310 PHILOSOPHICAL TKAXsACTIONS. [aNNO 1728. 



the same distance from the zenith as it had done in December, when it was 

 first observed. 



From the quick alteration of this star's dechnation about this time, it in- 

 creasing a second in 3 days, it was concluded, that it would now proceed 

 northward, as it before had gone southward, of its present situation ; and it 

 happened as was conjectured; for the star continued to move northward till 

 September following, when it again became stationary, being then near 20" 

 more northerly than in June, and no less than 39" more northerly than it was 

 in March. From September the star returned towards the south, till it arrived 

 in December to the same situation it was in at that time 12 months, allowing 

 for the difference of declination on account of the precession of the equinox. 



This was a sufficient proof, that the instrument had not been the cause of 

 this apparent motion of the star; but to find one adequate to such an effect 

 seemed a difficulty. A nutation of the earth's axis was one of the first things 

 that offered itself on this occasion; but it was soon found to be insufficient ; 

 for though it might have accounted for the change of declination in y Draconis, 

 yet it would not at the same time agree with the phenomena in other stars: 

 particularly in a small one almost opposite in right ascension to y Draconis, at 

 about the same distance from the north pole of the equator; for, though this 

 star seemed to move the same way, as a nutation of the earth's axis would have 

 made it, yet changing its declination but about half as much as y Draconis in 

 the same time, as appeared on comparing the observations of both made on the 

 same days, at different seasons of the year, this plainly proved that the apparent 

 motion of the stars was not occasioned by a real nutation, since if that had been 

 the cause, the alteration in both stars would have been nearly equal. 



The great regularity of the observations left no room to doubt, but that 

 there was some regular cause that produced this unexpected motion, which did 

 not depend on the uncertainty or variety of the seasons of the year. On com- 

 paring the observations with each other, it was discovered, that in both the 

 forementioned stars, the apparent difference of declination from the maxima, 

 was always nearly proportional to the versed sine of the sun's distance from the 

 equinoctial points. This was an inducement to think that the cause, whatever 

 it was, had some relation to the sun's situation with respect to those points. 

 But not being able to frame any hypothesis at that time sufficient to solve all 

 the phaenomena, and being very desirous to search a little further into this 

 matter: Mr. B. began to think of erecting an instrument for himself at VV'an- 

 sted, that having it always at hand, he might with the more ease and certainty 

 inquire into the laws of this new motion. The consideration likewise of being 

 able, by another instrument, to confirm the truth of the observations hitherto 



