﻿OF 
  TERRESTRIAL 
  MAGNETISM 
  AT 
  SEVERAL 
  PLACES, 
  ETC. 
  441 
  

  

  A 
  comparison 
  of 
  observations 
  5 
  and 
  7, 
  after 
  correcting 
  the 
  latter 
  by 
  

   an 
  approximate 
  coefficient 
  for 
  temperature, 
  gives 
  the 
  amount 
  of 
  daily 
  

   variation 
  to 
  be 
  allowed 
  for. 
  Assuming 
  the 
  progress 
  of 
  this 
  variation 
  

   to 
  be 
  in 
  proportion 
  to 
  the 
  time, 
  a 
  correction 
  is 
  deduced 
  for 
  number 
  6, 
  

   which 
  is 
  in 
  the 
  right 
  direction, 
  though 
  small 
  in 
  amount. 
  The 
  coeffi- 
  

   cient 
  deduced 
  from 
  5 
  and 
  6 
  is 
  m 
  = 
  .000,147. 
  The 
  coefficient 
  be- 
  

   fore 
  obtained 
  was 
  m 
  = 
  .000,117. 
  It 
  would 
  certainly 
  be 
  rash 
  to 
  infer 
  

   from 
  the 
  small 
  difference 
  thus 
  rendered 
  evident 
  between 
  the 
  values 
  of 
  

   m, 
  deduced 
  under 
  different 
  circumstances, 
  that 
  the 
  difference 
  resulted 
  

   from 
  these 
  circumstances. 
  The 
  times 
  of 
  vibration 
  at 
  corresponding 
  

   temperatures, 
  are 
  greater 
  in 
  the 
  second 
  set 
  of 
  observations 
  than 
  in 
  the 
  

   first, 
  as 
  well 
  as 
  the 
  differences 
  for 
  a 
  given 
  number 
  of 
  degrees. 
  It 
  is 
  

   possible 
  that 
  the 
  needle 
  may 
  have 
  undergone 
  a 
  slight 
  change 
  between 
  

   October 
  and 
  December, 
  a 
  question 
  which 
  future 
  observations 
  may 
  de- 
  

   termine. 
  As 
  far 
  as 
  the 
  application 
  to 
  the 
  observations 
  which 
  are 
  to 
  

   follow 
  is 
  concerned, 
  these 
  coefficients 
  are 
  so 
  near 
  to 
  each 
  other 
  that 
  

   either 
  might 
  be 
  adopted 
  without 
  sensibly 
  affecting 
  the 
  results. 
  The 
  

   difference 
  would 
  amount 
  to 
  but 
  .01 
  second 
  in 
  ten 
  vibrations, 
  for 
  ten 
  

   degrees 
  of 
  temperature. 
  From 
  8 
  and 
  9 
  uncorrected 
  for 
  diurnal 
  varia- 
  

   tion, 
  m 
  = 
  .000,145. 
  The 
  times 
  of 
  oscillation 
  being 
  nearly 
  equally 
  

   before 
  and 
  after 
  midnight, 
  about 
  which 
  time 
  the 
  march 
  of 
  the 
  intensity 
  

   begins 
  to 
  change 
  its 
  direction, 
  a 
  correction 
  deduced 
  from 
  preceding 
  

   observations, 
  would 
  probably 
  render 
  the 
  results 
  less 
  accurate 
  than 
  they 
  

   are 
  without 
  it. 
  

  

  The 
  very 
  close 
  agreement 
  of 
  the 
  two 
  numbers 
  just 
  given 
  for 
  the 
  

   coefficient, 
  strengthens 
  the 
  opinion, 
  that 
  the 
  difference 
  from 
  the 
  num- 
  

   ber 
  found 
  in 
  October 
  results 
  from 
  a 
  slight 
  change 
  in 
  the 
  magnetic 
  

   state 
  of 
  the 
  bar: 
  the 
  circumstances 
  in 
  the 
  second 
  and 
  third 
  sets 
  of 
  

   observations 
  having 
  been 
  so 
  very 
  different 
  as 
  to 
  local 
  magnetism. 
  

  

  We 
  infer 
  from 
  a 
  comparison 
  of 
  the 
  three 
  values 
  of 
  m, 
  that 
  a 
  co- 
  

   efficient 
  for 
  the 
  correction 
  for 
  temperature, 
  obtained 
  under 
  the 
  circum- 
  

   stances 
  of 
  the 
  second 
  and 
  third 
  sets 
  of 
  observations, 
  may 
  safely 
  be 
  

   applied 
  to 
  correct, 
  for 
  temperature, 
  the 
  observations 
  made 
  during 
  the 
  

   summer 
  and 
  autumn. 
  

  

  The 
  following 
  table 
  contains 
  the 
  observations 
  made 
  to 
  obtain 
  the 
  

   correction 
  to 
  apply 
  to 
  needles 
  C 
  and 
  B. 
  The 
  former 
  has 
  so 
  small 
  a 
  

  

  