Ml 



ciun INOMICAU 



' llfl.K, ASTRONOMICAL 



Ml 



Tab, viu. and ix. ; it Menu a (mailer one bad been uwd in the surrey 

 of Denmark a> early a* 1742. 



Bamaden undertook an altitude and azimuth circle for the obser- 

 vatory of Palermo in 1 788, and completed it in 1 789. Thin is described 

 by Piana ('Delia Specola Astronomic* di Palermo,' 17i*2), with four 

 plate* of the instrument and its detail*. (See, also, Pearson's ' Astro- 

 nomy,' voL ii., p. 418, plate xxiii.) It U with this circle that the 

 declinations of Piazza's celebrated catalogue were determined. In 

 Troughton perfected his altitude and azimuth circle; and in 

 1793 Mr. Francis WoUatton published in the ' Philosophical Trans- 

 actions ' a description, with a plate, of his transit circle : Borda's re- 

 peating circle seems to have been first used about 1787, in connecting 

 the meridians of Paris and Greenwich. The advantages of the cir- 

 cular form were now appreciated ; its superior strength and symmetry, 

 the elimination of any error of ezceutricity by opposite readings, and 



the diminution .if errors of division by increasing the inn. 

 readings, or by reversing the instrument, were placed beyond dispute. 

 Various alterations were made in parts of these instruments, ami in w 

 contrivances and conveniences applied, but nothing very novel in 

 principle was struck out till Troughton erected a six-feet mural 

 circle at Greenwich in 1812. After some controversy thin {> 

 been generally adopted in those British observatories which can afford 

 a second observer for the transit. 



The limits of the present work will not allow a detailed account 

 of any one instrument, and still less a description of tin- mnncp'U- 

 variations which have been introduced according to the views of astro- 

 nomers and artists. We shall give a general view of the tv, 

 essential forms, the mural meridian circle and the altitude and azimuth 

 circle, as constructed by Troughton and his school. 



The accompanying figure of the mural circle of Troughton ie 



Greenwich Mural Circle, by Troughton ; diameter ilx feet. 



from that prefixed to the Greenwich Observations for 1812, with the 

 subsequent alterations. The circle is framed as strongly as possible 

 (it might perhaps, in some cases, be advantageously cost in one piece), 

 and is fixed at right angles to, and concentric with, a long, h. .!!... 

 conical axis. This axis rest* and turns in two collars, one towards 

 each end of the cone, fixed at the front and back of a stone pier, three 

 IT four feet in depth, and there are four screws at the back for 

 adjusting the horizontal axis in inclination or azimuth. The circle is 

 divided on its edge to 5', upon a narrow ring of gold or other metal 

 let into the rim ; the divisions are read by six micrometer-microscopes 

 at A B, c D, E r. The telescope is fixed at right angles upon an axis 

 which works within the hollow conical axis of the circle, and can )>e 

 moved upon the circle into any position ; it is held fait by clamps 

 towards the object and eye-end. At the eye-end there are three or 

 more vertical, and one horizontal wire, and a micrometer, which carries 

 second horizontal wire parallel to the fixed wire. To prevent wear, 

 and to give ease and smoothness to the motion of the circle, two large 

 friction wheels are suspended in front of the pier from the arms of 

 two lerers, which by counterpoises may be made to support the whole 

 or part of the weight of the instrument. They press upwards upon 

 a ring on the axis, between the circle itself aad the pier. There are 



several small contrivances which need not here be mention..! 

 design is only to give an idea of the nature of the ii I '. >i ., 



more minute description the reader must commit Pearson's ' Ast i . >non \ \ .' 

 vol. ii. pp. 472, 488, plate xx. The above description belongn to the 

 mural circle as constructed by Troughton. In one tir two later in- 

 stances the collars have been exchanged l'-<>- Y.s. that, is. angular 

 notches in pieces of metal, in the hope of making the instrument serve 

 for a transit ; but this has not, we Iwlieve, yet been performed satis- 

 factorily. Dr. Robinson in the circle made for the observatory at 

 Armagh, preferred having the division* cut on the back of the instru- 

 ment, and the microscopes fixed on the edge of a circular pier. 

 with several other alterations (for which sec ' Mem. Ast. Soc.,' vol. 

 ix.) The divisions of the circle are cut by Troughton's method. 

 [GRADUATION.] 



After the instrument U erected on its pier, the axis must be placed 

 horizontally, or the plane of the circle vertical. This may be done by 

 a plumb-line apparatus, which generally accompanies the instrument ; 

 or more accurately by making a star pass the meridian wire at the 

 .!:: time when seen directly, and also when seen reflected in a tr. .u_-li 

 filled with mercury or water, 4c. The four screw* at the back ..t tin- 

 pier supply the means of effecting this. The second adjustment, or 



