“So Pennyroyal’s slipped through his fingers again,” said one wife.

“Surprise, surprise,” said another.

“This is exactly what happened at Arkangel,” said a third.

“Silence, wives!” Blinkoe shouted. “This is important!”

His fourth wife made a sour face. “Pennyroyal’s hardly worth the bother.”

“Poor, dear Professor Pennyroyal,” the fifth said weepily.

“Forget Pennyroyal,” bawled her husband, pulling off his hat and slipping on the radio headphones, tuning the transmitter to a secret wavelength, gesturing impatiently for wife number five to stop snivelling and turn the starting-handle. “I know people who will pay me well for what I’ve just learned! The trader Pennyroyal just left on was Anna Fang’s old ship!”

Tom had not realized until now how much he missed the company of other historians. Hester was always happy to hear the odd facts and stories that he recalled from his Apprentice days, but she could offer little in return. She had lived by her wits since she was just a child, and although she knew how to jump aboard a speeding town, how to catch and skin a cat and how to kick a would-be robber exactly where it hurt most, she had never bothered learning much about the history of her world.

Now, here was Professor Pennyroyal, his amiable personality filling the Jenny ’s flight deck. He had a theory or an anecdote about everything, and listening to him made Tom feel almost nostalgic for the old days in the London Museum when he had lived surrounded by books and facts and relics and scholarly debate.

“Now take these mountains,” Pennyroyal was saying, gesturing out of the starboard window. They were following a long spur of the Tannhausers southward, and the glow of lava in an active caldera flickered over the explorer’s face.