the upward
extension of the pillars adjacent to the winzes, shown in Figure
37. Always a careful balance must be cast as to the value of the ore
left, and as to the cost of a substitute, because every ore-pillar
can be removed at some outlay. Temporary pillars are not unusual,
particularly to protect roadways and shafts. They are, when left
for these purposes, removed ultimately, usually by beginning at
the farther end and working back to the final exit.

[Illustration: Fig. 41.--Horizontal plan at levels of Broken Hill.
Method of alternate stopes and ore-pillars.]

[Illustration: Fig. 42.--Longitudinal section of Figure 41.]

A form of temporary ore-pillars in very wide deposits is made use
of in conjunction with both filling and timbering (Figs. 37, 39,
40). In the use of temporary pillars for ore-bodies 100 to 250
feet wide at Broken Hill, stopes are carried up at right angles
to the strike, each fifty feet wide and clear across the ore-body
(Figs. 41 and 42). A solid pillar of the same width is left in the
first instance between adjacent stopes, and the initial series of
stopes are walled with one square-set on the sides as the stope is
broken upward. The room between these two lines of sets is filled
with waste alternating with ore-breaking in the usual filling method.
When the ore from the first group of alternate stopes (_ABC_, Fig.
42) is completely removed, the pillars are stoped out and replaced
with waste. The square-sets of the first set of stopes thus become
the boundaries of the second set. Entry and ventilation are obtained
through these lines of square-sets, and the ore is passed out of
the stopes through them.

[Illustration: Fig. 43.--Cross-section of stull support with waste
reenforcement.]

ARTIFICIAL PILLARS.--This system also implies a roof so strong
as not to demand continuous support. Artificial pillars are built
in many different ways. The method most current in fairly narrow
deposits is to reenforce stulls by packing waste above them (Figs.
43 and 44). Not only is it thus possible to economize in stulls by
using the waste which accumulates underground, but the principle
applies also to cases where the stulls alone are not sufficient
support, and yet where complete filling or square-setting is
unnecessary. When the conditions are propitious for this method, it
has the comparative advantage over timber systems of saving timber,
and over filling systems of saving imported filling. Moreover