Merthin was more surprised at Godwyn. How could an intelligent and educated man fail to see through Elfric? Perhaps it was less obvious to the person who was the object of the compliments.

Godwyn opened a small door and led the way up a narrow spiral staircase concealed in the wall. Merthin felt excited. He loved to enter the hidden passageways of the cathedral. He was also curious about the dramatic collapse, and eager to figure out its cause.

The aisles were single-storey structures that stuck out either side of the main body of the church. They had rib-vaulted stone ceilings. Above the vault, a lean-to roof rose from the outer edge of the aisle up to the base of the clerestory. Under that sloping roof was a triangular void, its floor the hidden side, or extrados, of the aisle’s vaulted ceiling. The four men climbed into this void to look at the damage from above.

It was lit by window openings into the interior of the church, and Thomas had had the foresight to bring an oil lamp. The first thing Merthin noticed was that the vaults, viewed from above, were not exactly the same in each bay. The easternmost formed a slightly flatter curve than its neighbour, and the next one – partly destroyed – looked as though it was different again.

They walked along the extrados, staying close to the edge where the vault was strongest, until they were as near as they dared go to the collapsed portion. The vault was constructed in the same way as the rest of the church, of stones mortared together, except that ceiling stones were very thin and light. The vault was almost vertical at its springing but, as it rose, it leaned inwards, until it met the stonework coming up from the opposite edge.

Elfric said: “Well, the first thing to do is obviously to rebuild the vaulting over the first two bays of the aisle.”

Thomas said: “It’s a long time since anyone in Kingsbridge built rib-vaulting.” He turned to Merthin. “Could you make the formwork?”

Merthin knew what he meant. At the edge of the vault, where the masonry was almost upright, the stones would stay in place by their own weight; but, higher up, as the curve turned towards the horizontal, some support was needed to keep everything in place while the mortar dried. The obvious method was to make a wooden frame, called formwork or centering, and lay the stones on top of that.

It was a challenging job for a carpenter, for the curves had to be just right. Thomas knew the quality of Merthin’s craftsmanship, having closely supervised the work Merthin and Elfric carried out at the cathedral over several years. However, it was tactless of Thomas to address the apprentice rather than the boss, and Elfric reacted quickly. “Under my supervision he can do it, yes,” he said.

“I can make the formwork,” Merthin said, already thinking about how the frame would be supported by the scaffolding, and the platform on which the masons would have to stand. “But these vaults were not built with formwork.”

“Don’t talk nonsense, boy,” Elfric said. “Of course they were. You know nothing about it.”

Merthin knew it was unwise to argue with his employer. On the other hand, in six months he would be competing with Elfric for work, and he needed people such as Brother Godwyn to believe in his competence. Also, he was stung by the scorn in Elfric’s voice, and he felt an irresistible desire to prove his master wrong. “Look at the extrados,” he said indignantly. “Having finished one bay, surely the masons would have re-used the same formwork for the next. In which case, all the vaults would have the same curve. But, in fact, they’re all different.”

“Obviously they didn’t re-use their formwork,” Elfric said irritably.

“Why wouldn’t they?” Merthin persisted. “They must have wanted to save on timber, not to mention the wages of skilled carpenters.”

“Anyway, it’s not possible to build vaulting without formwork.”

“Yes, it is,” Merthin said. “There’s a method-”

“That’s enough,” Elfric said. “You’re here to learn, not teach.”

Godwyn put in: “Just a minute, Elfric. If the boy is right, it could save the priory a lot of money.” He looked at Merthin. “What were you going to say?”

Merthin was half wishing he had not raised this subject. There would be hell to pay later. But he was committed now. If he backed off, they would think he did not know what he was talking about. “It’s described in a book in the monastery library, and it’s very simple,” he said. “As each stone is laid, a rope is draped over it. One end of the rope is tied to the wall, the other weighted with a lump of wood. The rope forms a right angle over the edge of the stone, and keeps it from slipping off its bed of mortar and falling to the ground.”

There was a moment of silence as they all concentrated, trying to visualize the arrangements. Then Thomas nodded. “It could work,” he said.

Elfric looked furious.

Godwyn was intrigued. “What book is this?”

“It’s called Timothy’s Book,” Merthin told him.

“I know of it, but I’ve never studied it. Obviously I should.” Godwyn addressed the others. “Have we seen enough?”

Elfric and Thomas nodded. As the four men left the roof space, Elfric muttered to Merthin: “Do you realize you’ve just talked yourself out of several weeks’ work? You won’t do that when you’re your own master, I’ll bet.”

Merthin had not thought of that. Elfric was right: by proving that formwork was unnecessary, he had also done himself out of a job. But there was something badly wrong with Elfric’s way of thinking. It was unfair to allow someone to spend money unnecessarily, just to keep yourself in work. Merthin did not want to live by cheating people.

They went down the spiral staircase into the chancel. Elfric said to Godwyn: “I’ll come to you tomorrow with a price for the work.”

“Good.”

Elfric turned to Merthin. “You stay here and count the stones in an aisle vault. Bring me the answer at home.”

“Yes.”

Elfric and Godwyn left, but Thomas lingered. “I got you into trouble,” he said.

“You were trying to boost me.”

The monk shrugged, and made a what-can-you-do gesture with his right arm. His left arm had been amputated at the elbow ten years ago, after infection set in to the wound he received in the fight Merthin had witnessed.

Merthin hardly ever thought about that strange scene in the forest – he had become used to Thomas in a monk’s robe – but he recalled it now: the men-at-arms, the children hiding in the bush, the bow and arrow, the buried letter. Thomas was always kind to him, and he guessed it was because of what happened that day. “I’ve never told anyone about that letter,” he said quietly.

“I know,” Thomas replied. “If you had, you’d be dead.”

Most large towns were run by a guild merchant, an organization of the leading citizens. Under the guild merchant were numerous craft guilds, each dedicated to a particular trade: masons, carpenters, leather tanners, weavers, tailors. Then there were the parish guilds, small groups centred on local churches, formed to raise money for priestly robes and sacred ornaments, and for the support of widows and orphans.

Cathedral towns were different. Kingsbridge, like St Albans and Bury St Edmunds, was ruled by the monastery, which owned almost all the land in and around the town. The priors had always refused permission for a guild merchant. However, the most important craftsmen and traders belonged to the parish guild of St Adolphus. No doubt this had started out, in the distant past, as a pious group that raised money for the cathedral, but it was now the most important organization in town. It made rules for the conduct of business, and elected an alderman and six wardens to enforce them. In the guild hall were kept the measures that standardized the weight of a woolsack, the width of a bolt of cloth, and the volume of a bushel for all Kingsbridge trade. Nevertheless, the merchants could not hold courts and dispense justice the way they did in borough towns – the Kingsbridge prior retained those powers for himself.