﻿134 
  PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  

  

  (5.) 
  This 
  may 
  not 
  be 
  strictly 
  true, 
  but 
  yet 
  is 
  assuredly 
  a 
  vei-y 
  marked 
  feature 
  in 
  the 
  statistics 
  of 
  

   the 
  question. 
  The 
  great 
  comet 
  of 
  1843, 
  whose 
  orbit 
  was 
  the 
  most 
  excentric 
  ever 
  known, 
  i. 
  e., 
  had 
  

   the 
  least 
  perihelion, 
  but 
  great 
  aphelion, 
  distance, 
  — 
  had 
  also 
  the 
  longest 
  and 
  narrowest 
  tail, 
  and 
  the 
  

   smallest 
  head 
  ; 
  consequently 
  the 
  nucleus 
  situated 
  near 
  the 
  centre 
  of 
  the 
  latter 
  was 
  most 
  excentrically 
  

   situated 
  in 
  the 
  gaseous 
  envelope. 
  Halley's 
  comet, 
  and 
  that 
  of 
  1811, 
  of 
  less 
  excentricity 
  of 
  orbit, 
  

   had 
  shorter 
  and 
  broader 
  tails 
  and 
  larger 
  heads 
  ; 
  and 
  their 
  nuclei, 
  consequently, 
  less 
  excentric 
  : 
  while 
  

   the 
  telescopic 
  comets 
  of 
  short 
  period, 
  and 
  aphelion 
  not 
  extraordinarily 
  greater 
  than 
  their 
  perihelion 
  

   distances, 
  exhibit 
  merely 
  somewhat 
  oval 
  masses 
  of 
  vapour. 
  

  

  6. 
  A 
  comet 
  revolves 
  on 
  an 
  axis 
  passing 
  through 
  the 
  nucleus, 
  and 
  at 
  right 
  

   angles 
  to 
  the 
  major 
  axis 
  of 
  the 
  envelope, 
  in 
  the 
  same 
  period 
  of 
  time 
  that 
  it 
  takes 
  

   to 
  revolve 
  about 
  the 
  sun 
  : 
  hence, 
  the 
  tail 
  being 
  turned 
  away 
  from 
  the 
  sun 
  in 
  the 
  

   normal 
  position, 
  is 
  turned 
  away 
  from 
  him 
  in 
  all 
  other 
  parts 
  of 
  the 
  orbit 
  also. 
  

  

  (6.) 
  Every 
  comet 
  has 
  invariably 
  been 
  observed 
  to 
  have 
  its 
  tail 
  turned 
  away 
  from 
  the 
  sun 
  in 
  

   every 
  part 
  of 
  its 
  orbit 
  ; 
  this 
  was 
  the 
  first 
  notable 
  fact 
  established 
  in 
  cometary 
  physics, 
  and 
  the 
  axiom 
  

   is 
  but 
  a 
  different 
  statement 
  of 
  it. 
  

  

  7. 
  This 
  axis 
  is 
  not 
  at 
  right 
  angles 
  to 
  the 
  plane 
  of 
  the 
  orbit, 
  but 
  variously 
  in- 
  

   clined 
  in 
  the 
  case 
  of 
  different 
  comets, 
  as 
  with 
  the 
  planets. 
  

  

  (7-) 
  There 
  is 
  no 
  reason 
  to 
  expect 
  the 
  contrary, 
  indeed, 
  analogy 
  rather 
  leads 
  us 
  to 
  this 
  conclusion, 
  

   and 
  it 
  may, 
  if 
  admitted, 
  be 
  sufficient 
  to 
  explain 
  the 
  apparent 
  want 
  of 
  symmetry 
  observed 
  in 
  the 
  tail 
  

   of 
  Halley's 
  comet, 
  that 
  of 
  1819, 
  and 
  most, 
  if 
  not 
  all, 
  which 
  have 
  been 
  the 
  subject 
  of 
  special 
  atten- 
  

   tion 
  ; 
  and 
  it 
  may 
  tend 
  to 
  account 
  for 
  some 
  of 
  the 
  differences 
  in 
  the 
  appearance 
  of 
  the 
  former 
  body 
  in 
  

   approaching 
  and 
  leaving 
  its 
  perihelion, 
  at 
  considerable 
  but 
  equal 
  distances 
  on 
  either 
  side 
  of 
  that 
  point. 
  

