sscT. iv.] DISSERTATION SECOND. 129 



axis of the ellipse in every case, or the whole ehange of posi- 

 tion, 40". It was the shorter axis which Bradley had ac- 

 tually actually observed in the case of y Draconis, that 

 star being very near the solstitial colure, so that its chan- 

 ges of declination and of latitude are almost the same. In 

 order to show the truth of his theory, he computed the 

 aberration of different stars, and, on comparing the results 

 with his observations, the coincidence appeared almost per- 

 fect, so that no doubt remained concerning the truth of 

 the principle on which he had founded his calculations. 

 He did not explain the rules themselves : Clairaut publish- 

 ed the first investigation of these in the Memoirs of the 

 Academy of Sciences for 1737. Simpson also gave a de- 

 monstration of them in his Essays, published in 1740. 



It has been remarked, that the velocity of light, as as- 

 sumed by Bradley, did not exactly agree with that which 

 Rbmer had assigned ; supposing the total amount of the 

 aberration 40£", it gave the time that light takes to come 

 from the sun to the earth 8' 13"; but it is proper to add, 

 that since the time of this astronomer, the velocity of light 

 deduced from the eclipses of Jupiter's satellites has been 

 found exactly the same. 



It is remarkable that the phenomenon thus discovered 

 by Bradley and Molyneux, when in search of the parallax 

 of the fixed stars, is in reality as convincing a proof of 

 the earth's motion in its orbit, as the discovery of that 

 parallax would have been. It seems, indeed, as satisfac- 

 tory as any evidence that can be desired. One only re- 

 grets, in reflecting on this discovery, that the phenomenon 

 of the aberration was not foreseen, and that, after being 

 predicted from theory, it had been ascertained from obser- 

 vation. As the matter stands, however, the discovery both 

 of the fact and the theory is highly creditable to its author. 



17 



