"Wait!" Marene placed a hand upon his arm. A sudden fear had come into her sparkling eyes. "If the gun makes a flash or a vibration of any kind, Zuldi will be sure to know. He will come here immediately!" Bob paused. "That's right," he said finally. Then, "But we've _got_ to test it! We can't wait until the black ship comes! We've got to be _sure_!" They stood there for a moment gazing into each other's eyes. Then Bob put his arm about her and drew her to him. "It doesn't matter so much about us," he told her. "We are doomed anyway. It's your father and my father and all the others in the city that we must think about! It is they...." He broke off short, his lips tight and his eyes blazing out of the window toward the far-away, dark horizon from whence the black ship would come. Several giant birds were soaring lazily through the pale sky. The young man seemed much older as he stood there. Zurk saw the lines of care in the cleanly cut face. And, at last, he recognized the youth. Bob Simms. The son of Captain Simms whose dead body had been dragged through the dusty streets by Zuldi and his men. "We've got to test it!" the young man said again. Zurk saw him put his shoulder determinedly against the piece and wheel the snout of it toward one of the soaring birds just outside the window. Zurk's photo-electric eyes saw the vibration of the charge slit through the pale sky scant inches from the bird as the youth pressed the release. But Bob had not seen that tell-tale vibration. He had seen only that the bird before him remained unharmed. The bitterness of defeat showed in his face. "Try again," urged Marene. Once again, the youth aimed the gun and pressed the release with trembling fingers. And once again Zurk's eyes saw the vibration miss its mark by inches. But he saw something else. He saw something no human eyes could see. Miles away on the far horizon, he saw another giant bird caught squarely by the charge. The bird disolved instantly into a smoky haze as every atom within it suddenly lost its valence. Zurk could have cried out with satisfaction had it not been for the tiny wire that shorted itself with white heat at the back of his neck. It made him realize once more his complete helplessness. Marene and the young man were standing there looking dejectedly at the gun. "It won't work," Bob groaned. "I've got to take it apart. I've got to try again." Zurk tried with all his might to tell the young man that the gun was a success. He tried until blackness sought to engulf him. Then his delicate ears picked up a sound vibration that sent a chill through his brain. He could hear the hurried scrape of many clawed feet in the street below. Zuldi and his men! They had detected the vibration of the gun! They were hurrying to investigate! And there was something else. Far above the distant horizon was a black speck that grew suddenly larger as it leaped forward through the pale sky.