Pyrometry  and  Areometry , See,  457 
comparative  expansibilities,  or  the  proportion  there  is 
between  the  expansibilities  of  bodies,  which  feem  to  me 
by  this  method  to  be  reduced  to  an  operation  as  Simple  as 
it  is  conclufive. 
Firft,  with  refpect  to  the  frame  of  my  Pyrometer, 
however  rude  it  be,  I have  hitherto  difeovered  no  de- 
feat in  it.  A point  of  fufpenfion  of  the  rods  which 
preferves  throughout  the  experiment  its  relative  por- 
tion to  the  Micro fcope,  is  the  eafieft  thing  to  obtain  by- 
means  of  this  deal  frame,  wdiich  is  not  fenlibly  affebted 
by  the  fmall  changes  in  the  air,  and  is  expofed  to  no  other 
variation : and  this  is  a great  point  gained  when  it  is  con- 
lidered,  that  hitherto  frames  had  a fenfible  effedt  upon 
the  indications  of  the  Pyrometers;  and  that  all  that  could 
be  done,  was  to  endeavour  to  obviate  this  influence. 
Again, 
entre  les  expanhbilites  des  corps,  qui  me  paroiffent  reduites  par  cette  route  a 
une  operation  auffi  limple  que  fure. 
D’abord  quant  a !a  monturede  mon  Pyrometre,  quelque  groiliere  quklle  foit, 
je  n’y  ai  trouve  jufqu’a  prefent  aucun  defaut.  Un  point  de  fufpenfion  des 
branches,  qui  conferve  pendant  Pexperience  fa  pofition  relative  a celle  du  Mr* 
crofcope,eft  la  chofe  la  plus  aifee  a obtenir  par  cette  monture  de  fapin  ; ks  petits 
changemens  de  Pair  ne  l’affe&ant  point  feniiblement,  et  n’etant  expofee  a,au- 
cune  autre  caufe  de  variation.  Or  c’eil  la  un  grand  point  d’obtenu,  quand  on 
conildere  que  jufqu’  ici  les  montures  influoient  toujours  effentiellement  fur  les 
indications  des  Pyrometres,  et  que  tout  ce  qu’on  pouvoit  faire,  etoit  de  chercher 
a en  corriger  les  effets. 
Yol.  LXVIII. 
I i i 
Enfuite* 
