ADDITIONS TO THE NEWTONIAN THEORY. 265 



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 Flamsteed thought he had discovered. 

 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. Bradley and Moly- 

 neux 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 empirical rules for the changes of 

 declination in different stars. At last, accident 

 turned his thoughts to the direction in which he 

 was to find the cause of the variations which 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 consequences of 

 this idea ; he found that it led to the empirical 



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



