C 256 3 
Having now fufficiently explained, what the cor- 
rection is, for a variation of the temperature of 
the air, and whence it arifes, I proceed to reduce 
M. de luc’s formula to bird’s Fahrenheit, and a 
fcale of English fathom. 
SECTION FIFTH. 
M. de luc’s rules reduced to English scales. 
T HE whole of this reduction I divide into 
three problems. 
PROBLEM FIRST. 
* To find the length of the fub tan gent of the at- 
mofipher i cal curve y in thoufiandths of a Paris toife, the 
mean temperature of the air being given in degrees of 
bird’s Fahrenheit. 
B Y the com pari fon of M. de luc’s fcale with 
bird’s Fahrenheit, it appears, that -{- i6| 
of the former correfponds to -j- 69,25 of the lat- 
ter. Hence 69,25 is the temperature in bird’s 
Fahrenheit, in which, the fubtangent, of the at- 
mofpherical curve, is equal to fo many ioooths 
of a Paris toife, as are expreffed by B, the fub- 
tangent of the Briggian fyftem. But the atmo- 
ipherical fubtangent is increafed or diminifhed by 
— of this quantity for every degree of M. de luc’s 
<2i S 
fcale 
