308 Professor Piazzi Smyth on a Centauri, 
speaking of 61 Cygni (849): “ The data in the case of a Cen- 
tauri are more uncertain. Since the year 1822, the distance 
has been steadily and pretty rapidly decreasing, at the rate 
of about half a second per annum, and that with very little 
changes in the angle of position. Hence it follows evidently, 
that the plane of its orbit passes nearly through the earth, 
and (the distance about the middle of 1834, having been 173”) 
it is very probable that either an occultation, like that ob- 
served in € Herculis, or a close approximation of the two 
stars, will take place about the year 1867.” 
This result being so very different from his, Captain Jacob 
has re-examined his observations and calculations, and has 
endeavoured to deduce limits within which the elements of 
the orbit must lie; his results, contained in several letters, 
are nearly as follows, and are given pretty fully, as so much 
importance is to be attached, in astronomical inquiry, to the 
fixation of distinct limits, within which it may be predicted 
that an unknown quantity will certainly be found: this it was 
which formed so great a difference between the researches of 
Le Verrier and those of Adams, with regard to the new planet 
Neptune, and made the former carry so much more conviction 
to the minds of those to whom they were communicated :— 
Since coming here (Madras)—writes Captain Jacob—I 
have got some rough measures of a Centauri (there not being 
proper instruments for the purpose), which give 244°:5 for 
the angle of position, and 6’-23 for the distance, at the epoch 
1849-63 ; these render probable a later perihelion passage, 
with less excentricity and inclination than what I assigned, 
but the difference cannot be very great. 
On looking at the whole observations to see what is really 
known on the subject ; it seems this much. The enclosed slip 
represents the area described between 1825 and 1848 (scale 
4” to an inch), as actually observed ; though allowing reason- 
Dunlop IS25 
able error of observation, say 1° in position and 05 in. dis- 
tanee, yet this will not produce much variation. | This,then 
