"They should be under water by then," pointed out Raeker mildly.

"And drifting again, I suppose. That's when everything will go to pieces at once, and we'll have a couple of screaming kids who'll probably start hitting switches right and left in the hope some miracle will bring them home."

"I don't know about the Drommian, but I think you do your daughter a serious injustice," replied Raeker.

"I've never known much about kids, but she strikes me as something pretty remarkable for her age. Even if you can't trust her, you'd better not let her know it."

"I realize that, and no one trusts her more than I do," was the weary answer. "Still, she is only a kid, and a lot of adults would have cracked before now. I can name one who's on the edge of it. Listen to them, down there."

Aminadorneldo's piercing tones were echoing from the speaker.

Radiation; Evaporation; Advection 117 "There's something on this side, Easy! Come and see this one."

"All right, 'Mina. Just a minute." Easy's small form could be seen for an instant on the screen, pa.s.sing through the control room from one side of the ship to the other, calling as she went, "It's probably another of those plant-eating things that are about as big as Nick's people. Remember, the ones we want stand up on end."

"This one does. Look!"

"Where?" Aminadorneldo must have been pointing; there was a moment of silence; then the girl's voice, "I still don't see anything; just a lot of bushes."

"It looked just like Nick. It stood beside that bush for a moment and looked at us, and then went away. I saw it." "Well, if you were right, it'll come back. We'll stand here and watch for it."

Rich looked at Raeker and shook his head dismally.

"That'll-" he began, but got no further. His sentence was interrupted by a sudden shriek from the speaker, SO' shrill that for a moment neither of them could tell who had uttered it.

VIII. RADIATION; EVAPORATION; ADVECTION.

JOHN and Nancy made steady progress into the west. Their journey so far had not been particularly difficult, though most of it had been made over ground not yet 118 CLOSE TO CRITICAL.

surveyed. They had fought with floaters and other carnivores a reasonable number of times, eaten the fruits of their victories when they felt hungry enough, and talked more or less incessantly. The talk was mostly speculation; they had learned more about the nature of their Teacher in the last few days than in the preceding sixteen years, but what they had learned seemed only to give rise to more questions. They were young enough to be surprised at this; hence the steady conversation, which was interrupted only by their reaching a region which seemed to match part of their map.

"We must have kept our direction pretty well," Nancy said after comparing the hills around them to those indicated on the sheet. "We were trying to hit the mapped region about here," she pointed, "and seem to be only a dozen miles to the north. Oliver mapped this place; it hasn't changed enough to be really doubtful.

We can head south, and be sure of ourselves in a few more miles."

"All right," agreed John. "You know, even if we are still a good many miles from either of our search areas, it wouldn't actually hurt to keep our eyes open for the machine."

Nancy sent the ripple that pa.s.sed for a shrug flickering down her scales. "It's hardly worth making a special effort. We'll be able to see it miles away, if it's as bright as f.a.gin said. I think we'd better concentrate on the map, just now, until we're sure we're where we're supposed to be."

"f.a.gin would have had something to say about that sentence," muttered John, "but I suppose you're right.

Let's get on.".

Two miles, twenty-five minutes, one brief fight, and one longisb quake later they were in a position to feel sure of themselves. Uniform as the solution-molded surface of Tenebra was, and rapid as its changes were, the present region matched the maps too closely to be coincidence. They spent a few minutes deciding whether it would be better to start gathering firewood for the night which was Radiation; Evaporation; Advection 119 not too many hours away or move closer to their first search area so as to waste less time in the morning, set- tled on the second alternative, and went on.

Nightfall was even closer when they stopped simultaneously. Neither needed to speak, since it was quite evi- dent to both that they had seen the same thing. Far to the south and somewhat to the west a light was shining.

For several seconds they stood looking at it. What they could see was not particularly brilliant-it was just enough to be noticeable; but light other than daylight on Tenebra can be explained only in a certain very few ways. So, at least, f.a.gin's students supposed.

After a moment's staring, they got out the maps once more and tried to judge where the source of the light might be. This was difficult, however, because it was next'to impossible to estimate the distance. The source itself was not directly visible, just the glow which fires, spotlights, and Altair itself produced in Tenebra's soupy envelope. The direction was plain enough, but it seemed likely that the actual source was either outside mapped territory altogether or in the poorly covered region Nick had done during the trip which had discovered the cave village. It seemed equally likely that they could not possibly reach the place before rainfall, but after the briefest of discussions they agreed to start out.

