Motion  by  Impact  and  Gravity.  333 
case  the  first.  Suppofe  a and  b to  reprefent  the 
magnitudes  of  two  fpherical  bodies,  and  a and  b their 
refpedtive  velocities  in  the  fame  direction ; fuppofe  a to 
be  greater  than  b , and  a will  overtake  b;  and  if  the  bo- 
dies are  non-elaftic,  they  will  proceed  together  in  the 
fame  diredtion  as  one  mafs : if  they  are  perfectly  elaftic, 
whatever  effedt  has  already  been  produced  by  the  colli- 
fion,  will  be  repeated;  and,  beeaufe  in  the  firft  cafe  there 
is  no  relative  velocity  after  the  ftroke,  in  the  fecond  the 
relative  velocity  before  and  after  the  ftroke  will  be  the 
fame,  and  in  contrary  diredtions;  and  in  either  cafe,  the 
motion  loft  by  the  ftriking  body  is  found  to  be  always 
equal  to  the  motion  communicated  to  b,  and  in  a con- 
trary diredtion.  In  this  fenfe  adtion  is  equal  to  re-adtion ; ; 
and  every  experiment  which  has  yet  been  produced, 
where  a clear  judgment  could  be  formed  of  the  effedt, 
has  confirmed  the  fame  thing.  All  the  experiments 
which  are  ufually  brought  to  determine  the  impreffions 
made,  upon  foft  bodies,  as  fnow,  clay,  &c.  are  abfolutely> 
unfit  for  the  purpofe.  The  circumftances,  which  take- 
place  in  the  produdlion  of  thefe  effedts,  are  fuch  as  we  : 
can  never  difcover.  The  diredlions  in  which  the  parti-  ■ 
cles  recede,  the  velocities  they  acquire,  their  mutual  ac-  ■ 
tions  upon  one  another,  and  laftly,  the  time,  in  which 
thefe  effedts  are  performed,  are  all  beyond  the  . reach  of . 
compu-  - 
