C 229 J 
fcales are general. It is not one of the leaft, that 
inftruments effentially different, as made at different 
elevations in the atmofphere, will continue to pals 
under the fame name. The error and confufion, 
which this may create, is remarkably inftanced, in 
what hath a&uaily happened to the thermometer of 
the celebrated reaumur. The ' inftrument, which, 
at this day pafies, all over Europe, under the name 
of Reaumur’s thermometer, is effentially different 
from his ; yet it was always fuppofed to be the fame, 
even among his own countrymen, till M„ de lUc de- 
tefted the miftake. At what time, upon what oc- 
cafion, or by whom, the change was introduced, is 
ftill unknown. 
Of ihe general principles of meafuring heights by the 
, barometer.. 
S it is my dehgn to compare the practical rules, 
which M. de luc hath deduced from ex- 
periment, with theory, it is neceflary, for the clearer 
arrangement of the argument, previoufly to ftate the 
general principles, upon which the meafurement of 
heights by the barometer depends. In doing this, Khali 
rather aim at perfpicuity than at brevity ; referring, 
however, for the demonflration of whatever hath been 
advanced before, to writers of approved authority. 
In any column of the atmofphere, refting 
perpendicularly upon any fmall portion of the 
earth’s furnace, the ' denfities of the air diminifh, 
as we afeend to greater heights 5 and if the 
aaelerative force of gravity were the fame 
CJ d 
SECTION SECOND. 
