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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  the 
  scaffolding 
  of 
  the 
  conceptions 
  of 
  mechanics, 
  and 
  to 
  H. 
  A. 
  Lorentz 
  

   the 
  liberation 
  of 
  the 
  conception 
  of 
  the 
  field 
  from 
  a 
  material 
  bearer; 
  

   according 
  to 
  Lorentz 
  the 
  physical 
  empty 
  space 
  (or 
  ether) 
  alone 
  

   figured 
  as 
  bearer 
  of 
  the 
  field; 
  in 
  Newton's 
  mechanics, 
  indeed, 
  space 
  

   had 
  not 
  been 
  devoid 
  of 
  all 
  physical 
  functions. 
  When 
  this 
  develop- 
  

   ment 
  had 
  been 
  completed, 
  no 
  one 
  any 
  longer 
  believed 
  in 
  directly 
  

   acting 
  instantaneous 
  forces 
  at 
  a 
  distance, 
  even 
  in 
  connection 
  with 
  

   gravitation, 
  though 
  a 
  field 
  theory 
  for 
  gravitation, 
  for 
  lack 
  of 
  sufficient 
  

   known 
  facts, 
  was 
  not 
  unmistakably 
  indicated. 
  The 
  development 
  of 
  

   the 
  theory 
  of 
  the 
  electromagnetic 
  field 
  also 
  led, 
  after 
  Newton's 
  

   hypothesis 
  of 
  action 
  at 
  a 
  distance 
  had 
  been 
  abandoned, 
  to 
  the 
  attempt 
  

   to 
  find 
  an 
  electromagnetic 
  explanation 
  for 
  Newton's 
  law 
  of 
  motion, 
  

   or 
  to 
  replace 
  that 
  law 
  by 
  a 
  more 
  accurate 
  law 
  based 
  on 
  the 
  field 
  

   theory. 
  These 
  efforts 
  were 
  not 
  crowned 
  with 
  full 
  success, 
  but 
  the 
  

   mechanical 
  basic 
  conceptions 
  ceased 
  to 
  be 
  regarded 
  as 
  foundation 
  

   stones 
  of 
  the 
  physical 
  conception 
  of 
  the 
  universe. 
  

  

  The 
  Maxwell-Lorentz 
  theory 
  led 
  inevitably 
  to 
  the 
  special 
  theory 
  

   of 
  relativity, 
  which, 
  by 
  destroying 
  the 
  conception 
  of 
  absolute 
  simul- 
  

   taneity, 
  negatived 
  the 
  existence 
  of 
  forces 
  at 
  a 
  distance. 
  Under 
  this 
  

   theory 
  mass 
  is 
  not 
  an 
  unalterable 
  magnitude, 
  but 
  a 
  magnitude 
  de- 
  

   pendent 
  on 
  (and, 
  indeed, 
  identical 
  with) 
  the 
  amount 
  of 
  energy. 
  The 
  

   theory 
  also 
  showed 
  that 
  Newton's 
  law 
  of 
  motion 
  can 
  only 
  be 
  con- 
  

   sidered 
  as 
  a 
  limiting 
  law 
  valid 
  only 
  for 
  small 
  velocities, 
  and 
  sub- 
  

   stituted 
  for 
  it 
  a 
  new 
  law 
  of 
  motion, 
  in 
  which 
  the 
  velocity 
  of 
  light 
  in 
  

   a 
  vacuum 
  appears 
  as 
  the 
  limiting 
  velocity. 
  

  

  THE 
  GENERAL 
  THEORY 
  OF 
  RELATIVITY 
  

  

  The 
  last 
  step 
  in 
  the 
  development 
  of 
  the 
  program 
  of 
  the 
  field 
  

   theory 
  was 
  the 
  general 
  theory 
  of 
  relativity. 
  Quantitatively 
  it 
  made 
  

   little 
  modification 
  in 
  Newton's 
  theory, 
  but 
  qualitatively 
  a 
  deep- 
  

   seated 
  one. 
  Inertia, 
  gravitation, 
  and 
  the 
  metrical 
  behavior 
  of 
  bodies 
  

   and 
  clocks 
  were 
  reduced 
  to 
  the 
  single 
  quality 
  of 
  a 
  field, 
  and 
  this 
  field 
  

   in 
  turn 
  was 
  made 
  dependent 
  on 
  the 
  bodies 
  (generalization 
  of 
  New- 
  

   ton's 
  law 
  of 
  gravitation 
  or 
  of 
  the 
  corresponding 
  field 
  law, 
  as 
  formu- 
  

   lated 
  by 
  Poisson). 
  Space 
  and 
  time 
  were 
  so 
  divested, 
  not 
  of 
  their 
  

   reality, 
  but 
  of 
  their 
  causal 
  absoluteness 
  (absoluteness 
  — 
  influencing, 
  

   that 
  is, 
  not 
  influenced), 
  which 
  Newton 
  was 
  compelled 
  to 
  attribute 
  to 
  

   them 
  in 
  order 
  to 
  be 
  able 
  to 
  give 
  expression 
  to 
  the 
  laws 
  then 
  known. 
  

   The 
  generalized 
  law 
  of 
  inertia 
  takes 
  over 
  the 
  role 
  of 
  Newton's 
  law 
  of 
  

   motion. 
  From 
  this 
  short 
  characterization 
  it 
  will 
  be 
  clear 
  how 
  the 
  

   elements 
  of 
  Newton's 
  theory 
  passed 
  over 
  into 
  the 
  general 
  theory 
  of 
  

   relativity, 
  the 
  three 
  defects 
  above 
  mentioned 
  being 
  at 
  the 
  same 
  time 
  

   overcome. 
  It 
  appears 
  that 
  within 
  the 
  framework 
  of 
  the 
  general 
  

   theory 
  of 
  relativity 
  the 
  law 
  of 
  motion 
  can 
  be 
  deduced 
  from 
  the 
  law 
  

  

  