power has to be
generated on the mine, the saving by the use of direct steam, generated
at the winding gear, is very considerable. Moreover, the cost of
haulage through a shaft for the extra distance from tunnel-level
to the surface is often less than the cost of transferring the
ore and removing it through the tunnel. The load once on the
winding-engine, the consumption of power is small for the extra
distance, and the saving of labor is of consequence. On the other
hand, where drainage problems arise, they usually outweigh all
other considerations, for whatever the horizon entered by tunnel,
the distance from that level to the surface means a saving of
water-pumpage against so much head. The accumulation of such constant
expense justifies a proportioned capital outlay. In other words,
the saving of this extra pumping will annually redeem the cost of
a certain amount of tunnel, even though it be used for drainage
only.

In order to emphasize the rapidity with which such a saving of
constant expense will justify capital outlay, one may tabulate the
result of calculations showing the length of tunnel warranted with
various hypothetical factors of quantity of water and height of lift
eliminated from pumping. In these computations, power is taken at
the low rate of $60 per horsepower-year, the cost of tunneling at
an average figure of $20 per foot, and the time on the basis of
a ten-year life for the mine.

Feet of Tunnel Paid for in 10 Years with Under-mentioned Conditions.

=============================================================
Feet of | 100,000 | 200,000 | 300,000 | 500,000 |1,000,000
Water Lift | Gallons | Gallons | Gallons | Gallons | Gallons
Avoided |per Diem |per Diem |per Diem |per Diem |per Diem
-----------|---------|---------|---------|---------|---------
100 | 600 | 1,200 | 1,800 | 3,000 | 6,000
200 | 1,200 | 2,400 | 3,600 | 6,000 | 12,000
300 | 1,800 | 3,600 | 5,400 | 9,000 | 18,000
500 | 3,000 | 6,000 | 9,000 | 15,000 | 30,000
1,000 | 6,000 | 12,000 | 18,000 | 30,000 | 60,000
=============================================================

The size of tunnels where ore-extraction is involved depends upon
the daily tonnage output required, and the length of haul. The
smallest size that can be economically driven and managed is about
6-1/2 feet by 6 feet inside the timbers. Such a tunnel, with single
track for a length of 1