474 PHILOSOPHICAL TRANSACTIONS. [aNNO 16Q2. 



the Asiatic pole, which I place about the meridian of the island of Celebes, to 

 be the fixed one, and consequently the American pole to be moveable. If this 

 be allowed, it is plain that the fixed poles are the poles of this external shell or 

 cortex of the earth, and the other two the poles of a magnetical nucleus, in- 

 cluded and moveable within the other. It likewise follows, that this motion is 

 westwards, and by consequence that the aforesaid nucleus has not precisely at- 

 tained the same degree of velocity with the exterior parts in their diurnal revo- 

 lution : but so very nearly equals it, that in 365 revolves the difference is 

 scarcely sensible. This I conceive to arise from the impulse by which this diurnal 

 motion was impressed on the earth, being given to the external parts, and from 

 thence in time communicated to the internal ; but not so as perfectly to equal 

 the velocity of the first motion impressed on, and still conserved by the super- 

 ficial parts of the globe. 



As to the quantity of this motion it is almost impossible to define it, both 

 from the nature of this kind of observation, which cannot be very accurately 

 performed ; as also from the small time these variations have been observed, and 

 their change discovered. It appears by all circumstances, that its period is of 

 many centuries of years, and as far as may be collected from the change of the 

 place, where there was no variation by reason of the equilibrium of the two 

 southern magnetical poles, viz. from Cape d'Agulhas to the meridian of St. 

 Helena, which is about 23° in about 90 years, and of the place where the 

 westerly variation is in its greatest deflection, being about half so much, viz. 

 from the isle of Diego Roiz to the south-west parts of Madagascar. We may 

 with some reason conjecture, that the American pole has moved westwards 46° 

 in that time, and that the whole period of it is performed in 700 years, or 

 thereabouts; so that the nice determination of this, and of several other parti- 

 culars in the magnetic system, is reserved for remote posterity; all that we can 

 hope to do, is to leave behind us observations that may be confided in, and to 

 propose hypotheses, which after ages may examine, amend, or refute. Only 

 here I must take leave to recommend to all masters of ships and all others, 

 lovers of natural truths, that they use their utmost diligence to make, or pro- 

 cure to be made, observations of these variations in all parts of the world, as 

 well in the north as south latitude, after the laudable custom of our East India 

 Commanders, and that they please to communicate them to the Royal Society, 

 in order to leave as complete a history as may be, to those who are hereafter to 

 compare all together, and to complete and perfect this abstruse theory. 



And by the way it will not be amiss to correct a received error in the practice 

 of observing the variation, which is, to take it by the amplitude of the rising 

 and setting sun, when his centre appears in the visible horizon ; whereas he 



