CLOSE TO CRITICAL

pressure. When you start the electrolysis units, some of the water will be decomposed; the oxygen will be piped outside the hull, but the hydrogen will be released on the other side of the membranes and gradually drive the air out of the cells. The old bathyscaphe used the same idea, only it didn't need the membranes to keep the two fluids from diffusing into each other."

"I see. How long will it take to make enougft gas to lift us?"

"I can't tell; we don't know the conductivity of the

atmosphere. Once you start things going, there's a bank of ammeters above the switches for each individual cell; if you'll give me their reading after things start, I'll try to calculate it for you."

"All right. Where are the— Oh, here; you labeled them decently. Upper right, a bank of twelve toggles, with a gang bar and a master?"

"That's it. You can see the meters above them. Close the lot, hit the master, and give the readings."

"All right." The thin arm reached up and out of the field of vision, and everyone could hear the switches click.

Easy pulled her hand back to her lap, settled back into the chair under her three hundred pounds of weight, eyed the dials one after another, and said, "The readings are all zero. What do I do now?"

V. PEREGRINATION; CONSIDERATION; ESTIVATION

NICK had chosen a fire on the landward side of the hill, so he was the first to have to consider the sea-level problem. In the home valley, of course, the water at night had never gotten more than thirty or forty feet deep; slow as the runoff was, enough always escaped at the valley foot to keep the village itself dry. He knew, from Fagin's lectures, that the water which flowed away must eventually reach something like a sea or lake; but not even Fagin had stopped to think of what would happen then—naturally enough; the surface of Earth's oceans compared to the volume of an average day's rainfall doesn't correspond to much of a sea-level rise, to put it mildly.

On Tenebra, the situation is a trifle different. There is no single giant sea basin, only the very moderate-sized lake beds, which are even less permanent than those of Earth. What this difference could mean in terms of

"sea" level might possibly have been calculated hi advance, but not by any of Nick's people.

At first, there was nothing to worry about. The great, cloudy drops drifted into sight from far above, settled downward, and faded out as the radiation from the fires warmed them a trifle. Then they came lower, and lower, until they were actually below the level of the hilltop on all sides.

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74 CLOSE TO CRITICAL

Once a sharp quake struck and lasted for half a minute or more, but when Nick saw that the spit of land joining the hill to the shore was still there, he put this from his mind. Something much more unusual was starting to happen. At home, raindrops which touched the ground after the latter had been cooled down for the night flattened into great, foggy half-globes and drifted around until a fire obliterated them; here they behaved differently. Drops striking the surface of the sea vanished instantly and by Nick's standards, violently. The difference in pressure and temperature made the reaction between oleum and water much less noticeable than it would be in an Earthly laboratory, but it was still quite appreciable.

After each such encounter, it could be seen that further raindrops falling on the same area faded out a little higher than usual for a few minutes; Nick judged correctly that some heat was being released by the reaction.

He had been watching this phenomenon for some time, interrupted twice by the need to relight his fire when a particularly close drop smothered it, when he noticed that the hill was not an island. This startled him a trifle, and he turned all his attention to the matter. The quake hadn't done it; he particularly recalled seeing the tombolo intact after the shaking was done. It didn't take him too long to conclude that if the land wasn't sinking, the sea must be rising; and a few minutes' close watch of the shore line proved that something of that sort was happening. He called the others, to tell them of what he had seen, and after a few minutes they agreed that the same thing was happening on all sides of the hill.

"How far will it come, Nick?" Betsey's voice was understandably anxious.

"I don't see how it can get this high," Nick1 answered. "After all, it hasn't risen as much as the water in our own valley would have by this time of night, and this hill is nearly as high as the village. We're safe enough."

It got a little harder to stick to this belief as the hours

Peregrination; Consideration; Estivation 75

passed and the sea grew higher. They could see the pools on shore swell and overflow into the main body; as time went on, more than one great river formed, carrying runoff from no one knew what drainage area.

Some of the rivers were frightening, their centers as high or higher than the hill itself before they spread out and merged with the sea. By this time the violence of water-meeting-acid had subsided; the sea, at least near the shore, was pretty dilute.

Of course, "near the shore" might be too casual a statement. No one on the hilltop could tell for certain just where the shore was now. The route they had followed was deep under the acid sea, and the only evidence that dry land existed was the rivers which still came into view above sea level.

The island that had been a hill shrank steadily. The cattle seemed unperturbed, but were driven inside the ring of fires. Then this had to be drawn in—or rather, others had to be built closer to the hilltop; and at last people and animals huddled together behind a single ring of glowing heat, while the sea bulged upward at their feeble protection. The raindrops were clear now; they had fallen from high enough levels to lose their suspended oxygen, and inevitably the last fires succumbed. Their heat had for many minutes past been maintaining a hollow in the surface of the sea; and as they cooled, the ocean reclaimed its own. Seconds after the last spark died every living being on the hilltop was unconscious, and a minute later only a turbulent dimple in the surface of the sea showed where the slightly warmer hilltop was covered. Nick's last thought was to the effect that at least they were safe from animals; they would be uncovered long before anything could get at them.

Apparently he wasn't quite right. When they woke up the next morning and brushed the thin frost of quartz crystals from their scales, all the people were there, but the herd seemed to have diminished. A count confirmed

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