382 PHILOSOPHICAL TRANSACTIONS. [ANNO 1687. 



iron, which draws the needle, and causes it to show a point on the horizon 

 much different from what it would, were it farther from the iron: which makes 

 it appear as if there is a considerable variation, where perhaps there is none at 

 all. We cannot therefore hope to be secure of any thing from the observations 

 we have at present, and especially from those made at sea, which are the most 

 considerable. This put me upon finding out some means, independent of ob- 

 servations, to discover the variation at sea ; but having considered that several 

 learned men of this age had proposed divers ways of making magnetical needles, 

 which should not be subject to variation, and that all these projects had failed 

 of effect; I judged that after all that they had done by means of the loadstone, 

 it was not to be hoped to draw any further advantage from it, since the stone 

 itself, as far as might be guessed from the experiments hitherto made, was sub- 

 ject to the same variation. 



I had quite given over this inquiry, when there accidentally came to my 

 hands a terella, or spherical loadstone, of 3 inches diameter, with which trying 

 some experiments, with a small needle, the foot of which might easily be 

 placed upon the stone, I soon observed what has been already noted by several, 

 viz. that this globe of magnet caused the needle to have the same changes as 

 are found in the compass in different parts of the world, as well in respect of 

 the direction towards the two poles, as of the inclination towards that which is 

 next it ; and on trial I satisfied myself that it was not possible to find the point 

 where the needle would stand indifferently in any position, which point would 

 have exactly showed me the pole of the stone ; but that the needle, however 

 placed, always directed itself one certain away. I determined by this means, as 

 well as I could, the point called the south pole ; but I was much surprised to 

 find it 18° distant from a cross deep engraven on the stone, which probably had 

 heretofore been the pole of this stone, as it had been observed by the person who 

 cut it. This change of the poles of this stone having revived my former thoughts 

 concerning the variation of the needle, I believed that if it were true that the 

 poles of the magnetical virtue changed in the loadstone, as we see they change 

 in the earth, we might hence derive great advantages, as to the variations of the 

 magnetical needle. For if this change of the poles in the loadstone were certain, 

 and that it was analogous to the change of the poles of the magnetic virtue of the 

 earth, it is not to be doubted but a terella, being suspended at liberty, would 

 remain immoveable, and that one point thereof would regard the pole of the 

 world, which might be called the true pole of the stone, while the poles of its 

 virtue would pass successively from one part to another, after the same manner 

 as they change in the earth. - 



Having well considered this hypothesis, and cleared up some doubts which I 



