C 612 ] 
to reach Bencoolen, and therefore refolved to (lay at 
the Cape, and to make their obfervations there ; and 
it was extremely fortunate they did fo, lor, by rea- 
fon of cloudy weather, Mr. Malkelyne was hinder- 
ed from making the proper obfervations, and in that 
cafe, the obfervation of the internal contad at the 
egrefs at Bencoolen, when compared with the fame 
obfervation 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 obfervations of the internal contad of Venus 
with the Sun’s limb, made at two different places, 
it is abfolutely neceffary that the difference of longi- 
tude between thefe two places be exadly determined. 
For this purpofe Mr. Mafon applied himfelf affiduoufly 
to obferve the eclipfes of Jupiter’s fatellites, and Mr. 
Green, the affiftant obferver at Greenwich, obferved as 
many of the fame eclipfes, as the unfavourable feafon 
would allow. Dr. Bevis and myfelf likewife obferved 
the fame eclipfes in Surry-Street, London. The in- 
fufficiency of thefe lort of obfervations in determining 
the longitude of places, where accuracy is required, is 
well known to thofe who have the pradice of them. 
By comparing the obfervations of the firffc and fe- 
cond fatellites made at the Cape with thofe made in 
Surry-Street*, the difference of longitude between 
Greenwich and the Cape, comes out, on a mean, 
T 13' 3 o'', and rejeding thofe of the fecond fa- 
tellite, which are always more uncertain than thole 
of the firft, I fix the difference of longitude between 
Greenwich and the Cape of Good Hope = i h 1 3 S' » 
which I have made ufe of in the following compu- 
tations. 
* Vid. Phil. Tranf. vol. lii. part 1. 
In 
