“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—­”

“My God, Paddy’s down there now,” cried Orton, suddenly seizing his telephone.  “Operator, give me the south tube—­quick—­what—­ they don’t answer?”

Out in the river above the end of the heading, where a short time before there had been only a few bubbles on the surface of the water, I could see what looked like a huge geyser of water spouting up.  I pulled Craig over to me and pointed.

“A blow-out,” cried Kennedy, as he rushed to the door, only to be met by a group of blanched-faced workers who had come breathless to the office to deliver the news.

Craig acted quickly.  “Hold these men,” he ordered, pointing to Capps and Shelton, “until we come back.  Orton, while we are gone, go over the entire day’s record on the telegraphone.  I suspect you and Miss Taylor will find something there that will interest you.”

He sprang down the ladder to the tunnel air-lock, not waiting for the elevator.  In front of the closed door of the lock, an excited group of men was gathered.  One of them was peering through the dim, thick, glass porthole in the door.

“There he is, standin’ by the door with a club, an’ the men’s crowdin’ so fast that they’re all wedged so’s none can get in at all.  He’s beatin’ ’em back with the stick.  Now, he’s got the door clear and has dragged one poor fellow in.  It’s Jimmy Rourke, him with the eight childer.  Now he’s dragged in a Polack.  Now he’s fightin’ back a big Jamaica nigger who’s tryin’ to shove ahead of a little Italian.”