450 NOTICE OF THE ORBIT OF THE 



that the orbit of the smaller star of « 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 i-easonable supposition of a nearly circular orbit, 

 seen in proiile, 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 John Herschel going to press, he knew of no micrometri- 

 cal measures subsequent to 1 838, but soon after that period, most fortunately for 

 the interests of sidereal astronomy. Captain Jacob 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 

 achi-omatic telescope from Dolloud, and on its an-ival 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 Jacob 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. Herschel's observations fully confirmed 

 Captain Jacob's views, who has now recomputed the orbit, including all the 

 known obseiwatious 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 1 00 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 



