Motion  by  ImpaSl  and  Gravity.  349 
raetry  have  been  already  applied  to  the  theory  of  motion 
in  numberlefs  cafes,  and  a variety  of  well  attefted  expe- 
riments have  been  clearly  explained  to  us  by  authors. 
The  laws  of  motion,  in  certain  cafes,  are  inconteftable, 
and  no  author  of  eminence  contradicts  them  : it  is  from 
a miftaken  application  of  thefe  laws  that  a difference  of 
opinion  has  arifen.  It  is  obvious,  that  the  laws  of  motion, 
as  defcribed  by  Sir  isaac  newton,  may,  in  a certain 
fenfe,  be  founded  on  experiment;  and  yet,  if  they  are 
extended  to  cafes  where  they  cannot  be  applied,  the  con- 
clufions  mult  ffcill  be  erroneous.  My  defign  in  thefe 
pages  is  to  point  out  diffinCtly  what  is  real  in  this  dif- 
ference of  opinion  from  what  is  merely  verbal,  and  to 
explain  the  caufes  of  it.  This,  which  perhaps  will  ap- 
pear to  have  never  been  done  with  fufficient  precifion, 
feems  to  be  the  moil;  effectual  way  of  preventing  mif- 
takes.  Geometry  and  algebra  will  lead  us  wrong,  if  our 
principles  are  ill  founded:  experiment  itfelf,  if  we  are 
not  extremely  careful,  will  deceive  us  in  forming  a ge- 
neral deduction,  or  what  is  called  a law  of  nature.  The 
controveriial  writings  of  the  moil  able  authors  will  em- 
barrafs  and  perplex  our  judgements;  but  when  we  have 
once  difcovered  the  grounds  of  their  mutual  miftakes 
and  mifappreheniions,  there  is  reafon  to  think,  that  we 
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