er
than men or animals, it sometimes pays (if the dip is steep enough)
to dump everything through winzes from one to two levels to a main
road below where mechanical equipment can be advantageously provided.
The cost of shaft-winding the extra depth is inconsiderable compared
to other factors, for the extra vertical distance of haulage can
be done at a cost of one or two cents per ton mile. Moreover, from
such an arrangement follows the concentration of shaft-bins, and of
shaft labor, and winding is accomplished without so much shifting
as to horizon, all of which economies equalize the extra distance
of the lift.

There are three principal methods of mechanical transport in use:--

1. Cable-ways.
2. Compressed-air locomotives.
3. Electrical haulage.

Cable-ways or endless ropes are expensive to install, and to work
to the best advantage require double tracks and fairly straight
roads. While they are economical in operation and work with little
danger to operatives, the limitations mentioned preclude them from
adoption in metal mines, except in very special circumstances such
as main crosscuts or adit tunnels, where the haulage is straight
and concentrated from many sources of supply.

Compressed-air locomotives are somewhat heavy and cumbersome, and
therefore require well-built tracks with heavy rails, but they
have very great advantages for metal mine work. They need but a
single track and are of low initial cost where compressed air is
already a requirement of the mine. No subsidiary line equipment is
needed, and thus they are free to traverse any road in the mine and
can be readily shifted from one level to another. Their mechanical
efficiency is not so low in the long run as might appear from the
low efficiency of pneumatic machines generally, for by storage of
compressed air at the charging station a more even rate of energy
consumption is possible than in the constant cable and electrical
power supply which must be equal to the maximum demand, while the
air-plant consumes but the average demand.

Electrical haulage has the advantage of a much more compact locomotive
and the drawback of more expensive track equipment, due to the
necessity of transmission wire, etc. It has the further disadvantages
of uselessness outside the equipped haulage way and of the dangers
of the live wire in low and often wet tunnels.

In general, compressed-air locomotives possess many attractions
for metal mine work, wher