Elfric went on: “I must tell guildsmen that I have already spoken to Prior Godwyn, who says the priory cannot afford to pay for this repair. We will have to finance it, as we financed the original cost of building the bridge, and be repaid out of penny tolls.”
They all groaned. There followed a long and bad-tempered discussion about how much money each member of the guild should put up. Merthin felt animosity building up towards him in the room. This was undoubtedly what Elfric had intended. Merthin kept looking at the door, willing Jeremiah to reappear.
Bill Watkin said: “Maybe Merthin should pay for the repairs, if it’s his design that’s at fault.”
Merthin could not stay out of the discussion any longer. He threw caution to the winds. “I agree,” he said.
There was a startled silence.
“If my design has caused the cracks, I’ll repair the bridge at my own expense,” he went on recklessly. Bridges were costly: if he was wrong about the problem, it could cost half his fortune.
Bill said: “Handsomely said, I’m sure.”
Merthin said: “But I have something to say, first, if guildsmen will permit.” He looked at Elfric.
Elfric hesitated, obviously trying to think of a reason for refusing; but Bill said: “Let him speak,” and there was a chorus of assent.
Elfric nodded reluctantly.
“Thank you,” said Merthin. “When an arch is weak, it cracks in a characteristic pattern. The stones at the top of the arch are pressed downwards, so that their lower edges splay apart, and a crack appears at the crown of the arch on the intrados – the underside.”
“That’s true,” said Bill Watkin. “I’ve seen that sort of crack many a time. It’s not usually fatal.”
Merthin went on: “This is not the type of cracking you’re seeing on the bridge. Contrary to what Elfric said, those arches are strong enough: the thickness of the arch is one twentieth of its diameter at the base, which is the standard proportion, in every country.”
The builders in the room nodded. They all knew that ratio.
“The crown is intact. However, there are horizontal cracks at the springing of the arch either side of the central pier.”
Bill spoke again. “You sometimes see that in a quadripartite vault.”
“Which this bridge is not,” Merthin pointed out. “The vaults are simple.”
“What’s causing it, then?”
“Elfric did not follow my original design.”
Elfric said: “I did!”
“I specified a pile of large, loose stones at both ends of the piers.”
“A pile of stones?” Elfric said mockingly. “And you say that’s what was going to keep your bridge upright?”
“Yes, I do,” Merthin said. He could tell that even the builders in the room agreed with Elfric’s scepticism. But they did not know about bridges, which were different from any other kind of building because they stood in water. “The piles of stones were an essential part of the design.”
“They were never in the drawings.”
“Would you like to show us my drawings, Elfric, to prove your point?”
“The tracing floor is long gone.”
“I did a drawing on parchment. It should be in the priory library.”
Elfric looked at Godwyn. At that moment the complicity between the two men was blatant, and Merthin hoped the rest of the guild could see it. Godwyn said: “Parchment is costly. That drawing was scraped and reused long ago.”
Merthin nodded as if he believed Godwyn. There was still no sign of Jeremiah. Merthin might have to win the argument without the help of the original plans. “The stones would have prevented the problem that is now causing the cracks,” he said.
Philemon put in: “You would say that, wouldn’t you? But why should we believe you? It’s just your word against Elfric’s.”
Merthin realized he would have to stick his neck out. All or nothing, he thought. “I will tell you what the problem is, and prove it to you, in daylight, if you will meet me at the riverside tomorrow at dawn.”
Elfric’s face showed that he wanted to refuse this challenge, but Bill Watkin said: “Fair enough! We’ll be there.”
“Bill, can you bring two sensible boys who are good swimmers and divers?”
“Easy.”
Elfric had lost control of the meeting, and Godwyn intervened, revealing himself as the puppetmaster. “What kind of a mockery are you planning?” he said angrily.
But it was too late. The others were curious now. “Let him make his point,” said Bill. “If it’s a mockery, we’ll all know soon enough.”
Just then, Jeremiah came in. Merthin was pleased to see that he was carrying a wooden frame with a large sheet of parchment stretched across it. Elfric stared at Jeremiah, shocked.
Godwyn looked pale and said: “Who gave you that?”
“A revealing question,” Merthin commented. “The lord prior doesn’t ask what the drawing shows, nor where it came from – he seems to know all that already. He just wonders who handed it over.”
Bill said: “Never mind all that. Show us the drawing, Jeremiah.”
Jeremiah stood in front of the scales and turned the frame round so that everyone could see the drawing. There at the ends of the piers were the piles of stones Merthin had spoken of.
Merthin stood up. “In the morning, I’ll explain how they work.”
Summer was turning into autumn, and it was chilly on the river bank at dawn. News had somehow got around that a drama would take place and, as well as the members of the parish guild, there were two or three hundred people waiting to see the clash between Merthin and Elfric. Even Caris was there. This was no longer merely an argument about an engineering problem, Merthin realized. He was the youngster challenging the authority of the old bull, and the herd understood that.
Bill Watkin produced two lads of twelve or thirteen, stripped to their undershorts and shivering. It turned out they were Mark Webber’s younger sons, Dennis and Noah. Dennis, the thirteen-year-old, was short and chunky, like his mother. He had red-brown hair the colour of leaves in autumn. Noah, the younger by two years, was taller, and would probably grow up to be as big as Mark. Merthin identified with the short redhead. He wondered whether Dennis was embarrassed, as Merthin himself had been at that age, to have a younger brother who was bigger and stronger.
Merthin thought Elfric might object to Mark’s sons being the divers, on the grounds that they might have been briefed in advance by their father and told what to say. However, Elfric said nothing. Mark was too transparently honest for anyone to suspect him of such duplicity, and perhaps Elfric realized that – or, more likely, Godwyn realized it.
Merthin told the boys what to do. “Swim out to the central pier, then dive. You’ll find the pier is smooth for a long way down. Then there’s the foundation, a great lump of stones held together with mortar. When you reach the river bed, feel underneath the foundation. You probably won’t be able to see anything – the water will be too muddy. But hold your breath for as long as you can and investigate thoroughly all around the base. Then come up to the surface and tell us exactly what you find.”
They both jumped into the water and swam out. Merthin spoke to the assembled townspeople. “The bed of this river is not rock but mud. The current swirls around the piers of a bridge and scours the mud out from underneath the pillars, leaving a depression filled only with water. This happened to the old wooden bridge. The oak piers were not resting on the river bed at all, but hanging from the superstructure. That’s why the bridge collapsed. To prevent the same thing happening to the new bridge, I specified piles of large rough stones around the feet of the piers. Such piles break up the current so that its action is haphazard and weak. However, the piles were not installed and so the piers have been undermined. They are no longer supporting the bridge, but hanging from it – and that’s why there are cracks where the pier joins the arch.”