She turned to look out of the galley porthole while she cleaned her teeth — anything rather than face her own reflection in the mirror above the basin. The sky was the colour of packet custard, streaked with rhubarb cloud. Three small black specks hung in the centre of the view. Flecks of dirt on the glass, thought Hester, but when she tried to rub them away with her cuff she saw that she was wrong. She frowned, then fetched her telescope and studied the specks for a while. Frowned some more.

When she reached the flight deck Tom was preparing to turn in for a nap. The gale had not abated, but they had flown clear of the mountains now and although the wind would slow them down there was no longer any danger of being blown through a volcanic plume or dashed against a cliff. Tom looked tired but contented, beaming at Hester as she ducked in through the hatchway. Pennyroyal sat in the co-pilot’s seat, a mug of the Jenny ’s best coffee in his hand.

“The professor’s been telling me about some of his expeditions,” Tom said eagerly, standing up to let Hester take the controls. “You wouldn’t believe the adventures he’s had!”

“Probably not,” agreed Hester. “But the only thing I want to hear about at the moment is why there’s a flight of gunships closing in on us.”

Pennyroyal squawked with fear, then quickly clamped a hand over his mouth. Tom went to the larboard window and looked where Hester pointed. The specks were closer now, and clearly airships; three of them, in line abreast.

“They might be traders, heading up to Airhaven,” he said hopefully.

“That’s not a convoy,” Hester said. “It’s an attack formation.”

Tom took the field-glasses from their hook under the main controls. The airships were about ten miles off, but he could see that they were fast and well-armed. They had some sort of green insignia painted on their envelopes, but otherwise they were completely white. It made them look absurdly sinister: the ghosts of airships, racing through the daybreak.