British Association, 111 



amount to half a second, and has been separately determined by the 

 admirable observations of Dr. Brinkleyj the third is the largest in 

 amount, being about 9", and completing its cycle in the time of a 

 complete revolution of the moon's nodes, or about eighteen years, 

 rather more : the exact determination of this vi^as the object of the 

 discussion of the observations of which he was now giving an account. 

 He then proceeded to give a general description of the method of 

 employing the observations, and the kind of observations to be se- 

 lected for this determination, showing that it was most important to 

 have a complete series of observations extending through the entire 

 period of the moon's nodes, made with the same instruments, and if 

 possible, by the same observer, or at least with the same system of 

 observation. The observations made at Greenwich under the super- 

 intendence of the late Mr. Pond were those selected. The tables of 

 Bessel were usedj his values of declination, nutation, and proper 

 motion were used, but Dr. Robinson had used his own values of aber- 

 ration and refraction. Upwards of 4000 observations of the pole- 

 star above were used, and in the results more than 2000 above and 

 below were combined to give the zero of polar distance ; the others 

 were used to watch for and detect any change in the instrument. He 

 then stated the principle on which the other stars were selected, viz. 

 that their altitudes secured them from uncertainties in refraction, and 

 that they should be such, that at least two-thirds of the nutation 

 should exist in the direction of their polar distances -, of such stars 

 fifteen were in the Greenwich Observation, but some of these could 

 not be used. They afforded about 8000 results, but 6000 only were 

 used J an accident which occurred to the instrument in 1820 ren- 

 dered useless about 1000 of these. The mean results of these ob- 

 servations being taken with the precautions which the paper pointed 

 out at length, the results of some required that the value of Lindenau 

 which is 8"*977 should be increased, while others required that it 

 should be diminished ; on the whole an increase of 0"'257 was ac- 

 quired, giving the result 9"*234, which differed only by sixteen thou- 

 sandths of a second from the number selected by Mr. Baily, and used 

 in his invaluable Catalogue. The learned author then proceeded to 

 notice and remove certain objections which he anticipated might be 

 made to the details of his method of reduction ; in the course of these 

 he stated, that as the corrections of Bessel's proper motions which 

 his work has given are all, except in one instance, negative, he in- 

 ferred that the Greenwich circle was undergoing some progressive 

 change of figure, making it show polar distances too great for about 

 30° south of zenith. If this were so, he observed, the sagacity of 

 Mr. Airy would not permit it to be long undetected. He then read 

 a table from these observations, showing that the declinations as ob- 

 tained from his calculations, though they differed materially from those 

 given by Pond himself in the Nautical Almanac for 1834', yet agreed 

 closely with those of Bessel, thus showing, that the difference between 

 these Catalogues arises solely from the different methods of reduction, 

 and exciting the wish, that the British Association might lend its aid 

 in reducing the whole of the Greenwich observations made by Pond. 



