Royal Society. :^7 
yet mentioaed; befides, the folid parts of the earth are not to be 
luppoled permeable by any other than fluid fubibnces, of which 
we know none that are any ways magnetical^ lb that the only 
way to render this motion intelligible and poffible, is, to luppole 
it to turn about the centre of the globe, having its centre of gra- 
vity fixed and immoveable in the lame common centre of the 
earth 5 and there is yet required, that this moving internal fub- 
ftance be loofe and detached from the external parts of the earth, 
whereon we live 5 for otherwile were it affixed thereto, the whole 
mull neceffarily move together : So then the external parts of 
the globe may well be reckoned as the fhell, and the internal as 
a Nucleus y or inner globe included within ours, with a fluid me- 
dium between, which, having the lame common centre and axis 
of diurnal rotation, may turn about with our earth each 24, 
hours 5 only this outer Iphcre, is to have its turbinating motion 
either fomewhat fwifter or flower than the internal ball 5 and 
even the difference of a minute in length of time, by many repe- 
titions becoming lenfible, the internal parts will by degrees recede 
from the external, and not keeping pace with each other, will 
appear gradually to move either to the eafc or weft by the diffe- 
rence of their motions : Now fuppofing fuch an internal fpherc 
having fuch a motion, we may refolvc the two great difflculties 
in the hypotheiis of the variation j for if this external fhell of 
earth be a magnet, having its poles at a diftance from the poles 
of diurnal rotation 3 and if the internal Nucleus be likewiie a 
magnet, having its poles in two other places diftinfl: alio fromi 
the axis • and thele latter by a gradual and flow motion change 
their place in reipeit of the external, we may then give a realbn- 
able account of the four magnetical poles, as likewife of the 
changes of the variations of the needle: The period of this mo- 
tion being wonderfully great, and there being hardly a century 
fince thcie variations have been duly oblerved, it will be very- 
hard to bring this hypotheiis to a Calculus, elpecially fince, tho* 
the variations do encreale and decreale regularly m the lame 
place, yet in different places, at no great diftance, there are 
found inch cafual changes thereof, as can no ways be accounted 
for by a regular hypotheiis, as depending upon the unequal and 
irregular diftribution of the magnetical matter within the fub- 
flance oF the external fhell or coat of the earth, which deflecls the 
needle from the polition it would acquire from the effect of the 
general raagnetilm of the whole ; of this the variations at London 
and 'Paris afford a notable inftancc^ for the needle has been con- 
ftantly about i ~ degree more calferly ;it "P^r/s thsn at Lcndoiiy 
D % tho^ 
