‘All right. With weight like mine I’d probably break the ladders anyhow. But make sure you suit up properly. Scarf, peaked hat, mask, heavy gloves.’

‘I know.’ Struggling, he pulled a tight coverall lined with gull feathers over his regular work clothes.

Kia went back to her gauges. She wasn’t about to say it, in fact she didn’t really trust herself to say anything, but today, at forty-three years old, she felt unreasonably proud of her only son. And of herself, she admitted, not to mention Engine Seventy-Four. People had always looked down on the House of the Beavers, Northland’s engineers, even though it was as old as any of the nation’s ancient guilds – and their nickname of the mechanikoi showed they had absorbed as much of the tradition of the learned Greeks who had once flocked to Northland as any of the more academic Houses. After all, without the Beavers’ work on the Wall and the various other sea-defence measures in the very beginning, Northland would not even exist, it would all have been lost under the sea before Ana was cold in her stony tomb in the Wall, probably. And these days the Beavers, equipped with new skills, worked harder than ever. The sages, nodding in deference to their inspirational founder Pythagoras, might lecture the world about the principles of heat flow and mechanical advantage, but it took a Beaver, one of the humble mechanikoi, crawling about in the dark and smoke and steam and sweat, to make it all work.

Except today it was only just working. All the Wall’s engines were labouring to cope with the flooding at the Wall’s base, caused by a brief warm spell. But that wasn’t the only stress on her engine. On such a day as this people must be flocking into the Wall for shelter, those who lived in smaller properties out on the plain – even those with homes in the chambered cloisters of Old Etxelur, which wasn’t as well equipped as the Wall. As they arrived, settling in the apartments and inns and taverns, they were all turning up the heat, and running deep steaming baths to soak away the cold of the day. She could see it all reflected in the bubbling levels of the gauges, hear it in the deep mechanical groans of the engine.

And now the engine itself was being choked by the snow.

What if Engine Seventy-Four failed? Then Seventy-Three and Seventy-Five would be quick to pack up too, as they strove too hard in an effort to compensate, and the whole system could come crashing down. Without heating – why, without even running cold water, for that had to be pumped up from the ground too, a fact not many citizens of the Wall were even aware of – what would become of the Wall and the citizens snug inside, like bees in their honeycombed hive? Seventy-Four must not fail – that was all. And on this shift it was Kia’s job, and Thux’s, to make sure of that.