﻿162 
  Prf. 
  J. 
  Tyndall 
  on 
  Acoustic 
  Reversibility. 
  [Jan. 
  7 
  } 
  

  

  behind 
  it 
  ; 
  while 
  the 
  action 
  was 
  insensible 
  when 
  these 
  positions 
  were 
  

   reversed. 
  

  

  Thus, 
  let 
  s 
  e, 
  fig. 
  2, 
  be 
  a 
  vertical 
  section 
  of 
  the 
  screen. 
  When 
  the 
  reed 
  

   was 
  at 
  A 
  and 
  the 
  flame 
  at 
  B 
  there 
  was 
  no 
  action 
  ; 
  when 
  the 
  reed 
  was 
  at 
  B 
  

  

  rig. 
  2. 
  

  

  B 
  

  

  and 
  the 
  flame 
  at 
  A 
  the 
  action 
  was 
  decided. 
  It 
  may 
  be 
  added 
  that 
  the 
  

   vibrations 
  communicated 
  to 
  the 
  screen 
  itself, 
  and 
  from 
  it 
  to 
  the 
  air 
  beyond 
  

   it, 
  were 
  without 
  effect 
  ; 
  for 
  when 
  the 
  reed, 
  which 
  at 
  B 
  is 
  effectual, 
  was 
  

   shifted 
  to 
  C, 
  where 
  its 
  action 
  on 
  the 
  screen 
  was 
  greatly 
  augmented, 
  it 
  

   ceased 
  to 
  have 
  any 
  action 
  on 
  the 
  flame 
  at 
  A. 
  

  

  "We 
  are 
  now, 
  I 
  think, 
  prepared 
  to 
  consider 
  the 
  failure 
  of 
  reversibility 
  

   in 
  the 
  larger 
  experiments 
  of 
  1822. 
  Happily 
  an 
  incidental 
  observation 
  of 
  

   great 
  significance 
  comes 
  here 
  to 
  our 
  aid. 
  It 
  was 
  observed 
  and 
  recorded 
  

   at 
  the 
  time 
  that 
  while 
  the 
  reports 
  of 
  the 
  guns 
  at 
  Villejuif 
  were 
  without 
  

   echoes, 
  a 
  roll 
  of 
  echoes, 
  lasting 
  from 
  20 
  to 
  25 
  seconds, 
  accompanied 
  every 
  

   shot 
  at 
  Montlhery, 
  being 
  heard 
  by 
  the 
  observers 
  there. 
  Arago, 
  the 
  

   writer 
  of 
  the 
  Report, 
  referred 
  these 
  echoes 
  to 
  reflection 
  from 
  the 
  

   clouds, 
  an 
  explanation 
  which 
  I 
  think 
  we 
  are 
  entitled 
  to 
  regard 
  as 
  pro- 
  

   blematical. 
  The 
  report 
  says 
  that 
  " 
  tous 
  les 
  coups 
  tires 
  a 
  Montlhery 
  y 
  

   etaient 
  accompagnes 
  d'un 
  roulement 
  semblable 
  a 
  celui 
  du 
  tonnerre." 
  I 
  

   have 
  italicized 
  a 
  very 
  significant 
  word 
  — 
  a 
  word 
  which 
  fairly 
  applies 
  to 
  

   our 
  experiments 
  on 
  gun-sounds 
  at 
  the 
  South 
  Foreland, 
  where 
  there 
  was 
  

   no 
  sensible 
  solution 
  of 
  continuity 
  between 
  explosion 
  and 
  echo, 
  but 
  which 
  

   could 
  hardly 
  apply 
  to 
  echoes 
  coming 
  from 
  the 
  clouds. 
  For 
  supposing 
  the 
  

   clouds 
  to 
  be 
  only 
  a 
  mile 
  distant, 
  the 
  sound 
  and 
  its 
  echo 
  would 
  have 
  

   been 
  separated 
  by 
  an 
  interval 
  of 
  nearly 
  ten 
  seconds. 
  But 
  there 
  is 
  no 
  

   mention 
  of 
  any 
  interval 
  ; 
  and 
  had 
  such 
  existed, 
  surely 
  the 
  word 
  " 
  fol- 
  

   lowed," 
  instead 
  of 
  " 
  accompanied," 
  would 
  have 
  been 
  the 
  one 
  employed. 
  

   The 
  echoes, 
  moreover, 
  appear 
  to 
  have 
  been 
  continuous, 
  while 
  the 
  clouds 
  

   observed 
  seem 
  to 
  have 
  been 
  separate. 
  " 
  Ces 
  phenomenes," 
  says 
  Arago, 
  

   " 
  n'ont 
  jamais 
  eu 
  lieu 
  qu'au 
  moment 
  de 
  l'apparition 
  de 
  quelques 
  nuages." 
  

   But 
  from 
  separate 
  clouds 
  a 
  continuous 
  roll 
  of 
  echoes 
  could 
  hardly 
  come. 
  

   "When 
  to 
  this 
  is 
  added 
  the 
  experimental 
  fact 
  that 
  clouds 
  far 
  denser 
  than 
  

   any 
  ever 
  formed 
  in 
  the 
  atmosphere 
  are 
  demonstrably 
  incapable 
  of 
  sensibly 
  

   reflecting 
  sound, 
  while 
  cloudless 
  air, 
  which 
  Arago 
  pronounced 
  echoless, 
  

   has 
  been 
  proved 
  capable 
  of 
  powerfully 
  reflecting 
  it, 
  I 
  think 
  we 
  have 
  

  

  