ALBERT C. RITCHIE, GOVERNOR. 689

Non-Gaseous Mine is a mine which has not been rated as
gaseous; or a mine which, having been rated as gaseous, has
been re-rated as non-gaseous as prescribed in the definition of
a gaseous mine, elsewhere in this section.

Operator means any firm, corporation, association or indi-
vidual operating any coal mine or part thereof, whether as an
owner, lessee or otherwise, and also includes the representa-
tive of such operator.

Panel means a unit area in a system of mining by which
the mine is divided into large rectangles or panels isolated
or surrounded by solid pillars of coal into which pairs of
entries are driven for the development of rooms and the
extraction of pillars.

Permissible. Unless the context requires a different sense,
"permissible" means the admission of a device, plan, formula,
method, machine or course of action by the United States Bu-
reau of Mines to a list, after actual test thereof.

Potential and Voltage. These words shall be taken as
synonymous, to mean electric pressure as determined by the
voltmeter.

Shaft means an opening, the axis of which is approxi-
mately vertical, extending downwards from the surface; and
shall include an underground shaft driven between two
levels.

Slope includes an inclined opening in a dipping coal bed
and an inclined tunnel to a coal bed.

Superintendent means the person who shall, on behalf of
the operator, have immediate supervision of one or more mines.

Tunnel means a level or inclined passageway driven mainly
through barren strata, It may or may not come to the sur-
face.

Ventilating Current means a current of air circulating
throughout the mine excavations, of such purity that it shall
contain not less than 19 per cent, of oxygen nor more than
(1 1/4) one and one-fourth per cent, carbon dioxide. The dis-
trict mine inspector shall, whenever he has reason to suspect
an excessive proportion of carbon dioxide, or insufficient oxy-
gen in the atmosphere of any mine, take such tests as the
Bureau may prescribe, and upon disclosure of a ventilating
current below or approaching minimum standard, require the
operator by written safety order to improve the ventilation
immediately.