3. Inexact setting or reading of a vernier. The use of a small, powerful pocket magnifying glass is helpful here. Also, when you have determined the  vernier  graduation  that  most  nearly  coincides with a limb graduation, it is a good idea to check your  selection  by  examining  the  graduations  on either side of the one selected. These should fall in coincidence with the limb counterparts by about the same amount. 4. Failure to line up the vertical cross hair with the  true  vertical  axis  of  the  object  sighted.  The effect  is  similar  to  that  of  not  plumbing  exactly over  the  station,  which  means  that  the  error increases  drastically  as  the  length  of  the  sight decreases. 5. Failure to bring the image of the cross hair or  that  of  the  object  sighted  into  clear  focus (parallax).  A  fuzzy  outline  makes  exact  alignment difficult. Common  mistakes  in  transit  work  are  the following: 1.   Turning   the   wrong   tangent   screw.   For example,  by  turning  the  lower  tangent  screw AFTER taking a backsight, you will introduce an error  in  the  backsight  reading. 2.  Forgetting  to  tighten  the  clamp(s),  or  a clamp  slipping  when  it  is  supposed  to  be  tight. 3.  Reading  in  the  wrong  direction  from  the index  (zero  mark)  on  a  double  vernier. 4.  Reading  the  wrong  vernier;  for  example, reading  the  vernier  opposite  the  one  that  was  set. 5. Reading angles in the wrong direction; that is,  reading  from  the  outer  row  rather  than  the inner row, or vice versa, on the horizontal scale. 6. Failure to take a full-scale reading before reading the vernier. For example, you may drop 20  to  30  min  from  the  reading,  erroneously recording only the number of minutes indicated on the vernier, such as 15°18´ instead of 15°48´. Do not be so intent on reading the vernier that you  lose  track  of  the  full-scale  reading  of  the circle. CARING  FOR  AND  MAINTAINING SURVEYING  INSTRUMENTS The accuracy and quality of a survey depend upon  the  condition  of  the  surveying  instrument and   the   experience   of   the   surveyor.   The   life expectancy  and  usefulness  of  an  instrument can   be   extended   considerably   by   proper   and careful  handling,  stowing,  and  maintenance. Undoubtedly,  by  simply  working  in  your  rating conscientiously,  you  will  become  experienced  in the  proper  use  of  the  instrument. As stated earlier, every instrument is accom- panied by an instruction manual that tells you not only the proper operation and components of the instrument  but  also  procedures  for  its  proper  care and maintenance. Study this instruction manual thoroughly  before  you  even  attempt  to  use  the instrument. Carrying and Stowing Every  transit,  theodolite,  or  level  comes equipped with a carrying box or case. The instru- ment and its accessories can be stowed in the case in a manner that ensures a minimum of motion during   transportation.   The   instrument   should ALWAYS  be  stowed  in  the  carrying  case  when it is not in use. Cleaning and Lubricating In general, all surveying instruments, equip- ment,   or   tools   must   be   cleaned   thoroughly immediately  after  you  have  used  them.  For example,  you  dust  off  the  transit  or  theodolite  and wipe it dry before placing it back in its case after each use. Remove all dust with a clean cloth. This applies particularly to the optical parts. Chamois leather  is  suitable  for  this  purpose,  but  it  is better to use a clean handkerchief than a soiled chamois  leather.  Use  no  liquid  for  cleaning  — neither  water,  petrol,  nor  oil.  If  necessary,  you can breathe on the lenses before polishing them. When  the  instrument  becomes  wet,  you  should remove  its  case  and  dry  it  thoroughly  at  room temperature as soon as you get home. If you leave the instrument in the closed case, the air inside the  hood  will  take  up  humidity  by  increasing temperature  and  will  in  time  diffuse  inside  the instrument.   While   cooling   off,   the   water   will condense and form a coating or tarnish that may make  any  sighting  with  the  telescope  and  reading of  the  circles  difficult. Remove any mud or dirt that may adhere to the  tripod,  range  pole,  level  rod,  and  so  forth, after each use. Clean each instrument, equipment, or tool after each use to eliminate the chance of forgetting it. This is important, especially when the surveying gear is made of a material that is susceptible  to  rust  action  or  decay. When lubricating the instruments, you must use the recommended lubricant for each part in conjunction  with  the  climatic  condition  in  your area.  For  instance,  it  is  recommended  that 13-26