VOL. LXXVII.] 



PHILOSOPHICAL TRANSACTIONS. 



237 



sions, observed at both Royal Observatories, he found Q™ 21'. From the ob- 

 servations made between 1761 and 1764 he found 9"" 28\ By the observations 

 made before 1700, 9™ 21^ And, from a comparison of mine and the Parisian 

 observations, with the intermediate help of his own made at Stockholm, 9"" 26': 

 and from the whole he inferred the difference of meridians to be 9"^ 25^ 



Twelve years having elapsed since Mr. Wargentin's comparison, I was desirous 

 to see what would result from the further observations made during that time, 

 and applied to the Comte Cassini, the respectable heir of the late M. Cassini de 

 Thury, and his successor at the Royal Observatory, and to the celebrated M. 

 Messier, to favour me with such of their observations of the eclipses of the first 

 satellite of Jupiter as had been made correspondent to mine. These they im- 

 mediately sent me in the most obliging manner, by which I am enabled to make 

 further inferences concerning the difference of our meridians, as exhibited in the 

 two following tables ; in reference to which it is to be understood, that all the 

 observations at Greenwich were made with a 46-inch achromatic telescope of 

 3.6 inches aperture, except a few otherwise noted as observed with a 6-feet New- 

 tonian reflector, whose aperture is 9.4 inches ; and once with a 2-feet Gregorian 

 reflector, whose aperture is 4.5 inches; and once with an 18-inch Gregorian 

 reflector, with an aperture of 4.4 inches, furnished with new metals of Mr. 

 Edwards's brilliant composition, described in the appendix to the Nautical Almanac 

 of the present year, which reflects as much of the incident light as an achro- 

 matic telescope transmits; and that in making out the columns, intitled difference 

 of meridians corrected, I have subtracted 7* from the immersions, and added as 

 much to the emersions, observed with the d-feet reflector, and added 13' to the 

 immersions, and subtracted as much from the emersions observed with the 2-feet 

 reflector, to reduce them to what they should have been probably observed at 

 with the 46-inch achromatic telescope, and added 5^ to the time of the emersion 

 observed on Sept. 5, 1784, at the Royal Observatory at Paris, with a 5-feet 

 reflector of Dollond, to reduce it to the 3-|- feet achromatic telescope. 



Difference of meridians of the Royal Observatories of Greenwich and Paris, by observations of 

 eclipses of Jupiter's first satellite, observed at both places. 



By Immersions. 



1779, Jan. 11 



18 



1780, Jan. 14 

 ■ Mar. 18 



25 

 1785, Oct. 1 



Mean of 6 imm. 



Circumstances of the observations at 

 Greenwich. 



6F. 



6 F. ; air a little hazy. 



Air very clear. 



Air very clear. 



[n contact with Jupiter's body. 



Circumstances of the observations at 

 the Royal Observatory at Paris. 



Limbs undulating. 

 Air very clear. 



Air hazy. 



Air a little hazy. 



A little hazy. 



