Shef nodded. “I know. I am wasting my time. I understand now how it can be done, but there are all kinds of things I do not understand. If I had a real weight here, like the one they really shot, then what kind of weight would I need to put in the other arm? It would be far greater than a dozen men could lift. And if it was as heavy as that, how could I wind down the long arm, the shooting arm? It would need some sort of a windlass. But I know now what the sound was that I heard just before the rock came over. It was the sound of someone cutting the rope, to release the rock.

“And there is another thing that bothers me even more. They shot one rock—that smashed the ram. If they had not hit with that one shot, the gate would have been down and all the machine-masters would be dead. They must have been very sure they could hit with the first shot.”

He swept suddenly at the lines he had been drawing in the dirt. “It is a waste of time. Do you see what I mean, Thorvin? There must be some sort of skill, some sort of craft, which would tell men where it would go without me having to try again and again. When I first saw the stockade round your camp by the Stour, I was amazed. I thought, how do the leaders know how many logs to bring with them to build a stockade that will hold all their men? But now I know how even the Ragnarssons do it. They notch a stick for each ship, ten notches to a stick, and then they throw the sticks down in turn in separate piles, one pile for each one of the three walls, or the four walls, or however many there are, and when there are no more sticks they pick the piles up and count them. And that is the reckoning of the greatest leaders and captains in the world. A pile of sticks. But what they have over there in the city is the knowledge of the Rome-folk, who could write in numbers as easily as they could write in letters. If I could learn to write in Roman numbers, then I would build a machine!”

Thorvin laid down the tongs and looked thoughtfully at the silver hammer displayed on his chest.

“You should not think the Rome-folk had the answers to everything,” he remarked. “If they had they would still be ruling England from York. And they were only Christians, when all is said and done.”

Shef jumped impatiently to his feet. “Hah! How do you explain the other instrument then? The one that shot the great arrow. I have thought and thought about that. Nothing will do. You could not make a bow big enough. The wood would break. But what can shoot except a bow?”