[* 220 \ 
SECTION FIRST. 
Of the variation of the point of boiling watir, and 
the COMPARISON of THERMOMETERS. 
T HE degree of heat wherewith any fluid boils, 
is invariably the fame, under a given preflure j 
but if the preflure be diminifhed or increafed, the 
boiling heat is diminifhed or increafed. 
Water, placed- under the exhaufled receiver, 
would be converted into fleam, with a degree of 
heat, far inferior to that, which is neceflary to its 
boiling, in the open air ; and under the preflure of 
its own? vapour, confined in papin’s digefler, it is 
faid to fuftain a degree of heat, without boiling, far 
exceeding that, which, in the open air, w’ould con- 
vert it into fleam. 
Hence it follows, that in climates, where the 
preflure of the atmofphere is liable to- confiderable 
change, the heat of boiling water, in the open air, 
will be different, at different times. Confequently, 
thermometers, made in different ftates of the ba- 
rometer, will difagree ; unlefs allowance hath been 
made, for the effedt of the variation of the barome- 
ter, upon accurate principles. 
If care were taken to adjuft the boiling point, to 
the mean height of the barometer, in every coun- 
try, the inftruments of the fame country would 
always be confident j but thofe of different coun- 
tries would ftill difagree ; that is, they would ex- 
prefs the fame temperature differently, though their 
fundamental intervals fhould be fimilarly divided;, 
for, in every fcale, the number of degrees above 
3 o*- 
