﻿138 
  PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  

  

  well 
  marked 
  changes 
  or 
  even 
  characteristics 
  at 
  any 
  part 
  of 
  their 
  orbits, 
  and 
  are 
  soon 
  lost 
  in 
  the 
  

   twilight 
  even 
  in 
  their 
  densest 
  states 
  ; 
  there 
  being 
  little 
  compression, 
  and 
  by 
  axiom 
  4, 
  little 
  substance 
  

   to 
  compress, 
  the 
  mean 
  distances 
  being 
  only 
  2-216, 
  3-502, 
  and 
  3 
  - 
  812, 
  the 
  earth's 
  distance 
  being 
  unity. 
  

  

  In 
  the 
  case 
  of 
  Halley's 
  comet, 
  however, 
  the 
  appearances 
  are 
  very 
  different, 
  the 
  excentricity 
  being 
  

   0-967, 
  and 
  the 
  semiaxis 
  major 
  17'988 
  ; 
  hence, 
  on 
  this 
  large 
  body 
  approaching 
  the 
  sun 
  and 
  under- 
  

   going 
  such 
  a 
  much 
  more 
  intense 
  degree 
  of 
  compression, 
  distinctly 
  marked 
  changes 
  were 
  seen 
  almost 
  

   from 
  day 
  to 
  day, 
  and 
  at 
  a 
  certain 
  distance 
  from 
  the 
  perihelion 
  it 
  was 
  of 
  great 
  brightness. 
  But 
  the 
  

   perihelion 
  distance 
  being 
  still 
  large, 
  about 
  half 
  that 
  of 
  the 
  earth, 
  or 
  near 
  fifty 
  millions 
  of 
  miles, 
  the 
  

   condensation 
  was 
  not 
  sufficient 
  to 
  enable 
  the 
  comet 
  to 
  be 
  seen 
  in 
  moderate 
  twilight, 
  and 
  hence 
  it 
  was 
  

   not 
  seen 
  after 
  the 
  perihelion 
  passage 
  for 
  more 
  than 
  two 
  months, 
  but 
  then 
  remained 
  visible 
  for 
  nearly 
  

   four 
  months, 
  so 
  that 
  it 
  was 
  lost 
  sight 
  of 
  at 
  about 
  six 
  months 
  after 
  perihelion 
  passage. 
  

  

  The 
  great 
  comet 
  of 
  1844-5 
  had 
  a 
  less 
  perihelion 
  distance, 
  viz., 
  about 
  25 
  millions 
  of 
  miles, 
  and 
  

   a 
  mean 
  distance 
  probably 
  much 
  greater, 
  hence 
  it 
  was 
  sufficiently 
  concentrated 
  in 
  the 
  neighbourhood 
  of 
  

   the 
  sun 
  to 
  force 
  itself 
  on 
  the 
  notice 
  of 
  men 
  within 
  a 
  week 
  after 
  perihelion 
  : 
  which 
  implies 
  a 
  very 
  much 
  

   greater 
  degree 
  of 
  brightness, 
  than 
  if 
  Halley's 
  comet 
  had 
  been 
  seen 
  as 
  early, 
  when 
  powerful 
  telescopes, 
  

   directed 
  by 
  means 
  of 
  an 
  accurate 
  ephenieris, 
  were 
  employed 
  in 
  the 
  search. 
  This 
  comet 
  remained 
  in 
  

   sight 
  between 
  three 
  and 
  four 
  months, 
  and 
  when 
  last 
  seen 
  was 
  a 
  faint 
  nebulosity 
  with 
  little 
  or 
  no 
  

   apparent 
  concentration 
  in 
  any 
  part. 
  

