﻿494 
  PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  THE 
  

  

  northern 
  point 
  exactly, 
  then 
  the 
  equator 
  would 
  be 
  represented 
  by 
  a 
  straight 
  line, 
  

   parallel 
  with 
  the 
  horizon, 
  and 
  elevated 
  above 
  it 
  at 
  an 
  angle 
  equal 
  to 
  the 
  colati- 
  

   tude 
  of 
  the 
  place. 
  And, 
  according 
  to 
  the 
  degree 
  to 
  which 
  the 
  spectator 
  turns 
  

   round 
  from 
  the 
  E. 
  or 
  W. 
  points 
  towards 
  the 
  north, 
  the 
  inclination 
  of 
  the 
  equatorial 
  

   line 
  will 
  vary, 
  from 
  the 
  angle 
  of 
  the 
  latitude 
  of 
  the 
  place, 
  to 
  perfect 
  horizontality. 
  

   This 
  varied 
  inclination 
  of 
  the 
  equator 
  has 
  been 
  strictly 
  attended 
  to 
  in 
  all 
  the 
  

   accompanying 
  cases 
  ; 
  but 
  it 
  has 
  been 
  found 
  advisable 
  for 
  simplicity 
  in 
  practice, 
  

   to 
  represent 
  the 
  declination 
  circles 
  also 
  b} 
  r 
  straight 
  lines, 
  for 
  there 
  is 
  hardly 
  any 
  

   sensible 
  difference 
  caused 
  thereby 
  in 
  the 
  central 
  region 
  of 
  the 
  picture, 
  where 
  all 
  

   the 
  important 
  part 
  of 
  the 
  subject 
  to 
  be 
  delineated, 
  lies 
  ; 
  and 
  although 
  the 
  con- 
  

   figuration 
  of 
  stars 
  near 
  the 
  borders 
  might 
  not 
  be 
  such 
  as 
  would 
  exactly 
  appear 
  to 
  

   the 
  eye 
  of 
  a 
  spectator, 
  or 
  as 
  they 
  should 
  be 
  represented 
  by 
  the 
  usual 
  rules 
  of 
  per- 
  

   spective, 
  still 
  the 
  amount 
  of 
  discordance 
  is 
  so 
  extremely 
  small, 
  that 
  the 
  unassisted 
  

   eye 
  would 
  hardly 
  perceive 
  it 
  ; 
  and, 
  what 
  is 
  more 
  important 
  than 
  having 
  a 
  repre- 
  

   sentation 
  perfectly 
  similar, 
  in 
  the 
  minutest 
  particular, 
  to 
  that 
  found 
  in 
  the 
  re- 
  

   tina, 
  — 
  the 
  particular 
  projection 
  of 
  the 
  sphere 
  which 
  was 
  actually 
  employed, 
  being 
  

   inserted 
  in 
  the 
  margin, 
  gives 
  just 
  as 
  good, 
  and 
  rather 
  simpler, 
  means, 
  than 
  would 
  

   have 
  been 
  available 
  on 
  the 
  other 
  plan, 
  for 
  the 
  identification 
  of 
  the 
  stars. 
  

  

  The 
  application 
  of 
  instrumental 
  measurement, 
  hereinbefore 
  described, 
  to 
  deter- 
  

   mine 
  the 
  phenomena 
  of 
  the 
  zodiacal 
  light, 
  is 
  believed 
  to 
  be 
  new 
  ; 
  and 
  the 
  obser- 
  

   vations 
  so 
  obtained, 
  seem 
  to 
  shew 
  very 
  decidedly, 
  especially 
  those 
  of 
  October 
  

   1844, 
  that 
  numerical 
  measures 
  of 
  the 
  place 
  of 
  the 
  apex 
  of 
  the 
  light 
  may 
  easily 
  be 
  

   obtained, 
  — 
  with 
  a 
  probable 
  error 
  of 
  not 
  more 
  than 
  two 
  degrees 
  : 
  so 
  that 
  vague 
  

   estimations 
  and 
  notes 
  of 
  mere 
  ideas 
  should 
  not 
  be 
  allowed 
  to 
  form 
  the 
  data 
  in 
  this 
  

   particular 
  branch 
  of 
  astronomy 
  any 
  longer. 
  

  

  The 
  general 
  results 
  to 
  be 
  deduced 
  from 
  the 
  data 
  given 
  in 
  the 
  Table, 
  are, 
  

   1st, 
  That 
  the 
  zodiacal 
  light 
  is 
  a 
  body 
  of 
  a 
  lenticular 
  form, 
  spread 
  out 
  nearly 
  

   in 
  the 
  plane 
  of 
  the 
  earth's 
  orbit, 
  and 
  extending 
  almost 
  equally 
  from 
  the 
  sun 
  in 
  

   every 
  direction. 
  Were 
  the 
  ordinary 
  European 
  observations 
  made 
  about 
  the 
  time 
  

   of 
  the 
  spring 
  equinox 
  the 
  only 
  ones 
  existing, 
  we 
  should 
  merely 
  be 
  entitled 
  to 
  

   conclude 
  the 
  existence 
  of 
  a 
  one-lobed 
  projection 
  from 
  the 
  sun 
  ; 
  but 
  when 
  we 
  

   combine 
  therewith 
  the 
  Cape 
  observations, 
  we 
  find 
  that 
  the 
  body 
  is 
  seen 
  all 
  through 
  

   the 
  year, 
  and 
  on 
  both 
  sides 
  of 
  the 
  sun, 
  of 
  pretty 
  nearly 
  the 
  same 
  size 
  and 
  shape, 
  

   viz., 
  a 
  curvilinear-sided 
  wedge, 
  in 
  which 
  the 
  light 
  continually 
  increases 
  from 
  the 
  

   borders 
  towards 
  the 
  centre 
  of 
  the 
  base, 
  or 
  the 
  actual 
  position 
  of 
  the 
  sun 
  ; 
  ap- 
  

   pearances 
  which 
  can 
  only 
  be 
  satisfied 
  by 
  a 
  lenticular 
  body 
  seen 
  in 
  section. 
  

  

  2d, 
  The 
  zodiacal 
  light 
  is 
  proved 
  to 
  be 
  excentrically 
  disposed 
  about 
  the 
  sun, 
  

   by 
  the 
  elongations 
  observed 
  east 
  and 
  west 
  on 
  the 
  same 
  day 
  being 
  different 
  ; 
  shew- 
  

   ing, 
  indeed, 
  at 
  various 
  times 
  an 
  excentricity 
  of 
  from 
  ^g 
  to 
  jg. 
  

  

  3d, 
  The 
  zodiacal 
  light 
  may 
  also 
  be 
  considered 
  to 
  rotate 
  about 
  the 
  sun, 
  and 
  to 
  

   be 
  brighter 
  in 
  some 
  parts 
  than 
  in 
  others 
  ; 
  because 
  it 
  is 
  observed 
  to 
  be 
  of 
  different 
  

   lengths 
  and 
  degrees 
  of 
  brightness 
  at 
  corresponding 
  periods 
  in 
  successive 
  years. 
  For 
  

  

  