“I wrote a letter to Martin, which I knew could not fail to bring him to Number 407, and to bring him alone. It suggested that somebody had discovered his true occupation, and was prepared to keep the matter secret, for a consideration. Then, on the Saturday morning, just before the men were due to leave, I walked openly into Number 407. I had kept an eye on their habits, and knew that before they left they packed away their tools in the back room on the ground floor. I could hear them there as I went in, and I made my way down to the cellar without anybody noticing me. All I did there was to put my poison device in the lamp holder in the small cellar⁠—I had heard the electrician tell the builder that the current would be connected that day⁠—hang the numbered counter on the switch, and remove the string by which the swing door could be pulled open from within. The trap was set, it remained only to ensure that Martin should have easy access to it.