The going was normal at first, but it gradually got rougher. This agreed with what they remembered of Nick's report on his trip. They also recalled his mention of a life form which lived in holes and was dangerous to pa.s.sers-by, but they encountered no sign of it just then. The light kept getting brighter, which was encouraging, but for several hours they failed to get any better idea of what was making it.

Then they began to get an impression that it was coming from a point above their level, and after another half hour they were both quite sure of this. The fact was hard to understand; f.a.gin had said that the bathyscaphe 120.

CLOSE TO CRITICAL couldn't fly because it was broken, and there had been no mention of a hill-at least, not of anything unusual in that respect-in the description of the machine's environment. As a matter of fact, they recalled, it had been stated to lie at the foot of a hill.

Then John remembered Nick's tale of a remarkably high hill in the region, and the two got out their maps once more. It seemed possible though far from certain, after careful checking, that the light was on the hill; but if that were the case it seemed to dispose of any remaining chance that they had found the bathyscaphe. Since the only other possibility they could envision was that Swift's people were there with a fire, a slight problem developed.

It would be raining before long, and travel without torches would be impossible. If the area ahead were ac- tually a camp of Swift's cave dwellers, approaching it with torches would simply be asking for capture. Of course, the chief might have accepted f.a.gin's offer, so that they would technically be allies; but from what John and Nancy knew of Swift they didn't want to take the chance. From one point of view, there was no reason to approach at all, since they were searching for the bathyscaphe rather than scouting the cave men; but this phase of the matter didn't occur to either of them. If it had, they would probably have insisted that they weren't sure the light wasn't from the crippled machine. Anyway, they kept trying to plan a method of approach to the light.

It was Nancy who finally worked it out. John didn't like the plan and didn't trust it. Nancy pointed out truthfully that she knew more physics than he did, and even if he didn't know what she was talking about he ought to take her word for it. He replied, equally truthfully, that he might be a mathematician rather than a chemist but even he knew enough about rain not to accept ideas like hers uncritically. Nancy finally won her point by the simple process of starting toward the light alone, giving John the choice of coming or staying behind. He came.

Radiation; Evaporation; Advection 121 Raeker would have liked to hear that argument. He had named the little creatures who had emerged from the stolen eggs quite arbitrarily, and still had no idea of the actual gender of any of them. Nancy's display of a human-feminine characteristic would have been fascinating if not very conclusive.

John watched the sky uneasily as they strode onward. Inwardly he knew perfectly well that the rain was not due for a while yet; but the mere fact of Nancy's defiance of the phenomenon made him abnormally conscious of it. By the time the first drops actually appeared far above, they were close enough to the light to see that something lay between them and the actual source-it was shining from behind a barrier of some sort, presumably a hill.

"Should we go over, or around?" John asked, when this fact became evident. "If we go up, we'll run into the rain sooner."

"That's a good reason for doing it," retorted Nancy. "If it is the cave people they won't be expecting us from that direction, and you'll see all the sooner that I'm right. Besides, I've never been up a really high hill, and Nick said this one was two or three hundred feet tall-remember?"

"I remember, but I'm not as sure as you seem to be that this is the hill he was talking about."

"Look at your map!"

"All right, I know we're close to it, but his notes were pretty rough; you know that as well as I do. There never was time to make a decent map of the country he covered, after he got back. We've been fighting or moving practically ever since."

"All right, you needn't make a thesis out of it. Come on." She led the way without waiting for an answer.

For some time there was no appreciable rise in the general ground level, though the number of ordinary hillocks remained about as usual. The first implication that Nancy might be right about the nature of the hill 122.

CLOSE TO CRITICAL.

was a change in the nature of the ground underfoot. Instead of the usual feldspar-rich granitic rock, heavily pitted with solution cavities, a darker, much smoother material became predominant. Neither of them had ever seen fresh lava, since Nick had brought back no specimens, and it took time for their feet to get used to it. The rain was getting very close to the surface now. There was no difficulty in dodging drops, since there was more light coming from ahead than Altair gave at high noon; the trouble was that Nancy was not bothering to dodge them. Theoretically she was right enough; they weie still cloudy with oxygen bubbles, and her body heat turned them into perfectly breathable air, but it took a while for John to follow her example. Habits are as hard to break for Tenebrites as for human beings.

