﻿PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  141 
  

  

  a 
  straight 
  rigid 
  rod, 
  in 
  defiance 
  of 
  the 
  law 
  of 
  gravitation, 
  nay, 
  even 
  of 
  the 
  received 
  

   laws 
  of 
  motion, 
  extending 
  (as 
  we 
  have 
  seen 
  in 
  the 
  comets 
  of 
  1680 
  and 
  1843)* 
  

   from 
  near 
  the 
  sun's 
  surface 
  to 
  the 
  earth's 
  orbit, 
  yet 
  whirled 
  round 
  unbroken; 
  in 
  

   the 
  latter 
  case 
  through 
  an 
  angle 
  of 
  180° 
  in 
  little 
  more 
  than 
  two 
  hours. 
  It 
  seems 
  

   utterly 
  incredible 
  that, 
  in 
  such 
  a 
  case, 
  it 
  is 
  one 
  and 
  the 
  same 
  material 
  object 
  

   which 
  is 
  thus 
  brandished." 
  

  

  This 
  and 
  much 
  more 
  to 
  a 
  similar 
  effect 
  might 
  be 
  quoted 
  from 
  Sir 
  J. 
  Her- 
  

   schel, 
  and 
  other 
  authors, 
  as 
  to 
  the 
  difficulties 
  experienced 
  in 
  the 
  usual 
  method 
  

   of 
  viewing 
  a 
  comet, 
  as 
  a 
  planetary 
  body 
  at 
  the 
  nucleus, 
  with 
  an 
  appendage 
  

   attached 
  to, 
  and 
  whirled 
  along 
  with 
  it 
  and 
  by 
  it. 
  No 
  wonder 
  that 
  doubts 
  were 
  

   expressed 
  as 
  to 
  the 
  attractive 
  force 
  of 
  the 
  small 
  nucleus 
  being 
  able 
  to 
  retain 
  

   within 
  its 
  grasp 
  portions 
  of 
  matter 
  thrown 
  out 
  to 
  such 
  distances 
  ; 
  and 
  that 
  fears 
  

   were 
  expressed 
  as 
  to 
  the 
  breaking 
  off 
  of 
  the 
  tail 
  ; 
  and, 
  because 
  some 
  thought 
  that 
  

   it 
  ought 
  to 
  bend 
  backwards 
  from 
  the 
  resistance 
  experienced 
  in 
  its 
  course, 
  there- 
  

   fore 
  they 
  said 
  that 
  they 
  did 
  see 
  it 
  bend. 
  Other 
  difficulties 
  also 
  follow 
  from 
  the 
  

   usual 
  mechanical 
  view 
  of 
  the 
  production 
  of 
  the 
  tail 
  at 
  perihelio, 
  as 
  has 
  been 
  

   stated 
  by 
  many, 
  from 
  the 
  heat 
  of 
  the 
  sun 
  causing 
  the 
  nucleus 
  to 
  throw 
  out 
  jets 
  

   of 
  vapour 
  on 
  the 
  side 
  of 
  that 
  luminary, 
  which 
  again 
  has 
  the 
  power 
  to 
  bend 
  them 
  

   back, 
  and 
  sending 
  them 
  streaming 
  past 
  the 
  nucleus 
  once 
  more, 
  forms 
  the 
  tail. 
  

   These 
  jets 
  of 
  vapour 
  ought 
  to 
  drive 
  the 
  nucleus 
  away 
  from 
  the 
  sun; 
  for 
  though 
  

   it 
  may 
  be 
  said 
  that 
  the 
  vapour, 
  being 
  nearly 
  imponderable, 
  should 
  not 
  produce 
  

   any 
  sensible 
  or 
  visible 
  effect 
  on 
  the 
  nucleus, 
  — 
  yet 
  the 
  nucleus 
  itself 
  is, 
  for 
  any- 
  

   thing 
  we 
  know, 
  as 
  imponderable 
  ; 
  indeed, 
  if 
  we 
  judge 
  of 
  the 
  masses 
  of 
  the 
  two 
  

   by 
  the 
  quantity 
  of 
  light 
  reflected, 
  which 
  is 
  almost 
  the 
  only 
  indication 
  we 
  have, 
  

   and 
  perhaps 
  not 
  a 
  very 
  bad 
  one, 
  then 
  the 
  mass 
  of 
  the 
  tail 
  in 
  most 
  cases 
  exceeds 
  

   that 
  of 
  the 
  nucleus, 
  i. 
  e., 
  if 
  the 
  quantity 
  of 
  light 
  reflected 
  by 
  the 
  whole 
  envelope 
  

   were 
  to 
  be 
  concentrated 
  into 
  a 
  single 
  point, 
  it 
  would 
  be 
  brighter 
  than 
  the 
  nucleus. 
  

