Hester pulled a telescope from one of the pockets of her coat and put it to her eye, twizzling the focusing ring until the blurry view came suddenly sharp. She was looking at a city: eight tiers of factories and slave-barracks and smut-spewing chimneys, a sky-train riding the slipstream, parasite airships sifting the exhaust plume for waste minerals, and down below, ghostly through veils of snow and powdered rock, the big wheels rolling.

“Arkangel!”

Tom took the telescope from her. “You’re right. It keeps to the northern foothills of the Tannhausers in summer, eating up scavenger towns as they come through the passes. The polar ice cap is much thicker now than it was in olden days, but there are still parts that are too thin to take Arkangel’s weight till summer’s end.”

Hester laughed. “Know-all.”

“I can’t help it,” Tom said. “I was an Apprentice Historian, remember? We had to memorize a list of the World’s Great Traction Cities, and Arkangel was right near the top, so I’m not likely to forget it.”

“Show off,” grumbled Hester. “I wish it had been Zimbra, or Xanne-Sandansky. You wouldn’t look so clever then.”

Tom was peering through the telescope again. “Any day now it’ll lift up its tracks and lower its iron runners and go skating off in search of ice cities and Snowmad scavenger towns to gobble up…”

For the present, however, Arkangel seemed content to trade. It was too vast to haul itself through the narrow passes of the Tannhausers, but airships were lifting from its harbours and flying south through the haze towards Airhaven. The first of them cut an arrogant swathe through the swirl of balloons around the floating town and swooped in to dock at strut six, just below Tom and Hester’s perch; they felt the faint vibration as its docking-clamps gripped the quay. It was a lean, short-range attack ship with a red wolf painted on its sable envelope and its name underneath in gothic script: the Clear Air Turbulence.