A22 PHILOSOPHICAL TRANSCATIONS. [aNNO I759. 



being disturbed at the time of an aurora borealis, as recorded in the 47 th vo- 

 lume of the Philosophical Transactions ; but is silent as to the cause. Mr. C. 

 had no opportunity of making observations of this sort himself, till the latter 

 end of the year 1756 ; but, after that time, he made near 4000, with an excel- 

 lent variation-compass, of about 9 inches in diameter. The number of days on 

 which these observations were taken, was 603 ; and the diurnal variation on 574 

 of them was regular ; that is, the absolute variation of the needle w^estward, was 

 increasing from about 8 or 9 o'clock in the morning till about I or 2 in the 

 afternoon, when the needle became stationary for some time ; after that, the 

 absolute variation westward was decreasing, and the needle came back again to 

 its former situation, or near it in the night, or by the next morning. The 

 diurnal variation is irregular when the needle moves slowly eastward in the latter 

 part of the morning, or westward in the latter part of the afternoon ; also when 

 it moves much either way after night, or suddenly both ways within a short 

 time. These irregularities seldom happen more than once or twice in a month, 

 and are always accompanied (so far as he had been able to observe) with an au- 

 rora borealis. Thus having explained what he means by the regular and irre- 

 gular diurnal variation, and showed, that this variation is generally regular ; he 

 then, in the first place, endeavours to account experimentally for the regular 

 variation ; then offers a conjecture concerning the cause of the regular varia- 

 tion : and, lastly, attempts to make it appear probable, that the aurora borealis 

 arises from the same cause. 



The attractive power of the magnet, (whether natural or artificial) will de- 

 crease while the magnet is heating, and increase w^hile it is cooling ; as will ap- 

 pear by the following experiments. 



Exper. 1. About e. n. e. from a compass a little more than 3 inches in dia- 

 meter, he placed a small magnet 2 inches long, half an inch broad, and -^\ of an 

 inch thick, parallel to the magnetic meridian ; and at such a distance, that the 

 power of the south end of the magnet was but just sufficient to keep the north 

 end of the needle to the n. e. point, or to 45 degrees. The magnet being 

 covered by a brass weight of 1 6 oz. about 2 oz. of boiling water was poured 

 into it, by which means the magnet was gradually heating for 7 or 8 minutes ; 

 and during that time the needle moved about three quarters of a degree west- 

 ward, and became stationary at 44°J ; in 9 minutes more it came back a 

 quarter of a degree, or to 44°-^ ; but was some hours before it gained its former 

 situation, and stood at 43''. n. b. The greater the power of the same magnet, 

 the more it will lose in a given degree of heat. 



Exper. 2. On each side of the compass, and parallel to the magnetic meridian, 

 he placed a strong magnet of the size above-mentioned ; so that the south ends 

 of laoth the magnets acted equally on the north end of the needle, and kept it 



