The walkway was one of three that extended at regular intervals from the very top of the dome’s arch, designed originally for the repair crews that had never appeared. The curve of the dome seemed to break the surface of the earth like the tip of a bent back, implying a vast body below ground. The figure was riding a gargantuan whaleback. He was buoyed up by the light that the dome trapped, that played on the underside of the glass and made the whole great edifice shine. He kept low, moving very slowly to avoid being seen. He had chosen the ladder on the Glasshouse’s north-western side, so as to avoid the trains on the Salacus Fields branch of the Sud Line. The tracks passed close by the glass on the opposite side of the dome, and any observant passengers would see the man crawling up the curved surface.

Eventually, after several minutes climbing, the intruder reached a metal lip that surrounded the apex of the great structure. The keystone itself was a single globe of limpid glass about eight feet in diameter. It sat perfectly in the circular hole at the dome’s apogee, suspended half in, half out like some great plug. The man stopped and looked out over the city, through the tips of the supporting struts and the thick suspension wires. The wind whipped about him, and he clung to the handholds with vertiginous terror. He looked up into the darkening sky, the stars dim to him from all the clotted light that surrounded him, that ebbed through the glass below his body.

He turned his attention to that glass, scanned its surface minutely, pane by pane.

After some minutes he raised himself and began to climb backwards down the rails. Down, fumbling with his feet, feeling for holds, gently probing with outstretched toes, pulling himself back towards the earth.

The ladder ran out twelve feet from the earth, and the man slid down on the grappling hook he had used to get up. He touched the dusty ground and looked around him.

“Lem,” he heard someone hiss. “Over here.”

Lemuel Pigeon’s companions were hiding in a gutted building at the edge of a rubble-strewn wasteground flanking the dome. Isaac was just visible, gesticulating at him from behind the doorless threshold.

Lemuel paced quickly across the thin scrub, treading on bricks and concrete overgrown and anchored with grass. He turned his back on the early evening light and slipped into the gloom of the burnt-out shell.

In the shadow before him crouched Isaac, Derkhan, Yagharek and the three adventurers. There was a pile of ruined equipment behind them, steam-pipes and conducting wires, the clasps from retort stands, lenses like marbles. Lemuel knew that the mess would resolve itself into five monkey-constructs as soon as they moved.

“Well?” demanded Isaac.

Lemuel nodded slowly.