﻿20 
  ON 
  THE 
  DIURNAL 
  VARIATION 
  

  

  instruments 
  used 
  or 
  the 
  localities 
  of 
  observation 
  may 
  perhaps 
  be 
  found 
  

   to 
  explain. 
  In 
  regard 
  to 
  the 
  latter 
  point, 
  there 
  was 
  of 
  course 
  a 
  marked 
  

   difference 
  in 
  the 
  nature 
  of 
  the 
  meteorological 
  phenomena 
  at 
  the 
  two 
  

   places. 
  In 
  the 
  whole 
  of 
  the 
  abstract 
  of 
  the 
  meteorological 
  observa- 
  

   tions 
  given 
  by 
  Lieut. 
  Foster, 
  there 
  is 
  no 
  record 
  of 
  rain, 
  and 
  no 
  notice 
  

   of 
  thunder 
  storms, 
  while 
  my 
  results, 
  obtained 
  during 
  part 
  of 
  the 
  last 
  

   month 
  of 
  summer 
  and 
  of 
  the 
  first 
  of 
  autumn, 
  were 
  diversified 
  by 
  

   steady 
  rains, 
  showers, 
  fogs, 
  &c. 
  

  

  In 
  showing 
  the 
  effects 
  of 
  meteorological 
  phenomena 
  on 
  the 
  varia- 
  

   tion, 
  I 
  purpose 
  to 
  appeal 
  to 
  the 
  direct 
  evidence 
  afforded 
  by 
  curve 
  No. 
  

   3 
  (Plate 
  I.*); 
  the 
  peculiarities 
  of 
  the 
  other 
  curves 
  may 
  afford 
  collateral 
  

   testimony, 
  but 
  comparatively 
  of 
  less 
  value. 
  The 
  morning 
  of 
  August 
  

   3 
  1st 
  was 
  clear 
  with 
  flying 
  white 
  clouds 
  ; 
  towards 
  two 
  o'clock 
  a 
  dark 
  cloud 
  

   came 
  up 
  from 
  the 
  westward 
  and 
  a 
  shower 
  fell 
  ; 
  this 
  was 
  not 
  accom- 
  

   panied 
  by 
  thunder 
  or 
  lightning, 
  and 
  soon 
  passed 
  over. 
  Just 
  at 
  the 
  

   beginning 
  of 
  this 
  shower 
  the 
  variation 
  was 
  3° 
  2 
  6'. 
  5 
  W., 
  and 
  as 
  soon 
  

   as 
  it 
  had 
  ceased, 
  the 
  needle 
  was 
  again 
  observed 
  and 
  was 
  found 
  but 
  

   3° 
  1 
  0'.7 
  W. 
  In 
  saying 
  that 
  there 
  was 
  no 
  extraordinary 
  electrical 
  excite- 
  

   ment 
  during 
  this 
  shower, 
  I 
  do 
  not 
  mean 
  to 
  assert 
  that 
  the 
  changes 
  of 
  

   the 
  variation 
  were 
  not 
  due 
  to 
  the 
  usual 
  electrical 
  changes 
  which 
  al- 
  

   ways 
  attend 
  such 
  phenomena. 
  Being 
  unprovided 
  with 
  means 
  of 
  esti- 
  

   mating 
  electrical 
  changes, 
  I 
  only 
  refer 
  to 
  the 
  visible 
  indications 
  of 
  

   lightning. 
  The 
  sun 
  now 
  shone 
  out, 
  and 
  at 
  3 
  P.M. 
  the 
  variation 
  had 
  

   increased 
  to 
  3° 
  25'. 
  8. 
  Towards 
  4 
  P.M. 
  clouds 
  began 
  to 
  gather, 
  and 
  

   at 
  5 
  h. 
  20 
  m. 
  black 
  clouds 
  to 
  the 
  N. 
  W. 
  indicated 
  an 
  approaching 
  gust. 
  

   At 
  the 
  moment 
  the 
  rain 
  commenced 
  the 
  needle 
  was 
  noted, 
  the 
  varia- 
  

   tion 
  had 
  decreased, 
  between 
  5 
  h. 
  30 
  m., 
  the 
  time 
  of 
  the 
  last 
  observa- 
  

   tion, 
  and 
  5 
  h. 
  48 
  m., 
  13 
  minutes. 
  The 
  rain 
  was 
  violent, 
  accompa- 
  

   nied 
  by 
  thunder 
  and 
  lightning. 
  At 
  7 
  P.M. 
  the 
  rain 
  had 
  ceased, 
  and 
  the 
  

   gust 
  had 
  passed 
  to 
  the 
  eastward, 
  the 
  needle 
  vibrated 
  violently 
  between 
  

   3° 
  32' 
  W. 
  and 
  3° 
  37' 
  W. 
  Such 
  irregularities 
  are 
  conclusive 
  as 
  to 
  the 
  

   influence 
  of 
  the 
  phenomena 
  attending 
  a 
  shower 
  and 
  a 
  gust. 
  The 
  va- 
  

   riations 
  noticed 
  will 
  be 
  seen, 
  from 
  curve 
  No. 
  3, 
  to 
  precede 
  the 
  effects 
  

   produced 
  on 
  the 
  thermometer; 
  after 
  a 
  general 
  coincidence, 
  the 
  curves 
  

   depart 
  from 
  each 
  other, 
  the 
  curve 
  of 
  variation 
  rising 
  abruptly, 
  and 
  that 
  

   of 
  temperature 
  but 
  slightly. 
  

  

  