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

  

  163 
  

  

  strong 
  reason 
  to 
  question 
  the 
  hypothesis 
  of 
  the 
  illustrious 
  Trench 
  

   philosopher. 
  

  

  And 
  considering 
  the 
  hundreds 
  of 
  shots 
  fired 
  at 
  the 
  South 
  Foreland, 
  

   with 
  the 
  attention 
  specially 
  directed 
  to 
  the 
  aerial 
  echoes, 
  when 
  no 
  single 
  

   case 
  occurred 
  in 
  which 
  echoes 
  of 
  measurable 
  duration 
  did 
  not 
  accompany 
  

   the 
  report 
  of 
  the 
  gun, 
  I 
  think 
  Arago's 
  statement 
  that 
  at 
  Yillejuif 
  no 
  

   echoes 
  were 
  heard 
  when 
  the 
  sky 
  was 
  clear 
  must 
  simply 
  mean 
  that 
  they 
  

   vanished 
  with 
  great 
  rapidity. 
  Unless 
  the 
  attention 
  were 
  specially 
  

   directed 
  to 
  the 
  point, 
  a 
  slight 
  prolongation 
  of 
  the 
  cannon-sound 
  might 
  

   well 
  escape 
  observation 
  ; 
  and 
  it 
  would 
  be 
  all 
  the 
  more 
  likely 
  to 
  do 
  so 
  if 
  

   the 
  echoes 
  were 
  so 
  loud 
  and 
  prompt 
  as 
  to 
  form 
  apparently 
  part 
  and 
  parcel 
  

   of 
  the 
  direct 
  sound. 
  

  

  I 
  should 
  be 
  very 
  loth 
  to 
  transgress 
  here 
  the 
  limits 
  of 
  fair 
  criticism, 
  or 
  

   to 
  throw 
  doubt, 
  without 
  good 
  reason, 
  on 
  the 
  recorded 
  observations 
  of 
  an 
  

   eminent 
  man. 
  Still, 
  taking 
  into 
  account 
  what 
  has 
  been 
  just 
  stated, 
  and 
  

   remembering 
  that 
  the 
  minds 
  of 
  Arago 
  and 
  his 
  colleagues 
  were 
  occupied 
  

   by 
  a 
  totally 
  different 
  problem 
  (that 
  the 
  echoes 
  were 
  an 
  incident 
  rather 
  

   than 
  an 
  object 
  of 
  observation), 
  I 
  think 
  we 
  may 
  justly 
  consider 
  the 
  sound 
  

   which 
  he 
  called 
  " 
  instantaneous 
  " 
  as 
  one 
  whose 
  aerial 
  echoes 
  did 
  not 
  dif- 
  

   ferentiate 
  themselves 
  from 
  the 
  direct 
  sound 
  by 
  any 
  noticeable 
  fall 
  of 
  

   intensity, 
  and 
  which 
  rapidly 
  died 
  into 
  silence. 
  

  

  Turning 
  now 
  to 
  the 
  observations 
  at 
  Montlhery, 
  we 
  are 
  struck 
  by 
  the 
  

   extraordinary 
  duration 
  of 
  the 
  echoes 
  heard 
  at 
  that 
  station. 
  At 
  the 
  South 
  

   Foreland 
  the 
  charge 
  habitually 
  fired 
  was 
  equal 
  to 
  the 
  largest 
  of 
  those 
  

   employed 
  by 
  the 
  French 
  philosophers 
  ; 
  but 
  on 
  no 
  occasion 
  did 
  the 
  gun- 
  

   sounds 
  produce 
  echoes 
  approaching 
  to 
  20 
  or 
  25 
  seconds' 
  duration. 
  It 
  

   rarely 
  reached 
  half 
  this 
  amount. 
  Even 
  the 
  syren-echoes, 
  which 
  were 
  

   more 
  remarkable 
  and 
  more 
  long-continued 
  than 
  those 
  of 
  the 
  gun, 
  never 
  

   reached 
  the 
  duration 
  of 
  the 
  Montlhery 
  echoes. 
  The 
  nearest 
  approach 
  to 
  

   it 
  was 
  on 
  the 
  17th 
  of 
  October, 
  1873, 
  when 
  the 
  syren-echoes 
  required 
  

   15 
  seconds 
  to 
  subside 
  into 
  silence. 
  

  

  On 
  this 
  same 
  day, 
  moreover 
  (and 
  this 
  is 
  a 
  point 
  of 
  marked 
  signi- 
  

   ficance), 
  the 
  transmitted 
  sound 
  reached 
  its 
  maximum 
  range, 
  the 
  gun- 
  

   sounds 
  being 
  heard 
  at 
  the 
  Quenocs 
  buoy, 
  which 
  is 
  16 
  j 
  nautical 
  miles 
  

   from 
  the 
  South 
  Foreland. 
  I 
  have 
  already 
  stated 
  that 
  the 
  duration 
  of 
  the 
  

   air-echoes 
  indicates 
  " 
  the 
  atmospheric 
  depths 
  " 
  from 
  which 
  they 
  come*. 
  

   An 
  optical 
  analogy 
  may 
  help 
  us 
  here. 
  Let 
  light 
  fall 
  upon 
  chalk, 
  the 
  

   light 
  is 
  wholly 
  scattered 
  by 
  the 
  superficial 
  particles 
  ; 
  let 
  the 
  chalk 
  be 
  

   powdered 
  and 
  mixed 
  with 
  water, 
  light 
  reaches 
  the 
  observer 
  from 
  a 
  far 
  

   greater 
  depth 
  of 
  the 
  turbid 
  liquid. 
  The 
  solid 
  chalk 
  typifies 
  the 
  action 
  of 
  

   exceedingly 
  dense 
  acoustic 
  clouds 
  ; 
  the 
  chalk 
  and 
  water 
  that 
  of 
  clouds 
  of 
  

   moderate 
  density. 
  In 
  the 
  one 
  case 
  we 
  have 
  echoes 
  of 
  short, 
  in 
  the 
  other 
  

   echoes 
  of 
  long 
  duration. 
  These 
  considerations 
  prepare 
  us 
  for 
  the 
  infer- 
  

   ence 
  that 
  Montlhery, 
  on 
  the 
  occasion 
  referred 
  to, 
  must 
  have 
  been 
  sur- 
  

   * 
  Phil. 
  Trans. 
  1874, 
  pt. 
  i. 
  p. 
  202. 
  

  

  