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freely without touching j is a flit of a lelTer fize, 
fufficient to pafs the wire, and admit the cover to 
come off without deranging the cord or index j c c 
are two fmall fcrews applied to two flits, by 
which the plate Aides lengthways, in order to adapt 
the hole a to the wire G I, at any place of the 
fludd I upon the index K L. 
Remarks on the preceding ConJiruBion, 
I ft. In this conftrudion the index K L being 12 
inches long, 4 inches from the extreme end are tiled 
fo narrow in the diredion in which it is feen by 
the eye, that any part of thefe 4 inches, lying over 
the divifions of the fcale, becomes an index there- 
to. The fcale itfelf Aides 4 inches, fo as to be 
brought under any part of the 4 inches of the index, 
attenuated as before mentioned. 
2dly. The pofition of the direding piece R R is 
fo determined, as to be parallel to a right line drawn 
through the point o upon the fcale, and the center 
K of the index; confequently, as the attenuated part 
of the index forms a part of a radius, or right 
line from the fame center, it follows, that when- 
ever the index points to 0 upon the fcale, though 
the fcale is moved nearer to or further from the 
center of the index, yet it produces no change in 
the place to which the index points. 
3dly. When the divided arch of the fcale is at 
10 inches from the center (that is, at its mean dif- 
tance) then the center of the arch and the center 
of the index are coincident. At other diftances, the 
extremes of which are 8 or 1 2 inches, the center of 
D d 2 the 
