Robert Sawyer - Foreigner

If Karshirl had been looking at Novato, instead of tipping her muzzle up at the tower, she’d have stopped before getting to the end of her explanation, since Novato’s face made it clear that she’d already thought of all this. “We tried that, of course,” said Novato. “The lifeboats accelerate quickly at first, but almost immediately seem to reach their maximum velocity. It seems the lifeboats are moving at something like one hundred and thirty kilopaces per day-tenth.”

“Good God!” said Karshirl, her eyelids strobing up and down. “That’s faster than even a runningbeast can manage.”

“Twice as fast, to be precise,” said Novato. “And it takes the lifeboats—wait for this— twenty days to make the round trip. Now, granted, there’s a lot of room for error—these are just back-of-a-sash calculations—but if you do the math, that would imply that the tower is on the order of thirteen thousand kilopaces tall.”

“But, good Novato, our entire world has a diameter of only twelve thousand kilopaces,” said Karshirl. “You can’t be seriously suggesting that the tower is taller than our world is wide. Something must be going on that we can’t see. The lifeboats must stop at the top for days on end, or else slow down once they get out of sight.”

Novato felt slightly surprised. She’d selected Karshirl for her own reasons, but was beginning to regret the choice. “Surely you wouldn’t discard data just because it doesn’t fit your expectations.”

“Oh, indeed,” said Karshirl, somewhat piqued. “I’m a good little scientist, too. However, I am also a structural engineer, which is something you are not. And I tell you, Novato, based on well-established engineering principles, that the tower cannot be as tall as you say. Look: stability is a real concern when building towers. You know the old story of the Tower of Howlee, told in the—the fiftieth, I think—sacred scroll? That was a tower that would reach up to the sky so that one could touch the other moons.” Novato nodded.

“But Howlee’s Tower is utterly impossible,” said Karshirl. “A sufficiently long, narrow object will buckle if it is held straight up.” She raised a hand. “Now, I know you’ve said that this tower is made out of stuff that’s harder than diamond. That’s irrelevant. No matter how great its compressive strength, such a tower will buckle if the ratio of its length to width goes above a certain value. In the old scroll, which was written long before we knew just how far away the other moons were, Howlee’s Tower was said to be twenty-five kilopaces tall, and had a base fifty paces on a side. You couldn’t build a tower like that out of any material. In fact, one can’t even build a scale replica of Howlee’s Tower, at any scale. It will buckle and collapse.”

“Because of the buffeting of the wind?” asked Novato.

“No, it’s not that. You can’t even build a scale model of Howlee’s Tower inside a sealed glass vessel, in which there are no air currents at all.”

“Why not?” said Novato.

Karshirl looked around vaguely, as if wishing she had something to draw a picture on. Failing to find anything, she simply turned back and faced Novato. “Let’s say you build a tower that’s a hundred paces tall and has a base of, oh, one square centipace.”

Novato’s tail swished in acceptance. “All right.”

“Well, visualize the top of this structure: it’s a flat roof, one square centipace in area.”

“Yes.”

“Consider the corners of that roof. There’s no way they will be perfectly even. One of them is bound to be a small fraction lower than the others. Even if they are all even originally, as the ground shifts even infinitesimally under the tower’s weight, one corner will end up lower than the others.”

“Ah, I see: the tower, of course, will lean toward the lowest corner, even if only very slightly.”

“Right. And when the tower does lean, that makes the lowest corner even lower, and the tower will lean some more, and on and on until the whole thing is leaning over like a tree in a storm—no matter how strong the building material is.”

“So the tower can’t be thirteen thousand kilopaces high,” said Novato.

“That’s right. Indeed, it can’t be anywhere near that high.”

Novato leaned back on her tail. “Obviously the pyramidal base gives the tower some stability, but the actual tower itself is only fourteen paces wide. How high could a tower that wide be?”

“Oh, I’m no Afsan,” said Karshirl. “I’d need to sit down with ink and writing leather to figure that out.”

“Roughly, though. How high? Remember, this tower extends well above the clouds.”

“And how high up are the clouds?” asked Karshirl.

“Oh, it varies. Say ten kilopaces. Could a tower fourteen paces wide be even that tall without collapsing in the manner you’ve described?”

Karshirl was silent for a time. “Ah, well, um, probably not,” she said at last.

Novato nodded. “So some other factor is at work here.” She gestured at the vast blue pyramid and the narrow four-sided tower thrusting up from its apex toward the vault of heaven. “Somehow, impossible as it seems, this tower does stand.”

No one normally sat in the Dasheter ’s lookout bucket when the ship was at rest. Still, even when just walking the decks, old Mar-Biltog couldn’t keep himself from occasionally scanning the horizon, so it was no surprise that he was the first to catch sight of them. He thumped the deck with his tail. The fools! Toroca said he’d warned them! Cupping his muzzle with his hands, Biltog shouted, “Boats approaching!”

Toroca, who happened to be passing fairly near, ran as fast as he could with his healing leg to the railing around the Dasheter ’s edge. Biltog had already made his way across the little bridge that joined the Dasheter’s forehull to its aft, and Toroca could hear his now-distant voice shouting again, “Boats approaching!”

And so they were: two long, orange boats. Typical Other designs. The lead boat contained five Others, each operating a pair of oars. They were packed in more tightly than Quintaglios could ever manage. The rear boat was too far away for Toroca to count its occupants, but it was likely a similar number.

In response to Biltog’s calls, Quintaglios were coming up the ramps onto the top deck. That was the worst thing that could happen. “No!” shouted Toroca. “Go below! Stay below!”

Babnol was emerging about ten paces away. Toroca pointed at her. “Get everyone below!”

“What’s happening?” she said.

“Get everyone below now! Others are coming!” Babnol reacted immediately, turning tail and heading back down the ramp. Toroca heard her entreating sailors to go to their cabins.

Toroca hurried toward the ropes that led up to the lookout’s bucket. He began to climb. When he got four body-lengths up, where he was sure the Others could see him, he waved his arm widely. “Go back!” he shouted in the Other tongue. “Stay away!”

The Dasheter ’s sails were furled, so he didn’t have to compete with their snapping, but the wind was in his face, stealing his words. “Go back!” he shouted again, and then, more plaintively, “Please! Please go back!”

The orange boats sliced through the water, approaching fast. Toroca thought about ordering the Dasheter ’s sails unfurled, for rowboats were no match for a sailing ship, but by the time the ropes could be untied and the red leather sheets had billowed out, the Others’ boats would have already arrived.

Toroca stopped waving. Even if they couldn’t hear him, they could surely see him. He made go-away gestures with his left hand. He hoped the sign of pushing away was universal, but in his lessons with Jawn such things had never come up. “Go back!” he shouted again in the Other language.