﻿2 
  ON 
  THE 
  DIURNAL 
  VARIATION 
  

  

  the 
  experience 
  obtained 
  when 
  occasion 
  might 
  offer 
  an 
  opportunity 
  of 
  

   prosecuting 
  more 
  extended 
  researches. 
  This 
  resolution 
  has 
  given 
  

   way 
  to 
  the 
  consideration, 
  that 
  magnetic 
  phenomena 
  have 
  not 
  yet 
  been 
  

   observed 
  with 
  that 
  closeness 
  of 
  scrutiny 
  to 
  which 
  other 
  branches 
  of 
  

   experimental 
  science 
  have 
  been 
  subjected 
  ; 
  that 
  the 
  inconvenience 
  of 
  

   hourly 
  observations 
  by 
  day 
  and 
  by 
  night 
  have 
  prevented 
  many 
  from 
  

   entering 
  this 
  particular 
  field 
  ; 
  and, 
  finally, 
  that 
  my 
  observations 
  appear 
  

   to 
  warrant 
  interesting 
  deductions 
  not 
  afforded 
  by 
  the 
  printed 
  observa- 
  

   tions 
  which 
  I 
  have 
  been 
  able 
  to 
  examine. 
  

  

  It 
  would 
  be 
  out 
  of 
  place, 
  in 
  a 
  brief 
  essay 
  like 
  the 
  present, 
  to 
  attempt 
  

   a 
  sketch 
  of 
  the 
  observations, 
  either 
  systematic 
  or 
  casual, 
  made 
  upon 
  

   the 
  diurnal 
  variation 
  of 
  the 
  needle; 
  the 
  references 
  which 
  will 
  be 
  

   made 
  to 
  other 
  experiments, 
  for 
  the 
  purpose 
  of 
  a 
  comparison 
  of 
  results, 
  

   will 
  supply 
  the 
  place 
  of 
  such 
  an 
  outline. 
  In 
  making 
  such 
  comparisons, 
  

   it 
  is 
  hardly 
  necessary 
  to 
  observe, 
  that 
  I 
  disclaim 
  any 
  intention 
  of 
  claim- 
  

   ing 
  for 
  my 
  results 
  any 
  more 
  weight 
  than 
  is 
  due 
  to 
  careful 
  and 
  frequent 
  

   observation 
  during 
  the 
  time 
  for 
  which 
  they 
  were 
  obtained. 
  

  

  In 
  the 
  following 
  account, 
  I 
  purpose, 
  first, 
  to 
  give 
  a 
  description 
  of 
  the 
  

   instrument, 
  and 
  of 
  its 
  location, 
  and 
  of 
  the 
  mode 
  of 
  observing 
  ; 
  next, 
  to 
  

   present 
  the 
  observations 
  ; 
  and, 
  lastly, 
  the 
  conclusions 
  which 
  they 
  may 
  

   warrant. 
  

  

  The 
  needle 
  was 
  thirty-six 
  inches 
  long, 
  .04 
  of 
  an 
  inch 
  thick, 
  broader, 
  

   in 
  its 
  horizontal 
  section, 
  at 
  the 
  middle 
  than 
  at 
  the 
  two 
  ends 
  : 
  it 
  was 
  

   supported 
  by 
  a 
  steel 
  pivot 
  playing 
  into 
  a 
  ruby 
  cap, 
  and 
  was 
  contained 
  

   in 
  a 
  prismatic 
  box 
  of 
  mahogany, 
  covered 
  by 
  sliding 
  plates 
  of 
  glass. 
  

   The 
  weight 
  of 
  the 
  needle 
  was 
  three 
  hundred 
  and 
  fifty-five 
  grains. 
  At- 
  

   tached 
  to 
  each 
  end 
  of 
  the 
  box 
  was 
  a 
  brass 
  arc, 
  divided 
  into 
  degrees 
  

   and 
  tenths. 
  The 
  zero 
  of 
  each 
  arc, 
  and 
  the 
  pivot 
  on 
  which 
  the 
  needle 
  

   rested, 
  were 
  in 
  the 
  same 
  line 
  ; 
  but 
  the 
  point 
  in 
  the 
  ruby 
  cap, 
  upon 
  which 
  

   the 
  needle 
  rested, 
  was 
  found 
  to 
  be 
  out 
  of 
  the 
  line 
  joining 
  the 
  two 
  ends. 
  

   On 
  which 
  account, 
  as 
  well 
  as 
  to 
  render 
  the 
  observations 
  more 
  accurate, 
  

   two 
  readings 
  were 
  always 
  made 
  : 
  one 
  on 
  the 
  scale 
  near 
  the 
  north 
  pole, 
  

   the 
  other 
  on 
  that 
  near 
  the 
  south 
  pole 
  of 
  the 
  needle. 
  The 
  nature 
  of 
  the 
  

   suspension 
  of 
  this 
  needle, 
  though 
  sufficiently 
  delicate 
  for 
  the 
  purposes 
  

   immediately 
  in 
  view, 
  did 
  not 
  warrant 
  my 
  using 
  its 
  oscillations 
  to 
  

   obtain 
  the 
  intensity 
  ; 
  its 
  shape 
  and 
  length 
  rendering 
  it, 
  besides, 
  un- 
  

  

  