We had gone no more than a hundred feet or so when Migul slowed our pace, and began to walk stooped over, with one of its abnormally long arms held close to the ground. The fingers were stiffly outstretched and barely skimmed the floor surface of the tunnel. As we passed through a spot of light I saw that Migul had extended from each of the fingertips an inch-long filament of wire, like finger nails. The Robot murmured abruptly, "Tugh's vibrations are here. I can feel them. He has passed this way recently." * * * * * Tugh's trail! I knew then that Tugh's body, touching this ground, had altered to some infinitesimal degree the floor-substance's inherent vibration characteristics. Vibrations of every sort are communicable from one substance to another. Tugh's trail was here--his vibration-scent--and like a hound with his nose to the ground, Migul's fingers with the extended filaments were feeling it. What strange sensitivity! What an amazing development of science was manifested in every move and act and word of this Robot! Yet, in my own Time-world of 1935, it was all crudely presaged: this now before me was merely the culmination. "He recently passed," said Migul. We stopped, I close beside the stooping metal figure. The Robot's voice was a furtive sepulchral whisper that filled me with awe. "How long ago?" I asked. "He passed here an hour or two ago, perhaps. The vibrations are fading out. But it was Tugh. Well do I know him. Put your hand down. Feel the vibrations?" "I cannot. My fingers are not that sensitive, Migul." A faint contempt was in the Robot's tone. "I forgot that you are a man." Then it straightened, and the extended filaments slid back into its fingers. It said softly, "There is one guard in this passage." My heart leaped. "A human or a Robot?" "A man. His name is Alent. He is at a gate that is too well fortified for any Robot to assail, but he will pass humans. It will be necessary for you to kill him." * * * * * I had no intention of doing that, but I did not say so. As we crept forward to where I saw that the tunnel made a bend, with the fortified gate just beyond it, there was in my mind that now I would do my best to separate from Migul, using this guard as my pretext, for he would doubtless pass me, but not the Robot. The palace was occupied, I assumed, by friendly humans. I could get them to locate Tina and Larry.... Then the flaws of this plan made themselves all too evident. Larry might be with Tugh, and without Migul I could not follow Tugh's trail. Worse than that, if I tricked Migul, the angered Robot would at once return to Mary. I shuddered at the thought. That would not do. I must try to get Migul past the guard. I whispered, "When we reach the gate you stay behind me. Let me persuade the guard." "You will kill him? You have the weapon. He is fortified against the Robot weapons, but yours will be strange to him." "We will see."