318 PHILOSOPHICAL TRANSACTIONS. [anNO 1728 



far as from the sun to us, in 8'" 13'. The near agreeincMit he met with among 

 the observations induces him to think that the maximum, as here fixed, cannot 

 differ so much as a second from the truth ; and therefore it is probable, that the 

 time which light spends in passing from the sun to us, inay be determined by 

 these observations within 5^ or 10'; which is such a degree of exactness, as we 

 can never hope to attain from the eclipses of Jupiter's satellites. 



Having thus found the maximum, or what the greatest alteration of declina- 

 tion would be, in a star placed in the pole of the ecliptic, Mr. B. next deduces 

 from it, according to the foregoing hypothesis, the alteration of declination in 

 one or two stars, at such times as they were actually observed, in order to see 

 how the hypothesis will correspond with the phaenomena through all the parts 

 of the year. 



It would be too tedious to set down the whole series of the observations ; he 

 therefore makes choice only of such as are most proper for the present purpose, 

 and begins with those of y Draconis. 



This star appeared farthest north about September 7? 1727, as it ought to 

 have done according to the hypothesis. The following table shows how much 

 more southerly the star was found to be by observation, in several parts of the 

 year, and how much more southerly it ought to be according to the hy- 

 pothesis. 



The difference The difference The dilference The difference 



ofdeclination of declination of declination of declination 



by observa. by the hypo. by observa. by the hypo. 



1727. D. II II 1728. D. II „ 



Oct. . . 20 . . 44 A\ March 24 37 38 



Nov. . . 17 . . 1 U 12 April . . 6 36 36^ 



Dec. . . 6 . . 1 7+ 1 8i May . . 6 28^ 29^ 



28 . . 25 26 June.. 5 \%\ 20 



1728 15 M^ 17 



Jan. . . 24 . . 34 34 July . . 3 1 U 1 U 



Feb. . . 10 . . 38 37 Aug. ..2 4 4 



March 7 ■ • 39 39 Sept. ..6 O o 



Hence it appears, that the hypothesis corresponds with the observations of tliis 

 star through all parts of the year ; for the small differences between them seem 

 to arise from the uncertainty of the observations, which is occasioned he thinks 

 chiefly by the tremulous or undulating motion of the air, and of the vapours in 

 it ; which causes the stars sometimes to dance to and fro so much, that it is 



