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used?’
    Rees shook his head, interest illuminating his tired face.
    Pallis described the simple sensorium of the tree. The tree was essentially a huge propeller. The great vegetable reacted to two basic forms of stimuli - gravity fields and light - and in their uncultivated state great forests of trees of all sizes and ages would drift through the clouds of the Nebula, their leaves and branchlets trapping starlight, the nourishment of drifting plants and animals, the moisture of fat rain clouds.
    Rees listened, nodding seriously. ‘So by rotating faster - or slower - the tree pushes at the air and can climb away from gravity wells or towards the light.’
    ‘That’s right. The art of the pilot is to generate a blanket of smoke to hide the light, and so to guide the flight of the tree.’
    Rees frowned, his eyes distant. ‘But what I don’t understand is how the tree can change its rotation speed.’
    Once again Pallis was surprised. ‘You ask good questions,’ he said slowly. ‘I’ll try to explain. The trunk is a hollow cylinder; it contains another, solid cylinder called the bole, which is suspended in a vacuum chamber. The trunk and the rest of the tree are made of a light, fine-fibred wood; but the bole is a mass of much denser material, and the vacuum chamber is crisscrossed with struts and ribs to keep it from collapsing. And the bole spins in its chamber; muscle-like fibres keep it whirling faster than a skitter.
    ‘Now - when the tree wants to speed its rotation it slows the bole a little, and the spin of the bole is transferred to the tree. And when the tree wants to slow it is as if it pours some of its spin back into the bole.’ He struggled for phrases to make it clearer; dim, half-understood fragments from Scientists’ lectures drifted through his mind: moments of inertia, conservation of angular momentum . . .
    He gave up with a shrug. ‘Well, that’s about the best I can explain it. Do you understand?’
    Rees nodded. ‘I think so.’ He looked oddly pleased with Pallis’s answer; it was a look that reminded the pilot of the Scientists he had worked with, a look of pleasure at finding out how things work.
    Gover, from the rim of the tree, watched them sullenly.
    Pallis stepped slowly back to his station at the trunk. How much education did the average miner get? He doubted Rees was even literate. As soon as a child was strong enough he was no doubt forced into the foundry or down to the crushing surface of the iron star, to begin a life of muscle-sapping toil . . .
    And he was forced there by the economics of the Nebula, he reminded himself harshly; economics which he - Pallis - helped to keep in place.
    He shook his head, troubled. Pallis had never accepted the theory, common on the Raft, that the miners were a species of subhuman, fit only for the toil they endured. What was the life span of the miners? Thirty thousand shifts? Less, maybe? Would Rees live long enough to learn what angular momentum was? What a fine woodsman he would make . . . or, he admitted ruefully, maybe a better Scientist.
    A vague plan began to form in his mind.
    Rees came to the trunk and collected his shift-end rations. The young miner peered absently around at the empty sky. As the tree climbed up towards the Raft, away from the Core and towards the edge of the Nebula, the air was perceptibly brightening.
    A distant sound carried over the sigh of the wind through the branches: a discordant shout, huge and mysterious.
    Rees looked questioningly at Pallis. The tree-pilot smiled. ‘That’s the song of a whale.’ Rees looked about eagerly, but Pallis warned, ‘I wouldn’t bother. The beast could be miles away . . .’ The pilot watched Rees thoughtfully. ‘Rees, something you haven’t told me yet. You’re a stowaway, right? But you can’t have any real idea what the Raft is like. So . . . why did you do it? What were you running from?’
    Rees’s brow creased as he considered the question. ‘I wasn’t running