"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 partition
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, hoarsely.

"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,