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 a hall. 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?’