‘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.

The wagon slowed. James latched the spring drive, applied the brakes to the rear wheel, and the wagon came to a slightly juddering halt. Without much dignity Ferron scrambled off his seat to the dirt floor.

They passed through an arched doorway and walked down further steps to emerge into a large chamber, walled with rough stone blocks and lit up by more torches and oil lamps. It was a cave, but a vast one; from its cathedral-like roof stalactites dangled like icicles.

And in the shadows obscure engines loomed, their metal flanks gleaming with oil. Monks scurried around the machines. There was a low hum of conversation, the clank of hammers on metal - and a shriek of released steam, which made them all jump.

A shadow like a bat’s rattled across the roof, and settled into a corner.

‘Welcome,’ James said, ‘to our manufactory.’

‘This is a cave,’ Ferron said, wondering.

‘Oh, yes,’ Grace said. ‘This shire is riddled with them. Limestone country, you see. And up above there’s nobody around for miles; the country has yet to recover from the Great Mortality. In fact we moved here after the plague, decades ago; already nearly a century had passed since Bacon’s first instructions, and we needed the room. In time the brothers have spread out through a whole complex of these caverns, quarrying out tunnels and passageways as they went. Like moles with tonsures!’ She seemed to find the notion comical. ‘An ideal place for work like this - heavy, noisy - if you want to keep it secret.’

They walked towards the machines. Ferron asked, ‘Secret? From whom?’