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Her name for it, and the name stuck. The archives give it a designation the concordance renders as a job rather than a noun, and the job is roughly move all the air. It fills a chamber I cannot see across — four hundred meters, taller than the Authority block, twelve intake throats each wide enough to fly a shuttle down. Primary atmospheric exchanger for half of Paragon, and no living person knew it existed.

It is also off-line, since the war's third year, and won't start.

"Interlock," Claire says, after ninety minutes. "It won't spin unless intake and exhaust sit inside four percent of each other. Correct engineering, and I hate it."

"Then we balance the network."

"We can't. The exhaust runs into the modern grid and the grid is a hundred patches deep. We'd have to rebuild half the city's return network before this thing consents to breathe." She sits back on her heels. "Which is why nobody ever has, including whoever tried in 2611."

I have spent nineteen years learning when a problem is finished. This one is finished. I tell her so.

Claire goes quiet for eleven minutes, which is the longest I have known her to be quiet while conscious. Then: "We don't have to balance it. We only have to make it believe it's balanced."

"That's the same statement."

"It absolutely is not." She's already pulling branch codes. "The interlock doesn't measure the city. It can't. It measures its own sensor loop, and the loop terminates at a return manifold on Sublevel Fifteen — the manifold feeding the scrubber bank we brought up two nights ago."

That takes me a moment.

"You want to use the scrubber bank as a sink."

"I want to use it as a liar. Bleed the exhaust into the scrubber circuit and hold it. The loop sees four percent, the interlock clears, the Lung starts. Once it's running its own output stabilizes the return network inside forty minutes, and then we drop the bleed and it never knows."

"And if the scrubber bank fails under the bleed?"

"Then Founders' Third has a bad hour and I've been extremely wrong with a councilor watching."

We do it at 16:40 on day thirty-one, Tomas on the scrubber bank, Helena on the surface feed, Rhyss behind us saying nothing.

The bleed goes in. The loop settles. The interlock clears at 16:52.

Then four hundred meters of chamber begins to move, and it reaches me through the floor before the air, and every light band along the spine dims a quarter as something enormous takes its share.

The Lung breathes.

Surface reports start at 17:30 and don't stop. Atmospheric index up in eleven districts. Then nineteen. Emergency reserve draw falling across half the city, which has never once happened in anyone's records. Helena stops relaying figures and starts relaying district names, and behind her a room full of engineers is making a noise I can hear through the link.

Claire sits on the deck plate with her back to an intake throat, filthy, blinking, spent.

I sit down beside her.

"I want to say something. Don't interrupt it."

"Uh oh."

"I have worked with the best engineers the Coalition and my own government produced. I have never met anyone who thinks the way you do. That solution was beyond me, and it would still have been beyond me in ten more years, because I would have kept trying to balance the network — correct the entire time, while the city died correct." I make myself finish. "You are the finest engineer I have ever stood beside. I wanted it said out loud, in front of the thing you started."

She doesn't answer at once, and when she does her voice isn't steady.

"You know that's going in my notes."

"I assumed."

CHAPTER 13

CLAIRE

Two days in the archive chamber before I understand what I'm reading, and when I do I have to go stand in the corridor.