﻿492 
  PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  THE 
  

  

  A 
  few 
  days 
  after 
  the 
  last 
  observation 
  I 
  left 
  the 
  Cape, 
  and 
  the 
  passage 
  thence 
  

   by 
  St 
  Helena 
  through 
  the 
  tropics, 
  was 
  so 
  uniformly 
  cloudy, 
  that 
  I 
  was 
  unable 
  to 
  

   obtain 
  another 
  satisfactory 
  look 
  at 
  the 
  zodiacal 
  light, 
  and 
  ever 
  since 
  my 
  residence 
  

   in 
  Edinburgh 
  in 
  the 
  middle 
  of 
  a 
  city 
  glowing 
  at 
  night 
  with 
  gas, 
  and 
  reeking 
  with 
  

   smoke, 
  and 
  under 
  a 
  sky 
  but 
  rarely 
  clear, 
  and 
  when 
  it 
  is 
  so, 
  not 
  unfrequently 
  

   illuminated 
  by 
  the 
  Aurora 
  Borealis, 
  I 
  have 
  been 
  equally 
  unfortunate. 
  

  

  To 
  be 
  able 
  to 
  make 
  a 
  good 
  observation 
  of 
  the 
  zodiacal 
  light, 
  the 
  sky 
  should 
  be 
  

   quite 
  free 
  from 
  clouds, 
  the 
  air 
  pure 
  and 
  transparent 
  ; 
  not 
  the 
  slightest 
  vestige 
  of 
  

   the 
  twilight 
  should 
  remain, 
  the 
  milky 
  way 
  should 
  be 
  far 
  from 
  the 
  neighbour- 
  

   hood, 
  the 
  moon 
  should 
  not 
  be 
  visible, 
  or 
  the 
  brighter 
  planets, 
  such 
  as 
  Venus 
  and 
  

   Jupiter. 
  If 
  these 
  circumstances 
  be 
  secured, 
  and 
  a 
  person 
  look 
  out 
  at 
  that 
  period 
  

   of 
  the 
  year, 
  as 
  hereinafter 
  detailed, 
  when 
  the 
  ecliptic 
  makes 
  a 
  large 
  angle 
  with 
  the 
  

   horizon 
  of 
  the 
  place 
  of 
  observation, 
  he 
  can 
  hardly 
  fail 
  to 
  see 
  the 
  phenomenon 
  in 
  the 
  

   most 
  marked 
  degree. 
  A 
  beginner 
  must 
  be 
  especially 
  cautioned 
  not 
  to 
  begin 
  looking 
  

   too 
  soon 
  in 
  the 
  twilight 
  to 
  discern 
  the 
  " 
  Sun's 
  atmosphere," 
  under 
  the 
  idea, 
  happily, 
  

   of 
  catching 
  it 
  before 
  all 
  traces 
  of 
  the 
  sun's 
  light 
  on 
  the 
  horizon 
  are 
  completely 
  

   gone 
  ; 
  and 
  he 
  should 
  also 
  be 
  forewarned 
  of 
  the 
  immense 
  influence 
  which 
  climate 
  

   and 
  geographical 
  position 
  have 
  on 
  the 
  visibility 
  and 
  apparently 
  on 
  the 
  form 
  and 
  size 
  

   of 
  the 
  phenomenon. 
  Thus, 
  in 
  b6° 
  N. 
  Lat., 
  and 
  still 
  less 
  further 
  north, 
  even 
  were 
  

   the 
  elongation 
  of 
  the 
  light 
  E. 
  and 
  W., 
  equal 
  in 
  every 
  respect, 
  it 
  would 
  still 
  never 
  

   appear 
  equally 
  visible, 
  and 
  would 
  but 
  seldom 
  be 
  seen 
  either 
  way. 
  In 
  the 
  summer 
  

   the 
  twilight 
  would 
  render 
  the 
  sight 
  impossible, 
  and 
  in 
  winter, 
  the 
  sun's 
  path 
  is 
  

   too 
  low 
  and 
  oblique. 
  In 
  the 
  spring 
  evenings, 
  the 
  light 
  would 
  be 
  well 
  seen, 
  be- 
  

   cause 
  then 
  the 
  twilight 
  is 
  of 
  a 
  moderate 
  length, 
  and 
  the 
  zodiacal 
  light 
  rises 
  at 
  an 
  

   angle 
  to 
  the 
  horizon 
  of 
  30° 
  greater 
  than 
  the 
  equator, 
  and 
  therefore 
  does 
  not 
  set 
  

   till 
  long 
  after 
  the 
  twilight 
  has 
  disappeared 
  ; 
  but 
  as 
  the 
  other 
  end 
  rises 
  in 
  the 
  

   morning 
  at 
  that 
  season, 
  necessarily 
  at 
  an 
  angle 
  30° 
  less 
  than 
  that 
  of 
  the 
  equator, 
  

   the 
  apex 
  in 
  its 
  standing 
  position, 
  hardly 
  rises 
  above 
  the 
  mists 
  of 
  the 
  horizon 
  be- 
  

   fore 
  the 
  twilight 
  illumines 
  the 
  sky. 
  In 
  the 
  month 
  of 
  September 
  matters 
  are 
  just 
  

   reversed, 
  and 
  the 
  zodiacal 
  light 
  rising 
  in 
  the 
  morning 
  at 
  a 
  greater 
  angle 
  than 
  the 
  

   equator, 
  is 
  then 
  well 
  seen, 
  but 
  is 
  not 
  at 
  all 
  visible 
  in 
  the 
  evening, 
  when, 
  from 
  the 
  

   standing 
  position 
  of 
  the 
  body, 
  the 
  apex 
  sets 
  very 
  soon 
  after 
  the 
  sun. 
  And 
  in 
  these 
  

   two 
  short 
  appropriate 
  seasons, 
  so 
  many 
  of 
  the 
  nights 
  may 
  be 
  rendered 
  untoward 
  

   by 
  clouds, 
  strong 
  moonlight, 
  and 
  other 
  causes, 
  that 
  an 
  opportunity 
  of 
  seeing 
  the 
  

   zodiacal 
  light 
  may 
  even 
  then 
  but 
  very 
  rarely 
  be 
  enjoyed. 
  

  

  Similarly 
  in 
  the 
  southern 
  hemisphere 
  in 
  56° 
  S. 
  Lat., 
  supposing 
  also, 
  as 
  before, 
  

   that 
  the 
  zodiacal 
  light 
  stretches 
  out 
  equally 
  from 
  the 
  sun 
  on 
  every 
  side 
  in 
  the 
  

   plane 
  of 
  his 
  equator, 
  the 
  two 
  most 
  favourable 
  opportunities 
  in 
  the 
  course 
  of 
  the 
  

   year 
  for 
  viewing 
  the 
  body 
  would 
  be, 
  in 
  the 
  evening 
  in 
  the 
  month 
  of 
  September, 
  

   and 
  in 
  the 
  morning 
  in 
  the 
  month 
  of 
  March. 
  And 
  that 
  this 
  would 
  really 
  be 
  the 
  

   case, 
  the 
  observations 
  made 
  in 
  Lat. 
  33 
  S. 
  sufficiently 
  attest. 
  

  

  