'Would you like to go with the inspector, M. Poirot?' asked the chief constable, 'or would you prefer to examine the study?' Poirot chose the latter alternative. Parker opened the door to us. His manner was smug and deferential, and he seemed to have recovered from his panic of the night before.

Colonel Melrose took a key from his pocket, and unlocking the door which let into the lobby, he ushered us through into the study.

'Except for the removal of the body, M. Poirot, this room is exactly as it was last night.' 'And the body was found - where?' As precisely as possible, I described Ackroyd's position.

The arm-chair still stood in front of the fire.

Poirot went and sat down in it.

'The blue letter you speak of, where was it when you left the room?' 'Mr Ackroyd had laid it down on this little table at his right hand.' Poirot nodded.

'Except for that, everything was in its place?' 'Yes, I think so.' 'Colonel Melrose, would you be so extremely obliging as to sit down in this chair a minute. I thank you. Now M. le docteur, will you kindly indicate to me the exact position of the dagger?' I did so, whilst the little man stood in the doorway.

'The hilt of the dagger was plainly visible from the door then. Both you and Parker could see it at once?' 'Yes.' Poirot went next to the window.

'The electric light was on, of course, when you discovered the body?' he asked over his shoulder.

I assented, and joined him where he was studying the marks on the windowsill.

'The rubber studs are the same pattern as those in Captain Paton's shoes,' he said quietly.

Then he came back once more to the middle of the room. His eye travelled round, searching everything in the room with a quick, trained glance.

'Are you a man of good observation. Doctor Sheppard?' he asked at last.

'I think so,' I said, surprised.

'There was a fire in the grate, I see. When you broke the door down and found Mr Ackroyd dead, how was the fire? Was it low?' I gave a vexed laugh.

'I - I really can't say. I didn't notice. Perhaps Mr Raymond or Major Blunt ' The little man opposite me shook his head with a faint smile.

'One must always proceed with method. I made an error of judgment in asking you that question. To each man his own knowledge. You could tell me the details of the patient's appearance - nothing there would escape you. If I wanted information about the papers on that desk, Mr Raymond would have noticed anything there was to see. To find out about the fire, I must ask the man whose business it is to observe such things. You permit ' He moved swiftly to the fireplace and rang the bell.

After a lapse of a minute or two Parker appeared.

'The bell rang, sir,' he said hesitatingly.

'Come in, Parker,' said Colonel Melrose. 'This gentleman wants to ask you something.' Parker transferred a respectful attention to Poirot.

'Parker,' said the little man, 'when you broke down the door with Dr Sheppard last night, and found your master dead, what was the state of the fire?' Parker replied without a pause.

'It had burned very low, sir.' It was almost out.' 'Ah!' said Poirot. The exclamation sounded almost triumphant. He went on: 'Look round you, my good Parker. Is this room exactly as it was then?' The butler's eye swept round. It came to rest on the windows.

'The curtains were drawn, sir, and the electric light was on.' Poirot nodded approval.

'Anything else?' 'Yes, sir, this chair was drawn out a little more.' He indicated a big grandfather chair to the left of the door between it and the window. I append a plan of the room with the chair in question marked with an X.

'Just show me,' said Poirot.

The butler drew the chair in question out a good two feet from the wall, turning it so that the seat faced the door.

'Voila ce qui est curieux,' murmured Poirot. 'No one would want to sit in a chair in such a position, I fancy. Now who pushed it back into place again, I wonder? Did you, my friend?' 'No, sir,' said Parker. 'I was too upset with seeing the master and all.' Poirot looked across at me.

'Did you, doctor?' I shook my head.

'It was back in position when I arrived with the police, sir,' put in Parker. 'I'm sure of that.' 'Curious,' said Poirot again.

'Raymond or Blunt must have pushed it back,' I suggested. 'Surely it isn't important?' 'It is completely unimportant,' said Poirot. 'That is why it is so interesting,' he added softly.

'Excuse me a minute,' said Colonel Melrose. He left the room with Parker.

'Do you think Parker is speaking the truth?' I asked.

'About the chair, yes. Otherwise I do not know. You will find, M. le docteur, if you have much to do with cases of this kind, that they all resemble each other in one thing.' 'What is that?' I asked curiously.

'Everyone concerned in them has something to hide.' 'Have I?' I asked, smiling.

Poirot looked at me attentively.

'I think you have,' he said quietly.

'But-' 'Have you told me everything known to you about this young man Paton?' He smiled as I grew red. 'Oh! do not fear. I will not press you. I shall learn it in good time.' 'I wish you'd tell me something of your methods,' I said hastily, to cover my confusion. 'The point about the fire, for instance?' 'Oh! that was very simple. You leave Mr Ackroyd at - ten minutes to nine, was it not?' 'Yes, exactly, I should say.' 'The window is then closed and bolted and the door unlocked. At a quater past ten when the body is discovered, the door is locked and the window is open. Who opened it?

Clearly only Mr Ackroyd himself could have done so, and for one of two reasons. Either because the room became unbearably hot but since the fire was nearly out and there was a sharp drop in temperature last night, that cannot be the reason, or because he admitted someone that way. And if he admitted someone that way, it must have been someone well known to him, since he had previously shown himself uneasy on the subject of that same window.' 'It sounds very simple,' I said.

'Everything is simple, if you arrange the facts methodically. We are concerned now with the personality of the person who was with him at nine-thirty last night. Everything goes to show that that was the individual admitted by the window, and though Mr Ackroyd was seen alive later by Miss Flora, we cannot approach a solution of the mystery until we know wno that visitor was. The window may have been left open after his departure and so afforded entrance to the murderer, or the same person may have returned a second time. Ah! here is the colonel who returns.' Colonel Melrose entered with an animated manner.

'That telephone call has been traced at last,' he said. 'It did not come from here. It was put through to Dr Sheppard at 10.15 last night from a public call office at King's Abbot station. And at 10.23 the night mail leaves for Liverpool.'