He did not stray from Brock Marsh. He had warned his friends in Salacus Fields that he would be out of touch, and those relationships were fluid anyway, relaxed, superficial. The only person he missed was Lin. Her work was keeping her at least as busy as him, and as the momentum of his research picked up, it was increasingly difficult to find times when they could meet.

Instead, Isaac sat up in bed and wrote her letters. He asked her about her sculpture, and he told her that he missed her. Every other morning or so he would stamp and post these letters in the box at the end of his street.

She wrote back to him. Isaac used her letters to tease himself. He would not let himself read them until he had finished his day’s work. Then he would sit and drink tea or chocolate in his window, sending his shadow out over the Canker and the darkening city, and read her letters. He was surprised at the sentimental warmth these moments made him feel. There was a degree of maudlin relish in the moods, but just as much affection, a real connection, a lack he felt when Lin was not there.

Within a week he had built a prototype of the crisis engine, a banging, spitting circuit of pipes and wire that did nothing more than produce noise in great gobbets and barks. He took it apart and rebuilt it. A little over three weeks later another untidy conglomerate of mechanical parts sprawled before the window, where the cages of winged things had gained their freedom. It was uncontained, a vague grouping of separate motors and dynamos and converters spread across the floor, connected by rough-and-ready engineering.

Isaac wanted to wait for Yagharek, but the garuda could not be contacted, living as he was like a vagrant. Isaac believed it to be Yagharek’s weird, inverted clutching at dignity. Living on the street he was beholden to none. The pilgrimage he had made across the continent would not end with him gratefully relinquishing his responsibility, his self-control. Yagharek was a deracinated outsider in New Crobuzon. He would not rely on, or be thankful to, others.

Isaac imagined him moving from place to place, sleeping on bare floors in deserted buildings, or curled up on roofs, huddled by steam-vents for heat. It might be an hour before he came to visit, or it might be weeks. It only took half a day of waiting before Isaac decided to test his creation in Yagharek’s absence.

In the belljar where the wires and tubes and flexing cables converged, Isaac had placed a piece of cheese. It sat there, drying slowly, while he hammered at the keys of his calculator. He was trying to mathematize the forces and vectors involved. He stopped often to take notes.