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The novice returned and said they had to wait for Bacon.

‘For, I’m told,’ said Joan, sweeping into the room, ‘today’s the day for his annual bath, and he’s not about to miss that for a couple of ragged refugees from the Outremer.’ She was dressed smartly, in a long crimson robe and a white wimple.

Saladin got to his feet and kissed his mother. ‘You don’t look ragged to me, Mother,’ he said.

‘Appearance is everything.’ She nodded stiffly. ‘I thank you for the stipend you send me. I spend it well, I hope.’

‘Spend it as you like.’

Her movements as she chose a chair and took her wine were firm and decisive. Her face was still young, he thought, still beautiful; she was only thirty-five years old. She was flushed, though, but not with health. Flushed with an impatience that had been building all the long years of her exile in England, as she saw it. She eyed her son. ‘No wife yet? No grandchildren for me?’

‘Not since I last saw you at Christmas,’ he said drily. ‘And no husband for you, Mother?’

‘A husband would only get in the way.’

‘He might provide you with an income,’ Thomas pointed out.

Joan snorted. ‘An income pledged to his ambitions, not mine. I’ve no use for that.’

Thomas looked at them both. ‘You are mother and son, but so different. Saladin is finding contentment. He lives simply; he uses the skills God has given him; slowly, patiently, he is building himself a place in this country. He asks nothing, and he resents nothing. But you, madam, are full of anger, aren’t you? Rage, even.’

‘Rage?’ Her cheeks coloured, her eyes glittered, and her lips were thin. ‘If you say so. You men of the cloth are so terribly wise.’

‘But, Mother,’ Saladin said, ‘what are you angry about?’

‘What do you think?’ she flared. ‘This is not my country. I despise the weakness of these Christians of the west, who cannot, it seems, summon the will to take back the lands which were lost - our home, Saladin. I have no wealth, no position. I am not respected here. Though my ancestors fought and died to win the Holy Land for Christendom, the people in this country even mock the way I speak. Can they not see who I am -
what
I am?’

Saladin was saddened. ‘And is this why you want to build your engines? To change the way people look at you?’

She stared him down.

But if this was true, Saladin saw with dismay, then his mother had no choice but to pursue her dream of engines of war; the logic of her personality dictated it. And, Saladin sensed with dread, he was destined to follow her.

The door crashed open. They all flinched.

A monk burst in, tall, skinny, agitated, and with his tonsured hair comically sticking up around his bald pate. He looked younger than thirty. ‘Thomas!’ he shouted without preamble. ‘Good to see you again. And this must be Joan of Jerusalem, and her son, fascinating, fascinating, you who brought the conundrum of time to my door, you who believe past and future are all muddled up.’

Thomas said, ‘Roger—’

The man dumped a leather folder on a low table, and kept talking. ‘And why should time
not
be mixed up? All is in flux, the world is an unstable place. Heraclitus pointed out that he was never able to dunk his foot in the same river twice, for it changes with every instant - you see? So why, then, should we imagine that even the river of time is inviolable and unchanging? Perhaps it is more like the fabled Meander in Phrygia, which changes its course with every season, endlessly seeking the perfection of its Platonic ideal. So, then, perhaps history is made and remade, cutting through our lives as a wandering river cuts through sandbanks, for ever seeking some new and more perfect shape. Why not, I say, why not? Shall we get to work?’

Joan turned to Thomas. ‘Who is this person?’

‘One of the liveliest minds of this new age of scholarship, that’s who,’ said Thomas.

‘One
of ...’

‘Which is why I took your puzzle to him. Joan, Saladin, this is Roger Bacon, born in Ilchester, trained in Oxford, and now lecturing in Westminster.’

‘Don’t forget Paris,’ Bacon put in.

‘I have been aware of his career since his student days - oh, a decade back now. You studied the classics at Oxford, did you not, Roger?’

‘And geometry, arithmetic, music, astronomy. I worked under Robert Grosseteste.’

‘The bishop of Lincoln—’

‘Who has led the reintroduction of the works of the Greeks into England.’

‘Roger lectured in Paris—’

‘I earned my master of arts degree there. I saw Alexander of Hales there, and twice saw William of Auvergne dispute...’

This fast-paced duologue was hugely confusing to Saladin, who had heard of none of these scholars.

‘I see myself now as a
dominus experimentorum,’
Bacon said.

Joan glanced at Thomas. ‘What does that mean?’

‘One who studies the physical world,’ Thomas said, ‘and tinkers with it, in the hope of learning more about the truths of God.’

‘I have always “tinkered”,’ Bacon said. ‘I once set up a candle and a mirror in a darkened room, and peered into the eye of a cat. Have you ever tried such a thing, brother Saladin?’

‘I can’t say I have.’

‘You see a carpet of dusky red vessels overlaid by a golden tracery. Quite beautiful, quite mysterious. My study in optics began with those first observations. And if you could look into the head of a man, what would you find? But I have never been able to persuade anybody to sit still long enough to let me see. Ah, well.’

‘I thought all truth was to be found in the Bible.’

‘Of course, and in the authorities of antiquity. I myself am one of Europe’s leading scholars on Aristotle,’ Bacon said without a shred of modesty. ‘But there are many routes to the same destination, which is God’s truth. The role of the natural philosopher is to understand how phenomena reveal that truth. Saint Augustine himself instructed us not to embarrass ourselves by quoting the word of God to contradict some fact of nature, because that would only reveal that we understood neither the word nor the nature. Experimentation: that is the way to that deeper truth, that final reconciliation. Or so I am coming to believe. Perhaps you have heard of the work of Master Peter de Maricourt, a Picard who once took the Cross, and subsequently—’

‘Yes, yes, Roger,’ Thomas cut in. ‘But perhaps we should get to the point?’

