On the Elements for reducing Polar Distances. 55 



at the time, by finding fragments of well-known argilla- 

 ceous ironstone amongst the spoil at the pit ground. 



I remain, &c. 



Coleford, Gloucestershire, DaVID MuSHET, 



20th April, 1812. 



XIII. On the Elements for reducing the Polar Distances of 

 the principal fixed Stars ivhich pass near the Zenith of 

 the Royal Observatory at Greenwich. Taken Jro7n Dr. 

 Bradley's Catalogue, by Air. Firminghr. 



Charles- street, Somers Town, 

 Q July 25, 181-.J. 



OiNCE the time that the reductions of the latitude and 

 longitude of the stations used in carrying on the trigono- 

 metrical survey of this country have been in part published, 

 many gentlemen engaged in the pursuit of practical astro- 

 nomy have either made use of them in settling the position 

 of theic observatories, or have employed themselves in the 

 verification of these results with a series of astronomical 

 observations made at the diflerent statii>ns or objects that 

 have been given in the survey. Some of these verifications 

 were attempted with sextants; but as the sextant itself 

 cannot be depended on generally, to an accuracy nearer 

 than 20 or 30 seconds, this combined with liit; errors 

 arising from the artificial horizon used with it render such 

 observations, in deductio:!s where an uncertainty of only a 

 few seconds is to be looked for, of little or no value, even 

 though a mean, from a great number of repetitions, should 

 be taken. The usual method which astionomers have 

 adopted to determine the difference in latitude between two 

 places, not very distant from each other, has been by ob- 

 serving the zenith distances at the two places of a number 

 of stars which pass near the zenith of each place; these, 

 when properly reduced, give the difference of latitude be- 

 tween the two places, or the number of degrees in the con- 

 tained arc. The instruments generally empUiyed in such 

 operations have been those denoted by the name of Zenith 

 Sectors, and they are the best instruments when well con- 

 structed for this purpose, admitting of a more accurate 

 measure, from being divided on an arc of a much larger 

 radius than can be used in the construction of instruments 

 for other astronomcial observations. The zenith sector in 

 the Roval Observatory at Greenwich is divided upon an arc of 

 twelve feet radius; its principal use there is for determining 

 the error in collimation of the two mural quadrants ; and 

 this has been done from a scries of observations upon the 

 D 4 meridional 



