( 445 ) 



XXX. — Notice of the Orbit of the Binary Star a Centauri, as recently determined by 

 Captain W. S. Jacob, Bombay Engineers. By Professor C. Piazzi Smyth, 

 F.R.S.E. 



(Read, AprU 5, 1848.) 



The object of this short notice is merely to submit to the Society some astro- 

 nomical results which were recently communicated to me in a letter from my 

 friend Captain Jacob, as they appeared not only to be of a highly interesting 

 nature in themselves, but imperatively to require being followed up farther, and 

 as the observer has lately been obUged by bad health to resign his situation in 

 India, it seemed advisable, for the purpose of procuring attention to the subject 

 elsewhere, to make its peculiarly interesting features as generally known as pos- 

 sible amongst scientific men ; and as a Centauri is already in a manner identified 

 with Scotland, through the researches of the late Professor Henderson, and his 

 determination of the parallax, no medium can be more appropriate than the 

 Transactions of the Royal Society of Edinburgh. 



The star a Centauri, situated in 14" 29°' A.R., and 150' 12' N.P.D., is in many 

 respects a notable object, and though its greatest claims to attention have all 

 arisen within the last few years, under the applications of the advanced astro- 

 nomy of the present day, yet even to the naked eye it has much to raise it above 

 the general crowd. It is a star of the first magnitude, and one of the brightest 

 indeed of that class, and is situated in a peculiarly splendid region of the sky, 

 the same as that occupied by the Southern Cross ; a constellation, by the way, 

 Avhich, on accoimt of its small dimensions, and the few stars it contains visible 

 to the naked eye, is by no means entitled to the too warm encomiums so lavishly 

 bestowed upon it so generally by the early Southern navigators and travellers. 

 (And here I may be perhaps allowed to point out an error in Purdy's Hydro- 

 graphy, where, amongst other fine qualities attributed to the Cross, he adds that 

 of its forming always a sort of clock to the inhabitants of the Southern hemi- 

 sphere ; for the longer diameter of the Cross standing vertical, as he says, at mid- 

 night, persons may always judge by the inclination of the Cross to the one side 

 or the other, when the middle of the night may have passed. Now the two stars 

 at either end of the longer diameter having the same right ascension, will cer- 

 tainly stand in a vertical line when on the meridian, but will of course only be 

 on the meridian above the pole at midnight, once in the course of the year.) The 

 region of the Cross, however, abundantly compensates for the poverty of the con- 

 stellation itself, for such is the general blaze of star-light from that part of the 

 sky, that a person is immediately made aware of its having risen above the hori- 



VOL. XVI. TAKT IV. 5 X 



