Pyrometry  and  Areometry , See.  487 
part  of  them  are  intended  only  for  the  difeovery  of  co- 
effeCts,  by  the  knowledge  of  thofe  which  are  more  evi- 
dent. Our  fearch  after  new  meafures  has  likewife  the 
fame  end  in  view.  If  we  wifh  for  an  Hygrometer,  an 
Electrometer,  a Photometer,  it  is  lefs  with  a defign  of 
arriving  by  means  of  them  to  a knowledge  of  the 
abfolute  or  relative  quantities,  of  moifture,  of  eledtri® 
fluid,  of  light,  than  to  endeavour  afterwards  to  connedt 
the  perceptible  effedts  of  thefe  caufes  upon  our  mea- 
fures, with  other  lefs  evident  effedts,  but  which  depend 
upon  them,  either  as  feparate  co-effedts,  or  as  modifica- 
tions of  other  effedts. 
With  all  this  the  general  problem  of  phyfical  mea- 
fures is  a complicated  one  from  the  firfl  outfet.  The 
flrft  objedt  of  all  thefe  meafures  is  to  know  the  exiftence 
of  a Ample  caufe  and  of  its  degrees  of  intenfity ; and  we 
have 
de  phyfique,  on  voit  que  le  plus  grand  nombre  n’eft  deftine  qu’a  determiner  des 
co-effets,  par  la  connoiffance  de  ceux  qui  font  le  plus  evidens.  Laplupart  aufft 
de  nos  recherches  de  nouvelles  mefures  tendent  a ce  meme  but.  Si  nous  defirons 
un  Hygrometre,  un  Ele&rometre,  un  Photometre,  e’eft  moins  pour  connoitre, 
en  les  obfervant*  les  quantites  abfolues  ou  relatives  de  l’humidite,  du  fluide  elec- 
trique,  de  la  lumiere;  que  pour  travailler  enfuite  a Her  les  effets  evidens  de  ces 
caufes  fur  nos  mefures,  a d’autres  effets  moins  evidens  qui  en  dependent,  ot* 
comme  co-effets  fepares,  ou  comme  modifications  d’autres  efiets. 
Cependant  le  probleme  general  des  mefures  phyfiques  eft  complique  des  fon 
premier  pas,  Connoitre  I’exiftence  d’une  caufe  fimple  et  fe$  degres  d’intenfite, 
eft 
