﻿PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  143 
  

  

  be 
  altered 
  as 
  the 
  comet 
  proceeded 
  in 
  its 
  orbit. 
  As 
  seen 
  from 
  the 
  sun, 
  the 
  comet 
  

   should 
  always 
  present 
  a 
  circular 
  body, 
  and 
  be 
  equally 
  illuminated 
  on 
  all 
  sides, 
  

   except 
  in 
  so 
  far 
  as 
  the 
  longer 
  axis 
  is 
  inclined 
  to 
  the 
  plane 
  of 
  the 
  orbit 
  : 
  when 
  the 
  

   comet 
  retreats 
  towards 
  the 
  aphelion, 
  the 
  point 
  of 
  view 
  from 
  the 
  earth 
  becoming 
  

   more 
  nearly 
  the 
  same 
  as 
  that 
  from 
  the 
  sun,— 
  the 
  comet 
  should 
  become 
  rounder 
  and 
  

   rounder, 
  as 
  well 
  as 
  larger 
  ; 
  and 
  this 
  is 
  found 
  actually 
  to 
  be 
  the 
  case, 
  — 
  the 
  tails 
  of 
  

   Halley's 
  and 
  those 
  of 
  1843 
  and 
  1844-5, 
  having 
  been, 
  at 
  and 
  about 
  the 
  perihelion 
  

   passage, 
  narrow, 
  and 
  intense, 
  and 
  becoming 
  at 
  the 
  last 
  instant 
  in 
  which 
  they 
  were 
  

   seen, 
  wide, 
  round, 
  and 
  diffuse. 
  

  

  This, 
  perhaps, 
  together 
  with 
  the 
  facts 
  of 
  phase 
  and 
  imperfect 
  transparency 
  

   (axioms 
  9 
  and 
  10), 
  is 
  sufficient 
  to 
  shew 
  the 
  importance 
  of 
  correcting 
  the 
  observa- 
  

   tions 
  for 
  both 
  the 
  terrestrial 
  and 
  the 
  solar 
  elements 
  of 
  effect 
  on 
  the 
  appearance 
  

   of 
  a 
  comet, 
  and 
  be 
  able 
  to 
  deduce 
  its 
  normal 
  condition. 
  That 
  there 
  may 
  be 
  other 
  

   changes 
  operating 
  is 
  very 
  probable, 
  but 
  be 
  that 
  as 
  it 
  may, 
  these 
  effects 
  of 
  

   geometry, 
  mechanics, 
  and 
  optics, 
  actually 
  exist 
  to 
  a 
  very 
  sensible 
  amount, 
  and 
  

   their 
  corrections 
  must 
  be 
  applied 
  before 
  we 
  can 
  expect 
  to 
  discover 
  any 
  of 
  the 
  

   residual 
  causes. 
  

  

  I 
  ought, 
  doubtless, 
  to 
  apologize 
  for 
  having 
  formed 
  opinions 
  different 
  from 
  Sir 
  

   J. 
  Herschel's 
  last, 
  as 
  he 
  is 
  confessedly 
  the 
  person, 
  above 
  all 
  others, 
  entitled 
  to 
  

   paramount 
  weight 
  in 
  cometary 
  physics 
  ; 
  and 
  it 
  may 
  be 
  that 
  I 
  have 
  not 
  properly 
  

   understood, 
  and 
  unintentionally 
  have 
  not 
  here 
  sufficiently 
  represented 
  the 
  reasons 
  

   upon 
  which 
  his 
  old 
  conclusions 
  have 
  been 
  altered, 
  and 
  on 
  which 
  he 
  has 
  thought 
  

   it 
  allowable 
  to 
  introduce 
  electrical 
  and 
  other 
  forces, 
  to 
  explain 
  phenomena 
  

   amongst 
  the 
  celestial 
  bodies 
  ; 
  and 
  I 
  would 
  therefore 
  refer 
  inquirers 
  to 
  his 
  works 
  

   themselves. 
  But 
  while 
  on 
  the 
  one 
  side, 
  I 
  hope 
  that 
  what 
  has 
  been 
  here 
  brought 
  for- 
  

   ward 
  with 
  regard 
  to 
  the 
  complete 
  body 
  of 
  a 
  comet 
  and 
  its 
  symmetrical 
  and 
  geome- 
  

   trical 
  form, 
  when 
  freed 
  of 
  the 
  effects 
  of 
  phase, 
  may 
  remove 
  one 
  of 
  the 
  objections 
  

   which 
  he 
  felt 
  to 
  allowing 
  the 
  all-sufficiency 
  of 
  gravity 
  acting 
  on 
  a 
  group 
  of 
  inde- 
  

   pendent 
  molecules 
  ; 
  on 
  the 
  other 
  side, 
  I 
  not 
  only 
  allow, 
  but 
  think 
  it 
  extremely 
  pro- 
  

   bable, 
  that 
  some 
  other 
  effects 
  besides 
  those 
  already 
  mentioned, 
  may 
  legitimately 
  

   occur 
  ; 
  and 
  if 
  heat 
  and 
  rotation 
  on 
  the 
  earth 
  produce 
  our 
  trade- 
  winds 
  and 
  hur- 
  

   ricanes, 
  much 
  greater 
  effects 
  may 
  follow 
  the 
  more 
  violent 
  alterations 
  of 
  tempera- 
  

   ture 
  and 
  velocity 
  of 
  motion 
  in 
  a 
  comet. 
  Further, 
  as 
  confirming 
  a 
  curious 
  feature 
  

   noticed 
  in 
  Halley's 
  comet, 
  by 
  Sir 
  John, 
  after 
  the 
  perihelion 
  passage, 
  viz., 
  a 
  long 
  

   thin 
  ray 
  posterior 
  to 
  the 
  nucleus, 
  to 
  which 
  it 
  might 
  perform 
  the 
  office, 
  he 
  suggest- 
  

   ed, 
  of 
  conveying 
  back 
  the 
  vapour 
  driven 
  off 
  in 
  front 
  at 
  perihelion 
  ; 
  I 
  would 
  men- 
  

   tion, 
  that 
  a 
  ray 
  of 
  the 
  same 
  sort 
  was 
  seen 
  posterior 
  to 
  the 
  nucleus 
  of 
  the 
  great 
  

   comet 
  of 
  1843, 
  of 
  extravagant 
  length 
  and 
  excessive 
  thinness, 
  appearing 
  as 
  a 
  very 
  

   fine 
  line 
  of 
  light, 
  and 
  traceable 
  for 
  many 
  degrees 
  up 
  the 
  tail 
  : 
  in 
  these 
  particu- 
  

   lars, 
  bearing 
  some 
  relation, 
  perhaps, 
  to 
  the 
  excentricity 
  of 
  the 
  orbit, 
  and 
  to 
  the 
  

  

  VOL. 
  XX. 
  PART 
  I. 
  2 
  Q 
  

  

  