"A lighthouse is – a tower, with a rotating spotlight. With a curved mirror, to shine out in a narrow cone, that moves around in a circle. So the light is concentrated enough to be visible, but can be seen from a lot of different angles. Trades off space and time, sort of.

I was thinking of reflected sunlight, for signal mirrors, but lighthouses are mostly useful at night. I guess you might want two different systems for day and night.

I'm not sure I'm visualizing what you have in mind by the crescent."

"A narrow cone could work well enough, too, I'll drop the crescent. Maybe they can sweep it up and down to catch other altitudes... Perhaps we'll suggest it to a town that looks like it could use a lighthouse."

"A cone is probably better for the two-dimensional case; the lighthouse was mostly just a metaphor. An ill-considered metaphor. in retrospect. If by crescent you meant something like a wide narrow arc, that could be better for the three-dimensional case at work here. I'm just not sure what arrangement of mirrors you'd use to get that. But I'm sure someone with more knowledge and expertise in optics than me could figure it out."

"Possibly. I mostly know shipcraft but I might be able to poke about with optics, too."

Nod.

"So where're we headed, anyway? I assume you haven't changed whatever plans you had before I showed up."

"We are headed to some random high-altitude farming village with a belly full of shinies. We will pick up lots of cheap food and then head back down towards towns and cities - probably Ernald's Dodge, weather is clear in that direction."

"...Well, now I'm wondering about the ecology of this place. I've just barely heard of a nitrogen cycle, but – if food tends to get exported from the high altitudes and imported to the low, which would make sense because of sunlight, how do the nutrients get back up to replenish the soil? Is someone running a freighter full of dung up from the lower levels?

Does your world irreversibly shrink a little every time someone throws anything overboard? Or off the edge of their island, I guess."

"Maybe we'll see a smog plume sooner or later. But these massive columns of dust and such from the surface get kicked up into the air and eventually settle over everything."

"...huh. So there's a cycle to it. All right.

That's good.

I wonder what I'm gonna do here. Do you think there's anything for an outworlder to do, the way there's people interested in my stuff? Scholars who'd want to ask me about stuff? That's probably not really a career, is it.

Maybe I can be a bookbinder. I bet you don't have a lot of industrial infrastructure around here. Tends to be large and heavy. And involve a— no, I suppose it can be made out of wood instead of metal. But still large and heavy."

"I mean, there is industry here and there. Glassmakers and things. Books are kind of expensive. There's always farming if you can't find something better."

"They would be, wouldn't they. All the more reason it needs doing. Maybe I can figure out how to build a printing press."

"Nah, there's printing presses. It's because paper's expensive."

"...oh. Well. Maybe I can make paper, then.

Or is the problem that all the arable farmland is used for food crops?"

"I'm not actually sure. Probably not all farmland is used optimally, anyway. Trees are rare enough that paper is hard, though. I think there was some kind of fake paper craze a while ago, made of plastic, but it was junk."

"Hm. I wonder if you could make paper out of the inedible parts of food crops. Stems and leaves and such. Might be already used as animal feed, I guess."

"That's an interesting idea. I'll do chemistry at it later, probably."

"I know how to make real paper. Plant fibers. Doing it with floatgrass leavings should be possible."

"...Floatgrass?"

"Where do you think we get our hydrogen? Floatgrass and other Cloudbank life makes it."

"Huh.

Well. That sounds useful. What are floatgrass leavings used for now? Feed, ballast, mulch... probably not ballast, grass isn't very dense I bet. Fuel for fires?"

"Feed, mulch. Fertilizer."

She nods. "Probably not gonna be a problem then, especially if people mulch old paper. ...I guess the economics probably aren't gonna work out for that, paper being too expensive to use for anything you're gonna throw out.

...uh. Where can I find the bathroom?"

"It's next to the galley - down this hall, right down the other hall, second door on the left."

"Thanks." She goes.

Now to see if she can figure out the alien facilities.