of the late Dr. Maskelyne. $ 
transit of Venus, which had been the occasion of the voy- 
age; but Dr. Maskelyne, who was furnished with an ex- 
cellent clock of Shelton’s regulated at Greenwich by Brad- 
ley, and which had been removed with all possible care, 
determined the number of vibrations it made less at St. 
Helena than at London, to judge from thence the diminu- 
tion of gravity. 
The second object of the voyage was the parallax of 
Sirius: this observation, like the other, failed, but it gave 
occasion to a curious and useful remark. To judge whether 
the star Sirius had any sensible parallax, it was requisite to 
have a better instrument than~La Caille’s, and to observe 
the star in favourable circumstances. The last point des 
pended on the astronomer, the first on the artist. The 
Royal Society had a sector made on purpose, which was 
completed only at the moment of departure, and therefore 
could not be tried at'Greenwich. What was Dr. Maske- 
lyne’s surprise when he found that this instrument, in- 
tended for the most delicate researches, gave him daily dif- 
ferences of 10”, 20” and 39” in the measure of the same 
angle! Carefully examining what might be the cause of 
these extraordinary variations, he ascertained it by certain 
trials, and endeavoured to correct it; but succeeded only 
in part. He reduced the error to 3”, yet this was insuffi- 
cient for the object he had in view*. He was therefore 
obliged to give it up: but this disappointment was the oc- 
* This error was occasioned by the plumb-line, at the top, being in a 
loop and hung over a cylinder of 1-20th of an inch in diameter, fixed to the 
centre of the sector. The telescope could not be directed to a star without 
giving this cylinder a motion of rotation equal to the distance of the star 
from the zenith: this motion, by the effect of adhesion to the cylinder, 
deranged the line from its first position; and the are which had passed undér 
the line was not then the true zenith distance of the star. Dr. Maskelyne 
had the cylinder filed to 1-70th of a line, and then the error was reduced 
to 3”, On this occasion no doubt the actual suspension was thought of, 
which consists in attaching the top of the plumb-line to a fixed point, trom 
which it might hang freely, opposite the point marked on the outer sur- 
face of the axisof the cylinder. By this means the plumb line will keep the 
same position, without variation, and the observed distance may be depended 
on. Jt may be asked whether the sector with which Bradley made his fine 
discoveries of aberration and nutation, had-not this defect. ‘he answer 
would be the same; for Bradley’s sector, made by Graham, was the model 
of the sector constructed by that celebrated artist and taken to Lapland, 
Bradléy could not then rely on the distances he had measured. Fortunately 
the error would be very nearly the sume for each star that he observed; he 
only wanted relative distances, and the sector gave them nearly as exact as 
if there had not been any error, This defect, which certainly existed in the 
sector of Lapland,did not prevent Lemonnier, 9n his return to France, from 
observing, like Bradley, all the variations produced by aberration, and 
hence fully to confirm the brilliant discoveries of the English a:tronomer. 
See Degré du Méridien entre Paris et Amiens, Paris 1740. ‘ 
A3 casion 