  

  But 
  the 
  great 
  comet 
  of 
  1843 
  is 
  again 
  the 
  decisive 
  test, 
  as 
  this 
  had 
  a 
  perihelion 
  distance 
  of 
  only 
  

   half 
  a 
  million 
  miles, 
  60,000 
  only 
  from 
  the 
  surface 
  of 
  the 
  sun 
  : 
  here, 
  therefore, 
  we 
  might 
  expect 
  to 
  see 
  

   the 
  brightness 
  excessive 
  at 
  and 
  about 
  the 
  perihelion 
  ; 
  but 
  the 
  subsequent 
  expansion, 
  on 
  account 
  of 
  

   the 
  great 
  mean 
  distance, 
  would 
  be 
  so 
  rapid 
  that 
  the 
  comet 
  would 
  be 
  soon 
  lost 
  sight 
  of 
  by 
  reason 
  of 
  

   faintness. 
  Accordingly, 
  Ave 
  find 
  that 
  this 
  comet 
  pressed 
  itself 
  on 
  men's 
  attention 
  one 
  day 
  only 
  after 
  the 
  

   perihelion 
  passage 
  ; 
  and 
  from 
  its 
  being 
  so 
  very 
  bright 
  then, 
  and 
  yet 
  seen 
  by 
  so 
  few, 
  there 
  can 
  be 
  little 
  

   doubt 
  but 
  that 
  it 
  might 
  have 
  been 
  observed 
  the 
  day 
  before, 
  if 
  it 
  had 
  been 
  looked 
  for 
  ; 
  and 
  would 
  have 
  

   been 
  so 
  seen, 
  were 
  not 
  staring 
  into 
  the 
  sun's 
  face 
  and 
  immediate 
  vicinity 
  rather 
  a 
  trying, 
  and, 
  conse- 
  

   quently, 
  an 
  unpleasant 
  occupation 
  to 
  most 
  eyes, 
  and 
  seldom 
  indulged 
  in, 
  especially 
  in 
  the 
  warmer 
  

   countries 
  of 
  the 
  south, 
  when 
  the 
  sun 
  might 
  have 
  been 
  that 
  day 
  unveiled 
  from 
  cloud, 
  and 
  was 
  high 
  in 
  the 
  

   sky. 
  But, 
  however, 
  even 
  the 
  day 
  after 
  the 
  perihelion 
  passage, 
  when 
  the 
  comet 
  must 
  have 
  been 
  much 
  

   less 
  dense 
  than 
  at 
  that 
  epoch, 
  it 
  was 
  quite 
  bright 
  enough 
  to 
  be 
  seen 
  throughout 
  the 
  day 
  within 
  two 
  

   degrees 
  of 
  the 
  sun, 
  and 
  was 
  then 
  about 
  one 
  degree 
  in 
  apparent 
  length 
  ; 
  four 
  days 
  after 
  it 
  had 
  increased 
  

   to 
  25 
  degrees, 
  in 
  a 
  fortnight 
  to 
  double 
  that 
  ; 
  in 
  a 
  month 
  it 
  was 
  so 
  faint 
  and 
  distended 
  as 
  to 
  be 
  lost 
  to 
  

   most 
  person's 
  eyes, 
  and 
  powerful 
  telescopes 
  only 
  kept 
  it 
  in 
  sight 
  a 
  few 
  days 
  longer. 
  Its 
  meteor-like 
  

   brightness 
  and 
  short 
  ephemeral 
  existence 
  were 
  subjects 
  of 
  general 
  remark 
  in 
  the 
  south. 
  

  

  This 
  instance 
  may 
  be 
  considered 
  to 
  settle 
  the 
  matter, 
  but 
  Mr 
  Hind's 
  interesting 
  comet 
  of 
  1847 
  

   as 
  a 
  later 
  instance, 
  and 
  a 
  well-marked 
  one 
  also, 
  is 
  very 
  deserving 
  of 
  mention. 
  He 
  discovered 
  this 
  on 
  

   February 
  6, 
  1847, 
  as 
  an 
  exceedingly 
  faint 
  nebulous 
  body 
  approaching 
  perihelion 
  ; 
  he 
  observed 
  the 
  

   gradual 
  condensation 
  in 
  the 
  head 
  and 
  appearance 
  of 
  nucleus 
  and 
  tail, 
  this 
  last 
  being 
  about 
  a 
  degree 
  

   long 
  on 
  March 
  9 
  ; 
  and 
  having 
  computed 
  the 
  orbit 
  and 
  found 
  the 
  time 
  of 
  perihelion 
  passage 
  to 
  be 
  March 
  

