Suspension of the Magnetic Needle. 91 
screw. If both sides of the instrument be made of glass, it 
may be filled with some transparent fluid, which will not 
cause the metal to rust, as spirit of wine, or oil of turpentine, 
that the needle may move more steadily, and not be sensibly 
influenced by ’heat. 
Expedients for rendering the instrument more portable 
might be easily contrived : but I hope no unprejudiced phi- 
losopher will pronounce it useless, because the spider's thread, 
which is so easily replaced, may break ; for sensibility is the 
only property of essential use in discoveries wherein a small 
degree of magnetic attraction is to be ascertained. 
EXPERIMENT XIII. 
The first use I made of my needle, suspended as above, was 
to try the polarity of several iron utensils ; and, as might be 
expected, they attracted or repelled the north end of the needle, 
according to their position with respect to the magnetic at- 
mosphere of the earth. A bar of soft iron, half an inch square, 
and nine inches long, moved the needle very sensibly at the 
distance of about three feet ; longer bars moved it at a much 
greater distance ; and if a bar was held horizontally, near the 
end of the needle, and at right angles, it might be made either 
to attract or repel, by moving it up or down only half an inch, 
so as to appear to change its attraction to repulsion at com- 
mand ; which has surprised persons unacquainted with the 
importance of position with respect to the earth's magnetic 
atmosphere. This polarity of position may be very sensibly 
perceived, by presenting small nails, or smaller bits of wire, 
above or below the needle, or with the remote end inclining 
towards the north or south ; which plainly demonstrates the 
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