﻿MECHANICAL 
  ACTION 
  OF 
  HEAT. 
  157 
  

  

  by 
  mechanical 
  power 
  employed 
  in 
  different 
  ways, 
  viz., 
  by 
  electric 
  currents 
  excited 
  

   by 
  the 
  rotation 
  of 
  a 
  magnet, 
  by 
  the 
  forcing 
  of 
  water 
  through 
  narrow 
  tubes, 
  by 
  the 
  

   agitation 
  of 
  water 
  and 
  oil 
  with 
  a 
  paddle, 
  by 
  the 
  compression 
  of 
  air, 
  and 
  by 
  the 
  fric- 
  

   tion 
  of 
  air 
  rushing 
  through 
  a 
  narrow 
  orifice. 
  The 
  value 
  of 
  the 
  depth 
  of 
  fall 
  equiva- 
  

   lent 
  to 
  a 
  rise 
  of 
  one 
  degree 
  of 
  Fahrenheit's 
  scale 
  in 
  the 
  temperature 
  of 
  a 
  mass 
  of 
  

   water, 
  as 
  determined 
  by 
  that 
  gentleman, 
  varies, 
  in 
  the 
  different 
  series 
  of 
  experi- 
  

   ments, 
  between 
  the 
  limits 
  of 
  760 
  feet 
  and 
  890 
  feet, 
  the 
  value 
  in 
  which 
  Mr 
  Joule 
  

   appears 
  to 
  place 
  the 
  greatest 
  confidence 
  being 
  about 
  780 
  feet. 
  

  

  Although 
  the 
  smallness 
  of 
  the 
  differences 
  of 
  temperature 
  measured 
  in 
  those 
  

   experiments 
  renders 
  the 
  numerical 
  results 
  somewhat 
  uncertain, 
  it 
  appears 
  to 
  me 
  

   that, 
  as 
  evidence 
  of 
  the 
  convertibility 
  of 
  heat 
  and 
  mechanical 
  power, 
  they 
  are 
  

   unexceptionable. 
  Nevertheless, 
  there 
  is 
  reason 
  to 
  believe 
  that 
  the 
  true 
  mecha- 
  

   nical 
  equivalent 
  of 
  heat 
  is 
  considerably 
  less 
  than 
  any 
  of 
  the 
  values 
  deduced 
  from 
  

   Mr 
  Joule's 
  experiments 
  ; 
  for 
  in 
  all 
  of 
  them 
  there 
  are 
  causes 
  of 
  loss 
  of 
  power, 
  

   the 
  effect 
  of 
  which 
  it 
  is 
  impossible 
  to 
  calculate. 
  In 
  all 
  machinery, 
  a 
  portion 
  of 
  

   the 
  power 
  which 
  disappears 
  is 
  carried 
  off 
  by 
  waves 
  of 
  condensation 
  and 
  expansion, 
  

   along 
  the 
  supports 
  of 
  the 
  machine, 
  and 
  through 
  the 
  surrounding 
  air 
  : 
  this 
  portion 
  

   cannot 
  be 
  estimated, 
  and 
  is, 
  of 
  course, 
  not 
  operative 
  in 
  producing 
  heat 
  within 
  the 
  

   machine. 
  It 
  is 
  also 
  impossible 
  to 
  calculate, 
  where 
  friction 
  is 
  employed 
  to 
  produce 
  

   heat, 
  what 
  amount 
  of 
  it 
  has 
  been 
  lost 
  in 
  the 
  production 
  of 
  electricity, 
  a 
  power 
  

   which 
  is, 
  no 
  doubt, 
  convertible 
  into 
  heat, 
  but 
  which, 
  in 
  such 
  experiments, 
  pro- 
  

   bably 
  escapes 
  without 
  undergoing 
  that 
  conversion. 
  To 
  make 
  the 
  determination 
  

   of 
  the 
  mechanical 
  equivalent 
  of 
  heat 
  by 
  electro-magnetic 
  experiments 
  correct, 
  it 
  

   is 
  necessary 
  that 
  the 
  whole 
  of 
  the 
  mechanical 
  power 
  should 
  be 
  converted 
  into 
  

   magnetic 
  power, 
  the 
  whole 
  of 
  the 
  magnetic 
  power 
  into 
  what 
  are 
  called 
  electric 
  

   currents, 
  and 
  the 
  whole 
  of 
  the 
  power 
  of 
  the 
  electric 
  currents 
  into 
  heat, 
  not 
  one 
  of 
  

   which 
  conditions 
  is 
  likely 
  to 
  be 
  exactly 
  fulfilled. 
  Even 
  in 
  producing 
  heat 
  by 
  the 
  

   compression 
  of 
  air, 
  it 
  must 
  not 
  be 
  assumed 
  that 
  the 
  whole 
  of 
  the 
  mechanical 
  power 
  

   is 
  expended 
  in 
  raising 
  the 
  temperature. 
  

  

  (3.) 
  The 
  best 
  means 
  of 
  determining 
  the 
  mechanical 
  equivalent 
  of 
  heat 
  are 
  

   furnished 
  by 
  those 
  experiments 
  in 
  which 
  no 
  machinery 
  is 
  employed. 
  Of 
  this 
  

   kind 
  are 
  experiments 
  on 
  the 
  velocity 
  of 
  sound 
  in 
  air 
  and 
  other 
  gases, 
  which, 
  

   according 
  to 
  the 
  received 
  and 
  well-known 
  theory 
  of 
  Laplace, 
  is 
  accelerated 
  by 
  

   the 
  heat 
  developed 
  by 
  the 
  compression 
  of 
  the 
  medium. 
  

  

  The 
  accuracy 
  of 
  this 
  theory 
  has 
  lately 
  been 
  called 
  in 
  question. 
  There 
  can 
  

   be 
  no 
  doubt 
  that 
  it 
  deviates 
  from 
  absolute 
  exactness, 
  in 
  so 
  far 
  that 
  the 
  magnitude 
  

   of 
  the 
  displacements 
  of 
  the 
  particles 
  of 
  air 
  is 
  neglected 
  in 
  comparison 
  with 
  the 
  

   length 
  of 
  a 
  wave. 
  It 
  appears 
  to 
  me, 
  however, 
  that 
  the 
  Astronomer-Royal, 
  in 
  his 
  

   remarks 
  on 
  the 
  subject 
  in 
  the 
  London 
  and 
  Edinburgh 
  Philosophical 
  Magazine 
  for 
  

   July 
  1849, 
  has 
  shewn, 
  in 
  a 
  satisfactory 
  manner, 
  that 
  although 
  the 
  effect 
  of 
  the 
  

   appreciable 
  magnitude 
  of 
  those 
  displacements, 
  as 
  compared 
  with 
  the 
  length 
  of 
  a 
  

  

  