greater in diameter, namely, 10 ft. (Fig. 3, Plate X), in which electric
motors, electric cutting machines, and similar apparatus, are being
tested in the presence of explosive mixtures of gas and dust and with
large amperage and high voltage, such as may be used in the largest
electrical equipment in mines.

The investigation as to the ability of insulation to withstand the
effects of acid mine waters has been very difficult and complicated. At
first it was believed possible that mine waters from nearby Pennsylvania
mines and of known percentages of acidity could be procured and kept in
an immersion tank at approximately any given percentage of strength.
This was found to be impracticable, as these waters seem to undergo
rapid change the moment they are exposed to the air or are transported,
in addition to the changes wrought by evaporation in the tank. It has
been necessary, therefore, to analyze and study carefully these waters
with a view to reproducing them artificially for the purpose of these
tests. Concerning the insulation, delicate questions have arisen as to a
standard of durability which shall be commensurate with reasonable cost.
These preliminary points are being solved in conference with the
manufacturers, and it is expected that the results will soon permit of
starting the actual tests.

_Safety-Lamp Investigations._--Many so-called safety lamps are on the
market, and preliminary tests of them have been made in the lamp
gallery, in Building No. 17 (Fig. 2, Plate X). After nearly a year of
endeavor to calibrate this gallery, and to co-ordinate its results with
those produced in similar galleries in Europe, this preliminary inquiry
has been completed, and the manufacturers and agents of all safety lamps
have been invited to be present at tests of their products at the
Pittsburg laboratory.