Motion  by  Impact  and  Gravity v 363 
NOulli  knew  the  prefervation  of  the  vires  viva  to  be  an 
infallible  confequence  of  perfect  elafticity  in  bodies;  or 
indeed  that  he  had  any  other  realbn  for  taking  that  prin- 
ciple for  granted,  but  becaufe  he  was  not  able  to  prove  it. 
All  the  inftances  that  are  ufually  brought  on  both  fides 
are  to  be  treated  in  a fimilar  way.  The  meaning  of  the 
terms  muft  firft  be  defined ; then  the  principles  affumed 
explained ; and  if  we  cannot  tell  at  firft  fight,  whether 
they  are  agreeable  to  experience  or  not,  as  is  frequently 
the  cafe;  we  muft  examine  into  their  confequences  by 
the  affiftance  of  geometry,  and  we  lhall  at  laft  arrive  at 
fome  Ample  principle,  the  exiftence  of  which  is  necef- 
farily  implied  in  the  original  hypothefis.  The  collifion 
of  fpherical  bodies  is  the  moft  Ample  way  of  communi- 
cating motion  from  one  to  another;  and  therefore  fuch 
examples  are  better  adapted  to  throw  light  on  a difputa- 
ble  queftion,  than  where  the  fuppofitions  are  more  per- 
plexed with  mechanical  contrivances.  Befides,  when  the 
theory  of  mechanics  is  well  underftood,  and  the  founda- 
tions of  error  difcovered,  the  fame  reafonings  are  eafily 
transferred  to  other  cafes,  and  fimilar  precautions  applied. 
Indeed  practical  artifts  have  little  to  do  with  the  fudden 
communication  of  motion  by  impact.  The  collifions  of 
bodies  are  too  violent  operations  to  enter  into  the  com- 
Vol.  LXVIII.  A n jpofition 
