When the door shut behind the last of the party he came over to the fire.

A slight gesture from Poirot enjoined me to stay behind the rest. I obeyed, going over to the fire and thoughtfully stirring the big logs on it with the toe of my boot.

"I don't know what to think," I said frankly. "What was the point? Why not go straight to Inspector Raglan with the truth instead of giving the guilty person this elaborate warning?"

I was puzzled. For the first time I was absolutely at sea as to Poirot's meaning. For a moment I was inclined to think that the scene I had just witnessed was a gigantic piece of bombast -- that he had been what he called "playing the comedy" with a view to making himself interesting and important. But, in spite of myself, I was forced to believe in an underlying reality. There had been real menace in his words -- a certain indisputable sincerity. But I still believed him to be on entirely the wrong tack.

"Well, my friend," he said quietly, "and what do you think of it all?"

Poirot sat down and drew out his case of tiny Russian cigarettes. He smoked for a minute or two in silence. Then: "Use your little grey cells," he said. "There is always a reason behind my actions."

"I thought, perhaps, that by making him believe you knew, you might force him out into the open -- not necessarily by confession. He might try to silence you as he formerly silenced Mr Ackroyd -- before you could act tomorrow morning."

Poirot nodded approvingly.

I hesitated for a moment, and then I said slowly: "The first one that occurs to me is that you yourself do not know who the guilty person is, but that you are sure that he is to be found amongst the people here tonight. Therefore your words were intended to force a confession from the unknown murderer?"

"Then I fail to understand you. Surely you are running the risk of letting the murderer escape by thus putting him on his guard?"

"A clever idea, but not the truth."

"A trap with myself as the bait! Merci, man ami, but I am not sufficiently heroic for that."

Poirot shook his head.

"He cannot escape," he said gravely. "There is only one way out -- and that way does not lead to freedom."

"You really believe that one of those people here tonight committed the murder?" I asked incredulously.

"I will take you the way that I have travelled myself. Step by step you shall accompany me, and see for yourself that all the facts point indisputably to one person. Now, to begin with, there were two facts and a little discrepancy in time which especially attracted my attention. The first fact was the telephone call. If Ralph Paton were indeed the murderer, the telephone call became meaningless and absurd. Therefore, I said to myself, Ralph Paton is not the murderer."

There was a silence for some minutes. Then Poirot tossed the stump of his cigarette into the grate and began to speak in a quiet, reflective tone.

"Yes, my friend."

"Which one?"

"I satisfied myself that the call could not have been sent by anyone in the house, yet I was convinced that it was amongst those present on the fatal evening that I had to look for my criminal. Therefore I concluded that the telephone call must have been sent by an accomplice. I was not quite pleased with that deduction, but I let it stand for the minute."

"I next examined the motive for the call. That was difficult. I could only get at it by judging its result. Which was -- that the murder was discovered that night instead ofin all probability -- the following morning. You agree with that?"

"In your manuscript you have drawn a neat little plan of the study. If you had it with you this minute you would see that -- the chair being drawn out in the position indicated by Parker -- it would stand in a direct line between the door and the window."

"Ye-es," I admitted. "Yes. As you say, Mr Ackroyd, having given orders that he was not to be disturbed, nobody would have been likely to go to the study that night."

"Tres bien. The affair marches, does it not? But matters were still obscure. What was the advantage of having the crime discovered that night in preference to the following morning? The only idea I could get hold of was that the murderer, knowing the crime was to be discovered at a certain time, could make sure of being present when the door was broken in -- or at any rate immediately afterwards."

"And now we come to the second fact -- the chair pulled out from the wall. Inspector Raglan dismissed that as of no importance. I, on the contrary, have always regarded it as of supreme importance."

"The window!" I said quickly.

"You, too, have my first idea. I imagined that the chair was drawn out so that something connected with the window should not be seen by anyone entering through the door. But I soon abandoned that supposition, for though the chair was a grandfather with a high back, it obscured very little of the window -- only the part between the sash and the ground. No, mon ami -- but remember that just in front of the window there stood a table with books and magazines upon it. Now that table was completely hidden by the drawn-out chair -- and immediately I had my first shadowy suspicion of the truth."

"Supposing that there had been something on that table not intended to be seen? Something placed there by the murderer? As yet I had no inkling of what that something might be. But I knew certain very interesting facts about it."

For instance, it was something that the murderer had not been able to take away with him at the time that he committed the crime. At the same time it was vital that it should be removed as soon as possible after the crime had been discovered. And so -- the telephone message, and the opportunity for the murderer to be on the spot when the body was discovered.

"Now four people were on the scene before the police arrived. Yourself, Parker, Major Blunt, and Mr Raymond."

