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?'

Grace shrugged. 'The seventh King Henry isn't long on his throne. These brothers haven't toiled for centuries to put bombards in the hands of one pretender to the English crown or another.'

Ferron nodded. 'We have a higher purpose than the ambitions of kings, we are waging a war which transcends all others. You have chosen the right course – you and your forebears, for centuries.' He had recovered his composure, James noticed, amused.

Now they were walking among the engines. They passed wheeled platforms, and huge hulls like steel houses, and blunt cannons whose mouths gaped, and more exotic forms yet, complicated masses of machinery with no clear purpose. On one bench lay a huge skeletal wing, twice the length of a man's body. There was a stink of oil and hot metal, the air was dense with steam, and the labouring monks, wide-eyed in the gloom, scurried out of their way.

James said, 'It might seem simple to translate given designs into actuality. In fact much of our work is at a more basic level, as we learn to make the components required by our engines. Steel hard enough to make screws and gears that will not shear was a particular challenge. Advanced cannons need just the right casting, loading, lighting and cooling if we are to increase their capacities and speed of fire.'

Ferron confessed, 'I understand little of what I am seeing.'

'The engineering detail doesn't matter,' Grace said. 'Our purpose is only to show you the scope of the work here. The practical demonstration aboveground later will show you all you need to know.'

Ferron smiled thinly. 'A demonstration? I'll look forward to it. And all of this comes from the fevered brow of this Roger Bacon?'

'He worked from the designs and recipes in the Codex of Aethelmaer, returned from its hiding place in Seville…'

Bacon had quickly abandoned his Aristotelian studies and had thrown himself into experimental, secretive research. He had recruited students and assistants, and had discreetly sounded out like-minded savants across Europe. He appointed a Picard called Peter de Maricourt as his domum experimentorum, and it was de Maricourt who had set out the design for the first laboratory-manufactory.

The work progressed quickly. But as he aged, Bacon himself became more difficult. He had always been a man who craved attention and recognition. He campaigned for the acquisition of more experimental knowledge about the natural world, and began to compile a vast encyclopaedia of all the known sciences. But he made plenty of enemies by expressing his strong contempt for those who did not share his passions, and he was severely disciplined by superiors who thought he was out of control. Bacon, ever grandiose, appealed over their heads, even direct to Pope Clement. The death of Clement ended his ambitions, and his career.

His superiors excluded him from the manufactory, and in the end actually imprisoned him for his indiscipline and suspected heresies; his whirling mind was confined to a cell for thirteen years. When released, he was exhausted; his final works remained incomplete.

Ferron listened to this soberly. 'But after Bacon, for two hundred years, underground, all invisible, the monks of his great manufactory have toiled at weapons of war. Yes? Quite remarkable. You know, I am told Colon has used Bacon's writings in trying to construct his own case for the monarchs. In his Opus Majus Bacon surveys geographical understanding, and argues, for example, against the existence of a Torrid Zone below the equator.'

Grace said, 'Perhaps we can use that to persuade Colon and his brother to accept these, the fruits of Bacon's genius.'

'It's possible.'

They moved through a low passage into another, smaller chamber. Here only enclosed oil lamps burned, and the murky air stank of dung and piss. Ferron recoiled, and with an impatient snap Grace summoned forward a novice, who presented each of them with a scented napkin to hold over their noses.

Here gunpowder was manufactured, according to Bacon's carefully researched recipes. It was kept separate for safety, and for the foul air; the brothers assigned to this work didn't last long.

James said, 'We mix the ingredients with mortars and pestles, or with these wooden stamps.' He showed Ferron a clunky device, all levers of iron and mallets of wood. 'We need to combine the powder into granules of varying sizes, depending on the application. Granule size determines burning rate; you don't want your powder to burn so rapidly it shatters your bombard's casing. So we mix up the powder with a binding agent. Sometimes it's water and wine, but in fact urine is best.'

'As I can smell,' Ferron said drily. 'And the ingredients?'

'The best charcoal is free of knots, and made of coppiced wood – hazel or ash, gathered in the spring and so full of sap. We import our sulphur from the volcanoes of Iceland, the purest in the world. The saltpetre is more difficult, and needs manufacture.' He showed Ferron a series of vats, from which a murky water was poured one to the next. 'Saltpetre is made from dung.'

The monks filled a pit with layers of quicklime, cow manure, wood ash and vegetable waste. They turned this regularly, moistening its surface. It was important that the matter was not allowed to get too wet, or too dry. After some months of this a whitish efflorescence would appear on the surface of the heap, which the monks scraped off and collected. The powder was dissolved in water, which was then passed through the series of vats.

'This is saltpetre,' James said. 'The Arabs call it "Chinese snow". The saltpetre stays dissolved where other salts precipitate out. It's an intricate technique, worked out over centuries by the Chinese among others-'

'The Arabs have such processes now. They've been firing cannon at Christians for a hundred years – more, I think.'

'Yes,' James said patiently, 'but thanks to the Codex, Bacon had saltpetre, and the recipe for black powder, decades earlier than otherwise. The scholars believe that as a result we have a lead of a century or more over the Arabs in the exploitation of these secrets. And that is how these engines will win the holy war.'

'It's quite an industry then,' Ferron said. 'All this material flowing into this dungeon, sulphur from the mountains of Iceland and manure from the farms of Derbyshire, and then the ingenuity of the burrowing monks here. I suppose it would be inappropriate if it did not take intelligence and effort to make this devilish dust, this gunpowder, that can slay so many men. But I wonder how its victims would feel if they knew the shot that killed them was propelled by an alchemy of dung?…'