68 On the Observed Motions of the Companion of Sirius, 



To which must be added, that the first hypothesis requires 

 an increase of distance between 1862*2 and 1863 '2 of 0"*8 ; 

 the third, a very slight diminution ; but observation indi- 

 cates a diminution of about O'^'oS, a quantity, to use Mr 

 Kutheriurd's expression,* so small that its existence cannot 

 be asserted wdth confidence." It is hardly conceivable that 

 the long and careful series of observations of Mr Kutherfurd 

 should be in error 3^*3 ; and also inconceivable that Professor 

 Bond's measures, agreeing as they do within 2° 20' among 

 themselves, should be in the mean 5°-4 erroneous. 



We have therefore nothing to oppose to the hypothesis 

 that the new companion is the disturbing body, but the 

 very improbable supposition that the small star partakes 

 very nearly in the great proper motion of Sirius without 

 physical connection ; or the second hypothesis, that the new 

 star is in the system, but with small mass. If this is the 

 case, the disturbing body must, in lieu of the small light of 

 the companion, have still less, or even be absolutely invisible. 

 It is consequently highly probable that the disturbing body has 

 been actually found ; that what was predicted by theory has 

 been confirmed by sight. The importance of continued obser- 

 vations on Sirius cannot be too highly felt. The companion 

 must be measured the coming year, and for several years ; 

 while Sirius itself should be re-observed with meridian in- 

 struments. So far as the right ascension element is con- 

 cerned, a series of observations is now in progress at Cam- 

 bridge ; while Captain Gilliss has most obligingly consented 

 to make a series of declination-observations at Washington ; 

 and the standard observatories at Greenwich and Paris will 

 doubtless continue their series of fundamental star observa- 

 tions, including, of course, Sirius. 



I am much obliged to Mr Eutherfurd for the communica- 

 tion of the details of his observations in 1863, and hope he 

 will publish them, together with similar details of those of 

 1862, and others to be made hereafter. The subject is one 

 where the co-operation of several observers is desirable. 

 Full certainty here can only be obtained after several years' 

 observations. 



* As before, American Journal of Science for May 1863, p. 407. 



