( ) 
of the Steel ground off by touching: And if theQiian- 
tity of Steel is, by rubbing, diminiflied, the Lightnefs 
is not owing to its having Magnetifm, but to its Defed 
of Matter. Before an Experiment of this Nature is 
made, the Piece of Steel ihould be well hardned, po- 
liflied, and wiped very clean, and if warm by rubbing 
or handling, {hould be permitted to cool before it is 
weighed. Tlien being weighed with a Brafs Beam, 
let it afterward be well touched on the foft Armour of 
a Magnet, then wiped clean, and permitted to cool, be-' 
fore it is weighed after the Touch. Care muft alfo be 
taken that no Iron Bar, or other Magnet, be in the 
Window, or any other Part of the Room above, or 
that underneath, large enough to affed it, which a Mag- 
net, the Brads of the Floor, or other Iron in the fame 
Room, or about the Operator, fuch as a Key, Knife, 
Buckles, or the like, may more or lefs do, according to 
their Diftance and ' Situation : And I have feena Brad in 
the Floor make the End of a large horizontal Needle 
dip to it at the Diftance of above an Inch. 
Mr. inhisfaid Book, fuppofesthe 
Surface of the Earth’s central Magnet to be diftant in 
Miles from the Surface of the Earth 3400, and ac- 
cordingly, Pag, 48, computes the Semidiameter of the 
faid Magnet to be about 575 ; both which Sums added, 
make the Semidiameter of the Earth to be about 3 975 
Miles ; which is about 7 Miles lefs than Mr Norwood 
makes it by reckoning 69^ Miles to a Degree, which 
multiplied by 360, makes 25*020 in the whole Circum- 
ference, and the Radius (by Fan Ceuleji^s Proportion 
of the Circumference to the Diameter) I find to be 
3982.05*66, (fc, which 7 Miles is but a Trifle in 3975*, 
or 3982. 
I (hall 
