[ a6i ] 
His reafon for not (hading his thermometer from 
the fun, was this : when one fide of the ther- 
mometer is expofed to the fun’s rays, the other 
is in its own fhade ; therefore, in this fituation, 
he thought it the juft meafure of the mean tempe- 
rature of the air, which is neither the temperature of 
that part on which the fun’s rays fall, nor of that 
from which they are intercepted, but lefs than the 
one, and greater than the other, or a mean between 
the twoW. I confefs, I fhould have expedfed an 
irregularity, from heat excited by the rays of light, 
in their paflage from the air into the glafs, and 
from the glals into the quickfilver; and fhould 
therefore have expofed my thermometer in the 
fhade ; but the fuccefs of M. de luc’s experiments- 
feems fufficiently to juftify the method he hath 
taken. 
I fhall clofe this fedtion with a brief folution of 
two phyfical problems, for which this feems the 
proper place. 
PROBLEM FOURTH. 
To compare the denjities of the air , at any given 
elevation above the Jurface of the earth , in different 
temperatures . 
I F the barometer hath been obferved, at any 
given elevation above the earth’s furface, in differ- 
ent temperatures, reduce the obferved heights of 
(«) Recherch,. fur les Modif. de l’Atm. §. 533 — 536. 
the 
