Pyrometry  and  Areometry , See.  541 
accompanied  with  a change  of  its  nature,  by  the  mix- 
ture of  vapours  and  exhalations,  which  may  occafion 
great  variations  in  the  law  of  dilatations;  confidering 
above  all,  that  the  changes  of  refringent  power  and  of 
denfity  are  two  co-effe£ts  of  very  different  nature,  the 
progreffions  of  which  may  differ  more,  than  thofe  of  the 
denfities  alone  in  different  bodies.  Here  then  are  com- 
plications of  complications,  which  may  very  likely  accu- 
mulate errors  in  the  intervals  of  the  proportions  fur- 
nifhed  by  experience  between  the  refradlions  and  the 
indications  of  the  Thermometer,  if  thofe  proportions 
were  taken  in  points  very  far  diftant.  The  application 
of  the  theory  of  refractions  to  the  practice  of  aftronomy 
is  as  delicate  as  important,  and  cannot  be  viewed  in  too 
many  lights : which  determines  me  not  to  infill  farther 
here 
fpherique  par  laehaleur,  font  probablement  acepmpagnes  de  changement  dans  fa 
nature  par  \&  melange  des  vapeurs  et  des  exhalaifons,  ce  qui  peut  rendre  la  lol 
de  fes  dilatations  tres  variable;  vu  furtout  que.  les  ehangemens  de  vertu  refrin- 
gente  et  de  denfite,  font  deux  co-eftets  de  nature  bien  differente,  et  dont  les 
marches  peuvent  s’ecarter  davantage,  que  ceiles  des  denfites  feules  dans  difFerens 
corps,  Voila  done  des  complications  de  complications,  qui  pourroient  bien 
accumuler  des  erreursdans  l’intervalle  des  rapports  four n is  par  Inexperience  entre 
les  refractions,  et  les  indications  du  Thermometre,  fi  ces  rapports  etoient  pris  . 
en  des  points  fort  eloignes.  ^application  de  la  theorie  des  refra&ions  a la  pra- 
tique de  l’aftronomie,  eft  au/H  delicate  qu’importante,  et  1’on  ne  fauroit  retwi- 
fagerr 
