VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 147 



moon's R.A. for 12 hours, he has constantly taken it from the Nautical Almanacs, 

 which give it sufficiently exact, provided some attention be paid to the increase or 

 decrease of the moon's motion. 



Were the following circumstances attended to, the results, he doubts not, 

 would be much more exact. 1st, Compare the observations to the same made in 

 several other places. 2dly, Let several and the same stars be observed at these 

 places. 3dly, Such stars as are nearest in r.a. and declination to the moon are 

 most preferable. 4thly, To get as near as possible an equal number of observa- 

 . tions of each limb, to take a mean of each set, and then a mean of both means. 

 5thly, To adjust the telescopes to the eye of the observer before the observation. 

 The latitude of York, by a medium of many observations, was, 53° 5^' A5". 

 And the mean declination of the magnetic needle was 23° AO' west. 



Sir H. Englefield, when at Scarborough, in Aug. and Sept. 1781, was so kind 

 as to observe, at noon, the height of his barometer and thermometer. Mr. P. 

 also made similar observations in the Observatory at York ; from which, by 8 

 comparisons, none disagreeing above 0.018 of an inch from the mean, he found 

 that the quicksilver at the sea stood 0.063 of an inch higher than at York. The 

 barometers were made by Ramsden, and they agreed together to 0.005 part of 

 an inch. 



XXI f^. Advertisement of the Expected Return of the Comet of 1532 and l66l 

 in the Year 1788. % the Rev. Nevil Maskelyne, D. D., F. R. S. &c. 

 p. 426. 



The comet of 1531, 1607, and l682, having returned in the year 1759, ac- 

 cording to Dr. Halley's prediction in his Synopsis Astronomiae Cometicae, first 

 published in the Philos. Trans, in 1705, and re-published with his Astronomical 

 Tables in 1749, there is no reason to doubt that all the other comets will return 

 after their proper periods, according to the remark of the same author. In the 

 first edition of the Synopsis he supposed the comets of 1532 and 1661, from the 

 similarity of the elements of their orbits, to be one and the same ; but in the 2d 

 edition he has seemed to lessen the weight of his first conjecture by not repeating 

 it. Probably he thought it best to establish this new point in astronomy, the 

 doctrine of the revolution of comets in elliptic orbits, as all philosophical matters 

 in the beginning should be, on the most certain grounds ; and feared that the 

 vague observations of the comet, made by Apian in 1532, might rather detract 

 from, than add to, the evidence arising from more certain data. Astronomers 

 however have generally acquiesced in his first conjecture of the comets of 1532 

 and 1661 being one and the same, and to expect its return to its perihelium ac- 

 cordingly in 1789. The interval between the passages of the comet by the peri- 



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