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.'
'Incendium Dei,' Saladin said, wide-eyed.
Joan stared at Bacon. 'Can it really be as simple as that?'
Bacon grinned. He now had a full hold on their attention, Saladin thought, and he knew it. 'Shall we try it?' He wiped the table clean of chalk with his sleeve, and began to scribble again. The three of them bent over to see. 'We begin with the key,' Bacon said. He wrote, INCENDIVM DEI
'The U replaced by V as you see. Next we eliminate duplicates.'
INCEDVM
'There is our key. So we construct our code. I tried out a forward substitution, but succeeded with a backward…' He scribbled rapidly.
'Now we reconstruct our message. That first B becomes a P, the M becomes R…'
BMQVK XESEF EBZKM BMHSM BGNSD DYEED OSMEM HPTVZ HESZS ZHVH PRGSL CVEVO VPALR PRMER PNYET TBVVT IERVR MHDSA MVEAE AMSM
Saladin stared at the new string, unreasonably disappointed. 'It's still nonsense.'
Bacon smiled, a magician with another trick to show. 'A simple transposition would be too easy. Our puzzle involves numbers as well as letters. Look at the 'sentence' again. Nine 'words' of five letters, and one of four. What sentence is as regular as that? What we have here is a simple string of letters, of length – how many, Thomas?'
'I'm not one of your Parisian students,' Thomas growled.
'Just answer,' Joan murmured.
'Forty-nine, then.'
'Good. What's significant about the number forty-nine?'
'Seven sevens,' said Saladin immediately.
'Very good!' said Bacon.
Thomas looked surprised. Saladin said, 'Some of the villagers think it's a lucky number. Seven times seven. That's how I know.'
'Seven squared,' Bacon said. 'That is surely a clue. So now, if we write out the decoded message again, not in these arbitrary blocks of five or four, but in a grid of seven by seven…'
'It still means nothing to me,' Joan said.
But Thomas was tracing the letters with a chalky fingertip. 'But if you read, not across, but down – else why put them in a grid at all? P – E – R… Give me that chalk, Roger.' He wrote out the letters, column by column, as a single line.
PERNVMERVPYTHAGOREIDESVMTESALPETRAMCARBVM SVLPVRVM
'Look at this string,' Thomas said, excited. 'Pythagorei – see it? Surely there is meaning here at last.'
'Good, good,' Bacon said. 'You can imagine the variants I explored before I hit on this correct route through the maze. Now all we have to do is find the breaks between the words…'
But Thomas was ahead of him, splitting the line with bold slashes.
PER / NVMERV / PYTHAGOREI / DESVMTE / SALPETRAM / CARBVM / SVLPVRVM
And there, for Saladin, the magic happened, a readable sentence emerging from a clamour of nonsense. He was the first to read it aloud: ''By Pythagoras's number take saltpetre, charcoal, sulphur.''
'Almost there,' said Bacon. 'Almost there.'
'But what does it mean?'Joan said.
'Well, Pythagoras's number is obvious. It is six.'
'It is?'
'Six is the perfect number,' said Saladin.
Thomas raised his eyebrows at him. 'And why is it perfect?'
'Because if you take the numbers that divide into it evenly…' Saladin took the chalk now, and wrote out, 1,2,3. 'If you add them up you get six again.' 1+2+3=6.
Bacon smiled. 'Once again you surprise us.'
Saladin felt sheepish. 'Another lucky number for the villagers.'
'In fact there are many perfect numbers,' Bacon said. 'Pythagoras did indeed study them. Twenty-eight is the next one. You see, it is divisible by-'
'Never mind,' Joan said hastily. 'So now we have this: 'By one, two, three take saltpetre, charcoal, sulphur.''
'Or,' Bacon said, 'three, two, one. In fact those proportions aren't quite correct, but near enough the range that a little trial and error gives you the right product. The value of experimentation,' he said, smiling.
Saladin was mystified again. 'What product?'
'Why, it's obvious – black powder. Haven't you heard of it? The Chinese have studied it for centuries, we're told. They call it the 'fire drug'. It's said they found it looking for an elixir of life! I had been hoping to obtain samples via the trade routes opened up by the Mongol empire, in order to verify its properties for myself. Now I can begin to experiment with its very manufacture.'
'The manufacture of what?' Joan demanded. 'What does this stuff do, you infuriating monk?'
He didn't seem insulted. 'Well, if you set fire to it-'
'Yes?'
'It explodes.'
XXII
They sat around the low table, heaped with Bacon's papers and covered with chalk scribbles.
'Explodes,' Joan said.
'Somebody,' Bacon said, 'your Weaver of the tapestry of time, Thomas, wants you to make explosions. Incendium Dei indeed. I wonder why.'
Joan glanced at Thomas. 'Have you told him of the engines?'
Thomas closed his eyes. 'No. Because I did not have your permission. And because, frankly, I was frightened where it might lead, if he knew.'
Bacon's eyes were wide. 'What engines? You must tell me.'
Thomas glanced at Joan. 'You see what I mean?'
Joan said, 'Well, we are committed. And perhaps this strange monk of yours can help us.' She described succinctly the legend of Sihtric and his machines of war, the plans now believed lost beneath the floor of the great mosque of Seville, in faraway al-Andalus.
'But you must retrieve this Codex,' Bacon said. 'You must!'
'Why?'
'Can't you see? Combine these engines of war, engines that roll and swim and even fly, with the black powder, with the Fire of God, and no man could stand before you. Think of it – a miniature Vesuvius loaded on each arrow!…'
Saladin's experience of explosions was limited. But once he had seen a forge blow itself apart. He tried to imagine such energies harnessed, launched, and used against the flesh of enemies.
'He's right,' he said reluctantly. 'You told us, Thomas, that Sihtric was dissatisfied with the engines he made.
