I went over now to the dummy, and after a few minute's work managed to wrench the dagger loose out of the armor. I brought the old weapon and placed its hilt in a hole near the top of the post where it fitted loosely, the point upward. After that I went again to the lever and gave another strong heave, and the post descended about a foot, to the bottom of the cavity, catching there with another clang. I withdrew the lever and the narrow strip of floor slid back, covering post and dagger, and looking no different from the surrounding surface. Then I shut the chancel gate, and we both stood well to one side. I took the spear-like lever, and gave the gate a little push, so that it opened. Instantly there was a loud thud, and something sang through the air, striking the bottom wall of the Chapel. It was the dagger. I showed Jarnock then that the other half of the post had sprung back into place, making the whole post as thick as the one upon the right-hand side of the gate. "There!" I said, turning to the young man and tapping the divided post. "There's the 'invisible' thing that used the dagger, but who the deuce is the person who sets the trap?" I looked at him keenly as I spoke. "My father is the only one who has a key," he said. "So it's practically impossible for anyone to get in and meddle." I looked at him again, but it was obvious that he had not yet reached out to any conclusion. "See here, Mr. Jarnock," I said, perhaps rather curter than I should have done, considering what I had to say. "Are you quite sure that Sir Alfred is quite balanced--mentally?" "He looked at me, half frightenedly and flushing a little. I realized then how badly I put it. "'I--I don't know,' he replied, after a slight pause and was then silent, except for one or two incoherent half remarks. "'Tell the truth,' I said. 'Haven't you suspected something, now and again? You needn't be afraid to tell me.' "'Well,' he answered slowly, 'I'll admit I've thought Father a little--a little strange, perhaps, at times. But I've always tried to think I was mistaken. I've always hoped no one else would see it. You see, I'm very fond of the old guvnor.' "I nodded. "'Quite right, too,' I said. 'There's not the least need to make any kind of scandal about this. We must do something, though, but in a quiet way. No fuss, you know. I should go and have a chat with your father, and tell him we've found out about this thing.' I touched the divided post.