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Abutment- In coal mining, (1) the weight of the rocks above a narrow roadway is transferred to the solid coal along the sides, which act as abutments of the arch of strata spanning the roadway; and (2) the weight of the rocks over a longwall face is transferred to the front abutment, that is, the solid coal ahead of the face and the back abutment, that is, the settled packs behind the face.

Acid deposition or acid rain�C Refers loosely to a mixture of wet and dry "deposition" (deposited material) from the atmosphere containing higher than "normal" amount of nitric and sulfuric acids. The precursors or chemical forerunners of acid rain formation result from both natural sources, such as volcanoes and decaying vegetation, and man-made sources, primarily emissions of sulfur and nitrogen oxides resulting from fossil fuel combustion.

Acid mine water- Mine water that contains free sulfuric acid, mainly due to the weathering of iron pyrites.

Active workings- Any place in a mine where miners are normally required to work or travel and which are ventilated and inspected regularly.

Adit- A nearly horizontal passage from the surface by which a mine is entered and dewatered. A blind horizontal opening into a mountain, with only one entrance.

Advance- Mining in the same direction, or order of sequence; first mining as distinguished from retreat.

Air split- The division of a current of air into two or more parts.

Airway- Any passage through which air is carried. Also known as an air course.

Anemometer- Instrument for measuring air velocity.

Angle of dip- The angle at which strata or mineral deposits are inclined to the horizontal plane.

Angle of draw- In coal mine subsidence, this angle is assumed to bisect the angle between the vertical and the angle of repose of the material and is 20�� for flat seams. For dipping seams, the angle of break increases, being 35.8�� from the vertical for a 40�� dip. The main break occurs over the seam at an angle from the vertical equal to half the dip.

Angle of repose- The maximum angle from horizontal at which a given material will rest on a given surface without sliding or rolling.

Anticline- An upward fold or arch of rock strata.

Aquifer- A water-bearing bed of porous rock, often sandstone.

Arching- Fracture processes around a mine opening, leading to stabilization by an arching effect.

Area (of an airway)- Average width multiplied by average height of airway, expressed in square feet.

Auger- A rotary drill that uses a screw device to penetrate, break, and then transport the drilled material (coal).

Auxiliary operations- All activities supportive of but not contributing directly to mining.

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