23 



A good series of measures was obtained by me again on the 

 evenings of the 16th and 17th. 



On the former evening it was compared with B.A.C. 489, 

 from which its approximate position was found to be 

 R.A., lh. 29m. 26s.") . 1n1 , , 

 Dec, 31° 21' south \ at 10h ; lm " 



On that evening I thought I could occasionally see a slight 

 stellar condensation, but this was not confirmed on the follow- 

 ing evening, when its appearance in the 8-inch equatorial was 

 that of a faint nebulous telescopic comet, without any defined 

 nucleus. As I have before said, it would bear no illumination 

 of the field, and the measures were taken at the comb of the 

 micrometer. The tail could at times be just traced in the field 

 as a slight elongation of the nebula. The reference star used 

 on the 17th was Lacaille No. 522. It cannot be found in any 

 other or modern catalogue, and its position, therefore, requires 

 verification. The approximate place of the comet deduced 

 from fourteen measures in E.A, and ten in declination was — 

 E.A., lh. 41m. 16s. ; Dec, 30° 18' at lOh. 5m. 



I have been unable to identify the comet with any previous 

 one. No orbit has been calculated, and in fact the few obser- 

 vations we have obtained are insufficient to give more than a 

 very rough approximation to its path. "Whether, therefore, it 

 is a new friend or a regular denizen of our system I cannot 

 say. _ 



With comets, as you are aware, the space through which we 

 can follow them is often too limited to enable us to deduce an 

 elliptical orbit. A parabola, and in some instances an hyper- 

 bola, will satisfy all the observations, and in that case no 

 period of revolution can be obtained ; in other words, we 

 cannot say whether the comet will soon return to us, or 

 whether it will go on to other systems far away in the infinite 

 space by which we are surrounded. "We are, however, helped 

 by knowing the velocity of its motion. For instance, it has 

 been shown that if a comet at a distance from the sun of about 

 92,000,000 travelled at a rate of 26 miles a second, it would 

 describe a parabola ; and, therefore, unless arrested in its 

 course by the action of one or rnore of the larger planets, it 

 would never return. 



Many comets to which a parabolic orbit has been assigned 

 no doubt move in very elongated ellipses, and will and do 

 return to us after long intervals. 



Now, the position of a comet's orbit in space is fixed : — 



1. By the inclination of the orbit to the ecliptic. 



2. The points of intersection with that plane — i.e., the 



longitude of its ascending and descending nodes. 



3. The longitude of perihelion. 



