They sank back. With his elbows slightly raising him, Jim watched the head of Talon hitching itself to the stern of the boat. The moon had risen high above the horizon. From where Jim lay he could see its yellow, horn-shaped disk. That, and a narrow segment of the star-strewn sky, was all that showed above the gunwales of the boat. The stars rolled with a lazy swing; the boat was throbbing, propelled evidently, by some invisible engine, over a calm, rolling sea, and in the silence Jim could hear the water slipping past the boat’s smooth sides. He wondered how far from shore they were? If he and Ren, with a leap, could plunge overboard, a mad, fool-hardy attempt, of course, but still he must see where they were, try and plan something. “Ren?” “Yes. What is it?” “Move over a little. I’m going to get behind you and sit up, see where we are, how far from shore.” Jim cautiously raised his head. He half expected a command from the rear of the boat. But none came. They were on a broad expanse of calm water. The moon made a yellow shimmering path into which they were heading. Jim sank back. It would have been folly to have attempted an escape. For a long time he and Ren lay quiet. An hour, perhaps, or more. The boat sped rapidly on. Its invisible engine made a hiss, and a line of bubbles rose from its sides. Jim had noticed them when he sat up; the boat seemed traveling on a continuous, rising mass of bubbles. There was a queer acrid smell in the air from the gas of them. Jim learned later from Talon the details of this boat. It was built of metal which, with its load, would barely float. Beneath its hull was a chamber through which the water circulated. A grid of wires was there; a current heated the wires, decomposed the water into its two component gases, hydrogen and oxygen. The bubbles were buoyant. The rising flow of them lifted the boat, so that in truth it skimmed forward upon the gas bubbles beneath it. The generation of gases was controlled, so that the boat floated high or low at will. The engine was similar. The forcible ejection of gases from a tube extending under water from its stern propelled it forward. The tube was movable, like a rudder, to give direction. An hour passed. Then the hairy brutemen who had been sitting quiet got to their feet, fumbled at the gunwales. An oval metal cover rolled from beneath the gunwales up like a canopy to enclose the boat overhead. Jim had taken a last swift look outside, before the arched metal cover rolled and closed them in. The boat was now making for a sheer wall of cliff that lay directly ahead. But in one place, for which they were steering, the cliff dropped sheer, unbroken into the water. Above the cliff, behind it, a jagged mountain range stood yellow in the moonlight, tumultuous, naked crags.