the Divisions of astronomical Instrumenls. 279 



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

 embracing the arc as a groove, and having a clamping, 

 screw underneath, for the purpose of fixing it firmly 10 

 the arc at any point required. To the top of the upright 

 piece of brass is attached a microscope, with aTnoveable 

 wire in its focus, pointing down to the division upon the 

 circle, not directly, however, but with an inclination to 

 the left of about 30". This inclination is given to it, in 

 order to make it point to ihe same division upon the circle, 

 which is immediately under the micrometer itself, when it 

 has been moved up as near to the micrometer as it is ca- 

 pable of ap[)roachinir« The microscope has attached to it 

 a small graduated circle of brass, and an index, by which 

 the seconds, and parts of a second, moved over by the 

 wirf are determined. 



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 ren- 

 ders additional strength necessary. It is fixed with its 

 broad convex side downwards upon two brass pillars, 

 screwed fast to the plane of the aziniuth circle, and standing 

 in a line parallel to the plane of the vertical circle at the 

 distance of about four inches from it, and on the right sule 

 of the pillars which support the micrometers belonging to » 

 this circle. The pillar, to which the left end of the arc is 

 fastened, is placed close to the lower micrometer of the 

 vertical circle, and the other contiguous to the elevated rim, 

 in which the divisions of the azimuth circle are cut. The 

 right end of the. arc reaches beyond this pillar about ten 

 inches. The pillars are of such a height, and so propor- 

 tioned to each other, that whilst the left end of the arc, 

 which lies horizontally, is raised to within about two inches 

 of the height at which the lowest point of the vertical cir- 

 cle is placed, the whole arc runs parallel to the circle 

 through an extent of something more than 90°. Upon 

 the arc a microscope, with a moveable wire in the focus, 

 is made to slide as in the former case, and to point to the 

 divisions upon the vertical circle, not directly, but with 

 an inclination of about 30^ to the left, in order that the 

 same division (which is the lowest upon the circle) may 

 be seen througli it and through the lower micrometer at 

 the same time. 



I will now proceed to show you in what manner the 

 examination of the divisions upon either circle may be 



S 4 performed. 



