OBSERVATIONS ON THE MAGNETIC FORCE. 
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a string, passing through a hole in the great ring, which, when drawn tight, 
releases the needle, by pulling the lever from under it. 
A third addition consists of two lenses, applied for the purpose of reading 
off the arc when the needle has settled ; they are of such focal length, and so 
placed, that the coincidence of the needle with the division can be correctly 
seen, when the eye is near to, or at a distance from, the lenses ; they are 
borne at the end of two arms, which turn upon a centre fixed to the outside 
of the great ring. 
