Astronomical Society. 315 



vations of the solstices made by Dr. Pearson at South Kilworth, 

 with similar observations of Bradley at the Royal Observatory. Dr. 

 Pearson remarks, that the most remote determination of the obli- 

 quity which has any claim to precision, is Dr. Bradley's ; and, ac- 

 cordingly, some one of his obliquities has been chosen by almost all 

 practical astronomers for the purpose of comparison with their own, 

 in order to determine the annual diminution. But, notwithstanding 

 the apparent facility with which the diminution maybe obtained, by 

 a comparison of determinations separated by a considerable interval 

 of time, no two astronomers agree in their results. Dr. Maskelyne 

 considered it to be — 0""52 ; Delambre adopted — 0"'48; Dr. Brink- 

 ley, — 0"-43; and Bessel, — 0"'457. These discrepancies, which 

 in the course of years amount to considerable quantities, demand 

 that the question of preference should be settled ; and this can only 

 be effected by practical methods. 



Bradley determined his summer and winter obliquities by separate 

 deductions, — a method which rendered the result dependent on the 

 assumed latitude of the place. He assumed the latitude of the ob- 

 servatory to be 51° 28' 40", which has since been shown to be too 

 great bj' at least one, if not two, seconds ; and, accordingly, eiII his 

 obliquities are affected with a corresponding error, which may ex- 

 plain the reason why the Greenwich winter obliquities are smaller 

 by some seconds than the summer ones. But it is easy to see that, 

 by combining the observations of two successive solstices, the lati- 

 tude may be eliminated; for, half the difference of the sun's extreme 

 meridian altitudes gives the obliquity due to the middle of the in- 

 terval, which, if the winter solstice be taken first, will be the vernal 

 equinox. In like manner, if we take three, or any odd number of 

 successive obliquities, separately deduced by means of an assumed 

 colatitude, the sum of the whole, divided by their number, will give 

 the mean obliquity belonging to the middle epoch, independent of 

 the assumed colatitude. 



Bradley's first recorded determination of an obliquity was in the 

 winter of 1753; and he observed seven winter solstices, and as many 

 summer ones, without interruption. Omitting the first determina- 

 tion, in order to have an odd number, and taking the thirteenth 

 part of the sum of the remaining thirteen half-yearly determinations, 

 we have an average obliquity corresponding to June 1757, viz. 

 23° 28' 13"-4-446. 



Dr. Pearson commenced his solstitial observations in June 1828, 

 and continued them till June 1838, thereby obtaining twenty-one 

 successive half-yearly obliquities, the sum of which gives an average 

 obliquity corresponding to June 1833. His result is 23° 27' 39"-2409, 

 and is therefore less tlian the average resulting from Bradley's de- 

 terminations by 34"-2037. Dividing this difference by 76, the num- 

 ber of years between the two epochs (1757 — 1833), the annual di- 

 minution is found = — 0"'4500. This accords very nearly with 

 the annual diminution adopted by Bessel in the Tabula? Regio- 

 montanae. 



'i"he instrument with which the observations were made, is an al- 



