» $2  Mr.  milker  on  the  Communication  of 
inference  of  reafon,  exclufive  of  all  experiment;  and 
yet,  if  words  have  any  meaning  at  all,  the  above  quota- 
tion inclines  us  to  think  fo.  It  is  true,  the  change  which 
is  produced  in  either  body  may  be  meafured  by  the 
aCtion  which  it  exerts  upon  the  other,  or  by  the  refin- 
ance which  it  meets  with  from  the  fame:  but  what  are 
we  to  underhand  by  aCtion  or  refill  an  ce,  until  they  are 
explained  by  more  intelligible  terms?  or,  when  they  are 
explained  by  terms  which  do  not  neceffarily  imply  the 
fame  thing,  how  do  we  know  that  their  meafures  are 
equal,  or  that  they  are  made  in  contrary  directions,  until 
thefe  truths  be  eflablillied  by  experiments?-  A law  of  na- 
ture is  not  merely  a deduction  of  reafon:  it  muft  be 
proved,  either  at  once  and  directly,  by  fome  fimple  and 
decifive  experiments;  or  if  that  cannot  be  done,  by  fuch 
experiments  as  enable  us  to  colled  its  exiftence  by  the 
affiftance  of  geometry.  However  obvious  thefe  reflec- 
tions may  appear,  I thought  it  necelfary  to  take  notice  of 
maclaurin’s  affertion ; becaufe  in  confequence  of  that 
and  fimilar  expreffions,  young  philofophers  are  ex- 
tremely puzzled  in  the  beginning  of  their  ftudies,  and 
becaufe  I have  known  fome,  who  are  more  experienced, 
affirm,  that  the  third  law  of  motion  is  nothing  more 
than  a definition.  I now  proceed  to  the  confideration  of 
particular  cafes. 
7 
CASE 
