BINARY STAR a CENTAURI. 449 



I have inserted here the observation of Ma?kelyne in 1761, with which, pro- 

 bably, Sir J. Heeschel was unacquainted ; it makes an apparently bad figure 

 among the rest, but is by no means to be left out on that account merely, seeing 

 the care and the superior means for that day with which the measures were made. 

 " Mr Fallowes' determinations," continues Sir John, " in this series, are open 

 to objection, from the decidedly inadequate instrumental means by which they 

 were furnished (a small altitude and azimuth circle). Mr Taylor's results also 

 rest on so few observations, as to entitle them to little weight. 



" Though it is obviously impracticable to deduce any elliptic elements from 

 such a series, there are some features which it is impossible not to recognise. 

 There can be no doubt that the distance has gone on decreasing since 1822 at 

 least; and the comparison of the measures least open to objection leads us to con- 

 clude that, for the ten years previous to 1838, the rate of decrease was jg, or a 

 little more than half a second per annum, which, if continued, wiU bring on an 

 occultation, or exceedingly close appulse, about the year 1867. The small amount 

 of variation in the angle of position shews that the plane of orbitual motion passes 

 nearly, but not quite through our system, while its actual tendency to increase 

 exemplifies the general law of increase of angular velocity, with diminution of 

 distance. Mr Fallowes' distance is probably too great by 3" or 4" ; but in the 

 long interval between 1750 and 1822 (at the former of which epochs the distance 

 must have been on the increase), there is room for a very much greater excursion 

 of the small star towards its apparent aphelion, so that, although we are sm-e that 

 the major axis of the real ovbii must materially exceed 24", it is impossible to say 

 how much it may exceed that limit. Taking, therefore, the co-efficient of parallax 

 for a Centauri, as determined by Professor Henderson, at 1", it wiU follow from 

 what has been said, that the real orbit of one star about the other cannot be so 

 small as that of the orbit of Saturn about the sun, and exceeds, in all probability, 

 that of the orbit of Uranus. 



" The plane of the orbit in the case of a Centauri, passing nearly thi-ough our 

 system, my method of approximating to the elliptic elements becomes inapplicable, 

 and for their determination, measures of the distance of the stars from each other 

 can alone be relied on. No subject more worthy of continued and diligent inquiry 

 can possibly be urged on the attention of southern astronomers.'' 



Thus the result arrived at, both by Professor Henderson and by Sir J. Herschel, 

 and which, though proved since to be erroneous, would have been probably con- 

 cluded by any one else from the same data, seems to be, that the smaller star 

 had been employed during the last century in gaining its aphelion, without any 

 sensible change of angle of position ; what the aphelion distance, the diameter 

 of the orbit, and the period of revolution, might be, no guess could be at- 

 tempted : but in his address, on the occasion of giving the gold medal to 

 Besset for his discovery of the parallax of 6 Cygni, Sir John Herschel stated, 



VOL. XVI. PART IV. 5 Y 



