s, more than in any other feature of the
equipment, must mechanical efficiency be subordinated to the other
issues.

FLEXIBILITY.--Flexibility in plant is necessary because volume and
head of water are fluctuating factors. In wet regions the volume
of water usually increases for a certain distance with the extension
of openings in depth. In dry climates it generally decreases with the
downward extension of the workings after a certain depth. Moreover,
as depth progresses, the water follows the openings more or less
and must be pumped against an ever greater head. In most cases
the volume varies with the seasons. What increase will occur, from
what horizon it must be lifted, and what the fluctuations in volume
are likely to be, are all unknown at the time of installation. If
a pumping system were to be laid out for a new mine, which would
peradventure meet every possible contingency, the capital outlay would
be enormous, and the operating efficiency would be very low during
the long period in which it would be working below its capacity. The
question of flexibility does not arise so prominently in coal-mines,
for the more or less flat deposits give a fixed factor of depth.
The flow is also more steady, and the volume can be in a measure
approximated from general experience.

RELIABILITY.--The factor of reliability was at one time of more
importance than in these days of high-class manufacture of many
different pumping systems. Practically speaking, the only insurance
from flooding in any event lies in the provision of a relief system
of some sort,--duplicate pumps, or the simplest and most usual
thing, bailing tanks. Only Cornish and compressed-air pumps will
work with any security when drowned, and electrical pumps are easily
ruined.

GENERAL POWER CONDITIONS.--The question of pumping installation
is much dependent upon the power installation and other power
requirements of the mine. For instance, where electrical power is
purchased or generated by water-power, then electrical pumps have
every advantage. Or where a large number of subsidiary motors can be
economically driven from one central steam- or gas-driven electrical
generation plant, they again have a strong call,--especially if
the amount of water to be handled is moderate. Where the water
is of limited volume and compressed-air plant a necessity for the
mine, then air-driven pumps may be the most advantageous, etc.

MECHANICAL EFFICIENCY.--The mechanical