“Straight?”

“Yes, straight—­no joints, no pockets where oil, moisture, and gases can collect.”

“Straight as lines, Kennedy,” he said with a sort of contemptuous defiance.

They were facing each other coldly, sizing each other up.  Like a skilful lawyer, Kennedy dropped that point for a moment, to take up a new line of attack.

“Capps,” he demanded, turning suddenly, “why do you always call up on the telephone and let some one know when you are going down in the tunnel and when you are coming out?”

“I don’t,” replied Capps, quickly recovering his composure.

“Walter,” said Craig to me quietly, “go out in the outer office.  Behind the telephone switchboard you will find a small box which you saw me carry in there this morning and connect with the switchboard.  Detach the wires, as you saw me attach them, and bring it here.”

No one moved, as I placed the box on a drafting-table before them.  Craig opened it.  Inside he disclosed a large disc of thin steel, like those used by some mechanical music-boxes, only without any perforations.  He connected the wires from the box to a sort of megaphone.  Then he started the disc revolving.

Out of the little megaphone horn, sticking up like a miniature talking-machine, came a voice:  “Number please.  Four four three o, Yorkville.  Busy, I’ll call you.  Try them again, Central.  Hello, hello, Central—­”

Kennedy stopped the machine.  “It must be further along on the disc,” he remarked.  “This, by the way, is an instrument known as the telegraphone, invented by a Dane named Poulsen.  It records conversations over a telephone on this plain metal disc by means of localised, minute electric charges.”

Having adjusted the needle to another place on the disc he tried again.  “We have here a record of the entire day’s conversations over the telephone, preserved on this disc.  I could wipe out the whole thing by pulling a magnet across it, but, needless to say, I wouldn’t do that—­yet.  Listen.”

This time it was Capps speaking.  “Give me Mr. Shelton.  Oh, Shelton, I’m going down in the south tube with those men Orton has sent nosing around here.  I’ll let you know when I start up again.  Meanwhile—­you know—­don’t let anything happen while I am there.  Good-bye.”

Capps sat looking defiantly at Kennedy, as he stopped the telegraphone.

“Now,” continued Kennedy suavely, “what could happen?  I’ll answer my own question by telling what actually did happen.  Oil that was smoky at a lower point than its flash was being used in the machinery—­not really three-hundred-and-sixty-degree oil.  The water-jacket had been tampered with, too.  More than that, there is a joint in the pipe leading down into the tunnel, where explosive gases can collect.  It is a well-known fact in the use of compressed air that such a condition is the best possible way to secure an explosion.