Motion  by  Impact  and  Gravity.  351 
Some  ufeful  writers,  who  have  copied  after  Sir  isaac 
newton,  have  talked  in  the  fame  way;  and  only  in- 
ereafed  the  ambiguity  by  being  more  diffufe.  Mr.  mac- 
eaurin  himfelf,  who  engaged  very  warmly  in  this  de- 
bate with  the  foreign  mathematicians,  and  who,  to  fay  the 
truth,  feems  to  have  underftocd  the  nature  of  the  con- 
troverfy  better  than  any  one.  elfe,  is  frequently  unguarded : 
in  his  expreflion.  In  chap.  II.  book  2.  of  his  account  of 
newton’s  difcoveries,  he  is  defcribing  the  laws  of  motion 
for  the  firft  time,  and. one  naturally  expects  a more  than 
ordinary  precifion  and  exa&nefs.  There  he  blames, 
very  juftly,,  the  oppofers  of  the  Newtonian  definition  of 
motion  for  miftaking  the  direction  in  which  the  motion,, 
loft. of  communicated,  ought  always  to  be  eftimated.  But 
in  p.  X2  2:W,  he  thus  exprefies  himfelf:  “ When  two 
“ bodies  meet,  each  endeavours  to  perfevere  in  its  ftate,. 
“ and  refills,  any  change;  and  becaufe  the  change,  which 
“ is  produced  in  either,  may  be  equally  meafured  by  the- 
“ aCtion,  which  it  exerts  upon  the  other,  or  by  the  re- 
“ fiftance,  which  it  meets  with  from  it,  it  follows,  that 
“ the  changes  produced  in  the  motions  of  each  are 
“ equal ; but  are  made  in  contrary  directions-.*  I cannot 
poffibly  conceive,  that  fo  Ikilful  and  accurate  a philofo- 
gher  could  beheve,  that  the  third  law  of  motion  was  an; 
( c)  0£lavo  edition. 
inference;- 
