“I have not only arranged this laboratory so as to reproduce Mrs. Popper’s seance-room,” began Craig afresh, “but I have had the cabinet placed in relatively the same position a similar cabinet occupies in Mr. Vandam’s private seance-room in the Vandam mansion.

“One night, Mr. Jameson and myself were visiting Mr. Vandam.  At precisely twelve-thirty we heard most unaccountable rappings from that cabinet.  I particularly noted the position of the cabinet.  Back of it ran a hallway.  That is duplicated here.  Back of this cabinet is a hallway.  I had heard of these rappings before we went, but was afraid that it would be impossible for me to catch the ghost red handed.  There is a limit to what you can do the first time you enter a man’s house, and, besides, that was no time to arouse suspicion in the mind of anyone.  But science has a way out of every dilemma.  I determined to learn something of these rappings.”

Craig paused and glanced first at Farrington, then at Mrs. Popper, and then at Mr. Vandam.

“Mr. Jameson,” he resumed, “will escort the doctor, the inspector, Mr. Farrington, Mrs. Popper, and Mr. Vandam into my imitation hall of the Vandam mansion.  I want each of you in turn to tiptoe up that hall to a spot indicated on the wall, back of the cabinet, and strike that spot several sharp blows with your knuckles.”

I did as Craig instructed tiptoeing up myself first so that they could not mistake his meaning.  The rest followed separately, and after a moment we returned silently in suppressed excitement to the room.

Craig was still standing by the table, but now the pendulums with the magnets and needles and the drums worked by clockwork were before him.

“Another person outside the Vandam family had a key to the Vandam mansion,” he began gravely.  “That person, by the way, was the one who waited, night by night, until Mrs. Vandam took the fatal capsule, and then when she had taken it apprised the old man of the fact and strengthened an already blind faith in the shadow world.”

You could have heard a pin drop.  In fact you could almost have felt it drop.

“That other person who, unobserved, had free access to the house,” he continued in the breathless stillness, “is in this room now.”

He was looking at O’Connor as if for corroboration.  O’Connor nodded.  “Information derived from the butler,” he muttered.

“I did not know this until yesterday,” Kennedy continued, “but I suspected that something of the sort existed when I was first told by Dr. Hanson of the rappings.  I determined to hear those rappings, and make a record of them.  So, the night Mr. Jameson and I visited Mr. Vandam, I carried this little instrument with me.”

Almost lovingly he touched the pendulums on the table.  They were now at rest and kept so by means of a lever that prevented all vibration whatever.