Motion  by  ImpaSt  and  Gravity.  377 
2d.  In  the  fecond  cafe,  the  fame  principle  is  taken  for 
granted  by  j.  Bernoulli.  We  have  exarpined  into  the 
confequences  of  this  author’s  folution,  and  fhewn  that 
his  hypothelis  will  prove  all  bodies  to  be  perfectly  elaftic. 
As  the  fteps  by  which  he  deceived  himfelf  are  here  ex- 
poled,  whoever  carefully  attends  to  thefe  two  examples 
cannot  eafily  miftake  in  any  cafe  that  may  occur.  It  is 
plain,  that  if. any  one  contends  for  the  equality  of  afliou 
and  re-action,  and  explains  thofe  terms  by  the  changes 
produced  in  the  abfolute  forces  of  the  bodies,  the  difpute  - 
is  not  merely  verbal. . 
3d.  When  a conclulion,  agreeable  to  experience,  is  de- 
duced from  any  hypcthelis,  it  does  not  therefore  necef- 
farily  follow,  that  the  hypothelis  is  univerfally  true,  not 
even  fuppoling  the  converfe  of  the  propolition  to  hold. 
In  this  third  cafe  it  is  fhewn,-  what  kind  of  anfwer  we  are 
to  give  fuch  reafoning.  The  confervatio  virium  vivarum 
is  never  to  be  admitted,  unlefs  its  connexion  with  limple 
fails,  which  are  inconteftable,  be  firft  made  out.  The 
folution  of  this  problem  depends  on  this,  that  the 
motion  loft  is  equal  to  the  motion  communicated  in 
a contrary  direction  after  the  property  of  the  lever 
is  taken  into  the  account ; and  therefore  the  nice 
agreement  of  Mr.  s me  a ton’s  experiments  with  the 
theory 
