49 4 -Mri de lug on Barometrical Meafures 
I was at fir ft as much furprized as fatisfied with this 
exatftnefs, which I did not expert notwithftanding my 
former experience ; and I fhould not have been furprized 
to find two or three fathoms difference either way. But, 
reflecting on it afterwards, I perceived that there ought 
to be more certainty in the barometric meafure for the 
depth of mines, than for the height of mountains ; and 
that for the two following reafons. 
Firft, that in making thefe obfervations along the 
ill aft s of mines we go but little from the fame vertical 
column, and by that we know with certainty the weight 
which compreftes the part which we meafure : whereas 
in mountains, even the fteepeft, the obfervations are made 
in columns of air which are far enough from one ano- 
ther to be liable to fenfible differences on that account; 
and 
Je fus d’abord auffi furpris que content de cette exactitude, a laquelle je nc 
m’attendois point malgre mes precedentes experiences; et je n’aurois pas ete 
etonne de trouver 2 au 3 toifes d’ecart dans l’un ou l’autre fens. Cependant, y 
refledmTant enfuite, j’ai compris qu’il doit y avoir plus de furete dans la mefure 
barometrique pour la profondeur des mines, que pour la hauteur des montagnes; 
et cela par les deux confiderations fuivantes. 
La premiere, qu’en faifant ces obfervations le long des puits des mines, on 
s’ecarte peu d’une meme colonne verticale; et l’on connoit ainfi furement le 
poids qui comprimela partie qu’on mefure. Aulieu que dans les montagnes, meme 
les plus rapides, les obfervations fe font dans des colonnes d’air affez eloignees 
I’^ne de i’autre, pour quhl puiffe y avoir des differences fenhbles a cet egard ; et 
que 
