﻿PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  133 
  

  

  served 
  ; 
  but, 
  at 
  the 
  actual 
  distances 
  at 
  which 
  they 
  are 
  viewed, 
  the 
  most 
  powerful 
  telescopes 
  never 
  

   show 
  these 
  atmospheres 
  in 
  the 
  same 
  manner 
  as 
  those 
  of 
  a 
  comet 
  ; 
  they 
  are 
  indicated 
  only 
  by 
  a 
  very 
  

   different 
  order 
  of 
  phenomena. 
  

  

  When 
  Uranus 
  was 
  first 
  discovered, 
  and 
  no 
  one 
  dreamt 
  of 
  planets 
  beyond 
  Saturn, 
  it 
  was 
  called 
  

   a 
  comet 
  ; 
  not 
  because 
  its 
  form 
  was 
  like 
  that 
  of 
  any 
  recognised 
  comet, 
  but 
  because 
  it 
  was 
  expected 
  

   that 
  its 
  orbit 
  would 
  prove 
  similar 
  ; 
  when, 
  however, 
  the 
  real 
  nature 
  of 
  its 
  path 
  was 
  discovered, 
  the 
  

   appellation 
  of 
  comet 
  was 
  quickly 
  retracted. 
  

  

  So 
  much 
  for 
  the 
  necessity 
  of 
  a 
  gaseous 
  envelope 
  ; 
  of 
  the 
  equal 
  importance 
  of 
  a 
  nucleus, 
  it 
  may 
  

   be 
  remarked, 
  that 
  although 
  some 
  comets 
  are 
  described 
  as 
  having 
  nuclei, 
  and 
  others 
  as 
  having 
  none 
  ; 
  

   this 
  turns 
  out 
  to 
  be 
  but 
  negative 
  testimony, 
  inasmuch 
  as 
  these 
  latter 
  bodies 
  have 
  always 
  been 
  the 
  

   fainter, 
  smaller, 
  and 
  more 
  distant 
  ones, 
  in 
  which 
  the 
  nucleus 
  should 
  have 
  been 
  so 
  much 
  the 
  more 
  

   difficult 
  to 
  distinguish 
  ; 
  and 
  if 
  it 
  has 
  not 
  been 
  actually 
  observed 
  itself, 
  there 
  has 
  at 
  least 
  been 
  in- 
  

   variably 
  noticed 
  in 
  every 
  recorded 
  comet, 
  some 
  one 
  point 
  where 
  the 
  gaseous 
  matter 
  was 
  visibly 
  more 
  

   concentrated 
  than 
  in 
  other 
  parts, 
  indicating 
  thus 
  a 
  virtual 
  or 
  a 
  dark 
  nucleus, 
  if 
  not 
  an 
  actual 
  and 
  

   a 
  reflective 
  one 
  : 
  while 
  observation, 
  combined 
  with 
  calculation, 
  has 
  satisfactorily 
  shewn, 
  that 
  in 
  

   comets 
  of 
  every 
  degree 
  of 
  size 
  and 
  excentricity, 
  the 
  mass 
  is 
  so 
  very 
  nearly 
  concentrated 
  in 
  this 
  

   nucleoid 
  centre, 
  that 
  that 
  need 
  alone 
  be 
  referred 
  to 
  in 
  all 
  determinations 
  of 
  the 
  orbit. 
  

  

  2. 
  The 
  nucleus 
  if 
  solid 
  and 
  material, 
  is 
  exceedingly 
  small. 
  

  

  (2.) 
  Every 
  advance 
  of 
  our 
  knowledge 
  has 
  tended 
  to 
  diminish 
  the 
  possible 
  size 
  of 
  the 
  solid 
  nuclei 
  

   of 
  comets, 
  planetary 
  perturbation 
  has 
  shewn 
  them 
  to 
  have 
  no 
  sensible 
  mass, 
  and 
  telescopic 
  observa- 
  

   tion 
  no 
  sensible 
  size 
  ; 
  and 
  in 
  the 
  cases 
  of 
  comets 
  of 
  all 
  sizes, 
  observers 
  have 
  witnessed 
  them 
  pass 
  

   over 
  stars 
  in 
  every 
  position, 
  except, 
  perhaps, 
  exactly 
  centrically 
  with 
  the 
  nucleus, 
  without 
  perceiving 
  

   any 
  obscuration 
  of 
  the 
  stellar 
  rays. 
  

