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  PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  

  

  owing 
  to 
  the 
  probable 
  minuteness 
  of 
  its 
  mass. 
  Cohesion 
  can 
  hardly 
  be 
  supposed 
  

   to 
  exist 
  in 
  a 
  gaseous 
  or 
  nebulous 
  body 
  of 
  such 
  tenuity 
  ; 
  so 
  that 
  the 
  only 
  bond 
  of 
  

   union 
  between 
  its 
  molecules 
  must 
  be 
  their 
  feeble 
  gravitation 
  to 
  each 
  other, 
  which 
  

   is 
  hardly 
  more 
  than 
  mere 
  juxtaposition 
  in 
  space. 
  Hence 
  we 
  must 
  regard 
  each 
  

   molecule 
  as 
  constituting 
  almost 
  a 
  separate, 
  independent 
  projectile, 
  describing 
  its 
  

   own 
  parabola 
  about 
  the 
  sun. 
  Now, 
  the 
  interval 
  between 
  two 
  or 
  more 
  parabolas 
  

   described 
  about 
  a 
  common 
  focus, 
  and 
  having 
  their 
  axes 
  coincident, 
  is 
  a 
  minimum 
  

   at 
  the 
  perihelion, 
  and 
  increases 
  as 
  we 
  recede 
  from 
  it" 
  in 
  the 
  sesquiplicate 
  ratio 
  of 
  

   the 
  radius 
  vector. 
  The 
  obervations 
  of 
  Encke's 
  comet, 
  which 
  Sir 
  John 
  treated 
  by 
  

   this 
  theory, 
  shewed 
  rather 
  a 
  more 
  rapid 
  rate 
  of 
  increase 
  and 
  decrease 
  ; 
  which 
  

   might, 
  he 
  thought, 
  be 
  readily 
  accounted 
  for 
  by 
  the 
  effect 
  of 
  the 
  brighter 
  back- 
  

   ground 
  of 
  sky 
  on 
  which 
  the 
  comet 
  was 
  projected 
  as 
  it 
  approached 
  its 
  perihelion, 
  

   and 
  vice 
  versa. 
  But 
  whether 
  any 
  other 
  forces 
  may 
  have 
  part 
  in 
  the 
  entire 
  pheno- 
  

   menon 
  presented, 
  he 
  concludes 
  that 
  the 
  property 
  above 
  pointed 
  out, 
  cannot 
  but 
  

   be 
  allowed 
  to 
  be 
  a 
  vera 
  causa, 
  and 
  to 
  have 
  some 
  share 
  in 
  the 
  production 
  of 
  the 
  

   effect. 
  

  

  To 
  the 
  latter 
  part 
  of 
  this 
  opinion 
  every 
  one 
  must 
  assent 
  ; 
  and 
  with 
  respect 
  

   to 
  the 
  want 
  of 
  agreement 
  between 
  theory 
  and 
  observation, 
  in 
  the 
  case 
  quoted 
  by 
  

   Sir 
  John 
  of 
  a 
  small 
  comet, 
  in 
  addition 
  to 
  the 
  observations 
  themselves 
  requiring 
  cor- 
  

   rection, 
  for 
  the 
  cause 
  he 
  has 
  mentioned, 
  the 
  theoretical 
  quantity 
  requires 
  it 
  also 
  on 
  

   account 
  of 
  the 
  greater 
  attraction 
  of 
  the 
  molecules 
  upon 
  each 
  other 
  at 
  the 
  perihelion, 
  

   by 
  reason 
  of 
  their 
  increased 
  proximity 
  ; 
  while, 
  moreover, 
  the 
  figure 
  of 
  the 
  comet, 
  and 
  

   the 
  direction 
  in 
  which 
  it 
  is 
  seen, 
  require 
  also 
  to 
  be 
  taken 
  into 
  consideration. 
  With 
  

   regard 
  to 
  large 
  comets, 
  which 
  seem 
  generally 
  to 
  have 
  been 
  thought 
  to 
  be 
  under 
  the 
  

   dominion 
  of 
  absolutely 
  different 
  laws, 
  the 
  decrease 
  and 
  concentration 
  of 
  the 
  tail 
  at 
  

   perihelio, 
  is 
  full}'' 
  accounted 
  for 
  by 
  this 
  12th 
  axiom, 
  as 
  well 
  as 
  some 
  other 
  pheno- 
  

   mena, 
  the 
  perplexing 
  nature 
  of 
  which, 
  when 
  viewed 
  by 
  the 
  light 
  of 
  any 
  other 
  theory, 
  

   may 
  be 
  gathered 
  by 
  the 
  account 
  given 
  by 
  Sir 
  J. 
  Herschel 
  himself, 
  at 
  the 
  conclusion 
  

   of 
  the 
  chapter 
  on 
  comets 
  in 
  his 
  work 
  of 
  last 
  year 
  {Outlines 
  of 
  Astronomy.) 
  

  

  " 
  It 
  is 
  in 
  a 
  physical 
  point 
  of 
  view 
  that 
  these 
  bodies 
  offer 
  the 
  greatest 
  stimulus 
  

   to 
  our 
  curiosity. 
  There 
  is, 
  beyond 
  question, 
  some 
  profound 
  secret 
  and 
  mystery 
  

   concerned 
  in 
  the 
  phenomena 
  of 
  their 
  tails. 
  Perhaps 
  it 
  is 
  not 
  too 
  much 
  to 
  hope 
  

   that 
  future 
  observations, 
  borrowing 
  every 
  aid 
  from 
  rational 
  speculation, 
  grounded 
  

   on 
  the 
  progress 
  of 
  physical 
  science 
  generally 
  (especially 
  those 
  branches 
  of 
  it 
  

   which 
  relate 
  to 
  the 
  ethereal 
  or 
  imponderable 
  elements), 
  may 
  ere 
  long 
  enable 
  us 
  

   to 
  penetrate 
  this 
  mystery, 
  and 
  to 
  declare 
  whether 
  it 
  is 
  really 
  matter, 
  in 
  the 
  ordi- 
  

   nary 
  acceptation 
  of 
  the 
  term, 
  which 
  is 
  projected 
  from 
  their 
  heads 
  with 
  such 
  extra- 
  

   vagant 
  velocity, 
  and 
  if 
  not 
  impelled, 
  at 
  least 
  directed, 
  in 
  its 
  course 
  by 
  a 
  reference 
  

   to 
  the 
  sun, 
  as 
  its 
  point 
  of 
  avoidance. 
  In 
  no 
  respect 
  is 
  the 
  question 
  as 
  to 
  the 
  

   materiality 
  of 
  the 
  tail 
  more 
  forcibly 
  pressed 
  on 
  us 
  for 
  consideration, 
  than 
  in 
  that 
  

   of 
  the 
  enormous 
  sweep 
  that 
  it 
  makes 
  round 
  the 
  sun 
  in 
  perihelio, 
  in 
  the 
  manner 
  of 
  

  

  