But suddenly Harwich squinted. Something _was_ moving toward him, skimming low over the ground, but not touching it. Something that glinted wickedly, and showed long, shadowy arms. It was no hallucination. Evan Harwich was sure of that! Fear came out of that numb fog into which his brain was settling. It gave him a last, feeble spurt of strength. He knew that here he must be facing a tiny part of Io's colossal riddle. He tried to crawl away from nameless danger, dragging Paul Arnold with him. He got behind a mass of million-year-old masonry, tufted with prickly plants. But the thing that pursued him, easily overcame his weak, instinctive effort to find concealment. Cold metal claws closed on him. He felt himself lifted upward, into the night. His mind toppled away into black nothingness. * * * * * Somehow, it wasn't the end of life. Harwich began to regain his senses, slowly. First he heard a distant, muffled clanging. For a long time before he paid any real attention to the fact, he was aware that strange warm rays were pouring down upon his body. They seemed to heal and soothe his aching muscles. He opened his eyes at last. Startled, he sat up. Around him was the warm glitter of glass and metal. His space suit was gone. He was in a crystalline cage, filled with warm, humid air. Odd gadgets, like ray lamps used in therapy, were fitted to the ceiling. Strange, tropical vegetation grew in the cage, and water tinkled somewhere. There was a kind of soothing quiet over the place, except for that distant clanging. There was a smoothness to everything; a mood of mechanical refinement and perfection. It was almost hypnotic, somehow. It dazed and quieted the senses. Paul Arnold, clad in the slacks and shirt he'd worn under his space armor, was lying on the floor beside Harwich. He was still unconscious, but he was breathing evenly. His color was much better than before. The rays from the roof above were slowly healing his weakened body. Evan Harwich shook the boy gently. "Wake up, Paul!" he urged. "This must be it! The center of Power! The place we wanted to find! Some kind of machine brought us!" Paul Arnold rubbed his eyes and sat up. Together, Harwich and the boy looked around through the crystal walls of the cage in which they were confined. "There--there's the Tower!" young Arnold stammered at last, pointing. It glittered in the faint morning sunshine. It was undoubtedly the same huge pinnacle that astronomers had photographed from the other moons of Jupiter. Only it was close, now, its details sharp and clear and real. Around its slender, tapered spire, thousands of feet aloft, the faintest of frosty aureoles clung; a ghostly light, like the sundogs of Earthly winter days. "The Tower must be the source of the Ionian force envelope, Evan!" Paul Arnold offered after a moment. "That light up there at its top almost proves it."