ut, the deadly white damp, that kills as
it strikes!

The hewers retreated, grumbling.

"We can stand it, with reliefs!" they declared.

But the Bureau man was adamant.

"Get back when you're told," he said shortly. "We'll get those men out
all right. Bring the gas gang here!"

Then it was that the researches of the trained workers of the Bureau
of Mines showed to their best advantage.

Along the gallery came a line of strange-eyed and humped figures,
inhuman of appearance, wearing the newly devised respirators by which
men can work in the most vitiated air without harm.

There are several types of these "gas masks," most of them based on
the principle of carrying compressed oxygen for breathing, and bearing
chambers containing chemicals which absorb the carbonic acid gas and
moisture of the exhaled breath. These masks proved their utility at
the great explosion at Courrieres in 1906, the greatest mining
disaster on record, when 1100 miners were killed.

[Illustration: INTO THE POISON-FILLED AIR!

Rescue-Crew of the U. S. Bureau of Mines, equipped with
oxygen-breathing apparatus, facing the deadly "damps."

_Courtesy of U. S. Bureau of Mines._]

[Illustration: U. S. BUREAU OF MINES RESCUE CAR.]

[Illustration: INTERIOR VIEW, SHOWING LIFE-SAVING EQUIPMENT.]

It was not long, however, before it became evident that there was a
limit to the usefulness of the respirators. Excellent as they were for
exploring galleries filled with poisonous gas, it was difficult to do
fast digging in them. The work slowed down.

"Look here, Mr. Owens," protested Otto, "if we don't go no faster'n
we're goin' now, it'll be a month afore we get through. Let us go in!
If the gas is bad, we'll take hour shifts, or half-hour shifts, or
ten-minute shifts, if it comes to that! The men'll tough it out as
long as they can!"

"What about it?" said the superintendent, to the Director of the
Bureau of Mines car.

"If the men are willing to take the risk! But we can purify the air to
some extent, anyway. I've a man down there with a Burrell gas
detector, which is several hundred times more sensitive than any
canary, so that we can keep a close watch on the air changes, and
there are plenty of tanks of compressed oxygen to be got. I've some
here in the car, and a telegram to Pittsburgh will bring us more in a
few hours. We can put in another bellows, too.

"This miner's right enough, about the digging. Fast work can't be done