Back and forth--watching it closely made him almost dizzy. What was its purpose? Was it a detection device, to locate him? Raf's hand went to his stun gun. What effect its rays might have on the box he had no way of knowing, but at that moment he was sorely tempted to try the beam out, with the oscillating machine as his target. The motion of the floating black thing became less violent, its swoop smoother as if some long-idle motor was now working more as its builders had intended it to perform. The swing made wide circles, graceful glides as the thing explored the air currents. Searching--it was plainly searching for something. Just as plainly it could not be hunting for him, for his presence on that roof would have been uncovered at once. But the machine was--it must be--out of sight of the warriors in the street. How could they keep in touch with it if it located what they sought? Unless it had some built-in signaling device. Determined to keep it in sight, Raf risked a jump from the parapet of the building where he had taken cover to another roof beyond, running lightly across that as the hound bobbed and twisted, away from its masters, out across the city in pursuit of some mysterious quarry.... * * * * * The climb which had looked so easy from the street proved to be more difficult when Dalgard actually made it. His hours of swimming in the river, the night of broken rest, had drained his strength more than he had known. He was panting as he flattened himself against the wall, his feet on one of the protruding bands of colored carving, content to rest before reaching for another hold. To all appearances the city about him was empty of life and, except for the certainty of the merpeople that the alien ship and its strange companion had landed here, he would have believed that he was on a fruitless quest. Grimly, his lower lip caught between his teeth, the scout began to climb once more, the sun hot on his body, drawing sweat to dampen his forehead and his hands. He did not pause again but kept on until he stood on the top of the shortened tower. The roof here was not flat but sloped inward to a cuplike depression, where he could see the outline of a round opening, perhaps a door of sorts. But at that moment he was too winded to do more than rest. There was a drowsiness in that air. He was tempted to curl up where he sat and turn his rest into the sleep his body craved. It was in that second or so of time when he was beginning to relax, to forget the tenseness which had gripped him since his return to this ill-omened place, that he touched--