seems there is no sound without air to convey it,' Khalid commented.

The visitors from the madressa were eager to inspect the apparatus, and to discuss its uses in trials of various sorts, and to speculate about what, if anything, remained in the globe when the air was pumped out. Khalid was adamant in his refusal to discuss this question, preferring to talk about what the demonstration seemed to be indicating about the nature of sound and its transmission.

'Echoes might elucidate this matter in another way,' one of the qadis said. He and all the other visiting witnesses were bright eyed, pleased, intrigued. 'Something strikes air, pushes it, and the sound is a shock moving through the air, like waves across water. They bounce back, like waves in water bounce when they strike a wall. It takes time for this movement to cross the intervening space, and thus echoes.'