Pyrom&try  and  Areometry , Sr c.  423 
enough  to  do  it,  and  told  me,  that  he  propofed  meafuring 
the  expanlions  of  bodies,  by  the  Micrometer  of  a Mi- 
crofcope ; by  which  means  he  fliould  obviate  the  greateft 
mechanical  difficulties.  He  added,  moreover,  that  he 
had  made  a firft  trial  of  his  method  a long  while  ago, 
and  was  allured  of  the  fuccefs. 
This  idea  ftruck  me,  and  being  very  delirous  of  fol- 
lowing it  in  my  prefent  need,  I determined,  if  I could  hit 
upon  any  method  within  my  compafs  of  ability,  to  un- 
dertake to  execute  it  myfelf. 
I found  many  difficulties  fo  long  as  I only  thought 
of  abfolute  meafures  of  the  expanlions  of  bodies.  I 
was  determined  not  to  fet  to  work  without  the  hope 
of  making  an  inftrument  that  Ihould  be  really  an  exadt 
one,  and  the  Micrometer  always  puzzled  me.  But 
coming  happily  to  reflebt  that  I did  not  want  abfolute 
meafures, 
de  le  faire.  II  me  dit  done,  qu’il  fe  propofoit  de  mefurer  les  expanfions  des 
corps,  au  moyen  du  Micrometre  d’un  Microfcope;  ce  qui  eviteroit  les  plus 
grandes  difficultes  mechaniques:  il  ajoutameme,  qu’il  avoit  fait  depuis  long  terns 
nn  premier  effai  de  cette  methode,  ct  qu’il  etoit  perfuade  du  fucces. 
Cette  idee  me  frappa;  et  defirant  beaucoup  d’en  faire  ufage  dans  mon  befoin 
prefent,  je  me  determinai  a entreprendre  de  l’executer  moi-meme,  fi  je  pouvois 
imaginer  quelque  moyen  qui  fut  a ma  portee. 
Je  trouvai  beaucoup  de  difficultes  a cette  entreprife,  tant  que  je  ne  fongeai 
qu’a  des  mefures  abfolues  des  expanfions  des  corps.  Je  ne  voulois  pas  mettre  la 
main  a l’oeuvre,  fans  avoir  l’efperance  de  faire  une  machine  vraiment  exa&e;  et 
le  Micrometre  m’embaralToit  toujours,  Mais  venant  heureufement  a confiderer, 
que 
