While I held a pocket flashlight Craig was busy concealing another instrument of his own in the little storeroom.  It seemed to be a little black disk about as big as a watch, with a number of perforated holes in one face.  Carelessly he tossed it into the top drawer of the chest under some old rubbish, shut the drawer tight and ran a flexible wire out of the back of the chest.  It was a simple matter to lay the wire through some bins next the storeroom and then around to the passageway down to the subterranean den of Brixton.  There Craig deposited a little black box about the size of an ordinary kodak.

For an hour or so we sat with Brixton.  Neither of us said anything, and Brixton was uncommunicatively engaged in reading a railroad report.  Suddenly a sort of muttering, singing noise seemed to fill the room.

“There it is!” cried Brixton, clapping the book shut and looking eagerly at Kennedy.

Gradually the sound increased in pitch.  It seemed to come from the ceiling, not from any particular part of the room, but merely from somewhere overhead.  There was no hallucination about it.  We all heard.  As the vibrations increased it was evident that they were shaping themselves into words.

Kennedy had grasped the black box the moment the sound began and was holding two black rubber disks to his ears.

At last the sound from overhead became articulate It was weird, uncanny.  Suddenly a voice said distinctly:  “Let American dollars beware.  They will not protect American daughters.”

Craig had dropped the two ear-pieces and was gazing intently at the Osram lamp in the ceiling.  Was he, too, crazy?

“Here, Mr. Brixton, take these two receivers of the detectaphone,” said Kennedy.  “Tell me whether you can recognise the voice.”

“Why, it’s familiar,” he remarked slowly.  “I can’t place it, but I’ve heard it before.  Where is it?  What is this thing, anyhow?”

“It is someone hidden in the storeroom in the basement,” answered Craig.  “He is talking into a very sensitive telephone transmitter and—­”

“But the voice—­here?” interrupted Brixton impatiently.

Kennedy pointed to the incandescent lamp in the ceiling.  “The incandescent lamp,” he said, “is not always the mute electrical apparatus it is supposed to be.  Under the right conditions it can be made to speak exactly as the famous ‘speaking-arc,’ as it was called by Professor Duddell, who investigated it.  Both the arc-light and the metal-filament lamp can be made to act as telephone receivers.”

It seemed unbelievable, but Kennedy was positive.  “In the case of the speaking-arc or ‘arcophone,’ as it might be called,” he continued, “the fact that the electric arc is sensitive to such small variations in the current over a wide range of frequency has suggested that a direct-current arc might be used as a telephone receiver.  All that is necessary is to superimpose a microphone current on the main