Gradually the slope of the dark rock began to increase. They were on a hill, and the light was close ahead, now. Rocks were silhouetted sharply against it, not more than a mile in front. Nancy stopped, not because of the rain but to take a final look around; and it was then that they both noticed something else.

In the first place, the raindrops were not falling straight; they were drifting horizontally as they descended, drifting in the same direction as the two were traveling. That was reasonable when one stopped to think; they had known about convection and advection currents almost as long as they could remember. It was the speed that was remarkable; the drops were heading toward the fire at a good two miles an hour. The air current that impelled them could actually be felt-and that was a major hurricane, for Tenebra. If the thing ahead was a fire, it was a bigger fire than f.a.gin's pupils had ever lighted or ever seen.

"If Swift lighted that, he must have touched off a whole map section," remarked John.

Nancy turned to him abruptly. "Johnny! Remember what happened last night, when Nick got the Teacher away from the caves? He did light fires over a good part Radiation; Evaporation; Advection 123 of a section! Do you suppose they could still be burning, and have spread like this?"

"I don't know." John stood still and thought for a moment or two. Then he referred to the map, easily legi- ble in the brilliant light. "I don't see how it could be," he said at length. "We're a lot closer to the caves than we were this morning, but not that close. Besides, the clear rain late at night should have put any fire out if there was no one to tend it."

"But if it were big enough, maybe it would stir up the air so there was always enough oxygen for it-feel this wind on our backs. Have you ever known anything like it?"

"No. Maybe you're right. We can go on and see, though; I still think it's more likely to be Swift. Are you still going to try that idea of yours?"

"Of course. It's all the better, with the wind carrying the drops as fast as this."

"I hope you're as right as you are reasonable." The two went on, somewhat more slowly since it was necessary to follow a rather tortuous path to keep their goal in sight among the drops. These were now reaching the surface in great numbers and remaining liquid, except for those parts most closely exposed to the body heat of the two travelers. It took a little longer than might have been expected, therefore, to get within two hundred yards of the rocks ahead, which from the absence of anything but light beyond them appeared to mark the top of the bill. At this point, Nancy decided that stealth was in order; so she brought the scary part of her plan into operation.

Finding an exceptionally large and still cloudy rain drop drifting downward at no great distance, she deliberately placed herself so as to be enveloped by it as it landed. Naturally, the bottom portion of the fifty-foot spheroid was obliterated at once by her body heat; but further descent of the drop finally hid her from view. The great, foggy blot of liquid began to follow the general pat- 124 CLOSE TO CRITICAL.

tern of activity of the others, moving slowly toward the light; and Nancy did her best to follow. This was not as easy as it might have been, even though the gas around her was perfectly breathable, since with no view of her surroundings it was nearly impossible to judge the rate of drift of the raindrop. The wind was some help, but not enough, and several times John could see her outline as she came too close to the edge of the volume of fog. He stayed where he was, not considering it cowardly to see how the experiment turned out before he tried it himself.

In one sense, the trial was a perfect success; that is, Nancy remained conscious as long as the drop lasted.

In another, however, there was something lacking. This lay in the failure of the drop to last long enough.

Suffering the a.s.sault of heat radiation both from Nancy within and the fire ahead, the thing abruptly faded out in a final surge of turbulence, leaving her in full view.

This turned out to be less of a catastrophe than it might have been. For three or four seconds after the vanishing of her concealment Nancy stood perfectly still; then she called out, making no effort to direct her voice away from the light ahead, "Johnny! Here, quick!"

Her companion leaped forward, taking a little but not much less care to dodge raindrops, and came to a halt at her side.

She had stopped perhaps five yards from the edge of a nearly vertical-sided pit, fully two miles across.

Her first few seconds of silence had been spent in telling herself how lucky she was that her shelter had not lasted a few seconds longer; then the blast of radiant heat coming from the floor of the crater, a scant hundred feet below, forced her to admit the matter was hardly one of luck. It could be seen from this vantage point that no raindrops at all approached the area except those which drifted up the slope of the hill from outside. The floor glowed visibly all over, and numerous patches were of almost dazzling bril- liance. These last looked suspiciously like liquid, though Radiation; Evaporation; Advection 125 the liquid possessed a remarkably sharp and well-defined surface.

