304 Professor Piazzi Smyth on a Centauri, 
medal to Bessel for his discovery of the parallax of 6’ Cygni, Sir 
John Herschel stated that the orbit of the smaller star of @ Cen- 
tauri 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 John Herschel going to press, he knew of no 
micrometrical measures subsequent to 1838, but soon after that pe- 
riod, most fortunately for the interests of sidereal astronomy, Cap- 
tain Jacob came into the field. On visiting 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 de- 
voted all his spare time with great perseverance 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 ob- 
servations 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 document 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 observations up to the present time ; 
and though this performance is to be considered but a first approxi- 
mation, still it will probably not be very much altered by future ob- 
servations 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 hay~ 
ing remained almost stationary in its orbit for the last 100 years, to 
have really made a whole revolution,how came it to pass that 
every observer in the interval saw it always in about the same posi- 
tion on the west, and never on the east of the large star? This ob- 
jection 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 
