Pyrometry  and  Areometry , 8tc,  427 
relation  of  the  expanfibilities  of  the  two  fubftances. 
Now  nothing  is  eafier  than  to  do  this  by  means  of  a Mi- 
crofcope,  furnifhed  with  a tingle  immoveable  wire;  for 
the  wire  being  fitted  to  a point  of  the  free  rod,  and  the 
two  rods  being  equally  warmed,  if  this  point  moves,  it  is 
a fign  that  it  is  not  that  which  is  fought  for,  which  will 
then  foon  be  found  by  pointing  the  Microfcope  higher 
or  lower  on  the  free  rod,  according  to  what  fhall  have 
been  indicated  by  the  firft  trials. 
Thus  then  will  the  relation  of  the  expanfibilities  of 
two  fubftances  be  procured  without  the  neceflity  of  hav- 
ing recourfe  to  any  Micrometer;  confequently,  without  the 
rilks  of  the  errors  thofe  inftruments  are  fubjedt  to,  when. 
they  are  ufed  in  very  nice  meafures.  All  that  will  be 
neceffary  for  the  exadtnefs  of  the  obfervations  will  only 
be, 
expanfibilities  des  deux  matieres.  Or  il  eft  tres  aife  de  le  trouver  par  le  moyen  d’un 
Microfcope  muni  d’un  feul  fil  immobile.  Car  en  ajuftant  ce  fil  fur  un  point  de  la 
branche  libre,  et  echauffant  egalement  les  deux  branches,  ft  ce  point  fe  meut, 
cc  ne  fera  pas  celui  qu’on  cherche ; mais  on  le  trouvera  bientot  par  tatonnement, 
en  pointant  le  Microfcope  plus  haut  ou  plus  bas  fur  la  branche  libre,  fuivant  que 
les  premieres  tentatives  l’auront  indique. 
On  aura  done  ainli  les  rapports  des  expanfibilites  de  deux  matieres,  fans 
befoin  de  Micrometre,  et  par  confequent  fans  etre  expofe  aux  erreurs 
qu’il  pourroit  introduire  dans  des  mefures  aufli  delicates:  et  tout  ce  qui 
♦ft  necelfaire  pour  I’exadlitude  ne  confifte  qu’a  s’aflurer,  qu’un  Microfcope 
E e e 2 et 
