Pyrometry  and  Areonietry , Sec.  537 
co-efFe£ts  are  not  followed  in  all  their  degrees.  Whilft, 
on  the  other  hand,  the  method  of  taking  the  fundamen- 
tal proportions  in  points  which  are  very  diftant,  under 
the  idea  of  leffening  the  effects  of  the  errors,  is  exactly 
that  which  accumulates  a greater  quantity  of  them  upon 
the  intermediate  cafes,  which  are  the  moft  frequent,  and 
often  the  only  ones  in  the  which  there  is  need  in  prac- 
tice of  knowing  the  co-effe£ts  by  one  another. 
One  muft  not,  therefore,  feek  the  power  of  the 
Thermometer  which  corredts  watches,  invented  by  the 
immortal  harrison,  by  trying  it  in  the  temperatures  of 
artificial  congelation  and  in  a ftove ; for  that  is  the  way 
of  deftroying  a great  part  of  its  correcting  effect,  in  the 
very  cafes  wherein  it  is  moft  neceflary,  by  accumulating 
on  them  the  deviations  of  two  co-effedts,  probably  very 
little  proportional,  namely,  the  changes  of  the  elaftic  force 
of 
fenftble,  quand  on  n’aura  pas  fuivi  ces  co-effets  dans  to  us  leu  rs  degres.  Tandis 
que  celle  de  chercher  leurs  rapports  en  des  points  fort  eloignes,  dans  l’idee  de 
diminuer  FefFet  des  erreurs,  eft  precifement  le  moyen  d’en  aceumuler  le  plus 
fur  les  cas  intermediaresj  qui  font  les  pins  frequens;  et  louvent  les  feuls  ou  i’on 
ait  befoin  de  connoitre  les  co*effe<fts  les  nnspar  les  autres. 
11  ne  faut  done  pas,  par  exemple,  chercher  le  pouvoir  du  Thermomttre  cor- 
re£leur  des  montres,  invente  par  l’immortel  harrison,  en  l’eprouvant  dans  les 
temperatures  d’une  congelation  artificielle  et  d’une  etuve  : car  e’eft  le  moyen  de 
lui  oter  une  grande  partie  de  cet  effet  corre£teur,  dans  les  cas  ou  il  eft  le  plus 
neceftaire;  puifque  e’eft  y accumuler  les  ecarts  de  deux  co-eftets  probablement 
tres  peu  proportionnels;  fa  voir  les  changemens  de  force  elaftique  d’un  reftbrt 
Vo L,  LX VIII.  T t t lpiral. 
