OF THE UNITED STATES. S3 1 



middle between the two columns ^.^Sf which support the axis 

 of the upper tek-scope and circle, and is connectiMl with them 

 by means of the plate // of alioiit half an inch thick, and tlie 

 cross piece /at the up[)crend of it. 



Upon the spheric centre piece aa is fixed a circular ring 

 hh. ha\ing; three radii in the direction of the three legs of the 

 instriinient. which hear at their ends the three verniers 1), 

 E, F. Nvith a clamp and tangent screw at V). This holds the 

 circle dd hy its lower plane, which is also that of the radii 

 of the circle. 



The divided limb of the circle /'/ is elevated aI)ovc the 

 other plane, so as to leave both on the inside and outside a 

 recess sufficient for the clamping; parts of the irujcr and the 

 outer vcrtiiers. The inner clamp and tangent screw at A 

 clamping and leading the upper |)art of the instrument, has 

 the three verniers A, B, C reaching upon the same division 

 of the limb ii, and affording readings for both the forward 

 and the backward motion of the telescope. These motions 

 are required in measuring an angle by multiplication. For, 

 alternating between the two clamps at A and at I), the circle 

 will, in one motion, move with the upner part of the instru- 

 ment, and in the other retiiain clamped to the stand part. 



The reading glass is on a deiached part, consisting of a 

 piece m about an inch and a half Ions, two-thirds of an inch 

 broad, and one-third of an inch thick, l)earing at one end 

 a small pillar, upon which the projecting artn revolves. This 

 brinfxs the magtiilyiny; glass, with its reflector over any vernier 

 required. At the other end a solid nob forms, at the same 

 time, a handle and counterpoise to tlie magnifuM-. The 

 reflector is a circular [jlate of brass lined below with plaister 

 of Paris. 



The two colunms gs^ rise from the strong plate// on both 

 sides of the centre, so that their bases trmch the base of the 

 central cone whicli forms the socket ol' the axis, 'fhey are 

 about two inches and a half in diameter at the base, and fif- 

 teen inches higli. Kach carries ujjon its top a solid piece ////. 



