﻿• 
  Theory 
  of 
  Gravity. 
  655 
  

  

  And 
  the 
  irrotational 
  motion 
  given 
  by 
  (40) 
  is 
  possible 
  even 
  

   when 
  matter 
  is 
  present. 
  Nothing 
  in 
  the 
  deductions 
  from 
  

   (48) 
  requires 
  the 
  equation 
  of 
  continuity, 
  and 
  on 
  restoring 
  J 
  

   the 
  formula 
  (44) 
  becomes 
  equivalent 
  to 
  (30). 
  

  

  Variation 
  of 
  F 
  and 
  cf> 
  in 
  (44) 
  leads 
  to 
  the 
  electromagnetic 
  

   equations 
  as 
  before. 
  These 
  are 
  unaffected 
  by 
  the 
  motion 
  of 
  

   aether. 
  If 
  we 
  are 
  willing 
  to 
  accept 
  a 
  four-dimensional 
  space 
  

   as 
  physically 
  real, 
  and 
  it 
  seems 
  to 
  me 
  that 
  this 
  is 
  almost 
  

   inevitable, 
  the 
  explanation 
  is 
  easy. 
  Each 
  element 
  of 
  aether 
  

   is 
  capable 
  of 
  a 
  polarization 
  defined 
  by 
  cf> 
  and 
  the 
  components 
  

   of 
  F. 
  The 
  polarization 
  can 
  be 
  varied 
  without 
  varying 
  the 
  

   aether's 
  motion, 
  and 
  the 
  energy 
  arising 
  hence 
  is 
  wholly 
  statical. 
  

   It 
  is 
  determined 
  entirely 
  by 
  the 
  position 
  occupied 
  in 
  the 
  

   statical 
  universe 
  by 
  charged 
  matter. 
  And 
  each 
  element 
  of 
  

   aether 
  as 
  it 
  moves 
  takes 
  up 
  without 
  u 
  lag 
  " 
  the 
  appropriate 
  

   polarization 
  of 
  this 
  statical 
  field. 
  

  

  The 
  electromagnetic 
  field 
  causes, 
  however, 
  a 
  volume 
  force 
  

   of 
  amount 
  

  

  ^V(H 
  2 
  -B 
  2 
  j 
  

  

  in 
  the 
  aether, 
  which 
  merely 
  alters 
  its 
  pressure 
  without 
  

   affecting 
  the 
  irrotational 
  flow. 
  

  

  All 
  this 
  will 
  hardly 
  satisfy 
  physicists 
  who 
  still 
  regard 
  a 
  

   fourth 
  dimension 
  as 
  merely 
  a 
  mathematician's 
  dream. 
  I 
  have 
  

   therefore 
  discussed. 
  again 
  in 
  § 
  9 
  how 
  the 
  theory 
  of 
  a 
  moving 
  

   aether 
  can 
  be 
  reconciled 
  with 
  the 
  facts 
  of 
  optics. 
  It 
  is 
  

   then 
  necessary 
  to 
  interpret 
  the 
  electromagnetic 
  equations 
  

   in 
  terms 
  of 
  the 
  local 
  time 
  at 
  any 
  point 
  of 
  the 
  aether. 
  The 
  

   results 
  may 
  placate 
  those 
  who 
  are 
  more 
  at 
  ease 
  in 
  three 
  

   dimensions 
  than 
  in 
  four. 
  

  

  § 
  6. 
  Matter 
  and 
  Mtrer. 
  

  

  My 
  main 
  concern 
  is 
  with 
  the 
  law 
  of 
  gravity 
  ; 
  it 
  is 
  not 
  my 
  

   purpose 
  to 
  discuss 
  here 
  the 
  various 
  hypotheses 
  which 
  may 
  be 
  

   adopted 
  as 
  to 
  the 
  nature 
  of 
  matter. 
  The 
  essential 
  point 
  of 
  

   my 
  theory 
  is 
  that 
  matter, 
  whatever 
  else 
  it 
  may 
  be, 
  is 
  an 
  

   aether-sink 
  — 
  or 
  an 
  aether-source. 
  Thus 
  I 
  have 
  so 
  far 
  borne 
  

   in 
  mind 
  that 
  we 
  may 
  wish 
  to 
  treat 
  matter 
  as 
  a 
  distinct 
  

   substance, 
  the 
  electric 
  fluid. 
  But 
  a 
  much 
  more 
  radical 
  treat- 
  

   ment 
  is 
  possible. 
  We 
  need 
  no 
  longer 
  suppose 
  that 
  there 
  are 
  

   two 
  distinct 
  substances, 
  aether 
  and 
  matter. 
  I 
  shall 
  now 
  

   postulate 
  that 
  there 
  is 
  but 
  one 
  substance, 
  a 
  fluid 
  whose 
  

   density, 
  motion, 
  and 
  pressure 
  are 
  everywhere 
  continuous. 
  

  

  In 
  " 
  aether 
  " 
  this 
  fluid 
  is 
  polarized, 
  in 
  " 
  matter 
  " 
  it 
  is 
  

  

  