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  VI. 
  — 
  Some 
  remarks 
  on 
  Theories 
  of 
  Cometary 
  Physics. 
  By 
  C. 
  Piazzi 
  Smyth, 
  Esq., 
  

   F.R.S.E., 
  Astronomer 
  Royal 
  for 
  Scotland, 
  and 
  Professor 
  of 
  Practical 
  Astro- 
  

   nomy 
  in 
  the 
  University 
  of 
  Edinburgh. 
  

  

  (Read 
  1st 
  April, 
  1850.) 
  

  

  While 
  the 
  physical 
  appearances 
  of 
  comets 
  have 
  ever 
  excited 
  such 
  intense 
  

   curiosit}*- 
  and 
  interest, 
  all 
  the 
  theories 
  concerning 
  them 
  are 
  generally 
  confessed 
  to 
  

   be 
  insufficient 
  to 
  explain 
  them 
  ; 
  and, 
  certainly, 
  if 
  we 
  may 
  judge 
  from 
  the 
  various 
  

   views 
  advocated 
  by 
  different 
  writers, 
  and 
  the 
  anomalous 
  forces 
  gratuitously 
  

   brought 
  in 
  to 
  support 
  the 
  different 
  hypotheses 
  — 
  it 
  is 
  so. 
  

  

  This 
  unsatisfactory 
  state 
  of 
  things, 
  so 
  different 
  from 
  that 
  in 
  which 
  is 
  the 
  

   theory 
  of 
  the 
  motions 
  of 
  comets, 
  — 
  seems 
  to 
  be 
  owing 
  partly 
  to 
  the 
  difficulty 
  of 
  

   making 
  the 
  necessary 
  observations 
  by 
  reason 
  of 
  the 
  undefinable 
  nature 
  of 
  the 
  bodies 
  

   themselves, 
  and 
  partly 
  from 
  the 
  untoward 
  circumstances 
  under 
  which 
  the 
  obser- 
  

   vations 
  must 
  be 
  made, 
  as 
  well 
  as 
  the 
  rareness 
  of 
  any 
  opportunities 
  offering. 
  

   Hence, 
  theories 
  are 
  built 
  upon 
  accounts 
  handed 
  down 
  from 
  old 
  astrological 
  times 
  

   when 
  men's 
  prejudices 
  would 
  have 
  prevented 
  them, 
  even 
  if 
  their 
  means 
  had 
  been 
  

   ample, 
  which 
  they 
  were 
  not, 
  from 
  giving 
  any 
  satisfactory 
  and 
  trustworthy 
  ac- 
  

   counts 
  of 
  the 
  phenomena 
  displayed 
  by 
  the 
  heavens 
  of 
  their 
  day. 
  

  

  Then, 
  again, 
  the 
  theories 
  appear 
  to 
  have 
  failed 
  from 
  attempting 
  too 
  much, 
  

   attempting 
  things 
  not 
  legitimately 
  within 
  their 
  reach 
  ; 
  it 
  would 
  have 
  been 
  enough 
  

   to 
  determine 
  the 
  laws 
  of 
  the 
  changes 
  which 
  the 
  tail 
  undergoes 
  during 
  the 
  orbit 
  

   of 
  a 
  comet 
  ; 
  but 
  in 
  place 
  of 
  this, 
  they 
  attempted 
  to 
  shew 
  why 
  the 
  tails 
  were 
  there, 
  

   and 
  how 
  they 
  came 
  into 
  existence. 
  This 
  is 
  as 
  much 
  as 
  in 
  the 
  planetary 
  theory 
  

   to 
  attempt 
  to 
  determine 
  why 
  Saturn 
  has 
  rings 
  ; 
  a 
  problem 
  which 
  would 
  have 
  

   eluded 
  the 
  grasp 
  even 
  of 
  Newton, 
  and 
  will 
  for 
  ever 
  remain 
  wrapped 
  up 
  in 
  the 
  

   mystery 
  of 
  creation 
  ; 
  enough 
  for 
  us 
  that 
  the 
  rings 
  are 
  there 
  ; 
  we 
  can 
  measure 
  

   their 
  diameter 
  and 
  thickness, 
  approximate 
  to 
  their 
  weight, 
  and 
  determine 
  the 
  

   laws 
  of 
  their 
  rotation, 
  and 
  alternate 
  appearance 
  and 
  disappearance 
  to 
  the 
  earth, 
  

   and 
  to 
  their 
  own 
  planet 
  ; 
  and 
  something 
  of 
  the 
  same 
  sort 
  we 
  may 
  expect 
  to 
  be 
  

   able 
  to 
  do 
  in 
  the 
  case 
  now 
  before 
  us. 
  

  

  It 
  has 
  been 
  remarked 
  that 
  theory 
  and 
  fact 
  sometimes 
  unite, 
  and 
  that 
  some- 
  

   thing 
  of 
  theory 
  is 
  necessary 
  to 
  enable 
  us 
  to 
  speak 
  correctly 
  of 
  facts. 
  Many 
  in- 
  

   stances 
  of 
  this 
  occur 
  in 
  the 
  history 
  of 
  most 
  of 
  the 
  sciences, 
  but 
  in 
  none 
  have 
  the 
  

   facts 
  been 
  more 
  misinterpreted 
  by 
  the 
  vulgar 
  feeling 
  of 
  the 
  senses, 
  in 
  the 
  absence 
  

   of 
  correct 
  theory, 
  than 
  in 
  the 
  case 
  of 
  the 
  physical 
  characteristics 
  of 
  comets. 
  No 
  

   phenomena 
  were 
  so 
  likely 
  to 
  be 
  misinterpreted 
  by 
  reason 
  of 
  the 
  strong 
  pre- 
  

   judices 
  almost 
  innate 
  in 
  men's 
  minds, 
  as 
  well 
  as 
  the 
  specious 
  and 
  inexplicable 
  

   character 
  of 
  the 
  appearances 
  themselves. 
  Accordingly, 
  because 
  the 
  tails 
  of 
  

  

  VOL. 
  XX. 
  PART 
  I. 
  2 
  N 
  

  

  