She felt a draught. Since the officer had forced the door it wouldn’t fit back in its frame.

Up on the Wall parapet above Engine Seventy-Four, Thux was struggling.

It felt like he’d been out here for hours, and maybe he had been, for it was getting darker and darker. He had a lantern fixed into a prepared socket on the growstone surface, but all he saw by its light was a bubble of snowflakes, flying horizontally. The wind was coming from the north, flat and hard and muscular, and it sucked every bit of warmth out of any exposed skin, out of his cheeks, out of his hands when he flipped back the clever little lids on his mittens to expose his fingers for the finer work. And though the snow wasn’t settling up here, except in the engine’s vents and chimneys, the wind was depositing ice on every surface, on the growstone under his feet, on the pipework and handholds and rungs he had to use to clamber about.

He’d been up here without a break since the engine had first started to struggle. But there was no choice. The vents were the problem, the vents that fed the great engine with the air it needed to burn its fuel, and expelled its excess heat and steam and smoke. The swirling snow was forever clogging them up. Even on the hot chimneys the snow fell and froze in place faster than it could be melted away. So Thux was following a dismal circuit, clambering around the growstone with his pack of tools at his waist, clearing snow and ice from the vents with hammer and chisel and, sometimes, when he couldn’t find any other way, with his mittened hands. He was exhausted, cold to the bone, wet through from snow that had got under his hood and down his collar and melted against his skin. And, when he thought about it, also hungry. He wondered how much longer he could keep this up. He was pretty sure there’d be no relief shift tonight. Every one of the mechanikoi available would be at work somewhere in the Wall’s battered infrastructure. If he gave up and went down he knew that his mother would insist on coming up here to take his place, and he couldn’t have that.

And Engine Seventy-Four couldn’t be allowed to fail, or any of the hundreds of engines implanted along the mighty length of the Wall. Not on a night like this. Up to now they had been winning; he couldn’t hear the engine, such was the howl of the wind and the muffling of his ears by his big fur cap, but he could feel its steady vibration through the growstone, like a beating heart buried within the Wall.

But now he heard an ominous groan from the Wall’s sea-facing surface. A sharp crack. A hiss of escaping steam. Something new, then.