Roughly, they corresponded to the arctic and antarctic regions on the outside of the sphere; the latitudes that alternated between decades of permanent sunlight and decades of permanent night as the hollow world swung around its vast, lazy orbit. He had actually emerged into the central space within one of the regions, without noticing anything strange about it at the time; there were lots of holes even in the smooth areas. It was the region between tunnel mouths that was different. He would have to be more attentive as he approached the other. He was hesitating whether to report the discovery and oversight now, or to wait until he had checked more details at the northern area, when he was interrupted.
“There—we can stop and make that check.” Carol’s voice interested him but did not improve his pessimism. The women must have a long, long way to travel before they reached the core, and all sorts of things could happen to the river before then. There were no rivers to be seen here, on smooth areas or rough; the only question was not whether something happened, but what?
“Here” came the Human voice. “This will do to drop.” Joe could not tell what she was talking about; Carol could presumably see. Some small tool, no doubt. “It would be better if we had a thread or fine wire to hang it from, so I could let go without any extra components, but I’ll be careful. We’ll each make several runs. Top of the robot—we know it’s level, so we can sight along it—down to level with the field disc. Ready? Three, two, one, zero.”
Joe listened, but not with full attention. He was well past Enigma’s center now, though with some hundreds of kilometers still separating him from even the nearer side. Should he hold the mappers and look personally to see whether there was water in the tunnels? The little machines should not be harmed by it if there were. Their hulls were liquid and gas tight. No, waiting would be useless precaution and a waste of time; mapping should start at once.
The Nethneen keyed a set of general commands to his small slaves. These should have the effect of forming them into a disc some ten kilometers in diameter and sending it ahead along his present flight path at several times his present speed. He could not see the mappers even at this fairly small distance, but watched his model expectantly, looking for resolution to improve shortly in the area approached by the disc. Hundreds of observing machines, feeding new signals to his computer from constantly changing positions, should give almost microscopic detail to the region immediately in front of the antenna system.
To his satisfaction, this seemed to be working. He was loss happy at the continued absence of water; he had now set his computer to note the presence of changing data at any given point in contrasting color, but only the shades he had assigned to geometric contour indication were visible so far.
Some twenty—no, twenty-three—concavities seemed to be tunnel mouths in the highly resolved region just ahead of the disc. A single key activated the principle mapping program, and three robots darted toward each of these. The others hovered for the moment, ready to form chains along the tunnels that could relay if necessary. It quickly became so, and once again mapping routine was underway.
“Forty-three centimeters per second squared.” Molly knew better than to address anyone in particular. “About two thirds the surface value. That’s discouraging one way, but better than I really dared to expect. It’s not too much cooler than the central temperature; that can’t possibly be changing linearly.”
“Rock’s a bad conductor. Even with this reflux condenser setup, most of the temperature change must occur fairly near the surface,” Carol pointed out.
“And at your end of the planet, you’re getting air currents from the central hollow,” Charley added.
Molly agreed, rather shortly. She was feeling less and less comfortable. The itch seemed to be getting worse, and she was having trouble seeing; she wanted to wipe her eyes. Blinking had no effect. Once or twice she had considered taking her helmet off briefly to attend to the sensation—she could have stood the temperature and pressure for the few seconds it should take, she was sure—but judgment overrode temptation for the time being. She wondered whether Carol was experiencing anything similar but decided it was better not to ask, at least for a while. If either of them because unable to see, things would be awkward; if both were blinded ...
The thought made her check her light once more. If Carol noticed this, or considered anything strange about it, she made no comment. The light itself seemed to be working properly, but the surrounding darkness suddenly seemed more oppressive. Darkness, and silence except for the sounds they made themselves and the voices coming through her translator—the river, in this gravity, seldom did anything audible—and the endless awareness of a planetful of rock surrounding them in all directions, ready to squeeze.
There was one good thing about this asteroidal gravity. She couldn’t really make herself feel that much would happen even if the outer part of the planet did fall in on them.
That was silly, of course. Silly to suppose it would happen; why should a hollow world, unstable by nature, pick the time she was visiting it to have its inevitable collapse? Sillier to suppose that if it did happen, it would have no personal effect. Even under this mere four percent of normal gravity, there were rocks she had seen on this trip, rocks that had certainly fallen from upper parts of the caves and passages they were traversing, which would have pressed Human and Shervah into thin films. They might have done it a little more slowly than on Earth or Nova Lidiska, but would that really be an improvement?
She twisted her mind firmly off that track.
There was no life, or anything she or Carol could recognize as life, now to be seen. Why? Temperature? Chemistry? Energy? There was no telling, nothing but guessing until the specimens in their collecting cans reached a lab. Even then there might only be more questions.
Charley, following what was evidently a different river, had reached a similar biological situation, he reported. He had not stopped to make a gravity check, but the temperature of the air around him was about the same as theirs, and he might well be closer to the center now than they were. Like them, he still had a river to follow.
But his river, he reported, was changing. He sounded puzzled.
“I took for granted that this stream must be getting deeper in this section, since it had become a good deal narrower.” No, his voice wasn’t so much puzzled aswronged. Someone was playing an unkind trick on him. “Now it’s turned into rapids, if you can call them that, with rocks sticking through the surface all over the place and water oozing around them. It just can’t be very deep, unless there was a channel ten or fifteen times as deep as it is wide that got filled by loose rubble and has water flowing between and around the rocks all the way down. That’s hard to believe.”
“Why?” asked Carol, predictably.
“I’d expect the rubble to fall, some decent fraction of the time, from the sides of the cut, and make it wider. I’ve seen canyons with pretty steep walls, but this is too much. Why would everything have come from above—especially when there isn’t always enough clearance above to supply fifty or sixty meters of rubble?”
“I grant that’s a better point,” the little Shervah conceded. “If it really isn’t so deep, though, what’s the alternative?”
“Even less attractive. The river is shrinking. It’s carrying less water.”
“Why is that unattractive?” Joe’s attention had been caught by Charley’s report, and his pessimism vanished as his mind found something to work on. “You have hot air from the core blowing outward, against the flow of the river—your reflux condenser, Molly. The river is simply evaporating and getting blown back up to where it can condense. The fact that water’s equilibrium vapor pressure is great enough to let it be liquid there, or even down here, is no guarantee that the actual vapor pressure is high enough in either place for the liquid to be present. My home world may not have air enough to give us weather, but even I know that much physical chemistry. Dynamic equilibrium is an interesting state to study and a useful one to produce industrially, but one never has the right to assume it’s the actual, current situation.”