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.

He scrambled across the growstone, crouching in the wind. Facing north, he lay on his belly, holding his hood to protect his face from the blasting wind, and looked down by the light of his lantern. The Wall face itself was caked with ice. And there was a broken pipe venting steam, just beneath the edge of the roof. The pipes here, stitching in and out of the growstone surface, were part of the radiator system; the steam would pass through these exposed sections and lose a little heat in the open air before being drawn back into the closed cycles of the engine. Now, Thux could see, even though the radiator pipes were usually blistering hot, the cold had won. The pipes had become coated with thick masses of ice, flowing forms that must have melted and refrozen as the heat battled the snow, and the fluid in one section of the pipe must have frozen and, expanding, split the pipe wide open. The engine was bleeding steam pressure, and, worse, if the radiator wasn’t working the engine would soon be overheating too.