the Subaltern at last pushed in to the uncovered
opening and crawled along the tunnel, flashing his electric torch
before him. Half-way to the end he felt a draught of cold air, and,
promptly extinguishing his lamp, saw a hole in the roof. His men were
alive all right, and not only alive but keeping on hard at work at the
end of the tunnel. When the collapse came they had gone back to where
their roof lay across the bottom of a shell-hole, pulled a plank out,
and--gone back to work.

When the tunnel reached a point under the German parapet it was turned
sharp to left and right, forming a capital T with the cross-piece
running roughly along the line of trench and parapet. Here there was
need of the utmost deliberation and caution. A pick could not be used,
and even a spade had to be handled gently, in case the sounds of
working should reach the Germans overhead. In some places the
Subaltern could actually hear the movements and footsteps of the enemy
just above him.

Twice the diggers disturbed a dead German, buried evidently under the
parapet. Once a significant crumbling of the earth and fall of a few
heavy clods threatened a collapse where the gallery was under the edge
of the trench. The spot was hastily but securely shored up with
infinite caution and the least possible sound, and after that the
Subaltern had the explosive charges brought along and connected up in
readiness. Then, if the roof collapsed or their work were discovered,
the switch at the shaft could still be pressed, the wires would still
carry the current, and the mine would be exploded.

At last the Subaltern decided that everything was ready. He carefully
placed his charges, connected up his wires again, cleared out his
tools, and emerged to report 'all ready.'

Now the 'touching off' of a good-sized mine is not a matter to be done
lightly or without due and weighty authority, and that because more is
meant to result from it than the upheaval of some square yards of earth
and the destruction of so many yards of enemy trench. The mine itself,
elaborate and labour-making as it may have been, is, after all, only a
means to an end. That end may be the capture of a portion of the ruins
of the trench, it may be the destruction of an especially strong and
dangerous 'keep,' a point of resistance or an angle for attack. It may
even be a mine to destroy a mine which is known to be tunnelling into
our own trenches, but in any case the explosio