﻿PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  139 
  

  

  been 
  visible 
  : 
  then 
  the 
  introduction 
  of 
  the 
  distance 
  from, 
  the 
  earth 
  does 
  not 
  appear 
  correct, 
  for 
  although 
  

   this 
  may 
  change 
  the 
  apparent 
  diameter 
  of 
  the 
  body, 
  it 
  does 
  not 
  at 
  all 
  alter 
  the 
  intrinsic 
  brightness 
  of 
  the 
  

   surface. 
  

  

  The 
  second 
  conclusion 
  he 
  was 
  led 
  to, 
  by 
  the 
  old 
  erroneous 
  idea 
  (to 
  use 
  his 
  own 
  words) 
  that 
  " 
  the 
  

   close 
  approach 
  of 
  the 
  comet 
  to 
  the 
  sun 
  would 
  be 
  likely 
  to 
  produce 
  a 
  tail 
  of 
  considerable 
  length 
  :" 
  

   but 
  in 
  place 
  of 
  so 
  doing 
  it 
  was 
  contracted 
  in 
  size 
  to 
  a 
  very 
  small 
  compass. 
  This 
  additional 
  instance 
  

   of 
  the 
  prevalence 
  of 
  an 
  idea 
  so 
  completely 
  the 
  reverse 
  of 
  the 
  fact, 
  will, 
  I 
  hope, 
  excuse 
  me 
  from 
  having 
  

   attempted 
  in 
  so 
  very 
  crude 
  a 
  manner 
  to 
  establish 
  what 
  appears 
  to 
  me 
  the 
  grand 
  statistical 
  truths 
  of 
  

   cometary 
  physics. 
  But 
  if 
  I 
  have 
  not 
  been 
  able 
  to 
  agree 
  with 
  Mr 
  Hind 
  in 
  his 
  physical 
  ideas, 
  I 
  must 
  

   express 
  my 
  testimony 
  of 
  his 
  high 
  standing 
  in 
  the 
  more 
  important 
  question 
  of 
  the 
  motions 
  of 
  comets 
  ; 
  

   here 
  he 
  has 
  indeed 
  filled 
  an 
  honourable 
  niche, 
  which 
  had 
  been 
  long, 
  if 
  not 
  always, 
  unfilled 
  in 
  the 
  

   cometary 
  credit 
  and 
  fame 
  of 
  this 
  country. 
  

  

  A 
  general 
  result 
  in 
  cometography, 
  certainly 
  following 
  the 
  establishment 
  of 
  this 
  axiom, 
  is, 
  that 
  

   when 
  the 
  length 
  of 
  the 
  tail 
  of 
  any 
  comet 
  of 
  celebrity 
  is 
  described 
  in 
  millions 
  of 
  miles, 
  a 
  very 
  favourite 
  

   method 
  with 
  most 
  writers, 
  it 
  will 
  be 
  absolutely 
  necessary 
  to 
  accompany 
  it 
  with 
  an 
  account 
  of 
  the 
  

   part 
  of 
  its 
  orbit, 
  where 
  the 
  comet 
  is 
  supposed 
  to 
  have 
  been 
  at 
  the 
  time 
  : 
  without 
  this, 
  the 
  statement 
  

   of 
  an 
  actual 
  length, 
  is 
  as 
  absurd 
  as 
  the 
  fixation 
  of 
  the 
  place 
  of 
  the 
  magnetic 
  pole, 
  without 
  a 
  date 
  

   being 
  attached, 
  

  

  11. 
  The 
  axis 
  of 
  the 
  tail 
  of 
  a 
  comet 
  is 
  straight 
  at 
  the 
  perihelion, 
  but 
  at 
  any 
  

   point 
  between 
  this 
  and 
  the 
  aphelion 
  is 
  curved, 
  and 
  is 
  concave 
  toward 
  the 
  latter, 
  

   the 
  radius 
  of 
  curvature 
  being 
  inversely 
  as 
  the 
  excentricity. 
  

