﻿PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  137 
  

  

  whole 
  body 
  of 
  the 
  comet 
  was 
  seen, 
  but 
  what 
  his 
  idea 
  of 
  the 
  true 
  complete 
  form 
  of 
  its 
  body 
  was, 
  he 
  

   does 
  not 
  give 
  ; 
  but, 
  as 
  he 
  states, 
  that 
  a 
  slight 
  change 
  of 
  brightness 
  near 
  the 
  head 
  was 
  " 
  the 
  only 
  

   thing 
  to 
  distinguish 
  the 
  nucleus 
  from 
  the 
  tail 
  at 
  that 
  point 
  ;" 
  and, 
  further, 
  " 
  that 
  every 
  part 
  of 
  the 
  

   tail 
  was 
  as 
  well 
  defined 
  as 
  the 
  moon 
  on 
  a 
  clear 
  day 
  ;" 
  it 
  would 
  appear 
  to 
  be 
  quite 
  safe 
  to 
  infer, 
  that 
  

   the 
  tail 
  was 
  not 
  forked 
  ; 
  and 
  that 
  the 
  base, 
  instead 
  of 
  a 
  broad 
  or 
  forked, 
  or 
  many-pointed 
  indistinct 
  

   termination, 
  was 
  as 
  well 
  and 
  sharply 
  outlined 
  as 
  the 
  limbs. 
  A 
  notable 
  distinction 
  this 
  to 
  every 
  

   subsequent 
  view 
  obtained 
  on 
  succeeding 
  days 
  ; 
  and, 
  indeed, 
  in 
  the 
  case 
  of 
  every 
  other 
  comet 
  what- 
  

   ever 
  observed 
  at 
  a 
  great 
  distance 
  from 
  the 
  sun, 
  — 
  when, 
  whatever 
  the 
  definition 
  of 
  the 
  limbs 
  of 
  the 
  

   tail, 
  the 
  termination 
  or 
  the 
  base 
  has 
  always 
  been 
  so 
  excessively 
  uncertain, 
  that 
  different 
  persons 
  have 
  

   varied 
  several 
  degrees 
  in 
  assigning 
  the 
  place 
  of 
  it. 
  

  

  Alas 
  ! 
  indeed, 
  that 
  the 
  practical 
  astronomy 
  of 
  the 
  present 
  day 
  did 
  not 
  take 
  better 
  account 
  of 
  

   this 
  unique 
  and 
  critical 
  instance 
  which 
  was 
  offered 
  by 
  the 
  skies 
  of 
  our 
  times 
  ; 
  centuries 
  may 
  elapse 
  

   before 
  another 
  such 
  instance 
  may 
  occur, 
  and 
  this 
  question 
  of 
  the 
  real 
  and 
  complete 
  form 
  of 
  a 
  comet 
  

   may 
  be 
  in 
  abeyance 
  as 
  long. 
  Something 
  may, 
  doubtless, 
  be 
  done 
  by 
  rigid 
  examination 
  of 
  all 
  the 
  

   persons 
  who 
  did 
  witness 
  the 
  phenomenon 
  in 
  the 
  comparatively 
  imperfect 
  form 
  of 
  the 
  day 
  after 
  the 
  

   perihelion 
  passage 
  ; 
  but 
  their 
  answers 
  would 
  not 
  be 
  very 
  safe 
  now, 
  so 
  many 
  years 
  after 
  the 
  event, 
  

   and 
  after 
  the 
  promulgation 
  of 
  a 
  particular 
  theory. 
  Something 
  might 
  also, 
  perhaps, 
  be 
  done, 
  by 
  

   careful 
  and 
  photometrical 
  observation 
  of 
  the 
  faintest 
  nebulae, 
  while 
  the 
  darker 
  part 
  of 
  a 
  comet's 
  

   tail, 
  if 
  it 
  exists, 
  must 
  be 
  passing 
  across 
  them. 
  But 
  this 
  is 
  a 
  very 
  unpromising 
  method, 
  for 
  comets, 
  

   at 
  all 
  periods, 
  attenuated, 
  become 
  so 
  exceedingly 
  diffuse 
  by 
  the 
  time 
  that 
  they 
  have 
  reached 
  a 
  

   sufficient 
  distance 
  from 
  the 
  sun, 
  to 
  be 
  viewed 
  for 
  any 
  length 
  of 
  time 
  in 
  a 
  dark 
  sky, 
  and 
  contrasted 
  

   therein 
  with 
  very 
  faint 
  nebulas, 
  — 
  that 
  we 
  can 
  hardly 
  expect 
  to 
  obtain 
  any 
  certain 
  indication 
  in 
  this 
  

   manner. 
  The 
  only 
  sure 
  way 
  is 
  for 
  the 
  comet 
  to 
  be 
  so 
  very 
  close 
  to 
  the 
  sun, 
  that 
  rays 
  from 
  some 
  

   part 
  or 
  other 
  of 
  his 
  surface 
  will 
  reach 
  every 
  portion 
  of 
  the 
  body 
  of 
  the 
  comet 
  directly, 
  i. 
  e., 
  without 
  

   having 
  to 
  pass 
  through 
  any 
  other 
  part 
  in 
  order 
  to 
  arrive 
  there. 
  

