The Black Sailor rose from his chair, and walked across to the bench, from which he picked up an apparatus made of wood and india rubber, not unlike a miniature cross-bow. “I amused myself in my spare time by making this,” he said. “As you see, it has a holder which just takes a counter. You would be surprised to see how far and how accurately it will project it. I have frequently practised with it in the country, and I have attained sufficient skill in its use to be able to place a counter where I like at any range up to a hundred feet or so. “Now, above my shop is a room, which I use for storage purposes. From the window of this room you can see into the window of Mr. Copperdock’s bedroom. I had merely to wait for a windless evening, fine enough for Mr. Copperdock to leave his window open, and seize my opportunity when I knew his house was empty. The opportunity came, and I took it. “It might have been thought that by delivering the counter in advance I had increased my difficulties. But, on the contrary, I had diminished them. The obvious improbability of Mr. Copperdock’s story, that he had found a counter on his bed at a time when it was demonstrable that nobody had entered the house, was merely another link in the chain of suspicion which I was forging round him. I allowed a few days to pass, and then, the following Saturday, I took the final step. “Now, even you, Dr. Priestley, failed to read the riddle of the broken hypodermic needle and the incrustation of potassium carbonate round the puncture. Yet it was ridiculously simple. I secured a piece of metallic potassium, and moulded it into the shape of a bullet to fit an air-gun which I possessed. Into the head of this bullet I inserted the broken end of a hypodermic needle, which I had already charged with the same drug as had been so effectual in the case of Mr. Colburn. Keeping this prepared bullet in paraffin, to avoid oxidation, I placed the air-gun ready upstairs, and waited for Mr. Copperdock’s return from the Cambridge Arms. He brought two friends home with him, and I was afraid at first that I should be compelled to postpone my attempt. It was essential to my scheme that he should be alone in the house when I fired the gun. “However, the two friends left after a while, and Mr. Copperdock entered his bedroom and proceeded to undress. I could just see the washing-stand through a gap in his curtains, and I waited until he was bending over this. Then I loaded the gun with my potassium bullet, and pulled the trigger. I saw Mr. Copperdock spin round, and then he passed out of my line of vision. But I knew I had succeeded, and I came quietly down here.