758 Col. roy’s ’Experiments for 
of air is above 32 0 of Fahrenheit’s thermometer in the 
fhade, add the equation correfponding to the tempera- 
ture and height of quickfilver in the mean barometer to 
the logarithmic altitude; when below 32 0 , fubtradt the 
equation from the logarithmic altitude ; the fum in the 
firlt cafe, and difference in the laft, gives the real height. 
Befides the table of equation for the air, adapted to the 
meafurement of the greateft acceflible heights the baro- 
meter can poJTibly be applied to in middle latitudes, I 
have annexed, for the ufe of thofe who may prefer fim- 
plicity, and frill doubt of the vertical diminution, a ther- 
mometrical fcale of the equation, fuited to Englifh and 
French meafures, with their refpedtive thermometers. It 
will readily be conceived, that the divifions, expreffing 
the 1 oooth parts in this fcale (r \ are unequal, fince they 
follow the inverfe ratio of the thermometrical compared 
with the manometrical degrees. Where thefe laft are 
the greateft, as between 5 2 0 and 7 2 0 , the divifions ex- 
preffing the equation are the fmalleft, becaufe a greater 
(r) Any fcale of this kind, tinlefs it had been mechanically divided by a 
•mathematical inflrument-maker, could not be rendered very exa£l ; and it may 
be expected, that the imperfections in the original will be augmented in copy- 
ing by the engraver, notwithftanding the utmoft care on his part : wherefore, 
on the left-hand fide of the plate, I have annexed tire number of degrees and 
decimal parts of Fahrenheit, below the temperature of 9i°.88, correfponding 
to every xlgjrth parts of the equation, by which means the unequal fcale may, 
at any time, bo divided with fufScient accuracy. 
number 
