“I’d save them if I could.”

“We can do nothing. It is the road builders we want. Too primitive to threaten us, advanced enough to know answers to questions. We will find an isolated vehicle and swoop down on it.”

In midafternoon Louis took over the flying.

The swamp became a river that arched away to spinward, wide of its original bed. The crude road followed the new river. The original bed ran more nearly to port, in careful S-curves, with an occasional stretch of rapids or waterfall. It was dry as bone, running into bone-dry desert. The swamp must have been a sea before it silted up.

Louis dithered, then followed the original bed.

“I think we’ve got the timing right,” he told Chmeee. “Prill’s people evolved long after the engineers were gone. Of all the intelligent races here, they were the most ambitious. They built the big, grand cities. Then that odd plague knocked out most of their machinery. Now we’ve got the Machine People, and they could be the same species. The Machine People built the road. They did it after the swamp formed. But I think the swamp formed after Prill’s people’s empire collapsed.

“So what I’m doing is looking for an old Prill People city. We could get lucky and find an old library or a map room.”

They had found cities scarce during the first expedition. Today they traveled for some hours without seeing anything except, twice, a cluster of tents, and once, a sandstorm the size of a continent.

The floating city was still ahead of them, edge on, hiding detail. A score of towers reared around the edge; inverted towers dropped from nearer the center.

The dry river ended in a dry sea. Louis cruised along the shore, twenty miles up. The sea bed was strange. It was quite flat, except where artfully spaced islands with fluted edges rose from the bottom.

Chmeee called, “Louis! Set us on autopilot!”

“What have you found?”

“A dredge.”

Louis joined Chmeee at the telescope.

He had taken it for part of one of the bigger islands. It was huge and flat, disc-shaped, the color of seabottom mud. Its top would have been below sea level. Its seamless rim was angled like the blade of a wood planer. The machine had stalled up against the island it had dredged from the sea bottom.

So this was how the Ringworld engineers had kept the sludge flowing into the spillpipes. It wouldn’t flow of itself; the sea bottoms were too shallow. “The pipe blocked,” Louis speculated. “The dredge kept going till it broke down, or till something cut the power—something like the superconductor plague. Shall I call the Hindmost?”

“Yes. Keep him satisfied …”

But the Hindmost had bigger news.

“Observe,” he said. He ran a quick succession of holograms on one of the screens. A bracket poked up and out from the rim wall, with a pair of toroids mounted at its tip. Another bracket, seen from farther away; and in this picture a spill mountain showed at the foot of the rim wall. The spill mountain was half the size of the bracket. A third bracket showed. A fourth, with structures next to it. A fifth—“Hold it!” Louis cried. “Go back!”

The fifth bracket stayed on the screen for a moment. Its tip held nothing at all. Then the Hindmost flipped back to the fourth hologram.

It was somewhat blurred by the probe’s velocity. There was heavy lifting machinery anchored to the rim wall next to the bracket: a crude fusion generator; a powered winch; a drum and a hook floating unsupported below it. The cable depending from the drum must be invisibly thin, Louis thought. It could be shadow square wire.

“A repair team already at work? Uurrr. Are they mounting attitude jets or dismounting them? How many are mounted?”

“The probe will tell us,” the Hindmost said. “I direct your attention to another problem. Recall to your mind those toroids that circle the waist of the one intact Ringworld spacecraft. We surmise that they generate the electromagnetic scoop fields for Bussard ramjets.”

Chmeee studied the screen. “The Ringworld ships were all of the same design. I wondered why. You may be right.”

Louis said, “I don’t understand. What has—”

Two one-eyed snakes looked out of a screen at him. “Halrloprillalar’s species built part of a transportation system that would give them endless room to colonize and explore. Why didn’t they continue? All of the Ringworld was theirs through the rim transport system. Why would they make the effort to reach the stars?”

It made an ugly pattern. Louis didn’t want to believe it, but it fit too well. “They got the motors for free. They dismounted a few of the Ringworld attitude jets, built ships around them, and reached the stars. And nothing went obviously wrong. So they dismounted a few more. I wonder how many they used.”

“The probe will tell us in time,” the puppeteer said. “They seem to have left a few motors still mounted. Why did they not move the Ringworld back into position before the instability grew so great? Chmeee’s question is a good one. Are motors being remounted, or stolen to be used in ships so that a few more of Halrloprillalar’s race may escape?”

Louis’s laugh was bitter. “How does this sound? They left a few jets in place. Then came a plague that killed off most of their machinery. Some of them panicked. They took all the ships they had, and they built more ships in a hurry and dismounted most of the attitude jets to do it. They’re still at it. They’re leaving the Ringworld to its fate.”

Chmeee said, “Fools. They did it to themselves.”

“Did they? I wonder.”

“But this is just the possibility I find ominous,” said the puppeteer. “Would they not have taken as much of their civilization as they could move? Certainly they would have taken transmutation machinery.”

Oddly, Louis was not even tempted to laugh. But what answer could he make?

The kzin found an answer. “They would take all they could reach. Anything near the spaceport ledges. Anything near the rim wall, where the rim transport system was available. We must search inward, and we must search out the Repair Center. Any of Prill’s people found there would have been trying to save the Ringworld, not leave it.”

“Perhaps.”

Louis said, “It would help if we knew just when the plague started eating their superconductors.”

If he thought the Hindmost would flinch, he was wrong. The puppeteer said, “You will likely learn that before I do.”

“I think you know already.”

“Call me if you learn anything.” The snaky heads disappeared.

Chmeee was looking at him strangely, but he said nothing. Louis returned to the flight controls.

The terminator line was a vast shadow encroaching from spinward when Chmeee spotted the city. They had followed a sand-filled riverbed to port of the dry sea. The river forked here, and the city nestled in the fork.

Prill’s people had built tall, even where there was no obvious need. The city had not been wide, but it had been tall, until floating buildings smashed down into the lesser structures below. One slender tower still stood, but at a slant. It had driven itself like a spear into the lower levels. A road ran from port, along the outer edge of one branch of the dry river, then across a bridge so massively braced that it had to belong to the Machine People. Halrloprillalar’s people would have used stronger materials or would have floated it.

Chmeee said, “The city will have been looted.”

“Well, yes, given that someone built a road to do the looting. Why don’t you take us down anyway?”

“Your monkey curiosity?”

“Maybe. Just circle the tanj thing, give us a closer look.”

Chmeee dropped the lander fast enough to put them in free fall. The kzin’s fur was almost all grown out, a glossy and handsome orange coat, and a reminder of Chmeee’s new youth. Adolescence wasn’t helping his temper. Four Man-Kzin wars, plus a few “incidents” … Louis kept his mouth shut.