HERE THE TIMBER GOES.

Whole forests are cut down to hold up the mine galleries. On the
strength of this work the lives of the miners depend.

_Courtesy of the Wigham Coal Co._]

[Illustration: GEOPHONE EXPERT LISTENING FOR TAPPING OF SURVIVORS.

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

[Illustration: BUILDING THE WALL FOR THE "SAND-HOGS."

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

If, indeed, it were safe to blast, it might be possible to get rid of
the obstruction by the use of a heavy blast and then rush through and
grab the men. But this was impossible. The Burrell tester showed a
large proportion of methane gas or fire damp, and a blast of any size
might easily start an explosion which would not only wreck the mine,
but also kill every member of the rescue parties, while affording no
chance of getting the imprisoned men.

How could the wall be taken down, without allowing the gas to
percolate through?

"Stand back, men," said the official, "here come the 'sand hogs,'
now."

Amazed, the colliers retreated from the coal face to give place to a
very different group of men. Small and wiry folk, these, dressed in an
entirely different fashion from the miners. The respirators gave them
the same goggle-eyed goblin faces. Not one of them had ever been in a
coal mine before.

With a speed and dexterity that showed their knowledge of the work,
these men proceeded to build up, at the side of the gallery, close to
the point where the last obstruction still held, a solid face of
concrete, and rapidly cemented it to the somewhat elaborate
construction that had been in process of making all the preceding day,
and to which Otto had paid no heed.

It was not long before it became evident that a completely closed room
was being made. Other gangs came along, carrying strange screw-doors
of iron, and a multitude of devices new to the eyes of miners.
Everything had been measured and prepared above-ground. It remained
only to throw the material together, according to a prearranged plan.

By midnight, all was ready.

Three "sand hogs," with a gallant young doctor who had volunteered,
prepared to enter.

A steady throbbing sound told that machinery connected with an outlet
pipe--solidly embedded in the cement--had been set in motion. The
newly made walls threatened to bulge inwards, and the signal was given
to stop.

Then a rushing noise was heard in the inlet pipe, similarly embedded.
The outer of the double doors