[ 2 35 ] 
SECTION THIRD, 
Of the equation for the difference of temperature of 
the quicksilver in the barometers, at the different 
STATIONS. 
HE preceeding rub, however, mull: not be 
expected to give an accurate refult, even in 
that particular temperature of the attnofphere, to 
which it is adapted, unleis the fpecific gravity of the 
quickfllver, in the barometer, hath been the fame at 
both Rations, at the time of obfervation. If the 
fpecific gravity hath been different, at the different 
Rations, the heights of the quickfllver, in the ba- 
rometer, will not have been proportional to the den- 
fities of the air ; that is, to the forces by which they 
have been fuRained : and the calculation, built upon 
the fuppofition that they werefo, becomes- erroneous. 
The fpecific gravity of every material fubRance 
varies with its temperature. If the (temperature of 
the quickfRver, therefore, hath been different, at 
the different Rations, the difference of elevation,, 
found by the foregoing rule, will require corre&ion, 
though the mean temperature of the air may have 
been fuch as it prefcribes h). No particular tempera* 
ture of the quickfllver is neceRary to the accuracy 
of the refult of the preceeding rule, or to render the 
(?) Obferve, that the temperature of the quickfllver, in the 
portable barometer, will not be neceffarily the fame with that of 
the circumambient air, at the place and time of obfervation 
there will generally be a confiu'erable difference. 
Hh 2 
cor* 
