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Dr. Hallejr, who was firfl aware of the unequal 
returns of this comet in its former appearances, 
which he found to have been alternately of 75 and 
'76 years, was likewife the firft who alhgned their 
true cauie. He afcribed it, as T faid above, to the 
nearer or more diftant approaches of the planets of 
our iyftem ; and having obferved that the comet we 
are fpeaking of came very near Jupiter in the 
fummer of 1681, above a year before its laft ap- 
pearance, and remained feveral months in the neigh- 
bourhood of that planet, he judged that circum- 
ftance alone fufficient to have coniiderably retarded 
its motion, and prolonged the duration Of its revo- 
lution. Hence he concluded that its return was not 
to be expedled till the latter end of 1758, or the 
beginning of the next year. 
Dr. Halley obferves, in confirmation of this opi- 
nion, that the adion of Jupiter upon Saturn is alone 
fufficient to alter the duration of Saturn’s period one 
whole month ; and he adds, how much greater ir- 
regularities muft not a comet be liable to, which at 
its remotefl diftance gets near four times farther 
from the fun than Saturn, and whofe velocity in 
drawing near the'fun needs but a very fmall increafe 
to change itselliptic into a parabolic curve. 
Dr. Halley does not determine more exadly the 
time of the return of the comet of 1682; neither 
could he do it but by determining exadly the effed 
of the neighbourhood of Jupiter, which muft very 
fenfibly affed the velocity with which the comet was 
moving towards the fun. Befides, regard muft be had, 
not only to this approach to Jupiter in 1681, but 
likewife to the other approaches to this and all the 
other 
