ADDITIONS TO THE NEWTONIAN THEORY. 465 



who was then Professor of Astronomy at Oxford, and afterwards As- 

 tronomer Royal at Greenwich. Molyneux and Bradley, iu 1725, began 

 a series of observations for the purpose of ascertaining, by observations 

 near the zenith, the existence of an annual parallax of the fixed stars, 

 which Hooke had hoped to detect, and Flamsteecl thought he had dis- 

 covered. Bradley 7 soon found that the star observed by him had a 

 minute apparent motion different from that which the annual parallax 

 would produce. He thought of a nutation of the earth's axis as a 

 mode of accounting for this ; but found, by comparison of a star on the 

 other side of the pole, that this explanation would not apply. Bradlev 

 and Molyneux then considered for a moment an annual alteration of 

 figure in the earth's atmosphere, such, as might affect the refractions, 

 but this hypothesis was soon rejected. 8 In 1727, Bradley resumed his 

 observations, with a new instrument, at Wanstead, and obtained em- 

 pirical rules for the changes of declination of different stars. At last, 

 accident turned his thoughts to the direction in which he was to find 

 the cause of the variations whicli he had discovered. Being in a boat 

 on the Thames, he observed that the vane on the. top of the mast gave 

 a different apparent direction to the wind, as the boat sailed one way 

 or the other. Here was an image of his case : the boat represented 

 the earth moving in different directions at different seasons, and the 

 wind represented the light of a star. He had now to trace the conse- 

 quences of this idea ; he found that it led to the empirical rules, which 

 he had already discovered, and, in 1729, he gave his discovery to the 

 Royal Society. His paper is a very happy narrative of his labors and 

 his thoughts. His theory was so sound that no astronomer ever con- 

 tested it ; and his observations were so accurate, that the quantity 

 which he assigned as the greatest amount of the change (one nineteenth 

 of a degree) has hardly been corrected by more recent astronomers. 

 It must be noticed, however, that he considered the effects in declina- 

 tion only ; the effects in right ascension required a different mode oi 

 observation, and a consummate goodness in the machinery of clocks, 

 which at that time was hardly attained. 



/ 



Sect. 4. Discovery of Nutation. 



WHEN Bradley went to Greenwich as Astronomer Royal, he con- 

 tinued with perseverance observations of the same kind as those by 

 which he had detected Aberration. The result of these was another 



7 Rigaud's Bradley. 8 Rigaud, p. xxiii. 



VOL. I 20 



