unconscious. This
is Clem Swinton talking."

"How is the air?"

"Getting bad, now."

"Keep your courage up! We'll have you out soon!"

The hewers set to work in high spirits, hoping that every blow of the
pick would drive through.

Then:

"Stop work, men!" said the Bureau chief suddenly.

The men stared at him, amazed at the order. All stopped, however,
except old Otto, who continued to use his pick-axe steadily.

The official grabbed him by the shoulder and spun him round with none
too gentle a hand.

"Stop, you thick-head, when you're told!"

"What for? We'll be through this wall in an hour!"

"You'll have a hole through it, maybe. But what good will that do?"

Otto stared at the official amazed, and the Bureau of Mines man went
on:

"You've had to start working in a respirator, after all, haven't you?
Why? Because of white damp! Haven't you got sense enough to see what
would happen as soon as you drove a hole through big enough to let the
white damp in and not big enough to get the men out? How long do you
think they'd last in this air, in their weakened state?"

Otto looked at him a moment, and then nodded his head.

"You're right, boss," he admitted. "I'm a fool. I'd never ha' thought
o' that. But what are you goin' to do?"

"You don't seem to know enough to use your eyes," the official
answered, shortly, "and they told me you were one of the best men in
the mine! What do you suppose we've been doing all this cement
construction along this gallery for the last couple of shifts?"

"I hadn't stopped to think," admitted Otto, taken aback.

"Well, you'll have a chance to do some thinking, now."

In effect, it was not surprising that Otto should not be able to see a
way out of the difficulty, for the problem was a serious one.

The proportion of white damp, or carbon monoxide, in the air where the
rescuers had now been compelled to work in respirators, was strong
enough to kill a man in ten or fifteen minutes. In the undoubtedly
weakened state of the three survivors, a lesser time than this would
suffice to be fatal.

If, in the course of digging away the obstruction which remained
between the rescuers and the entombed men, a small hole were made, or
if the rocks should lie in such a manner that there were
interstices between, Clem and his comrades would succumb before a
sufficiently large breach could be made in the wall whereby they might
be dragged through to liberty.

[Illustration: W