Paddy let the air out of the lock by turning on a valve leading to the outside, normal atmosphere.  Thus he let the air out rapidly at first until we had got down to half the pressure of the tunnel.  The second half he did slowly, and it was indeed tedious, but it was safe.  There was at first a hissing sound when he opened the valve, and it grew colder in the lock, since air absorbs heat from surrounding objects when it expands.  We were glad to draw sweaters on over our heads.  It also grew as misty as a London fog as the water-vapour in the air was condensed.

At last the hiss of escaping air ceased.  The door to the modern dungeon of science grated open.  We walked out of the lock to the elevator shaft and were hoisted up to God’s air again.  We gazed out across the river with its waves dancing in the sunlight.  There, out in the middle, was a wreath of bubbles on the water.  That marked the end of the tunnel, over the shield.  Down beneath those bubbles the sand-hogs were rooting.  But what was the mystery that the tunnel held in its dark, dank bosom?  Had Kennedy a clue?

“I think we had better wait around a bit,” remarked Kennedy, as we sipped our hot coffee in the dressing-room and warmed ourselves from the chill of coming out of the lock.  “In case anything should happen to us and we should get the bends, this is the place for us, near the medical lock, as it is called—­that big steel cylinder over there, where we found Orton.  The best cure for the bends is to go back under the air—­recompression they call it.  The renewed pressure causes the gas in the blood to contract again, and thus it is eliminated—­sometimes.  At any rate, it is the best-known cure and considerably reduces the pain in the worst cases.  When you have a bad case like Orton’s it means that the damage is done; the gas has ruptured some veins.  Paddy was right.  Only time will cure that.”

Nothing happened to us, however, and in a couple of hours we dropped in on Orton at the hospital where he was slowly convalescing.

“What do you think of the case?” he asked anxiously.

“Nothing as yet,” replied Craig, “but I have set certain things in motion which will give us a pretty good line on what is taking place in a day or so.”

Orton’s face fell, but he said nothing.  He bit his lip nervously and looked out of the sun-parlour at the roofs of New York around him.

“What has happened since last night to increase your anxiety, Jack?” asked Craig sympathetically.

Orton wheeled his chair about slowly, faced us, and drew a letter from his pocket.  Laying it flat on the table he covered the lower part with the envelope.

“Read that,” he said.