Crystals in which one or more parts, regularly arranged, are in reverse
position with reference to the other part or parts. They often appear
externally to consist of two or more crystals symmetrically united, and
sometimes have the form of a cross or star. They also exhibit the
composition in the reversed arrangement of part of the faces, in the
striae of the surface, and in re-entering angles; in certain cases, the
compound structure can only be surely detected by an examination in
polarized light. Fay
A pair of parallel entries, one of which is an intake air course and the
other a return air course. Rooms can be worked from both entries. Often
called double entry. Fay
The laminae or thin plates in repeated or polysynthetic twins.
CF:polysynthetic twinning; repeated twinning.
A statement or statements of the symmetrical relationships between the
members of a twin crystal, e.g., the twin plane or the twin axis that
resolves one crystal individual into congruity with another in a twin
crystal. The twin law cannot be an element of symmetry of the point group
of the twin parts, although it may be an element in a point group with
higher symmetry in the same crystal system.
See:twin axis
The special and characteristic method according to which twin crystals of
any mineral are formed. Fay
In a twin crystal, a plane normal to the twinning axis. Fay
A type of instrument used to detect and mark twinning and determine the
sense of orientation in etched sections. Am. Mineral., 2
An instrument employing a directed beam of light used to examine etched
wafers for twinning. Am. Mineral., 2
A packer designed so that a borehole can be sealed simultaneously at two
separated points. Long
In a twin crystal, a plane through which one twin individual forms a
mirror image of the other, or the plane at right angles to the axis about
which one individual is rotated with respect to the other. The twin plane
is commonly the composition plane across which the individuals are joined.
With rare exceptions, a twin plane is a possible crystal face for the
crystal individuals.
An L-shaped conveyor in which the carrying surface and guard gradually
exchange their functional duties.
A drill made by twisting a length of steel of rectangular or oval section
into a spiral form, hence the term twist drill. Many hand-operated coal
drills are of this type, and the rotation of the drill spiral removes the
cuttings from the hole. See also:auger; coal auger. Nelson
Splice made by holding the bared wires side by side. Half of their length
is bent back to form a loop at the end. The loop is then twisted around
the main shank of wire. Carson, 1
In the asbestos products industry, one who twists together two or more
strands of wire and asbestos yarn for use in weaving asbestos products,
such as brake linings. DOT
A force, such as the force on the shaft of a rotating motor.
Morris
The breaking off of a member of the drill string, caused by excessive
torsional stress. Long
A device permitting rotation of a small crystal about two orthogonal axes
for optical observation. CF:goniometer
Consist of two or more unmixed, commercially manufactured, prepackaged
chemicals, including oxidizing chemicals, flammable liquids, or solids
that are not independently classified as explosives. When combined,
however, the mixture is classified as an explosive and is stored,
transported, and handled as an explosive. Cote
See:roller rock bit
An arrangement, using two auxiliary fans, for ventilating a mine tunnel or
hard heading. It consists of an exhausting fan with rigid ducting to
within about 100 ft (30 m) of the face, and a forcing fan using a flexible
duct discharging air about 20 ft (6 m) from the face. The ducts of the two
units overlap by at least 30 ft (9 m) to minimize the recirculation of
air. The air delivered by the forcing fan does not exceed about one-third
of that removed by the exhaust fan.
See also:overlap auxiliary ventilation; auxiliary ventilation;
recirculation of air; reversible auxiliary ventilation. Nelson