Raeker, or even Easy, would have recognized a volcano at once; but the phenomenon was completely outside the experience and education of f.a.gin's pupils. Raeker had noted, in pa.s.sing, Nick's earlier reference to the conical shape of the high hill he had reported; the geologists had also paid some attention to it, and even placed it on the list of things to be investigated more fully; but that was as far as matters had gone. Nick had said nothing to suggest that the thing was active-or rather, nothing the men had recognized as such evidence; he had mentioned wind. As a matter of fact, it had not been nearly so violent when he had pa.s.sed, some three terrestrial months before. Only its size and shape had been worthy of note.

"You know," John remarked after some minutes of silence, "this would be a wonderful place for a village.

We wouldn't need to keep fires going."

"How about food?" countered Nancy. "The plants growing on this dark rock are different from the ones we're used to; maybe the cattle wouldn't eat them."

"That would be easy enough to find out-"

"Anyway, that's not the a.s.signment just now. This light obviously isn't what we're looking for, though I admit it's interesting. We'd better get on with the job."

"It's raining," John pointed out, "and there was no suggestion that we should search through the night as well as by day. This would seem a perfect place to sleep, at least."

"That's true enough-" Nancy's agreement was interrupted suddenly. Some three hundred yards to their left, a segment of the pit's edge about fifty yards long and ten or fifteen deep cracked loose with a deafening roar and plunged downward. In that gravity even Tenebra's atmosphere was an ineffective brake, and a good ten or fifteen thousand tons of well-cemented volcanic detritus made its way effortlessly through the red-hot crust of nearly 126 CLOSE TO CRITICAL.

solid lava at the foot of the ledge. The results left no doubt about the liquid state of the hotter material-or would have left none had the two explorers still been watching. They weren't; they were on their way downhill in the direction from which they had come before the ma.s.s of rock was completely detached.

Even as he ran, John had time to feel lucky that the incident had waited to happen until Nancy had agreed with him about what a good camping spot the place was. Needless to say, he did not mention this aloud.

Even John was not bothering to dodge raindrops at the moment, much less talk on irrelevant subjects.

They covered nearly a mile down the slope before stopping. The light was still quite ample to permit read- ing the maps, and it took only a few minutes to convince them both that this was indeed the tall, conical hill which Nick had reported. With this settled, however, neither could quite decide what to do about it.

The natural urge was to return to the camp to report the phenomenon to f.a.gin; against this, however, lay the fact that they had another a.s.signment to complete, which involved life and death.

"This can wait a day," John pointed out. "We can perfectly well camp right here, search our areas tomorrow, and then go back as was planned. We can't drop everything for one new discovery."

"I suppose not," agreed Nancy with some slight reluctance, "but we certainly can't camp here. There isn't enough fuel for a dozen hours on this black stone, to say nothing of the rest of the night; and the raindrops are starting to get clear."

"That I had noticed," replied John. "We'd better get going, then. Just a minute; there's enough here to make a torch. Let's get one started; we may be a little pressed for time later."

Nancy agreed with this observation, and ten minutes later they were on their way once more with John carry- Radiation; Evaporation; Advection 127 ing a glowing torch and Nancy the material for two more, all that the vegetation within convenient reach afforded. They headed toward a region which their maps showed as having slightly higher hills than usual, so as to avoid finding themselves in a lake bed before morning. Both were becoming a trifle uneasy, in spite of Nick's earlier success at all-night travel; but they were distracted once more before getting really worried.

Again a light showed ahead of them. It was harder to perceive, since the brilliance from behind was still great, but there was no doubt that a fire of some sort was on one of the hilltops ahead of them.

"Are you going to sneak up on this one the way you did on the other?" queried John.

Nancy glanced at the now dangerously clear raindrops and did not condescend to answer. Her companion had expected none, and after a moment asked a more sensible question.

"What about this torch? If we can see that fire, anyone near it can see us. Do you want to put it out?"

Nancy glanced upward-or rather, shifted her attention in that direction by a subtle alteration in the positions of her visual spines, which acted rather like a radio interferometer system, except that they were sensitive to much shorter wavelengths. "We'd better," she said. "There's plenty of light to dodge the drops."

John shrugged mentally and tossed the glowing piece of wood under a settling raindrop. The two slipped up toward the distant light.

