368  Mr.  milker  on  the  Communication  of 
It  is  now  extremely  eafy  to  trace  thefe  expreffions 
back  again  in  a contrary  order,  and  to  fhew,  that  if  thefe 
laft  equations  are  true,  the  original  one  muft  be  true  alfo  j 
that  px  v2  muft  necefiarily  be  equal  to  —r-  + p or, 
which  is  the  fame  thing,  that  the  body  (^multiplied  into 
the  fquare  of  its  velocity,  and  added  to  the  fum  of  all  the 
products  which  arife  by  multiplying  every  particle  into 
the  fquare  of  its  refpedlive  velocity,  is  equal  to  the  body 
(^multiplied  by  the  fquare  of  the  velocity  which  it  would 
have  acquired  by  the  fame  defcent  in  vacuo. 
Now  this  is  to  give  the  argument  its  full  force ; and 
fince  the  conclufions  are  confirmed  by  repeated  experi- 
ments, as  the  author  himfelf  allures  us,  it  is  prefumed, 
that  the  premifes  can  be  liable  to  no  juft  exception.  If 
we  do  not  think  with  the  advocates  for  this  docftrine,  that 
the  vires  viva  muft  always  remain  the  fame  from  the 
thing  itfelf,  they  will  force  our  aflent  by  the  teftimony  of 
experience,  and  oblige  us  to  admit  their  principles  when 
we  find  it  impoflible  to  deny  the  confequences. 
A prudent  philofopher  is  always  afraid  to  pronounce 
generally  concerning  the  exiftence  of  caufes,  which  are 
attended  with  a variety  of  circumftances,  and  are  com- 
plex in  their  operations.  To  fay  that  the  quantity  of 
force  in  bodies  remains  invariably  the  fame,  feems  to  be 
a propo- 