Bacon smiled, utterly in control. ‘Quite right, Father, quite right. You!’ He jabbed a finger at the novice, who jumped. ‘More wine for our guests. And bring a lamp over here.’ He sat before the low table, opened his leather wallet and extracted papers that he proceeded to spread out. ‘We have a deep mystery to unravel.’

Saladin murmured to Thomas, ‘He’s quite a showman, isn’t he?’

‘And he knows it. But it’s not necessarily good for him. Ah, Roger, Roger, how your busy head distracts your pious heart!’

But they sat before Roger Bacon, wide-eyed, as he began to reveal the truth of the
Incendium
Dei cipher.

‘We begin with your fragment of coded text, as Thomas presented it to me,’ Bacon said. He spread out a parchment on the table:

 
BMQVK XESEF EBZKM BMHSM BGNSD DYEED OSMEM HPTVZ
HESZS ZHVH

 
‘I was intrigued by the puzzle...’

‘I knew he would be,’ Thomas whispered to Joan. ‘Very useful thing about scholars, that curiosity. He didn’t even ask for a fee.’

Bacon glanced at Saladin. ‘You. Tell me what you see.’

‘I’m no scholar—’

‘Just answer.’

‘I see ten words,’ Saladin said. ‘Latin letters, not Arabic. I recognise none of the words, though.’

‘And nor should you, for they aren’t words at all. Even these groupings are a decoy, I quickly realised. This is no sentence. Look at them! What sentence has words of such regular lengths?’

‘It is written in a cipher,’ said Joan. ‘That much is obvious.’

‘Yes! But what cipher? What do we known about ciphers? You, Thomas?’

‘Just get on with it, Roger.’

‘Oh, very well. The first cipher was used by the Spartans, long before the birth of Christ. They had a device called a
scytale.
You would wrap a strip of leather around a baton, and write out your message; once unwrapped the letters are scrambled, you see, illegible to anyone who doesn’t have a baton of the same dimensions. Tacitus wrote of codes and ciphers, as did the Greek Polybius. Julius Caesar used a substitution cipher, which depended on a simple cyclic displacement of the alphabet. Caesar used a displacement of three positions, while Augustus later used one.’

‘I’ll be the one to ask,’ Joan said heavily. ‘What is a “simple cyclic displacement”?’

Bacon reached for a bit of chalk and scribbled on the tabletop. ‘You write out your alphabet. A, B, C, D. And you write it out again with the letters shifted through three spaces, say. D, E, F, G. You have the word you wish to encode, say “CAESAR”. And you exchange the true letters for the shifted ones. So C becomes F, A is D, E is H ...’

‘I understand,’ said Joan.

‘Now, history tells us there have been ciphers a good deal more sophisticated than that. Polybius himself described a bilateral substitution system, which means... never mind! Happily for your weary brains, I soon concluded we aren’t dealing with anything much more complex than Caesar’s substitutions.’

‘Why do you believe that?’ Saladin asked.

‘This is a message in the Latin alphabet, not Arabic or Persian or Greek. So it is surely a Latin message. The Moors of Spain are developing extremely advanced cryptographic systems, I’m told. But a thousand years after the Caesars, we Latin scholars still lag behind the rest in our ciphers as in everything else. One point on which I kept an open mind was which alphabet we are using here.’

‘The Latin one,’ said Saladin.

‘Ah, but
which
Latin? Caesar used twenty-three letters. We use twenty-five, for we have added U and J. I thought it most likely the classic alphabet was the one employed.

‘So I began my analysis. A common technique in breaking ciphers is to study the distribution of letters. The most common symbol is likely to correspond to a common letter in plain language - E perhaps, or
S
, or T. But this fragment is too short to enable such a count. I experimented with
scytales
of various dimensions, to no avail. And I tried all the possible cyclic permutations, with no luck either. With all the permutations exhausted, I racked my brains for a new way forward.’

Joan murmured, ‘And in the end, after much heroic struggle, you found a way, did you?’

Bacon blithely ignored her sarcasm. ‘A simple variant on cyclic substitution is to use a key.’

‘A key?’ Saladin asked.

‘Caesar, for instance, could have used his own name.’ He wrote it out: CAESAR. ‘We must eliminate repetitions.’ He crossed out the second A. ‘Now we use this five-letter key as the foundation of our cipher.’ He wrote out a twenty-three Latin letter alphabet with a code beneath it:

‘You see? A substitution with the shift depending on the key word, and with those letters removed. So the word CAESAR now encodes as—’ He wrote it out:

 
ECRQCP

 
‘It’s a poor sort of code,’ Saladin observed. ‘The last few letters are transcribed without change.’

‘You’re a practical man, I can see that,’ Bacon said. ‘That’s true. But there are easy variants. The simplest is to put the key word at the end of the alphabet, not the beginning, and to proceed backwards.’ He scribbled quickly,

‘Now all the letters save one have a different symbol.’

‘This is all very well,’ Joan said, ‘but
we
don’t have a key, do we?’

‘Oh, but we do,’ Bacon said. ‘You gave it to me - or rather, to Thomas.’

‘I did? What key?’

‘It was in the letter you received. From your cousin in Spain. The phrase she was particularly interested in, that appeared to be left incomplete on the scrap of parchment you held.’