annot be cheaply accomplished under
the conditions prevailing in most metal regions. Although such
shafts would have a longer life, the duration of timbered shafts
is sufficient for most metal mines. It follows that, as timber
is the cheapest and all things considered the most advantageous
means of shaft support for the comparatively temporary character
of metal mines, to get the strains applied to the timbers in the
best manner, and to use the minimum amount of it consistent with
security, and to lose the least working space, the shaft must be
constructed on rectangular lines.

[Footnote *: Octagonal shafts were sunk in Mexico in former times.
At each face of the octagon was a whim run by mules, and hauling
leather buckets.]

[Footnote **: The economic situation is rapidly arising in a number
of localities that steel beams can be usefully used instead of
timber. The same arguments apply to this type of support that apply
to timber.]

The variations in timbered shaft design arise from the possible
arrangement of compartments. Many combinations can be imagined,
of which Figures 9, 10, 11, 12, 13, and 14 are examples.

[Illustration: FIG. 9. FIG. 10. FIG. 11. FIG. 12. FIG. 13. FIG.
14.]

The arrangement of compartments shown in Figures 9, 10, 11, and
13 gives the greatest strength. It permits timbering to the best
advantage, and avoids the danger underground involved in crossing
one compartment to reach another. It is therefore generally adopted.
Any other arrangement would obviously be impossible in inclined
or combined shafts.

SIZE OF SHAFTS.--In considering the size of shafts to be installed,
many factors are involved. They are in the main:--

_a_. Amount of ore to be handled.
_b_. Winding plant.
_c_. Vehicle of transport.
_d_. Depth.
_e_. Number of men to be worked underground.
_f_. Amount of water.
_g_. Ventilation.
_h_. Character of the ground.
_i_. Capital outlay.
_j_. Operating expense.

It is not to be assumed that these factors have been stated in
the order of relative importance. More or less emphasis will be
attached to particular factors by different engineers, and under
different circumstances. It is not possible to suggest any arbitrary
standard for calculating their relative weight, and they are so
interdependent as to preclude separate discussion. The usual result
is a compromise between the demands of all.

Certain factors, however, dictate a minimum position, whic