At Ghalib's mocking call Robert turned away from Moraima and looked towards the river. Hisham was standing on a wall along the bank.

And Ghalib had somehow climbed up onto a waterwheel. As it turned, he was climbing up from one spoke to the next, as if clambering over a treadmill. He was soaked to the skin, his red turban bright, and he was laughing. 'Hey, Moraima – hey, God's warrior! Look at me, look at me!'

Moraima laughed, but she clamped her hand over her mouth. 'Allah preserve him. He'll get himself killed.'

Robert strode towards the waterwheel, pushing through a gathering crowd of laughing onlookers. 'Get down off there, you idiot!'

Hisham threw a mock punch at Robert. 'You're just jealous because Moraima's looking at him, not you.'

Robert glared. 'Unless you shut up she will be looking at you when I push your teeth down your throat.'

Hisham returned the challenge for one heartbeat, then backed off.

'Hey, Christian.' Ghalib was calling again. 'Watch this.' Now he was heading for the wheel's mighty axle. He was spun around the hub, turning head-over-heels with each revolution. The wood was soaked by spray and was slippery.

Moraima ran forward. 'Get down! Oh, get down, you fool!'

Ghalib grabbed a strut with one hand, then threw himself backwards, flinging out the other hand, so he was splayed out over the hub, turning over and over on the wheel. 'Hey, look at me! I'm crucified! I'm Jesus on the cross!'

He actually got a laugh from the onlookers, and a smattering of applause. Hisham played up. He pulled his shirt over his head, and wailed in a loud, high voice. 'And I'm His mother the Virgin! Oh, my son, my only son, what have those awful Romans done to you?'

On the wheel, Ghalib kept grinning, but his expression was forced, and Robert saw he was tiring.

Then his right hand slipped from the wood. He dangled from his left arm, and flailed, trying to get a fresh hold with his right hand. But the wheel turned remorselessly, and he flipped over, and his left hand started to slip too. He tried desperately to grab onto something, anything.

And he pushed his right arm inside the wheel, into the machinery. Robert heard a distant crunch, like an owl chewing a mouse's bone. Ghalib didn't even scream. He fell down the face of the wheel, his right arm dangling like a blood-soaked rag, and splashed into the water.

The watching people just stared. The wheel turned as if Ghalib had never existed.

Robert ran over the cobbles and climbed onto the bank wall beside Hisham. There was barely any room between the wall and the turning wheel. But there was Ghalib, floating in the water, apparently unconscious. The water all around him was stained red. And soon he would be drawn into the wheel's machinery again.

Moraima grabbed Robert's arm. 'You must help him.'

Cursing, he kicked off his boots for the second time that day. Then he jumped, feet first, his arms tucked in at his sides, and plummeted down into the water.

Weary from the heat and light, Orm and Sihtric sat in the shade of an awning and sipped water laced with lemon juice. They looked out over the scaffolding that encased the arbalest.

Sihtric said, 'The principles are simple, but the devil is in the detail, Orm. Our ambition constantly outruns our capabilities. On the arbalest, for instance, I've lost count of the number of spring shafts we've stripped, the bow arms we've fractured. We learn, step by step.' He riffled through the much-thumbed sketches of Aethelmaer's designs, with elaborations and annotations added by Moorish scholars. 'It is as if the wretched Aethelred was given a glimpse of the future. And we poor fools labour to build the machines of another century with the tools and materials of this.

'But there remains the central mystery of the Incendium Dei, which will give the punch to these weapons. Look, here is the scrap of cipher Aethelmaer left.' He pointed to a line of spidery lettering, headed simply Incendium Dei:

'I saw this before, at Westminster,' Orm said. 'It meant nothing to me then, nor does it now.'

'Nor me, and that's the problem. Well, nobody said it would be easy.'

'And you've devoted your life to this stuff ever since Hastings?'

Sihtric shrugged. 'After Harold fell, after my Menologium lost its value, I had no purpose. I needed a new goal.'

'You could,' Orm pointed out, 'have found some parish to serve. There has been plenty of suffering among the English these last twenty years.'

Sihtric smiled, almost sadly. 'Me, a humble parish priest? After I was nearly a king-maker? I don't think so. I wanted power – that's the truth and I don't deny it. I had no other purpose in mind. And I saw Aethelmaer's designs as a way to achieving that power.'

'So you found a way to live here.'

'It took time. You may remember I had a contact in Ibn Sharaf of Toledo, the noted astronomer, who corresponded with me in London. He gave me a start. After that I found a place in a monastery. I quickly learned Arabic, which is the language of government here. I made some money translating the Bible into Arabic, for other Mozarabs. There are Christians here who have grown up reading only Arabic. Imagine that!'

'And you too are a Mozarab,' Orm said. 'A "nearly Arab". You are defined by what you are not. Tolerated or not, I don't think I would like to live with such a label.'

'Few do,' conceded Sihtric. 'And there are boundaries to that tolerance. The Moors are clannish, Orm. You can't just find the local lord and offer him your services, as in England. With the Moors it's all family and patronage and who you know – devilish hard to break into. And under Islamic law there are limits to the tax you can impose on a Muslim, but you can tax Christians as much as you like. And then, Mozarabs are excluded from the higher levels of government, from power. It's actually a good career move to do as Ibn Hafsun's family once did, and convert. But then I am a priest; that course is excluded to me.' The bitterness in his voice was obvious. 'We survive, we Mozarabs. But we are a cowed people.'

'And yet you prospered.'

'Well, I formed a relationship with the vizier, Ibn Tufayl. I told him my goals; I showed him Aethelmaer's designs. He sponsored my work. This is an age of war. I think he regards my work as a worthwhile investment: a relatively small outlay for a possibly handsome return.'

Orm frowned. 'What sort of relationship?' This seemed to him the central mystery of Sihtric's life here.

But Sihtric would only say, 'There are some things it's better an innocent like you never learns, Orm.'

Orm, irritated and patronised, tried another approach. 'Ibn Tufayl works for the emir in Seville. If he turns your arbalest on other Moors, that's one thing. But what if he turns it on the armies of the Christian kings? Have you thought about that? You're building these machines with Moorish money. But who are they for?'

Sihtric glanced around, as if they might be overheard. 'That particular truth is murky. I came here seeking power and influence for myself, that's all, ignoble as it is. But while here I have discovered a higher purpose.'

Orm laughed. 'You always did have ideas above your station, priest.'

'Yes, well, I'll have to show you, in good time, and then we'll see what you have to say about it. And in the meantime, we have another murky truth to explore. Don't we, Orm?'

'You mean Eadgyth's Testament.' He felt uncomfortable, even though he had come all this way to discuss this.