Royal Society^ 129 
needle, and repels the fcjuth end. That is a mutuble pole, which 
may be north or loath, as you hold it 5 and a fixed pole, that 
which does not change, however you hold it. The ipecies of the 
pole, whether north or Ibuth, may be found by paffing the iron 
rod thro' cork or wood, and then leaving it to Iwim on water, it 
will turn to its proper pole; but this way is (low, and not accurate 5 
a better way to try, for inftance, a north pole, is to hold the iron 
perpendicular to the horizon, and to try, whether being held under 
the north end of the needle, it attra61:s it 3 but a yet better way is to 
try whether the upper end of the rod attradt the'louth end needle; 
for attraction is more fenfible than repulfion. A fixed north pole 
may be made after all the ways, and with all the rods that you 
can make a fixed fouth pole, but not en the contrary ; for there 
are many cales, wherein you can make a fixed north pole, but not 
a fixed fouth pole; and whatever way you obtain a fixed fouth 
pole, it is weaker than a fixed north pole made the lame way ; 
applying a needle to an eredt bar, beginning at the top, and i'o 
d-own, the needle turns not at the middle, but nearer ; of Ibme 
rods you cannot make a fixed fouth pole primarily, yet you may 
conlequentially ; fo you may make one end a fixed north pole, 
and then the other end of thofe rods may, without more a-do, 
become a fixed north pole; but this doth not always hold, for 
the one may be a fixed north pole, and the other may be a muta- 
ble pole. Fire deflroys all fixed poles, whether made by the 
magnet, or otherwife; but it increafes, or rather lels impedes that 
magnetifm, which proceeds from the earth ; a wire or rod of iron 
heated at one end, that end becomes a mutable pole, but more vi- 
gorous whilft hot than cold; the vigour of mutable poles is more 
in great than Irnall rods; but it is otherwirein fixed poles. Heat 
the end of a rod of iron red hot, or heat all the rod, and cool 
that ignited end northwards, it will be a fixed north pole; if 
cooled Ibuth, it becomes a fixed Ibuth pole; this Gilbert 3lx\A 
others aflert from experience, but this holds only in Ibme cales ; 
*viz> if the rod is fliort, you cannot make a fixed pole that way. 
Take a round wire, whole diameter is f inch, and length 
10 inches, you cannot produce a fixed pole by ignition; but if 
this wire was longer, as luppofe 50 inches, or never lb much 
longer, it is capable of a fixed pole by ignition. Again, take a 
round rod 30 inches long, and i inch in diameter, this rod is not 
capable of a fixed pole at that length, tho' the lefler was capable 
thereof at the fame length ; and experiments give reafon to think, 
that there is not any rod or bar of iron ever lo thick, but which, 
if it had lufficient length, would be capable of a fixed pole by 
Vol, hi. " R. bate 
