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“Yes.”

“Well, they don’t just take from it, they put back as well. And what they feed us can make us more cautious… or at least, that’s the case when they’re fully alert.”

Freya had heard something like that when she was a child, but she’d thought it was just a folk superstition.

“Are you serious?”

“I’ve seen the difference it makes,” Hanna insisted.

“But why would it work like that?”

“If your survival depended entirely on your sister’s, wouldn’t you want to do everything you could to discourage her from killing you both?”

Freya said, “Yes—but I expect she’d already be quite keen on staying alive herself.”

“Of course.” Hanna looped the needle through one last time, made a knot, then cut the thread. “But she’d also be capable of taking into account considerations to which her brothers were entirely oblivious. Do any of our brothers know what’s happened with the geysers? Why we’re building the tower? What’s at stake here?”

“So you think some of the women have started silencing their brothers’ qualms? They know that what we’re doing is worth the risk—and they want that to be enough to keep them going.”

“I think they want a fairer fight,” Hanna replied. “They want to conquer their own fears, one against one—instead of four against one, with three adversaries who can’t be reasoned with at all.”

Freya went to find Gro, to tell her that she was leaving for the village.

“Have you decided on a punishment?” Gro asked.

“I’m not going to punish them,” Freya replied. “They know what they did wrong. Now we need to find a way to make it safe.”

Gro was bemused. “The tripods will meet up soon, and the temptation will be gone. Why not banish anyone who tries the same stunt again, and leave it at that?”

Freya said, “The first level of tripods will meet up soon, but what about the next one? And the next? Do you think people will climb up and down all those stairs, just because we asked them to, long after we’re dead?”

“Maybe not,” Gro conceded.

“This thing isn’t ours anymore.” Freya laughed. “If it ever was. All we can leave behind is what we’ve learned: about the strength of ice, about the way the roots grow, about gravity, torques and forces. The rest is in other people’s hands, and the ones who’ll matter most haven’t even been born yet.”

She returned to the medical tent with a cart, then set off across the ice.

PART TWO

5

Leander was not the brightest star in the sky, but it was the brightest that ever crossed behind Tvíburi. Rosalind set up the telescope to track it, lit a lamp and prepared the comparator.

She had chosen a night when the timing would be as close to perfect as the various celestial constraints allowed. Leander needed to approach Tvíburi’s dark side, which meant the occultation had to take place well before midnight, when the sunlit portion of the twin was still skewed to the west. But then it was a question of balancing that requirement with the need to ensure that the sky was equally dark for both measurements: the one just before the occultation, and the one just before Leander set. Some of her friends had argued that it wasn’t strictly necessary to do both on the same night, but Rosalind found the idea of a prolonged delay troubling. She was confident that she could trim and fuel a lamp in such a way that it would give out constant illumination from dusk to dawn, but keeping it burning longer, let alone extinguishing and reigniting it, would be inviting inconsistency. If she ended up dead, it wasn’t going to be for a foolish reason like that.

Well before the occultation, she stopped down the light from the lamp and adjusted the comparator’s iris until the switch back and forth between starlight and lamplight induced no perceptible change in brightness. Simply staring at Leander and assigning a number to the strength of its light would have been a hopeless task, but the comparator made even the slightest difference from the reference source jump out at her. She wrote down the current size of the iris; it told her nothing on its own, but if the same setting worked again halfway to the second measurement, that would reassure her that the lamp’s output had remaining steady.

When she stepped away from the instruments to stretch her neck, she took care to shield her eyes from the light of Tvíburi, lest they lose their sensitivity. She didn’t need to watch the two bodies drawing nearer in the sky; with Joanna’s help, she’d calculated all the angles in advance and etched the landmarks she needed to be aware of into the wheel that turned the scope.

The night was cold, but there were tents around her on three sides, sparing her from the north wind. The tents hid the base of the tower, but she could see starlight glinting off the slender spike that rose up from the final tripod. She hunted for a softer, internal glow of lamplight showing through the ice, but it was late, and all her friends were probably asleep. That made her feel a little hard-done by, as if they’d owed it to her to keep a vigil, but she was the one who’d rebuffed their offers to join her on the ground for the measurements themselves. She hadn’t wanted the distraction.

Rosalind returned to the telescope, and nudged it along its preset arc until Leander was centered in the view again. She flipped to the lamp and back a few times, to reassure herself that nothing about the setup had drifted unexpectedly, but any change that had occurred was subtler than her powers of discernment. She stroked the edge of the wheel gently with her thumb, gauging the time that remained. It was strange: she’d been less anxious than this before most of her jumps, even those from untried heights. Maybe the risk here wasn’t as great as she was telling herself; if she made a mistake, one of her friends was sure to pick it up when they repeated the whole procedure. But to be complacent would still be foolish.

As she followed Leander along its path, the unlit eastern limb of Tvíburi crept into view, a bland, almost featureless gray against the deeper darkness speckled with stars. The contrast made the sister world seem closer and more quotidian than ever: it was almost as if she’d just pointed the telescope at the edge of a building, or someone was holding their hand in front of the lens. Half a dozen faint stars dimmed and disappeared behind the twin’s disk; Rosalind watched them carefully, building up a sense of the progression as it would apply to her target—trusting Joanna’s mathematics, but prepared to salvage the situation if it turned out that she’d mis-transcribed the results, or if the wheel on the scope’s mount had slipped.

The furrows on the wheel declared that the light from Leander had begun to graze the upper atmosphere of Tvíburi. Rosalind waited; she didn’t expect a visible effect until the path cut deeper.

At the halfway mark any dimming was still impossible to detect in isolation, but when she switched to the lamp and back a few times, she found she had to shrink the iris slightly before the two points of light appeared equally bright again. As Leander moved closer to the edge of the disk, she forced herself to stop trying to spot changes by anything other than the official method—but there was an unmistakable blurring of the star’s image, with the once-sharp point shimmering into a faintly trembling ellipse.

Moments before the occultation, Rosalind flicked the comparator, adjusted the iris, checked and rechecked. She had matched the lights again, perfectly, she was sure of it—and then Leander vanished from sight.