Now four novices approached, each bearing an iron egg. These were slim ovals, each the size of a sleeping pigeon, with sprawling tails. The novices trod warily, nervous, trying not to tremble. They hung the eggs from James’s belt, and he tested the leather tabs he had to pull to release them.

Next his engine had to be assembled around his body.

First came the ‘muscle’, as he thought of it. This was a box of canes several feet tall. It had complicated ‘shoulder’ mechanisms, and at its heart was a powerful crossbow as thick as his arm, already wound back. It had quickly been learned that a man’s muscles were too feeble to beat the great wings; but the crossbow would suffice. This frame was lowered onto his shoulders and strapped to his torso by a cradle of leather bands.

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.