Yagharek concentrated, and began to scan the inside of the dome with a hard, rigorous eye. He focused carefully, went into something of a hunter’s trance.

He started looking at the edges of the dome. He took in the whole inside circumference in one long, slow sweep, then spiralled his vision carefully towards the centre, examining and investigating the circle of houses and streets a little way in, and then further in again.

In this exacting, methodical way, he could cast his eye on every nook and cranny of the Glasshouse’s surfaces. His eyes stopped briefly, momentarily, on imperfections of the red stone, then moved on.

As the day came closer to its end, the nervousness of the cactus people seemed to increase.

Yagharek came to the end of his scanning sweep. There was nothing immediate, nothing clearly wrong that leapt out at him. He turned his attention to the inside of the roof immediately around him, looking for some purchase.

It would not be easy. Some way from him the girders coalesced around the heavy glass globe, but on the underside of the glass they were not as protuberant. He believed that with some effort he could climb them: as, probably, could Lemuel and perhaps Derkhan or one or two of the adventurers. But it was hard to imagine Isaac clinging so close and holding his bulk suspended, crawling hundreds of yards of dangerous metal piping to the earth.

The sun was low outside. Even with the languorous summer evenings, time was short.

He felt someone tap his back. Yagharek raised his head up, lifting it out of the inverted bowl into the air of New Crobuzon, air that felt suddenly chill.

Behind him, Shadrach was crouched on the glass. He wore a mirror-helm, and held out a similar piece cobbled together from plate iron towards Yagharek.

Shadrach’s helmet looked different. Yagharek’s was a rude piece of rescued metal. Shadrach’s was intricate, wired and valved with copper and brass. At the top was a socket, with holes to screw in some fitting. It was only the mirrors that seemed a makeshift addition.

“You forgot this,” Shadrach said in his gentle voice, waving the helmet. “No waved flag, no word from you for twenty minutes. I’m here to check you’re alive and all right.”

Yagharek showed him the girders inside the dome. He and Shadrach discussed the problem of Isaac in urgent whispered tones.

“You must go down,” said Yagharek. “You must go by the sewers, with Lemuel as your guide. You must find your way as fast as you can into the dome. Send some of the mechanical monkeys to me, to aid me if I am attacked. Look inside.”