408 PROFESSOR C. PIAZZI SMYTH ON THE 



difficult pair no doubt to observe, on account both of the difference of size in the 

 components and their great distance apart ; and some error, perhaps a typical 

 one, seems included in the Hi distance. But putting that on one side, there is 

 running through the whole period, a large and rapid change of position angles 

 with very little change of distance. Now this would in most cases be at once 

 put down as indicative of the distinguishing features of Unary, as contrasted to 

 proper, motion : but in that case what an angular subtense for a stellar orbit, 

 something like 5' of space altogether ; or 5 times as large as the utmost stretch 

 ever given to a Centauri, hitherto known as the largest orbit presented by any 

 star-system in either Northern or Southern hemisphere of the sky. The case 

 therefore requires caution; while both the Cycle in 1844, and the Spec. Hart, in 

 1860, think that the question is still an open one ; and at the latter date, that 

 "years must yet elapse" before it can be either confirmed or contradicted. 



The Elchies observation however helps us to a conclusion at once ; for, taking 

 that as the later date, and for the earlier one the mean of the four sets given 

 l)etween 1821 and 1836, which must surely give a good determination for its 

 mean epoch, and computing thence the annual proper motion, we find that it 

 chances to be in such a direction as to alter the position much and the distance 

 little, giving finally for the proper motion of A on B in a straight line. 



In R.A. + 0"-57 and Decl. + 0"-36, 



while the absolute proper motion of A, as determined by Meridian Instruments, 

 and recorded in the British Association Catalogue, is 



In R-A. + 0"-51, and Decl. + 0"-38. 



So remarkably near an agreement as this, must, I think, be held to settle now 

 beyond all cavil, that — 



In CharacMr, this is an optical pair only. 



(23.) 7 DELPHINI, R. A. 20'^ 40™ 17^^ and Decl. + 15" 37' 54", January 1, 1862. 



" .^ °" Colour. Position. Distance. Date. Authority, 



ponents. nitucle. ■' 



A ... e , 



B ... AB 274 9 11-82 177974 H-. 



A ... 



B ... AB 273 13-50 1800 P. 



A ... 



B ... AB 273 26 11-83 182334 a 



A ... 



B ... AB 273 43 12-31 1823-68 H^ and S. 



