Now the two of them stood in the engine room, watching the labouring of their steel beast with some anxiety.

This chamber, with its rough plastered-over growstone walls, was entirely embedded within the body of the Wall, not far below its parapet. Shelves were cluttered with the paraphernalia of their profession: precision screws, gears, transmission chains, camshafts, pistons, valves. The engine itself was a massive cylinder that filled the room. Its big rocking arm converted the heat of good Albian coal to motive force, the force that helped keep the ground by the face of the Wall pumped dry, and worked elevators within the Wall, and lifted cargo cranes on the sea-facing side. The whole apparatus was surrounded by condenser pipes and feeds that kept the engine working to its best theoretical ability, and bled steam and hot water off into the body of the Wall to keep its inhabitants in the warmth and comfort to which they had been accustomed for centuries, even in the hardest winter. Engine Seventy-Four was dumb, but it was big and strong and reliable – just as Kia liked to say of her son, not inaccurately. But today it was in trouble. You didn’t need to read the liquid-level gauges showing pressure and temperature and steam output and all the rest to know that; you could feel it, standing in this growstone pen before the labouring beast.

As Kia and Thux stood there bewildered, a few flakes of snow came drifting down the ventilation shafts from the outside world, quickly melting in the heat of the engine room.

‘It’s overheating,’ Kia said.

‘I don’t understand,’ Thux replied. ‘I know it’s snowing—’

‘I’ve never known it to snow so hard before, I have to say. Certainly not this early in the winter.’

‘But the loops should be too hot to be affected by the frost.’ Big radiator loops were embedded in the Wall’s outer surface, to enable the engines, buried in the body of the Wall, to lose heat. ‘In fact,’ Thux said, ‘the colder it is outside the better the heat loss. Right? What then, Mother? Owl’ A mass of snow came tumbling down a shaft, straight onto his head and shoulders. Splashing onto the floor it quickly melted, leaving a shallow puddle. As Kia tried not to laugh, Thux brushed the residue off his hair and shoulders. ‘How could that happen? It’s impossible.’

Kia peered up the shaft. ‘Not if the snow’s falling fast enough. Maybe ice is forming on the protective grilles . . .’ She snapped her fingers. ‘That’s it. The exhaust shafts must be blocked by ice and snow. Our baby’s choking. One of us will have to go up and clear it. I’ll go,’ she said immediately.

‘No,’ he snapped.

‘You’ve never seen conditions like this before.’

‘Well, nor have you. My job, Mother. I’m younger than you. And it wouldn’t cost as much to replace me.’ He went to a locker and began to pull out his kit.