10 An Account of the Repeating Circle, 



publicity ; on the continent, astronomers and others have writ- 

 ten a great deal about it, and the resuhs of thousands of ob- 

 servations have been pubhshed ; the greater part of which 

 were made on Polaris,- a star, to which, on account of its slow 

 motion, this instrument is peculiar^ adapted. Although the 

 altitude and azimuth uistrument, as a portable one, was pro- 

 duced about the year 1792, we find no description of it in 

 print, until the article Circle appeared in Rees's and Brew- 

 ster's Encyclopedias*; the latter of which is referred to for the 

 use of those who may wish to see a more detailed account of 

 both the instruments under consideration, than can be given 

 in the following brief descriptions. 



Description of the Repeating Circle. 



The lowest part of this instrument is a strong tripod, having 

 at its extremities three steady foot-screws ; one of which, at 

 least, should stand upon a well knov,-n apparatus, for the pur- 

 pose of supplying a slower and finer motion to the upper part, 

 than can be given by the screw itself. This apparatus should 

 support that particular foot which during observation is di- 

 rected to the meridian, or is opposite to the object observed. 

 In the centre of the tripod is fixed a strong vertical axis, of a 

 height sufficient for allowing head-room for observing con- 

 veniently when the telescope is pointed towards the zenith. A 

 pillar of the same height with the axis, is nicely fitted at both 

 ends upon the latter, and both together, when the axis is ver- 

 tical, produce a steady azimuthal motion. To the lower end of 

 the pillar is fixed an azimuth circle: and to the higher end, a 

 cross piece ; on the two extremities of which stand, about five 

 inches apart, two upright bars for supporting a cross axis, to 

 which the principal circle by its centre-work is attached, and 

 round which axis the circle may be turned into any position 

 from one side of the pillar to the other. A semicircle is fast- 

 ened to one end of the cross axis, which, together with a clamp 

 attached to one of the upright bars, affords the means of se- 

 curing the circle in any position. The principal circle, or 

 that of repetition, has (affixed to the middle of its plane, and 

 opposite to the one divided) centre-work, the length of which 



* It is true that the late Rev. Francis Wollaston, in the Phil.Trans. for 1 7f).3, 

 gave a description of a two-feet circle which had an azimuth. That instru- 

 ment, however, was solely designed for n meridian one, and was in fact quite 

 unfit for any other purpose. The same gentleman, in the Appendix to his 

 Fascicuhis, points out the best means of using an altitude and azimuth circle 

 (pro])erly so called), but without giving any descrijjtion. The Wcstbury 

 circle described in the Phil. Trans, for 1806, "although well constructed for 

 obser\-ing azimuths, was not designed for taking transits, and besides was 

 not a portable instrument. 