  

  8. 
  A 
  quicker 
  rotation 
  round 
  the 
  longer 
  axis 
  of 
  the 
  body 
  also 
  appears 
  to 
  

   exist. 
  

  

  (8.) 
  This 
  seemed 
  to 
  be 
  almost 
  proved 
  by 
  some 
  of 
  the 
  changes 
  which 
  took 
  place 
  in 
  the 
  head 
  of 
  

   the 
  great 
  comet 
  of 
  1843, 
  night 
  after 
  night, 
  in 
  the 
  earlier 
  part 
  of 
  its 
  apparition 
  ; 
  for 
  instance, 
  a 
  double- 
  

   winged 
  head, 
  laterally, 
  one 
  night, 
  becoming 
  a 
  single 
  and 
  centrically 
  winged, 
  or 
  rather 
  a 
  tailed-head 
  

   the 
  next 
  night 
  ; 
  but 
  when 
  a 
  body 
  is 
  seen 
  for 
  so 
  very 
  short 
  a 
  space 
  of 
  time, 
  for 
  a 
  few 
  minutes 
  only 
  in 
  

   twenty-four 
  hours 
  ; 
  and 
  sometimes, 
  perhaps, 
  for 
  several 
  days, 
  even 
  that 
  short 
  glimpse 
  is 
  prevented 
  

   by 
  clouds, 
  — 
  it 
  becomes 
  extremely 
  difficult 
  to 
  separate 
  in 
  such 
  a 
  body 
  as 
  a 
  comet, 
  in 
  which 
  there 
  is 
  

   nothing 
  decided 
  and 
  tangible, 
  and 
  fixed 
  either 
  in 
  size 
  or 
  brightness, 
  any 
  indications 
  of 
  revolution 
  

   from 
  those 
  of 
  the 
  other 
  motions 
  and 
  changes 
  which 
  are 
  going 
  on 
  simultaneously. 
  But 
  it 
  seems 
  a 
  

   point 
  well 
  worthy 
  of 
  attention, 
  and 
  to 
  be 
  proved 
  or 
  disproved. 
  

  

  9. 
  A 
  comet 
  shines 
  by 
  reflected 
  light, 
  and 
  shews 
  a 
  sensible 
  phase 
  ; 
  the 
  quan- 
  

   tity, 
  form, 
  and 
  position, 
  therefore, 
  of 
  its 
  component 
  matter, 
  cannot 
  be 
  judged 
  of 
  

   by 
  the 
  eye 
  alone. 
  

  

  (9.) 
  That 
  comets 
  shine 
  by 
  reflected 
  light, 
  is 
  considered 
  to 
  have 
  been 
  proved 
  by 
  Arago's 
  polarizing 
  

   experiment 
  ; 
  and 
  was 
  inferred 
  before 
  by 
  every 
  analogy 
  in 
  the 
  planetary 
  system 
  ; 
  but 
  all 
  appearance 
  

   of 
  phase 
  has 
  been 
  denied, 
  this, 
  therefore, 
  requires 
  a 
  little 
  explanation. 
  The 
  supposed 
  absence 
  of 
  phase 
  

   has 
  been 
  attributed 
  to 
  the 
  excessive 
  tenuity 
  of 
  the 
  matter 
  of 
  the 
  comet, 
  and 
  the 
  case 
  has 
  been 
  illustrated 
  

   by 
  reference 
  to 
  the 
  thin 
  clouds 
  often 
  seen 
  in 
  the 
  west 
  after 
  sunset, 
  or 
  in 
  the 
  east 
  before 
  sunrise, 
  

   glowing 
  in 
  the 
  solar 
  rays, 
  literally 
  drenched 
  with 
  light, 
  and 
  exhibiting 
  no 
  distinction 
  of 
  light 
  and 
  

   dark 
  side. 
  A 
  little 
  examination 
  of 
  this 
  instance 
  would 
  have 
  shewn 
  that 
  the 
  conclusion 
  is 
  not 
  so 
  

   safe 
  ; 
  the 
  whole 
  of 
  the 
  cloud 
  being 
  so 
  bright, 
  the 
  difference 
  of 
  illumination 
  of 
  the 
  two 
  sides 
  of 
  it 
  is 
  

  

  

  