Mr.  de  luc  on 
486 
When  from  experiments,  fometimes  very  nice,  we 
have  affured  ourfelves  that  two  or  more  effects  con- 
ftantly  go  together  in  certain  relations,  we  may  content 
ourfelves  with  obferving  the  moft  evident  of  them, 
and  depend  upon  the  exiftence  of  the  others,  as  if  they 
were  immediately  obferved.  This  leads  us  from  relation 
to  relation  to  the  difcovery  of  operations  of  Nature  which 
before  were  entirely  hidden : and  nothing  can  be  more 
ufeful  to  man,  than  fometimes  to  examine,  how  he  rea- 
foned  upon  thofe  objects  before  he  was  guided  by- 
experience. 
Thefeinftancesof  the  connexions  of  effects, difcovered, 
and  afterwards  meafured  one  by  the  other,  are  now  be- 
come fo  frequent  in  natural  philofophy  that  it  would  be 
ufelefstoinfiftupon  them : and  indeed  when  oneconfiders 
our  phyfical  inftruments,  one  may  fee  that  the  greateft 
part 
Quand  par  des  experiences,  fouvent  tres  delicates,  nous  nous  fommes  allures 
que  deux  ou  plufieurs  effets  marchent  toujours  enfemble  dans  certains  rapports, 
nous  pouvons  nous  contenter  d’obferver  le  plus  evident,  et  compter  furl’exiftence 
des  autres  comme  s’ils  etoient  immediatement  manifeftes;  ce  qui  nous  conduit, 
de  rapport  en  rapport,  a decouvrir  des  operations  de  la  Nature,  qui  avant  cela 
nous  etoient  entierement  voilees : et  rien  n’eft  plus  necelTaire  a l’Homme,  que 
d’examiner  quelquefois  comment  il  en  raifonnoit  avant  qu’il  eut  ces  fecours. 
Les  exemples  de  ces  liaifons  d’efFets,  decouvcrts,  et  mefu'res  enfuite  les  uns  par 
les  autres,  font  aujourd’hui  li  multiplies  dans  la  Phyfique,  qu’il  feroit  inutile 
4’infifler  fur  ce  point:  et  quand  on  confidere  meme  Fenfemble  de  nos  machines 
