I could see a length of green cable proceeding from the wall-plug _out_ through the open window. The cable attached to the instrument which Gatton held did not come from the proper connection at all, but came _in_ through the window, and was evidently connected with something outside in the garden! "What does this mean, Gatton?" I cried. Evidently as deeply mystified as I, Gatton placed the telephone on the little table and fully opening the window, leaned out. "Hullo!" he cried. "The cable leads up to the roof of the tool-shed!" "To the roof of the tool-shed!" I echoed incredulously. But Gatton did not heed my words, for: "What the devil have we here?" he continued. He was hauling something up from the flower-bed below the window, and now, turning to me, he held out ... a second telephone! "Why, Gatton!" I cried, and took it from his hand, "_this_ is the authentic instrument! See! It is connected in the proper way!" "I see quite clearly," he replied. "It was simply placed outside, whilst a duplicate one was substituted for it. I observe a ladder against the shed. Let us trace the cable attached to the duplicate." The ladder was one used by Coates about the garden; and now, climbing out of the window, Gatton mounted it and surveyed the roof of the lean-to which I used as a tool-shed. "Ha!" he exclaimed. "A gas cylinder!" "What!" He fingered the green cable. "This is not cable at all," he cried; "it's _covered tubing_! Do you see?" He descended and rejoined me. "You see?" he continued. "A call from the exchange would ring the bell in the ante-room here. This devilish contrivance"--he pointed to the false telephone--"is really hollow. The weight of the receiver hermetically closes the end of the tube, no doubt. But any one answering the call and taking up the duplicate instrument would receive the full benefit of the contents of the cylinder which lies up there on the roof!" "My God, Gatton!" I muttered. "The fiends! But why was the contrivance not removed?" "They hadn't time," he said grimly. "They had not counted on the death-grip of the victim!" I heard a car come racing up to the gate, followed by the sound of many excited voices. "At last we know where the gray mist came from," I said, as Gatton and I walked through the cottage to meet the new arrivals. "We know more than that," he retorted. "We know how _Sir Marcus_ died!" "Gatton!" I cried excitedly, as we approached a group waiting in the porch--"do you mean--" He looked at me grimly. "I mean," he said slowly, "that I have not forgotten the _gas-plug_ in the wall of that recess in the supper-room at the Red House! The only thing I was doubtful about (the means by which the victim was induced to admit the gas into the room) is now as clear as daylight." "You are right, Gatton," I agreed. "The same trick has succeeded twice."