ld pay for a shaft
1,000 feet deep. At a depth of 1,000 feet, at least six levels
might be necessary. The tramming of ore by hand through such a
distance would cost about double the amount to hoist it through
a shaft and transport it mechanically to the dressing plant at
surface. The aggregate cost and operation of barren levels therefore
soon pays for a second shaft. If two or more shafts are in question,
they must obviously be set so as to best divide the work.

Under Cases IV, V, and VI,--that is, deep-level projects,--ventilation
and escape become most important considerations. Even where the
volume of ore is within the capacity of a single shaft, another
usually becomes a necessity for these reasons. Their location is
affected not only by the locus of the ore, but, as said, by the time
required to reach it. Where two shafts are to be sunk to inclined
deposits, it is usual to set one so as to intersect the deposit at
a lower point than the other. Production can be started from the
shallower, before the second is entirely ready. The ore above the
horizon of intersection of the deeper shaft is thus accessible from
the shallower shaft, and the difficulty of long rises or crosscuts
from that deepest shaft does not arise.

CHAPTER VIII.

Development of Mines (_Continued_).

SHAPE AND SIZE OF SHAFTS; SPEED OF SINKING; TUNNELS.

SHAPE OF SHAFTS.--Shafts may be round or rectangular.[*] Round
vertical shafts are largely applied to coal-mines, and some engineers
have advocated their usefulness to the mining of the metals under
discussion. Their great advantages lie in their structural strength,
in the large amount of free space for ventilation, and in the fact
that if walled with stone, brick, concrete, or steel, they can be
made water-tight so as to prevent inflow from water-bearing strata,
even when under great pressure. The round walled shafts have a longer
life than timbered shafts. All these advantages pertain much more to
mining coal or iron than metals, for unsound, wet ground is often
the accompaniment of coal-measures, and seldom troubles metal-mines.
Ventilation requirements are also much greater in coal-mines. From
a metal-miner's standpoint, round shafts are comparatively much
more expensive than the rectangular timbered type.[**] For a larger
area must be excavated for the same useful space, and if support
is needed, satisfactory walling, which of necessity must be brick,
stone, concrete, or steel, c