elevation  above  or  below  an  adopted  datum  is known.  Usually  designated  as  a  BM,  such  a mark  is  sometimes  further  qualified  as  a  PBM (permanent bench mark) or as a TBM (temporary bench  mark). BILL   OF   MATERIALS—List   of   materials needed for a given project placed directly above the  title  block;  not  normally  found  on  con- struction  drawings. BISECT—To  divide  into  two  equal  parts. BITUMEN—A   mixture   of   hydrocarbons   of natural  or  pyrogenous  origin,  or  a  combination of  both;  frequently  accompanied  by  nonmetallic derivatives,    which  may  be  gaseous,  liquid, semisolid,  or  solid,  and  which  are  completely soluble  in  carbon  disulfide. BLAST-FURNACE   SLAG—The   nonmetallic product,  consisting  essentially  of  silicates  and aluminosilicates  of  lime  and  of  other  bases,  which is developed simultaneously with iron in a blast furnace. BLAZE—A   mark   made   on   the   trunk   of   a standing tree by chipping off a spot of bark with an axe. It is used to indicate a trail, a boundary, location  for  a  road,  a  tree  to  be  cut,  and  so  on. BORDER LINES—Dark lines defining the inside edge  of  the  margin  on  a  drawing. BREAK  LINES—Lines  used  to  reduce  the graphic  size  of  an  object  generally  to  conserve paper  space.  There  are  two  types. Long—Thin ruled line with freehand zigzag. Short—Thick,  wavy  freehand  line. BROKEN  SECTION—See  partial  section. BUBBLE  AXIS  (LEVEL  VIAL)—The  horizontal line tangent to the upper surface of the centered bubble, which lies in the vertical plane through the  longitudinal  axis  of  the  bubble  tube. CABINET  PROJECTION—A  single  view  of  an object having one face in orthographic projection and  depicting  all  three  dimensions  (length,  width, and height). The projection lines are parallel and at an oblique angle with the plane of projection (generally 45°). The lengths of the receding lines are   foreshortened   to   make   the   object   appear optically   correct. CALIBRATION—The  determination  in  terms  of an adopted unit and by mechanical interpolation based on the values obtained by standardization of   the   supplementary   marks   on   a   measuring instrument or device. Also, the determination of the   values   of   the   divisions   of   a   circle   as proportional  parts  of  a  circumference. CAVALIER PROJECTION—A single view of an object having one face in orthographic projection and showing all three dimensions. The projection lines are parallel and at an oblique angle with the plane  of  projection  (generally  45°).  The  lengths of all object lines are drawn to scale and do not appear  optically  correct. CENTER LINES—Lines that indicate the center of a circle, arc, or any symmetrical object; consist of alternately long and short dashes evenly spaced. CHAIN  (Gunter’s)—A  unit  of  distance  formerly much  used  in  land  measurement  and  a  term frequently  found  in  deed  descriptions.  A  chain equals  66  ft,  4  rods,  1/80  mi. CIRCLE—A   plane   closed   figure   having   every point  on  its  circumference  (perimeter)  equidistant from  its  center. CIRCUIT  CLOSURE—In  leveling,  the  amount by  which  the  algebraic  sum  of  the  measured differences  of  elevation  around  a  circuit  fails  to equal  the  theoretical  closure,  zero. CIRCUMFERENCE—The  length  of  a  line  that forms  a  circle. CIRCUMSCRIBED   FIGURE—A   figure   that completely  encloses  another  figure. CLOCKWISE   ANGLE—A   horizontal   angle measured  from  left  to  right.  A  clockwise  angle may  have  between  0°  and  360°.  Azimuths  are clockwise  angles  measured  from  either  north  or south. CLOSED   TRAVERSE—A   traverse   that   starts and ends at the same point or at stations whose positions  have  been  determined  by  other  surveys. (See   CONNECTING   TRAVERSE   and   LOOP TRAVERSE.) AI-3