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.