  

  The 
  fact 
  of 
  this 
  great 
  and 
  invaluable 
  opportunity 
  having 
  been 
  lost, 
  would 
  seem 
  to 
  shew 
  that 
  it 
  

   is 
  highly 
  desirable 
  that 
  extra 
  meridian 
  observations 
  should 
  be 
  made 
  and 
  watched 
  for 
  by 
  some 
  public 
  

   observatory 
  in 
  its 
  official 
  routine, 
  instead 
  of 
  being 
  abandoned 
  altogether 
  to 
  amateurs. 
  It 
  is 
  high 
  time 
  

   that 
  our 
  observatories 
  should 
  be 
  placed 
  in 
  the 
  clearer 
  climates 
  of 
  some 
  of 
  the 
  colonies, 
  and 
  that 
  the 
  

   most 
  favourable 
  geographical 
  positions 
  should 
  be 
  sought 
  for, 
  rather 
  than 
  the 
  most 
  convenient 
  places 
  in 
  

   a 
  social 
  point 
  of 
  view 
  ; 
  for 
  this 
  results 
  in 
  smoky 
  towns 
  in 
  our 
  own 
  beclouded 
  country 
  being 
  selected 
  as 
  

   the 
  places 
  where 
  the 
  stars 
  are 
  if 
  possible 
  to 
  be 
  obsei-ved. 
  

  

  10. 
  The 
  gaseous 
  envelope 
  is 
  of 
  extreme 
  tenuity, 
  is 
  elastic, 
  and 
  with 
  regard 
  

   to 
  light 
  is 
  slightly 
  reflective 
  and 
  imperfectly 
  transparent 
  ; 
  it 
  decreases 
  in 
  size, 
  but 
  

   increases 
  in 
  density, 
  and 
  light 
  reflective 
  power 
  in 
  approaching 
  the 
  perihelion, 
  and 
  

   the 
  reverse 
  when 
  receding 
  from 
  it 
  ; 
  and 
  this 
  occurs 
  in 
  a 
  degree 
  proportioned 
  to 
  

   the 
  excentricity 
  of 
  the 
  orbits 
  of 
  the 
  comets. 
  

  

  (10.) 
  That 
  the 
  gaseous 
  envelope 
  of 
  a 
  comet 
  is 
  of 
  extreme 
  tenuity, 
  and 
  is 
  elastic, 
  slightly 
  reflec- 
  

   tive 
  and 
  imperfectly 
  transparent, 
  is 
  apparently 
  confessed 
  on 
  all 
  hands, 
  and 
  is 
  proved 
  by 
  the 
  pheno- 
  

   mena 
  presented 
  by 
  every 
  comet. 
  That 
  it 
  increases 
  in 
  density 
  and 
  light-reflective 
  power 
  with 
  its 
  

   proximity 
  to 
  the 
  perihelion, 
  and 
  that 
  this 
  occurs 
  in 
  a 
  degree 
  proportioned 
  to 
  the 
  excentricity 
  of 
  the 
  

   orbit, 
  requires, 
  at 
  least 
  the 
  latter 
  part 
  does, 
  that 
  the 
  instances 
  on 
  which 
  it 
  is 
  founded 
  should 
  be 
  men- 
  

   tioned 
  ; 
  for 
  though 
  the 
  contraction 
  in 
  size 
  of 
  small 
  comets 
  on 
  approaching 
  the 
  sun 
  had 
  been 
  remarked, 
  

   yet 
  some 
  had 
  maintained 
  it 
  to 
  be 
  accompanied 
  by 
  a 
  decrease 
  in 
  density, 
  by 
  an 
  actual 
  evaporation 
  and 
  

   disappearance 
  at 
  perihelio 
  ; 
  and 
  no 
  one 
  that 
  I 
  am 
  acquainted 
  with 
  had 
  applied 
  it 
  to 
  the 
  larger 
  comets 
  

   also, 
  or 
  compared 
  the 
  degree 
  of 
  it, 
  with 
  the 
  excentricity 
  of 
  the 
  orbit. 
  

  

  With 
  regard 
  to 
  the 
  effect 
  of 
  excentricity 
  of 
  orbit, 
  a 
  small 
  proportion 
  of 
  it 
  should 
  make 
  a 
  comet 
  

   visible 
  for 
  a 
  long 
  period 
  on 
  either 
  side 
  of 
  the 
  perihelion, 
  from 
  the 
  lesser 
  degree 
  of 
  attenuation 
  and 
  expan- 
  

   sion 
  of 
  its 
  substance 
  at 
  a 
  distance 
  therefrom 
  ; 
  and 
  it 
  should 
  also 
  be 
  lost 
  in 
  the 
  sun's 
  rays 
  for 
  a 
  consi- 
  

   derable 
  time 
  at 
  and 
  about 
  the 
  perihelion 
  passage, 
  from 
  the 
  matter 
  never 
  being 
  compressed 
  into 
  a 
  suffi- 
  

   ciently 
  dense 
  body 
  to 
  be 
  visible 
  in 
  the 
  blaze 
  of 
  day. 
  This 
  rule 
  appears 
  well 
  borne 
  out 
  by 
  both 
  small 
  

   and 
  large 
  comets 
  ; 
  the 
  small 
  ones, 
  for 
  instance 
  Encke's, 
  Biela's, 
  and 
  Faye's, 
  which 
  have 
  for 
  the 
  ratio 
  

   of 
  the 
  excentricity 
  to 
  the 
  semiaxis 
  major, 
  the 
  numbers 
  respectively, 
  0-847, 
  0*755, 
  0-555, 
  shew 
  no 
  very 
  

  

  