Immediately our guards had marched us back to the door through which we had entered, and across the ante-room beyond through another door and into a short, broad hall along the sides of which rested the great tower's lift-cage. We were ordered into one of the cages, our guards holding their heat-pistols full upon us now, and then as a stud was pressed and the motors' power was turned through the cage's powerful vertical tube-propellers, those tube-propellers drove us up with a thin whistling of air up through the narrow shaft the cages moved in, up until in a moment more we had stopped and were emerging into a similar hall on the great tower's hundredth floor. From that hall we moved into a short corridor that ran the width of the great tower, which at this height was but a hundred or more feet in diameter, its slender pinnacle tapering as it rose to its tip, while much of that pinnacle's space was occupied by the great connections which carried the city's electrical power down from the mighty tower's tip. Along that corridor we went, one lined with solid metal doors on either side, and finally were halted before one of those doors. Then one of our guards drew from a pocket a small instrument resembling an electric torch, from which he flashed a tiny beam into a transparent-fronted little opening in the wall beside the door. At once there came a clicking of locks, and the door swung open, its locks unbolted by the beam of light or force, rather, whose vibratory rate was exactly tuned to affect a delicate receiver tuned to the same frequency, set in the wall and controlling the lock. These vibration-locks, indeed, had long ago replaced the old, clumsy keys, and were far safer in that they responded only to one certain frequency vibration out of the millions possible, and thus could be opened only by one who knew the correct frequency. Now, as the door swung open, our guards pointed inside with their heat-pistols and perforce we stepped within, the door snapping shut behind us. We found ourselves in a small, metal-walled cell some ten feet in length and half that in width, furnished with but a few metal bunk-racks swung from the walls. At its farther end from us was the only opening beside the door, a small square window that was quite open and unbarred, and that looked out over all the colossal mass of the great air-city of Berlin, a giant field of blazing lights stretching far around and beneath the great tower in which we were prisoned. Then, as we gazed about the little cell with our eyes becoming accustomed to its lack of light, we made out suddenly a figure standing near its window, a dark, erect figure who seemed watching us for the moment and who then was striding across the cell toward us. "Brant!" he exclaimed, as his eyes made out our faces through the dusk. "Brant--and you were with the ships that attacked the city but now--you were captured in some way!"