Slowly, but surely, the conviction had been gaining upon me that, let the mystery of River Hall be what it would, no ordinary explanation could account for the phenomena which it had presented to tenant after tenant; and my own experiences in the house, slight though they were, tended to satisfy me there was something beyond malice or interest at work about the place. The very peace vouchsafed to me seemed another element of mystery, since it would certainly have been natural for any evil-disposed person to inaugurate a series of ghostly spectacles for the benefit of an investigator like myself; and yet, somehow, the absence of supernatural appearances, and the presence of that shadowy human being who thought it worth while to track my movements, and who had at last left tangible proof of his reality behind him in the snow, linked themselves together in my mind. "If there is really anyone watching me," I finally decided, "there must be a deeper mystery attached to River Hall than has yet been suspected. Now, the first thing is to make sure that some one is watching me, and the next to guard against danger from him." In the course of the day, I made a, for me, curious purchase. In a little shop, situated in a back street, I bought half a dozen reels of black sewing-cotton. This cotton, on my return home, I attached to the trellis-work outside the drawing-room window, and wound across the walk and round such trees and shrubs as grew in positions convenient for my purpose. "If these threads are broken to-morrow morning, I shall know I have a flesh-and-blood foe to encounter," I thought. Next morning I found all the threads fastened across the walks leading round by the library and drawing-room snapped in two. It was, then, flesh and blood I had come out to fight, and I decided that night to keep watch. As usual, I went up to my bedroom, and, after keeping the gas burning for about the time I ordinarily spent in undressing, put out the light, softly turned the handle of the door, stole, still silently, along the passage, and so into a large apartment with windows which overlooked both the library and drawing-room. It was here, I knew, that Miss Elmsdale must have heard her father walking past the door, and I am obliged to confess that, as I stepped across the room, a nervous chill seemed for the moment to take my courage captive. If any reader will consider the matter, mine was not an enviable position. Alone in a desolate house, reputed to be haunted, watching for some one who had sufficient interest in the place to watch it and me closely. It was still early--not later than half-past ten. I had concluded to keep my vigil until after midnight, and tried to while away the time with thoughts foreign to the matter in hand. All in vain, however. Let me force what subject I pleased upon my mind, it reverted persistently to Mr. Elmsdale and the circumstances of his death.