h may
be considered as a datum from which to start consideration.

_First_, a winding engine, in order to work with any economy, must
be balanced, that is, a descending empty skip or cage must assist
in pulling up a loaded one. Therefore, except in mines of very
small output, at least two compartments must be made for hoisting
purposes. Water has to be pumped from most mines, escape-ways are
necessary, together with room for wires and air-pipes, so that at
least one more compartment must be provided for these objects.
We have thus three compartments as a sound minimum for any shaft
where more than trivial output is required.

_Second_, there is a certain minimum size of shaft excavation below
which there is very little economy in actual rock-breaking.[*]
In too confined a space, holes cannot be placed to advantage for
the blast, men cannot get round expeditiously, and spoil cannot be
handled readily. The writer's own experience leads him to believe
that, in so far as rock-breaking is concerned, to sink a shaft
fourteen to sixteen feet long by six to seven feet wide outside
the timbers, is as cheap as to drive any smaller size within the
realm of consideration, and is more rapid. This size of excavation
permits of three compartments, each about four to five feet inside
the timbers.

[Footnote *: Notes on the cost of shafts in various regions which
have been personally collected show a remarkable decrease in the
cost per cubic foot of material excavated with increased size of
shaft. Variations in skill, in economic conditions, and in method
of accounting make data regarding different shafts of doubtful
value, but the following are of interest:--

In Australia, eight shafts between 10 and 11 feet long by 4 to
5 feet wide cost an average of $1.20 per cubic foot of material
excavated. Six shafts 13 to 14 feet long by 4 to 5 feet wide cost
an average of $0.95 per cubic foot; seven shafts 14 to 16 feet
long and 5 to 7 feet wide cost an average of $0.82 per cubic foot.
In South Africa, eleven shafts 18 to 19 feet long by 7 to 8 feet
wide cost an average of $0.82 per cubic foot; five shafts 21 to
25 feet long by 8 feet wide, cost $0.74; and seven shafts 28 feet
by 8 feet cost $0.60 per cubic foot.]

The cost of timber, it is true, is a factor of the size of shaft,
but the labor of timbering does not increase in the same ratio.
In any event, the cost of timber is only about 15% of the actual
shaft cost, even in localities