. It has deficiencies in reliability and greater
complication of machinery than direct air.

AIR-COMPRESSION.--The method of air-compression so long accomplished
only by power-driven pistons has now an alternative in some situations
by the use of falling water. This latter system is a development
of the last twelve years, and, due to the low initial outlay and
extremely low operating costs, bids fair in those regions where
water head is available not only to displace the machine compressor,
but also to extend the application of compressed air to mine motors
generally, and to stay in some environments the encroachment of
electricity into the compressed-air field. Installations of this
sort in the West Kootenay, B.C., and at the Victoria copper mine,
Michigan, are giving results worthy of careful attention.

Mechanical air-compressors are steam-, water-, electrical-, and
gas-driven, the alternative obviously depending on the source and
cost of power. Electrical- and gas- and water-driven compressors
work under the disadvantage of constant speed motors and respond
little to the variation in load, a partial remedy for which lies
in enlarged air-storage capacity. Inasmuch as compressed air, so
far as our knowledge goes at present, must be provided for drills,
it forms a convenient transmission of power to various motors
underground, such as small pumps, winches, or locomotives. As stated
in discussing those machines, it is not primarily a transmission
of even moderate mechanical efficiency for such purposes; but as
against the installation and operation of independent transmission,
such as steam or electricity, the economic advantage often compensates
the technical losses. Where such motors are fixed, as in pumps
and winches, a considerable gain in efficiency can be obtained by
reheating.

It is not proposed to enter a discussion of mechanical details of
air-compression, more than to call attention to the most common
delinquency in the installation of such plants. This deficiency
lies in insufficient compression capacity for the needs of the
mine and consequent effective operation of drills, for with under
75 pounds pressure the drills decrease remarkably in rapidity of
stroke and force of the blow. The consequent decrease in actual
accomplishment is far beyond the ratio that might be expected on
the basis of mere difference of pressure. Another form of the same
chronic ill lies in insufficient air-storage capacity to pr