Pyrometry  and  Ar  comet ry,  See.  ye  j 
nation  of  the  two  fubftances,  which  would  preferve  the 
fame  length  to  the  pendulum  between  two  determined 
temperatures. 
To  make  this  {till  ealier,  it  would  be  poffible  to  con- 
ftrufit  the  grid-iron  in  fuch  a manner  as  that  its  correc- 
tive rods  being  fixed  by  fc-rews,  the  proportions  of  their 
lengths  might  be  changed,  till  the  bell-metal  perfectly 
compenfate  for  the  lengthening  of  the  glafs  between  the 
tw’O  fixed  temperatures,  of  which  I fhall  fpeak  hereafter. 
•For  there  would  be  nothing  to  correct  by  this  method, 
but  the  difference  of  expanfibilities  of  the  fubftances 
employed,  comparatively  with  the  mean  expanfibility  of 
fubftances  of  the  fame  denomination,  which  would  have 
been  firft  difeovered  by  experiments  made  for  that  pur- 
pofe.  I {hall  refume  hereafter  this  correction  of  the  pen- 
dulum. 
courte,  la  combi naifon  des  deux  matieres  qui  conferveroit  fenflblement  au  pen- 
dule,  une  meme  longueur  dans  des  variations  determinees  de  temperature. 
Pour  rendre  ce  moyen  plus  commode,  on  pourroit  ailement  conffruire  la  grille 
de  maniere,  que  fes  branches  correctives  etant  retenues  dans  leurs  traverfes  par 
des  vis,  on  put  changer  les  rapports  de  leurs  longueurs,  jufqu’a  ce  que  le  metal 
de  cloche  compenfat  parfaitement  l’allongement  du  verre  entre  les  deux  tempera- 
tures fixees,  dont  je  parlerai  ci-apres.  Car  on  n’auroit  a,  corriger  par  cette 
methode,  que  les  differences  d’expanfibilite  des  matieres  employees,  comparative- 
ment  a l’expanlibilite  moyenne  des  matieres  de  meme  nom,  qu’on  anroit  trouvee 
premierement  par  des  experiences  faites  a ce  deffein.  Je  reviendrai  dans  la  iuite 
H h h a a cette 
