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An idea seemed to be suggested to him by the sign. He wheeled and entered the place. Inquiry brought out a caretaker who showed us several rooms unoccupied, among them one vacant on the first floor.

Kennedy looked it over carefully, as though considering whether it was just the place he wanted, but ended, as I knew he intended, in hiring it.

“I can’t move my stuff in for a couple of days,” he told the caretaker. “Meanwhile, I may have the key, I suppose?”

He had paid a good deposit and the key was readily forthcoming.

The hiring of the ground floor room accomplished without exciting suspicion, Kennedy and I made a hasty trip up to his own laboratory, where he took a small box from a cabinet and hurried back to the taxicab which had brought us uptown.

Back again in the bare room which he had acquired, Craig set to work immediately installing a peculiar instrument which he took from the package.

It seemed to consist of two rods much like electric light carbons, fixed horizontally in a wooden support with a spindle-shaped bit of carbon between the two ends of the rods. Wires were connected with binding screws at the free ends of the carbon rods.

First Craig made a connection with an electric light socket from which he removed the bulb, cutting in a rheostat. Then he attached the free wires from the carbons to a sort of telephone headgear and switched on the current.

“What is it?” I asked curiously.

“A geophone,” he replied simply.

“And what is a geophone?” I inquired.

“Literally an earth-phone,” he explained. “It is really the simplest form of telephone, applied to the earth. You saw what it was. Any high school student of physics can make one, even with two or three dry batteries in circuit.”

“But what does it do?” I asked.

“It is really designed to detect earth vibrations. All that is necessary is to set the carbon stick arrangement, which is the transmitter of this telephone, on the floor, place myself at the other end and listen. A trained ear can readily detect rumblings. Really it is doing in a different and often better way what the seismograph does. This instrument is so sensitive that it will record the slamming of a cellar door across the street. No one can go up those stairs next door without letting me know it, no matter how cautious he is about it.”

Craig stood there some minutes holding the thing over his ears and listening intently.

“The vibrodyne machine isn’t running,” he remarked finally after repeated adjustments of the geophone. “But someone is in that little room under Creighton’s workshop. I suspected that something was down there after that watch crystal test of mine. Now I know it. I wonder what the man is doing?”

There was no excuse yet, however, for breaking into the room on the other side of the wall and under Creighton’s. Kennedy went out and watched. Though we waited some time nobody came out. He went back to our own room in the rear of the first floor. Though we both listened some time, neither of us could now hear a sound through the geophone except those made by passing trolleys and street vehicles.

Inquiry about the neighborhood did not develop who was the tenant or what was his business. In fact the results were just the reverse. No one seemed to know even the business conducted there. The room back of the locked door which Miss Laidlaw had passed was shrouded in mystery.

Nothing at all of any value was being recorded by the geophone when Kennedy glanced quickly at his watch. “If we are to see Miss Laidlaw and meet that Mrs. Barry, we had better be on our way,” he remarked hurriedly.

Miss Laidlaw was living in a handsome apartment on Central Park, West. We entered and gave our cards to the man at the door of her suite, who bowed us into a little reception room. We entered and waited.