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.