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  PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  

  

  ing 
  forth 
  from 
  one 
  side 
  only 
  of 
  the 
  nucleus, 
  that 
  body 
  should 
  be 
  driven 
  far 
  in 
  the 
  

   opposite 
  direction 
  : 
  but 
  by 
  comparing 
  its 
  observed 
  daily 
  places 
  during 
  the 
  peri- 
  

   helion 
  passages 
  with 
  the 
  computed, 
  we 
  find 
  that 
  no 
  deviations 
  from 
  any 
  such 
  

   anomalous 
  causes 
  are 
  ever 
  experienced. 
  

  

  All 
  these 
  difficulties, 
  however, 
  vanish 
  on 
  considering 
  the 
  envelope 
  of 
  a 
  comet 
  

   to 
  consist 
  of 
  separate 
  molecules, 
  each 
  constituting 
  an 
  independent 
  projectile, 
  and 
  

   bound 
  together 
  only 
  by 
  their 
  mutual 
  gravitation 
  and 
  the 
  laws 
  mentioned 
  above 
  ; 
  

   for 
  then 
  the 
  size, 
  character, 
  and 
  position 
  of 
  a 
  comet 
  being 
  given 
  at 
  the 
  perihelion, 
  

   which 
  we 
  must 
  look 
  on 
  as 
  the 
  normal 
  state, 
  all 
  its 
  principal 
  variations 
  of 
  appear- 
  

   ance 
  during 
  the 
  rest 
  of 
  the 
  orbit 
  may 
  be 
  readily 
  computed 
  ; 
  and 
  the 
  return 
  of 
  

   every 
  particle 
  of 
  the 
  envelope 
  to 
  perihelion, 
  or, 
  vulgarly 
  speaking, 
  the 
  retention 
  of 
  

   the 
  tail 
  by 
  the 
  nucleus, 
  will 
  be 
  no 
  more 
  surprising, 
  nor 
  deviating 
  from 
  ordinary 
  

   laws, 
  than 
  the 
  return 
  of 
  the 
  nucleus 
  itself: 
  certainly 
  there 
  is 
  nothing 
  " 
  in 
  defiance 
  

   of 
  the 
  laws 
  of 
  gravitation, 
  and 
  even 
  of 
  the 
  received 
  laws 
  of 
  motion," 
  as 
  stated 
  

   by 
  a 
  supporter 
  of 
  the 
  other 
  theory 
  to 
  be 
  the 
  case 
  with 
  that. 
  

  

  But 
  following 
  this 
  principle 
  further, 
  we 
  may 
  expect 
  that, 
  in 
  the 
  multitude 
  of 
  

   molecules 
  moving 
  about 
  amongst 
  each 
  other, 
  occasional 
  conglomerations 
  may 
  occur, 
  

   after 
  passing 
  the 
  proximity 
  of 
  some 
  large 
  planet, 
  whose 
  attraction 
  acting 
  much 
  

   more 
  strongly 
  on 
  one 
  part 
  of 
  the 
  envelope 
  than 
  the 
  other, 
  will 
  so 
  much 
  alter 
  the 
  

   motion 
  of 
  the 
  particles 
  therein, 
  that 
  they 
  will, 
  after 
  some 
  revolutions, 
  gradually 
  

   collect 
  together 
  at 
  a 
  distance 
  from 
  the 
  nucleus, 
  and 
  at 
  length 
  separate 
  and 
  become 
  

   a 
  distinct 
  comet. 
  Such 
  a 
  case 
  having 
  actually 
  occurred 
  under 
  our 
  own 
  eyes, 
  four 
  

   years 
  ago, 
  with 
  Biela's 
  comet, 
  when 
  it 
  was 
  not 
  at 
  the 
  time 
  under 
  the 
  immediate 
  

   influence 
  of 
  any 
  planet 
  in 
  particular, 
  nor 
  in 
  any 
  trying 
  part 
  of 
  its 
  orbit 
  round 
  the 
  

   sun, 
  adds 
  much 
  additional 
  weight 
  to 
  this 
  view 
  of 
  the 
  constitution 
  of 
  such 
  bodies. 
  

  

  This 
  brings 
  us 
  to 
  the 
  second 
  portion 
  of 
  the 
  subject, 
  viz., 
  the 
  corrections 
  which 
  

   should 
  be 
  applied 
  to 
  apparent 
  observations 
  to 
  deduce 
  the 
  real 
  phenomena. 
  

  

  A 
  comet 
  being 
  an 
  elongated, 
  gaseous, 
  elastic, 
  and 
  semitransparent 
  body, 
  

   varying 
  in 
  size 
  and 
  density 
  with 
  its 
  distance 
  from 
  the 
  sun, 
  evidently 
  requires 
  

   many 
  different 
  corrections, 
  according 
  to 
  the 
  point 
  of 
  view 
  and 
  the 
  distance 
  from 
  

   which 
  it 
  is 
  seen, 
  to 
  give 
  an 
  idea 
  of 
  its 
  real 
  nature 
  at 
  the 
  instant 
  of 
  observation 
  ; 
  

   and 
  needs 
  other 
  corrections, 
  to 
  reduce 
  it 
  from 
  that 
  instant 
  to 
  some 
  other 
  in 
  which 
  

   it 
  is 
  in 
  a 
  normal 
  condition. 
  This 
  normal 
  condition 
  is 
  plainly 
  in 
  perihelio 
  (though 
  

   a 
  better 
  general 
  comparison 
  of 
  the 
  volumes 
  of 
  different 
  comets 
  would 
  be 
  obtained 
  

   by 
  reducing 
  each 
  to 
  its 
  mean 
  distance, 
  as 
  they 
  would 
  then 
  be 
  all 
  of 
  much 
  more 
  nearly 
  

   equal 
  density), 
  and 
  viewed 
  at 
  right 
  angles 
  to 
  the 
  line 
  of 
  its 
  larger 
  axis. 
  This 
  is 
  a 
  

   position 
  which 
  seems 
  generally 
  to 
  have 
  been 
  taken 
  for 
  granted, 
  though 
  it 
  never 
  oc- 
  

   curs 
  even 
  approximately. 
  If 
  we 
  could 
  see 
  a 
  comet 
  at 
  the 
  instant 
  of 
  its 
  perihelio, 
  the 
  

   plane 
  of 
  its 
  orbit 
  being 
  inclined 
  90° 
  to 
  that 
  of 
  the 
  ecliptic, 
  and 
  the 
  radius 
  vector 
  

   being 
  infinitely 
  small, 
  the 
  above 
  view 
  would 
  be 
  nearly 
  obtained, 
  but 
  would 
  gradually 
  

  

  