north—it  will  probably  come  to  rest  a  little  off the magnetic meridian. The most common cause of sluggishness is weakening of the magnetism of the  needle.  A  needle  may  be  demagnetized  by drawing it over a bar magnet. The needle should be  drawn  from  the  center  of  the  bar  magnet toward the end, with the south end of the needle drawn over the north end of the magnet and vice versa. On each return stroke, lift the needle well clear  of  the  magnet. Sometimes the cause of a sluggish needle is a blunt  point  on  the  pivot.  This  may  be  corrected by  sharpening  the  pivot  with  a  fine  file. If the compass is not level when a bearing or azimuth   is   being   read,   the   reading   will   be incorrect.  A similar error will exist if the compass is equipped with sighting vanes and one or more of  them  are  bent.  To  check  for  bent  compass vanes, you set up and level the compass, and then sight  with  the  vanes  on  a  plumb  bob  cord. The most common personal error the observer can  make  in  compass  work  is  MISREADING. This  is  caused  by  the  observer’s  eye  not  being vertically  above  the  compass  at  the  time  of  the reading.  Other  common  mistakes  are  reading  a needle  at  the  wrong  end  and  setting  off  local attraction  or  declination  in  the  wrong  direction when  the  compass  is  being  oriented. ESTABLISHING DIRECTIONS BY  TRANSIT Directions  are  similarly  determined  by  the  use of a transit. This can be done by measuring the size of the horizontal angle between the line whose direction is sought and a reference line. With a transit, however, you are expected to do this with considerably  more  accuracy  and  precision  than with  a  surveyor’s  compass.  Some  of  the  basic procedures associated with the proper operation of   the   instrument   will   be   discussed   in   the following  paragraphs. Setting Up the Transit The  point  at  which  the  line  of  sight,  the horizontal axis, and the vertical axis of a transit meet is called the INSTRUMENT CENTER. The point on the ground over which the center of the instrument   is   placed   is   the   INSTRUMENT POINT,   TRANSIT   POINT,   or   STATION.   A wooden stake or hub is usually marked with a tack when   used   as   a   transit   station   or   point.   To prevent  jarring  or  displacement  of  the  transit, avoid those stations having loose planking, those having soft or marshy ground, and those having other  conditions  that  would  cause  the  legs  of the  tripod  to  move.  The  following  steps  are recommended when you are setting up a transit over  a  station  point: 1. Center the instrument as closely as possible over the definite point by suspending a plumb line from a hook and chain beneath the instrument. The plumb string is tied with a slipknot, so that you  can  adjust  the  height  of  the  plumb. 2. Move the tripod legs as necessary until the plumb  bob  is  about  1/4  in.  short  of  being  over the tack, meanwhile maintaining a fairly level foot plate.  Spread  the  tripod  legs,  and  apply  sufficient pressure to the legs to make sure of their firmness in  the  ground.  Make  sure  to  loosen  the  wing nuts  to  rid  the  static  pressure  in  them  before retightening. 3. Turn the plates so that each plate level is parallel to a pair of opposite leveling screws. (It is  common  practice  to  have  a  pair  of  opposite leveling screws in line with the approximate line of sight.) The leveling screws are then tightened to firmness, but not tight. Rotate opposing pairs of  leveling  screws  either  toward  each  other  or away from each other until the plate bubbles are centered. If the plumb bob is not directly over the center of the tack, you may loosen two adjacent leveling screws enough to free the shifting plate. Relevel the instrument if the bubbles become off-center. During  breezy  conditions,  you  may  shield  the plumb  line  with  your  body  when  setting  up  an instrument.  Sometimes  in  windy  locations,  it  may be  necessary  to  construct  a  wind  shield. Setting  and  leveling  the  transit  rapidly  requires a skill on your part that you will learn and develop through   consistent   practice.   You   should   take advantage  of  any  opportunity  that  you  can  to train  yourself  and  increase  your  skills  in  handling surveying  instruments.  Again,  when  setting  up  or operating  a  transit,  you  should  remember  the following   points: 1.  The  plate  bubble  follows  the  direction  of the  left  thumb  when  you  are  manipulating  the leveling screws. 2.  You  should  always  check  to  see  if  the plumb bob is still over the point after leveling it. If the plumb bob has shifted, you should recenter the instrument. 3. While loosening the two adjacent leveling screws, you can shift the transit head laterally. 13-8