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  XXXIII. 
  — 
  Contributions 
  to 
  a 
  Knowledge 
  of 
  the 
  Phenomena 
  of 
  the 
  Zodiacal 
  Light. 
  

  

  By 
  Professor 
  C. 
  Piazzi 
  Smyth. 
  

  

  (Read 
  February 
  7, 
  1848.) 
  

  

  When 
  preparing 
  to 
  make 
  a 
  night 
  journey 
  over 
  one 
  of 
  the 
  plains 
  of 
  South 
  Africa, 
  

   in 
  the 
  month 
  of 
  June 
  1843, 
  a 
  friend 
  called 
  my 
  attention 
  to 
  the 
  peculiar 
  appearance 
  

   of 
  the 
  sky 
  in 
  the 
  west, 
  as 
  offering 
  a 
  very 
  decided 
  proof, 
  " 
  agreeably 
  with 
  theory," 
  

   that 
  there 
  was 
  no 
  " 
  Solar 
  atmosphere" 
  to 
  be 
  seen 
  at 
  that 
  season 
  of 
  the 
  year. 
  

  

  On 
  looking 
  in 
  the 
  direction 
  mentioned, 
  the 
  last 
  portion 
  of 
  the 
  twilight 
  was 
  

   just 
  visible, 
  and 
  forming 
  a 
  peculiarly 
  level 
  line 
  above 
  the 
  place 
  where 
  the 
  sun 
  had 
  

   set, 
  for 
  an 
  extent 
  in 
  azimuth, 
  of 
  perhaps 
  40°, 
  and 
  at 
  a 
  height 
  of 
  about 
  5°. 
  All 
  the 
  

   gorgeous 
  colours 
  which 
  had 
  attended 
  the 
  setting 
  of 
  the 
  sun 
  had 
  long 
  since 
  vanished, 
  

   and 
  there 
  only 
  remained 
  sufficient 
  light 
  within 
  the 
  flattened 
  arc 
  described, 
  to 
  

   make 
  the 
  space 
  included 
  between 
  it 
  and 
  the 
  horizon 
  appear 
  light 
  blue, 
  while 
  all 
  

   the 
  rest 
  of 
  the 
  sky 
  had 
  attained 
  a 
  deeper 
  colour, 
  nay 
  was 
  almost 
  black, 
  and 
  thickly 
  

   spangled 
  with 
  small 
  as 
  well 
  as 
  large 
  stars. 
  

  

  There 
  most 
  decidedly 
  was 
  not 
  then 
  any 
  symptom 
  of 
  the 
  so-called 
  " 
  way 
  of 
  

   the 
  twilight 
  shooting 
  upward." 
  But 
  as 
  soon 
  as 
  the 
  last 
  illuminated 
  portion 
  

   of 
  the 
  western 
  sky 
  had 
  set, 
  the 
  phenomenon, 
  i. 
  e., 
  the 
  zodiacal 
  light, 
  appeared 
  

   in 
  an 
  unmistakeable 
  manner, 
  rising 
  up 
  in 
  the 
  ecliptic, 
  to 
  a 
  height 
  of 
  50\ 
  with 
  a 
  

   breadth 
  of 
  perhaps 
  12° 
  at 
  the 
  horizon 
  ; 
  and 
  forming, 
  with 
  the 
  vast 
  extent 
  of 
  its 
  

   illuminated 
  surface, 
  and 
  the 
  regularity 
  of 
  its 
  contour, 
  one 
  of 
  the 
  most 
  remark- 
  

   able 
  objects 
  in 
  the 
  starry 
  sky. 
  The 
  form 
  was 
  that 
  usually 
  described, 
  viz., 
  a 
  wedge 
  

   pointing 
  upwards, 
  with 
  curved 
  sides, 
  and 
  of 
  excessively 
  indefinite 
  outline 
  ; 
  but 
  

   still, 
  as 
  far 
  as 
  could 
  be 
  judged, 
  free 
  from 
  any 
  irregularities 
  ; 
  while 
  the 
  light, 
  which 
  

   was 
  more 
  delicate 
  and 
  transparent 
  than 
  that 
  of 
  the 
  milky 
  way, 
  increased 
  in 
  

   intensity 
  transversely 
  from 
  either 
  side 
  to 
  the 
  central 
  longitudinal 
  axis 
  ; 
  and 
  

   augmented 
  also 
  in 
  the 
  axis, 
  from 
  the 
  apex 
  downwards, 
  until 
  overpowered 
  by 
  the 
  

   haze 
  on 
  the 
  horizon. 
  

  

  Now, 
  two 
  circumstances 
  worthy 
  of 
  notice, 
  were 
  pointed 
  out 
  by 
  this 
  night's 
  ob- 
  

   servation, 
  firstly, 
  that 
  persons 
  did 
  not 
  always 
  know 
  exactly 
  when 
  to 
  look 
  for 
  the 
  

   zodiacal 
  light, 
  nor 
  what 
  sort 
  of 
  object 
  to 
  expect 
  ; 
  and, 
  secondly, 
  that 
  the 
  theory 
  

   was 
  greatly 
  in 
  error 
  ; 
  and, 
  for 
  an 
  astronomical 
  matter, 
  grievously 
  wrong. 
  Both 
  

   classes 
  of 
  mistakes 
  may 
  have 
  been 
  brought 
  about 
  in 
  no 
  small 
  degree 
  by 
  the 
  inju- 
  

   dicious 
  mixing 
  up 
  of 
  erroneous 
  theoretical 
  and 
  speculative 
  views 
  with 
  the 
  simple 
  

   nomenclature 
  of 
  the 
  phenomenon. 
  All 
  that 
  can 
  be 
  asserted 
  from 
  a 
  single 
  obser- 
  

   vation, 
  and, 
  perhaps 
  indeed, 
  from 
  all 
  the 
  observations 
  that 
  have 
  been 
  made, 
  up 
  to 
  

   the 
  present 
  time, 
  is 
  that 
  a 
  light 
  appears 
  in 
  the 
  zodiac, 
  and 
  if 
  it 
  be 
  called 
  " 
  the 
  

  

  VOL. 
  XX. 
  PART 
  III. 
  6 
  Q 
  

  

  