James's shoulder units had to be tested. These were elaborate arrangements of rods, shafts, gears, pulleys and ropes at either side of the muscle frame that would translate the crossbow's unwinding into complex movements, up and down, twisting. The shoulders, carefully oiled, worked flawlessly. The novices added a vertical 'tail' of wood and feathers fixed on a strut to the back of the central frame.

And now came the wings, each borne on the backs of more sweating novices. They climbed stepladders to either side of James, and pushed the wings' joints into their attachments in the shoulder units and strapped them into place with more leather belts. The wings, spread, were like tents made of young fir, fustian, starched taffeta and feathers, and the morning light shadowed their internal skeletons.

The supervising friar ordered one last test of the mechanism. With utmost caution the wings, still supported by the novices, were lifted and lowered, twisting as they did so, and the feathers spread and closed, each pasted by hand to its own tiny cog wheel. The flight of birds had been carefully studied by Bacon and his followers for two hundred years. It was clear that the air was a fluid through which birds swam, as fish and seals swam through water. This elaborate machinery had been designed, after generations of paper designs, model-making and trial and error, to copy exactly that flapping, swimming motion.

But at this moment the theory, the mechanics, didn't matter at all to James, compared to the sheer beauty of the engine above him. He was thrilled that he had used his master's seniority to become the soul of this fantastic creation.

The supervising friar, a blunt practical man with wild grey hair around his tonsure, now faced James. 'Ready?'

James grinned and nodded.

The friar yanked at a rope. The crossbow was unlatched, and flexed, and immediately its elastic energy was poured through gears and pulleys into the shoulders. The wings flapped and twisted, James's harness tugged at his chest, and he was dragged up into the air.

The ground fell away, and the novices' upturned faces were like coins on a table. They were clapping and cheering. The landscape opened up beneath him, and he saw the shape of the limestone ridge from which he had launched, and the plain before it, and the ruined village where engines crawled and gunpowder flashed.

His heart raced with excitement, and a bit of fear, and he felt his loins tense up. He had admitted to nobody, not even his confessor, the extraordinary erotic thrill this hurling into the air gave him. If he could never have a woman, at least he had this. And as the air washed around him the face of Grace Bigod swam into his mind, elegant, cold, sneering.

Already the crossbow was running down, but it had lifted James high enough for his purpose. He had to work quickly. He pulled strings to latch the crossbow, and others to lock the wings, outstretched with the feathers closed and banked. Then, with a grunting effort, he leaned forward, and the leather cradle into which he was strapped pivoted, so that he was suspended beneath the wings, belly down.

He glided forward, wings rigid as a coasting seagull's. He was falling, of course, falling like a dead leaf. But he should reach the battlefield, and that was enough for him to do his job.

He looked down at the ruined village. More hapless novices were defending a 'fortress', crudely constructed of heaped-up stone from the village. They were equipped with weapons of a conventional sort, crossbows, longbows, arquebuses and cannon, and had even had some rudimentary training in using them.

But huge machines crawled relentlessly towards the village, spitting fire. James made out the gun carriage nicknamed the 'organ-pipes'. Between two massive wheels was suspended an axle with a triangular cross-section. On each of the axle's three faces had been fixed a dozen cannon, in a row like the pipes of a church organ. These fired together, lit simultaneously by spring-loaded flints. Then, dragged by unhappy mules, the engine trundled forward until the axle turned and another bank of cannon was brought into play.

Other engines displayed different solutions to the challenge of multiplying fire. Here was a great wheel set horizontally, mounted with cannon spread radially like the petals of a daisy, which turned and spat fire when each gun was brought to the right position.

And now, shaking itself to life with a series of crashing explosions, here came the most spectacular machine of all. It was the testudo, named by the brothers after the famous formation of the Roman legions. It was a great shell of steel, immensely heavy, tough enough to withstand a direct hit from any of the defenders' petty weapons. Cannon fired from ports in all the forward angles, and as it advanced it shook and shuddered, smoke billowing from its ports from the internal detonations that drove it forward. It was unstoppable, inhuman. The testudo simply crushed a crumbling stone wall beneath its great hidden wheels, and the defenders, their shot and bolts and arrows simply bouncing off the mighty shell, fled, probably terrified out of their wits by the noise alone.

From James's elevated vantage he could smell nothing of the battle; he could hear little save the distant crump of explosions and the shouting of men like birds' cries, sounds drowned by the hiss of the wind in his wings, and his own rapid breathing. It was like watching a battle played out with toys, he thought – distant and abstract enough to quash his own conscience, and to allow him to savour the exhilaration of his extraordinary flight over these engines of carnage.

Now it was time for James himself to deliver the finishing blow. He tugged on guide ropes so that his wooden bird swooped to a line that led straight to the mock fortress. He glanced to his left to the observers' canvas pavilion, to see if Bartolomeo Colon and the rest were watching him. He saw Grace in a bright purple gown. He grinned, and the wind was cold on his teeth, and his heart beat even faster.

He banked over the fortress. As his huge shadow crossed the village some of the defenders ran, their superstitious fear overwhelming them, though they knew it was James. Now he pulled at the leather tags at his belt, one, two, three, four. The metal eggs were released and fell straight down, their bird-like shapes cutting through the air, the fins at their backs stabilising their fall.

All four landed in the heart of the fortress, splashing fire as they hit. The noise of the explosions hit him, and a sudden updraught of hot air pushed him higher. He whooped with an unreasonable joy. James was a man of peace, but he was young enough to relish the sheer exhilaration of such a complicated and dangerous game.

And as the fires bloomed he thought of Grace, Grace pushed down before him, Grace begging for his forgiveness – begging him to stop.

He looked over at the pavilion. Grace and the others were standing and pointing – not towards James and the fortress, but east. James craned his neck to see that way.

Something was wrong.