Saladin nodded. 'If you know the Mongols are coming you can arm against them.'

'We tried that,' Joan said. 'But nobody wants to believe in the coming of the Mongols until they are on the doorstep.

'What we could anticipate was the plight of the refugees – those poor folks driven ahead of the Mongols' advance, in Asia, Persia, Europe. So we set up caravan stops. We supplied food and water, blankets. We even hired Saracen doctors. And we bought up the land to which they had to flee.'

'We made money out of the terrified,' Saladin said. He seemed faintly disgusted.

'We saved lives,' Joan said sternly. 'There are far more ignoble ways of making a living, Saladin. And if we had not, our family could not have survived here.'

'Think of it as a miracle,' Thomas said to Saladin. 'Everyone knows that the First Crusade's dazzling successes all depended on miracles. Perhaps God has miraculously assisted your family, for purposes yet to be revealed. Think of it that way.'

'But now,' Joan said, 'things have changed.'

'How so?'

'For one thing,' Thomas said, 'the Mongols have turned back. We must discuss the meaning of this in due course.'

Joan said, 'And then there is this letter from Subh the Moor, our distant cousin. She drops a hint that the Codex, Sihtric's engine designs, may not be lost after all…' Subh had said that a copy of the lost plans might have been buried under the floor of the mosque in Seville. 'If Robert ever knew this,' Joan said, 'he did not tell his son, or at any rate it was not written down.'

Now Saladin's face was full of a boyish wonder, pleasing to Thomas's dry heart. 'Buried under a mosque! What a story!'

'It may be just that,' Thomas warned. 'A story. But Subh has taken it seriously enough to write to you.'

Joan said, 'Subh believes that all these fragments of prophecy, in her possession and ours, may be put back together into a whole, the prophetic lore reassembled for the first time since the age of Sihtric himself.'

'And so that's why she wrote to you?' Saladin asked. 'But what does she want? We are Christian, she is Muslim. The Christians are destroying her country. Perhaps she intends to trick us, as that old priest Sihtric intended to trick the Moors.'

Thomas said, 'Even if that is her intention it need not be fulfilled. If we could get hold of these designs, if we could build the Engines of God, we could strike a devastating blow against Islam.'

Saladin studied him, curious. 'That sounds a very military ambition for a monk.'

'Not military. Evangelical.' He told them of Saint Francis of Assisi, founder of his order, whom Thomas himself had, thrillingly, met as a novice. 'The first rule Francis wrote for our order was a command for a global mission to "all peoples, races, tribes, and tongues, all nations and all men of all countries, who are and who shall be". Perhaps these Engines will enable my brothers to advance their most holy mission – even if not a life is taken with them.'

Joan said, 'So we share common goals, my family and your order.'

'Oh, yes.'

'So what do you think we should do about this, Thomas?'

'You could write to this cousin in Cordoba, or travel there. Or you could come to England – perhaps we could bring her there.'

Joan frowned. 'The Muslims would have us out of Jerusalem for good. It's not the best time to leave, chasing a dream. And I'm not sure if writing back to my Moorish cousin about this matter is advisable. For now, let us study the matter further. Subh can wait.' But she frowned again at her letter. 'You know, there are so many puzzles here. Thomas, what of this phrase, Incendium Dei?'

Thomas said, 'Something Subh's own ancestress, Moraima, evidently remembers of the lost Codex.'

"'The Fire of God."'

'More than "fire",' Thomas said. 'It is a word with passion. It means conflagration. Ruin. Perhaps it is a phrase associated with the bit of the manuscript from which Robert tore his corner…'

A thought occurred to him. He pawed through his notes once more, looking for the transcription of the ripped-through phrase with the coded words. All those incomplete letters, I, V, M – was it possible that Subh's Latin phrase would complete that puzzle?

But he was exhausted by the heat. With apologies to Joan, promising to discuss all this further, he gathered up his documents and made his way to bed, his head buzzing with the enigma of the Incendium Dei, the Fire of God.

Among Subh's many admirable qualities was an antipathy to wasting time. And so, on only his second day in Seville, Peter was to meet Moorish scholars who would inspect the fragmentary weapons plans he had retrieved from the wreckage of the schemes of long-dead Sihtric.

He was in no state for this, with his hair and skin caked with dust and his clothes stiff with stale sweat. At Subh's suggestion, or perhaps it was an order, he took a bath, Moorish style. For the price of an English penny he had the dirt scraped and sweated out of his skin, he was shaved and his hair chopped back, and he had the ache of the mule ride kneaded out of his back by a masseur, a huge Moor with biceps like a bull's thighs.

When he got back to Subh's house he found his own clothes had been taken away for mending and cleaning, to be replaced by a set of clean, crisp white robes that would have suited any Moor. There was even a turban.

'My,' Subh said, when she met him at the gate. 'Has your skin been oiled? You even smell civilised.'

His head full of her perfume, he could only reply, 'Lady, I'll take that as a compliment.' He offered her his arm.

They walked the short distance to Seville's royal palace, which the Moors called the al-qasr al-Mubarak, the Blessed Palace. Somehow it didn't surprise Peter at all that Subh was held in high enough esteem by the emir to have been granted the use of rooms in his palace to meet her scholars.

They were met by one of the emir's staff, a sleek chamberlain with a scalp shaved smooth and polished oak-brown. He led them on a gentle walk through the rooms of the palace, which led from one to the other in the indirect Moorish style, filled with water-reflected light from patios and gardens. In some of the rooms they glimpsed people at leisure, wives and princes perhaps, and palace staff who worked on the business of running the emirate.

'I'm impressed you organised this so quickly,' he said.

'We may need to be hasty, if the rumours I've been hearing about the Christians' plans are true. We have a few years at best before Seville is besieged the way Cordoba was. So we must get on with it. But tell me – do you despair of the quality of our scholars in these latter days? After all the great age of the caliphate is long gone.'

'Not at all. I have studied Ibn Rushd, for instance, whom we know as Averroes. An astronomer, physician, philosopher, who is generally believed to have produced the best commentary on Aristotle since antiquity, and stirred up some trouble with it. Ibn Bajjah, a teacher of medicine, and tutor of Ibn Rushd. Al-Jayyani, who wrote commentaries on Euclid. Al-Maghribi, famous for trigonometry. Al-Zarqali, the foremost astronomer-'

'Enough. You have convinced me. You know,' she mused, 'these scholars are heroes to you. But my son's heroes are men like Tariq and Saladin. Warriors. Makes you think.'

'I'll tell you something else about Muslim learning. Many Arabic words have no Latin equivalents, because the scholarship is so much more advanced. So when the work of Ibn Rushd, for instance, is translated into Latin, the Arabic words are copied over as they are. Alkali. Camphor. Borax. Elixir. Algebra. Azure. Zenith. Nadir. Zero. Cipher-'

'Enough of your lists, boy!'

'Do you think that people in England and Germany will end up speaking Arabic without even knowing it?'