I was just going to abandon my investigations and return to my own room, when, more by chance than design, I knelt down for a moment at the little altar. As I was about to rise I noticed something rather odd. I listened attentively. It was certainly remarkable. As I knelt I could just hear a low, continuous hissing sound. Directly I moved away it ceased. As I tried it several times with the same invariable result, I became seriously puzzled to account for it. What devilry could be at work to produce this? Was it possible that some one was playing a trick on _me_?--and if so, by what means? I glanced rapidly round, and as I did so a mad thought struck me. I hurried across to the fountain and put my ear close to the swan's mouth, from which a tiny jet of water was issuing. The low, scarcely audible noise that the water made as it flowed out through the swan's bill was exactly the same sound I had heard nearly twenty feet away at the altar. The enormity of the situation stunned me for a moment, then gradually, piece by piece, the plot revealed itself. The shape of the gallery was a true oval, a geometrical ellipse, the extraordinary acoustic properties of which I knew well. This peculiarly shaped gallery contained two foci--one towards each end--and the nature of the curve of the walls was such that sound issuing from either focus was directed by reflection at various points to the other focus, and to the other focus alone. Even across an enormous distance between such would be the case. The swan's mouth was evidently at one focus; the position of a man's head as he knelt at the altar would be without the slightest doubt at the other. Could the pipe be used as a speaking-tube when the water was turned off? I felt so excited by this extraordinary discovery that it was only with an effort that I maintained my self-control. I knew that presence of mind was absolutely necessary in order to expose this horrible scheme. I left the gallery and passed through the conservatory. Here I found the gardener arranging some pots. I chatted to him for a few moments. He looked surprised at seeing me up at such an unusual hour. "Can you tell me how the fountain in the gallery is turned on or off?" I asked. "Yes, I can, sir," he replied; "the pipe runs along outside this stand, and here's the tap." I went across and looked at it. In the leaden pipe that was fastened to the wall were two nuts, which could be turned by a small spanner, and between them was a brass cap, which fitted on to a circular outlet from the pipe. "What is this used for?" I asked, pointing to the little outlet which was closed by the cap. "We screw the hose on there, sir, to water the flowers." "I see," I answered; "so when you use the hose you shut off the water from the fountain in the gallery." "That's it, sir, and a wonderful deal of trouble it saves. Why it was never done before I can't think." "When was it done, then?" I asked. My heart was beating fast.