of extremely high prices.

_Third_, three reasons are rapidly making the self-dumping skip
the almost universal shaft-vehicle, instead of the old cage for
cars. First, there is a great economy in labor for loading into
and discharging from a shaft; second, there is more rapid despatch
and discharge and therefore a larger number of possible trips;
third, shaft-haulage is then independent of delays in arrival of
cars at stations, while tramming can be done at any time and
shaft-haulage can be concentrated into certain hours. Cages to
carry mine cars and handle the same load as a skip must either
be big enough to take two cars, which compels a much larger shaft
than is necessary with skips, or they must be double-decked, which
renders loading arrangements underground costly to install and
expensive to work. For all these reasons, cages can be justified only
on metal mines of such small tonnage that time is no consideration
and where the saving of men is not to be effected. In compartments
of the minimum size mentioned above (four to five feet either way)
a skip with a capacity of from two to five tons can be installed,
although from two to three tons is the present rule. Lighter loads
than this involve more trips, and thus less hourly capacity, and,
on the other hand, heavier loads require more costly engines. This
matter is further discussed under "Haulage Appliances."

We have therefore as the economic minimum a shaft of three compartments
(Fig. 9), each four to five feet square. When the maximum tonnage
is wanted from such a shaft at the least operating cost, it should
be equipped with loading bins and skips.

The output capacity of shafts of this size and equipment will depend
in a major degree upon the engine employed, and in a less degree
upon the hauling depth. The reason why depth is a subsidiary factor
is that the rapidity with which a load can be drawn is not wholly a
factor of depth. The time consumed in hoisting is partially expended
in loading, in acceleration and retardation of the engine, and in
discharge of the load. These factors are constant for any depth,
and extra distance is therefore accomplished at full speed of the
engine.

Vertical shafts will, other things being equal, have greater capacity
than inclines, as winding will be much faster and length of haul less
for same depth. Since engines have, however, a great tractive ability
on inclines, by an increase in the size of skip it is usually po