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  PROFESSOR 
  WILLIAM 
  THOMSON 
  ON 
  THE 
  

  

  4. 
  Considering 
  it 
  as 
  thus 
  established, 
  that 
  heat 
  is 
  not 
  a 
  substance, 
  but 
  a 
  

   dynamical 
  form 
  of 
  mechanical 
  effect, 
  we 
  perceive 
  that 
  there 
  must 
  be 
  an 
  equiva- 
  

   lence 
  between 
  mechanical 
  work 
  and 
  heat, 
  as 
  between 
  cause 
  and 
  effect. 
  The 
  first 
  

   published 
  statement 
  of 
  this 
  principle 
  appears 
  to 
  be 
  in 
  Mayer's 
  " 
  Bemerkungen 
  

   iiber 
  die 
  Krafte 
  der 
  unbelebten 
  Natur," 
  * 
  which 
  contains 
  some 
  correct 
  views 
  

   regarding 
  the 
  mutual 
  convertibility 
  of 
  heat 
  and 
  mechanical 
  effect, 
  along 
  with 
  a 
  

   false 
  analogy 
  between 
  the 
  approach 
  of 
  a 
  weight 
  to 
  the 
  earth 
  and 
  a 
  diminution 
  of 
  

   the 
  volume 
  of 
  a 
  continuous 
  substance, 
  on 
  which 
  an 
  attempt 
  is 
  founded 
  to 
  find 
  

   numerically 
  the 
  mechanical 
  equivalent 
  of 
  a 
  given 
  quantity 
  of 
  heat. 
  In 
  a 
  paper 
  

   published 
  about 
  fourteen 
  months 
  later, 
  " 
  On 
  the 
  Calorific 
  Effects 
  of 
  Magneto- 
  

   Electricity 
  and 
  the 
  Mechanical 
  Value 
  of 
  Heat,"f 
  Mr 
  Joule 
  of 
  Manchester 
  

   expresses 
  very 
  distinctly 
  the 
  consequences 
  regarding 
  the 
  mutual 
  convertibility 
  of 
  

   heat 
  and 
  mechanical 
  effect 
  which 
  follow 
  from 
  the 
  fact, 
  that 
  heat 
  is 
  not 
  a 
  sub- 
  

   stance 
  but 
  a 
  state 
  of 
  motion 
  ; 
  and 
  investigates 
  on 
  unquestionable 
  principles 
  the 
  

   " 
  absolute 
  numerical 
  relations,"" 
  according 
  to 
  which 
  heat 
  is 
  connected 
  with 
  

   mechanical 
  power 
  ; 
  verifying 
  experimentally, 
  that 
  whenever 
  heat 
  is 
  generated 
  

   from 
  purely 
  mechanical 
  action, 
  and 
  no 
  other 
  effect 
  produced, 
  whether 
  it 
  be 
  by 
  

   means 
  of 
  the 
  friction 
  of 
  fluids 
  or 
  by 
  the 
  magneto-electric 
  excitation 
  of 
  galvanic 
  

   currents, 
  the 
  same 
  quantity 
  is 
  generated 
  by 
  the 
  same 
  amount 
  of 
  work 
  spent, 
  and 
  

   determining 
  the 
  actual 
  amount 
  of 
  work, 
  in 
  foot-pounds, 
  required 
  to 
  generate 
  a 
  

   unit 
  of 
  heat, 
  which 
  he 
  calls 
  " 
  the 
  mechanical 
  equivalent 
  of 
  heat." 
  Since 
  the 
  

   publication 
  of 
  that 
  paper, 
  Mr 
  Joule 
  has 
  made 
  numerous 
  series 
  of 
  experiments 
  for 
  

   determining 
  with 
  as 
  much 
  accuracy 
  as 
  possible 
  the 
  mechanical 
  equivalent 
  of 
  heat 
  

   so 
  defined, 
  and 
  has 
  given 
  accounts 
  of 
  them 
  in 
  various 
  communications 
  to 
  the 
  

   British 
  Association, 
  to 
  the 
  Philosophical 
  Magazine, 
  to 
  the 
  Eoyal 
  Society, 
  and 
  to 
  

   the 
  French 
  Institute. 
  

  

  5. 
  Important 
  contributions 
  to 
  the 
  Dynamical 
  Theory 
  of 
  Heat 
  have 
  recently 
  

   been 
  made 
  by 
  Eankine 
  and 
  Clausius 
  ; 
  who, 
  by 
  mathematical 
  reasoning 
  ana- 
  

   logous 
  to 
  Carnot's 
  on 
  the 
  motive 
  power 
  of 
  heat, 
  but 
  founded 
  on 
  an 
  axiom 
  con- 
  

   trary 
  to 
  his 
  fundamental 
  axiom, 
  have 
  arrived 
  at 
  some 
  remarkable 
  conclusions. 
  

   The 
  researches 
  of 
  these 
  authors 
  have 
  been 
  published 
  in 
  the 
  Transactions 
  of 
  this 
  

   Society, 
  and 
  in 
  Poggendorff's 
  Annalen, 
  during 
  the 
  past 
  year 
  ; 
  and 
  they 
  are 
  

   more 
  particularly 
  referred 
  to 
  below 
  in 
  connection 
  with 
  corresponding 
  parts 
  of 
  the 
  

   investigations 
  at 
  present 
  laid 
  before 
  the 
  Eoyal 
  Society. 
  

  

  6. 
  The 
  object 
  of 
  the 
  present 
  paper 
  is 
  threefold, 
  — 
  

  

  (1.) 
  To 
  shew 
  what 
  modifications 
  of 
  the 
  conclusions 
  arrived 
  at 
  by 
  Carnot, 
  and 
  

   by 
  others 
  who 
  have 
  followed 
  his 
  peculiar 
  mode 
  of 
  reasoning 
  regarding 
  the 
  motive 
  

  

  * 
  " 
  Annalen" 
  of 
  Wohler 
  and 
  Liebig, 
  May 
  1842. 
  

  

  f 
  British 
  Association, 
  August 
  1843, 
  and 
  Philosophical 
  Magazine, 
  September 
  1843. 
  

  

  