es
used in mining. Taking the steam consumption of a good surface
steam plant at 15 pounds per horse-power hour, the efficiency of
rotary pumps with well-insulated pipes is probably not over 50%,
and of direct-acting pumps from 40% down to 10%.

The advantage of all steam-pumps lies in the low capital outlay,--hence
their convenient application to experimental mining and temporary
pumping requirements. For final equipment they afford a great deal
of flexibility, for if properly constructed they can be, with slight
alteration, moved from one horizon to another without loss of relative
efficiency. Thus the system can be rearranged for an increased
volume of water, by decreasing the lift and increasing the number
of pumps from different horizons.

COMPRESSED-AIR PUMPS.--Compressed-air transmission has an application
similar to direct steam, but it is of still lower mechanical efficiency,
because of the great loss in compression. It has the superiority
of not heating the workings, and there is no difficulty as to the
disposal of the exhaust, as with steam. Moreover, such pumps will
work when drowned. Compressed air has a distinct place for minor
pumping units, especially those removed from the shaft, for they
can be run as an adjunct to the air-drill system of the mine, and
by this arrangement much capital outlay may be saved. The cost of
the extra power consumed by such an arrangement is less than the
average cost of compressed-air power, because many of the compressor
charges have to be paid anyway. When compressed air is water-generated,
they have a field for permanent installations. The efficiency of
even rotary air-driven pumps, based on power delivered into a good
compressor, is probably not over 25%.

ELECTRICAL PUMPS.--Electrical pumps have somewhat less flexibility
than steam- or air-driven apparatus, in that the speed of the pumps can
be varied only within small limits. They have the same great advantage
in the easy reorganization of the system to altered conditions of
water-flow. Electricity, when steam-generated, has the handicap
of the losses of two conversions, the actual pump efficiency being
about 60% in well-constructed plants; the efficiency is therefore
greater than direct steam or compressed air. Where the mine is
operated with water-power, purchased electric current, or where
there is an installation of electrical generating plant by steam or
gas for other purposes, electrically driven pumps take pr