"'I should like to show you, sir, a curious optical effect there is in this room. Would you mind standing on that flagstone there?'
"I came round the bed to it, and my foot had just touched it when I was jerked backwards and fell to the floor.
"'Beg your pardon, sir,' said the inspector behind me. 'I had to satisfy myself that you didn't know of the trap. See here.'
"He knelt down beside the big flagstone and touched it lightly with his fingers. It was exactly balanced by a big iron pin through the centre, and it now swung open, showing a dark shaft going far down into the earth.
"'You mean that they are down there?' I said.
"'Not a doubt. Each of them, as is only natural, tried the floor as well as the walls, and moved the bed for the purpose. That finished them. It's the merest chance that I didn't go down the shaft myself.'
"'Well,' I said, 'the sooner we go down there the better. Where can we get a rope?'
"The inspector picked up a small tin match-box and emptied out the matches into the palm of his hand. 'Listen,' he said. He flung the box down the shaft. We listened, and listened, but heard no sound. 'See?' he said. 'That's deep. No use to get a rope there. Anyone who fell down there is dead. That's been a well, I should say.'
"I was angry with the man's cock-surety, and said that I was going down in any case. A rope was brought and attached to a lighted lantern. The lantern was lowered, and in a few yards went out. The experiment was tried again and again, and each time the lantern was extinguished by the foul air. It was hopeless. No human being could have lived for five minutes down there.
"I rose from the floor, put on my coat, and turned to the inspector. 'This explains nothing,' I said. 'On the morning that Dr Ash was missed I went in here with Mr Stavold, and we found the bed placed as it had been the night before, immediately over this trap. If Dr Ash fell down it how did he put the bed back after him? The same thing applies to Mr Stavold; again the bed was left over the trap.'