Way above the heads of the roof-people, bloated airships ploughed across the sky. Noisy predators. Grubby specks of light and dark, moving edgily in the night’s cloud.

To Isaac’s relief, the plateau at the top of the hill of layered slate was flat, and about fifteen feet square. Large enough. He wagged his gun, indicating that Andrej should sit, which the old man did, collapsing slowly and precipitously into the far corner. He huddled in on himself, hugging his knees.

“Yag,” said Isaac. “Keep watch, mate.” Yagharek dropped the final twist of the cable he had hauled up, and stood sentry at the edge of the little open space, looking down across the gradient of the massive roof. Isaac staggered under the full weight of the sack. He put it down and began to unpack the equipment.

Three mirrored helmets, one of which he put on. Derkhan took the others, gave one to Yagharek. Four analytical engines the size of large typewriters. Two large chymico-thaumaturgical batteries. Another battery, this one metaclockwork, a khepri design. Several connecting cables. Two large communicators’ helmets, of the type used by the Construct Council on Isaac to trap the first slake-moth. Torches. Black powder and ammunition. A sheaf of programme cards. A clutch of transformers and thaumaturgic converters. Copper and pewter circuits of quite opaque purpose. Small motors and dynamos.

Everything was battered. Dented, cracked and filthy. It was a sad pile. It looked like nothing at all. Rubbish.

Isaac squatted beside it and began to prepare.

His head wobbled under the weight of his helmet. He connected two of the calculating engines, linking them into a powerful network. Then he began a much harder job, connecting the rest of the various oddments into a coherent circuit.

The motors were clipped to wires, and they to the larger of the analytical engines. The other engine he tinkered with internally, checking subtle adjustments. He had changed its circuitry. The valves within were no longer simply binary switches. They were attuned specifically and carefully to the unclear and the questionable; the grey areas of crisis mathematics.

He snapped small plugs into receivers and wired up the crisis engine to the dynamos and transformers that converted one uncanny form of energy into another. A discombobulated circuit spread out across the flat little roofspace.

The last thing he pulled from the sack and connected to the sprawling machinery was a crudely welded box of black tin, about the size of a shoe. He picked up the end of the cable—the enormous work of guerrilla engineering that stretched more than two miles to the huge hidden intelligence of the Griss Twist dump. Isaac deftly unwound the splayed wires and connected them to the black box. He looked up at Derkhan, who was watching him, her gun trained on Andrej.