Hester turned back to the window in time to see one of the approaching ships fire a rocket to windward, signalling the Jenny Haniver to heave to and allow herself to be boarded. She wasn’t sure she believed Pennyroyal, but there was something threatening about those ships. She was certain they hadn’t just encountered the Jenny by chance. They’d been sent to find her.

She touched Tom’s arm. “Go.”

Tom heaved on the rudder controls, swinging the Jenny about until she was steering north with the gale behind her. He pushed a sequence of brass levers forward and the rumble of the engines rose to a higher pitch. Another lever, and small air-sails unfolded, semi-circles of silicon-silk stretched between the pods and the flanks of the gasbag, adding a little extra thrust to help shove the Jenny through the sky.

“We’re gaining!” he shouted, peeking into the periscope at the grainy, upside-down image of the view astern. But the Fox Spirits were persistent. They altered course to match the Jenny ’s, and coaxed more power out of their own engines. Within an hour they were close enough for Tom and Hester to make out the symbol painted on their flanks — not the broken wheel of the Anti-Traction League, but a jagged green lightning bolt.

Tom scanned the greyish landscape below, hoping for a town or city where he might seek sanctuary. There were none, except for a couple of slow-moving Lapp farming towns leading their herds of reindeer across the tundra far to the east, and he could not reach them without the Fox Spirits cutting him off. The Tannhauser Mountains barred the horizon ahead, their canyons and pumice-clouds offering the only hope of shelter.

“What shall we do?” he asked.

“Keep going,” said Hester. “Maybe we can lose them in the mountains.”

“What if they rocket us?” whimpered Pennyroyal. “They’re getting terribly close! What if they start shooting at us?”