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.
The cover closed overhead. A tiny green light winked on. Within the boat, lurid in the green glow, the four brutemen moved about with swift activity; the soft voice of Talon was directing them; his great head was raised on his hands as he followed their movements.
They bolted the metal over, adjusted other mechanisms which now came into use at the stern. A lessening of the flow of gas from beneath the hull; the water filled the chamber there. The rear power tube now pointed downward, to dip the bow. Other tubes, one on each side below the water line, pointed upward, with powerfully ejected streams of gas.