472 PHILOSOPHICAL TRANSACTIONS. j^ANNO 16Q2. 



it. the variation of the needle gradually decreases ; but whether, on the con- 

 trary, it increases in those places which lie more westerly than that tract where- 

 in the highest east variation is found ; or how it may be in the vast Pacific Sea, 

 we have not experience enough to ascertain ; only we may by analogy infer, that 

 both the east and west variations gradually increase and decrease after the same 

 rule. 



These phaenomena, being well understood and duly considered, sufficiently 

 evince, that the whole magnetical system has one, or perhaps more motions ; 

 that the moving force is very great, as extending its effects from pole to pole ; 

 and that its motion is not per saltum, but a gradual and regular motion. 



Now considering the structure of our terraqueous globe, it cannot be well 

 supposed that a very great part of it can move within it, without notably 

 changing its centre of gravity, and the equilibrium of its parts, which would 

 produce very wonderful effects in changing the axis of diurnal rotation, and 

 occasion strange alteration in the surface of the sea, by inundations and re- 

 cessions, such as history never yet mentioned. Besides, the solid parts of the 

 earth are not to be supposed permeable by any other than fluid substances, of 

 which we know none that are any ways magnetical. So that the only 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 fixed and immoveable in the same 

 common centre of the earth : and there is yet required, that this moving in- 

 ternal substance be loose, and detached from the external parts of the earth, 

 whereon we live; for otherwise, were it affijied to the earth, the whole must 

 necessarily move together. 



So then the external parts of the globe may well be considered as the shell, 

 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 re- 

 petitions becoming sensible, 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 east or west by the difference of their motions. 



Now supposing such an internal sphere, having such a motion, we may 

 solve the two great difficulties in my former hypothesis. For if this exterior 

 shell of earth be a magnet, having its poles at a distance from the poles of 

 diurnal rotation; and if the internal nucleus be likewise a magnet, having its 

 v, poles in two other places distant also from the axis; and these latter, by a 

 gradual and slow motion, change their place in respect of the external, we may 



