UF THli UNITED STATES. 3t9 



An arm j\ similar to that for the clamp, rises [)erpen(licu 

 lar to the level, IVom the centre [)iece ot" the hack alhidade 

 to the circii micro lice. Tlierc it hcnds over to (/,• and has a 

 circular ring lasteiicd to it, going lounii the circiind'eience 

 and forming a complete semicircle ddd^ which is again fas- 

 tened near both endd to two knees r, extending from the 

 level frame. This semicircle is divided upon the flat outer 

 part, like the moveable quadrant ol a glolje. Upon this ring 

 small pieces e, e slide by means of springs on their inner 

 part, and present their projecting part to a small arm g, ada|)t- 

 ed to tlie fiont telescope, by which this is siop[)ed u|)on the 

 proper zenith distance to which the stops t', e have been 

 placed. This piece g should be a very liglit piece of brass, 

 made to spiing or give way when it comes in contact with 

 the stops ; so that it could not all'ect the pcjsition of the ttle- 

 ■icope in coming accidentally in contact. 



The motion of the horizontal axis of the circle is stopped 

 by the screw ?/, which presses to this axis an arrangetnent 

 similar to the stop of a windmill ; and the small motions are 

 made by the screw at w?, at the end of two artns, one of 

 which is fast to this arrangement and the other to the centre 

 piece of the horizontal axis q. To make this motion easy, 

 the screw at m consists of two parts of unequal tlneads or 

 paths, eacli going in the nut of its respective arm, and tlierc- 

 by causing the motion of the circle itself to arise only from 

 the difference of the two screws. This motion is therefore 

 as small as that arising from any other tangent screw on the 

 instrumefit; and the use of the screws in the legs of the in- 

 strument can be fully dispensed with in this respect. 



Tlie horizontal circle of the instrument is lixed to the co- 

 nical radii, which form at the same time the legs of it. 'I'he 

 clamping and three readings A, B, C are adapted to the co- 

 lumn forming the socket of the vertical axis of tlic instru- 

 ment. A magnifier revolving round tlie lowermost part of 

 the colunm serves equally lor all three readings. 



To stop the horizontal motion of the upper circle in the 

 proper situation in respect to the azimuth, when stars arc 



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