Mr.  de  luc  on 
did  not  find  that  the  direct  rays  of  the  Sun  heated  irregu- 
larly the  glafs  of  the  ball  of  a Thermometer  when  clean, 
which  may  be  eafily  feen  by  looking  at  the  experiment 
mentioned  in  p.  56,  57.  of  the  fecojid  volume  of  my 
work:  confequently,  when  thefe  diredt  rays  act  upon  the 
air,  it  is  a caufe  of  heat  which  fhould  not  be  negledted. 
I am  {till  therefore  of  opinion,  that  it  is  better  to  ob- 
ferve  the  Thermometer  in  the  Sun  than  in  the  fliade; 
and  that  the  correction  for  the  heat  of  the  air  may 
Hand  fuch  as  this  method  of  obferving  requires  it. 
There  are  always  adting  caufes  enough  in  the  column 
of  air  weighing  upon  the  inferior  Barometer,  which  can- 
not be  known  in  the  fuperior  ftation,  for  us  not  to  negledt 
any  of  the  ordinary  caufes  which  may  be  perceived. 
It 
foleil  echauffaffent  irregulierement  le  verre  bien  net  de  la  boule  d’un  Thermo- 
metre; ce  qu’on  pourra  voir  aifement  par  l’experience  rapportee  aux  p.  56  et  57 
du  fecond  volume  de  mon  ouvrage:  et  ainli  quand  ces  rayons  directs  agiffent  fur 
l’air,  c’eftune  caufe  deehaleur  qui  ne  me'femble  pas  devoir  etre  negligee. 
Je  crois  done  toujours  qu’il  convient  mieux  d’obferver  le  Thermometre  au 
foleil  qu’a  l’ombre,  et  de  lailfer  la  correftion  pour  la  chaleur  de  Pair  propor- 
tionnee  a cette  methode.  II  refte  toujours  alfez  de  caufes  agilfantes  dans  la  co- 
lonne  d’airquipefe  immediatement  fur  le  Barometre  inferieur,  qu’on  ne  peutpas 
connoitre  a la  ftation  fuperieure,  pour  qu’on  ne  doive  negliger  aucune  des  caufes 
communes  qui  font  faififfables. 
1 
II 
