VOL. LII.] PHILOSOPHICAL TRANSACTIONS. 651 



Cape, and to make their observations there, and it was extremely fortunate they 

 did so, for, by reason of cloudy weather, Mr. Maskelyne was hindered from 

 making the proper observations, and in that case, the observation of the internal 

 contact at the egress at Bencoolen, when compared with the same observation 

 at Greenwich, could have determined nothing with regard to the parallax of 

 the sun. ! 



To determine the parallax of the sun, by means of the observations of the 

 internal contact of Venus with the sun's limb, made at two different places, it 

 is absolutely necessary that the difference of longitude between these two places 

 be exactly determined. For this purpose Mr. Mason applied himself assiduously 

 to observe the eclipses of Jupiter's satellites, and Mr. Green, the assistant ob- 

 server at Greenwich, observed as many of the same eclipses, as the unfavourable 

 season would allow. Dr. Bevis and myself likewise observed the same eclipses 

 in Surry-street, London. The insufficiency of these sort of observations in de- 

 termining the longitude of places where accuracy rs required, is well known to 

 those who have the practice of them. 



By comparing the observations of the first and second satellites made at the 

 Cape with those made in Surry-street, the difference of longitude between 

 Greenwich and the Cape comes out, on a mean, =: 1^ IS"" 30% and rejecting 

 those of the 2d satellite, which are always more uncertain than those of the first, 

 I fix the difference of longitude between Greenwich and the Cape of Good Hope 

 __ .jh J 3m 35$^ which I have made use of in the following computations. 



In order to ascertain the sun's parallax with more certainty, I have compared 

 the observation of the internal contact at the egress at the Cape, with the obser- 

 vations of the same contact made at 1 5 different places in Europe. But before 

 proceeding any farther I shall mention the times of such observations whence I 

 had them ; and the longitudes of those places, and whence I likewise had those. 



Internal contact at h. ra. s. From Longitude h. m. s. From. 



Greenwich at. . 8 19 <>• . Phil- Trans from the Cape . . = 1 13 35 w. Phil. Trans. 



Shirburn castle .... at. . 8 15 10. . ditto from Greenwich =0 4 1 w. ditto. 



8 13 14. . 



Saville house at. . 8 18 22. . ditto from. Greenwich = 30 w. ditto. 



Leskeard at. . 8 21.. ditto from Greenwich = 18 32 w. ditto. 



Paris at. . 8 28 2 J. . ditto from Greenwich = 0910 e. Con. des Terns. 



8 28 29 



Bologna at. . 9 4 54, . ditto from Paris = 36 5 e. ditto 



9 5 



Rome at. . 9 9 36. . a private letter from Paris .... .= 40 37 e. ditto 



Drontheim at. . 9 1 49. . a private letter from Greenwich = O 43 58 e. a private letter 



Upsal at. . 9 28 3. . Phil. Trans from Paris =1 1 10 e. Phil. Trans. 



9 28 9 

 Stockholm at.. 9 30 8. . ditto from Paris =1 3 10 e. Con. des Terns. 



9 30 11 



Hernosand at. . ,y 28 52. . Swedish acts . . from Paris . . . . = I 2 12 e. Phil. Trans. 



Calmar at. . 9 23 40 . ditto from Stockholm =0 6 2? w. Swedish acts " 



4 O 2 



