He darted around toward the hinged side of the trap, where he knew that the black horror could not fire at him, and devoted his attention to another window. He would have reinforced the barricade with more ingots, but he realized that by spending his nearly exhausted strength that way he would be defeating his own purpose. A dozen times he jabbed up viciously with the bar before a tiny crack appeared in the round pane of glass. The trapdoor behind him was being shaken violently. An ingot on top of the pile was jarred from its place and crashed to the floor. Yes, the window was giving. A small hole appeared in it. A pair of shiny black forearms had forced their way from under the edge of the trapdoor. Slowly and mightily the shoulders of the monster surged upward. The door was rising, and this time it did not seem that it would sink back. Shelby had finished his task. Now, with the upper end of the bar thrust through the opening he had made in the window, and the lower end resting in a slight depression in the floor, he proceeded to climb it to safety. His head and shoulders were through the hole when the monster at last burst its way into the room below. But the thing was just an instant too late to hinder him. Sweating and bloody, Shelby drew himself to the roof and staggered over to the landing stage. Yes, his plane was there. The night air, and the flush of success was refreshing him. His exaltation leaped higher and higher as his plane swept him up from the summit of the tower of the mysterious Selba. A wild refrain was drumming in his mind: "Hekalu Selba is dead! I have killed him!" There was nothing more to do but notify the Municipal Air Patrol--an S. O. S. with his siren would accomplish that. They would raid the tower. If any of the Martian's fellow plotters sought to continue with the project the Earthman's new weapon would take care of them. Shelby was reaching for the siren button, and then a terrific explosion thundered up from somewhere below, and several hundred yards to his right. He saw the orange flash, and then, in an instant the whole city went dark. Another crash came and another. Shelby saw a dark form glide through the air. From far beneath him he heard a troubled murmur mixed with the din of colliding vehicles. Sirens shrieked. In the distance to his right, a great plume of lurid flame blossomed in the sky. The low purr of a machine gun sounded behind him, and he heard the almost inaudible tick-tick of poisoned needle-darts piercing the fuselage of his craft. He zoomed sharply upward for a thousand feet, and then glanced back. There was a dim shadow out there--he was being followed. But this discovery, and the realization that the city was attacked made but a vague impression upon his fast-dimming mind. The warm fluid that oozed from his shoulder, making his clothing sodden and sticky, had all but drained his vital energy.