“I've seen that,” he said. “In De Chelonian Mobile. There was a drawing.”
“Oh, it's the walking Library,” said Didactylos. “Yes. You're right. Illustrating the principle of reaction. I never asked Urn to build a big one. This is what comes of thinking with your hands.”
“I took it round the lighthouse one night last week,” said Urn. “No problems at all.”
“Ankh-Morpork is a lot further than that,” said Simony.
“Yes, it is five times further than the distance between Ephebe and Omnia,” said Brutha solemnly. “There was a scroll of maps,” he added.
Steam rose in scalding clouds from the whirring ball. Now he was closer, Brutha could see that half a dozen very short oars had been joined together in a star-shaped pattern behind the copper globe, and hung over the rear of the boat. Wooden cogwheels and a couple of endless belts fiIled the intervening space. As the globe spun, the paddles thrashed at the air.
“How does it work?” he said.
“Very simple,” said Urn. "The fire makes-
“We haven't got time for this,” said Simony.
“-makes the water hot and so it gets angry,” said the apprentice philosopher. “So it rushes out of the globe through these four little nozzles to get away from the fire. The plumes of steam push the globe around, and the cogwheels and Legibus's screw mechanism transfer the motion to the paddles which turn, pushing the boat through the water.”
“Very philosophical,” said Didactylos.
Brutha felt that he ought to stand up for Omnian progress.
“The great doors of the Citadel weigh tons but are opened solely by the power of faith,” he said. “One push and they swing open.”
“I should very much like to see that,” said Urn.
Brutha felt a faint sinful twinge of pride that Omnia still had anything he could be proud of.
“Very good balance and some hydraulics, probably.”
“Oh.”
Simony thoughtfully prodded the mechanism with his sword.
“Have you thought of all the possibilities?” he said.
Urn's hands began to weave through the air. "You mean mighty ships ploughing the wine-dark sea with no- he began.
“On land, I was thinking,” said Simony. “Perhaps . . . on some sort of cart . . .”
“Oh, no point in putting a boat on a cart.”
Simony's eyes gleamed with the gleam of a man who had seen the future and found it covered with armor plating.
“Hmm,” he said.
“It's all very well, but it's not philosophy,” said Didactylos.
“Where's the priest?”
"I'm here, but I'm not a-
“How're you feeling? You went out like a candle back there.”
“I'm . . . better now.”
“One minute upright, next minute a draftexcluder.”