Puffs of smoke bloomed high in air--but any hit on such a fast-moving target, from so unstable a platform, would have been a miracle. The enemy screeched overhead, and an instant later flame and thunder erupted all around the running spider. The machine stumbled and for a moment seemed going down, but it righted itself and staggered on. Dworn shook his ringing head and saw the flier banking steeply half a mile away, while a second and a third were climbing against the sky, gaining altitude to dive. They couldn't last another thirty seconds, couldn't even hope to reach the doubtful cover of the rocks.... Up ahead, two hundred yards, was a low mound, only a few feet high, the only nearby elevation of any sort. And it was plainly artificial, though wind-piled sand had softened its outlines; others like it were scattered around the periphery of the great sink, and Dworn guessed their nature as he saw a column of the aluminum crawlers beginning to emerge from the side of the one just ahead. It must be the other end of a tunnel such as they had discovered among the cliffs.... He nudged Qanya urgently, shouted, "Head for that!" She gave him a fleeting, wide-eyed look. The mound's low swell could furnish no shelter for the towering spider, and the tunnel mouth was of course much too small to enter. But she veered without slackening speed in the direction indicated. Dworn abandoned the useless guns. The mound, with a gleaming line of crawlers still parading out of it, swept closer; and at the same time the desert echoed back the screaming onrush of the two new attackers. Dworn wrenched open the cabin door with one hand. His other arm circled Qanya's waist, dragged her away from the controls. She cried in uncomprehending shock as he swung her before him into the open doorway. They swayed there, high above the speeding ground, wind whipping at them as the spider pounded blindly on. The mound loomed immediately at hand. Dworn prayed that he had judged the moment right, and with a mighty leap launched both of them out into space. A pistoning steel leg barely missed them. Even as they fell, the air was torn by explosions as the swooping fliers opened fire. * * * * * Dworn hit the ground with almost stunning force. His hold on the girl was broken and he was rolled helplessly over and over by his own momentum. But he fetched up on hands and knees, bruised and breathless but unhurt. From the corner of his eye he saw Qanya sitting up dizzily, half-buried in the drifted sand that had broken their fall. Apparently she too was uninjured, but she was staring in horrified fascination after her runaway machine. The spider careened onward, no hand at its controls. It hit the line of crawling little machines coming from underground; it knocked one spinning end over end, and stepped squarely on another, stamping it flat. It recovered its balance amazingly, and loped on, even though one leg was buckling beneath it--