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“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.

“It would all seem so natural, even if discovered,” explained Kennedy rapidly. “The smoking oil – smoking just as an automobile often does – is passed into the compressed-air pipe. Condensed oil, moisture, and gases collect in the joint, and perhaps they line the whole distance of the pipe. A spark from the low-grade oil-and they are ignited. What takes place is the same thing that occurs in the cylinder of an automobile where the air is compressed with gasoline vapour. Only here we have compressed air charged with vapour of oil. The flame proceeds down the pipe – exploding through the pipe, if it happens to be not strong enough. This pipe, however, is strong. Therefore, the flame in this case shoots out at the open end of the pipe, down near the shield, and if the air in the tunnel happens also to be surcharged with oil-vapour, an explosion takes place in the tunnel – the river bottom is blown out – then God help the sand-hogs!

“That’s how your accidents took place, Orton,” concluded Kennedy in triumph, “and that impure air – not impure from carbon dioxide, but from this oil-vapour mixture – increased the liability of the men for the bends. Capps knew about it. He was careful while he was there to see that the air was made as pure as possible under the circumstances. He was so careful that he wouldn’t even let Mr. Jameson smoke in the tunnel. But as soon as he went to the surface, the same deadly mixture was pumped down again – I caught some of it in this flask, and – “