'It's the shape that gives it away.' A woman stepped forward now, her face twisted into a fearful rage.

'What shape?'

'The fish! Everybody knows that's a Christian sign. I've seen it daubed all over the walls of the mosque in Cordoba, a desecration. I've seen it for myself! Doesn't that prove he's a Christian?'

'Oh, for the love of Allah.'

But the mob began to growl again. Ludicrous the root cause might be, but the situation was dangerous.

Ibrahim nodded to Abdul. 'Captain. Take this man.'

Abdul muttered orders. Two of the troops went onto the bridge to seize the 'spy', who did not resist when they took his arms. The rest lined up briskly alongside Ibrahim, forming a barrier between the mob and their prey. They kept their swords in their scabbards, however.

Ibrahim raised his arms again. 'You can see we have taken this man. If he is a spy, we will soon discover the truth and will do something about it. So you don't need to pursue him any further. Go to your homes – go to your prayers. But you with the torches,' he said with a note of command, 'douse them in the river first. The city is too crowded to risk a fire.'

He turned away without waiting to see if they complied. But he murmured to Abdul, 'Make sure they obey.' He turned to the captive. 'And you,' he said, 'are in my charge.' He handed him back his bit of parchment with the fish-ship designs.

The man took it. 'Thank you.'

Although his voice was gruff, Ibrahim thought he recognised his accent. He stepped forward and, carefully, not wishing to exacerbate any injuries, he lifted back the man's hood. His hair was bright blond.

'Peter.'

'Hello, Ibrahim.' The English scholar grinned, then winced as he cracked his bruised lips.

The palace was as crowded as the rest of the city; anybody who could find shelter with the emir did so. But Ibrahim found an empty room where he arranged for Peter's wounds to be treated by a doctor, and ordered a girl to take him to the baths, and he called for a new set of clothes to replace those ruined by the mob.

By the time Ibrahim came to find him, late that afternoon, Peter was transformed. Sitting on a heap of floor cushions, he gazed out of the arched doorway into the light. His hair had been cut, his stubble shaved, and his skin cleansed of blood. He showed no trace of the beating he had received, save for the sheen of salve applied to his bruises and broken lips, and a little neat stitching in the wound on his forehead. But he had aged since Ibrahim had last seen him; now in his late twenties, he was a little thicker around the neck, his skin of his face less fresh, a little peppering of grey in that golden hair.

The battered bit of parchment, with its images of ships, rested on a low table.

Ibrahim sat down, and Peter offered him orange tea. 'I should thank you,' he began. 'I owe you my life.'

'I'd have done it for anybody. It's my job.'

'Which you do very well, everybody says so-'

'If you'd gone home to England you wouldn't have been in peril in the first place.'

'Why would I want to do that? It's much more interesting here. You know, I believe it's been four years since we last met. It took you a year to fall out with your mother, as I recall,' he said drily.

'And you're still working on this nonsense, after all this time.' Ibrahim reached forward and took the parchment. 'The Engines of God.'

'Four years isn't long,' Peter said. 'Not for a project like this. You have no idea how much ground must be laid before you can take a single step.'

'Why a fish?'

'Pardon?'

'Why build a boat shaped like a fish?'

'Because that's what the sketches say. We are still working from the Sihtric designs.' He meant the sketches he had been able to recover from the records of Sihtric's clerk. It had been a long time since Ibrahim had heard the archaic name of that long-dead priest. Peter went on, 'Oh, I can make deeper guesses about why. A fish is comfortable in the water, isn't it? Its smooth shape simply glides through that mysterious substance. Well, then, it stands to reason that if you make a boat with the same profile, it will be similarly advantaged. That's just my guess, though. I don't know.

'Progress is slow, Ibrahim. Well, you saw that, before you flounced out of the project. The sketches are partial, incomplete. Many of them are scribbles that would mean far more to the clerk who made them than to us, for whom they were never intended. We have to guess at so much – sizes, weights, materials, gearing. Very often we ask the impossible of our artisans: steel cogs of unimaginable fineness and accuracy, wooden wheels of a seamless perfection. Sometimes we simply don't have the correct materials at all. And, what's still more difficult, we have to make guesses as to the machines' purposes in the first place.'

Ibrahim looked at the designs again. 'It looks as if these stick men are totally enclosed in their fish-boat.'

'So they are. Can you see, they operate their oars and paddles through seals in the skin of the ship, which appears to be a fine metal shell. We are using beaten copper. Some of us speculate that the ship might be sealed so that it can travel not just on the surface of the water, but beneath it.'

'How is that possible?'

'Do you really want the details? Look, there are bladders here which, if filled with water, might cause the craft to sink, and if pumped out could make it float. It would certainly make sense of the fish shape, wouldn't it? And think of the advantage, Ibrahim. A boat that could float under your enemy's fleet, all unseen, and attack from below.'

Ibrahim tossed aside the bit of parchment. 'This is such a waste of time. It always was.'

'The emir may not think so when we demonstrate our weapons to him.'

'And when will that be?'

Peter shifted, uncomfortable. 'We have a number of designs, partially realised… We aren't ready yet.'

'Allah preserve us, but the Christian armies are close. Surely even a bookworm like you is aware of that.'

'Of course I am. We're all working as hard as we can, and as fast.'

'What of your conscience, Peter? Are you happy as a Christian to be arming Muslims?'

'I think of myself as a scholar before I'm a Christian. And this is a scholarly project, whatever else it is. I'm curious, Ibrahim. Anyway, perhaps our weapons, if they deter Fernando, will prevent war, rather than provoke it. Have you thought of that? In a way we're alike, aren't we, Ibrahim? Both striving to save people from harm, in our different ways.'

Ibrahim thought this was all artifice, and he said nothing. The thoughtful young man he had met five years ago was being eroded away by ambition and a certain flavour of greed – not greed for wealth, but for accomplishment and recognition. He had seen it in scholars before, in his time at the court. Such men would do anything to stand out from their peers.

Peter was watching him. 'You know, we do miss you, Ibrahim. When I first met you I took you for a bone-headed dolt. A slab of righteous muscle.'

'I wasn't twenty years old!'

'Now you're five years older, and your true qualities are emerging. You're no soldier, for all you wear that scimitar at your waist. You're far more than that. You have a set of skills your mother could put to good use – organisation, leadership. You should make your peace with Subh. She misses you.'

'My relationship with my mother is not a matter for you, but for my conscience. And I believe I put my skills to good use here. There is an emergency in the city. Again, even you must be aware of that…'

Seville's crisis had now lasted nearly a decade, since Cordoba's fall. The city was flooded with refugees from the lost cities to the north. There was a perennial shortage of food, because of the abandonment of the city's hinterland and the disruption to river trade. Every so often the poor sanitation would cause an outbreak of cholera or typhoid or some other hideous disease. Rumours that Fernando's armies had been glimpsed in the heat-haze of the horizon periodically swept the fearful city, causing panic and rioting.