   30-269, 
  Gr. 
  M. 
  T., 
  and 
  that 
  the 
  distance 
  from 
  the 
  sun 
  was 
  then 
  only 
  four 
  millions 
  of 
  miles, 
  he 
  called 
  

   general 
  attention 
  to 
  the 
  circumstance 
  under 
  the 
  hope 
  that, 
  1st, 
  the 
  comet 
  might 
  be 
  seen 
  in 
  daylight 
  on 
  

   that 
  day 
  ; 
  and, 
  2d, 
  that 
  a 
  long 
  tail 
  might 
  be 
  visible 
  in 
  the 
  evening 
  after 
  sunset. 
  In 
  the 
  former 
  he 
  was 
  

   borne 
  out 
  by 
  the 
  fact, 
  for 
  he 
  observed 
  the 
  comet 
  himself 
  with 
  a 
  refractor 
  of 
  7 
  inches 
  aperture 
  at 
  ll 
  h 
  

   a.m., 
  within 
  two 
  degrees 
  of 
  the 
  sun, 
  and 
  three 
  other 
  persons 
  are 
  recorded 
  to 
  have 
  witnessed 
  it 
  too. 
  I 
  

   examined 
  that 
  part 
  of 
  the 
  sky 
  myself 
  on 
  the 
  occasion, 
  but 
  with 
  a 
  telescope 
  of 
  only 
  3 
  '7 
  inches 
  aperture 
  

   could 
  see 
  nothing 
  : 
  Mr 
  Hind 
  himself 
  found 
  it 
  a 
  very 
  difficult 
  object 
  to 
  observe, 
  so 
  that 
  the 
  sizes 
  of 
  the 
  

   two 
  instruments 
  may 
  be 
  taken 
  as 
  giving 
  some 
  measure 
  of 
  its 
  visibility. 
  In 
  the 
  latter 
  supposition 
  he 
  

   was 
  not 
  confirmed 
  ; 
  for 
  no 
  person 
  saw 
  a 
  tail 
  after 
  sunset, 
  and 
  he 
  himself 
  says 
  that 
  the 
  tail 
  which 
  he 
  

   saw, 
  exceedingly 
  faint 
  certainly, 
  in 
  the 
  telescope 
  in 
  the 
  day 
  time, 
  very 
  nearly 
  at 
  the 
  epoch 
  of 
  the 
  peri- 
  

   helion 
  passage, 
  was 
  only 
  40" 
  long, 
  — 
  but 
  the 
  90th 
  part 
  of 
  its 
  length 
  21 
  days 
  before. 
  

  

  He 
  was 
  led 
  to 
  the 
  first 
  conclusion 
  by 
  the 
  consideration, 
  that 
  the 
  intensity 
  of 
  the 
  light 
  would 
  vary 
  

  

  as 
  -jg 
  — 
  2 
  (when 
  r 
  is 
  the 
  comet's 
  radius 
  vector, 
  and 
  A 
  its 
  true 
  distance 
  from 
  the 
  earth), 
  whence 
  the 
  

  

  comet 
  should 
  be 
  at 
  the 
  time 
  of 
  perihelion 
  230 
  times 
  brighter 
  than 
  that 
  on 
  March 
  8, 
  when 
  it 
  was 
  just 
  

   perceptible 
  to 
  the 
  naked 
  eye. 
  (Royal 
  Astronomical 
  Society's 
  Monthly 
  Notices, 
  vol. 
  vii., 
  p. 
  248.) 
  But 
  

   here 
  it 
  will 
  be 
  seen 
  that 
  with 
  regard 
  to 
  the 
  distance 
  from 
  the 
  sun 
  and 
  perihelion, 
  the 
  intensity 
  of 
  solar 
  

   illumination 
  alone 
  is 
  taken 
  account 
  of 
  ; 
  but 
  the 
  concentration 
  of 
  the 
  comet 
  at 
  the 
  perihelion 
  must 
  have 
  

   greatly 
  assisted 
  the 
  effect, 
  and 
  without 
  this 
  it 
  seems 
  pretty 
  certain 
  that 
  the 
  comet 
  would 
  not 
  have 
  

  

  

  