ASTRONOMY OF EIGHTEENTH CENTURY. 249 



and that were they all suddenly and simultaneously blotted out of 

 existence, whole generations of men might afterwards live their term 

 and pass to their graves without perceiving the (jifiference. 



In the year 1725 two English astronomers, MOLYNEUX and BRADLEY 

 (1692 1762), began to observe a star in the constellation of the 

 Dragon (y Dmconis), which star passed very nearly vertically over the 

 Observatory at Kew. Observations made on the 3rd, 5th, nth, and 

 1 2th of December, not having indicated any material difference in 

 the place of the star, they were about to be discontinued for the time, 



FIG. 120. BRADLEY. 



when on the i7th Bradley, having from curiosity repeated his obser- 

 vation, perceived that the star passed rather more to the southward 

 than before. This was at first attributed to the uncertainty or errors 

 of observation ; but another observation on the 171)1 showed the star 

 passing still more to the southward. These appearances surprised 

 the two astronomers, for they evidently could not be due to the annual 

 parallax. It was first supposed that some change in the position of 

 the instrument had occurred ; but this idea was soon excluded by the 

 observed regularity of the motion. At the beginning of March the 

 star was found 20" more southerly than at the time of the first obser- 

 vation ; by the middle of April it began to return to the north ; in 

 September it began to return to the south; and in December, 1726, 

 it occupied the same position as before, allowance being made for the 

 precession of the equinoxes. Bradley at first supposed that a change 

 of the position of the earth's axis would account for these appearances, 

 but observations on other stars were found inconsistent with this sup- 

 position. He had a sector (that is, a portion of a graduated circle) 

 erected, having a radius of 1 2 1 feet, and an arc ranging over 6 on each 

 side of the zenith, or vertical point. He devoted anpther year of obser- 



