﻿ISAAC 
  NEWTON 
  EINSTEIN 
  203 
  

  

  infinitely 
  short 
  period 
  (differential 
  law) 
  does 
  Newton 
  arrive 
  at 
  a 
  

   formula 
  which 
  is 
  applicable 
  to 
  all 
  motions. 
  He 
  takes 
  the 
  conception 
  

   of 
  force 
  from 
  the 
  already 
  highly 
  developed 
  theory 
  of 
  statics. 
  He 
  

   is 
  only 
  able 
  to 
  connect 
  force 
  with 
  acceleration 
  by 
  introducing 
  the 
  new 
  

   conception 
  of 
  mass, 
  which, 
  indeed, 
  is 
  supported 
  curiously 
  enough 
  

   by 
  an 
  apparent 
  definition. 
  To-day 
  we 
  are 
  so 
  accustomed 
  to 
  forming 
  

   conceptions 
  which 
  correspond 
  to 
  differential 
  quotients 
  that 
  we 
  can 
  

   hardly 
  realize 
  any 
  longer 
  how 
  great 
  a 
  capacity 
  for 
  abstraction 
  was 
  

   needed 
  to 
  pass 
  across 
  a 
  double 
  barrier 
  to 
  the 
  general 
  differential 
  laws 
  

   of 
  motion, 
  with 
  the 
  further 
  need 
  to 
  evolve 
  the 
  conception 
  of 
  mass. 
  

  

  But 
  this 
  was 
  still 
  a 
  long 
  way 
  from 
  the 
  causal 
  comprehension 
  of 
  the 
  

   phenomena 
  of 
  motion. 
  For 
  the 
  motion 
  was 
  only 
  determined 
  by 
  the 
  

   equation 
  of 
  motion 
  if 
  the 
  force 
  was 
  given. 
  Newton 
  had 
  the 
  idea, 
  to 
  

   which 
  he 
  was 
  probably 
  led 
  by 
  the 
  laws 
  of 
  the 
  planetary 
  motions, 
  

   that 
  the 
  force 
  acting 
  on 
  a 
  mass 
  is 
  determined 
  by 
  the 
  position 
  of 
  all 
  

   masses 
  at 
  a 
  sufficiently 
  small 
  distance 
  from 
  the 
  mass 
  in 
  question. 
  

   Not 
  until 
  this 
  connection 
  was 
  realized 
  was 
  a 
  completely 
  causal 
  com- 
  

   prehension 
  of 
  the 
  phenomena 
  of 
  motion 
  obtained. 
  How 
  Newton, 
  

   proceeding 
  from 
  Kepler's 
  laws 
  of 
  the 
  motion 
  of 
  planets, 
  solved 
  this 
  

   problem 
  for 
  gravitation 
  and 
  so 
  discovered 
  the 
  identity 
  of 
  the 
  nature 
  

   of 
  gravity 
  with 
  the 
  motive 
  forces 
  acting 
  on 
  the 
  stars 
  is 
  common 
  

   knowledge. 
  It 
  is 
  only 
  the 
  combination 
  of 
  — 
  

  

  (Law 
  of 
  motion) 
  -f( 
  Law 
  of 
  attraction) 
  

  

  through 
  which 
  is 
  constituted 
  that 
  wonderful 
  thought-structure 
  which 
  

   enables 
  the 
  earlier 
  and 
  later 
  conditions 
  of 
  a 
  system 
  to 
  be 
  calculated 
  

   from 
  the 
  conditions 
  ruling 
  at 
  one 
  particular 
  time, 
  in 
  so 
  far 
  as 
  the 
  

   phenomena 
  occur 
  under 
  the 
  sole 
  influence 
  of 
  the 
  forces 
  of 
  gravitation. 
  

   The 
  logical 
  completeness 
  of 
  Newton's 
  system 
  of 
  ideas 
  lay 
  in 
  the 
  fact 
  

   that 
  the 
  sole 
  causes 
  of 
  the 
  acceleration 
  of 
  the 
  masses 
  of 
  a 
  system 
  

   prove 
  to 
  be 
  the 
  masses 
  themselves. 
  

  

  On 
  the 
  basis 
  sketched 
  Newton 
  succeeded 
  in 
  explaining 
  the 
  motions 
  

   of 
  the 
  planets, 
  moons, 
  comets, 
  down 
  to 
  fine 
  details, 
  as 
  well 
  as 
  the 
  ebb 
  

   and 
  flow 
  of 
  the 
  tides 
  and 
  the 
  jjrecessional 
  movement 
  of 
  the 
  earth 
  — 
  

   this 
  last 
  a 
  deductive 
  achievement 
  of 
  peculiar 
  brilliance. 
  It 
  was, 
  no 
  

   doubt, 
  especially 
  impressive 
  to 
  learn 
  that 
  the 
  cause 
  of 
  the 
  movements 
  

   of 
  the 
  heavenly 
  bodies 
  is 
  identical 
  with 
  the 
  force 
  of 
  gravity, 
  so 
  

   familiar 
  to 
  us 
  from 
  everyday 
  experience. 
  

  

  SIGNIFICANCE 
  OF 
  NEWTON'S 
  ACHIEVEMENT 
  

  

  The 
  significance, 
  however, 
  of 
  Newton's 
  achievement 
  lay 
  not 
  only 
  

   in 
  its 
  provision 
  of 
  a 
  serviceable 
  and 
  logically 
  satisfactory 
  basis 
  for 
  

   mechanics 
  proper 
  ; 
  up 
  to 
  the 
  end 
  of 
  the 
  nineteenth 
  century 
  it 
  formed 
  

   the 
  program 
  of 
  all 
  theoretical 
  physical 
  research. 
  All 
  physical 
  phe- 
  

  

  