﻿314 
  Geological 
  Survey 
  of 
  Louisiana 
  [Sect. 
  

  

  1820, 
  he 
  must 
  run 
  on 
  one 
  variation, 
  and 
  to 
  follow 
  another 
  in 
  the 
  

   same 
  region 
  but 
  established 
  bj^ 
  a 
  different 
  surveyor 
  with 
  a 
  dif- 
  

   ferent 
  compass 
  at 
  a 
  different 
  time 
  he 
  must 
  run 
  on 
  a 
  different 
  var- 
  

   iation. 
  In 
  other 
  words, 
  putting 
  aside 
  occasional 
  outright 
  poor 
  

   work, 
  the 
  lack 
  of 
  a 
  definite 
  knowledge 
  of 
  magnetic 
  declination 
  

   and 
  its 
  secular 
  v-ariation 
  on 
  the 
  part 
  of 
  the 
  previous 
  surveyors 
  

   has 
  often 
  been 
  the 
  source 
  of 
  a 
  vast 
  amount 
  of 
  trouble, 
  mental 
  and 
  

   financial. 
  Unfortunately, 
  even 
  to-day, 
  the 
  surveyor 
  with 
  an 
  

   ordinar}^ 
  compass 
  has 
  no 
  ready 
  means 
  of 
  determining 
  the 
  amount 
  

   of 
  this 
  variation 
  ; 
  for 
  to 
  determine 
  the 
  number 
  of 
  degrees 
  and 
  

   minutes 
  that 
  the 
  axis 
  of 
  his 
  needle 
  makes 
  with 
  a 
  true 
  merid- 
  

   ian 
  line 
  means 
  that 
  the 
  latter 
  is 
  already 
  established 
  or 
  known. 
  

   But 
  such 
  a 
  known 
  line 
  does 
  not 
  exist 
  in 
  his 
  parish, 
  nor 
  has 
  he 
  

   the 
  means 
  of 
  establishing 
  one. 
  

  

  Need 
  of 
  meridian 
  lines. 
  — 
  The 
  above 
  statements 
  are 
  sufficient 
  to 
  

   show 
  that 
  if 
  compass 
  surveying 
  be 
  continued, 
  it 
  is 
  quite 
  time 
  

   that 
  some 
  systematic 
  records 
  be 
  kept 
  of 
  what 
  each 
  surveyor 
  

   means 
  by 
  "due 
  north 
  " 
  or 
  any 
  other 
  direction 
  he 
  gives, 
  basing 
  

   his 
  statement 
  on 
  the 
  direction 
  assumed 
  by 
  his 
  undamped 
  com- 
  

   pass 
  needle. 
  

  

  A 
  well 
  kept 
  needle 
  will 
  usually 
  settle 
  twice 
  within 
  5' 
  of 
  the 
  

   same 
  place. 
  With 
  proper 
  care, 
  it 
  can 
  be 
  read 
  to 
  5' 
  of 
  arc. 
  

   Poor 
  needles 
  may 
  mislead 
  to 
  the 
  extent 
  of 
  30' 
  or 
  more. 
  . 
  A 
  line 
  

   then 
  that 
  is 
  within 
  i' 
  of 
  the 
  true 
  meridian 
  of 
  the 
  place 
  is 
  

   sufficiently 
  accurate 
  for 
  all 
  compass 
  work. 
  Any 
  higher 
  degree 
  

   of 
  refinement, 
  however 
  desirable 
  and 
  satisfactory 
  on 
  general 
  

   principles, 
  is 
  wholly 
  unappreciated 
  in 
  compass 
  surveying. 
  

  

  How 
  established. 
  — 
  But 
  as 
  before 
  stated, 
  such 
  a 
  line 
  can 
  be 
  

   established 
  by 
  day 
  whenever 
  the 
  sun 
  is 
  shining 
  ; 
  or 
  by 
  night 
  

   whenever 
  the 
  circumpolar 
  stars 
  are 
  visible, 
  to 
  a 
  still 
  greater 
  

   degree 
  of 
  accuracy 
  by 
  a 
  rather 
  unpretentious 
  engineer's 
  transit. 
  

  

  Instructions 
  for 
  this 
  operation 
  have 
  been 
  so 
  frequently 
  pub- 
  

   lished 
  in 
  works 
  and 
  reports 
  relating 
  to 
  surveying 
  and 
  surveys 
  

   that 
  it 
  seems 
  quite 
  unnecessary 
  to 
  repeat 
  them 
  here. 
  However 
  

   we 
  cannot 
  refrain 
  from 
  suggesting 
  that 
  in 
  place 
  of 
  the 
  usual 
  

   paragraph 
  of 
  instruction 
  in 
  night 
  work 
  which 
  reads 
  somewhat 
  

   as 
  follow 
  : 
  "Let 
  an 
  assistant 
  place 
  a 
  light 
  corresponding 
  in 
  

   brightness 
  to 
  that 
  of 
  the 
  stars 
  at 
  a 
  point 
  seemingly 
  exactly 
  

  

  