was opened and the four men stepped in,
entering a tiny ante-chamber. They closed the outer door, which was
absolutely air-tight, opened the inner one, and passed into the
chamber built against the coal face, made of solid cement except for a
circle of coal a yard in diameter.

A minute or two later, could be heard, faintly, the high screech of
some rapid-cutting machine.

When Otto came back on his next shift, at 2 o'clock on Friday morning,
the sand hogs were still working.

Curiosity overcame the old miner's desire not to seem ignorant.

"Just what is that, sir?" he asked the Bureau official, who was still
on watch.

"That you, Otto? So you want to know, now, do you? Well, that's a sort
of lightly made caisson, or air-tight chamber, with an air-lock or
double door. It's used a good deal for working under water, but for
the job we have here, it doesn't have to be very solidly built.

"It's simple enough, when you think it out. We just cemented it up,
put in an air-pump to take out the gassy air that was in it, and then
turned in compressed air, with a pressure of a little more than one
atmosphere, just enough to keep any of the gas from entering the hole
that is being dug through the coal pillar."

"Why can't gas get in? Gas'll go through coal."

"Because the pressure from inside is bigger than from outside. The
compressed air is leaking through the coal and driving any gas away."

"Why didn't you let us get in there to finish the job, if that's all
there is to it?" protested Otto, indignant that strangers should have
the glory of the final rescue, after the miners had done so much.

"Because you couldn't stand it. Those men are sand hogs. They're used
to working in compressed air. Just as soon as a man gets into a
pressure of two or three atmospheres, unless he's mighty careful he's
apt to get dangerously ill. His blood absorbs too much air. While he's
under compression, he doesn't feel it so much, but if he comes out of
the compression too quickly, the surplus air in his blood can't come
out as slowly as it ought, and little bubbles form in the blood
current. That's deadly. Sometimes these bubbles cause a terrible
caisson disease known as the 'bends,' when all the muscles and joints
are affected; or it may give a paralysis known as 'diver's palsy,'
because divers working in compressed atmospheres suffer the same way;
all too often, it causes sudden death. So you see, Otto, it's not a
chance a man ought t