C ] 
limply taking the difference of the logarithms of 
the heights of the barometer, or will be equal 
to log. B — log. b ; and in any other degree of 
heat, will be greater or lefs, in proportion as 
the rarity of the air is greater or lefs, than in 
the fixt temperature ; or greater or lefs by --jth 
part of the whole, for every degree of the ther- 
mometer reckoned from the fixt temperature i6|; 
and confequently the height of the place will be 
expreffed generally in French toifes, by this formula 
log. B — log. b -{- log. B — log. b x — ~ 5 ' 6 - 
— log. B — log. b x i -f- 
c — lbj 
215 
To reduce this 
formula to Englifh meafure, and to the fcale of 
Fahrenheit’s thermometer, we Ihould firft premife 
fome particulars. The French foot is to the Eng- 
lifh foot, as 1,06575 t0 as was fotmd by a very 
accurate experiment: fee Phil. Tranf. Vol. LVIII. 
for 1768, p. 326; and it is well known, that the 
point of freezing, on Fahrenheit’s thermometer, is 
at 3.2, and that of boiling water at 212, or the 
interval between them 180 degrees. But M. de 
luc’s point of boiling water 80, was marked when 
the barometer was at 27 French inches; and it is 
the cuftom of our principal Englifh workmen to 
mark the point of boiling water, 212, on Fahrenheit’s 
thermometer, when the barometer hands at 30 
inches, which is equal to 28 inches, 1,8 lines French 
meafure; or 1 3,8 lines higher than M.de luc’s baro- 
meter, when he let off the point of boiling water 
on his thermometers ; and it is well known, that the. 
heat of boiling water varies with the weight of 
the atmofphere : M. de luc finds, by his experi- 
ments. 
