At one point he stopped and moved the thing vertically up along the wall, from floor to ceiling.

"That's a gas pipe," he said simply.

"What's the instrument?" I asked.

"A new apparatus for finding pipes electrically, which I think can be just as well applied to finding other things concealed in walls under plaster and paper."

He paused to adjust the thing. "The electrical method," he went on, "is a special application of well-known induction balance principles. You see one set of coils receives an alternating or vibrating current. The other is connected with this telephone. First I established a balance so that there was no sound in the telephone."

He moved the thing about. "Now, when the device comes near metal-piping, for example, or a wire, the balance is disturbed and I hear a sound.

That was the gas pipe. It is easy to find its exact location. Hulloa--"

He paused again in a corner, back of Gaskell's desk and appeared to be listening intently.

A moment later he was ruthlessly breaking through the plaster of the beautifully decorated wall.

Sure enough, in there was a detectaphone, concealed only a fraction of an inch beneath the paper, with wires leading down inside the part.i.tion in the direction of the cellar.

CHAPTER XI

THE INFERNAL MACHINES

He ripped the little mechanical eavesdropper out, wires and all, but he did not disconnect the wires, yet.

We traced it out, and down into the cellar the wires led, directly, and then across, through a small opening in the foundations into the next cellar of an apartment house, ending in a bin or storeroom.

In itself the thing, so far, gave no clew as to who was using it or the purpose for which it had been installed. But it was strange.

"Someone _was_ evidently trying to get something from you, Mr. Gaskell,"

remarked Craig pointedly, after we returned to the Gaskell library. "Why do you suppose he went to all that trouble?"

Gaskell shrugged his shoulders and averted his eyes.

"I've heard of a yacht outside New York harbor," added Craig casually.

"A yacht?"

"Yes," he said nonchalantly, "the _Furious_."

Gaskell met Kennedy's eye and looked at him as though Craig had some occult power of divination. Then he moved over closer to us.

"Is that detectaphone thing out of business now?" he asked, hoa.r.s.ely.

"Yes."

"Absolutely?"

"Absolutely."

Gaskell leaned over.

"Then I don't mind telling you, Professor Kennedy," he said in a low tone, "that I am letting a friend of mine from London use that yacht to supply some allied warships on the Atlantic with news, supplies and ammunition, such as can be carried."

Kennedy looked at him keenly, but for some moments did not answer. I knew he was debating on how he might properly dove-tail this with Burke's case, ethically.

"Someone is trying to find out from eavesdropping just what your plans are, then," remarked Craig thoughtfully, with a significant tap on the detectaphone.

A moment later he turned his back to us and knelt down. He seemed to be wrapping the detectaphone up in a small package which he put in his pocket and closing the hole in the wall as best he could where he had ripped the paper.

"All I ask of you," concluded Gaskell, as we left a few minutes later, "is to keep your hands off that phase of things. Find the incendiary--yes; but this other matter that you have forced out of me--well--hands off!"

On our way downtown to keep the appointment Kennedy had made with Burke the night before, he stopped at the laboratory to get a heavy parcel which he carried along.

We found Burke waiting for us, impatiently, at the Customs House.

"We've just discovered that the liners over at Hoboken have had steam up for a couple of days," he said excitedly. "Evidently they are waiting to make a break for the ocean--perhaps in concert with a sortie of the fleets over in Europe."

"H-m," mused Kennedy, looking fixedly at Burke, "that complicates matters, doesn't it? We must preserve American neutrality."

He thought a moment. "I should like to go aboard the revenue cutter. May I?"

"Surely," agreed Burke.

A few moments later we were on the _Uncas_, Kennedy and Burke in earnest conversation in low tones which I did not overhear. Evidently Craig was telling him just enough of what he had himself discovered so as to enlist Burke's services.

The captain in charge of the _Uncas_ joined the conversation a few moments later, and then Kennedy took the heavy package down below. For some time he was at work in one of the forward tanks that was full of water, attaching the thing, whatever it was, in such a way that it seemed to form part of the skin of the ship.

Another brief talk with Burke and the captain followed, and then the three returned to the deck.

"Oh, by the way," remarked Burke, as he and Kennedy came back to me, "I forgot to tell you that I have had some of my men working on the case and one of them has just learned that a fellow named Petzka, one of the best wireless operators,--a Hungarian or something--has been engaged to go on that yacht."

"Petzka?" I repeated involuntarily.

"Yes," said Burke, in surprise, "do you know anything about him?"

I turned to Kennedy.

"Not much," replied Craig. "But you can find out about him, I think, through his wife. He used to be one of my students. Here's her address.

She's very anxious to hear from him. I'm sure that if you have any news she will be only too glad to receive it."

Burke took the address and a little while later we went ash.o.r.e.

I was not surprised when Kennedy proposed, as the next move, to revisit the cellar in the apartment next to Gaskell's house. But I was surprised at what he said, after we had reached the place.

All along I had supposed that he was planning to wait there in hope of catching the person who had installed the detectaphone. That, of course, was a possibility, still. But in reality he had another purpose, also.