Pistols in hand, Carnes and the doctor crawled over the crest and joined the advance. Again and again the rifle spoke from the hollow and was answered by the vicious barks of the rifles in the hands of the guides, Carnes and the doctor resting their pistols on rocks and sending an occasional bullet toward the loophole. The conditions of light and the moving target were not conducive to good marksmanship on the part of the besieged man, and none of the attackers were hit. Presently Walter succeeded in sending a bullet through the loophole. The rifle barrel suddenly disappeared. With a shout the four men rose from their cover and advanced toward the building at a run. As they did so an ominous whirring sound came from the apparatus in front of the house and a sudden chill filled the air. "Back!" shouted Dr. Bird. "Back below the hill if you value your lives!" He turned and raced at full speed toward the sheltering crest of the hill, the others following him closely. The whirring sound continued, and the concave reflector turned with a grating sound on its gears. As the path of its rays struck the ground the rocks became white with frost and one rock split with a sharp report, one fragment rolling down the slope, carrying others in its trail. * * * * * With panic-stricken faces the four men raced toward the sheltering crest, but remorselessly the reflector swung around in their direction. The intense cold numbed the racing men, cutting off their breath and impeding their efforts for speed. "Stop!" cried the doctor suddenly. "Fire at that reflector! It's our only chance!" He set the example by turning and emptying his pistol futilely at the turning mirror. Bill, Walter and Carnes followed his example. Nearer and nearer to them came the deadly ray. Bill was the nearest to its path, and he suddenly stiffened and fell forward, his useless gun still grasped in his hands. As his body struck the ground it rolled down hill for a few feet, the deadly ray following it. His head struck a rock, and Carnes gave a cry of horror as it broke into fragments. Walter threw his rifle to his shoulder and fired again and again at the rotating disc. The cold had became intense and he could not control the actions of his muscles and his rifle wavered about. He threw himself flat on the ground, and, with an almost superhuman effort, steadied himself for a moment and fired. His aim was true, and with a terrific crash the reflector split into a thousand fragments. Dr. Bird staggered to his feet. "It's out of order for a moment!" he cried. "To the house while we can!" As swiftly as his numbed feet would allow him, he stumbled toward the house. The muzzle of the rifle again projected from the loophole and with its crack the doctor staggered for a moment and then fell. Walter's rifle spoke again and the rifle disappeared through the loophole with a spasmodic jerk. Carnes stumbled over the doctor.