‘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.’

XXVII

XXVII

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.’