Free The Last Hero by Terry Pratchett

go over the Edge just before dawn tomorrow," Ponder explained.
    Everyone turned to Leonard, who was watching a seagull.
    "Mr da Quirm?" said Lord Vetinari.
    "What? Oh. Yes." Leonard blinked. "Yes. The device will be ready, although the privy is giving me problems."

The Lecturer in Recent Runes fumbled in the capacious pockets of his robe. "Oh dear, I believe I have a bottle of something... the sea always affects me that way, too."
    "I was rather thinking of problems associated with the thin air and low gravity," said Leonard. "That's what the survivor of the Maria Pesto reported. But this afternoon I feel I can come up with a privy that, happily, utilises the thinner air of altitude to achieve the effect normally associated with gravity. Gentle suction is involved."
    Ponder nodded. He had a quick mind when it came to mechanical detail, and he'd already formed a mental picture. Now a mental eraser would be useful,
    "Er... good," he said. "Well, most of the ships will fall behind the barge during the night. Even with magically assisted wind we dare not venture closer than thirty miles to the Rim. After that, we could be caught in the current and swept over the Edge."
    Rincewind, who had been leaning moodily over the rail and watching the water, turned at this.
    "How far are we from the island of Krull?" he said.
    "That place? Hundreds of miles," said Ponder. "We want to keep well away from those pirates."
    "So... we'll run straight into the Circumfence, then?"
    There was technically silence, although it was loud with unspoken thoughts. Each man was busy trying to think of a reason why it would have been far too much to expect him to have thought of this, while at the same time being a reason why someone else should have. The Circumfence was the biggest construction ever built; it extended almost a third of the way around the world. On the large island of Krull, an entire civilisation lived on what they recovered from it. They ate a lot of sushi, and their dislike for the rest of the world was put down to permanent dyspepsia.
    In his chair, Lord Vetinari grinned in a thin, acid way.
    "Yes indeed," he said. It extends for several thousands of miles, I understand. However, I gather the Krullians no longer keep captive seamen as slaves. They simply charge ruinous salvage rates."
    "A few fireballs would blow the thing apart," said Ridcully.
    "That does rather require you to be very close to it, though," said Lord Vetinari. "That is to say, so close to the Rimfall that you would be destroying the very thing that is preventing you from being swept over the Edge. A knotty problem, gentlemen."
    "Magic carpet," said Ridcully. "Just the job. We've got one in —"
    "Not that close to the Edge, sir," said Ponder, dismally. "The thaumic field is very thin and there are some ferocious air currents."
    There was the crisp rattle of a big drawing pad being turned to the next page.
    "Oh, yes," said Leonard, more or less to himself.
    "Pardon me?" said the Patrician.
    "I did once design a simple means whereby entire fleets could be destroyed quite easily, my lord. Only as a technical exercise, of course."
    "But with numbered parts and a list of instructions?" said the Patrician.
    "Why, yes , my lord. Of course. Otherwise it would not be a proper exercise. And I feel sure that with the help of these magical gentlemen we should be able to adapt it for this purpose."
    He gave them a bright smile. They looked at his drawing. Men were leaping from ships in flames, into a boiling sea.
    "You do this sort of thing as a hobby, do you?" said the Dean.
    "Oh, yes. There are no practical applications."
    "But couldn't someone build something like that?" said the Lecturer in Recent Runes. "You practically include glue and transfers!"
    "Well, I daresay there are people like that," said Leonard diffidently. "But I am sure the government would put a stop to things before they went too far."
    And the smile on Lord Vetinari's face was one that probably even Leonard of