“Come, now, Tom,” he rejoined, argumentatively.  “You know as well as I do what sort of people those gamblers are—­superstitious as the deuce.  I did this once before to-day.  This is a good time to do it again, before they persuade themselves that there is nothing in that story which we printed in the Star.  That fellow is in there now, probably in that room where we were, and it is possible that they may reassure him and settle his fears.  Now, just suppose a murder had been committed in a room, and you knew it, and heard groanings and mutterings—­from nowhere, just in the air, about you, overhead—­what would you do, if you were inclined to be superstitious?”

Before I could answer, he had resumed the antics which before I had found so inexplicable.

“Cut out and run, I suppose,” I replied.  “But what has that to do with the case?  The groanings are here—­not there.  You haven’t been able to get in over there to attach anything, have you?  What do you mean?”

“No,” he admitted, “but did you ever hear what you could do with a microphone, a rheostat, and a small transformer coil if you attached them properly to a direct-current electric lighting circuit?  No?  Well, an amateur with a little knowledge of electricity could do it.  The thing is easily constructed, and the result is a most complicated matter.”

“Well?” I queried, endeavouring to follow him.

“The electric arc,” he continued, “isn’t always just a silent electric light.  You know that.  You’ve heard them make noises.  Under the right conditions such a light can be made to talk—­the ‘speaking arc,’ as Professor Duddell calls it.  In other words, an arc light can be made to act as a telephone receiver.”

I could hardly believe the thing possible, but Garrick went on explaining.

“You might call it the arcophone, I suppose.  The scientific fact of the matter is that the arc is sensitive to very small variations of the current.  These variations may run over a wide range of frequency.  That suggested to Duddell that a direct-current arc might be used as a telephone receiver.  All that you need is to add a microphone current to the main arc current.  The arc reproduces sounds and speech distinctly, loud enough, even, to be heard several feet away from the light.”

He had cut out the microphone again while he was talking to me.  He switched it in again with the words, “Now, get ready, Tom.  Just one more; then we must hurry around in that car of ours and watch the fun.”

This time he was talking into the microphone.  In a most solemn, sepulchral voice he repeated, “Let the slayer of Rena Taylor beware.  She will be avenged!  Beware!  It will be a life for a life!”

Three times he repeated it, to make sure that it would carry.  Then, grabbing up his hat and coat, he dashed out of the room, past the surprised policeman at the door, and took the steps in front of the house almost at a bound.