Kennedy smiled.  “I have provided for that, Walter, in the way I installed the thing.  I took good care that we could not be cut off that way.  We can hear everything ourselves, but we cannot be overheard.  He knows nothing.  You see, I took advantage of the fact that additional telephones or so-called phantom lines can be superposed on existing physical lines.  It is possible to obtain a third circuit from two similar metallic circuits by using for each side of this third circuit the two wires of each of the other circuits in multiple.  All three circuits are independent, too.

“The third telephone current enters the wires of the first circuit, as it were, and returns along the wires of the second circuit.  There are several ways of doing it.  One is to use retardation or choke-coils bridged across the two metallic circuits at both ends, with taps taken from the middle points of each.  But the more desirable method is the one you saw me install this afternoon.  I introduced repeating-coils into the circuits at both ends.  Technically, the third circuit is then taken off from the mid-points of the secondaries or line windings of these repeating coils.

“The current on a long-distance line is alternating in character, and it passes readily through a repeating-coil.  The only effect it has on the transmission is slightly reducing the volume.  The current passes into the repeating-coil, then divides and passes through the two line wires.  At the other end the halves balance, so to speak.  Thus, currents passing over a phantom circuit don’t set up currents in the terminal apparatus of the side circuits.  Consequently, a conversation carried on over the phantom circuit will not be heard in either side circuit, nor does a conversation on one side circuit affect the phantom.  We could all talk at once without interfering with each other.”

“At any other time I should be more than interested,” remarked Brixton grimly, curbing his impatience to be doing something.

“I appreciate that, sir,” rejoined Kennedy.  “Ah, here it is.  I have the central down in the village.  Yes?  They will hold the boat for us?  Good.  Thank you.  The nine-o’clock train is five minutes late?  Yes—­what?  Count Wachtmann’s car is there?  Oh, yes, the train is just pulling in.  I see.  Miss Brixton has entered his car alone.  What’s that?  His chauffeur has started the car without waiting for the Count, who is coming down the platform?”

Instantly Kennedy was on his feet.  He was dashing up the corridor and the stairs from the den and down into the basement to the little storeroom.

We burst into the place.  It was empty.  Janeff had cut the wires and fled.  There was not a moment to lose.  Craig hastily made sure that he had not discovered or injured the phantom circuit.

“Call the fastest car you have in your garage, Mr. Brixton,” ordered Kennedy.  “Hello, hello, central!  Get the lodge at the Brixton estate.  Tell them if they see the engineer Janeff going out to stop him.  Alarm the watchman and have the dogs ready.  Catch him at any cost, dead, or alive.”