August 1, 1890.] 



KNOWLEDGE. 



185 



274 - ■tiBI, 



(about f second) the star is not within the 

 reach of small telescopes. 



7. Strui-e 2173. — In this pair a complete 

 revolution has been described since its dis- 

 covery by Struve in 1829. The components 

 are nearly equal, about 6th magnitude ; 

 but the present distance is less than one 

 second of arc. Dimer's orbit, with a period 

 of about 45i years, seems to be a fjood one, 

 as it represents recent measures satisfac- 

 torily. The iral orbit is nearly circular, 

 but owing to its high inclination (nearly 

 81°) the (ijijiarent orbit is a very elongated 

 ellipse. 



Fig. 7. — A Centauki. Real Okbit. 



Fig. 8. — Apparent Orbit of 70 Ophiuchi. 



8. Siiiits. — The faint companion to this 

 brilliant star was discovered by .\lvan Clark 

 in 1862. Some irregularities in the proper 

 motion of Sirius led Bessel, in 18-14, to 

 suggest the existence of a disturbing body, 

 and Peters, in 1851, calculated an hypo- 

 thetical orbit for the supposed companion, 

 lie found a period of about fifty years, ^\•ith 

 an ellipse of large eccentricity. An investi- 

 gation was also made by Safford in 1861, 

 and when Alvan Clark (.llscovered the now 

 well-known companion its position was 

 found to agree fairly well with that of 

 Safford's hjiwthetical body. The com 

 panion, which is of about the tenth mag- 

 nitude, has been regularly observed smee, 

 and several orbits have been computed 

 giving periods of about fifty years. Kecent 

 measures, however, show, I thmk, that 

 this period is somewhat too small. My 

 period of 58-47 years (computed in 1889) 

 represents all the measures fairly well. 

 Observing with the 36-inch refractor of the 

 Lick Observatory Mr. Biu-nham says:— 

 " The companion to Sirius is a very easy 



