Then I laughed.

There was a dead silence for a minute and a half.

"You're mad," I said.

"No," said Poirot placidly. "I am not mad. It was the little discrepancy in time that first drew my attention to you -- right at the beginning."

"Discrepancy in time?" I queried, puzzled.

"But yes. You will remember that everyone agreed -- you yourself included -- that it took five minutes to walk from the lodge to the house -- less if you took the short cut to the terrace. But you left the house at ten minutes to nine -- both by your own statement and that ofParker, and yet it was nine o'clock when you passed through the lodge gates. It was a chilly night -- not an evening a man would be inclined to dawdle; why had you taken ten minutes to do a five minutes' walk? All along I realized that we had only your statement for it that the study window was ever fastened. Ackroyd asked you if you had done so -- he never looked to see. Supposing, then, that the study window was unfastened? Would there be time in that ten minutes for you to run round the outside of the house, change your shoes, climb in through the window, kill Ackroyd, and get to the gate by nine o'clock? I decided against that theory since in all probability a man as nervous as Ackroyd was that night would hear you climbing in, and then there would have been a struggle. But supposing that you killed Ackroyd before you left -- as you were standing beside his chair? Then you go out of the front door, run round to the summer-house, take Ralph Paton's shoes out of the bag you brought up with you that night, slip them on, walk through the mud in them, and leave prints on the window ledge, you climb in, lock the study door on the inside, run back to the summer-house, change back into your own shoes, and race down to the gate. (I went through similar actions the other day, when you were with Mrs Ackroyd -- it took ten minutes exactly.) Then home -- and an alibi -- since you had timed the dictaphone for half-past nine."

"My dear Poirot," I said in a voice that sounded strange and forced to my own ears, "you've been brooding over this case too long. What on earth had I to gain by murdering Ackroyd?"

"I will confess to you that it was my greatest stumbling block when I found that a call had actually been put through to you from King's Abbot station. I at first believed that you had simply invented the story. It was a very clever touch, that. You must have some excuse for arriving at Fernly, finding the body, and so getting the chance to remove the dictaphone on which your alibi depended. I had a very vague notion of how it was worked when I came to see your sister that first day and inquired as to what patients you had seen on Friday morning. I had no thought of Miss Russell in my mind at that time. Her visit was a lucky coincidence, since it distracted your mind from the real object of my questions. I found what I was looking for. Among your patients that morning was the steward of an American liner. Who more suitable than he to be leaving for Liverpool by the train that evening? And afterwards he would be on the high seas, well out of the way. I noted that the Orion sailed on Saturday, and having obtained the name of the steward I sent him a wireless message asking a certain question. This is his reply you saw me receive just now."

"Safety. It was you who blackmailed Mrs Ferrars. Who could have had a better knowledge of what killed Mr Ferrars than the doctor who was attending him? When you spoke to me that first day in the garden, you mentioned a legacy received about a year ago. I have been unable to discover any trace of a legacy. You had to invent some way of accounting for Mrs Ferrars's twenty thousand pounds. It has not done you much good. You lost most of it in speculation -- then you put the screw on too hard, and Mrs Ferrars took a way out that you had not expected. If Ackroyd had learnt the truth he would have had no mercy on you -- you were ruined for ever."

"And the telephone call?" I asked, trying to rally. "You have a plausible explanation of that also, I suppose?"

He held out the message to me. It ran as follows: "Quite correct. Dr Sheppard asked me to leave a note at a patient's house. I was to ring him up from the station with the reply. Reply was 'No answer.'"

"All this," I said, "is very interesting -- but hardly in the sphere of practical politics."

"It was a clever idea," said Poirot. "The call was genuine. Your sister saw you take it. But there was only one man's word as to what was actually said -- your own!"

"You think not? Remember what I said -- the truth goes to Inspector Raglan in the morning. But, for the sake of your good sister, I am willing to give you the chance of another way out. There might be, for instance, an overdose of a sleeping draught. You comprehend me? But Captain Ralph Paton must be cleared -- ça va sans dire. I should suggest that you finish that very interesting manuscript of yours -- but abandoning your former reticence."

I yawned.

"You seem to be very prolific of suggestions," I remarked. "Are you sure you've quite finished?"

"Now that you remind me of the fact, it is true that there is one thing more. It would be most unwise on your part to attempt to silence me as you silenced M. Ackroyd. That kind of business does not succeed against Hercule Poirot, you understand."

"Well, well," I said, with a slight yawn, "I must be off home. Thank you for a most interesting and instructive evening."

Poirot also rose and bowed with his accustomed politeness as I passed out of the room.

"My dear Poirot," I said, smiling a little, "whatever else I may be, I am not a fool." I rose to my feet.