an Experiment on Heat . j i j 
I formed a cylinder of thin pafteboard, fix inches diameter 
■and fixteen inches long. The infide I lined with rabbit fkin, 
laying the fur fmooth ; a thin ring of pafteboard was placed 
in the middle. One end was clofed with a bottom of the 
fame pafteboard ; the other was open. This cylinder was 
placed in the center of another wider cylinder, alfo of pafte- 
board, which had likewife a bottom of pafteboard. It was fo 
placed, as that the outer cylinder was diftant from the inner 
one inch and a half; at the bottom and fides the fpace between 
was filled with Eider down, fuffered to rife to as great a bulk 
as it would from its own elafticity. The two cylinders were 
-even at top, and the fpace between them fhut by a cover of 
pafteboard. In the fide of the machine, a little below the 
middle of the inner cylinder, a pafteboard tube was made to 
pafs through the outer and open into the inner, half an inch 
wide, for the infertion of a thermometer. A fimilar tube was 
placed a little from the middle, towards the other end of the 
fmalleft cylinder* A circular plate, of pafteboard, fix inches 
diameter, and about one-eighth thick, weighing i oz» 102 grs. 
was pufhed down the inner cylinder, until it was flopped by 
the ring-. A circle of flint glafs, ground flat and parallel on 
both fides, was fixed over the mouth of the inner cylinder fo 
as not to obftru<ft any part of it. 
A fimilar apparatus, as exadlly as poftible, was formed, ex- 
cepting that the circular plate in the middle of the inner cylin- 
der was iron-, of the fame dimenfions with the pafteboard one, 
and weighing 12 oz. 62 grs. Thefe apparatus’s were fet in a 
warm expofure for feveral months, to dry* 
The circular plates, which were deftined to receive the direfl 
rays of the fun, were placed as nearly perpendicular to the 
inner cylinder as poftible. They were both covered with a 
Z z 2 black 
