﻿X42 
  Transactions, 
  — 
  Miscellaneous, 
  

  

  Fig. 
  1 
  represents 
  the 
  field 
  of 
  view 
  of 
  a 
  positive 
  eye-piece 
  of 
  higli 
  mag- 
  

   nifying 
  power. 
  In 
  this 
  are 
  arranged, 
  in 
  the 
  manner 
  shown 
  in 
  the 
  figure, 
  

   images 
  of 
  wires 
  for 
  ordinary 
  use 
  and 
  of 
  webs 
  for 
  more 
  dehcate 
  observations. 
  

  

  Fig. 
  2 
  is 
  the 
  position 
  cu-cle. 
  This 
  is 
  made 
  of 
  very 
  stout 
  block-tin, 
  and 
  

   is 
  wired 
  at 
  the 
  back 
  to 
  prevent 
  its 
  warping. 
  Its 
  circumference 
  is 
  divided 
  

   into 
  degrees 
  (the 
  minutes 
  are 
  to 
  be 
  estimated). 
  The 
  circle 
  is 
  fastened 
  on 
  a 
  

   central 
  cap, 
  like 
  that 
  which 
  is 
  used 
  for 
  a 
  sun-shade, 
  so 
  that 
  the 
  circle 
  can 
  be 
  

   screwed 
  on 
  to 
  the 
  eye-piece 
  with 
  facility. 
  Every 
  care 
  must 
  be 
  taken 
  to 
  set 
  

   the 
  plane 
  of 
  the 
  circle 
  at 
  right 
  angles 
  to 
  the 
  axis 
  of 
  the 
  telescope. 
  

  

  Before 
  the 
  circle 
  is 
  put 
  on 
  the 
  eye-piece, 
  the 
  index 
  I, 
  fig. 
  3, 
  is 
  placed 
  

   on 
  the 
  telescope, 
  tube 
  T", 
  and 
  temporarily 
  secured 
  by 
  means 
  of 
  the 
  clamp- 
  

   and 
  screw 
  Cs. 
  Then 
  the 
  circle 
  is 
  put 
  on, 
  and 
  the 
  a^^paratus 
  will 
  be 
  in 
  

   the 
  condition 
  represented 
  in 
  fig. 
  3. 
  If 
  the 
  telescope 
  used 
  is 
  equatorially 
  

   mounted 
  and 
  properly 
  adjusted, 
  it 
  may 
  be 
  now 
  turned 
  on 
  a 
  double 
  star 
  in 
  

   any 
  part 
  of 
  the 
  heavens 
  ; 
  if 
  it 
  is 
  an 
  alt. 
  azimuth, 
  a 
  star 
  must 
  be 
  chosen 
  on 
  

   or 
  near 
  the 
  meridian, 
  the 
  nearer 
  the 
  better. 
  The 
  star, 
  or 
  rather 
  one 
  of 
  the 
  

   component 
  stars, 
  is 
  now 
  made 
  to 
  run 
  along 
  between 
  the 
  wires 
  TT, 
  fig. 
  1, 
  by 
  

   turning 
  the 
  eye-piece 
  tube 
  of 
  the 
  telescope 
  round 
  until 
  it 
  does 
  so. 
  Then 
  

   the 
  index 
  I, 
  fig. 
  3, 
  must 
  be 
  made 
  to 
  point 
  accurately 
  to 
  the 
  zero 
  of 
  the 
  

   position 
  circle, 
  and 
  be 
  firmly 
  secured 
  there 
  by 
  means 
  of 
  the 
  clamp. 
  

  

  Next 
  the 
  eye-piece 
  tube 
  is 
  turned 
  round 
  until 
  the 
  line 
  joining 
  the 
  centres 
  

   of 
  the 
  two 
  stars 
  is 
  exactly 
  parallel 
  to 
  the 
  two 
  wires. 
  Then 
  the 
  cbcle 
  

   indication 
  is 
  read 
  off, 
  and, 
  if 
  necessary, 
  180° 
  must 
  be 
  added 
  to 
  the 
  angle 
  so 
  

   obtained. 
  Then, 
  evidently, 
  the 
  angle 
  of 
  position 
  with 
  the 
  meridian 
  has 
  

   been 
  obtained. 
  Several 
  observations 
  of 
  the 
  same 
  star 
  on 
  different 
  nights 
  

   should 
  be 
  taken. 
  It 
  is 
  advantageous, 
  too, 
  to 
  use 
  different 
  parts 
  of 
  the 
  cu'cle 
  

   as 
  the 
  zero 
  point. 
  If 
  this 
  be 
  done, 
  the 
  mean 
  of 
  all 
  the 
  observations 
  will 
  be 
  

   a 
  very 
  close 
  approximation 
  to 
  the 
  truth. 
  

  

  Having 
  found 
  the 
  angle 
  of 
  position, 
  we 
  next 
  proceed 
  to 
  obtain 
  the 
  

   distance. 
  This 
  operation 
  should 
  be 
  attempted 
  only 
  in 
  the 
  very 
  finest 
  

   weather. 
  The 
  writer 
  always 
  measures 
  distances 
  either 
  in 
  morning 
  or 
  

   evening 
  twilight, 
  or 
  in 
  full 
  moonlight 
  when 
  the 
  moon 
  is 
  near 
  the 
  meridian. 
  

   Thus 
  the 
  illumination 
  difficulty 
  is 
  avoided. 
  

  

  The 
  clepsydra, 
  the 
  use 
  and 
  construction 
  of 
  which 
  will 
  easily 
  be 
  under- 
  

   stood 
  from 
  the 
  section 
  of 
  it 
  given 
  in 
  fig. 
  4, 
  is 
  placed 
  in 
  a 
  convenient 
  position 
  

   near 
  the 
  telescope. 
  The 
  tanks 
  T 
  and 
  T" 
  are 
  filled 
  with 
  water, 
  the 
  eyepiece 
  

   tube 
  is 
  turned 
  i^oUnd 
  as 
  in 
  the 
  previous 
  operation, 
  until 
  one 
  of 
  the 
  com- 
  

   ponents 
  of 
  the 
  double 
  star 
  runs 
  along 
  the 
  mre 
  TT.or 
  the 
  web 
  w.w. 
  Then 
  

   the 
  star 
  is 
  recalled 
  and 
  raised 
  in 
  the 
  field 
  a 
  litttle, 
  so 
  that 
  it 
  may 
  transit 
  the 
  

   obliqixe 
  wii-e 
  TW, 
  or 
  the 
  obhque 
  web 
  w.T. 
  The 
  instant 
  that 
  the 
  first 
  star 
  

   is 
  bisected 
  by 
  the 
  wire 
  or 
  web, 
  the 
  lever 
  is 
  pressed 
  sharply 
  down 
  to 
  the 
  peg 
  P 
  

  

  