tneafuring Heights zvith the Barometer. 729 
lower than thofe attached to their refpedtive barometers, 
until they had remained a confiderable time in the fame 
fituation, equally lhaded from the Sun, when they always 
agreed: whence it followed, that in thefe obfervations, 
the mean temperature of the air, and equation depending 
upon it, might always have been determined very near 
the truth, from the temperature of the quickfilver in the 
tubes, as fhewn by the attached thermometers, without 
ever confulting the detached ones. Let us now fee what 
were the circumftances attending the obfervations on 
Saleve. 
Mr. de luc’s lowermoft barometer flood in the 
ground-ftory of a houfe near Geneva, where it remained 
unaltered during the whole of his experiments ; while 
the detached thermometer, indicating the temperature 
of the air, was expofed on a fmall eminence, at a little 
diftance, directly to the Sun’s rays : hence we find that, in 
the obfervations of high temperatures, the bottom of the 
column of air is often 1 a° or 1 5 0 , and in one cafe x 8°, 
hotter than the quickfilver in the tube. And even in the 
loweft temperatures, the bottom of the column of air is 
generally hotter than the quickfilver within doors, con- 
trary to common experience in this country: for in Eng- 
land, in winter, the exterior air in the lhade is always 
colder than the interior air. This circumfiance gives 
rcafaa 
5 A 2 
