Most aviators decorated their flight-deck shrines with pictures of their ancestors, but neither Tom nor Hester had any images of their dead parents, so they had tacked up a photograph of Anna Fang which they unearthed from a trunk in the cabin when the Jenny was being repaired. Hester said a little prayer to her, hoping that she would be a friend to them down in the Sunless Country.

It was as she stood up to go back to Tom that she happened to glance out across the ice and saw the cluster of lights. At first she thought that they were just a reflection of that strange fire in the sky which Tom had been so pleased by — but these were steady points, not changing colour, just twinkling a little in the frosty air. She went closer to the shattered window. The cold made her eye water, but after a while she made out a dark bulk around the lights, and a pale drift of fog or steam above them. She was looking at a small ice city, about ten miles to leeward, heading north.

Trying to ignore her strange, ungrateful sense of disappointment, she went to rouse Tom, patting his face until he groaned and stirred and said, “What is it?”

“Some god’s got a soft spot for us,” she said. “We’re saved.”

By the time he reached the flight deck the city was closer, for the fortunate wind was blowing them almost directly towards it. It was a small, two-tiered affair, skating along on broad iron runners. Tom trained the binoculars on it and saw its curved and sloping jaws, closed to form a snow-plough, and the huge, hooked stern-wheel that propelled it across the ice. It was an elegant city, with crescents of tall white houses on its upper tier and some sort of palace complex near the stern, but it had a faintly mournful air, and there were patches of rust, and many lightless windows.

“I don’t understand why we didn’t pick up their beacon,” Hester was saying, fumbling with the controls of the radio set.