﻿OF 
  THE 
  HORIZONTAL 
  NEEDLE. 
  3 
  

  

  suitable 
  to 
  such 
  observations, 
  and 
  inducing 
  the 
  fear 
  that 
  it 
  might 
  be 
  

   liable 
  to 
  changes 
  of 
  magnetism, 
  which 
  would 
  have 
  been 
  fatal 
  to 
  such 
  

   results. 
  

  

  The 
  relative 
  amount 
  of 
  variation 
  being 
  the 
  object 
  sought, 
  it 
  was 
  only 
  

   necessary 
  that. 
  the 
  needle 
  box 
  should 
  remain 
  in 
  a 
  fixed 
  position 
  during 
  

   the 
  observations. 
  Wishing, 
  however, 
  further 
  to 
  determine 
  the 
  mean 
  

   variation 
  of 
  the 
  place 
  of 
  observation, 
  I 
  employed, 
  for 
  tracing 
  a 
  meridian 
  

   line, 
  the 
  best 
  means 
  which 
  were 
  at 
  hand 
  ; 
  namely, 
  equal 
  altitudes 
  of 
  

   the 
  sun 
  before 
  and 
  after 
  noon, 
  as 
  shown 
  by 
  the 
  passage 
  of 
  the 
  bright 
  

   image 
  of 
  a 
  circular 
  opening 
  in 
  a 
  metallic 
  plate 
  attached 
  to 
  a 
  style, 
  

   over 
  a 
  horizontal 
  circle, 
  the 
  centre 
  of 
  which 
  corresponded 
  with 
  the 
  ver- 
  

   tical 
  passing 
  through 
  the 
  middle 
  of 
  the 
  opening. 
  This 
  method 
  is 
  well 
  

   known, 
  and 
  needs 
  no 
  particular 
  description 
  here 
  ; 
  the 
  platform 
  used 
  

   was 
  of 
  wood, 
  the 
  top 
  planed 
  and 
  levelled 
  by 
  a 
  spirit 
  level. 
  Only 
  a 
  

   portion 
  of 
  the 
  form 
  was 
  truly 
  level, 
  and 
  this 
  portion 
  was 
  used 
  in 
  

   observing 
  the 
  altitudes. 
  Concentric 
  circles 
  were 
  drawn 
  upon 
  the 
  

   platform, 
  their 
  centre 
  being 
  in 
  the 
  vertical 
  passing 
  through 
  the 
  centre 
  

   of 
  a 
  round 
  hole 
  in 
  a 
  copper 
  plate 
  attached 
  to 
  a 
  style 
  ; 
  the 
  style 
  was 
  

   fastened 
  to 
  the 
  middle 
  of 
  the 
  south 
  side 
  of 
  the 
  platform. 
  The 
  shadow 
  

   of 
  the 
  plate 
  first 
  fell 
  upon 
  the 
  edge 
  of 
  the 
  platform 
  at 
  10^ 
  A.M., 
  and 
  

   left 
  it 
  at 
  2 
  P.M. 
  ; 
  nine 
  observations 
  were 
  made 
  of 
  the 
  passage 
  of 
  the 
  

   centre 
  of 
  the 
  image 
  of 
  the 
  aperture 
  over 
  different 
  circles, 
  from 
  which 
  

   the 
  meridian 
  line 
  was 
  determined. 
  The 
  points 
  given 
  by 
  seven 
  ob- 
  

   servations 
  were 
  in 
  the 
  line 
  ; 
  and 
  of 
  those 
  given 
  by 
  two 
  others, 
  the 
  far- 
  

   thest 
  departed 
  but 
  one 
  minute 
  and 
  a 
  half 
  from 
  the 
  same 
  line. 
  The 
  

   limit 
  of 
  the 
  error 
  by 
  this 
  method 
  is 
  small, 
  though 
  much 
  beyond 
  that 
  

   which 
  other 
  methods 
  would 
  have 
  furnished. 
  I 
  repeat, 
  however, 
  that 
  

   this 
  error 
  affects 
  only 
  the 
  mean 
  variation, 
  and 
  not 
  the 
  horary 
  varia- 
  

   tions. 
  

  

  This 
  platform, 
  upon 
  which 
  the 
  needle 
  rested, 
  was 
  supported 
  upon 
  

   three 
  posts, 
  about 
  six 
  inches 
  high, 
  and 
  firmly 
  planted 
  ; 
  to 
  these 
  it 
  was 
  

   attached 
  by 
  wooden 
  pins 
  : 
  the 
  same 
  kind 
  of 
  pins 
  had 
  been 
  used 
  in 
  the 
  

   construction 
  of 
  the 
  platform 
  itself. 
  The 
  location 
  of 
  the 
  platform 
  was 
  

   in 
  a 
  garden, 
  more 
  than 
  forty 
  feet 
  from 
  the 
  house, 
  and 
  fifteen 
  from 
  a 
  

   small 
  paling 
  which 
  formed 
  the 
  inclosure. 
  The 
  garden 
  was 
  upon 
  the 
  

   side 
  of 
  a 
  hill, 
  the 
  ground 
  sloping 
  towards 
  a 
  meadow 
  : 
  a 
  hill 
  enclosed 
  

  

  