  

  (11.) 
  This 
  I 
  will 
  not 
  attempt 
  to 
  lay 
  much 
  stress 
  upon 
  ; 
  but 
  certainly 
  the 
  tails 
  of 
  the 
  comets 
  of 
  

   Halley, 
  of 
  1843, 
  and 
  of 
  1844-5, 
  were 
  sensibly 
  straight 
  near 
  the 
  perihelion 
  ; 
  and 
  the 
  two 
  latter 
  

   became 
  curved 
  after 
  it, 
  the 
  former 
  more 
  than 
  the 
  latter, 
  and 
  they 
  were 
  concave 
  to 
  the 
  direction 
  in 
  

   which 
  they 
  we're 
  proceeding 
  ; 
  precisely 
  the 
  reverse 
  of 
  the 
  general 
  belief, 
  which 
  states 
  them 
  to 
  bend 
  

   backwards 
  at 
  the 
  extremity 
  of 
  the 
  tail, 
  as 
  if 
  experiencing 
  some 
  resistance, 
  when 
  whirled 
  round 
  the 
  

   perihelion 
  with 
  such 
  exceeding 
  velocity. 
  

  

  The 
  direction 
  in 
  which 
  those 
  two 
  comets 
  were 
  proceeding 
  at 
  the 
  time 
  was 
  towai'ds 
  the 
  aphelion 
  ; 
  

   and 
  I 
  have 
  not 
  had 
  any 
  opportunity 
  of 
  examining 
  a 
  large 
  comet 
  coming 
  up 
  to 
  the 
  perihelion. 
  The 
  

   great 
  comet 
  of 
  1843 
  would 
  have 
  been 
  sufficient 
  to 
  settle 
  the 
  question, 
  but 
  I 
  have 
  only 
  heard 
  of 
  one 
  

   person 
  (a 
  Commissariat 
  officer 
  voyaging 
  from 
  New 
  South 
  Wales 
  to 
  the 
  Cape), 
  who 
  saw 
  it 
  in 
  the 
  

   eastern 
  skies 
  before 
  sunrise 
  and 
  the 
  perihelion 
  passage 
  ; 
  and 
  he 
  had 
  made 
  no 
  observations. 
  

  

  12. 
  The 
  molecules 
  composing 
  the 
  envelope 
  of 
  a 
  comet 
  are 
  only 
  held 
  together 
  

   by 
  their 
  mutual 
  gravitation, 
  each 
  constituting 
  almost 
  a 
  separate 
  projectile, 
  and 
  

   describing 
  its 
  own 
  parabola 
  about 
  the 
  sun. 
  

  

  The 
  12th 
  axiom 
  is 
  Sir 
  John 
  Heeschel's, 
  and 
  taken 
  in 
  conjunction 
  with 
  the 
  

   others, 
  seems 
  generally 
  to 
  explain 
  all 
  the 
  principal 
  variations 
  in 
  appearance, 
  and 
  

   affords 
  ground 
  for 
  testing 
  each 
  exactly 
  by 
  calculation, 
  and 
  thereby 
  of 
  ascertain- 
  

   ing 
  what 
  residual 
  phenomena 
  may 
  be 
  due 
  to 
  laws 
  others 
  than 
  those 
  of 
  gravita- 
  

   tion, 
  mechanics, 
  and 
  optics. 
  

  

  After 
  alluding 
  to 
  the 
  observed 
  concentration 
  of 
  Encke's 
  comet 
  near 
  perihelio, 
  

   and 
  the 
  error 
  of 
  attempting 
  to 
  account 
  for 
  it 
  by 
  the 
  pressure 
  of 
  a 
  supposed 
  aether 
  

   in 
  the 
  vicinity 
  of 
  the 
  sun, 
  Sir 
  John 
  says 
  {Royal 
  Astronomical 
  Society's 
  Memoirs, 
  

   vol. 
  vi.), 
  " 
  It 
  appears 
  to 
  me 
  that 
  the 
  phenomenon 
  is 
  (if 
  not 
  wholly, 
  at 
  least 
  par- 
  

   tially) 
  explicable 
  on 
  a 
  much 
  less 
  gratuitous 
  supposition, 
  viz., 
  that 
  of 
  the 
  extremely 
  

   feeble 
  attractive 
  force 
  by 
  which 
  the 
  matter 
  of 
  a 
  comet 
  must 
  be 
  held 
  together, 
  

   vol. 
  xx. 
  part 
  i. 
  2 
  p 
  

  

  