49 8 Mr. de luc on Barometrical Meafures 
At that diftance there are doubtlefs feme caufes of va- 
riations which are irremediable; fince the formula fup- 
pofes that the obfervations are made in the fame column 
of air. It is therefore only in the cafes in which that 
fuppoiltion approaches near the truth that we can hope 
to perfect the rule. But this can only be by introducing 
new conditions into it; that is, other modifications of 
the air, of which we have not as yet taken any account. 
In meditating on the caufes of the diverfity of refults 
in experiments, it has always appeared to me, that the 
differences of the effects of heat on the air, according to 
the different ftates of it, was the principal ; that is, that the 
air not being always of the fame nature, heat, that grand 
caufe, whofe effects we ought principally to determine, 
does not always adt equally. Befides the particular ex- 
periments 
A cette diftance fans doute il y a des caufes d’ecarts qui font irremediables; 
puifque la formule fuppofe que les obfervations font faites dans une meme co- 
lonne d’air. Ce n’efl done que pour les cas ou cette fuppofition approche de la 
verite, qu’on peut efperer de perfedlionner la regie, Mais ce ne fera qu’en y in- 
troduifant de nouvelles conditions ; e’eft a dire d’autres modifications de l’air 
dont on n’a pas tenu compte jufqu’ ici, 
£n etudiant les caufes de la diverfite des refultats dans les experiences, il m’a 
toujours paru, que la difference des effets de la chaleur fur ]’air, fuivant qu’il eft 
lui meme conftitue, etoit la principale: e’eft a dire, que Fair n’etant pas tou- 
jours de meme nature, la chaleur, cette grande caufe dont il faut principalement 
determiner les effets, n’y agit pas toujour egalement. Outre les experiences 
I particulieres. 
