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  PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  

  

  comets 
  were 
  seen 
  chiefly 
  at 
  or 
  about 
  the 
  perihelion 
  passages, 
  they 
  were 
  said 
  to 
  

   be 
  produced 
  then 
  ; 
  to 
  have 
  been 
  shot 
  out, 
  and 
  then 
  drawn 
  in 
  again, 
  or 
  dissipated 
  ; 
  

   and 
  numerous 
  have 
  been 
  the 
  theories 
  to 
  explain 
  this 
  creation 
  and 
  extinction. 
  

   And 
  yet 
  of 
  all 
  the 
  facts 
  that 
  have 
  been 
  ascertained, 
  if 
  any 
  of 
  them 
  can 
  be 
  so 
  con- 
  

   sidered, 
  with 
  regard 
  to 
  the 
  physical 
  appearances 
  of 
  comets, 
  of 
  none 
  may 
  we 
  be 
  

   more 
  sure 
  than 
  that 
  the 
  tails 
  of 
  comets, 
  in 
  place 
  of 
  being 
  largest, 
  or 
  existing 
  only 
  

   at 
  the 
  perihelion 
  point 
  of 
  the 
  orbits, 
  are 
  then 
  the 
  smallest. 
  Comets 
  of 
  every 
  size, 
  

   (the 
  distinction 
  of 
  those 
  said 
  to 
  be 
  with 
  and 
  without 
  tails 
  is 
  visionary, 
  or 
  rather 
  

   the 
  tail 
  is 
  equally 
  a 
  part 
  of 
  the 
  general 
  body 
  of 
  the 
  comet, 
  as 
  the 
  so-called 
  head, 
  

   and 
  obeys 
  the 
  same 
  laws), 
  when 
  accurately 
  observed, 
  have 
  always 
  been 
  found 
  to 
  

   decrease 
  in 
  coming 
  to 
  perihelion, 
  and 
  to 
  increase 
  in 
  size 
  in 
  retreating 
  therefrom 
  ; 
  

   this 
  condensation 
  of 
  substance, 
  producing 
  more 
  power 
  to 
  reflect 
  light 
  at 
  that 
  

   period 
  of 
  the 
  orbit, 
  when, 
  from 
  the 
  closer 
  proximity 
  to 
  the 
  sun, 
  there 
  is 
  more 
  

   light 
  to 
  reflect. 
  These 
  two 
  causes 
  combining, 
  and 
  both 
  increasing 
  most 
  rapidly 
  

   with 
  comets 
  of 
  great 
  excentricity 
  and 
  small 
  perihelion 
  distance, 
  occasions 
  the 
  

   sight, 
  all 
  of 
  a 
  sudden, 
  of 
  a 
  long 
  cometric 
  ray 
  in 
  our 
  skies, 
  when 
  the 
  previous 
  

   night, 
  or 
  at 
  least 
  the 
  previous 
  clear 
  night, 
  there 
  was 
  none 
  bright 
  enough 
  to 
  catch 
  

   men's 
  eyes. 
  As 
  the 
  comet 
  leaves 
  the 
  sun, 
  the 
  tail 
  or 
  body 
  expands, 
  and 
  partly 
  

   from 
  its 
  consequent 
  greater 
  rarity, 
  and 
  the 
  diminishing 
  intensity 
  of 
  its 
  solar 
  illu- 
  

   mination, 
  is 
  lost 
  to 
  our 
  sight 
  ; 
  and 
  only 
  the 
  denser 
  roundish 
  portion 
  about 
  the 
  

   head 
  remains 
  visible. 
  This 
  is 
  likewise 
  expanding, 
  and 
  is 
  at 
  length 
  also 
  lost 
  sight 
  

   of 
  for 
  the 
  same 
  reason. 
  In 
  like 
  manner 
  a 
  comet 
  reappears, 
  first 
  the 
  oval 
  mass 
  

   about 
  the 
  head, 
  and 
  then 
  the 
  tail 
  gradually 
  strengthens 
  ; 
  but 
  its 
  aspect 
  will 
  

   materially 
  depend 
  not 
  only 
  on 
  its 
  distance 
  from 
  the 
  sun, 
  but 
  on 
  our 
  distance 
  from 
  

   it, 
  and 
  the 
  direction 
  of 
  our 
  line 
  of 
  sight 
  with 
  the 
  longer 
  axis 
  of 
  the 
  body. 
  

  

  Having 
  had 
  the 
  good 
  fortune 
  to 
  see 
  a 
  rather 
  large 
  number 
  of 
  comets, 
  both 
  

   great 
  and 
  small, 
  and 
  under 
  circumstances 
  favourable 
  above 
  the 
  average, 
  I 
  hope 
  

   that 
  I 
  may 
  be 
  of 
  some 
  service 
  to 
  theorists, 
  by 
  stating 
  what 
  data 
  may 
  be 
  looked 
  

   upon 
  as 
  well 
  fixed 
  with 
  regard 
  to 
  these 
  phenomena 
  ; 
  by 
  pointing 
  out 
  some 
  cor- 
  

   rections 
  which 
  are 
  absolutely 
  necessary 
  to 
  be 
  made 
  upon 
  the 
  observations, 
  before 
  

   any 
  good 
  and 
  safe 
  grounds 
  for 
  theorizing 
  can 
  be 
  procured, 
  but 
  which 
  corrections 
  

   never 
  have 
  been 
  made 
  ; 
  and 
  by 
  pointing 
  out 
  the 
  most 
  probable 
  method 
  of 
  im- 
  

   proving 
  the 
  observations 
  themselves, 
  which, 
  as 
  at 
  present 
  conducted, 
  are 
  by 
  no 
  

   means 
  satisfactory. 
  

  

  With 
  regard 
  then 
  to 
  the 
  physical 
  nature 
  of 
  comets, 
  we 
  may 
  take 
  the 
  follow- 
  

   ing 
  as 
  axioms 
  : 
  — 
  

  

  1. 
  A 
  comet 
  consists 
  of 
  a 
  nucleus, 
  and 
  one 
  or 
  more 
  gaseous 
  envelopes. 
  

  

  (1.) 
  No 
  instance 
  has 
  ever 
  been 
  recorded, 
  at 
  least 
  since 
  the 
  fabulous 
  days 
  of 
  astronomy, 
  of 
  a 
  

   comet 
  having 
  ever 
  been 
  seen 
  without 
  some 
  gaseous 
  appendage, 
  forming, 
  indeed, 
  a 
  distinctive 
  feature 
  

   at 
  a 
  distance 
  from 
  every 
  other 
  body 
  of 
  the 
  solar 
  system 
  ; 
  at 
  a 
  distance, 
  because 
  very 
  close 
  to 
  one 
  of 
  

   the 
  planets, 
  especially 
  one 
  or 
  two 
  of 
  the 
  asteroids, 
  something 
  of 
  its 
  atmosphere 
  might 
  be 
  ob- 
  

  

  