Bahram said, 'With the aid of an echoing cliff we could perhaps time the speed of sound.'

'The speed of sound!' Iwang said. 'Very nice!'

'A capital idea, Bahram,' Khalid said. He checked to make sure his clerk was noting all done or said. He unscrewed the stopcock all the way and removed it, so that they all heard the noisy clanging of the alarm as he reached into the flask to turn off the device. It was strange that the clapper should have been so silent before. He rubbed his scalp with his right wrist. 'I wonder,' he said, 'if we could establish a speed for light too, using the same principle.'

'How would it echo?' Bahram asked.

'Well, if it were aimed at a distant mirror, say… a lantern unveiled, a distant mirror, a clock that one could read very precisely, or start and stop, even better…'

Iwang was shaking his head. 'The mirror might have to be very far away to give the recorder time to determine an interval, and then the lantern flash would not be visible unless the mirror were perfectly angled.'

'Make a person the mirror,' Bahram suggested. 'When the person on the far hill sees the first lantern light, he reveals his, and a person next to the first person times the appearance of the second light.'

'Very good,' several people said at once. Iwang added, 'It may still be too fast.'

'It remains to be seen,' Khalid said cheerfully. 'A demonstration will clarify the issue.'

With that Esmerine and Fedwa wheeled in the ice tray and its 'demonstration of sherbets' as Iwang termed it, and the crowd fell to, talking happily, Iwang speaking of the thin sound of goraks in the high Himalaya where the air itself was thin, and so on.

The Khan Confronts the Void

So Iwang brought Khalid back out of his black melancholy, and Bahram saw the wisdom of Iwang's approach to the matter. Every day now, Khalid woke up in a hurry to get things done. The businesses of the compound were given over to Bahram and Fedwa and the old hands heading each of the shops, and Khalid was distracted and uninterested if they came to him with matters of that sort. All his time was taken by conceiving, planning, executing and recording his demonstrations with the void pump, and later with other equipment and phenomena. They went to the great western city wall at dawn when all was quiet, and timed the sound of wood blocks slapped together and their returning echoes, measuring their distance from the wall with a length of string one third of an li long. Iwang did the calculations, and soon declared that the speed of sound was something like two thousand li an hour, a speed that everyone marvelled at. 'About fifty times faster than the fastest horse,' Khalid said, regarding Iwang's figures happily.

'And yet light will be much faster,' Iwang predicted.

'We will find out.'

Meanwhile Iwang was puzzling over the figures. 'There remains the question of whether sound slows down as it moves along. Or speeds up for that matter. But presumably it would slow, if it did anything, as the air resisted the shock.'

'Noise gets quieter the further away it is,' Bahram pointed out. 'Maybe it gets quieter rather than slower.'

'But why would that be?' Khalid asked, and then he and Iwang were into a deep discussion of sound, movement, causation and action at a distance. Quickly Bahram was out of his depth, being no philosopher, and indeed Khalid did not like the metaphysical aspect of the discussion, and concluded as he always did these days: 'We will test it.'

Iwang was agreeable. Ruminating over his figures, he said, 'We need a mathematics that could deal not only with fixed speeds, but with the speed of the change of a speed. I wonder if the Hindus have considered this.' He often said that the Hindu mathematicians were the most advanced in the world, very far ahead of the Chinese. Khalid had long ago given him access to all the books of mathematics in his study, and Iwang spent many hours in there reading, or making obscure calculations and drawings, on slates with chalk.

The news of their void pump spread, and they frequently met with the interested parties in the madressas, usually the masters teaching mathematics and natural philosophy. These meetings were often contentious, but everyone kept to the ostentatiously formal disputation style of the madressa's theolo ical debates.

Meanwhile the Hindu caravanserai frequently sheltered booksellers, and these men called Bahram over to have a look at old scrolls, leatheror wood bound books, or boxes of loose leaved pages. 'Old One Hand will be interested in what this Brahmagupta has to say about the size of the earth, I assure you,' they would say grinning, knowing that Bahram could not judge.

'This one here is the wisdom of a hundred generations of Buddhist monks, all killed by the Mughals.'

'This one is the compiled knowledge of the lost Frengis, of Archimedes and Euclid.'

Bahram would look through the pages as if he could tell, buying for the most part by bulk and antiquity, and the frequent appearance of numbers, especially Hindu numbers, or the Tibetan ticks that only Iwang could decipher. If he thought Khalid and Iwang would be interested, he haggled with a firmness based on ignorance, 'Look this isn't even in Arabic or Hindi or Persian or Sanskrit, I don't even recognize this alphabet! How is Khalid to make anything of this?'

'Oh, but this is from the Deccan, Buddhists everywhere can read it, your Iwang will be very happy to learn this!'

Or, 'This is the alphabet of the Sikhs, their last guru invented an alphabet for them, it's a lot like Sanskrit, and the language is a form of Punjabi,' and so on. Bahram came home with his finds, nervous at having spent good money on dusty tomes incomprehensible to him, and Khalid and Iwang would inspect them, and either page through them like vultures, congratulating Bahram on his judgment and haggling skills, or else Khalid would curse him for a fool while Iwang stared at him, marvelling that he could not identify a Travancori accounting book full of shipping invoices (this was the Deccan volume that any Buddhist could read).

Other attention drawn by their new device was not so welcome. One morning Nadir Devanbegi appeared at the gate with some of the Khan's guards. Khalid's servant Paxtakor ushered them across the compound, and Khalid, carefully impassive and hospitable, ordered coffee brought to his study.

Nadir was as friendly as could be, but soon came to the point. 'I argued to the Khan that your life be spared because you are a great scholar, philosopher and alchemist, an asset to the khanate, a jewel of Samarqand's great glory.'

Khalid nodded uncomfortably, looking at his coffee cup. He lifted a finger briefly, as if to say, Enough, and then muttered, 'I am grateful, effendi.'

'Yes. Now it is clear that I was right to argue for your life, as word comes to us of your many activities, and wonderful investigations.'

Khalid looked up at him to see if he were being mocked, and Nadir lifted a palm to show his sincerity. Khalid looked down again.

'But I came here to remind you that all these fascinating trials take place in a dangerous world. The khanate lies at the centre of all the trade routes in the world, with armies in all directions. The Khan is concerned to protect his subjects from attack, and yet we hear of cannon that would reduce our cities' walls in a week or less. The Khan wishes you to help him with this problem. He is sure you will be happy to bring him some small part of the fruits of your learning, to help him to defend the khanate.'

'All my trials are the Khan's,' Khalid said seriously. 'My every breath is the Khan's.'

Nadir nodded his acknowledgement of this truth. 'And yet you did not invite him to your demonstration with this pump that creates a void in the air.'