Along the bottom of the completed tube we followed Paddy and Capps.  On we trudged, fanned by the moist breath of the tunnel.  Every few feet an incandescent light gleamed in the misty darkness.  After perhaps a hundred paces we had to duck down under a semicircular partition covering the upper half of the tube.

“What is that?” I shouted at Paddy, the nasal ring of my own voice startling me.

“Emergency curtain,” he shouted back.

Words were economised.  Later, I learned that should the tunnel start to flood, the other half of the emergency curtain could be dropped so as to cut off the inrushing water.

Men passed, pushing little cars full of “muck” or sand taken out from before the “shield”—­which is the head by which this mechanical mole advances under the river-bed.  These men and others who do the shovelling are the “muckers.”

Pipes laid along the side of the tunnel conducted compressed air and fresh water, while electric light and telephone wires were strung all about.  These and the tools and other things strewn along the tunnel obstructed the narrow passage to such an extent that we had to be careful in picking our way.

At last we reached the shield, and on hands and knees we crawled out into one of its compartments.  Here we experienced for the first time the weird realisation that only the “air” stood between us and destruction from the tons and tons of sand and water overhead.  At some points in the sand we could feel the air escaping, which appeared at the surface of the river overhead in bubbles, indicating to those passing in the river boats just how far each tunnel heading below had proceeded.  When the loss of air became too great, I learned, scows would dump hundreds of tons of clay overhead to make an artificial river bed for the shield to stick its nose safely through, for if the river bed became too thin overhead the “air” would blow a hole in it.

Capps, it seemed to me, was unusually anxious to have the visit over.  At any rate, while Kennedy and Paddy were still crawling about the shield, he stood aside, now and then giving the men an order and apparently forgetful of us.

My own curiosity was quickly satisfied, and I sat down on a pile of the segments out of which the successive rings of the tunnel were made.  As I sat there waiting for Kennedy, I absently reached into my pocket and pulled out a cigarette and lighted it.  It burned amazingly fast, as if it were made of tinder, the reason being the excess of oxygen in the compressed air.  I was looking at it in astonishment, when suddenly I felt a blow on my hand.  It was Capps.

“You chump!” he shouted as he ground the cigarette under his boot.  “Don’t you know it is dangerous to smoke in compressed air?”

“Why, no,” I replied, smothering my anger at his manner.  “No one said anything about it.”

“Well, it is dangerous, and Orton’s a fool to let greenhorns come in here.”