As Shelby stumbled up the dark spiral he heard the thing below tearing at the closed grill. He knew that it could not delay the thing for long. He had just reached the trapdoor at the top of the long climb, when a muffled 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.