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that of the town, I determined to obferve at the fame 
time the variations of the humor in the open air for 
a whole day together. There are doubtlefs many 
varieties in this relpedt ; nor indeed fhall I determine 
from this obfervation, any thing more than the date 
of the open air during one day, and in one parti- 
cular fpot. 
90. I made my obfervation the 13th of Septem- 
ber 1772, in a garden fituated to ihe weft of our 
lake, and only feparated from it by another garden 
and fome buildings. There I hung up a couple of 
hygrometers which I kept perfectly infulated, one of 
which had no other frame but a fcale fixt to its tube, 
and the other was in a frame whole opening at the 
height of the ivory tube was of a confiderable fize. 
They were four feet and a half above the ground, 
and at the diftance of a foot from each other. A 
piece of pafteboard about 12 inches in breadth, 
placed at a foot’s diftance from the hygrometer which 
was not mounted, was intended to fhelter it from 
the fun. Each hygrometer had a thermometer clofe 
to it, the ball of which was not in contact with any 
thing. I have proved in my work, that it is ne- 
ceftary to keep this ball infulated, in order to obferve 
the heat of the free air. 
91. One of the hygrometers I made ufe of for 
this obfervation, was at 93, and the other at 96^ 
in my room the night before. In order to correct 
this difference, which I fhould fuppofe to be in pro- 
portion to their height, I would always add about 
Tr to the height of that one which kept itfelf the 
loweft, that there might be no difference between 
them* but fuch as fhould be produced by the dif- 
ference 
