sections would be equally obliging.
One of the engineers was carefully going along the rim of the caisson with a spirit level, to check that it was sinking vertically. Presently he gave the thumbs-up signal, which Lawrence acknowledged in the same manner. There had been a time when, like any regular spacehog, he could carry out an extended and fairly technical conversation by sign-language alone. This was an essential skill of the trade, for radio sometimes failed and there were occasions when one did not wish to clutter up the limited number of channels available.
“Ready for Number Two!” he said.
This would be tricky. The first section had to be held rigid while the second was bolted to it without altering the alignment. One really needed two cranes for this job, but a framework of I-beams, supported a few centimeters above the surface of the dust, could carry the load when the crane was otherwise engaged.
No mistakes now, for God's sake! he breathed silently. Number-two section swung off the sledge that had brought it from Port Roris, and three of the technicians manhandled it into the vertical. This was the sort of job where the distinction between weight and mass was vital. That swinging cylinder weighed relatively little, but its momentum was the same as it would be on Earth, and it could pulp a man if it managed to trap him on one of those sluggish oscillations. And that was something else peculiar to the Moon—the slow-motion movement of this suspended mass. In this gravity, a pendulum took two-and-a-half times as long to complete its cycle as it would on Earth. This was something that never looked quite right, except to a man who had been born here.
Now the second section was upended and mated to the first one. They were clamped together, and once again Lawrence gave the order to lower away.
The resistance of the dust was increasing, but the caisson continued to sink smoothly under its own weight.
“Eight meters gone,” said Lawrence . “That means we're just past the halfway mark. Number-three section coming up.”
After this, there would only be one more, though Lawrence had provided a spare section, just in case. He had a hearty respect for the Sea's ability to swallow equipment. So far, only a few nuts and bolts had been lost, but if that piece of caisson slipped from the hook, it would be gone in a flash. Though it might not sink far, especially if it hit the dust broadside on, it would be effectively out of reach even if it was only a couple of meters down. They had no time to waste salvaging their own salvage gear.
There went number three, its last section moving with almost imperceptible slowness. But it was still moving; in a few minutes, with any luck at all, they would be knocking on the cruiser's roof.
“Twelve meters down,” said Lawrence . “We're only three meters above you now, Selene. You should be able to hear us at any minute.”
Indeed they could, and the sound was wonderfully reassuring. More than ten minutes ago Hansteen had noticed the vibration of the oxygen inlet pipe as the caisson scraped against it. You could tell when it stopped, and when it started moving again.
There was that vibration once more, accompanied this time by a delicate shower of dust from the roof. The two air pipes had now been drawn up so that about twenty centimeters of their lengths projected through the ceiling, and the quickdrying cement which was part of the emergency kit of all space vehicles had been smoothed around their points of entry. It seemed to be working loose, but that impalpable rain of dust was far too slight to cause alarm. Nevertheless, Hansteen thought that he had better mention it to the skipper, who might not have noticed.
“Funny,” said Pat, looking up at the projecting pipe. “That cement should hold, even if the pipe is vibrating.”
He climbed up on a seat, and examined the air pipe more closely. For a moment he said nothing; then he stepped down, looking puzzled and annoyed—and more than a little worried.
“What's the trouble?” Hansteen asked quietly. He knew Pat well enough now to read his face like an open book.
“That pipe's pulling up through the roof,” he said. “Someone up on the raft's being mighty careless. It's shortened by at least a centimeter, since I fixed that plaster.” Then Pat stopped, suddenly aghast. “My God,” he whispered, “suppose it's our own fault, suppose we're still sinking.”
“What if we are?” said the Commodore, quite calmly. “You'd expect the dust to continue settling beneath our weight. That doesn't mean we're in danger. Judging by that pipe, we've gone down one centimeter in twenty-four hours. They can always give us some more tubing if we need it.”
Pat laughed a little shamefacedly.
“Of course—that's the answer. I should have thought of it before. We've probably been sinking slowly all the time, but this is the first chance we've had to prove it. Still, I'd better report to Mr. Lawrence—it may affect his calculations.”
Pat started to walk toward the front of the cabin; but he never made it.
CHAPTER 25
It had taken Nature a million years to set the trap that had snared Selene and dragged her down into the Sea of Thirst . The second time, she was caught in a trap that she had made herself.
Because her designers had no need to watch every gram of excess weight, or plan for journeys lasting more than a few hours, they had never equipped Selene with those ingenious but unadvertised arrangements whereby spaceships recycle all their water supply. She did not have to conserve her resources in the miserly manner of deep-space vehicles; the small amount of water normally used and produced aboard, she simply dumped.
Over the past five days, several hundred kilos of liquid and vapor had left Selene, to be instantly absorbed by the thirsty dust. Many hours ago, the dust in the immediate neighborhood of the waste vents had become saturated and had turned into mud. Dripping downward through scores of channels, it had honeycombed the surrounding Sea. Silently, patiently, the cruiser had been washing away her own foundations. The gentle nudge of the approaching caisson had done the rest.
Up on the raft, the first intimation of disaster was the flashing of the red warning light on the air purifier, synchronized with the howling of a radio klaxon across all the space-suit wave bands. The howl ceased almost immediately, as the technician in charge punched the cutoff button, but the red light continued to flash.
A glance at the dials was enough to show Lawrence the trouble. The air pipes—both of them—were no longer connected to Selene. The purifier was pumping oxygen into the Sea through one pipe and, worse still, sucking in dust through the other. Lawrence wondered how long it would take to clean out the filters, but wasted no further time upon that thought. He was too busy calling Selene.
There was no answer. He tried all the cruiser's frequencies, without receiving even a whisper of a carrier wave. The Sea of Thirst was as silent to radio as it was to sound.
They're finished, he said to himself; it's all over. It was a near thing, but we just couldn't make it. And all we needed was another hour.
What could have happened? he thought dully. Perhaps the hull had collapsed under the weight of the dust. No—that was very unlikely; the internal air pressure would have prevented that. It must have been another subsidence. He was not sure, but he thought that there had been a slight tremor underfoot. From the beginning he had been aware of this danger, but could see no way of guarding against it. This was a gamble they had all taken, and Selene had lost.
Even as Selene started to fall, something told Pat that this was quite different from the first cave-in. It was much slower, and there were scrunching, squishing noises from outside the hull which, even in that desperate moment, struck Pat as being unlike any sounds that dust could possibly make.
Overhead, the oxygen pipes were tearing loose. They were not sliding out smoothly, for the cruiser was going down stern first, tilting toward the rear. With a crack of splintering Fiberglas, the pipe just ahead of the air-lock galley ripped through the roof and vanished from sight. Immediately, a thick jet of dust sprayed into the cabin, and
