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  PROFESSOR 
  WILLIAM 
  THOMSON 
  ON 
  THE 
  

  

  this 
  : 
  — 
  The 
  total 
  external 
  thermal 
  effect 
  is 
  determined 
  when 
  air 
  is 
  allowed 
  to 
  

   expand, 
  through 
  a 
  small 
  orifice, 
  from 
  one 
  vessel 
  into 
  another 
  previously 
  ex- 
  

   hausted 
  by 
  an 
  air-pump. 
  Here 
  the 
  first 
  mechanical 
  effect 
  produced 
  by 
  the 
  expand- 
  

   ing 
  gas, 
  is 
  vis 
  viva 
  generated 
  in 
  the 
  rushing 
  of 
  the 
  air. 
  By 
  the 
  time 
  equilibrium 
  

   is 
  established, 
  all 
  this 
  mechanical 
  effect 
  has 
  been 
  lost 
  in 
  fluid 
  friction 
  (there 
  being 
  

   no 
  appreciable 
  mechanical 
  effect 
  produced 
  externally 
  in 
  sound, 
  which 
  is 
  the 
  only 
  

   external 
  mechanical 
  effect, 
  other 
  than 
  heat, 
  that 
  can 
  be 
  produced 
  by 
  the 
  motions 
  

   of 
  a 
  fluid 
  within 
  a 
  fixed 
  rigid 
  vessel) 
  ; 
  and 
  no 
  truth 
  in 
  physical 
  science 
  can 
  be 
  

   more 
  certain 
  than 
  that 
  by 
  the 
  time 
  thermal 
  as 
  well 
  as 
  mechanical 
  equilibrium 
  

   is 
  established 
  at 
  the 
  primitive 
  temperature, 
  the 
  contents 
  of 
  the 
  two 
  vessels 
  must 
  

   have 
  parted 
  with 
  just 
  as 
  much 
  more 
  heat 
  than 
  they 
  would 
  have 
  parted 
  with, 
  had 
  

   the 
  air 
  in 
  expanding 
  pushed 
  out 
  a 
  piston 
  against 
  an 
  external 
  resisting 
  force, 
  as 
  is 
  

   equivalent 
  to 
  the 
  mechanical 
  effect 
  thus 
  produced 
  externally. 
  Hence, 
  if 
  the 
  two 
  

   vessels 
  and 
  the 
  tube 
  connecting 
  them 
  be 
  immersed 
  (as 
  they 
  are 
  in 
  Joule's 
  first 
  

   set 
  of 
  experiments 
  with 
  this 
  apparatus) 
  in 
  one 
  vessel 
  of 
  water, 
  and 
  if, 
  after 
  time 
  

   is 
  allowed 
  for 
  the 
  pressure 
  and 
  temperature 
  of 
  the 
  air 
  to 
  become 
  the 
  same 
  in 
  the 
  

   two 
  vessels, 
  the 
  water 
  be 
  found 
  to 
  have 
  neither 
  gained 
  nor 
  lost 
  heat 
  (it 
  being 
  un- 
  

   derstood, 
  of 
  course, 
  that 
  the 
  air 
  and 
  all 
  other 
  matter 
  external 
  to 
  the 
  water 
  are 
  at 
  

   an 
  absolutely 
  constant 
  temperature 
  during 
  the 
  experiment), 
  then, 
  for 
  the 
  tempera- 
  

   ture 
  of 
  the 
  experiment, 
  Mayer's 
  hypothesis 
  is 
  perfectly 
  confirmed 
  ; 
  but 
  any 
  final 
  

   elevation 
  or 
  depression 
  of 
  temperature 
  in 
  the 
  water, 
  would 
  show 
  that 
  the 
  work 
  

   due 
  to 
  the 
  expansion 
  is 
  either 
  greater 
  than 
  or 
  less 
  than 
  the 
  absolute 
  equivalent 
  of 
  

   the 
  heat 
  absorbed. 
  

  

  12. 
  Mr 
  Joule's 
  second 
  experiment 
  on 
  the 
  same 
  apparatus, 
  in 
  which 
  he 
  exa- 
  

   mined 
  separately 
  the 
  external 
  thermal 
  effects 
  round 
  each 
  of 
  the 
  two 
  vessels, 
  and 
  

   round 
  a 
  portion 
  of 
  the 
  tube 
  containing 
  the 
  small 
  orifice 
  (a 
  stop-cock) 
  has 
  sug- 
  

   gested 
  to 
  me 
  a 
  method 
  which 
  appears 
  still 
  simpler, 
  and 
  more 
  suitable 
  for 
  obtain- 
  

   ing 
  an 
  excessively 
  delicate 
  test 
  of 
  Mayer's 
  hypothesis 
  for 
  any 
  temperature. 
  It 
  

   consists 
  merely 
  in 
  dispensing 
  with 
  the 
  two 
  vessels 
  in 
  Joule's 
  apparatus, 
  and 
  sub- 
  

   stituting 
  for 
  them 
  two 
  long 
  spirals 
  of 
  tube 
  (instead 
  of 
  doing 
  this 
  for 
  only 
  one 
  of 
  the 
  

   vessels, 
  as 
  Joule 
  does 
  in 
  his 
  third 
  experiment 
  with 
  the 
  same 
  apparatus) 
  ; 
  and 
  in 
  

   forcing 
  air 
  continuously 
  through 
  the 
  whole. 
  The 
  first 
  spiral 
  portion 
  of 
  the 
  tube, 
  

   up 
  to 
  a 
  short 
  distance 
  from 
  the 
  orifice, 
  ought 
  to 
  be 
  kept 
  as 
  nearly 
  as 
  possible 
  at 
  the 
  

   temperature 
  of 
  the 
  atmosphere 
  surrounding 
  the 
  portion 
  containing 
  the 
  orifice, 
  

   and 
  serves 
  merely 
  to 
  fix 
  the 
  temperature 
  of 
  the 
  entering 
  air. 
  The 
  following 
  

   investigation 
  shows 
  what 
  conclusions 
  might 
  be 
  drawn 
  by 
  experimenting 
  on 
  the 
  

   thermal 
  phenomena 
  of 
  any 
  fluid 
  whatever 
  treated 
  in 
  this 
  manner. 
  

  

  13. 
  Let 
  j!? 
  be 
  the 
  uniform 
  pressure 
  of 
  the 
  fluid 
  in 
  the 
  first 
  spiral, 
  up 
  to 
  a 
  short 
  

   distance 
  from 
  the 
  orifice, 
  and 
  let 
  p 
  be 
  the 
  pressure 
  a 
  short 
  distance 
  from 
  the 
  

   orifice 
  on 
  the 
  other 
  side, 
  which 
  will 
  be 
  uniform 
  through 
  the 
  second 
  spiral. 
  Let 
  t 
  

   be 
  the 
  constant 
  external 
  temperature, 
  and 
  let 
  the 
  air 
  in 
  both 
  spirals 
  be 
  kept 
  as 
  

  

  