The Air was noticeably thinner. Dura felt her chest strain at the stuff and her heart pumped as it sought to power her muscles; from various points in her body she heard the small pops of pressure equalizing. She knew what was happening, of course. The Air had two main components, a neutron superfluid and an electron gas. The neutrons were thinning out; more pressure here was supplied by the gas of free electrons. When she held up her hand before her face she could see the ghostly sparkle of electrons around her fingers, bright in the gloom and evoking dim highlights from the crowding leaves.

But now her vision seemed to be failing. The Air was growing poor at carrying the high-frequency, high-velocity sound waves which allowed her to see. And, worse, the Air — thin as it was — was losing its superfluidity. It started to feel sticky, viscous; and as she moved she began to feel a breeze, faint but unquestionably present, plucking at her face and hair-tubes, impeding her motion.

She found herself trembling at the thought of this sticky stuff congealing in the fine network of capillaries which powered her muscles — the network which sustained her very being.

Human Beings weren't meant to live up here. Even pigs spent no more time close to the Crust than they had to. She heaved at the sludgelike Air, feeling it curdle within her capillaries, and longed for the open space of the Mantle beneath the roof-forest, for clean, fresh, thick Air.

In all directions around her the tree trunks filled the world. As it became progressively more difficult to see, the trunks, parallel, curving slightly to follow the Magfield, seemed suddenly artificial, sinister in their regularity, like the threads of some huge Net around her. She found herself gripped by a slow panic. Her chest heaved at the unsatisfying Air, the breath noisy in her throat. It took a strong, conscious effort to keep moving, an exercise of will just to keep her hands working at the tree trunk.

She was concerned for Farr. Even in the gloom she could see how distressed he was: his face was white and seemed to be bulging, his eyes half-closed; he seemed barely aware of where he was, and he moved along the trunk stiffly.

Dura forced herself to look away, to carry on. There was no help she could give him. Not now. The best she could do was to move on, to bring home the results of a successful hunt. And as Adda had said, the boy was probably safer with her than anywhere else...

At least Adda was close to Farr. Dura found herself offering up a simple, childlike message of thanks to the watchful Xeelee for the old man's presence and support.

—

When the climb ended it was with a suddenness that startled her.