“My first move was to make the acquaintance of the builder who was carrying out the alterations to Number 407, and from him I learnt in the course of conversation the name of the new tenant. He asked me to come and look at the place, an invitation which I accepted without too great a show of eagerness. In the next few days I visited the place again, more than once. I wanted the builder’s men to get accustomed to the sight of me. For this purpose I was Mr. Ludgrove the local resident, of course. As a matter of fact, most of the builder’s friends went to have a look at the place. There is always an interest in vacant premises in a street like this.
“Dr. Morlandson had provided me with a number of celluloid capsules, containing pure prussic acid. I took an ordinary electric lamp, broke it, and connected the ends of the leading-in wires by a piece of fine foil, which would become red hot on the passage of a current. Round this I wrapped a few strands of nitrated cellulose, and to this I fixed the celluloid capsule. The action of this device was very simple. If a current were passed through it, the cellulose would inflame and set fire to the capsule, thus releasing the prussic acid. Some of the vapour of this might catch fire, but enough would be left to be fatal to anybody breathing it in a confined space. And I knew of a confined space most suitably adapted to the purpose.
“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.
“I knew about the arrangement whereby the keys were kept at the sweet shop. I had announced that I was going into the country that day, travelling into Essex by the London, Tilbury and Southend from Fenchurch Street. I took the Underground, apparently to Aldgate, but got out at Aldersgate Street. From here I walked down the Barbican, saw Martin leave his office, go to lunch, and finally book at Aldersgate Street to Praed Street. I immediately telephoned from one of the boxes there to Mr. Briggs, who had the keys, saying that I was the tenant of Number 407, and asking him to look out for a Mr. Martin, and give him the keys. I then went on to Fenchurch Street, spent a happy and profitable day in the country, and returned home to hear of the death of Mr. Martin. I was, I confess, slightly annoyed to find that it had been attributed to suicide. But then, I had never anticipated that Martin would bring an automatic with him. But perhaps I weary you with these details, Dr. Priestley?”
“Not in the least!” replied the Professor fervently. He glanced furtively at the clock. It was already past ten o’clock.
Chapter XXII.
The Death Chamber
“What I have to tell you now may interest you more nearly, Dr. Priestley,” continued the Black Sailor. “Having disposed of five of my intended victims, I had to remove Copperdock. But I wished first of all to concentrate as much suspicion as possible on him before his death. You will perceive the idea, no doubt. If the murders were to cease with Copperdock’s death, there being already a vague suspicion that he had caused them, further enquiries would not be pursued very briskly.
“For this reason, I appeared to Copperdock that evening as he was coming out of the Cambridge Arms. The disguise was very imperfect, since I had merely smeared the dye on my beard in the street, and the scar was made of sticking-plaster. It would not have borne a moment’s close inspection, but it was good enough for Copperdock, as I felt sure it would be. I escaped from him without difficulty, and, having taken a wet sponge with me, removed the scar and dye at the first opportunity and came quietly home. I beat Mr. Copperdock by about five minutes.
“As I had expected, he told Whyland the story of his adventure, and I was in a position to swear that he and I had been alone in the street. The result was to discredit his veracity still further. I then thought it time to act. You will remember that Copperdock said he found a numbered counter on his bed? That was the exact truth, although it seemed impossible, and nobody believed him. But it was capable of the simplest explanation in the world. Look here.”
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.