ripping crash echoed up dimly from far beneath him. The gate was down!

Feverishly he struggled with the heavy trap. Normally it would not have been difficult for him to lift the rectangle of aluminum alloy; but wounded as he was, forcing his numbing limbs to obey him required almost super-human effort. When he had at last succeeded in hoisting it on its hinges, he could again hear the soft padding of hurrying feet.

The engineer found himself in a large room, one wall of which was curved, conforming to the outer contour of the cylindrical tower. Scattered illumination globes gave a dim light to the place. The room was evidently a storehouse for Hekalu's laboratory supplies. Complex mechanisms stood about, evidently waiting to be installed. There were hundreds of metal drums presumably containing chemicals. There were bolts of heavy fabric and stacks of ingots neatly corded. Set in the ceiling of the chamber were several circular windows through the heavy glass of which bright stars shone. Directly above was the roof, and but a few paces distant, the landing stage!

Escape seemed tantalizingly near, but with sinking heart, Shelby noted that there was no easy means of ascent to the roof. He'd have to try to smash one of those windows. But the monster hurrying up the spiral claimed his immediate attention.

Deeply thankful for the peculiar eccentricities of Martian architecture, he hurriedly proceeded to pile ingots on the closed trapdoor. Each of these ingots weighed well over a hundred and fifty pounds. Fortunately for the wounded Earthman, the distance he had to carry them was only a few feet.

CHAPTER IV

Capture!

He had transferred five to their new position before his pursuer arrived beneath the trap and began to push upward mightily upon it. Shelby transferred several more ingots to the pile just to make sure that the monster could not enter. Then, fighting off the diaphanous veil of unconsciousness that was trying to drop over him, he looked about for something with which to effect his escape.

A long bar of metal caught his eye. He seized it, and with all his strength thrust upward at one of the ceiling windows. But the thick glass, crisscrossed by rods of metal, was not easily shattered.

A rattling noise attracted his attention. He glanced back toward the trap. His pile of ingots was trembling as if shaken by a miniature earthquake. The door was rising upward! It settled back and rose again. An inch crack appeared, and through it Shelby could see two eyes and the muzzle of a pistol. He leaped out of range just in time to avoid the bullet that whizzed across the room and flattened itself against the wall.

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.

Somehow he began to doubt that he had killed Selba. It had been only a dream, and the monstrous thing that had sought his life had been a dream too. Hekalu was pursuing him now, trying to kill him! The idea took hold, for he could no longer distinguish fancy from reality. It brought to him a vague fear which would have been completely out of place with him had he not been so near gone from loss of blood. It was like a child's fear of the dark.

He began to fly towards home in a wild zigzag course like a dazed bat, but this favored him, for it enabled him to avoid the darts from the pursuing plane. Luckily he remembered that while under fire combat fliers do not make use of their automatic pilots except as a last resort, for these devices cannot direct the complex movements necessary in dodging enemy bullets. Automatically Shelby watched the guiding instruments and followed their directions.

Several times he signaled with his siren, but no one answered him. Thousands of sirens were hooting, and the Air Patrol was very busy. The darkness, the explosions and the muffled roar from the streets continued.

Two ideas now possessed Shelby's mind and he clung to them with the grim persistence of a wounded tiger. One was to get home, secure his weapon and rush it to the federal authorities. The other was to hurry to Janice Darell.

Presently his plane bounded down awkwardly on the landing platform of the building in which his apartment was located. He stumbled out, and down the dark stair. The elevators were not working. Somehow he found his door and unlocked it. He groped toward the wall safe. It was open, and the little black case which contained the unfinished atomic ray projector was gone. A neat round hole had been drilled in the metal door of the safe.

The view-phone bell was ringing. Shelby stumbled to the instrument and moved its switches. The view-plate did not work but he heard a faint voice which he recognized as Jan's. "Is that you, Austin?" it said. "Can't you help me? Something is out there. It has me cornered in my room. It has killed old Rufus. The house police—" There the connection snapped.

A wild surge of anger quickened the engineer's weakly beating heart. He tried to reach the door, and then he felt a stinging sensation in the back of his neck. A needle-dart charged with a sleep-producing drug had struck him. He slumped to the floor.

A