“See, I release this lever—­now, let no one in the room move.  Watch the needles on the paper as the clockwork revolves the drums.  I take a step—­ever so lightly.  The pendulums vibrate, and the needles trace a broken line on the paper on each drum.  I stop; the lines are practically straight.  I take another step and another, ever so lightly.  See the delicate pendulums vibrate?  See, the lines they trace are jagged lines.”

He stripped the paper off the drums and laid it flat on the table before him, with two other similar pieces of paper.

“Just before the time of the rapping I placed this instrument in the corner of the Vandam cabinet, just as I placed it in this cabinet after Mr. Jameson conducted you from the room.  In neither case were suspicions aroused.  Everything in both cases was perfectly normal—­I mean the ‘ghost’ was in ignorance of the presence, if not the very existence, of this instrument.

“This is an improved seismograph,” he explained, “one after a very recent model by Prince Galitzin of the Imperial Academy of St. Petersburg.  The seismograph, as you know, was devised to register earthquakes at a distance.  This one not only measures the size of a distant earthquake, but the actual direction from which the earth-tremors come.  That is why there are two pendulums and two drums.

“The magnetic arrangement is to cut short the vibrations set up in the pendulums, to prevent them from continuing to vibrate after the first shock.  Thus they are ready in an instant to record another tremor.  Other seismographs continue to vibrate for a long time as a result of one tremor only.  Besides, they give little indication of the direction from which the tremors come.

“I think you must all appreciate that your tiptoeing up the hall must cause a far greater disturbance in this delicate seismograph than even a very severe earthquake thousands of miles away, which it was built to record.”

He paused and examined the papers sharply.

“This is the record made by the ‘ghost’s’ walk the other night,” he said, holding up two of them in his left hand.  “Here on the table, on two other longer sheets, I have records of the vibrations set up by those in this room walking to-night.

“Here is Mr. Jameson’s—­his is not a bit like the ghost’s.  Nor is Mr. Vandam’s.  Least of all are Dr. Hanson’s and Inspector O’Connor’s, for they are heavy men.

“Now here is Mr. Farrington’s”—­he bent down closely, “he is a light man, and the ghost was light.”

Craig was playing with his victim like a cat with a mouse.

Suddenly I felt something brush by me, and with a swish of air and of garments I saw Mrs. Popper fling herself wildly at the table that bore the incriminating records.  In another instant Farrington was on his feet and had made a wild leap in the same direction.

It was done so quickly that I must have acted first and thought afterward.  I found myself in the midst of a melee with my hand at his throat and his at mine.  O’Connor with a jiu-jitsu movement bent Farrington’s other arm until he released me with a cry of pain.