358  Mr.-  milner  on  the  Communication  of 
idea  of  the  grounds  of  that  contention  which  has  iub- 
fifted  fo  many  years.  We  here  fee  Bernoulli  taking  it 
for  granted,  that  the  quantity  of  force  in  elaftic  bodies  is 
no  ways  aftedled  by  their  mutual  actions,  whether  direct 
or  oblique ; and  the  moll  furprizing  circumftance  is,  that 
he  Ihould  not  fo  much  as  hint  at  any  apparent  difficulty 
in  the  prefent  cafe,  after  he  had  been  fo  very  diffufe.in 
illuftrating  others  which  were  much  more  fimple.  No 
doubt  he  believed  this  principle  to  be  a diredt  confe- 
quence  of  the  equality  of  adtion  and  re-adlion,  and  there- 
fore it  is  plain  he  could  not  mean  the  fame  things  by  thofe 
terms  as  we  do  at  prefent.  He  believes  no  force  is  gained 
or  loll:  by  impadt ; he  defines  force  by  quantity  of  matter 
and  fquare  of  the  velocity  conjointly;  and  in  eflimating 
the  velocity,  he  pays  no  regard  to  the  diredtion  in  which 
the  bodies  are  moved.  Let  us  not  cavil  at  his  words : we 
cannot  miflake  his  meaning.  The  queflion  is,  how  far 
thefe  notions  are  agreeable  to  experience ; how  far  they 
are  confiftent  with  fome  other  principles  which  are  in- 
conteflable,  and  which  he  himfelf  has  admitted : for  in- 
fiance,  he  admits  it  as  an  undoubted  principle,  that  the 
quantity  of  motion  in  any  fyflem  of  bodies  is  preferved 
invariable,  when  eftimated  in  a given  diredlion,.  in  all 
their  collifions  and  mutual  adtions  upon  one  another; 
and  in  this  he  entirely  agrees  with  the  followers  of  Sir 
ISAAC 
