Dura frowned at this. "Wouldn't it be easier to stop the caravan, rather than try to change the pigs over in flight?"

Kae laughed. "Hardly. Dura, when the caravan is assembled, back on the edge of the upflux, it takes several days, usually, for the pig-teams to haul it up to speed. And once this mass of wood is moving, it's much easier to maintain its motion than to keep stopping and starting it. Do you see?"

Dura sighed inwardly. "I know what momentum is. So you don't even stop when you sleep?"

"We sleep in shifts. We sleep tied up to nets and cocoons fixed to the trunks themselves." Kae pointed to the nearest pig-team. "We rotate the pigs in flight. It isn't so difficult to steer a caravan; all you have to do is follow the vortex lines downflux until you get to the South Pole... Dura, a caravan like this never stops moving, once it sets off from the edge of the hinterland. Not until it's within sight of Parz itself. Then the pig-teams are turned around, and the caravan's broken up to be taken into the City."

Dura tried to envisage the distance from the upflux to Parz. "But at this speed it must take months to reach the City."

"A full year, generally."

"A year?" Dura frowned. "But how can the City wait that long for lumber?"

"It can't. But it doesn't have to." Kae was smiling, but there didn't seem to be any impatience in her tone at Dura's slowness. "At any time, there's a whole stream of caravans like this, heading for the City from all around the circumference of the hinterland. From the point of view of Parz there's a steady flow of the wood it needs."

"Rauc knew on exactly which day to come down to the caravan. In fact, you and Brow were waiting to greet us."

"Yes. We were on time. We always are, Dura; all the caravans are, right across the hinterland. It's all carefully planned."

Dura thought of dozens, hundreds perhaps, of caravans like this, endlessly converging on Parz with their precious lumber... and all on time. She felt awed at the idea of humans being able to plan and act systematically on such a scale, and with such precision.

They moved on along the length of the caravan. In some places the trunks had been opened up to expose the green glow of the wood's nuclear-burning core. Humans moved around the glowing spots and circles, purposeful and busy. There were nets and lengths of rope trailing from the trunks, and Dura saw sleep cocoons, tools, clothes, food bales tucked into the nets. In one place there was a little clutch of infants and small children, safely confined inside a fine-meshed net.

"Why," she said, "the caravan's like a little City in itself. A City on the move. There are whole families here."