be further strengthened by
building waste pillars against them, in which case the arrangement
merges into the system of artificial pillars.

[Illustration: Fig. 28.--Longitudinal section of stull-supported
stope.]

[Illustration: Fig. 29.--Longitudinal section showing square-set
timbering.]

[Illustration: Fig. 30.--Square-set timbering on inclined ore-body.
Showing ultimate strain on timbers.]

Square-sets (Figs. 29 and 30), that is, trusses built in the opening
as the ore is removed, are applicable to almost any dip or width
of ore, but generally are applied only in deposits too wide, or to
rock too heavy, for stulls. Such trusses are usually constructed on
vertical and horizontal lines, and while during actual ore-breaking
the strains are partially vertical, ultimately, however, when the
weight of the walls begins to be felt, these strains, except in
vertical deposits, come at an angle to lines of strength in the
trusses, and therefore timber constructions of this type present
little ultimate resistance (Fig. 30). Square-set timbers are sometimes
set to present the maximum resistance to the direction of strain,
but the difficulties of placing them in position and variations in
the direction of strain on various parts of the stope do not often
commend the method. As a general rule square-sets on horizontal
lines answer well enough for the period of actual ore-breaking. The
crushing or creeps is usually some time later; and if the crushing
may damage the whole mine, their use is fraught with danger.
Reenforcement by building in waste is often resorted to. When done
fully, it is difficult to see the utility of the enclosed timber,
for entire waste-filling would in most cases be cheaper and equally
efficient.

[Illustration: Fig. 31.--"Cribs."]

There is always, with wood constructions, as said before, the very
pertinent danger of subsequent crushing and of subsidence in after
years, and the great risk of fires. Both these disasters have cost
Comstock and Broken Hill mines, directly or indirectly, millions of
dollars, and the outlay on timber and repairs one way or another
would have paid for the filling system ten times over. There are
cases where, by virtue of the cheapness of timber, "square-setting"
is the most economical method. Again, there are instances where the
ore lies in such a manner--particularly in limestone replacements--as
to preclude other means of support. These cases are being yearly
more and