450 NOTICE OF THE ORBIT OF THE 
that the orbit of the smaller star of a Centauri might subtend the large angle of 
about 1 minute. As it had been actually observed at an elongation of 28” on one 
side of the large star, the very reasonable supposition of a nearly circular orbit, 
seen in profile, would, in course of time, give the same distance on the opposite 
side. Both authorities also predicted the probability of an appulse of the same 
stars somewhere about the year 1867. 
At the time of Sir Joun HERsScHEL going to press, he knew of no micrometri- 
cal measures subsequent to 1838, but soon after that period, most fortunately for 
the interests of sidereal astronomy, Captain Jacos came into the field. On visit- 
ing the Cape from India, where he had been engaged in the great Trigonometrical 
Survey, he spent most of his time at the Observatory, and not only witnessed, 
but took part in the parallax observations of a Centauri. He then ordered a good 
achromatic telescope from Dollond, and on its arrival in India, after his return 
there, erected a small observatory, and devoted all his spare time with great per- 
severance and eminent success to that most difficult species of observation,—viz. 
the double stars. 
About a year ago, he wrote to me to send him out all the old observations 
known of @ Centauri, for the two stars were approaching more and more rapidly, 
and his own observations seemed to give a most unexpected orbit. The first docu- 
ment which reached him was Professor HENDERSON’s memoir on the parallax, 
and then Captain Jacos found that he had been forestalled as to the actual facts of 
an appulse being shortly to be expected, though he indeed fixed the time as being 
very much closer at hand, bringing it from 1867 to 1851 ; but as to the idea that 
the small star had only been gaining its aphelion, without sensible alteration of 
angle of position since 1751,—he found, on computing the orbit, that within that 
interval it had made a whole revolution, or had altered its angle of position by 
360°. The subsequent arrival of Sir J. HerscneEt’s observations fully confirmed 
Captain JAcos’s views, who has now recomputed the orbit, including all the 
known observations up to the present time; and though this performance is to be 
considered but a first approximation, still it will probably not be very much al- 
tered by future observations in any of the important elements. 
The difficulty that might be started at the first mention of this new opinion, 
would be, that supposing the small star, instead of having remained almost sta- 
tionary in its orbit for the last 100 years, to have really made a whole revolu- 
tion,—how came it to pass that every observer in the interval saw it always in 
about the same position on the west, and never on the east of the large star? This 
objection is fully met by the extraordinary nature of the orbit, which turns out 
much more nearly like that of a comet than of a planet, the greatest distance 
being 21°85,” and the least 0-5,” in consequence of which, the small star moves 
with such surpassing rapidity at its periaster, actually 2° 40’ per day; that it is 
but a very short space of time on the eastern side of its primary, and when at its 
