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N 2 the fcale, difpofed in the arch of a circle, 
and containing loo equal divifions. This fcale is 
attached to the brafs Hiding ruler Q^P, which moves 
upon the direding piece R R, fixed by fcrews to the ' 
board, which makes the frame or bafe of the whole j 
and the fcale and ruler, N Q P, Is retained in any 
place, nearer to or further from the center K, as may 
be required, by the fcrew S. 
Fig. 4. reprefents in profile, the Aiding piece, 
and Audd I. (fig. 3.), which traverfes upon that 
part of the index next the center K ; and which can, 
by the two fcrews of the Audd, be retained upon 
any part of the index that is made parallel ; and 
wliich is done for 3 or 4 inches from the center,, 
for that purpofe. The Audd is filed to the edges, 
like the fulcrum of a fcale beam, one being formed 
on the underfide, the other upon the upper, and as 
near as may be to one another. An hook formed 
at the lower end of the wire loops C I, retains the 
index by the lowermoA edge of the Audd, while 
the weight M hangs by a fmall hook upon the up- 
per edge: by thefe means the index is kept Aeady, 
and the cords Arained by the weight, with very 
little fridion or burthen upon the central Audd K. 
Fig. 5. is a parallelogram of plate brafs, to keep 
out duA, which is attached to the upper edge of. 
the box cover H, and ferves to Aiut the part of the 
box cover necefifarily cut away, to give leave for 
the wire G I to traverfe with the Aiding Audd (fig. 
4.) nearer to, or further from, the center of the 
index K j and where in (fig. 5.) a is an hole about 
4 of an inch diameter, for the wire G I to pafs 
through, in the rifing and falling of the index, 
freely 