Parker I eliminated at once, since at whatever time the crime was discovered, he was the one person certain to be on the spot. Also it was he who told me of the pulled-out chair. Parker, then, was cleared (of the murder, that is. I still thought it possible that he had been blackmailing Mrs Ferrars). Raymond and Blunt, however, remained under suspicion since, if the crime had been discovered in the early hours of the morning, it was quite possible that they might have arrived on the scene too late to prevent the object on the round table being discovered."

"As soon as I learned that a representative of a dictaphone company had called, the idea of a dictaphone took root in my mind. You heard what I said in this room not half an hour ago? They all agreed with my theory -- but one vital fact seems to have escaped them. Granted that a dictaphone was being used by Mr Ackroyd that night -- why was no dictaphone found?"

"Now what was that object? You heard my arguments tonight in reference to the scrap of conversation overheard?"

"I never thought of that," I said.

"We know that a dictaphone was supplied to Mr Ackroyd. But no dictaphone has been found amongst his effects. So, if something was taken from the table -- why should not that something be the dictaphone? But there were certain difficulties in the way. The attention of everyone was, of course, focused on the murdered man. I think anyone could have gone to the table unnoticed by the other people in the room. But a dictaphone has a certain bulk -- it cannot be slipped casually into a pocket. There must have been a receptacle of some kind capable of holding it."

"You are like Mr Raymond. You take it for granted that what was heard at nine-thirty was Mr Ackroyd's voice speaking into a dictaphone. But consider this useful invention for a little minute. You dictate into it, do you not? And at some later time a secretary or a typist turns it on, and the voice speaks again."

I interrupted. "By why remove the dictaphone? What was the point?"

"You see where I am arriving? The figure of the murderer is taking shape. A person who was on the scene straightaway, but who might not have been if the crime had been discovered the following morning. A person carrying a receptacle into which the dictaphone might be fitted"

(1) They might really have been made by Ralph Paton. He had been at Fernly that night, and might have climbed into the study and found his uncle dead there. That was one hypothesis.

Poirot nodded. "Yes, I meant that. At nine-thirty Mr Ackroyd was already dead. It was the dictaphone speaking -- not the man."

"You mean --?" I gasped.

"Possibly. But we must not exclude the likelihood of some mechanical device having been applied -- something after the nature of a time lock, or even of a simple alarm clock."

"And the murderer switched it on. Then he must have been in the room at that minute?"

(2) There was the possibility that the footmarks might have been made by somebody else who happened to have the same kind of studs in his shoes. But the inmates of the house had shoes soled with crepe rubber, and I declined to believe in the coincidence of someone from outside having the same kind of shoes as Ralph Paton wore. Charles Kent, as we know from the barmaid of the Dog and Whistle, had on a pair of boots "clean dropping off him."

But in that case we must add two qualifications to our imaginary portrait of the murderer. It must be someone who knew of Mr Ackroyd's purchase of the dictaphone and also someone with the necessary mechanical knowledge."

"I had got thus far in my own mind when we came to the footprints on the window ledge. Here there were three conclusions open to me.

"You know the result of her investigations. Ralph Paton had had a pair of boots with him. The first question I asked him when he came to my house yesterday morning was what he was wearing on his feet on the fatal night. He replied at once that he had worn boots -- he was still wearing them, in fact -- having nothing else to put on.

(3) Those prints were made by someone deliberately trying to throw suspicion on Ralph Paton. To test this last conclusion, it was necessary to ascertain certain facts. One pair of Ralph's shoes had been obtained from the Three Boars by the police. Neither Ralph nor anyone else could have worn them that evening, since they were downstairs being cleaned. According to the police theory, Ralph was wearing another pair of the same kind, and I found out that it was true that he had two pairs. Now for my theory to be proved correct it was necessary for the murderer to have worn Ralph's shoes that evening -- in which case Ralph must have been wearing yet a third pair of footwear of some kind. I could hardly suppose that he would bring three pairs of shoes all alike -- the third pair of footwear were more likely to be boots. I got your sister to make inquiries on this point -- laying some stress on the colour, in order -- I admit it frankly -- to obscure the real reason for my asking.

"So we get a step further in our description of the murderer -- a person who had the opportunity to take these shoes of Ralph Paton's from the Three Boars that day." He paused, and then said, with a slightly raised voice: "There is one further point. The murderer must have been a person who ^ had the opportunity to purloin that dagger from the silver table. You might argue that anyone in the house might have done so, but I will recall to you that Flora Ackroyd was very positive that the dagger was not there when she examined the silver table." He paused again.

"Let us recapitulate -- now that all is clear. A person who was at the Three Boars earlier that day, a person who knew Ackroyd well enough to know that he had purchased a dictaphone, a person who was of a mechanical turn of mind, who had the opportunity to take the dagger from the silver table before Miss Flora arrived, who had with him a receptacle suitable for hiding the dictaphone -- such as a black bag, and who had the study to himself for a few minutes after the crime was discovered while Parker was telephoning for the police. In fact -- Dr. Sheppard, in fact."