Inside the northern turret library, they both relaxed a little. Moonlight was filtering faintly from the high windows.

Winter crossed the floor and pointed at a brass label. ‘If there’s any more information, it’ll be here: Animals, Real and Mythological. Last time we reached as far as there.’

So that narrowed it down, Joe thought, eyeing the broad section of shelves she was indicating.

‘Start at the bottom, work up?’ he asked Winter in a whisper.

‘Top down. We’ll be tired later.’ She went to wheel over one of the stepladders and climbed up to reach the highest shelf.

They spent the night scampering up and down the ladders, pausing to read – hunched over the lantern – waving each other over if they needed a second opinion. It was much more awkward, sharing the light, squinting at the page in the dim glow of the lantern.

As time went on, and they still had no new facts, Joe grew more anxious. He read till his eyes hurt. He read about animals he’d never heard of, and some he wasn’t sure were real. He read about the sphinx, the lion, the centaur, the 117cheetah, the phoenix and the elephant. He read about long-dead giant lizards, snakes, frogs, newts. But not a word about shadow dragons.

He was ready to give up. He sat there, wriggling the stiff­ness from his shoulders and blinking the dryness from his eyes, when he saw the moment that Winter found something.

She was sitting on the floor in her usual tatty grey dress, her cloak pooling around her in dark folds.

It was as if she had scented prey. She sat taller, reading quickly, her lips moving as she did.

‘What?’ Joe called, forgetting to be quiet.

‘I’ve found something. This is it!’ She clasped the book to her chest, eyes glistening. ‘It says: the life cycle of dragons and others of the genus.’

‘What’s a genus?’

‘I don’t know, maybe a grouping of animals? But listen: eggs are laid by mature dragons. In their dormant state they may survive decades – even up to one hundred years in the case of the rare shadow dragon.’

‘So they’re alive!’ Joe felt a warm rush of relief pass through him. ‘I mean, they could be. They’re not dead.’ He remembered that spark against his finger. He’d known it, deep down, but this was proof.

‘Wait there’s more: For hatching to occur, optimal conditions must prevail, that is, the presence of heat and moisture.’

‘What’s optim … and prevail?’ That all sounded complicated.

‘Let me finish! I’m just getting to the important part,’ 118Winter said. ‘For the rarer shadow dragon, to survive the long dormant years the egg is thick and hard. It hatches at the time of volcanic hot water flows. It may only succeed in the presence of the hatching agent silverblue, common to the region of Sartola. However, if no silverblue is pres­ent, the eggshells cannot thin and break, and the shadow dragons will die within.’