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'And how do you load a crossbow?'

Orm grunted. 'Depends. The old-fashioned sort, you bend over, put your foot on a stirrup, catch the bowstring in a hook on your belt, and straighten up until you've got the string in the lock. The newer sort you have a little hand-crank to draw back the string. You put your bolt in the groove, and press a lever to release it.'

'The principles are just the same here. Look at this.' Long metal screws had been built into the body of the stock. 'These are used to draw back the string. It isn't hard; a single man can turn that wheel, down there. Or you can use a mule. And look, see how the carriage wheels tip outward? That's to give the base more stability. Here's a tilting platform so you can raise the bow, and aim the flight of the bolt. And here, you see, anchors lock the crossbow to the ground and reduce recoil.'

'And have you fired such a thing?'

'Only in tests. We're still refining the design. How well do your hand bows perform?'

Orm shrugged. 'A range of two or three hundred paces. You can pierce chain mail.'

Sihtric grinned. 'This beauty should have a range of miles. And it will pierce masonry. Thus, one of Aethelmaer's designs, all but realised – all but ready to be deployed in war. Tell me you're not impressed.'

Orm pursed his lips, and walked around the machine once more. 'Yes. I'm impressed by what you've built. But imagine this in war. It would take a long time to load, longer to haul it around the countryside to aim it – and it could be destroyed by a single burning arrow.'

Sihtric sighed. 'All right. But what if I told you that instead of just knocking down a bit of wall, my arbalest could deliver a bolt capable of laying waste an entire fort, even a city? A single bolt! What then?'

Orm grimaced. 'That sounds a fever dream, and an ugly one.'

'But Aethelmaer had such dreams, or Aethelred did. I can show them to you, sketched in the notes. Dreams of a super-weapon – Aethelmaer called it Incendium Dei, the Conflagration of God. Perhaps it is something like Greek fire – I don't know. But the only clues he left for it are encrypted, and it remains beyond the capabilities even of the scholars of al-Andalus to decipher. Later I'll show you the Codex itself – much study remains to be done on it. But first, come. I'll show you how we work here.'

Leading their horses, they walked away from the mighty arbalest and through the open-air workshop. Orm glanced with interest at the tools of the carpenters and metal-workers. He had ordered several swords in the course of a long fighting life, and had come to appreciate the metal-workers' art; casting the immense screws of the arbalest must have set them significant challenges.

On some of the tables models were set out, intricate wooden toys that looked like birds or beetles or fish. They were models made from designs even more astonishing than the great arbalest, Orm saw, engines that flew and swam and crawled. Some of them were sliced open so you could see the wooden skeleton within, and the bodies of tiny men working oars or hauling on wheels. The boy trapped in Orm's battered warrior's body longed to hold these gadgets, to play with them.

In one of the tents a wooden floor had been set down, a few paces across. Its surface was incised into rows, along which stones the size of fists painted black or white were lined up. Two scholars argued in rapid Arabic over a parchment. In response to their commands a boy jumped about over the board, moving stones from one row to the next. Occasionally he apparently made a mistake, and his reward was a volley of abuse, but when he got it right the scholars forgot the boy and argued over the patterns he conjured.

'So,' Sihtric said. 'Any idea what this is, Orm?'

Orm shrugged. 'Some kind of game?'

Sihtric snorted. 'This is deadly serious. The scholars are working out the trajectories of an arbalest bolt. We are developing an aiming system, you see. And the boy with his counters on the board is figuring the numbers for the scholars as they call out the sums.'

Orm frowned. 'I don't see any numbers.'

'But they are here nonetheless, represented by the beads in their columns. This is called an abacus, Orm. It's a counting system. You can add, subtract. You can even multiply numbers together with ease.'

Orm scoffed. 'Everybody knows you can't figure numbers beyond nine hundred.'

'Using this, you can go as high as you like. With such gadgets a ten-year-old Moorish child can count better than the King of England. I'm not surprised you haven't heard of this, or of Arabic mathematics in general. Mark my words, one day everybody in Europe will be counting this way.'

'Turning prophet again, Sihtric? Well, we won't be around to see it, one way or another.'

'True. But it's this sort of learning I came here to discover, and to exploit. Ah, here we are. My copy of Aethelred's original sketches.' It was a well-thumbed compendium of parchments – a document Orm hadn't seen for twenty years, since the day he had met Aethelmaer in Westminster. 'The Engines of God…'

XIII

At Ghalib's mocking call Robert turned away from Moraima and looked towards the river. Hisham was standing on a wall along the bank.

And Ghalib had somehow climbed up onto a waterwheel. As it turned, he was climbing up from one spoke to the next, as if clambering over a treadmill. He was soaked to the skin, his red turban bright, and he was laughing. 'Hey, Moraima – hey, God's warrior! Look at me, look at me!'

Moraima laughed, but she clamped her hand over her mouth. 'Allah preserve him. He'll get himself killed.'

Robert strode towards the waterwheel, pushing through a gathering crowd of laughing onlookers. 'Get down off there, you idiot!'

Hisham threw a mock punch at Robert. 'You're just jealous because Moraima's looking at him, not you.'

Robert glared. 'Unless you shut up she will be looking at you when I push your teeth down your throat.'

Hisham returned the challenge for one heartbeat, then backed off.

'Hey, Christian.' Ghalib was calling again. 'Watch this.' Now he was heading for the wheel's mighty axle. He was spun around the hub, turning head-over-heels with each revolution. The wood was soaked by spray and was slippery.

Moraima ran forward. 'Get down! Oh, get down, you fool!'

Ghalib grabbed a strut with one hand, then threw himself backwards, flinging out the other hand, so he was splayed out over the hub, turning over and over on the wheel. 'Hey, look at me! I'm crucified! I'm Jesus on the cross!'

He actually got a laugh from the onlookers, and a smattering of applause. Hisham played up. He pulled his shirt over his head, and wailed in a loud, high voice. 'And I'm His mother the Virgin! Oh, my son, my only son, what have those awful Romans done to you?'

On the wheel, Ghalib kept grinning, but his expression was forced, and Robert saw he was tiring.

Then his right hand slipped from the wood. He dangled from his left arm, and flailed, trying to get a fresh hold with his right hand. But the wheel turned remorselessly, and he flipped over, and his left hand started to slip too. He tried desperately to grab onto something, anything.

And he pushed his right arm inside the wheel, into the machinery. Robert heard a distant crunch, like an owl chewing a mouse's bone. Ghalib didn't even scream. He fell down the face of the wheel, his right arm dangling like a blood-soaked rag, and splashed into the water.

The watching people just stared. The wheel turned as if Ghalib had never existed.

Robert ran over the cobbles and climbed onto the bank wall beside Hisham. There was barely any room between the wall and the turning wheel. But there was Ghalib, floating in the water, apparently unconscious. The water all around him was stained red. And soon he would be drawn into the wheel's machinery again.

Moraima grabbed Robert's arm. 'You must help him.'

Cursing, he kicked off his boots for the second time that day. Then he jumped, feet first, his arms tucked in at his sides, and plummeted down into the water.