﻿OF 
  TERRESTRIAL 
  MAGNETISM 
  AT 
  SEVERAL 
  PLACES, 
  ETC. 
  433 
  

  

  a 
  theoretical 
  point 
  of 
  view, 
  as 
  the 
  one 
  just 
  referred 
  to. 
  This 
  is, 
  sim- 
  

   ply, 
  to 
  note 
  the 
  time 
  of 
  beginning 
  and 
  ending 
  of 
  a 
  considerable 
  num- 
  

   ber 
  of 
  vibrations, 
  with 
  the 
  arcs 
  of 
  vibration 
  at 
  the 
  commencement 
  

   and 
  end. 
  

  

  In 
  practice, 
  the 
  needle 
  having 
  been 
  made 
  to 
  oscillate, 
  and 
  having 
  

   arrived 
  at 
  the 
  arc 
  previously 
  fixed 
  upon 
  for 
  beginning 
  the 
  observations, 
  

   the 
  time 
  of 
  passing 
  the 
  meridian 
  is 
  noted. 
  The 
  oscillations 
  continu- 
  

   ing, 
  when 
  the 
  arcs 
  have 
  decreased 
  to 
  the 
  point 
  intended 
  for 
  terminating 
  

   the 
  experiment, 
  the 
  time 
  of 
  passage 
  is 
  again 
  observed. 
  The 
  interval 
  

   is 
  the 
  time 
  of 
  making 
  the 
  observed 
  number 
  of 
  oscillations. 
  If 
  it 
  were 
  

   necessary 
  to 
  count 
  this 
  number 
  without 
  any 
  checks 
  upon 
  the 
  count- 
  

   ing, 
  the 
  method 
  would 
  be 
  tedious 
  and 
  liable 
  to 
  mistakes, 
  but 
  this 
  is 
  

   not 
  the 
  case. 
  If 
  the 
  time 
  of 
  a 
  given 
  number, 
  for 
  example, 
  of 
  ten 
  os- 
  

   cillations 
  has 
  been 
  found 
  approximately, 
  and 
  the 
  time 
  of 
  passage 
  of 
  the 
  

   needle 
  over 
  the 
  meridian 
  be 
  always 
  observed 
  when 
  the 
  same 
  end 
  is 
  

   moving 
  in 
  the 
  same 
  direction, 
  there 
  can 
  be 
  no 
  doubt 
  of 
  the 
  number 
  of 
  

   vibrations 
  corresponding 
  to 
  an 
  observed 
  interval, 
  until 
  the 
  difference 
  

   between 
  the 
  quotient 
  of 
  the 
  observed 
  interval 
  by 
  a 
  number 
  greater 
  or 
  

   less 
  by 
  two 
  than 
  the 
  true 
  number 
  of 
  oscillations, 
  is 
  less, 
  than 
  the 
  limit 
  

   of 
  accuracy 
  with 
  which 
  the 
  time 
  of 
  the 
  supposed 
  number 
  of 
  oscilla- 
  

   tions 
  is 
  known. 
  

  

  To 
  furnish 
  convenient 
  numbers 
  for 
  calculating 
  the 
  time 
  of 
  ten 
  oscil- 
  

   lations, 
  we 
  observed 
  usually 
  at 
  the 
  end 
  of 
  fifty 
  or 
  one 
  hundred 
  oscilla- 
  

   tions, 
  a 
  number 
  much 
  below 
  the 
  limit 
  allowed 
  by 
  the 
  condition 
  just 
  

   referred 
  to. 
  An 
  example 
  will 
  serve 
  to 
  show 
  how 
  fully 
  this 
  method, 
  

   when 
  properly 
  applied, 
  is 
  to 
  be 
  relied 
  upon. 
  

  

  Philadelphia, 
  September 
  19th, 
  1835. 
  Needle 
  A. 
  

  

  Observed 
  times 
  of 
  passage 
  over 
  the 
  magnetic 
  meridian. 
  

  

  h. 
  m. 
  s. 
  

  

  P. 
  M. 
  5 
  35 
  17.8 
  

  

  31 
  18.0 
  

  

  37 
  18.0 
  

  

  43 
  18.0 
  

  

  The 
  time 
  of 
  ten 
  vibrations 
  being 
  known 
  to 
  be 
  between 
  36 
  seconds 
  

   and 
  36.4 
  seconds, 
  there 
  can 
  be 
  no 
  doubt 
  as 
  to 
  the 
  number 
  of 
  vibra- 
  

   tions 
  corresponding 
  to 
  either 
  of 
  the 
  intervals 
  deduced 
  from 
  the 
  observa- 
  

  

  