[ 2 34 ] 
tion, for the logarithm of the ratio of the obferved 
heights of the quicklilver in the atmofpherical fyftem. 
Thofe, however, who have attempted to determine 
the lyflem, by this method, have hitherto agreed but 
ill in their conclufions. The fadt is, that the length 
of this fubtangent is very different at different times. 
The caufes and quantity of its variation will be con- 
fidered in another place. It appears, from M. de 
luc’s experiments, that, though otherwife fubjedt to 
change, it is conflant in a given temperature. And 
that when the temperature of the air is -{- 16*. of 
his fcale, the difference of the tabular logarithms of 
the heights of the quickfilver in the barometer, 
gives the difference of elevation in loooths of a 
Paris toile W. This is the rule, which he hath derived 
from a great number of experiments made at very 
different elevations: and the truth of it being ad- 
mitted, it is a neceffary confequence, that the num- 
ber, which is the modulus of Briggs’s fyftem, expreffes 
the length of the fubtangent of the atmofpherical 
curve, fuch as it is in that temperature, in loooths 
of a Paris toife. 
( b ) Recherch. fur les Modif. derAtmofp'n. §. 588. 
SECTION 
