﻿1875.] 
  Prof. 
  J. 
  Tyndall 
  on 
  Acoustic 
  Reversibility. 
  165 
  

  

  as 
  to 
  form 
  an 
  acute 
  angle 
  with 
  each 
  other. 
  At 
  the 
  end 
  of 
  one 
  is 
  the 
  

   vibrating 
  reed 
  r 
  ; 
  opposite 
  the 
  end 
  of 
  the 
  other, 
  and 
  in 
  the 
  prolonga- 
  

   tion 
  of 
  P 
  0, 
  is 
  the 
  sensitive 
  flame 
  /, 
  a 
  second 
  sensitive 
  flame 
  (/') 
  being 
  

   placed 
  in 
  the 
  continuation 
  of 
  the 
  axis 
  of 
  M 
  N. 
  On 
  sounding 
  the 
  reed, 
  the 
  

   direct 
  sound 
  through 
  M 
  N 
  agitates 
  the 
  flame/'. 
  Introducing 
  the 
  square 
  of 
  

   calico 
  ah 
  at 
  the 
  proper 
  angle, 
  a 
  slight 
  decrease 
  of 
  the 
  action 
  on/' 
  is 
  

   noticed, 
  and 
  the 
  feeble 
  echo 
  from 
  a 
  b 
  produces 
  a 
  barely 
  perceptible 
  agitation 
  

   of 
  the 
  flame/. 
  Adding 
  another 
  square, 
  cd, 
  the 
  sound 
  transmitted 
  by 
  

   a 
  b 
  impinges 
  on 
  cd 
  ; 
  it 
  is 
  partially 
  echoed, 
  returns 
  through 
  ah, 
  passes 
  along 
  

   PO, 
  and 
  still 
  further 
  agitates 
  the 
  flame/. 
  Adding 
  a 
  third 
  square, 
  ef, 
  

   the 
  reflected 
  sound 
  is 
  still 
  further 
  augmented, 
  every 
  accession 
  to 
  the 
  echo 
  

   being 
  accompanied 
  by 
  a 
  corresponding 
  withdrawal 
  of 
  the 
  vibrations 
  from 
  

   /' 
  and 
  a 
  consequent 
  stilling 
  of 
  that 
  flame. 
  

  

  With 
  thinner 
  calico 
  or 
  cambric 
  it 
  would 
  require 
  a 
  greater 
  number 
  of 
  

   layers 
  to 
  intercept 
  the 
  entire 
  sound 
  ; 
  hence 
  with 
  such 
  cambric 
  we 
  should 
  

   have 
  echoes 
  returned 
  from 
  a 
  greater 
  distance, 
  and 
  therefore 
  of 
  greater 
  

   duration. 
  Eight 
  layers 
  of 
  the 
  calico 
  employed 
  in 
  these 
  experi- 
  

   ments, 
  stretched 
  on 
  a 
  wire 
  frame 
  and 
  placed 
  close 
  together 
  as 
  a 
  

   kind 
  of 
  pad, 
  may 
  be 
  taken 
  to 
  represent 
  a 
  very 
  dense 
  acoustic 
  cloud. 
  

   Such 
  a 
  pad, 
  placed 
  at 
  the 
  proper 
  angle 
  beyond 
  1ST, 
  cuts 
  off 
  the 
  sound, 
  

   which 
  in 
  its 
  absence 
  reaches 
  /', 
  almost 
  as 
  effectually 
  as 
  an 
  imper- 
  

   vious 
  solid 
  plate* 
  : 
  the 
  flame 
  /' 
  is 
  thereby 
  stilled, 
  while 
  / 
  is 
  far 
  more 
  

   powerfully 
  agitated 
  than 
  by 
  the 
  reflection 
  from 
  a 
  single' 
  layer. 
  With 
  the 
  

   source 
  of 
  sound 
  close 
  at 
  hand, 
  the 
  echoes 
  from 
  such 
  a 
  pad 
  would 
  be 
  

   of 
  insensible 
  duration. 
  Thus 
  close 
  at 
  hand 
  do 
  I 
  suppose 
  the 
  acoustic 
  

   clouds 
  surrounding 
  Villejuif 
  to 
  have 
  been, 
  a 
  similar 
  shortness 
  of 
  echo 
  

   being 
  the 
  consequence. 
  

  

  A 
  further 
  step 
  is 
  here 
  taken 
  in 
  the 
  illustration 
  of 
  the 
  analogy 
  between 
  

   light 
  and 
  sound. 
  Our 
  pad 
  acts 
  chiefly 
  by 
  internal 
  reflection. 
  The 
  

   sound 
  from 
  the 
  reed 
  is 
  a 
  composite 
  one, 
  made 
  up 
  of 
  partial 
  sounds 
  

   differing 
  in 
  pitch. 
  If 
  these 
  sounds 
  be 
  ejected 
  from 
  the 
  pad 
  in 
  their 
  

   pristine 
  proportions, 
  the 
  pad 
  is 
  acoustically 
  white 
  ; 
  if 
  they 
  return 
  with 
  

   their 
  proportions 
  altered, 
  the 
  pad 
  is 
  acoustically 
  coloured. 
  

  

  In 
  these 
  experiments 
  my 
  assistant, 
  Mr. 
  Cottrell, 
  has 
  rendered 
  me 
  ■ 
  

   material 
  assistance. 
  

  

  * 
  January 
  13th. 
  — 
  Since 
  this 
  was 
  written 
  I 
  hare 
  sent 
  the 
  sound 
  through 
  fifteen 
  

   layers 
  of 
  calico, 
  and 
  echoed 
  it 
  back 
  through 
  the 
  same 
  layers, 
  in 
  strength 
  sufficient 
  to 
  

   agitate 
  the 
  flame. 
  Thirty 
  layers 
  were 
  here 
  crossed 
  by 
  the 
  sound. 
  

  

  