310 Royal Astronomical Society. 



pears to equal that of the best instruments of similar construction 

 hitherto made, and the astronomical observations confirm this esti- 

 mate. 



Mr. Henderson next investigates the extent of the changes in 

 the position of the centre of motion, and the formula for repre- 

 senting them. It appears, that from an unsteadiness in the support 

 of one of the pivots of the axis, the centre frequently changes its 

 position by irregular quantities ; and that, owing to the non-circular 

 figures of the pivots, other changes in the position of the centre of 

 motion take place while the circle revolves on its axis. Formulae 

 for expressing the variations occasioned by the latter cause are 

 given, which are constant for the same positions of the circle. But 

 neither of these causes operates in the least degree to affect the 

 mean of the readings of equidistant microscopes ; and, on the 

 whole, Mr. Henderson is of opinion, that the accuracy of the 

 astronomical observations made with the instrument are not im- 

 paired to any sensible extent by any of the anomalies which have 

 been mentioned. 



VI. Supplement to a paper on the Latitude and Longitude of 

 the Cape of Good Hope. By Mr. Henderson. See Monthly 

 Notices, vol. ii. p. 183, or Memoirs, vol. vi. p. 125. 



The preceding determination, from Mr. Fallows's observations 

 in 1829 and 1830, gave the longitude of the Cape Observatory 

 l h 13 m 55 s *8 East. Mr. Henderson deduces l h 13 m 54 s *4 from his 

 own observations in 1832 and 1833. The mean is l h 13 m 55 s ' 1, 

 which, rejecting the s * 1, Mr. Henderson proposes to adopt, until 

 the uncertainty is removed. The difference of longitude of Green- 

 wich and the Cape is determined from 17 observations of the moon's 

 first limb, and 6 observations of the second ; that of Cambridge and 

 the Cape from 16 and 8 observations of the first and second limb. 



VII. Observations of the Comet of 1830, made at Ascension 

 Island, by the late Captain Henry Foster, R.N., F.R.S., and re- 

 duced by Mr. Henderson. 



VIII. Addition to a letter to Professor Airy, dated Nov. 11, 1833, 

 on the Sun's Parallax. By Mr. Henderson. See Monthly Notice 

 for April 1834, vol. iii. page 39. 



Mr. Henderson has compared the Altona observations, Ast.Nach., 

 No. 240, with his own at the Cape. He finds for the values of /x 

 (see preceding reference) from the planet's north and south limb, 

 •0721 and -0507; from the mean of which he deduces 9"*105 for 

 the mean horizontal parallax. The mean of all the determinations 

 is 9"*028, the result being too great, as was also that of a similar 

 method of observation in the time of Lacaille. 



IX. A letter from Professor Santini to the Secretary, dated 

 Sept. 17, 1833, containing results of computations of the perturba- 

 tions of Biela's Comet. 



The letter gives the results of a memoir inserted in the Annali 

 delle Scienze, &c. Padua. The re-examination of the computations 

 was undertaken to detect the difference between the results of 

 MM. Santini and Damoiseau, as to the time of the perihelion 

 passage. An error discovered by M. Santini in the constants of the 