   Hence 
  with 
  the 
  extravagant 
  rapidity 
  and 
  the 
  enormous 
  quantity 
  of 
  vapour 
  rush- 
  

  

  * 
  This 
  is 
  not 
  stated 
  with 
  perfect 
  correctness, 
  at 
  least 
  with 
  regard 
  to 
  the 
  comet 
  of 
  1843, 
  

   which 
  might 
  have 
  had 
  a 
  tail 
  of 
  that 
  length 
  some 
  days 
  after 
  the 
  perihelion 
  passage, 
  when 
  it 
  had 
  

   grown 
  with 
  the 
  rapid 
  increase 
  of 
  its 
  radius 
  vector; 
  hut 
  the 
  first 
  day 
  after 
  the 
  perihelion 
  passage, 
  

   the 
  tail 
  was 
  observed 
  to 
  be 
  only 
  double 
  the 
  sun's 
  diameter 
  (excluding 
  inclination), 
  and 
  its 
  distance 
  

   must 
  then 
  have 
  been 
  100 
  times 
  greater 
  than 
  at 
  the 
  perihelion 
  ; 
  so 
  that 
  if 
  the 
  sesquiplicate 
  ratio 
  holds 
  

   good, 
  we 
  shall 
  have 
  for 
  that 
  epoch 
  a 
  size 
  not 
  very 
  different 
  from 
  planetary 
  bodies. 
  (A 
  curious 
  meet- 
  

   ing 
  this 
  must 
  be 
  of 
  the 
  molecules 
  brought 
  for 
  an 
  instant 
  into 
  such 
  close 
  proximity, 
  after 
  having 
  been 
  

   separated 
  for 
  ages 
  by 
  distances 
  so 
  vast 
  and 
  inconceivable 
  as 
  they 
  must 
  be 
  at 
  the 
  aphelion 
  ; 
  and 
  when 
  

   separating 
  for 
  their 
  diverse 
  orbits, 
  what 
  speculations 
  on 
  their 
  next 
  meeting, 
  not 
  in 
  thunder, 
  lightning, 
  

   and 
  in 
  rain, 
  but 
  in 
  light 
  and 
  heat 
  unspeakable. 
  On 
  the 
  last 
  occasion, 
  February 
  1843, 
  the 
  heat 
  was 
  

   equal 
  (according 
  to 
  Sir 
  J. 
  Herschel) 
  to 
  47,000 
  of 
  our 
  suns, 
  1900 
  whereof 
  are 
  sufficient 
  to 
  melt 
  the 
  

   very 
  rocks. 
  Such, 
  at 
  least, 
  must 
  have 
  been 
  the 
  heat, 
  if 
  the 
  comet 
  travelled 
  at 
  that 
  part 
  of 
  its 
  orbit 
  only 
  

   at 
  the 
  mean 
  rate 
  of 
  the 
  earth 
  ; 
  but 
  the 
  velocity 
  was 
  really 
  vastly 
  greater, 
  and 
  the 
  heat 
  much 
  modified 
  

   thereby. 
  The 
  degree 
  to 
  which 
  velocity 
  in 
  the 
  heavenly 
  spaces 
  may 
  modify 
  distance, 
  in 
  respect 
  of 
  heat, 
  

   is 
  one 
  still 
  open 
  for 
  inquiry 
  ; 
  and 
  the 
  result, 
  in 
  the 
  case 
  of 
  our 
  own 
  earth, 
  as 
  far 
  as 
  it 
  may 
  have 
  been 
  

   very 
  imperfectly 
  examined 
  into, 
  would 
  lead 
  us 
  to 
  expect 
  that 
  the 
  above 
  proportion 
  would 
  be 
  greatly 
  

   reduced.) 
  

  

  