﻿OF 
  TERRESTRIAL 
  MAGNETISM 
  AT 
  SEVERAL 
  PLACES, 
  ETC. 
  429 
  

  

  that 
  the 
  magnetism 
  of 
  the 
  earth 
  really 
  undergoes 
  frequent 
  and 
  consi- 
  

   derable 
  changes. 
  This 
  latter 
  point 
  seems 
  to 
  be 
  generally 
  conceded, 
  

   but 
  we 
  do 
  not 
  think 
  upon 
  sufficient 
  grounds. 
  

  

  Mr 
  Harris,* 
  of 
  Plymouth, 
  had 
  already 
  pointed 
  out 
  the 
  effect 
  of 
  cur- 
  

   rents 
  of 
  air 
  within 
  the 
  apparatus 
  in 
  which 
  the 
  needle 
  is 
  inclosed, 
  as 
  

   rendering 
  this 
  method 
  of 
  experiment 
  objectionable. 
  He 
  had 
  proposed 
  

   to 
  remove 
  this 
  objection 
  by 
  oscillating 
  the 
  needles 
  in 
  a 
  rarefied 
  medium, 
  

   and 
  had 
  devised 
  an 
  apparatus 
  for 
  this 
  purpose. 
  Mr 
  R. 
  W. 
  Fox 
  had 
  

   noticed 
  the 
  same 
  defectf 
  It 
  first 
  occurred 
  to 
  us 
  in 
  full 
  force 
  when 
  

   observing 
  for 
  the 
  correction 
  for 
  temperature. 
  This 
  source 
  of 
  error 
  

   was 
  particularly 
  active 
  in 
  the 
  case 
  of 
  the 
  needle 
  of 
  the 
  smallest 
  mass, 
  

   oscillating 
  rapidly, 
  and, 
  as 
  is 
  usual, 
  in 
  large 
  arcs, 
  at 
  the 
  commencement 
  

   of 
  the 
  motion. 
  To 
  get 
  rid 
  of 
  this 
  cause 
  of 
  error, 
  we 
  resorted 
  to 
  the 
  

   method 
  proposed 
  by 
  Mr 
  Harris, 
  and 
  had 
  constructed 
  a 
  stationary, 
  

   and 
  also 
  a 
  portable 
  apparatus 
  for 
  vibrating 
  in 
  a 
  rarefied 
  medium. 
  The 
  

   stationary 
  apparatus 
  was 
  intended 
  to 
  investigate 
  the 
  supposed 
  changes 
  

   of 
  the 
  horizontal 
  intensity 
  from 
  day 
  to 
  day, 
  and 
  the 
  portable 
  apparatus 
  

   to 
  repeat 
  the 
  observations 
  made 
  in 
  the 
  first 
  series 
  at 
  several 
  different 
  

   places. 
  

  

  Our 
  expectations 
  of 
  the 
  superior 
  accuracy 
  of 
  observations 
  in 
  a 
  rarefied 
  

   medium, 
  were 
  not 
  disappointed. 
  We 
  had 
  laid 
  aside 
  our 
  first 
  series 
  of 
  

   results 
  as 
  unsatisfactory, 
  and 
  now 
  propose 
  to 
  use 
  them 
  only 
  when 
  the 
  

   observations 
  were 
  very 
  numerous. 
  A 
  comparison 
  of 
  their 
  mean 
  error 
  

   with 
  that 
  of 
  the 
  series 
  in 
  the 
  rarefied 
  medium, 
  will 
  serve 
  to 
  show, 
  that 
  

   they 
  have 
  little 
  weight 
  when 
  the 
  number 
  is 
  not 
  very 
  much 
  multiplied, 
  

   in 
  comparison 
  with 
  the 
  latter. 
  

  

  It 
  was 
  stated 
  that 
  arrangements 
  had 
  been 
  made 
  by 
  Mr 
  Harris 
  for 
  

   determining 
  the 
  relative 
  intensities 
  at 
  certain 
  places 
  in 
  England, 
  by 
  a 
  

   needle 
  oscillating 
  in 
  rarefied 
  air; 
  but 
  we 
  believe 
  that 
  our 
  results 
  are 
  

   the 
  first 
  of 
  this 
  kind 
  which 
  have 
  been 
  offered 
  to 
  the 
  public. 
  The 
  

   method 
  of 
  vibrations 
  in 
  air 
  continues 
  to 
  be 
  used, 
  and, 
  if 
  our 
  views 
  of 
  

   its 
  imperfections 
  are 
  correct, 
  the 
  importance 
  of 
  our 
  results 
  is 
  consi- 
  

  

  * 
  W. 
  Snow 
  Harris, 
  in 
  the 
  Trans, 
  of 
  the 
  Royal 
  Society 
  of 
  London, 
  1831, 
  pp. 
  67, 
  68, 
  &c. 
  

   t 
  R. 
  W. 
  Fox, 
  in 
  the 
  London 
  and 
  Ed. 
  Philos. 
  Mag., 
  Vol. 
  I., 
  p. 
  310, 
  &c. 
  

  

  