The khan tested his legs again surreptitiously, then looked down as Torogene chuckled behind him. He turned his horse to see her standing there. He knew he was thin and pale from too much time indoors. His joints hurt and he ached for wine so that his mouth grew dry at the thought of it. He had promised Torogene he would drink fewer cups each day. More than that, she had made him swear a solemn oath. He had not told her about the set of enormous cups the kilns were firing for him. His word was iron, but wine was one of his few remaining joys.

'Don't stay out if you feel yourself getting tired,' Torogene said. 'Your officers can wait for another day if they have to. You must build your strength back slowly.'

He smiled at her tone, wondering if all wives became mothers to their husbands at some point. He could not help glancing at Sorhatani at that thought, still as lean and strong as a herdboy. There was one who should not go to waste in a cold bed. He could not remember when he had last felt honest lust outside of dreams. His body felt worn out, withered and old. Yet the sun shone weakly and the autumn sky was blue. He would ride along the canal to see the new works. Perhaps he would even bathe in the river that fed it, if he could bring himself to enter the icy waters.

'Do not set my city on fire while I am gone,' he said gruffly.

She smiled at his tone.

'I cannot promise, but I'll try,' Torogene replied. She reached out and touched his foot in the stirrup, holding it hard enough for him to feel the pressure. He did not need to speak the love he felt for her; he just reached down and touched her cheek before digging in his heels and clattering through the gate.

Sorhatani's sons came with him. Kublai held the reins of three packhorses, laden high with supplies. Ogedai watched as the young man clucked to them, so full of life that it was almost painful to see. He had not told Kublai his memories of Tolui's death. He was not yet ready to tell that story, with all the pain that continued to that cold day.

It took half the morning to reach the river. His stamina had melted away over so many months of inactivity. His arms and legs were leaden by the time he dismounted and he had to struggle not to cry out as his thighs cramped. He could already hear the rippling cracks across the valley and in the distance smoke hung like morning mist. The air had the tinge of sulphurous bitterness he remembered from the Sung border. To his surprise, he found it almost pleasant to breathe in the exotic scent.

Sorhatani and her sons made camp around him, setting up a small ger on dry ground by the bank and starting tea on the stove. While it brewed, Ogedai mounted once again. He clicked his tongue to catch Kublai's attention and the young man leapt into his saddle to join him, his face bright with excitement.

Together, both men rode across a sunlit field to where Khasar readied the ordnance crews for inspection. Ogedai could see the old general's pride in the new weapons from a distance. He too had been on the Sung border and seen their destructive potential. Ogedai rode up slowly. He felt no sense of urgency or hurry. His glimpse at the greater night had given him a long perspective. It was just harder to care about the smaller things. Having Kublai with him was a reminder that not everyone shared his long view. The sight of the polished bronze guns had Kublai practically sweating.

Ogedai suffered through the formalities with his uncle. He declined the invitation of tea and food and finally gestured for the gunners to begin.

'You might want to dismount and hold your horse, my lord khan,' Khasar said.

He looked thin and weary, but his eyes were bright in his enthusiasm. Ogedai wasn't touched by uncle's mood. His legs felt weak and he did not want to stumble in front of such men. He took a moment to remind himself that he was in the eye of the nation once again. One slip and his weakness would reach every ear.

'My horse was at the Sung border,' he replied. 'He will not bolt. Kublai? You should do as he says.'

'Very well, my lord,' Khasar said formally. He clasped his hands behind his back as he gestured sharply to the gunnery teams. They stood in groups of four, carrying sacks of black powder as well as a range of odd-looking equipment. Kublai drank it all in, fascinated.

'Show me,' Ogedai said to them.

Khasar snapped orders and Ogedai watched from the saddle as the first team checked their weapon had blocks against the massive, studded wheels. A warrior placed a reed in a hole in the tube, then lit a taper from a lamp. When the taper touched the reed, there was a spark, then an explosion that sent the cannon rocking back. The blocks barely held it and the weapon leapt and crashed back down. Ogedai did not see the ball that came flying out, but he nodded, deliberately calm. His horse flicked up its ears, but then bent to crop at the grass. Kublai had to slap his gelding on the face, shocking it out of panic. The young man snarled at the animal. He would not be shamed by seeing his horse break free and run in front of the khan. However, he was more than thankful he was not in the saddle.

'Fire the rest together,' Ogedai said.

Khasar nodded proudly and eight other teams inserted their reeds into the touch-holes and lit tapers.

'On my mark, gunners. Ready? Fire!'

The crash was extraordinary. The teams had practised outside the city for weeks and the guns fired almost together, with only a slight delay. This time, Ogedai saw blurs vanishing across the valley, one or two skipping along the ground. He smiled at the thought of a line of horses or men in the path of such weapons.

'Excellent,' he said.

Khasar heard and chuckled, still delighted at controlling thunder.

Ogedai's gaze drifted to the lines of heavy catapults beyond the guns. They could launch barrels of gunpowder for hundreds of feet. His engineers had learned from the Chin, but they had improved the powder, so that it burned faster and more fiercely. Ogedai did not understand the process, nor care. What mattered was that the weapons worked.

More men waited by the catapults, standing perfectly at attention. Ogedai suddenly realised that he did not feel tired. The explosions and bitter smoke had invigorated him. Perhaps because of that, he noticed how Khasar's shoulders had slumped. The older man wore his exhaustion for all to see.

'Are you ill, uncle?' he said.

Khasar shrugged with a wince. 'I have lumps in my shoulders. The things make it hard to move my arm, that's all.'

His yellow complexion gave the lie to his words and Ogedai frowned as his uncle went on.

'The shamans say I should have them cut out, but I won't let those butchers have me, not yet. Half the men they cut don't walk out again, maybe more.'

'You should,' Ogedai said softly. 'I don't want to lose you yet, uncle.'

Khasar snorted. 'I'm like the hills, boy. A few lumps won't stop me.'

Ogedai smiled.

'I hope not. Show me more, uncle,' he said. When Ogedai and Kublai returned to the small camp by the river, the morning was almost over and the tea was long past stewed and undrinkable. The gunnery went on behind them, using vast stocks of the powder to train the men who would play a vital role in future battles. Khasar could be seen striding up and down the lines, in his element.

Sorhatani saw that her son's flushed face was smudged with soot. Both the khan and Kublai reeked of sulphurous fumes and Arik-Boke and Hulegu could only look on with transparent envy. Sorhatani left her sons to make fresh tea and walked over to where Ogedai had dismounted.

He stood at the river's edge and stared over it, shading his eyes against the sun. The noise of the waterfall hid Sorhatani's steps as she came up behind him.

'Kublai is chattering like a bird,' she said. 'I take it the demonstration went well.'