*6% METHOD OF EXAMINING ASTRONOMICAL INSTRUMENTS 



struction of this instrument does not differ materially froni 

 that of other similar instruments, with which you are well 

 acquainted, and I shall not therefore waste your time by 

 Apparatus for giving you a particular description of it. For the purpose 

 SSsbas?* 9 of examining the divisions upon the two circles, I procured 

 an apparatus to be prepared by Mr. Cary, which will be 

 Tery easily explained. To the face of the rim which sur- 

 rounds the azimuth circle, and with its left end close to the 

 stand which supports the micrometer on the east side, an. 

 arc of brass, concentric with the circle itself, and a little 

 more than 90° in length, an inch in breadth, and one eighth 

 of an inch in thickness, is firmly fixed by screws, so as to 

 have the plane parallel to the plane of the circle, and a 

 small portion of its lower surface resting upon the extreme 

 part of the rim. The screws pass through a brass arc, 

 which is fastened to this at right angles, and lies with its 

 broad side against the face of the rim. Upon the first 

 mentioned arc, a strong upright piece of brass, about six 

 inches in length, is made to slide, the lower part of it em- 

 bracing the arc as a groove, and having a clamping screw 

 underneath, for the purpose of fixing it firmly to the arc at 

 any point required. To the top of the upright piece of 

 brass is attached a microscope, with a movable wire in its 

 focus, pointing down to the division upon the circle, not 

 directly, however, but with an inclination to thG left of 

 about 30°. This inclination is given to it, in order to make 

 it point to the same division upon the circle, which is im- 

 mediately under the micrometer itself, when it has been 

 moved up as near to the micrometer, as it is capable of ap- 

 proaching. The microscope has attached to it a small gra- 

 duated circle of brass, and an index, by which the seconds, 

 and parts of a second, moved over by the wire, are deter- 

 mined. 



To the vertical circle there is likewise an arc applied, of 

 the same length and breadth as the former, but somewhat 

 thicker, and of a radius exceeding that of the circle by about 

 two inches. This greater thickness is given to it, on account 

 of its being supported in a manner which renders additional 

 strength necessary. It is fixed with its broad convex side 

 downwards upon two brass pillars^ screwed fast to the 



plane 



