'Why? What's so important?'

'Joan has had a letter,' the monk said. 'From Roger Bacon.' And he produced a parchment.

It was more than a year since Bacon had successfully decoded the Incendium Dei scrap. Since then he had thrown himself into a vast and secretive project of experimental research, spending his time and energy and all the money he could get hold of on books and instruments and tables, hiring assistants and instructing savants.

'He has been puffed up by his own success in cracking the code.'

'That doesn't surprise me,' Saladin said.

'And now he's looking for engines that can deliver God's fire.'

'But the Codex is buried under the mosque.'

'True. But he's scoured Christendom and beyond for ideas on what might be possible. Look at what he speaks of – wagons that move without horses by means of a miraculous force, like the reaping chariots of the ancients, and machines for flight, and so on. He even says there is no doubt that such instruments were built in ancient times and are still being built today. He says he knows of a scholar who tried to build the flying machine…'

'He speaks of this Aethelmaer of the legend.'

'Perhaps. Or he may mean Ibn Firnas of Cordoba, who also built a flying machine some centuries ago.'

Saladin smiled thinly. 'In Jerusalem I knew a man who sold flying carpets making much the same sort of promises. Well, Bacon is undoubtedly eager to keep our money flowing. Is it possible the ancients really did have such machines, Thomas? In a way it makes the Sihtric designs more plausible, if they are memories of the past, rather than dreams of the future.'

'Well, Aristotle proposes that time is like a great wheel, going around and around endlessly, so that the past is the future, and there's no difference. But such speculations don't help us in any practical way. What we need is to get our hands on the Codex, and get it to Bacon, and then see what he can do with it-'

There was a spark of light. It came from one of the towers that bristled on the walls of the city, there and gone in a flash, like the sun reflecting from a bit of armour. Saladin stood up to see better. A tiny cloud of white smoke rose from the tower.

All around the camp, men were standing, pointing. In the flat light they looked skinny, skeletal, dead men besieging a city of the dead.

Long heartbeats after the flash of light, a muffled crump reached Saladin's ears, like thunder from a distant storm.

'What,' Thomas asked, 'was that?'

'I think we'd better go and find out,' Saladin said.

'I'll get your mule for you, friar,' Michael said. 'If it isn't in somebody's pot by now.'

The thunder-mouth had been set up on a look-out platform on top of a tower set in the city walls. Peter and Subh had led Ibrahim up there to see.

The thunder-mouth was a copper tube, shining in the sun. Its muzzle protruded over the battlements, pointing at the Christians' scattered camp. A brazier enclosed the rear of the tube, and when Ibrahim got there a fire was already burning hotly, so intense that it had turned that part of the cylinder red-hot. The brazier was being tended by two of Peter's scholars, who poked at the fire nervously.

Subh looked on with a complacent pride.

Ibrahim walked around the thunder-mouth cautiously. 'Quite a job to haul this thing up here,' he said.

'Oh, yes,' said his mother. 'That alone was a marvel.'

'Well, I hope it's worth the effort.' But, looking at the device now, he doubted it. In the glimmering shadows of the old Roman water tank Peter's machines had looked mysterious, potent, even magical. But here the slim copper tube and the brazier looked absurd, a toy beside the massive stone reality of the walls. 'Does the emir know about this?'

'You're his eyes and ears,' Subh murmured. 'When it all goes to plan, when Christian soldiers are scattered like wheat stalks in a storm, then you can tell him what we have done.'

Ibrahim looked at Peter. This mention of the slaughter of Christians didn't evoke any reaction in him. Obsessed with his machines and his ambitions, in the thrall of Subh, the man was quite without conscience, Ibrahim saw; he was a lost soul.

Peter nodded at the scholars. 'Let's get on with it.'

The two of them approached the thunder-mouth carrying a heavy pail of water between them. Ibrahim saw they were going to tip the water into a kind of funnel mounted over the brazier.

'All that water is coming out of somebody's ration,' Ibrahim said weakly.

'This will put an end to rationing,' Subh said.

Peter pointed. 'The water, poured in here, goes straight down into the hot barrel. It immediately flashes to steam. And steam, as you know, requires more space than water. The steam will roar up the tube and shoot the iron lump out over the walls, as a man spits out a pea, propelling it with his breath, spit it away and into the Christian lines. I am confident of the range. We have tested smaller models; the arithmetic is simple.'

'It will seem a miracle,' Subh said. 'The explosion of the steam, the roar of it as it gushes out of the thunder-mouth – and the iron ball itself, a mass heavier than a man, flying miles through the air. A mouth of thunder indeed.'

'But you haven't tested it,' Ibrahim said.

'Only smaller models. What else could we do, in the conditions of the siege?'

'And what better way to prove it,' Subh said silkily, 'than against live Christians?'

Peter stood straight. 'Do it,' he said to the scholars.

The cowering scholars tipped their great bucket. The water gurgled into the funnel, and through a length of copper pipe that fed it straight through the brazier's coals and into the cylinder. The thunder-mouth shuddered. And in that last heartbeat Ibrahim snatched his mother's arm and pulled her back, putting his body between her and the engine.

Ibrahim was slammed in the back as if by an immense hot fist, and he was thrown forward. An enormous noise crashed painfully into his head. Steam washed over him in a moment, scorching, gone.

He found himself sprawled over his mother. He pushed himself up. His back was tight and sore, burned. His mother seemed unharmed. Lying on the floor, looking up at him, her lips moved. But he could not hear a word she said.

In fact, he realised, he couldn't hear anything at all. He noticed blood trickling from his mother's ears and pooling in her throat. When he touched the sides of his own face, his fingers came away sticky with blood. He felt shocked. He had never heard such a noise, never in his life.

He stood up and turned around.

The thunder-mouth was destroyed, ripped open. The brazier was shattered, its hot coals scattered smoking on the platform. The two scholars lay on their backs, unmoving. He saw with wonder that misshapen bits of copper were embedded in the stone wall.

And Peter writhed on the floor. Blood pumped from a dozen wounds punched into body. His face was all but gone, Ibrahim saw, horrified, the skin scorched away, though some awful chance had left his eyeballs intact, staring terrified from lidless sockets.

The thunder-mouth tipped up silently. Ibrahim saw the tube nod down over the battlements, and an iron ball rolled harmlessly out to fall straight down the wall to the ground below.