Mr.  d e luc  on 
45® 
therefore  is,  that  it  would  be  of  fome  advantage,  if, 
upon  every  pendulum  intended  for  very  delicate  obfer- 
vations,  there  were  made  the  experiment  of  the  compara- 
tive lengthening  of  the  l'ubftances  which  compofe  it;, 
marking  the  center  of  ofcillation  upon  the  lens  by  the 
known  methods,  and  pointing  a Microfcope  upon  this 
center  whilft  the  heat  is  made  to  vary. 
By  ufing  for  thefe  pendulums  glafs  and  bell-metal, 
the  firft  of  which  is  not  at  all  affected  by  water,  and  the 
other  very  little  (which  little  might  even  be  prevented 
by  varnilhing  it)  it  would  be  eafy  to  obferve  the  effects  of 
the  heat  upon  them  in  water,  by  means  of  a tin  veflel,. 
having  a glafs  facing  the  center  of  ofcillation.  By  pouring 
water  of  different  temperatures  into  this  veflel,  whilft 
the  Microfcope  remains  pointed  to  that  center,  one  would 
difeover,  in  the  fureft  and  fhorteft  manner,  that  combi- 
nation: 
done  qu’on  obtiendroit  plus  d’exa£Fitude  fi  Ton  faifoit  immediatementfur  chaque 
pendule  deftine  a des  obfervations  delicates,  1’experience  de  l’allongement  com- 
paratif  des  matieres  qui  le  compofent,  en  marquant  fur  la  fentille,  par  les 
methodes  connues,  le  centre  d’ofcillation,  et  pointant  un  Microfcope  fur  ce 
centre,  tandis  qu9on  feroit  varie-r  la  chaleur. 
En  employant  a ces  pendules  le  verre  et  le  metal  de  eloclie,  dont  le  premier 
n’eft  point  affe&e  par  l’eau,  et  l’autre  Fell;  tres  peu,  et  pourroit  meme  etre  ver- 
niffe,  il  feroit  aife  encore  d’y  obferver  les  effets  de  la  chaleur  dans  l’eau,  au 
moyen  d’une  caiffe  de  fer  blanc,  qui  auroit  une  glace  vis  a vis  du  centre  d’ofcilla- 
tion.  En  y verfant  de  l’eau  a diverfes  temperatures  tandis  que  le  Microfcope 
feroit  poirtfe  fur  ce  centre,  on  trouveroit  de  la  maniere  la  plus  fu re  et  la  plus 
y courte 
