[ l6 3 ] 
ments, this rule, that an increafe of I line in the 
height of the barometer, raifes the quickfilver 
of the thermometer, placed in boiling water, by 
TT ‘ TT th part of the interval between the freezing 
point and that of boiling water : he afterwards 
indeed found, that this rule would not anfwer for 
fuch large variations of the barometer, as take place 
in afcending to very great heights above the earth’s 
fnrface ; vid. JLJfai fur les variations du chal: de 
I'eau bouill: but it is accurate enough for any 
fmall variation of the barometer, on one fide or 
other of its mean height in thefe lowed: regions of 
the atmofphere. The change therefore of the boiling 
point on Fahrenheit’s fcale, for a change of i line 
in the barometer, will be — 0,16 ; therefore 
13.8 lines will caufe 0,16x13,8 = 2,2 degrees of 
Fahrenheit’s fcale ; and a thermometer, whofe point 
of boiling water was marked 212, when the ba- 
rometer flood at 30 Englifh inches=28 inches, 
1.8 lines French meafure, will, when the barome- 
ter defcends to 27 French inches, fink 2,2 degrees 
ill boiling water, or to 209,8, or in round num- 
bers to 210 degrees, which is diftant only 178 
from 32, the point of freezing. Hence an ex- 
tent of 8o° of M. de luc’s thermometer anfwers 
to an extent of 178 of our Fahrenheit’s thermome- 
ter ; and putting F for the degrees of this thermo- 
meter, correfponding to C of M. de luc’s, we (hall 
have C : F — 32 :: 80 : 178, and C=F — 32 x T W 
which fubflituted in M. de luc’s formula , gives 
log. B — log. b X 1 + = 
= log. B — log. hi+- 
Y 2 
— 32*rnr 
215 
