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the caufe of the irregular variation ; and, laftly, at- 
tempt to make it appear probable, that the aurora 
borealis arifes from the fame caufe. 
The attractive power of the magnet, (whether na- 
tural, or artificial) will decreaje while the magnet is 
heating , and increafe while it is cooling ; as will ap- 
pear by the following experiments. 
Experiment i. About E. N. E. from a compafs, a 
little more than three inches in diameter, I placed a 
linall magnet two inches long, half an inch broad, 
and three- twentieths of an inch thick, parallel to the 
magnetic meridian ; and at fuch a diftance, that the 
power of the fouth end of the magnet was but juft 
iufficient to keep the north end of the needle to the 
N. E. point, or to 4y degrees. The magnet being 
covered by a brafs weight of fixteen ounces, about 
two ounces of boiling water was poured into it, by 
which means the magnet was gradually heating for 
feven or eight minutes ; and during that time, the 
needle moved about three quarters of a degree weft- 
ward, and became ftationary at 44 0 i ; in nine mi- 
nutes more, it came back a quarter of a degree, or to 
44 0 i but was fome hours before it gained its for- 
mer lituation, and flood at 45°. A LB. I he greater 
the power of the fame magnet, the more it will lofe 
in a given degree of heat. 
Exp. 2. On each fide of the compafs, and parallel 
to the magnetic meridian, I placed a flrong magnet 
of the fize above-mentioned ; fo that the fouth ends 
of both the magnets added equally on the north end 
of the needle, and kept it in the magnetic meridian ; 
but if either of the magnets was removed, the needle 
was attracted by the other, fo as to hand at 45 de- 
grees. 
