﻿164 
  Prof. 
  J. 
  Tyndall 
  on 
  Acoustic 
  Reversibility 
  \ 
  [Jan. 
  7, 
  

  

  rounded 
  by 
  a 
  highly 
  diacoustic 
  atmosphere 
  ; 
  while 
  the 
  shortness 
  of 
  the 
  

   echoes 
  at 
  Villejuif 
  shows 
  the 
  atmosphere 
  surrounding 
  that 
  station 
  to 
  

   have 
  been 
  acoustically 
  opaque. 
  

  

  Have 
  we 
  any 
  clue 
  to 
  the 
  cause 
  of 
  the 
  opacity 
  ? 
  I 
  think 
  we 
  have. 
  

   Villejuif 
  is 
  close 
  to 
  Paris, 
  and 
  over 
  it, 
  with 
  the 
  observed 
  light 
  wind, 
  was 
  

   slowly 
  wafted 
  the 
  air 
  from 
  the 
  city. 
  Thousands 
  of 
  chimneys 
  to 
  windward 
  

   of 
  Villejuif 
  were 
  discharging 
  their 
  heated 
  currents 
  ; 
  so 
  that 
  an 
  atmosphere 
  

   non-homogeneous 
  in 
  a 
  high 
  degree 
  must 
  have 
  surrounded 
  that 
  station. 
  

   At 
  no 
  great 
  height 
  in 
  the 
  atmosphere 
  the 
  equilibrium 
  of 
  temperature 
  

   would 
  be 
  established. 
  The 
  non-homogeneous 
  air 
  surrounding 
  Villejuif 
  

   is 
  experimentally 
  typified 
  by 
  our 
  screen 
  with 
  the 
  source 
  of 
  sound 
  close 
  

   behind 
  it, 
  the 
  upper 
  edge 
  of 
  the 
  screen 
  representing 
  the 
  place 
  where 
  

   equilibrium 
  of 
  temperature 
  was 
  established 
  in 
  the 
  atmosphere 
  above 
  the 
  

   station. 
  In 
  virtue 
  of 
  its 
  proximity 
  to 
  the 
  screen, 
  the 
  echoes 
  from 
  our 
  

   sounding-reed 
  would, 
  in 
  the 
  case 
  here 
  supposed, 
  so 
  blend 
  with 
  the 
  direct 
  

   sound 
  as 
  to 
  be 
  practically 
  indistinguishable 
  from 
  it, 
  as 
  the 
  echoes 
  at 
  Vil- 
  

   lejuif 
  followed 
  the 
  direct 
  sound 
  so 
  hotly, 
  and 
  vanished 
  so 
  rapidly, 
  that 
  

   they 
  escaped 
  observation. 
  And 
  as 
  our 
  sensitive 
  flame, 
  at 
  a 
  distance, 
  

   failed 
  to 
  be 
  affected 
  by 
  the 
  sounding 
  body 
  placed 
  close 
  behind 
  the 
  card- 
  

   board 
  screen, 
  so, 
  I 
  take 
  it, 
  did 
  the 
  observers 
  at 
  Montlhery 
  fail 
  to 
  hear 
  

   the 
  sounds 
  of 
  the 
  Villejuif 
  gun. 
  This 
  is 
  the 
  explanation 
  of 
  Arago's 
  dif- 
  

   ficulty 
  which 
  I 
  have 
  the 
  honour 
  to 
  submit 
  to 
  the 
  Royal 
  Society. 
  

  

  Received 
  January 
  4, 
  1875. 
  

  

  Something 
  further 
  may 
  be 
  done 
  towards 
  the 
  experimental 
  elucidation 
  of 
  

   this 
  subject. 
  The 
  facility 
  with 
  which 
  sounds 
  pass 
  through 
  textile 
  fabrics 
  

   has 
  been 
  already 
  illustrated*, 
  a 
  layer 
  of 
  cambric 
  or 
  calico, 
  or 
  even 
  of 
  thick 
  

   flannel 
  or 
  baize, 
  being 
  found 
  competent 
  to 
  intercept 
  but 
  a 
  fraction 
  of 
  the 
  

   sound 
  from 
  a 
  vibrating 
  reed. 
  Such 
  a 
  layer 
  of 
  calico 
  may 
  be 
  taken 
  to 
  

   represent 
  a 
  layer 
  of 
  air 
  differentiated 
  from 
  its 
  neighbours 
  by 
  temperature 
  

   or 
  moisture 
  ; 
  while 
  a 
  succession 
  of 
  such 
  sheets 
  of 
  calico 
  may 
  be 
  taken 
  

   to 
  represent 
  successive 
  layers 
  of 
  non-homogeneous 
  air. 
  

  

  Fig. 
  3. 
  

  

  Two 
  tin 
  tubes 
  (M 
  N 
  and 
  P, 
  fig. 
  3) 
  with 
  open 
  ends 
  are 
  placed 
  so 
  

   * 
  Phil. 
  Trans. 
  1874, 
  pt. 
  i. 
  p. 
  208. 
  

  

  