"To-night," continued his son, "I got in at about half-past nine. I had had such a rush, in one way and another, that the incident had quite lost its hold on my imagination; I hadn't forgotten it, of course, but I was not thinking of it when I unlocked the door. In fact I didn't begin to think of it again until, in slippers and dressing-gown, I had settled down for a comfortable read. There was nothing, absolutely nothing, to influence my imagination--in that way. The book was an old favourite, Mark Twain's _Up the Mississippi_, and I sat in the armchair with a large bottle of lager beer at my elbow and my pipe going strong." Becoming restless in turn, the speaker stood up and walking to the fireplace flicked off the long cone of grey ash from his cigar. He leant one elbow upon the mantel-piece, resuming his story: "St. Paul's had just chimed the half-hour--half-past ten--when my pipe went out. Before I had time to re-light it, came the damnable smell again. At the moment nothing was farther from my mind, and I jumped up with an exclamation of disgust. It seemed to be growing stronger and stronger. I got my pipe alight quickly. Still I could smell it; the aroma of the tobacco did not lessen its beastly pungency in the smallest degree. "I tilted the shade of my reading-lamp and looked all about. There was nothing unusual to be seen. Both windows were open and I went to one and thrust my head out, in order to learn if the odour came from outside. It did not. The air outside the window was fresh and clean. Then I remembered that when I had left my chambers in the afternoon, the smell had been stronger near the door than anywhere. I ran out to the door. In the passage I could smell nothing; but--" He paused, glancing at his father. "Before I had stood there thirty seconds it was rising all about me like the fumes from a crater. By God, sir! I realised then that it was something ... following me!" Dr. Cairn stood watching him, from the shadows beyond the big table, as he came forward and finished his whisky at a gulp. "That seemed to work a change in me," he continued rapidly; "I recognised there was something behind this disgusting manifestation, something directing it; and I recognised, too, that the next move was up to me. I went back to my room. The odour was not so pronounced, but as I stood by the table, waiting, it increased, and increased, until it almost choked me. My nerves were playing tricks, but I kept a fast hold on myself. I set to work, very methodically, and fumigated the place. Within myself I knew that it could do no good, but I felt that I had to put up some kind of opposition. You understand, sir?" "Quite," replied Dr. Cairn quietly. "It was an organised attempt to expel the invader, and though of itself it was useless, the mental attitude dictating it was good. Go on."