It was an ordinary fire this time, they could see as they approached. Unfortunately, there was no one visible near it, and the vegetation was not dense enough to hide anyone of ordinary size unless he were deliberately seeking to use it for the purpose. This suggested possible trouble, and the two explorers circled the hill on which the blaze stood with the most extreme caution, looking for traces of whoever had been there in the past few hours. Not 128 CLOSE TO CRITICAL.

having the tracking skill of the cave dwellers, they found no signs of people. After two full circuits and some low-voiced discussion, they were forced to conclude that either whoever made the fire was still on the hill but remarkably well hidden, or else the fire itself had been started by something a trifle unusual.

The latter hypothesis would probably not have occurred to them had it not been for their recent experience with the volcano. There seemed no way, however, to decide between the possibilities by reason alone.

Closer investigation was in order and, with a constant expectation of hearing the sharp voice of Swift echoing about them, they set to it. Very carefully, exam-* ining every bush, they went up the slope.

The climb bore some resemblance to a scientific experiment, in that its completion eliminated both of the hypotheses and left them completely without ideas for a moment. It was only for a moment however; as the two loomed up beside the small fire, which had quite obviously been laid by intelligent hands, a shout sounded from the next hilltop, three hundred yards away.

"John! Nancy! Where did you come from?" The startled investigators recognized simultaneously the voice of Oliver and the fact that they had been a little hasty in eliminating possibilities; obviously they had missed a trail, since neither Oliver nor Dorothy could fly. Neither said anything about it aloud; each decided in private that the different vegetation of the area was responsible.

When Oliver and his companion came back to the fire from the separate hilltops to which they had taken on sighting John's torch, it quickly transpired that they, too, had seen the light of the volcano and had come to investigate it. Their adventures had been very similar to those of John and Nancy, except that neither of them had tried hiding in raindrops. Oliver and Dorothy had been an hour or so ahead of the others, and had found a good supply of fuel, so they were well set for the night.

"I'll bet Jim and Jane will be with us before the night's Radiation; Evaporation; Advection 129 over," remarked Nancy when both parties had completed thek exchange of information. "Their search areas were even closer to this place than yours, Oliver, and unless they went 'way off course coming across country they must have seen the big light, too."

"Maybe they thought it was better to stick to their a.s.signed job," remarked John.

"Isn't investigating bright lights part of the job?" retorted his partner. "As for me, if they're not here in an hour or two I'm going to start worrying about them. This fire-hill couldn't possibly be missed or ignored, and you know it."

No one had a suitable answer for this, but no one was really impressed by the reasoning, since they had all spent some time in discussion before coming to check the mountain. At any rate, the hours pa.s.sed without the predicted appearance. If Nancy was worrying, she failed to show it; certainly none of the others were.

It was a very quiet night, and there was nothing to worry about. The hours were pa.s.sing, but that was normal; the light was getting brighter, but there was the peculiar hill to account for that; the rain was decreasing, but the hill might account for that, too. The fire was using up its fuel with unusual speed, but there was plenty of fuel. Doubtless the wind was responsible-none of them had ever experienced such a wind, and an air current one could actually feel would no doubt do many queer things. The four explorers stood by their fire and dozed, while the wind grew fiercer.

IX. DEDUCTION; EDUCATION; EXPERIMENTATION.

"DADDY! Dr. Raeker! 'Mina's right; it's Nick!" Easy's voice was close to hysteria. The men glanced at each other, worried frowns on their faces. Rich gestured that Raeker should do the answering, but his expression pleaded eloquently for care. Raeker nodded, and closed his own microphone switch.

"Are you sure it's actually Nick, Easy?" he asked in as matter-of-fact a voice as he could manage. "He's supposed to have stayed at the camp, you know. There are six others actually searching, supposedly in pairs; do you see two of them, there?"

"No," replied Easy in a much calmer voice. Her father sank back in his chair with a thankful expression on his face. "There was only one, and I saw him just for a second. Wait-there he is again." Raeker wished he could see the girl's face, but she was shouting her messages from one of the observing chambers and was well out of pickup range of the vision transmitter. "I can still see only one of them, and he's mostly hidden in the bushes-just his head and shoulders, if you can call them that, sticking up. He's coming closer now. He must see the 'scaphe, though I can't tell where he's looking, or what he's looking with. I'm not sure whether he's the same size, but he certainly is the same shape. I don't see how you'd ever tell them apart."