126 



DISSERTATION SECOND. 



[part tl 



friend Molyneux, in the end of the year 1 725, 1 were occu- 

 pied in searching for the parallax of the fixed stars by 

 means of a zenith sector, constructed by Graham, the most 

 skilful instrument maker of that period. The sector was 

 erected at Kew ; it was of great radius, and furnished with 

 a telescope twenty-four feet in length, with which they pro- 

 posed to observe the transits of stars near the zenith, ac- 

 cording to a method that was first suggested by Hooke, 

 and pursued by him so far as to induce him to think that 

 he had actually discovered the parallax of y Draconis, the 

 bright star in the head of the dragon, on which he made 

 his observations. They began their observations of the 

 transits of the same star on the 3d of December, when the 

 distance from the zenith at which it passed was carefully 

 marked. By the observations of the subsequent days the 

 star seemed to be moving to the south ; and about the 

 beginning of March, in the following year, it had got 20" 

 to the south, and was then nearly stationary. In the 

 beginning of June it had come back to the same situa- 

 tion where it was first observed, and from thence it continu- 

 ed its motion northward till September, when it was about 

 20" north of the point where it was first seen, its whole 

 change of declination having amounted to 40". 



This motion occasioned a good deal of surprise to the 

 two observers, as it lay the contrary way to what it would 

 have done if it had proceeded from the parallax of the 

 star. The repetition of the observations, however, con- 

 firmed their accuracy ; and they were afterwards pursued 

 by Dr. Bradley, with another sector constructed also by 

 Graham, of a less radius, but still of one sufficiently great 

 to measure a star's zenith distance to half a second. It 

 embraced a larger arch, and admitted of the observations 



' Phil. Trans. Vol. XXXV. p. 697. 



