Pyrometry  and  Areometry , See.  47  5 
ever  irregularities  there  might  be  in  the  obfervations, 
they  would  certainly  arife  only  from  the  bodies  that  may- 
be compared  to  it,  and  might  for  that  reafon  be  more 
eafily  afeertained  and  determined.  It  has  even  another 
ufeful  property  for  fuch  a purpofe,  and  that  is,  its  being 
one  of  the  leaf:  dilatable  of  all  bodies,  from  which  it 
would  almoft  always  happen  that  it  lliould  be  the  rod  ot 
glafs  which  would  be  fixed,  the  other  being  Ihorter ; 
which  would  prevent  making  any  changes  in  the  appa- 
ratus. 
Glafs,  as  I have  faid  before,  would  likewife  be  an  ufeful 
fubftance  for  the  pendulum;  fince  one  might  depend 
upon  the  conftancy  of  the  progrefs  of  its  variations  by 
heat.  It  is  true,  indeed,  that  its  fragility  would  be  an  ob- 
jection to  ufing  it  in  common  clocks;  but  the  aftronomer, 
accuftomed 
larites  qu’on  appercevroit  dans  les  obfervations,  ne  viendroient  furement  que  des 
corps  qui  lui  feroient  compares,  et  pourroient  etre  par  la  plus  aifement  con- 
ftatees  et  determinees.  II  a meme  encore  pour  cet  ufage  une  autre  proprietc 
•utile;  e’eft  d’etre  un  des  moins  dilatables  des  corps:  par  la  il  arriverolt  prefque 
toujours  que  ce  feroit  la  branclie  de  verre  qui  feroit  fixee,  1’autre  etant  plus 
courte;  ce  qui  epargneroit  des  changemens  dans  l’appareil. 
Le  verre  feroit  encore,  comrae  je  l’ai  deja  dit,  une  matiere  precieufe  pour  le 
pendule;  puis  qu’on  pourroit  compter  fur  la  conftance  de  fa  marclie  dans  les  va- 
riations de  la  chaleur.  Sa  fragilite  feroit  fans  doute  une  objedlion  pour  les  pen- 
L 1 1 2 dules 