  

  The 
  old 
  observers 
  have 
  certainly 
  spoken 
  of 
  very 
  large 
  nuclei, 
  but 
  they 
  evidently 
  meant 
  rather 
  

   the 
  head, 
  which, 
  in 
  some 
  comets 
  at 
  certain 
  parts 
  of 
  the 
  orbit, 
  presents 
  in 
  small 
  telescopes 
  an 
  ap- 
  

   pearance 
  of 
  planetary 
  opacity 
  and 
  definition. 
  

  

  Such 
  was 
  the 
  case 
  with 
  the 
  great 
  comet 
  of 
  1843, 
  for 
  three 
  or 
  four 
  days 
  after 
  having 
  passed 
  its 
  

   perihelion 
  ; 
  in 
  small, 
  telescopes 
  it 
  was 
  difficult 
  to 
  avoid 
  believing 
  in 
  the 
  existence 
  of 
  an 
  actual 
  planetary 
  

   nucleus 
  of 
  very 
  notable 
  size 
  ; 
  but 
  the 
  fourteen 
  feet 
  reflector 
  of 
  the 
  Cape 
  Observatory 
  shewed 
  the 
  

   borders 
  of 
  this 
  head 
  to 
  be 
  filmy, 
  and 
  exhibited 
  small 
  stars 
  shining 
  through 
  it 
  ; 
  day 
  after 
  day 
  it 
  expanded 
  

   and 
  became 
  less 
  defined, 
  until 
  at 
  last 
  it 
  ceased 
  to 
  present 
  a 
  solid 
  appearance 
  in 
  any 
  telescope 
  ; 
  and 
  at 
  

   no 
  time 
  was 
  there 
  anything 
  larger 
  than 
  a 
  stellar 
  point, 
  to 
  which 
  the 
  attribute 
  of 
  hard 
  or 
  heavy 
  matter 
  

   might 
  be 
  expected 
  to 
  apply. 
  

  

  3. 
  The 
  nucleus 
  is 
  excentrically 
  situated 
  in 
  the 
  gaseous 
  body. 
  

  

  (3.) 
  The 
  nucleus 
  actual 
  or 
  virtual, 
  has 
  never 
  been 
  observed 
  in 
  the 
  middle 
  of 
  the 
  envelope, 
  but 
  

   always 
  nearer 
  one 
  end 
  than 
  the 
  other, 
  the 
  envelopes 
  too, 
  never 
  being 
  round, 
  but 
  invariably 
  more 
  or 
  

   less 
  elongated. 
  

  

  4. 
  Comets 
  of 
  longest 
  period 
  have 
  the 
  largest 
  bodies. 
  

  

  (4.) 
  This 
  is 
  the 
  general 
  result 
  of 
  cometary 
  statistics, 
  but 
  need 
  not 
  be 
  any 
  more 
  strictly 
  true, 
  than 
  

   that 
  the 
  largest 
  planets 
  are 
  all 
  at 
  the 
  greatest 
  distances 
  from 
  the 
  sun 
  ; 
  they 
  are 
  not 
  strictly 
  ranged 
  in 
  the 
  

   order 
  of 
  distance 
  agreeably 
  with 
  size, 
  but 
  as 
  a 
  general 
  rule 
  merely, 
  the 
  smaller 
  are 
  closer 
  to 
  the 
  sun 
  

   than 
  the 
  larger 
  planets. 
  In 
  the 
  same 
  way 
  the 
  telescopic 
  comets, 
  when 
  sufficiently 
  numerous 
  obser- 
  

   vations 
  have 
  been 
  obtained, 
  have 
  almost 
  always 
  been 
  found 
  to 
  have 
  short 
  periods, 
  and 
  those 
  very 
  

   brilliant 
  ones, 
  with 
  not 
  only 
  long 
  but 
  broad 
  and 
  dense 
  tails, 
  have 
  invariably 
  been 
  found 
  to 
  be 
  of 
  long 
  

   period. 
  

  

  5. 
  Those 
  comets 
  whose 
  orbits 
  have 
  the 
  greatest 
  excentricity, 
  are 
  the 
  most 
  

   excentrically 
  situated 
  in 
  their 
  envelopes, 
  or, 
  vulgarly 
  speaking, 
  have 
  the 
  longest 
  

   tails. 
  

  

  