﻿160 
  Prof. 
  J. 
  Tyndall 
  on 
  Acoustic 
  Reversibility 
  . 
  [Jan. 
  7, 
  

  

  have 
  been 
  recognized 
  as 
  the 
  cause 
  of 
  the 
  observed 
  difference 
  ; 
  but 
  the 
  air 
  

   at 
  the 
  time 
  was 
  calm, 
  the 
  slight 
  motion 
  of 
  translation 
  actually 
  existing 
  

   being 
  from 
  Villejuif 
  towards 
  Montlhery, 
  or 
  against 
  the 
  direction 
  in 
  which 
  

   the 
  sound 
  was 
  best 
  heard. 
  

  

  So 
  marked 
  was 
  the 
  difference 
  in 
  transmissive 
  power 
  between 
  the 
  two 
  

   directions, 
  that 
  on 
  the 
  22nd 
  of 
  June, 
  while 
  every 
  shot 
  fired 
  at 
  Montlhery 
  

   was 
  heard 
  " 
  a 
  merveille 
  " 
  at 
  Villejuif, 
  but 
  one 
  shot 
  out 
  of 
  twelve 
  fired 
  at 
  

   Villejuif 
  was 
  heard, 
  and 
  that 
  feebly, 
  at 
  the 
  other 
  station. 
  

  

  With 
  the 
  caution 
  which 
  characterized 
  him 
  on 
  other 
  occasions, 
  and 
  which 
  

   has 
  been 
  referred 
  to 
  admiringly, 
  by 
  Faraday*, 
  Arago 
  made 
  no 
  attempt 
  to 
  

   explain 
  this 
  anomaly. 
  His 
  words 
  are 
  : 
  — 
  " 
  Quant 
  aux 
  differences 
  si 
  

   remarquables 
  d'intensite 
  que 
  le 
  bruit 
  du 
  canon 
  a 
  tou 
  jours 
  present 
  es 
  

   suivant 
  qu'il 
  se 
  propageaient 
  du 
  nord 
  au 
  sud 
  entre 
  Villejuif 
  et 
  Montlhery, 
  

   ou 
  du 
  sud 
  au 
  nord 
  entre 
  cette 
  seconde 
  station 
  et 
  la 
  premiere 
  ; 
  nous 
  ne 
  

   chercherons 
  pas 
  aujourd'hui 
  a 
  l'expliquer, 
  parce 
  que 
  nous 
  ne 
  pourrions 
  

   offrir 
  au 
  lecteur 
  que 
  des 
  conjectures 
  denuees 
  de 
  preuves"f. 
  

  

  I 
  have 
  tried, 
  after 
  much 
  perplexity 
  of 
  thought, 
  to 
  bring 
  this 
  sub- 
  

   ject 
  within 
  the 
  range 
  of 
  experiment, 
  and 
  have 
  now 
  to 
  submit 
  to 
  the 
  

   Royal 
  Society 
  a 
  possible 
  solution 
  of 
  the 
  enigma. 
  The 
  first 
  step 
  was 
  

   to 
  ascertain 
  whether 
  the 
  sensitive 
  flame 
  referred 
  to 
  in 
  my 
  recent 
  

   paper 
  in 
  the 
  Philosophical 
  Transactions 
  could 
  be 
  safely 
  employed 
  

   in 
  experiments 
  on 
  the 
  mutual 
  reversibility 
  of 
  a 
  source 
  of 
  sound 
  and 
  

   an 
  object 
  on 
  which 
  the 
  sound 
  impinges. 
  Now 
  the 
  sensitive 
  flame 
  

   usually 
  employed 
  by 
  me 
  measures 
  from 
  18 
  to 
  24 
  inches 
  in 
  height, 
  while 
  

   the 
  reed 
  employed 
  as 
  a 
  source 
  of 
  sound 
  is 
  less 
  than 
  a 
  square 
  quarter 
  

   of 
  an 
  inch 
  in 
  area. 
  If, 
  therefore, 
  the 
  whole 
  flame, 
  or 
  the 
  pipe 
  which 
  

   fed 
  it, 
  were 
  sensitive 
  to 
  sonorous 
  vibrations, 
  strict 
  experiments 
  on 
  rever- 
  

   sibility 
  with 
  the 
  reed 
  and 
  flame 
  might 
  be 
  difficult, 
  if 
  not 
  impossible. 
  

   Hence 
  my 
  desire 
  to 
  learn 
  whether 
  the 
  seat 
  of 
  sensitiveness 
  was 
  so 
  

   localized 
  in 
  the 
  flame 
  as 
  to 
  render 
  the 
  contemplated 
  interchange 
  of 
  flame 
  

   and 
  reed 
  permissible. 
  

  

  The 
  flame 
  being 
  placed 
  behind 
  a 
  cardboard 
  screen, 
  the 
  shank 
  of 
  a 
  

   funnel 
  passed 
  through 
  a 
  hole 
  in 
  the 
  cardboard 
  was 
  directed 
  upon 
  the 
  

   middle 
  of 
  the 
  flame. 
  The 
  sound-waves 
  issuing 
  from 
  the 
  vibrating 
  reed 
  

   placed 
  within 
  the 
  funnel 
  produced 
  no 
  sensible 
  effect 
  upon 
  the 
  flame. 
  

   Shifting 
  the 
  funnel 
  so 
  as 
  to 
  direct 
  its 
  shank 
  upon 
  the 
  root 
  of 
  the 
  flame, 
  

   the 
  action 
  was 
  violent. 
  

  

  To 
  augment 
  the 
  precision 
  of 
  the 
  experiment, 
  the 
  funnel 
  was 
  connected 
  

   with 
  a 
  glass 
  tube 
  3 
  feet 
  long 
  and 
  half 
  an 
  inch 
  in 
  diameter, 
  the 
  object 
  

   being 
  to 
  weaken 
  by 
  distance 
  the 
  effect 
  of 
  the 
  waves 
  diffracted 
  round 
  the 
  

   edge 
  of 
  the 
  funnel, 
  and 
  to 
  permit 
  those 
  only 
  which 
  passed 
  through 
  

   the 
  glass 
  tube 
  to 
  act 
  upon 
  the 
  flame. 
  - 
  

  

  Presenting 
  the 
  end 
  of 
  the 
  tube 
  to 
  the 
  orifice 
  of 
  the 
  burner 
  (h, 
  fig. 
  1), 
  

  

  * 
  Researches 
  in 
  Chemistry 
  and 
  Physics, 
  p. 
  484. 
  

   t 
  Connaissance 
  des 
  Temps, 
  1825, 
  p. 
  370. 
  . 
  

  

  