was loud, even piercing, and falling swiftly in pitch. Then it suddenly stopped.
A few seconds later it came again, repeating the same sequence. It had all the mournful, compelling quality of a lighthouse siren sending out its warnings into the fog-shrouded night. There was a message here, and an urgent one. It was not designed for their ears, but they understood it. Then, as if to make doubly sure, it was reinforced by the lights themselves.
They dimmed almost to extinction, then started to flash. Brilliant beads, like ball lightning, raced along the six narrow valleys that had once illuminated this world. They moved from both poles toward the sea in a synchronized, hypnotic rhythm that could have only one meaning. “To the sea!” the lights were calling. “To the sea!” And the summons was hard to resist; there was not a man who did not feel a compulsion to turn back, and to seek oblivion in the waters of Rama.
“Hub Control!” Norton called urgently. “Can you see what’s happening?”
The voice of Rousseau came back to him. He sounded awed, and more than a little frightened.
“Yes, Skipper. I’m looking across at the Southern Hemisphere. There are scores of biots over there—including some big ones. Cranes, bulldozers… lots of scavengers. And they’re all rushing back to the sea faster than I’ve ever seen them move before. There goes a crane—right over the edge! Just like Jimmy, but going down a lot quicker…. It smashed to pieces when it hit…. And here come the sharks; they’re tearing into it…. Ugh, it’s not a pleasant sight….
“Now I’m looking at the plain. There’s a bulldozer that seems to have broken down…. It’s going round and round in circles. Now a couple of crabs are tearing into it, pulling it to pieces…. Skipper, I think you’d better get back right away.”
“Believe me,” Norton said with deep feeling, “we’re coming just as quickly as we can.”
Rama was battening down the hatches, like a ship preparing for a storm. That was Norton’s overwhelming impression, though he could not have put it on a logical basis. He no longer felt completely rational. Two compulsions were warring in his mind: the need to escape and the desire to obey those bolts of lightning that flashed across the sky, ordering him to join the biots in their march to the sea.
One more section of stairway. Another ten-minute pause, to let the fatigue poisons drain from his muscles. Then on again. Another two kilometers to go, but better not to think about that….
The maddening sequence of descending whistles abruptly ceased. At the same moment the fireballs racing along the slots of the Straight Valleys stopped their seaward strobing; Rama’s six linear suns were once more continuous bands of light.
But they were fading fast, and sometimes they flickered, as if tremendous jolts of energy were being drained from waning power sources. From time to time there were slight tremors underfoot. The bridge reported that Rama was still swinging with imperceptible slowness, like a compass needle responding to a weak magnetic field. This was perhaps reassuring; it was when Rama stopped its swing that Norton would really begin to worry.
All the biots had gone, so Rousseau reported. In the whole interior of Rama, the only movement was that of human beings, crawling with painful slowness up the curving face of the north dome.
Norton had long since overcome the vertigo he had felt on that first ascent, but now a new fear was beginning to creep into his mind. They were so vulnerable here, on this endless climb from plain to hub. Suppose that Rama, when it had completed its attitude change, started to accelerate?
Presumably its thrust would be along the axis. If it was in the southward direction, that would be no problem; they would be held a little more firmly against the slope they were ascending. But if it was toward the north, they might be swept off into space, to fall back eventually on the plain far below.
He tried to reassure himself with the thought that any possible acceleration would be very feeble. Dr. Perera’s calculations had been most convincing. Rama could not possibly accelerate at more than a fiftieth of a gravity, or the Cylindrical Sea would climb the southern cliff and flood an entire continent. But Perera had been in a comfortable study back on Earth, not with kilometers of overhanging metal apparently about to crash down upon his head. And perhaps Rama was designed for periodic flooding.
No, that was