
Now considering the structure of our Terraqueous Globe, it cannot be well supposed that a very great part thereof can move within it, without notably changing its Centre of Gravity and the Equilibre of its parts, which would produce very wonderful Effects in changing the Axis of Diurnal Rotation, and occasion strange alteration in the Sea's Surface, by Inundations and Recesses thereof, such as History never yet mentioned. Besides, the solid parts of the Earth are not to be granted permeable by any other fluid substances, of which we know none that are any ways Magnetical. So that the ony way to render this Motion intelligible and possible is, to suppose it to turn about the Centre of the Globe, having its Centre of Gravity fixt and immoveable in the same common Centre of the Earth : And there is yet required that this moving internal Substance be loose and detached from the external parts of the Earth, whereon we live ; for otherwise were it affix'd thereto, the whole must becessarily move together.
So then the External Parts of the Globe may well be reckoned as Shall, and the Internal as a Nucleus or inner Globe included within ours, with a fluid medium between Which having the same common Centre and Axis of diurnal Rotation, may turn about with our Earth each 24 hours ; only this outer Sphere having its turbinating Motion some small matter either swifter or slower than the internal Ball. And a very minute difference in length of time, by many repetitions becoming sensible ; the Internal parts will by degrees recede from the External, and not keeping pace with one another will appear gradually to more either Eastwards of Westwards by the difference of their Motions.
Now supposing such an Internal Sphere having such a Motion, we shall solve the two great difficulties we encountred in my former Hypothesis. For if this exterior Shell of Earth by a Magnet having its Poles at a distance from the Poles of Diurnal Rotation ; and if the Internal Nucleus be likewise a Magent, having its Poles in two other places distant also from the Axis ; and these latter by a gradual and slow Motion change their place in respect to the External ; we may then give a reasonable account of the four Magnetical Poles I presume to have demonstrated in No. 148 of these Transactions; as likewise of the changes of the Needles Variations, which till now hath been unattempted.
An Account of the cause of the Change of the Variation of the Magnetical Needle, with an Hypothesis of the Structure of the Internal parts of the Earth: as it was proposed to the Royal Society in one of their late Meetings. By Edm. Halley. Philosophical Transactions, no. 195, 1692.





