With a last reluctant stare at the wreckage lying deep in the water—was that a gleam of copper he could see deep down?—Shef turned away. The mystery would wait. So would Hund. So would Svandis, who had said nothing to him since her bitter words on the quayside, was now sitting with one of the desperately hurt Greek sailors who had survived both fire and water. There were more urgent things to do. The enemy had tried a probing raid at the south gate already this morning, as if to show they were not dismayed. Time to anticipate them for a change.
On the deck of the Fafnisbane they were already making ready to stream a kite in the strong morning breeze. Tolman was already being fitted into his harness. Shef patted him encouragingly on the head which was all that could be seen sticking out of the sling—why did he seem to shrink away? Still sore from his fall, probably. Then he looked again at the rig. The top surface was seven feet wide and four feet long, each side four feet long again and three feet deep. Now, how much cloth was that? Think of it as so many squares a foot each way. Turning to the sand tray which two men now carried round behind him ready for instant use, Shef began to move the counters and write the signs, muttering to himself. All round, eighty square feet of cloth. And Tolman, he knew, weighed sixty-eight pounds. He himself weighed one hundred and eighty-five. If, then, you needed a foot of surface to lift a pound of weight…
“The wind makes a difference,” Hagbarth cut in on his mutterings. “The stronger the wind, the better the lift.”
“We can figure that in too,” Shef replied. “See what have we now…”
“Fresh breeze, enough to drive us at four knots under full sail.”
“Call that four then. And how many knots would we be going when you had to reduce sail?”
“Maybe ten.”
“Well that’s ten. So if we multiply eighty by four we have a lift of… three hundred and twenty, but if we had a ten-knot breeze we would have a lift of… eight hundred.”
“Enough to lift a walrus,” countered Hagbarth skeptically. “Which it wouldn’t.”
“All right, we’re allowing too much for the wind, but if you call a four-knot breeze one, and a ten-knot breeze two, or even one-and-a-half…”
Shef spoke eagerly, fascinated by the new experience of exact calculation. Ever since Solomon had shown him the basis of algorism, the methods of al-Khwarizmi, he had been seeking problems to turn it on. The answers might be wrong. To begin with. But he was sure this was the tool he had been searching for half his life.
A voice broke in, disapproving. Cwicca’s. “He’s up now. If you’d like to watch, that is. He is risking his neck.” Cwicca growing sarcastic and Hund aggressive, Shef noted with a part of his attention. Time to think of that another day.
Tolman was high in the sky, not flying free but streaming on the end of his line, already two hundred, three hundred feet up, blown out across the harbor, higher than the topmost turret of the walled city. Like a watchtower in the sky. From the besiegers on the hillside someone shot an arrow, which looped hopelessly short. Shef licked his lips as he stared up at him. Now he had the basis of calculation, he was determined to make a kite with enough surface to lift him too. If it were a one-to-one relationship, he would need a kite maybe twelve foot by six by four. But how much did the kite itself weigh? There was an answer to everything.
Cwicca turned back from his place by the ropes. “He’s pulled three times. He’s seen something. And he’s pointing north.”
North. And the probing attack from the south. “Winch him in,” Shef ordered. He had no doubt that this was the Emperor’s famous machine approaching. The “War-Wolf.”
The road which snaked along the coast towards the north wall of Septimania made its final turn some two hundred double paces short of the massive wooden gate, normally open for the passage of trade, now closed and barred, the towers either side of it bristling with breast-bows and crossbows, traction catapults and torsion dart-throwers. None of that could save the defenders once their gate was battered down. And reinforced oak though it was, criss-crossed with iron, one blow from “War-Wolf” would destroy it. This Erkenbert knew.