'Cannonball flights could be described precisely,' Khalid said.

'Yes. More easily than most things, because a cannonball pursues a single line. A curved line, but not something like an eagle's flight, say, or a person in his daily rounds. The mathematics for that would be…' Iwang became lost, jerked, came back to them. 'What was I saying?'

'Cannonballs.'

'Ah. Very possible to measure them, yes.'

'Meaning if you knew the speed of departure from the gun, and the angle of the gun…'

'You could say pretty closely where it was going to land, yes.'

'We should tell Nadir about this privately.'

Khalid worked up a set of tables for calculating cannon fire, with artful drawings of the curves describing the flight of shot, and a little Tibetan book filled with Iwang's careful numerics. These items were placed in an ornate carved ironwood box, encrusted with silver, turquoise and lapis, and brought to the Khanaka in Bokhara, along with a gorgeous damasked breastplate for the Khan. The steel rectangle at the centre of this breastplate was a dramatic swirl of white and grey steel, with iron flecks very lightly etched by a treatment of sulphuric acids and other caustics. The pattern was called by Khalid the Zeravshan Eddies, and indeed the swirl resembled a standing eddy in the river, spinning off the foundation of the Dagbit Bridge whenever the water was high. It was one of the handsomest pieces of metalwork Bahram had ever seen, and it seemed to him that it, and the decorated box filled with Iwang's mathematics, made for a very impressive set of gifts for Sayyed Abdul Aziz.

He and Khalid dressed in their best finery for their audience, and Iwang joined them in the dark red robes and conical winged hat of a Tibetan monk, indeed a lama of the highest distinction. So the presenters were as impressive as their presents, Bahram thought; although once in the Registan, under the vast arch of the gold-covered Tilla Karia Madressa, he felt less imposing. And once in the company of the court he felt slightly plain, even shabby, as if they were children pretending to be courtiers, or, simply, bumpkins.

The Khan, however, was delighted by the breastplate, and praised Khalid's art highly, even putting the piece on over his finery and leaving it there. The box he also admired, while handing the papers inside to Nadir.

After a few moments more they were dismissed, and Nadir guided them to the Tilla Karia garden. The diagrams were very interesting, he said as he looked them over; he wanted to inquire more closely into them; meanwhile, the Khan had been informed by his armourers that cutting a spiral into the insides of their cannon barrels had caused one to explode on firing, the rest to lose range. So Nadir wanted Khalid to visit the armourers and speak to them about it.

Khalid nodded easily, though Bahram could see the thought in his eyes; once again he would be taken away from what he wanted to be doing. Nadir did not see this, though he watched Khalid's face closely. In fact, he went on cheerfully to say how much the Khan appreciated Khalid's great wisdom and craft, and how much all the people of the Khanate and in Dar al Islam generally would owe to Khalid if, as seemed likely, his efforts helped them to stave off any further encroachments of the Chinese, reputed to be on the march in the west borders of their empire. Khalid nodded politely, and the men were dismissed.

Walking back along the river road, Khalid was irritated. 'This trip accomplished nothing.'

'We don't know yet,' Iwang said, and Bahram nodded.

'We do. The Khan is a. He sighed. 'And Nadir clearly thinks of us as his servants.'

'We are all servants of the Khan,' Iwang reminded him.

That silenced him.

As they came back towards Samarqand, they passed by the ruins of old Afrasiab. 'If only we had the Sogdian kings again,' Bahram said.

Khalid shook his head. 'Those are not the ruins of the Sogdian kings, but of Markanda, which stood here before Afrasiab. Alexander the Great called it the most beautiful city he ever conquered.'

'And look at it now,' Bahram said. 'Dusty old foundations, broken walls…'

Iwang said, 'Samarqand too will come to this.'

'So it doesn't matter if we are at Nadir's beck and call?' Khalid snapped.

'Well, it too will pass,' Iwang said.

Jewels in the Sky

Nadir asked for more and more of Khalid's time, and Khalid grew very restive. One time he went to Devanbegi with a proposal to build a complete system of drains underneath both Bokhara and Samarqand, to move the water of the scores of stagnant pools that dotted both cities, especially Bokhara. This would keep the water from becoming foul, and decrease the number of mosquitos and the incidence of disease, including the plague, which the Hindu caravans reported to be devastating parts of Sind. Khalid suggested sequestering all travellers outside the city whenever they heard such news, and causing delays in caravans that came from affected areas, to be sure of cleanliness. A purification delay, analogous to the spiritual purifications of Ramadan.

But Nadir ignored all these ideas. An underground system of pipes, though common in Persia from before the invasions of the Mongols, was too expensive now to contemplate. Khalid was being asked for military aid, not physic. Nadir did not believe he knew anything about physic.

So Khalid returned to his compound and put the whole place to work on the Khan's artillery, making every aspect of the cannons a matter for demonstrations, but without trying to learn anything of primary causes, as he called them, except occasionally in motion. He worked on metal strength with Iwang, and made use of Iwang's mathematics to do cannonflight studies, and tried a number of methods to cause the cannonballs to spiral reliably in flight.

All this was done with reluctance and ill humour; and only in the afternoon, after a nap and a meal of yoghurt, or late in the evening, after smoking from one of his narghiles, did he recover his equanimity, and pursue his studies with soap bubbles and prisms, air pumps and mercury scales. 'If you can measure the weight of air you should be able to measure heat, up to temperatures far beyond what we can distinguish with our blisters and ouches.'

Nadir sent his men by on a monthly basis to receive the latest news of Khalid's studies, and from time to time dropped by himself unannounced, throwing the compound into a flurry, like an anthill hit by water. Khalid was polite at all times, but complained to Bahram bitterly about the monthly request for news, particularly since they had very little. 'I thought I escaped the moon curse when Fedwa went through menopause,' he groused.

Ironically, these unwelcome visits were also losing him allies in the madressas, as he was thought to be favoured by the treasurer, and he could not risk telling them the real situation. So there were cold looks, and slights in the bazaar and the mosque; also, many examples of grasping obsequiousness. It made him irritable, indeed sometimes he rose to a veritable fury of irritability. 'A little power and you see how awful people are.'

To keep him from plunging back into black melancholy, Bahram scoured the caravanserai for things that might please him, visiting the Hindus and the Armenians in particular, also the Chinese, and coming back with books, compasses, clocks, and a curious nested astrolabe, which purported to show that the six planets occupied orbits that filled polygons that were progressively simpler by one side, so that Mercury circled inside a decagon, Venus a nonagon just large enough to hold the decagon, Earth an octagon outside the nonagon, and so on up to Saturn, circling in a big square. This object astonished Khalid, and caused night-long discussions with Iwang and Zahhar about the disposition of the planets around the sun.