﻿Ill] 
  

  

  Special 
  Report 
  No. 
  7 
  : 
  Meridian 
  Lines 
  

  

  315 
  

  

  beneath 
  Polaris 
  at 
  its 
  elongation, 
  or 
  let 
  him 
  hold 
  the 
  fine 
  point 
  

   ot 
  an 
  illuminated 
  pencil 
  on 
  a 
  wooden 
  plug 
  or 
  board 
  fixed 
  to 
  the 
  

   ground;" 
  we 
  would 
  suggest 
  placing 
  an 
  illuminating 
  scale 
  (see 
  

   Fig. 
  7) 
  at 
  600 
  to 
  1000 
  feet 
  from 
  the 
  transit 
  some 
  little 
  time 
  

   before 
  the 
  elongation 
  and 
  then 
  watch 
  the 
  star 
  move 
  in 
  azimuth 
  

   from 
  I, 
  2, 
  to 
  say 
  8 
  and 
  return. 
  

  

  A 
  fairly 
  good 
  scale 
  can 
  be 
  improvised 
  as 
  follows 
  : 
  Take 
  a 
  new 
  

   piece 
  of 
  tin 
  about 
  6x14 
  inches 
  and 
  cut 
  slits 
  at 
  intervals 
  of 
  from 
  

   .05 
  to 
  .10 
  ft. 
  according 
  to 
  the 
  power 
  of 
  the 
  telescope 
  and 
  the 
  

   distance 
  available. 
  

  

  Fio^. 
  7. 
  — 
  Illuminated 
  scale 
  for 
  night 
  zvork. 
  

  

  Fractional 
  parts 
  of 
  interspaces 
  can 
  be 
  estimated 
  to 
  the 
  nearest 
  

   y^ 
  z 
  . 
  (?., 
  o.oi 
  ft., 
  if 
  the 
  instrument 
  has 
  no 
  micrometer 
  attach- 
  

   ment. 
  

  

  When 
  the 
  scale 
  is 
  placed 
  at 
  such 
  a 
  distance 
  as 
  to 
  cause 
  one 
  

   space 
  to 
  represent 
  about 
  20", 
  it 
  follows 
  that 
  readings 
  are 
  made 
  

   to 
  about 
  2" 
  of 
  arc. 
  If 
  when 
  the 
  various 
  readings 
  have 
  all 
  been 
  

   reduced 
  to 
  the 
  elongation 
  and 
  there 
  is 
  no 
  serious 
  discordance 
  

   shown, 
  it 
  is 
  safe 
  to 
  conclude 
  that 
  the 
  average 
  is 
  a 
  close 
  approxi- 
  

   tion 
  to 
  the 
  true 
  elongation. 
  

  

  Other 
  circumpolar 
  stars 
  giving 
  an 
  opposite 
  elongation 
  the 
  

   same 
  night 
  can 
  also 
  be 
  observed 
  with 
  the 
  scale 
  reversed. 
  The 
  

   true 
  elongation 
  of 
  each 
  can 
  be 
  computed 
  from 
  the 
  several 
  obser- 
  

   vations 
  on 
  the 
  same, 
  and 
  all 
  can 
  be 
  used 
  in 
  the 
  final 
  location 
  of 
  

   the 
  meridian 
  line. 
  

  

  The 
  position 
  of 
  the 
  scale 
  on 
  the 
  subjacent 
  board 
  should 
  be 
  

   accurately 
  marked, 
  and 
  in 
  the 
  morning 
  a 
  fine, 
  straight 
  steel 
  pin 
  

  

  