SEARCHING FOR ONE THING AND FINDING ANOTHER. 237 



is usual in such cases) traced as a fact in observation 

 before it was clearly seen as a consequence of reasoning. 

 This fact, the ' aberration of light ' (or the ' aberration of 

 the fixed stars,' as it is called), ' the greatest astronomical 

 discovery of the eighteenth century, belongs to Bradley, 

 who was then Professor of Astronomy at Oxford, and after- 

 wards Astronomer Koyal at Greenwich. Molyneux and 

 Bradley, in 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 Flamstead thought 

 he had discovered. Bradley 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 Molyneux then considered for 

 a moment an annual alteration of figure of the earth's 

 atmosphere, such as might affect the refractions ; but this 

 hypothesis was soon rejected. 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 rules which he had already 



