“And to whom may it be dangerous?” I heard a voice inquire over my shoulder.  It was Kennedy.  “To Mr. Jameson or the rest of us?”

“Well,” answered Capps, “I supposed everybody knew it was reckless, and that he would hurt himself more by one smoke in the air than by a hundred up above.  That’s all.”

He turned on Kennedy sullenly, and started to walk back up the tunnel.  But I could not help thinking that his manner was anything but solicitude for my own health.  I could just barely catch his words over the tunnel telephone some feet away.  I thought he said that everything was going along all right and that he was about to start back again.  Then he disappeared in the mist of the tube without even nodding a farewell.

Kennedy and I remained standing, not far from the outlet of the pipe by which the compressed air was being supplied in the tunnel from the compressors above, in order to keep the pressure up to the constant level necessary.  I saw Kennedy give a hurried glance about, as if to note whether any one were looking at us.  No one was.  With a quick motion he reached down.  In his hand was a stout little glass flask with a tight-fitting metal top.  For a second he held it near the outlet of the pipe; then he snapped the top shut and slipped it back into his pocket as quickly as he had produced it.

Slowly we commenced to retrace our steps to the air-lock, our curiosity satisfied by this glimpse of one of the most remarkable developments of modern engineering.

“Where’s Paddy?” asked Kennedy, stopping suddenly.  “We’ve forgotten him.”

“Back there at the shield, I suppose,” said I.  “Let’s whistle and attract his attention.”

I pursed up my lips, but if I had been whistling for a million dollars I couldn’t have done it.

Craig laughed.  “Walter, you are indeed learning many strange things.  You can’t whistle in compressed air.”

I was too chagrined to answer.  First it was Capps; now it was my own friend Kennedy chaffing me for my ignorance.  I was glad to see Paddy’s huge form looming in the semi-darkness.  He had seen that we were gone and hurried after us.

“Won’t ye stay down an’ see some more, gintlemen?” he asked.  “Or have ye had enough of the air?  It seems very smelly to me this mornin’—­I don’t blame ye.  I guess them as doesn’t have to stay here is satisfied with a few minutes of it.”

“No, thanks, I guess we needn’t stay down any longer,” replied Craig.  “I think I have seen all that is necessary—­at least for the present.  Capps has gone out ahead of us.  I think you can take us out now, Paddy.  I would much rather have you do it than to go with anybody else.”

Coming out, I found, was really more dangerous than going in, for it is while coming out of the “air” that men are liable to get the bends.  Roughly, half a minute should be consumed in coming out from each pound of pressure, though for such high pressures as we had been under, considerably more time was required in order to do it safely.  We spent about half an hour in the air-lock, I should judge.