OF THE UMTEli sTATlis. ;89c{ 



the piece //. so as to cmhiace it hv two small pieces, and is 

 screwed on from below In- a fmgei- screw. 



The other end of tiic axis bears outside of its support a 

 circle of six inches in diameter, divided on silver. Upon 

 this revolves an alhidade of three arms, the two horizontal 

 arms ?/, u serving to read on both sides by verniers whicli are 

 attached to them and to hold the spirit level. The vertical 

 arm r, which is formed of the same piece of metal as the 

 vernier arms, clamp these and the level, l)y the linger screw. 

 The level being adjusted by liand, when so clamped, the te- 

 lescope will, in revolving, read vertical angles with suHkicnt 

 approximation to serve for finding a star, or determining 

 any other elevation, within a certain degree of accuracy. 



The adjustment of the axis of the telescope and tlic llnal 

 levelling of the instrument arc effected by means of a large 

 spiiit level w, suspended by hooks frojn both ends of the 

 axis, outside of the supports //,//. As the space immediately 

 under the axis is not iree, it hangs on each side close to the 

 columns. It is purposely without adjusting screws, and is 

 therefore brought to adjustment by the filing of its hooks; 

 and as the level is ground to a regular curve inside, the nice 

 adjustment is made by two small ivory scales sliding upon 

 the level, by two sections of tube holding to it by their spring. 

 This is the arrangement of all the larger levels of the instru- 

 ments, excepting those of the repeating circles. 



Below the horizontal circle is a verification telescope, sus- 

 pended in hooks from two opposite conical radii 6, b, exactly 

 similar in size and construction to the upper telescope, but 

 of course witliout a vertical circle. 



, The eye end has a micrometer arrangement similar to the 

 upper telescope, with one fixed wire in the direction of the 

 length of the micrometer and three wires perpendicular to 

 it, moveable by the micrometer screw. The whole micro- 

 meter arrangement stands at an inclination of about i.j° with 

 the horizon, so as to increase the chances of intersection 

 with any disfinct object within the field of the telescope. 



VOL. II. Q s 



