The _Esperance_ sailed across the live boundary and into the dark sea. As the boat went on, the sound dimmed.... "Definitely loudest," said Terry absorbedly, "at the edge of the circle of fish. At the line the fish couldn't cross to escape. It is if there were an electric fence in the sea. It felt like that, too. But there isn't any fence." Davis asked evenly, "Question: what holds them crowded?" Terry said again, groping in his mind, "They act like fish in a closing net. I've seen something like this once, when a purse-seine was hauled. Those fish were frantic because they couldn't get away. Just like these." "Why can't they get away?" asked Davis grimly. "We haven't seen anything holding them." "But we heard something," pointed out Deirdre. "The hum. That may be what closes them in." Her father made a grunting noise. "We'll see about that." He moved away, back to the stern. In moments, the _Esperance_ was beating upwind. Presently, she headed back toward her previous position, but outside the brightness. Terry could see dark silhouettes moving about near the yacht's wheel. Then he saw another brightness at the eastern horizon, but that was in the sky. Almost as soon as he noticed it, the moon peered over the edge of the world, and climbed slowly to full view, and then swam up among the lower-hanging stars. Immediately, the look of the sea was different. The waves no longer seemed to race the darkness with only star glitters on their flanks. The figures at the _Esperance's_ stern were now quite distinct in the moonlight. "You said a very sensible thing, Deirdre," said Terry. "I thought of the fish-driving paddle and its effects, but I was ashamed to mention it. I thought it would sound foolish. But when you said it, it didn't." "I have a talent," said Deirdre, "for making foolish things sound sensible. Or perhaps the reverse. I'm going to say a sensible thing now. We haven't had dinner. I'm going to fix something to eat." "You won't get anybody to go belowdecks right now!" said Terry. "I thought of that," she told him. "Sandwiches." She went below. Terry continued to watch, while figures at the stern of the schooner went through an involved process of visual measurement. It was not simple to determine the dimensions of a patch of shimmering light flashes from a boat in motion. But presently, Davis came toward him. "It's thirteen hundred yards across," he told Terry. "Plus or minus twenty." "I didn't expect all this," Davis said, frowning. "I've been making guesses and hoping fervently that I was wrong. And I have been, but each time the proof that I was wrong has led to new guesses, and I'm afraid to think those guesses may be right." "I can't begin to guess yet," said Terry. "You will!" Davis assured him. "You will! You try to add up things.... A half-mile-wide patch of foam that piles up thirty feet above the sea...."