VOL. XVIII.l PHILOSOPHICAL TRANSACTIONS, €?% 



A better way to try, for instance, a north pole, is to hold this iron perpendi- 

 cular to the hofizon, and to try whether, being held under the north end of 

 the needle, it attracts it. But a yet better way is to try whether the upper end 

 of the rod attracts the south-end of the needle, for attraction is more sensible 

 than expulsion. 



2. A fixed north pole may be made with all the ways and rods that you can 

 make a fixed south pole, but not on the contrary ; for there are many cases 

 wherein you can make a fixed north pole, but not a fixed south pole; and what- 

 ever way you get a fixed south pole, it is weaker than a fixed north pole made 

 the same way. Applying a needle to an erect bar, beginning at the top, and 

 so down, the needle turns not at the middle, but nearer. Of some rods you 

 cannot make a fixed south primarily, yet you may consequentially : so you may 

 make one end a north pole, and then the other end of those rods may, with- 

 out more ado, become a fixed south pole ; but this does not always hold, for 

 the one may be a fixed north pole, the other may be a mutable pole. 



3. Fire destroys all fixed poles, whether made by the magnet, or otherwise ; 

 but it increases, or rather less obstructs that magnetism which proceeds from 

 the earth ; a wire or rod of iron heated at one end, that end is a mutable pole, 

 but more vigorous while hot than if cold : or the ignited end held downwards, 

 will attract the said end of the needle more vigorous, than if cold ; and so if 

 held upwards, it more attracts the north end. The vigour of mutable poles is 

 more in great than small rods, but it is otherwise in fixed poles. 



4. Heat the end of a rod of iron red-hot, or heat all the rod, and cool that 

 ignited end northward, it will be a fixed north pole; if cooled south, it be- 

 comes a fixed south pole. This Gilbert and others assert from experience. But 

 this holds only in some cases, viz. if the rod is short, you cannot make a fixed 

 pole that way. Take a round wire, its diameter f inch, and length 10 inches, 

 you cannot produce a fixed pole by ignition; but if this wire is longer, as suppose 

 30 inches long, or ever so much longer, it is capable of a fixed pole by ignition. 

 Again, take a round rod 30 inches long, and 1 inch diameter ; this rod is not 

 capable of a fixed pole at that length, though the lesser was capable at that 

 length. And thus my experiments give me reason to think that there is no rod 

 or bar of iron ever so thick, but which, if it had length enough, would be 

 capable of a fixed pole, by bare ignition, for of that only I speak in this para- 

 graph ; and there is no rod ever so short, but which if you make it sufficiently 

 thin, is capable of a fixed pole. So, when in a rod I could not obtain a fixed 

 pole at 21 inches length in that thickness, I could by making the rod thinner 

 produce a fixed pole, even in the length of one inch and less, and the pole 



4 K 2 



