﻿1875.] 
  Prof. 
  J. 
  Tyndall 
  on 
  Acoustic 
  Reversibility. 
  159 
  

  

  The 
  solar 
  radiation 
  to 
  which 
  the 
  comet 
  was 
  subjected 
  would 
  be 
  inade- 
  

   quate 
  to 
  account 
  for 
  this 
  state 
  of 
  things 
  directly. 
  Is 
  there 
  chemical 
  

   action 
  set 
  up 
  within 
  the 
  comet 
  by 
  the 
  sun's 
  heat 
  ? 
  Is 
  the 
  comet's 
  light 
  

   due 
  to 
  electricity 
  in 
  any 
  form 
  excited 
  by 
  the 
  effect 
  of 
  the 
  solar 
  radiation 
  

   upon 
  the 
  matter 
  of 
  the 
  comet 
  ? 
  Are 
  we 
  to 
  look 
  for 
  the 
  source 
  of 
  the 
  

   light 
  to 
  the 
  friction 
  of 
  the 
  particles 
  of 
  the 
  cometary 
  matter 
  which 
  has 
  

   been 
  thrown 
  into 
  violent 
  agitation 
  by 
  the 
  comet's 
  approach 
  to 
  the 
  sun 
  ? 
  

  

  The 
  comet 
  was 
  unfavourably 
  situated 
  for 
  examination 
  from 
  my 
  obser- 
  

   vatory, 
  as 
  it 
  was 
  seen 
  on 
  a 
  background 
  of 
  sky 
  illuminated 
  by 
  the 
  lights 
  

   of 
  London 
  ; 
  and 
  as 
  it 
  approached 
  the 
  horizon, 
  it 
  became 
  partially 
  concealed 
  

   by 
  the 
  chimneys 
  of 
  some 
  neighbouring 
  houses. 
  Nearly 
  the 
  whole 
  of 
  the 
  

   time 
  that 
  the 
  comet 
  was 
  visible 
  was 
  consumed 
  in 
  the 
  observations 
  with 
  

   the 
  spectroscope 
  ; 
  and 
  a 
  few 
  sketches 
  only 
  were 
  made 
  of 
  the 
  appearances 
  

   presented 
  by 
  the 
  head 
  of 
  the 
  comet 
  in 
  the 
  telescope. 
  

  

  The 
  two 
  sketches 
  which 
  accompany 
  this 
  note 
  (Plate 
  6) 
  were 
  made 
  on 
  

   July 
  13 
  and 
  14. 
  On 
  the 
  latter 
  evening 
  the 
  fainter 
  parts 
  of 
  the 
  coma, 
  

   which 
  are 
  wanting 
  in 
  the 
  sketch, 
  were 
  rendered 
  invisible 
  by 
  the 
  bright 
  

   background 
  of 
  sky. 
  Two 
  of 
  the 
  phases 
  presented 
  by 
  the 
  intersection 
  of 
  

   the 
  envelopes 
  are 
  shown 
  in 
  the 
  sketches. 
  The 
  narrow 
  black 
  channel 
  

   behind 
  the 
  nucleus 
  passed 
  on 
  the 
  right-hand 
  side 
  of 
  the 
  nucleus 
  (as 
  seen 
  

   in 
  the 
  telescope), 
  where 
  it 
  terminated 
  in 
  a 
  small 
  round 
  extension, 
  pre- 
  

   senting 
  something 
  of 
  the 
  appearance 
  of 
  a 
  black 
  pin, 
  with 
  its 
  head 
  by 
  the 
  

   side 
  of 
  the 
  nucleus. 
  

  

  III. 
  "On 
  Acoustic 
  Reversibility. 
  " 
  By 
  J. 
  Tyndall, 
  D.C.L., 
  

   LL.D., 
  F.R.S. 
  Received 
  December 
  31, 
  1874. 
  

  

  On 
  the 
  21st 
  and 
  22nd 
  of 
  June, 
  1822, 
  a 
  Commission 
  appointed 
  by 
  the 
  

   Bureau 
  of 
  Longitudes 
  of 
  Prance 
  executed 
  a 
  celebrated 
  series 
  of 
  experi- 
  

   ments 
  on 
  the 
  velocity 
  of 
  sound. 
  Two 
  stations 
  had 
  been 
  chosen, 
  the 
  one 
  

   at 
  Yillejuif, 
  the 
  other 
  at 
  Montlhery, 
  both 
  lying 
  south 
  of 
  Paris, 
  and 
  11*6 
  

   miles 
  distant 
  from 
  each 
  other. 
  Prony, 
  Mathieu, 
  and 
  Arago 
  were 
  the 
  

   observers 
  at 
  Yillejuif, 
  while 
  Humboldt, 
  Bouvard, 
  and 
  Gray-Lussac 
  were 
  

   at 
  Montlhery. 
  Guns, 
  charged 
  sometimes 
  with 
  2 
  lbs. 
  and 
  sometimes 
  with 
  

   3 
  lbs. 
  of 
  powder, 
  were 
  fired 
  at 
  both 
  stations, 
  and 
  the 
  velocity 
  was 
  deduced 
  

   from 
  the 
  interval 
  between 
  the 
  appearance 
  of 
  the 
  flash 
  and 
  the 
  arrival 
  of 
  

   the 
  sound. 
  

  

  On 
  this 
  memorable 
  occasion 
  an 
  observation 
  was 
  made 
  which, 
  as 
  far 
  as 
  

   I 
  know, 
  has 
  remained 
  a 
  scientific 
  enigma 
  to 
  the 
  present 
  hour. 
  It 
  was 
  

   noticed 
  that 
  while 
  every 
  report 
  of 
  the 
  cannon 
  fired 
  at 
  Montlhery 
  was 
  

   heard 
  with 
  the 
  greatest 
  distinctness 
  at 
  Yillejuif, 
  by 
  far 
  the 
  greater 
  

   number 
  of 
  the 
  reports 
  from 
  Yillejuif 
  failed 
  to 
  reach 
  Montlhery. 
  Had 
  

   wind 
  existed, 
  and 
  had 
  it 
  blown 
  from 
  Montlhery 
  to 
  Yillejuif, 
  it 
  would 
  

  

  