Pyrometry  and  Areometry , &c.  459 
We  are  very  far  from  being  able  to  perceive,  in  the 
quantities  of  expansions,  thofe  differences  which  might 
arife  from  the  imperfection  of  this  meafure. 
One  may  take  therefore  a rod  of  fome  fubftance,  glafs 
for  inftance,  and  put  at  the  end  of  it  a convenient  clafp 
for  holding  other  rods  of  different  fubftances : thus,  by 
feparately  comparing  their  expansions  with  that  of  the 
rod  of  glafs,  by  the  position  of  the  immoveable  point, 
one  will  obtain  the  proportion  of  their  expansibility 
with  that  of  glafs,  and  consequently  with  each  other. 
Nor  may  folids  only  be  Subjected  to  thefe  experiments, 
but  fluids  alfo : for  by  enclofing  them  in  a cylindrical 
- tube  of  glafs,  the  expansibility  of  which  is  known,  they 
will  be  as  rods,  which  may  be  thus  Submitted  to  the  fame 
experi- 
ciine  confequence,  Nous  fommes  bien  loin  de  pouvoir  reconnoitre  dans  les 
quantites  des  expanlions,  les~  differences  qui  pourroient  refulter  de  l’imperfe&ion 
de  cette  mefure. 
On  pourra  done  avoir  une  branclie  d’une  certaine  matiere,  de  verre  par  exem- 
ple,  a l’un  des  bouts  de  laquelle  on  aura  ajufie  une  pince  commode,  pour  y fixer 
d’autres  branches  de  diverfes  matieres : et  comparant  ainfi  feparement  leur  expan- 
fion  avec  celle  de  la  branche  de  verre,  par  la  pofition  du  point  immobile,  on 
aura  les  rapports  de  leur  expanfibilite  avec  celle  du  verre,  et  par  confequent  entre 
dies. 
Ce  ne  fera  pas  feulement  les  folides  qu’on  pourra  foumettre  a ces  experiences; 
mais  les  fluides.  Car  en  les  renfermant  dans  un  tuyau  de  verre  cylindrique,  dont 
l'expanfibilite  foit  connue,  on  en  fera  comnie  des  branches,  qu’on  pourra  ainfi 
l i i 2 foumettre.< 
