'It's all gone now. There was upheaval at court, a fire in the Forbidden City, a lot of bad omens – the eunuch admirals were retired, the ships broken up. The mandarins at court adhere to the principles of tao – order, stability, harmony of all things. That sort of thinking doesn't sit well with the exploration of the unknown. I suppose in the end the Chinese decided China is world enough for them.

'But in the heyday of these tremendous ships, only a few decades ago, the Chinese ventured far over the oceans – around India, as far as the coast of Africa, and to the south-east, where they discovered huge masses of land and strange peoples unknown to Europeans.

'Listen to me. I once met a man who had worked in the Forbidden City. In a zoo there, he said, the eunuch explorers had brought back specimens from a dry southern land. There were strange skinny people with black skin and flat noses and curly hair. There were trees that kept their leaves and shed their bark. There were huge creatures with faces like a deer's and back legs like huge levers, that carried their young around in a flap of skin on their belly…'

Harry smiled. 'I too have heard such tavern tales.'

'All right, all right. I'll tell you this. The Chinese learned far more about the shape of the world than any European, thanks to us Moors, and thanks to their own expertise.

'Do you know any navigation, cousin? To fix your position on the round earth you need to know two numbers, your latitude and longitude. Latitude tells you how far north you are of the equator. That's easy. You just look for how high the Pole Star is; the higher in the sky, the further north you must have sailed, until it would be over your head if you sailed all the way to the north pole itself.

'Longitude, the angular distance travelled east or west, is trickier, for the sky itself spins about the earth. The Chinese developed a method using eclipses of the moon. Such events are visible all across the world. A legion of astronomers scattered across the world, all studying the elevation of the stars at that precise moment, would be able to map the earth's curve-'

Harry held up his hands. 'Enough. I'm better at figuring accounts than the geometry of the stars.'

'My point is that the Chinese know how big the world is. And they would tell you that it would be a long journey if you were to try to sail west from Lisbon, say, to China. But on the other hand,' Abdul said thoughtfully, 'that big Ocean Sea has plenty of room for an unknown continent or two. The Chinese never sailed far enough to find out.'

Geoffrey thought this through. 'Then you're saying,' he said carefully, 'that the prophecy of the Dove, the invasion of Europe by people from the west, could have a basis in truth.'

'I'm saying it's not impossible.' Abdul looked at the two of them. 'All this will take years to come to fruition, one way or another. The monarchs have other matters to deal with before they fund Ocean crossings. And the Engines of God need development before they kill anyone save by accident. We have time yet to deflect history's course.'

Harry's heart sank at that thought. 'So we can't be rid of this any time soon.'

'Not yet,' said Geoffrey grimly. 'Be patient.'

The Derbyshire country under its lid of low cloud was a dark green mouth, damp and enclosing, and the abandoned village was a field of worn-down hummocks. Though it was not long after noon, the light already seemed to be fading. As he followed James and Grace into the village, Friar Diego Ferron, tall, thin, almost spectral, held up the hem of his expensive robe, as if trying to avoid any contact with the English mud.

James couldn't help but see the murky, unsatisfactory English December day through Ferron's eyes. A greater contrast to the dry brilliance of southern Spain was hard to imagine. After all they were here to impress another man from the Mediterranean, Bartolomeo Colon, the brother of navigator Cristobal. Bartolomeo had come to England to seek support for Cristobal's adventure from King Henry, for after three years of fruitlessly pestering the Spanish monarchs Cristobal was casting his net wider. Grace and Ferron had seized the chance to impress one of the Colons with a demonstration of their Engines of God. If Ferron was instantly put off by the English weather, would Bartolomeo be too?

But then Diego Ferron was a uniquely unpleasant man, James told himself. Though they had worked together for seven years now on the continuing development of the Engines of God and on following the progress of Cristobal Colon, Ferron's stern, cruel piety appealed to James no more now than it ever had.

So James was spitefully glad when a hatch in the ground opened up under Ferron's feet, and the friar jumped back.

Grace said quickly, 'There's no need for alarm. Prepare to be impressed, brother. James?'

James led Grace and Ferron down muddy steps into a dark hall in the earth, leading off into the dark. Lamps burned in alcoves on the walls, and a greyer light diffused into the corridor from air vents.

A wagon was waiting at the bottom of the stair. A squat platform, it had a large crossbow-like mechanism mounted on its upper surface, and a fifth wheel attached to a rudder on a pivot at the back. With no horse or bullock in sight, there seemed no way it could be moved. James guided Grace and a bewildered Ferron to sit on two leather seats at the front of the vehicle. He himself took the rear seat, took hold of the rudder, and unclipped a latch on the crossbow.

The wagon moved off down the corridor, smoothly and silently. Ferron sat bolt upright, his large delicate hands white as they gripped the edge of his seat.

James, enjoying the moment, said nothing of the wagon, but described the background to work that had progressed in utter secrecy for more than two centuries since the time of Roger Bacon. 'We are working in a continuing tradition. In ancient times, thinkers like Archimedes applied their intellect to the design of weapons and defences. In more recent decades engineers like Taccola, Buonaccorso Ghiberti and Francesco di Giorgio Martini have developed military treatises. And we have had some fruitful correspondence with an artist and philosopher called Leonardo da Vinci, who is developing war engines for the Duke of Milan. But our engines are rather more advanced than his – of course we have had some centuries' start…'

Ferron had said nothing since the wagon began to move. Now he spoke at last, his voice tight. 'This cart of yours.'

'Yes?'

'It has no horses. No bullock. No slaves to pull it.'

'Of course not.'

'Yet it moves. What witchcraft is this?'

James grinned behind Ferron's back. 'No witchcraft. It propels itself. This mechanism – you see, it is rather like a crossbow – when wound back stores energy which, if released, is transmitted to gears that drive the wheels.

'Most of our designs are based on five simple machines studied since antiquity: I mean the winch, the lever, the pulley, the wedge and the screw. As to energy sources we use weights, heat – I mean trapped steam – human and animal muscle, wind or water power, and spring energy, as on this wagon. That, and Bacon's black powder. The principle of the wagon is simple. The engineering challenge was in designing differential gears so the wheels can move independently…'

Grace leaned back. 'Enough,' she whispered to James. 'We're here to impress the man, not to terrify him.'

James nodded. But he couldn't be bothered to suppress his grin. Thirty-three years old, he felt confident and in command – and he felt like taking a little petty revenge on these rather monstrous figures who had dominated his life.