285 



On the Discordances between the Sun's observed and computed Right 

 Ascensions, as determined at the Blackman-street Observatory, in 

 the Years 1821 and 1822 ,- with Experiments to show that they did 

 not originate in instrumental Derangement. Also a Description of 

 the seven-feet Transit with which the Observations were procured, and 

 upon which the Experiments were made. By James South, Esq. 

 F.R.S. Communicated June 1,1826. Read June 8, 1826. [Phil. 

 Trans. 1826,^. 423.] 



Astronomers have generally admitted that a certain discordance 

 between the observed transits of the sun and its computed right ascen- 

 sion takes place, and have been accustomed to refer it to instrumental 

 error, arising from the action of the solar rays on the parts of the 

 transit instrument used. 



The validity of this explanation, and the true state of the facts, are 

 what the author proposes to investigate in this communication. 



He commences his paper with a full description of the transit in- 

 strument used in the observations, (the work of Mr. Troughton,) and 

 of which, as accurate drawings have already been exhibited to the 

 Society, no account need now be given, further than to mention the 

 connexion of the cones which form the axis and the tube by tension 

 bars, acting in their interior by means of differential screws, and thus 

 drawing the cones in forcible contact with the spherical shell, which 

 forms the centre of the instrument. 



This instrument was erected in 1820, and its adjustments being 

 made, it was subjected for two months to every species of trial to as- 

 certain their permanence, which proving satisfactory, the regular se- 

 ries of observations were commenced. 



These observations are stated at length in the tables attached to 

 this paper and forming part of it. The right ascensions of the sun, 

 deduced from them, are compared with those deduced from the com- 

 putations in the Nautical Almanac reduced to the meridian of Black- 

 man-street ; and at each observation, the state of defence from, or 

 exposure to the sun's rays, of the instrument is stated. This was 

 purposely varied as much as possible, and in many instances the sun 

 was purposely allowed to shine for a full hour on the brace covered 

 with black cloth, to produce as great a difference of temperature as 

 possible in them, which was estimated by an attached thermometer, 

 and in some cases amounted to 16. Yet no appreciable error in the 

 times of transit was found to arise from this severe trial, especially 

 when the means of several days' observations were taken, whether 

 the computed places of the sun by the Nautical Almanac were used, 

 or the corresponding observed transits of the pole star were taken 

 as zero point. 



But the general result of the whole series of observations is, that 



the sun's observed right ascension is always in excess of his computed. 



This deviation goes in many cases to the extent of a whole second 



of time. The mean excess is from 0"'6 to 0"'7 ; and it appears from 



the tables here given to be subject to periodical increase and diminu- 



