Pyrometry  and  Areometry , See.  437 
The  firft  phyfical  difficulty  I met  with  was  not  new 
to  me : I had  already  met  with  it  in  two  different  ma- 
chines in  which  I had  made  ufe  of  metals;  in  the  one, 
to  mark  the  variations  of  the  heat;  in  the  other,  to  com- 
penfate  the  effe£ts  of  it : it  was  the  irregular  dilatations 
of  metals. 
The  laft  of  thefe  machines,  which  is  that  on  which  I 
have  bellowed  the  molt  care,  and  which  I have  ftudied 
with  the  greateft  attention,  corrects  the  effedts  of  the 
heat  upon  a Barometer,  and  upon  an  Hygrometer  of  my 
firft  conftrudfion,  which  is  joined  to  it.  A ftrong  rod  of 
well  hardened  brafs  fupports  upon  an  edge,  at  a con- 
venient diftance  from  the  center  of  motion,  a lever, 
which  holds  the  fcale  of  the  Barometer  fufpended,  and 
makes  it  rife  or  fall  by  the  dilatation  or  condenfa- 
tion  of  the  brafs  rod,  as  the  quickfilver  rifes  or  falls  in 
the 
La  premiere  des  difficulties  phyliques  que  j’ai  rencontrees  ne  m’etoit  pas  in- 
connue:  je  1’avois  deja  eprouvee  en  deux  machines  differentes  ou  j’avois  em- 
ploye des  metaux,  dans  l’une  pour  marquer  les  variations  de  la  chaleur,  et  dans 
l’autre  pour  en  compenfer  les  effets;  e’eft  1’irregularite  des  dilatations  des  metaux. 
La  derniere  de  ces  machines,  qui  eft  celle  a laquelle  j’avois  donne  le  plus  de 
foin,  et  que  j’ai  etudiee  avec  le  plus  d’attention,  corrige  les  effets  de  la  chaleur 
fur  un  Barometre,  et  fur  un  Hygrometre  de  ma  premiere  conftru£lion  qui  lui  eft 
joint,  Une  forte  branche  de  leton,  bien  durcie  a la  filiere,  foutient  fur  un 
tranchant,  a une  diftance  convenable  du  centre  de  mouvement,  un  levier  qui 
tient  fufpendue  l’echelle  du  Barometre,  et  qui  la  fait  monterou  defeendre,  par  la 
dilatation  ou  condenfation  de  la  branche  de  leton,  comme  le  mercure  monte  ou 
defeend 
