WITH A FOURTEEN FEET REFLECTOR. 193 



belonging to it are probably not those of estimation, but of rough and difficult 

 measurement. The errors of star (1) are on account of its distance from the 

 nebula, without any intermediate stars recorded; and the remainder, except 

 (2,) are all within very close limits. 



It miay be farther remarked, that the errors in h. 2008 were generally about 

 four times as large as in h. 1991; also, in h. 2092-3 they were three or four 

 times as large at similar distances, and in some instances on the remote con- 

 fines, much larger. More time and pains were spent on the first of the three, 

 partly in order to ascertain how much error was necessarily included in the 

 method of estimation. 



35. The places of the following stars, h. 1991, 24; 1i. 2008, 6,9, 23, 25; h. 

 2092-3, 129, 144, differ in a slight degree, in the catalogues, from the results 

 of measurement. These are cases in which the measures are rendered some- 

 what uncertain on account of extreme faintness, and are afterwards found to 

 disagree with the original chart. The general closeness with which the esti- 

 mations coincide with good measures authorize, in these few instances, an 

 equal, or greater confidence in the former. 



36. The artificial scale of magnitudes mentioned in the observation of Aug. 

 9th, Article 19, was converted into the common nomenclature by comparison 

 with various stars, incidentally noticed by Sir J. Herschel in this respect. The 

 magnitudes assigned, in his several catalogues of double stars, to the three in- 

 dividuals of the triple star in h. 1991, and those of six stars described by him 

 as forming a trapezium, or oval, at the bifurcation of the Nebula Cygni, (see 

 Article 59,) are principally relied on. 



The first column in the following catalogues contains the number of the star 

 in order of right ascension; the second, its magnitude; the third, its mean right 

 ascension; and the fourth, its mean declination, at the epoch A. D. 1830.0. 



The right ascension is given to the nearest tenth of a second of time, except 

 that of the stars measured by the micrometer, which are distinguished from 

 the rest by the addition of a second decimal. So the declination is in even 

 seconds for the estimated, and descends to tenths for the measured places. 



Note. — It appears to the committee that the coefficient 2 in Mr. Mason's formula for correcting' 

 the observed difference of right ascension should be omitted, and this correction be made only half 

 as great as Mr. Mason's. The error from this source does not, on the average, amount to more than 

 OM in time in the following catalogue. 

 VII. — 2 Y 



