﻿6 
  ON 
  THE 
  DIURNAL 
  VARIATION 
  

  

  The 
  greatest 
  difference 
  of 
  the 
  numbers 
  of 
  the 
  second 
  and 
  third 
  

   column 
  in 
  the 
  foregoing 
  table 
  is 
  at 
  the 
  beginning, 
  as 
  might 
  have 
  been 
  

   expected, 
  where 
  it 
  amounts 
  to 
  nearly 
  four 
  minutes, 
  and 
  shows 
  that 
  the 
  

   first 
  two 
  observations 
  are 
  of 
  little 
  value 
  ; 
  the 
  greatest 
  subsequent 
  varia- 
  

   tion 
  from 
  the 
  mean 
  difference 
  is 
  about 
  one 
  minute 
  and 
  a 
  half. 
  

  

  To 
  have 
  a 
  perspicuous 
  view 
  of 
  the 
  results 
  of 
  the 
  observations 
  for 
  

   every 
  twenty-four 
  hours, 
  I 
  traced, 
  at 
  the 
  close 
  of 
  each 
  series, 
  a 
  curve, 
  

   the 
  ordinates 
  of 
  which 
  represented 
  the 
  variation, 
  and 
  the 
  equidistant 
  

   abscissae 
  the 
  hours. 
  By 
  writing, 
  at 
  the 
  side 
  of 
  each 
  ordinate, 
  the 
  

   remarks 
  on 
  the 
  weather, 
  the 
  view 
  was 
  rendered 
  more 
  complete. 
  To 
  

   compare 
  the 
  progress 
  of 
  the 
  variation 
  with 
  that 
  of 
  the 
  temperature, 
  a 
  

   curve 
  was 
  traced 
  below 
  the 
  former, 
  in 
  which 
  the 
  ordinates 
  represented 
  

   the 
  temperatures. 
  The 
  crude 
  observations 
  suggested 
  by 
  the 
  visible 
  

   representation 
  of 
  the 
  day's 
  results, 
  given 
  in 
  chart 
  No. 
  1, 
  (see 
  Plate 
  I.*) 
  

   are 
  not 
  here 
  added 
  ; 
  it 
  may 
  not 
  be 
  amiss, 
  however, 
  to 
  state, 
  that 
  my 
  

   object 
  at 
  this 
  time 
  in 
  tracing 
  the 
  curve 
  of 
  temperature, 
  as 
  well 
  as 
  that 
  

   of 
  variation, 
  was 
  to 
  ascertain 
  whether 
  the 
  directive 
  force 
  underwent 
  

   correlative 
  changes 
  with 
  the 
  temperature, 
  as 
  the 
  intensities 
  were 
  sup- 
  

   posed 
  to 
  do 
  by 
  M. 
  Kupffer 
  from 
  his 
  first 
  experiments.* 
  

  

  Two 
  maxima 
  of 
  westerly 
  variation, 
  and 
  two 
  minima 
  were 
  distinctly 
  

   seen 
  in 
  this 
  day's 
  results. 
  The 
  first 
  maximum 
  was 
  at 
  noon 
  day 
  ; 
  the 
  

   second 
  between 
  midnight 
  and 
  1 
  A.M. 
  The 
  first 
  minimum 
  was 
  be- 
  

   tween 
  6^-and 
  7 
  in 
  the 
  afternoon, 
  and 
  the 
  second 
  between 
  8^ 
  and 
  9 
  in 
  

   the 
  morning. 
  There 
  is, 
  on 
  the 
  contrary, 
  but 
  one 
  tide 
  of 
  temperature, 
  

   rising 
  until 
  between 
  2 
  and 
  3 
  in 
  the 
  afternoon, 
  descending 
  with 
  vari- 
  

   able 
  rapidity 
  until 
  6 
  in 
  the 
  morning, 
  half 
  an 
  hour 
  after 
  sunrise. 
  The 
  

   descent 
  of 
  the 
  curve 
  is 
  interrupted 
  by 
  the 
  fog, 
  and 
  it 
  was 
  not 
  unnatural 
  

   to 
  suppose 
  this 
  to 
  have 
  its 
  effect 
  on 
  the 
  hour 
  of 
  the 
  minimum. 
  The 
  

   hour 
  of 
  evening 
  minimum 
  variation 
  differs, 
  it 
  will 
  be 
  observed, 
  from 
  

   that 
  obtained 
  by 
  Canton 
  and 
  Gilpin, 
  suggesting 
  a 
  close 
  attention 
  to 
  

   the 
  results 
  furnished 
  by 
  the 
  subsequent 
  observations. 
  

  

  In 
  the 
  following 
  tables 
  the 
  mean 
  of 
  the 
  observed 
  variations 
  shown 
  

   by 
  the 
  north 
  and 
  south 
  pole 
  of 
  the 
  needle 
  is 
  given. 
  The 
  hours 
  from 
  

  

  * 
  Recherches 
  sur 
  les 
  Variations 
  de 
  la 
  duree 
  Moyenne 
  des 
  oscillations 
  horizontales 
  de 
  l'ai- 
  

   guille 
  aimente, 
  &c. 
  A. 
  T. 
  Kupffer, 
  Annales 
  de 
  China, 
  et 
  de 
  Phys. 
  vol. 
  35 
  (1827). 
  

  

  