,000 feet, with one turn-out, permits handling
up to 500 tons a day with men and animals. If the distance be longer
or the tonnage greater, a double track is required, which necessitates
a tunnel at least 8 feet wide by 6-1/2 feet to 7 feet high, inside
the timbers.

There are tunnel projects of a much more impressive order than those
designed to operate upper levels of mines; that is, long crosscut
tunnels designed to drain and operate mines at very considerable
depths, such as the Sutro tunnel at Virginia City. The advantage
of these tunnels is very great, especially for drainage, and they
must be constructed of large size and equipped with appliances
for mechanical haulage.

CHAPTER IX.

Development of Mines (_Concluded_).

SUBSIDIARY DEVELOPMENT;--STATIONS; CROSSCUTS; LEVELS; INTERVAL
BETWEEN LEVELS; PROTECTION OF LEVELS; WINZES AND RISES. DEVELOPMENT
IN THE PROSPECTING STAGE; DRILLING.

SUBSIDIARY DEVELOPMENT.

Stations, crosscuts, levels, winzes, and rises follow after the
initial entry. They are all expensive, and the least number that
will answer is the main desideratum.

STATIONS.--As stations are the outlets of the levels to the shaft,
their size and construction is a factor of the volume and character
of the work at the levels which they are to serve. If no timber
is to be handled, and little ore, and this on cages, the stations
need be no larger than a good sized crosscut. Where timber is to
be let down, they must be ten to fifteen feet higher than the floor
of the crosscut. Where loading into skips is to be provided for,
bins must be cut underneath and sufficient room be provided to
shift the mine cars comfortably. Such bins are built of from 50 to
500 tons' capacity in order to contain some reserve for hoisting
purposes, and in many cases separate bins must be provided on opposite
sides of the shaft for ore and waste. It is a strong argument in
favor of skips, that with this means of haulage storage capacity
at the stations is possible, and the hoisting may then go on
independently of trucking and, as said before, there are no idle
men at the stations.

[Illustration: Fig. 15.--Cross-section of station arrangement for
skip-haulage in vertical shaft.]

[Illustration: Fig. 16.--Cross-section of station arrangement for
skip-haulage in vertical shaft.]

It is always desirable to concentrate the haulage to the least
number of levels, for many reasons. Among them is that, where haulage
is c