24 Mr. Cavallo’s Magnetic'al 
fituation or direction mud; be determined by all thofe attractions 
confidered together, viz. from their common center of aCtion ; 
thirdly, that by removing or altering the degrees of attraction 
of fome of thofe bodies which are fituate on one fide of the 
magnetic meridian, more than of thofe fituated on the other 
fide, the above-mentioned common center of attractions, and, 
of courfe, the direction of the magnetic needle muft be altered, 
which in faCt is the variation of the needle ; and, laftly, that 
this alteration in the attractions of fome of the ferruginous 
bodies in the earth muft undoubtedly take place, it being occa- 
sioned by the parts of the earth being irregularly heated and 
cooled, by the aCt ion of volcanoes which decompofe or other- 
wife alter large maffes of ferruginous fubftance, by earth- 
quakes which remove ferruginous bodies from their original 
places, and we may add alfo by the aurora borealis ; for though 
we are as yet ignorant of the caufe of that furprifing phaeno- 
■menon, it is however certain, that the magnetic needle has 
been frequently difturbed when the aurora borealis appeared 
very itrcng. 
The magnetic needle, therefore, being neceflarily affeCted 
by thofe caufes, it feems unneceflary to have recourfe to other 
hypothetical caufes which are not eftablifhed on aCtual expe- 
rience. 
In order to exemplify this explanation of the variation in a 
familiar manner, I made the following experiment, with the 
account of which I (hall conclude this Paper. Four earthen 
vefle-ls were difpofed round the magnetic needle, two near its 
fouth, and the other two near its north pole, but not at equal 
diftances. In one of thofe veflels there was placed a natural 
.magnet ; the fecond contained feveral fmall bits of magnetic 
iteel mixed with earth ; and in each of the other two there were 
put 
