220 PHILOSOPHICAL TRANSACTIONS. [aNNO J 787. 



Had Dr. Bradley lived longer, for the benefit of astronomy, to publish his 

 valuable observations ; or had they been since published by another hand, which 

 unfortunately they hitherto have not; these remarks might have been unneces- 

 sary, and perhaps even the occasion for them might never have occurred; as it 

 would have then appeared on what foundation the latitude of this observatory h*ad 

 been established, and what differences of meridians between Greenwich and the 

 other principal observatories of Europe, resulted from the observed eclipses of 

 Jupiter's satellites and other celestial phenomena. However, having formerly 

 been apprised by Dr. Bradley himself, of several particulars of moment relative 

 to his observations, and particularly of the method which he used for settling his 

 latitude and refractions, after he became possessed of the new instruments in 

 J750, and being assisted with some of his manuscript calculations, with the 

 addition of my own observations, I flatter myself I can throw the light wanted 

 on the question, and obviate the principal difficulty, that relative to the differ- 

 ence of latitude of Greenwich and Paris, and reduce the difference of meridians 

 within smaller limits, notwithstanding Dr. Bradley's original observations had 

 been removed from this observatory, in which they were made, before I came 

 here, and have not yet been restored to it. 



Dr. Bradley having been furnished by government, in the year 1750, with a 

 brass mural quadrant of 8 feet radius, constructed by that excellent artist Mr. 

 John Bird, an instrument far superior to any before used in the practice of as- 

 tronomy, assiduously observed with it the pole star, and other stars lying to the 

 north of the zenith, for upwards of 3 years; and then removed it to the oppo- 

 site side of the wall, making it change place with the iron quadrant of the same 

 radius constructed by Mr. Graham, also an excellent instrument, though infe- 

 rior to this, and commenced a regular series of observations of the sun, planets, 

 and fixed stars, which have been ever since continued in the same manner. 

 Moreover, the temperature of the air, shown by the barometer and thermo- 

 meter, is affixed to each observation; and the zenith point of the quadrant set- 

 tled from time to time, by the help of a zenith sector of 12^ feet radius, turned 

 alternately contrary ways, the same with which Dr. Bradley had before made his 

 two useful and admirable discoveries of the aberration of light and the nutation 

 of the earth's axis. 



By the observations of the pole star and other circumpolar stars, above and 

 below the pole, Dr. Bradley got the apparent zenith distance of the pole; by 

 the apparent and equal zenith distances of the sun at the two equinoxes, having 

 at the same time opposite right ascensions, as found from comparing his observed 

 transits over the meridian with those of fixed stars, after the manner used bv 

 Mr. Flamsteed for deducing the right ascensions of the fixed stars, he found the 

 apparent zenith distance of the equator, which lessened by parallax and added to 



